Effects of Non-Identical Rayleigh Fading on Differential Unitary

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					IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 57, NO. 5, MAY 2009                                                                                             1359




           Effects of Non-Identical Rayleigh Fading on
           Differential Unitary Space-Time Modulation
                                                                     Meixia Tao, Member, IEEE



   Abstract—Non-identical fading distribution in a multiple-input                             both time and space. A number of unitary ST signal sets
multiple-output (MIMO) channel, including unequal average                                     have been designed, including orthogonal codes [3]–[5], cyclic
channel gains and fade rates, often occurs when antennas are not                              group codes [6], and Cayley differential codes [7].
co-located. In this paper, we present an analytical study of the
effects of non-identical Rayleigh fading on the error performance                                It is commonly assumed in the design and performance
of differential unitary space-time modulation (DUSTM). The                                    analysis of space-time coding that the channels on different
fading processes for different transmit-receive antenna pairs are                             transmit-receive antenna pairs are statistically identical. The
assumed to be independent and time-variant. We find that the
                                                                                              assumption typically holds when antennas in the system
maximum-likelihood (ML) differential detector of DUSTM over
such channels is involved except for differential cyclic group                                are co-located and hence the channel path loss, as well as
codes. The conventional detector is proved to be asymptotically                               potential shadowing, experienced by each signaling branch is
optimal in the limit of high signal-to-noise ratio (SNR) over static                          the same to each other. There are many occasions, however,
fading channels. Applying the distribution of quadratic forms of                              that the antennas are not necessarily co-located. For instance,
Gaussian vectors, we then derive closed-form expressions for the
                                                                                              in distributed antenna systems [8], [9], the antennas are
exact error probabilities of two specific unitary classes, namely,
cyclic group codes and orthogonal codes. Simple and useful                                    geographically distributed at different radio ports and are
asymptotic bounds on error probabilities are also obtained. Our                               connected together through high-speed cables. It is natural to
analysis leads to the following general findings: (1) equal power                              expect different path loss as well as fade rates on different
allocation is asymptotically optimal, and (2) non-identical channel                           links. Similarly, in aeronautical telemetry communications
gain distribution degrades the error performance. Finally, we
                                                                                              [10], multiple antennas can be placed at different parts of
also introduce a water-filling based power allocation to exploit
the transmit non-identical fading statistics.                                                 the air vehicle and hence they experience different attenuation
                                                                                              during maneuvers. Cooperative communications among mo-
   Index Terms—Differential detector, error probability analysis,
independent and non-identical channels, Rayleigh fading, space-
                                                                                              bile nodes in a network is another important scenario. After
time modulation.                                                                              knowing each other’s data to be sent, the cooperating nodes
                                                                                              can form a virtual multiple-antenna system and employ space-
                                                                                              time coding in a distributed manner [11], [12]. Clearly, the
                             I. I NTRODUCTION                                                 different signaling branches in the cooperation phase can have

T     HE use of multiple antenna elements promises con-
      siderable diversity and multiplexing gains in wireless
communication systems. This motivated enormous develop-
                                                                                              unequal fading statistics. In all the aforementioned MIMO (or
                                                                                              virtual MIMO) communication settings, the resulting channels
                                                                                              can be modeled as independent and non-identically distributed
ment of multiple-input multiple-output (MIMO) techniques                                      (i.n.i.d) fading.
in the context of space-time (ST) coding and modulation                                          The goal of this paper is to study the effects of non-
in the last decade. Existing ST techniques can be broadly                                     identical fading distribution on the performance of existing ST
divided into coherent schemes and non-coherent schemes,                                       codes, in particular differential unitary space-time modulation.
based on whether or not instantaneous channel knowledge is                                    There are two issues to be addressed. First, whereas uniform
needed by the receiver. As channel estimation is waived, non-                                 power allocation in the spatial domain for both coherent and
coherent schemes can not only reduce receiver complexity                                      non-coherent ST codes is capacity-achieving in traditional
but also lower transmission overhead required for sending                                     independent and identically distributed (i.i.d) fading, it may
pilot symbols. Among the non-coherent schemes, differential                                   not be so in i.n.i.d fading. Therefore, it is of interest to
unitary space-time modulation (DUSTM) [1], [2] is known                                       investigate the optimal power allocation among the distributed
for its good error performance and high spectral efficiency.                                   antennas (or cooperating nodes). Second, the conventional
DUSTM is often viewed as a multiple-antenna counterpart of                                    differential detector for DUSTM over i.i.d channels may no
differential phase-shift-keying (DPSK) modulation, where the                                  longer be optimal in the maximum-likelihood (ML) sense.
signal constellation is a set of unitary matrices spread over                                 Hence, optimal differential detector is to be discussed.
   Paper approved by G. M. Vitetta, the Editor for Equalization and Fading                       Attempts have been made recently to study the effects of
Channels of the IEEE Communications Society. Manuscript received October                      non-identical channels in MIMO systems from different as-
15, 2007; revised February 8, 2008.
   This work was presented in part at the IEEE International Conference on                    pects. The outage probability of mutual information and power
Communications, Beijing, China, May 2008. This work is supported in part by                   control over distributed multiple-input single-output (MISO)
the Doctoral Fund of the Ministry of Education of China (No. 20082481002).                    channels with independent Rayleigh fading are studied in [13].
   The author is with the Department of Electronic Engineering, Shanghai Jiao
Tong University, Shanghai 200240, P. R. China (e-mail: mxtao@sjtu.edu.cn).                    The bit error probabilities (BEP) of coherent orthogonal space-
   Digital Object Identifier 10.1109/TCOMM.2009.05.070534                                      time block codes (OSTBC) over i.n.i.d Rayleigh/Riciean and
                                                                      0090-6778/09$25.00 c 2009 IEEE


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1360                                                                                                  IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 57, NO. 5, MAY 2009



Nakagami fading channels are analyzed in [14] and [15],                                          antenna elements at both the transmitter side and receiver
respectively. In [16], the authors derived the BEP of differ-                                    side are not necessarily co-located. At each time block k,
ential OSTBC, i.e., the orthogonal-design based DUSTM [3],                                       a set of log2 L information bits are mapped onto a data
[5], over independent and semi-identically distributed (i.s.i.d)                                 matrix D[k] ∈ V, where V = {Di , 0 ≤ i < L} denotes
Rayleigh channels, where the non-identical fading occurs at                                      a unitary space-time signal constellation with cardinality L.
the receiver side only. The study in [16] shows that in i.s.i.d                                  Each element of V is an M × M unitary matrix, satisfying
channels the ML differential detector (DD) for differential                                      Di DH = IM , for 0 ≤ i < L. For the special case of
                                                                                                       i
OSTBC is still on a per symbol basis but should weight                                           differential cyclic group codes [2], [6], the constellation set V
the output from each receive antenna according to its fading                                     forms a group under matrix multiplication and each element
statistics. Moreover, the ML detector significantly outperforms                                   of it is a diagonal matrix. In the case of differential OSTBC,
the conventional one at high signal-to-noise ratio (SNR) region                                  each element Di is a linear mapping of a set of P M-ary PSK
when the channel fluctuates rapidly over time.                                                    modulated information symbols, denote as {sp = ejθp }P ,     p=1
   In this paper, we extend the previous work in [16] to                                                                     1    P
                                                                                                 and is given by D[k] = √P p=1 (Φp cos θp + jΨp sin θp ).
a general framework of DUSTM over i.n.i.d time-varying                                           Here the set of encoding matrices {Φp , Ψp }P are chosen
                                                                                                                                                 p=1
Rayleigh fading channels. We first show that for a general                                        subject to certain orthogonality constraints [17].
unitary space-time constellation the ML differential detector                                       Let S[k − 1] denote the M × M dimensional code matrix
needs to perform joint optimization of the current data matrix                                   at the (k − 1)-th time block. The data matrix D[k] is then
and the previously transmitted signal matrix. However, for                                       differentially encoded as
cyclic group codes, it is independent of the previous signals
and differs from the conventional DD only by appropriate                                                                       S[k] = D[k]S[k − 1],
weights. The conventional DD is shown to be asymptotically
optimal in the limit of high SNR over static fading channels.                                    where the initial code matrix S[0] is an arbitrary unitary
We then apply the well-established distribution of quadratic                                     matrix. The actual signal matrix to be transmitted at time block
forms of Gaussian variables to derive the error performance                                      k over M antennas is given by
for two specific unitary classes: orthogonal codes and cyclic
group codes. For cyclic group codes, closed-form expressions                                                                 X[k] =          Es S[k]Σ1/2 ,     (1)
for the exact pairwise error probabilities (PEP) with both ML
and conventional DD at arbitrary channel fluctuation rates                                        where Es is the total transmit power, and Σ1/2 =
                                                                                                       √           √
are derived. For orthogonal codes, closed-form expressions                                       diag{ ε1 , . . . , εM } is the diagonal power allocation matrix.
for the exact BEP with conventional DD in static fading                                          The power allocation coefficients εm ’s are subject to the
are derived. Furthermore, simple asymptotic bounds on error                                      constraint M εm = M and to be optimized.
                                                                                                               m=1
probabilities for both codes are obtained. These bounds lead to                                     Since the transmission is on a per block basis, we assume
several useful findings applied to any DUSTM design. Lastly,                                      the channel is block-wise time-varying with each block con-
we propose a water-filling based power control to exploit                                         taining M symbol intervals. Let H[k] denote the M × N
the transmit non-identical fading statistics. This is carried                                    channel matrix of the k-th transmission block, where the
out by minimizing the Chernoff bound of approximate error                                        (m, n)-th entry hmn [k] represents the fading coefficient from
probabilities under a total power constraint.                                                    the m-th transmit antenna to the n-th receive antenna. Each
   The rest of the paper is organized as follows. In Section                                     {hmn [k]}k is modeled as a complex Gaussian wide-sense
II we present the system model of DUSTM over i.n.i.d                                             stationary random process with zero mean and autocorrelation
time-varying flat Rayleigh fading channels. The optimal and                                       function 2Rmn [l] = E[hmn [k]h∗ [k − l]], and is independent
                                                                                                                                   mn
suboptimal detectors are presented in Section III. The analysis                                  for different m and n. The channel variance and block
                                                                                                                                           2
of error probabilities is presented in Section IV, followed by                                   correlation coefficient are denoted as σmn = 2Rmn [0] and
                                                                                                                                                            2
the derivation of transmit power control in Section V. Some                                      ρmn = Rmn [1]/Rmn [0], respectively. The parameters {σmn }
numerical examples are illustrated in Section VI. Finally,                                       and {ρmn } represent the unequal average channel gains and
Section VII offers some concluding remarks.                                                      unequal channel fluctuation rates, respectively.
   Notations: E[·] denotes expectation over the random vari-                                        Let Y[k] denote the M × N received signal matrix from N
ables within the brackets. Tr(A) and ReTr(A) stand for the                                       antennas at the k-th transmission block. It is modeled as
trace and the real part of the trace of matrix A, respectively.
  · 2 represents the squared Frobenius norm. IM is the M ×M                                                             Y[k] =           X[k]H[k] + W[k],       (2)
identity matrix. diag(a1 , . . . , aM ) is the diagonal matrix with
element am on the m-th diagonal. Superscripts (·)T , (·)∗ ,                                      where W[k] is the complex-valued additive white Gaussian
and (·)H denote transpose, conjugate, and conjugate trans-                                       noise matrix whose entries are i.i.d with zero mean and
pose, respectively. Notation        and ⊗, respectively, represent                               variance N0 .
the Hadamard product and Kronecker product. Res[f (x), p]
denotes the residue of function f (x) at pole x = p.                                                                  III. D IFFERENTIAL D ETECTION
                   II. S YSTEM M ODEL                                                              Detection techniques of DUSTM over time-varying fading
 Consider a communication system with M transmit and                                             channels have evolved from traditional one-shot differential
N receive antennas over a flat Rayleigh fading channel. The                                       detection based on two consecutive blocks of received signals

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TAO: EFFECTS OF NON-IDENTICAL RAYLEIGH FADING ON DIFFERENTIAL UNITARY SPACE-TIME MODULATION                                                                             1361



[2] to more advanced sequence detection which jointly pro-                                    over i.n.i.d channels:
cesses multiple blocks, e.g., [18]–[22]. While these multiple-                                                                                 N
symbol based sequence detectors are able to attain coherent-                                                 ˆ
                                                                                                             DML = arg max max                       H˜˜ ˜
                                                                                                                                                    yn SCn SH yn .      (7)
like performance, they are difficult to analyze. To quantita-                                                                    D∈V S−1
                                                                                                                                              n=1
tively study the effects of non-identical fading statistics, we
focus on the one-shot differential detection for the convenience                                 The complexity of the ML detector is proportional to the
of analytical tractability. In this section, we first discuss a                                product of the constellation size L and the total number of
general structure of the one-shot optimal differential detector                               all possible previous code matrices. For those constellations
of DUSTM in the ML sense over the considered channel                                          with group structure, S−1 also belongs to the signal set V and
model. Simplified detectors under certain constraints are then                                 hence the complexity is in the order of L2 . For constellations
discussed. For notation brevity, we define γ0 = Es /N0 as                                      without group structure, such as OSTBC, the total number of
                                                    2
the total transmit SNR, and define γmn = εm σmn γ0 as the                                      possible S−1 can grow rapidly as L increases.
SNR on the branch between transmit antenna m and receive                                         For the semi-identical channel [16] and with equal power
antenna n, for 1 ≤ m ≤ M and 1 ≤ n ≤ N . In our high                                          allocation, we have γmn = γn and ρmn = ρn , for all m.
SNR assumption, all γmn ’s approach infinity as γ0 → ∞, but                                    It follows that both Cn0 and Cn1 become scaled identity
the ratios between one another are kept constant and finite.                                   matrices. Therefore, the ML detector no longer depends on
We also omit the time index k in the data matrix D[k] and                                     S−1 and is simplified to
rewrite S[k − 1] as S−1 hereafter as only one data matrix is
processed at one time.                                                                                                              N
                                                                                                                                                     I
                                                                                                  ˆ
                                                                                                  DML,semi = arg max                    wn yn
                                                                                                                                            H
                                                                                                                                                            I   DH   yn ,
                                                                                                                           D∈V                       D
                                                                                                                                  n=1
A. ML Detection of a General Constellation
                                                       T
                                                                                              where wn = γn ρn /[(1+γn)2 −(ρn γn )2 ]. This detector differs
   Define Y = Y [k − 1], Y [k] . Given D and S−1 ,
                           T                   T
                                                                                              from the conventional detector for i.i.d channel [2] at the
it can be shown easily that the column vectors of the                                         weights wn ’s only. The weights exploit the knowledge of
sufficient statistics Y, denoted as yn for n = 1, . . . , N ,                                  fading statistics and total transmit SNR, and are optimal for
are mutually independent Gaussian vectors with zero mean                                      any unitary constellation including OSTBC [16].
and covariance (see (3) at top of next page). Here, Kni ’s,
for i = 0, 1 and n = 1, . . . , N , are M × M diagonal
matrices defined as Kn0 = diag{γ1n , . . . , γMn } and Kn1 =                                   B. ML Detection of Cyclic Group Codes
diag{ρ1n γ1n , . . . , ρMn γMn }. Applying the formula for the
determinant of a partitioned matrix [23], we can show that                                       It is shown in [6] that every full-diverse unitary constellation
the determinant of the covariance matrix Λn is independent                                    having a group structure can be made equivalent to a cyclic
of the data matrix D and the previous code matrix S−1 . The                                   (also called diagonal in [2]) group for odd M , and either a
inverse of Λn , however, in general depends not only on D, but                                cyclic group or dicyclic group for even M . Therefore, the
also on S−1 . Using the Sherman-Morrison-Woodbury formula                                     cyclic group constellations are of particular interest to us.
for the inverse of the matrix of the form A + BCD [23] and                                    Because of the diagonal structure inherent in cyclic groups,
utilizing the diagonal structure of Kni , we obtain the inverse                               the code matrix S−1 is always diagonal as long as the
of Λn as                                                                                      initial matrix S[0] is diagonal, say IM . Since multiplication
                                                                                              commutes for diagonal matrices, we have S−1 Cni SH = Cni ,
                                                                                                                                                      −1
                                         1 ˜       Cn0       Cn1       ˜                      for all i and n. Therefore, the ML detector for cyclic group
      (Λn )−1        =    1
                          N0 I2M     −   N0 S                          SH ,          (4)
                                                   Cn1       Cn0                              codes reduces to:
                                                           ˜
                                                           Cn                                                              N
                                                                                                                                           I
                                                                                              ˆ
                                                                                              DML,c = arg max                   yn
                                                                                                                                 H
                                                                                                                                                         I Cn1 DH    yn (8)
where matrices Cni , for i = 0, 1 and n = 1, . . . , N , are also                                                  D∈V                   Cn1 D
                                                                                                                          n=1
diagonal, whose m-th diagonals, for m = 1, . . . , M , are given
by                                                                                            which can be further expressed as:
                                    γmn [1 + γmn (1 − ρ2 )]                                   ˆ
                                                                                              DML,c = arg max ReTr (Y[k − 1]                          W)H DH Y[k] ,         (9)
             Cn0            =                          mn
                                                             ,                       (5)                           D∈V
                     m             (1 + γmn )2 − (ρmn γmn )2
                                            γmn ρmn
             Cn1            =                                .                       (6)      where W is an M ×N matrix with the (m, n)-th entry formed
                     m             (1 + γmn )2 − (ρmn γmn )2                                  by the m-th diagonal of Cn1 and repeated as:
   The ML differential detector of D is to choose the candidate                                                                        γmn ρmn
 ˆ
D ∈ V that maximizes the joint likelihood function of the                                                        wmn =                                   .             (10)
                                                                                                                               (1 + γmn )2 − (ρmn γmn )2
received signal matrix Y over all possible S−1 . Note that the
dependence of ML detection on S−1 also arises in transmit-                                    It is clear from (9) that the ML DD for cyclic group codes
correlated channels as mentioned in [24] and [25]. But no                                     resembles the conventional DD but applies a weight wmn to
explicit ML decision metric is given therein due to the lack                                  the (m, n)-th element of Y[k − 1]. The resulting Hadamard
of closed-form expression for (Λn )−1 . Applying (4), we have                                 product Y[k−1] W behaves as the equivalent channel matrix
the quadratic-form based ML differential detector of DUSTM                                    as in coherent receivers.

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                                  Λn      = E[yn yn ]
                                                  H

                                                 S−1                0                 Kn0        Kn1              SH
                                                                                                                   −1        0
                                          = N0                                   ·                          ·                               + N0 I2M .                            (3)
                                                  0                DS−1               Kn1        Kn0               0       SH DH
                                                                                                                            −1

                                                                  ˜
                                                                  S                                                        ˜
                                                                                                                           SH




C. Asymptotically Optimal Detection of a General Constella-                                      where the pairwise decision variable zij is defined as
tion in Static Channels                                                                                                                 N
   In static fading channels the channel coefficients are as-                                                                  zij =          yn Ωij,n yn ,
                                                                                                                                              H
                                                                                                                                                                                 (14)
sumed to remain unchanged over the duration of two trans-                                                                              n=1
mission blocks. Therefore it has ρmn = 1 for all m and n. In                                     with Ωij,n given by
the limit γmn → ∞ for all m and n, the matrices Cn0 and
Cn1 defined in (5) (6) all approach (1/2)IM . Applying these                                                                   0                       Cn1 (Di − Dj )H
                                                                                                       Ωij,n =                                                                   (15)
into (7), we obtain the asymptotically optimal detector:                                                                Cn1 (Di − Dj )                       0
                                         N                                                       for ML detection and
                                                       I
        ˆ
        DAO        =      arg max             yn
                                               H
                                                                 I DH           yn . (11)                                           0      (Di − Dj )H
                               D∈V                     D                                             Ωij,n = Ωij =                                                      , ∀n     (16)
                                       n=1
                                                                                                                                (Di − Dj )      0
This is identical to the conventional DD [2]. Hence we
                                                                                                 for conventional detection. Since each vector yn is inde-
conclude that the conventional DD is suboptimal in i.n.i.d
                                                                                                 pendent and zero-mean complex Gaussian distributed, the
time-varying channels but asymptotically optimal (high SNR)
                                                                                                 pairwise decision variable zij is a quadratic form of Gaussian
in i.n.i.d static channels.
                                                                                                 vectors. Therefore, the evaluation of PEP can be carried out by
                                                                                                 using the well-established techniques in, e.g., [24, Appendix
                  IV. E RROR P ROBABILITY A NALYSIS                                              A]. We summarize the results in the following proposition.
  In this section, we derive the error performance of DUSTM                                        Proposition 1: The exact pairwise error probability Pe,ij
with two specific constellation designs: cyclic group codes                                       of differential cyclic group codes over i.n.i.d time-varying
and orthogonal codes. Through the analysis, we obtain several                                    Rayleigh fading channels is (17) at the bottom of the page,
general findings that are applicable to an arbitrary DUSTM                                        where
design.                                                                                                                               (ρmn γmn )2
                                                                                                                amn      =                                dij,m                  (18)
                                                                                                                                (1 + γmn )2 − (ρmn γmn )2
A. Pairwise Error Probability for Cyclic Group Codes                                             with dij,m being the m-th diagonal entry of the difference
                                                                                                 matrix (Di − Dj )(Di − Dj )H , and
  Since the exact bit or block error probability of a cyclic
group code V with L > 2 elements is usually not computable,                                                                     1,                       ML detector
we resort to the union bound by summing up pairwise error                                                  bmn      =                                                   . (19)
                                                                                                                                wmn in (10),             conv. detector
probabilities. In specific, the block error probability (BkEP)
for equiprobable elements is bounded by                                                          In the case of ML differential detection, an alternative expres-
                                                                                                 sion is given by
                                                 L−1 L−1
                                             1                                                                                      π/2 N      M                        −1
                         Pe,UB       =                      Pe,ij ,                   (12)                                1                                    amn
                                             L                                                           Pe,ij      =                                   1+                   dθ. (20)
                                                 i=0 j=0                                                                  π     0      n=1 m=1
                                                                                                                                                             4 sin2 θ
                                                     j=i

where Pe,ij denotes the PEP of deciding in favor of data                                              Proof: See Appendix A.
matrix Dj given that Di is sent. In the following we derive                                         Although the expression for Pe,ij is exact and in closed
the exact expressions for Pe,ij and the asymptotics.                                             form, it does not offer much insight on the effects of channel
  Based on the quadratic form of the ML and suboptimal                                           parameters. Therefore, useful asymptotic bounds are desirable.
detectors given in (8) and (11) respectively, the PEP can be                                     We first consider the asymptotic Pe,ij over static channels
expressed as                                                                                     with ρmn = 1, for all m, n. Then we conduct the error floor
                                                                                                 analysis in time-varying channels. The results are summarized
                          Pe,ij      =       P (zij < 0|Di ) ,                        (13)       in the following two corollaries.


                                                       ⎧                                                                                                        ⎫
                                                       ⎨                                     1                                                  1         1   1 ⎬
                Pe,ij      = −                   Res                                                                          , s = bkl           +         +                    (17)
                                                       ⎩s       M         N              1        1
                                                                                                       −     s
                                                                                                                   −    1 2                     2         4 akl ⎭
                                     1≤k≤M                      m=1       n=1 amn        4   +   amn        bmn         2
                                     1≤l≤N




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TAO: EFFECTS OF NON-IDENTICAL RAYLEIGH FADING ON DIFFERENTIAL UNITARY SPACE-TIME MODULATION                                                                             1363



   Corollary 1: The asymptotic PEPs of differential cyclic                                      Corollary 2: The pairwise error floor of differential cyclic
group codes at high SNR with ML and conventional differen-                                    group codes with ML differential detection over i.n.i.d time-
tial detection over i.n.i.d static Rayleigh fading channels are                               varying Rayleigh fading channels is independent of the un-
the same and given by                                                                                                        2
                                                                                              equal average channel gains {σmn } but depends on the fading
                           2M N − 1
                                                M              −N
                                                                       γgm     −M N           correlation coefficients {ρmn } only. It is given by (24) at the
  lim Pe,ij       =                                  dij,m                             ,      bottom of the page. In the case where ρm,n = ρ and ρ ≈ 1
γ0 →∞                        MN                m=1
                                                                        2
                                                                                     (21)     but ρ = 1, the pairwise error floor is simplified as

where γgm is the geometric mean of {γmn }, given by                                                        lim
                                                                                                         γ0 →∞
                                                                                                                     Pe,ij =
                             1/MN                                                                       ρmn =ρ≈1
                                    , and 2MN −1 denotes the
            N     M
γgm =       n=1   m=1 γmn                  MN                                                                                           M               −N            −MN
binomial coefficient.                                                                                             2M N − 1                                      ρ2
                                                                                                                                              dij,m                     (25)
                                                                                                                                                                          .
     Proof: Assuming static fading channels with ρmn = 1                                                           MN                                        1 − ρ2
                                                                                                                                       m=1
and taking the limit γ0 → ∞, one finds from (18) and (10)
that amn → γmn dij,m /2 and wmn → 1/2. As a result, the                                            Proof: The proof of the first equation in the
M N poles, where the residues are evaluated in Proposition 1,                                 corollary is straightforward by observing that amn →
all approach the constant 1 for ML DD and the constant 1/2                                    ρ2 dij,m /(1 − ρ2 ) when γ0 → ∞ in (17) (with bmn = 1).
                                                                                                mn               mn
for conventional DD. Using the residue equation of a function                                 If ρm,n = ρ and ρ ≈ 1 but ρ = 1, the M N positive poles
f (x) at a pole p of multiplicity v                                                           all approach 1/2. Using the formulas (22) and (23) again, we
                                                                                              prove the second equation in the corollary.
                              1          dv−1
   Res [f (x), p] =                 lim       [(x − p)v f (x)]                       (22)        Corollary 2 concludes that the irreducible error floors
                           (v − 1)! x→p dxv−1                                                 achieved over i.n.i.d channels (with ML detection) and tra-
and applying the formula                                                                      ditional i.i.d channels are the same, as long as their fade rates
                                                                                              are the same (ρm,n = ρ, ∀m, n). Moreover, the error floor
  dm             (−1)m (m + k − 1)!
     (x + p)−k =                    (x + p)−(m+k) (23)                                        decreases exponentially with M N when ρ is very close to
 dxm                  (k − 1)!
                                                                                              but not equal to one. This condition on ρ typically holds if
in (17), we arrive at the asymptotic results in (21) for both                                 the normalized Doppler frequency of the channel is much less
ML and conventional DD.                                                                       than one.
    Corollary 1 leads to several insights. First, the result that
the asymptotic PEPs of ML and conventional DD are the
same is consistent with the finding in Section III-C that the                                  B. Bit Error Probability for Orthogonal Codes
conventional DD is in fact asymptotically optimal for a general                                  In this subsection we derive the error performance of
constellation without assuming a specific signal structure.                                    differential OSTBC. Only the conventional DD and static
Second, the traditional diversity product design criterion1 for                               channels are considered. The analysis for time-varying fading
i.i.d channels [1], [2] still applies to i.n.i.d channels. That is,                           or ML detection is so far not tractable. Since the data matrix
                     M
the minimum of m=1 dij,m over all distinct pairs (Di , Dj )                                   D is a linear combination of P information symbols as
should be maximized. Using the arithmetic-geometric-mean                                      mentioned in Section II, the differential detector (11) reduces
                                                  N      M
inequality (i.e. γgm ≤ γam = 1/M N n=1 m=1 γmn ,                                              to P independent symbol-by-symbol detectors. The details are
where the equality holds if and only if γmn ’s are all the same),                             given in [5] or [16, eq.(12)]. Hence, instead of PEP, BEP is
we can see that the non-identical channel distribution will de-                               derived.
grade the error performance compared with the identical case                                     As shown in [16, eq.(15)-(17)], the BEP conditioned on
if the total received SNR is kept the same. Furthermore, after                                symbol sp is the same for all p, and can be expressed as
rewriting γgm as γ0 ( m=1 εm )1/M ( n=1 m=1 σmn )1/MN , 2
                         M                 N      M

where εm is the power allocation coefficient defined in (1)                                                           Pb (α) = P (zp (α) < 0|sp = 1),                     (26)
subject to the constraint M εm = M , it follows that γgm
                               m=1                                                            where the decision phasor zp (α) is defined as
is maximized when εm = 1 for all m. Therefore, equal power
allocation is asymptotically optimal in static channels.                                                                                  N
                                                                                                                           zp (α) =           yn Ωp yn
                                                                                                                                               H
                                                                                                                                                                        (27)
  1 Assume   the full-rank criterion is already satisfied.                                                                               n=1



                                                     ⎧                                                                                                               ⎫
                                                     ⎨                                         1                                           1             1   1 − ρ2 ⎬
       lim Pe,ij           =    −              Res                                                                                     , s= +              +      kl
                                                                                                                                                                       (24)
     γ0 →∞                                           ⎩s        M         N  ρ2 dij,m
                                                                             mn                 1
                                                                                                    +
                                                                                                         1−ρ2
                                                                                                            mn
                                                                                                                    − s−        1 2        2             4 ρ2 dij,k ⎭
                                                                                                                                                              kl
     ρmn <1                         1≤k≤M                      m=1       n=1 1−ρ2               4       ρ2 dij,m                2
                                                                                mn                       mn
                                    1≤l≤N

                                            ⎧                                                                                                                      ⎫
                                            ⎨                                    1                                          cγkl                          1 + 2γkl ⎬
    Pb (α)      =      −              Res                                                                            , s=                          1+   1+ 2 2      (28)
                                            ⎩s       N         M
                                                                        1 + 2cγmn s − (1 + 2γmn )s2                       1 + 2γkl                          c γkl  ⎭
                           1≤k≤M                     n=1       m=1
                           1≤l≤N




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with the Hermitian matrix Ωp given by                                                            where E = Di − Dj . Given that Di is sent, substituting (2)
                                                                                                 into (30) yields
                            0                          cos αΦH + j sin αΨH
Ωp       =                                                   p           p
                                                                                             ,             zij     =              ˆ       ˆ
                                                                                                                         2ReTr{Es HH DH EH}
                   cos αΦp − j sin αΨp                          0                                                                     i
                                                                                                                                       ˆ
                                                                                                                          + 2ReTr{ Es H D EW[k − 1]}
                                                                                                                                         H H
                                                                                                                                                             i
and α is some angle that depends on the symbol modulation
                                                                                                                                                        η1
scheme. For BPSK, the exact BEP is obtained by letting α =
0, and for QPSK with Gray mapping we have α = −π/4.                                                                          + 2ReTr{                     ˆ
                                                                                                                                                Es WH [k]EH}
   Proposition 2: The exact bit error probability Pb (α) of                                                                                       η2
differential OSTBC over i.n.i.d static Rayleigh fading channels                                                               + 2ReTr{W [k]EW[k − 1]},
                                                                                                                                                  H
                                                                                                                                                                                (31)
with conventional differential detection is√(28) (see the bottom
                                                                                                                                                       η3
of the previous page), where c = cos α/ P .
     Proof: See Appendix B                                                                       where H = S−1 Σ1/2 H with H being the channel matrix in
                                                                                                         ˆ
   At high SNR, all the M N positive poles, where the residues                                   static fading. It can be easily shown that, conditioned on H, ˆ
are evaluated, approach the constant c. Therefore, we obtain                                     the noise terms η1 and η2 are independent and real-valued
the asymptotics of Pb (α) as follows by applying the definition                                   zero-mean Gaussian variables with variance 2N0 Es EH 2      ˆ
of residue as in the proof of Corollary 1.                                                       for each. By neglecting the second-order noise term η3 , which
   Corollary 3: The asymptotic BEP of differential OSTBC                                         has diminishing effect at high SNR (i.e., Es /N0              1),
over i.n.i.d static Rayleigh fading channels with conventional                                   and noting DH E + EH Di = EH E, the pairwise decision
                                                                                                                 i
differential detection is                                                                        variable zij can be approximated as a Gaussian variable with
                                                                                                 mean Es EH 2 and variance 4N0 Es EH 2 . As a result, the
                                                                                                              ˆ                             ˆ
                                                                       −MN
                            2M N − 1              2 cos2 α                                       conditional probability of zij < 0 can be expressed in the form
  lim Pb (α) =                                             γgm                  ,     (29)       of standard Q-function [26]. Further, applying the inequality
 γ0 →∞                        MN                     P
                                                                                                 Q(x) ≤ 1 exp (−x2 /2), we obtain the Chernoff bound of the
                                                                                                           2
where γgm is the geometric mean of {γmn }.                                                       approximate PEP as
  The implications in Corollary 3 are the same as those                                                                             1         γ0                 2
in Corollary 1. Therefore, we readily extend the following                                                          Pe                E exp −     ˆ
                                                                                                                                                 EH                  .
                                                                                                                                    2         8
remarks to the general DUSTM.
  Remarks: (1) Non-identical fading degrades the error perfor-                                     Obtaining the distribution of EH 2 is difficult in general if
                                                                                                                                     ˆ
mance compared with the identical case given the same total                                      E is the difference matrix of an arbitrary unitary constellation.
received SNR; (2) Equal power allocation is asymptotically                                       Fortunately, by utilizing the diagonal structure of cyclic group
optimal in i.n.i.d static fading channels.                                                       codes, it is clear that EH 2 can be expressed as a weighted
                                                                                                                            ˆ
                                                                                                 sum of absolute squares of M N i.i.d complex Gaussian
                                                                                                                                           2
                                                                                                 variables with weights given by εm σmn dij,m . Hence, the
                     V. T RANSMIT P OWER C ONTROL                                                above expectation can be evaluated as
                                                                                                                                N     M        2                         −1
   Given the unequal channel gain distribution among different                                                             1            γ0 εm σmn dij,m
                                                                                                          Pe,ij                      1+                                       . (32)
transmit antennas, it is intuitive to use power control to                                                                 2 n=1 m=1           8
improve the error performance, especially when the total
                                                                                                    We now find the optimal power allocation coefficients εm ’s
transmit power is small. To simplify investigation, we consider
                                                                                                 to minimize the bound in (32) for a dominant error pair,
static channels only in this section. Moreover, as shown in
                                                                                                 which consequently provides a good result in minimizing the
Section VI and [16], the conventional detector performs almost
                                                                                                 overall block error probability. The dominant error pair of a
the same as the ML detector in i.n.i.d static fading channels.
                                                                                                 cyclic group code is the data matrix pair that has the small-
Hence, we assume conventional DD here.                                                                 M                                           M
                                                                                                 est m=1 dij,m [2], denoted as ζ = min             m=1 dij,m .
   Both the exact PEP in Proposition 1 for cyclic group codes                                                                                                0≤i<j<L
and the exact BEP in Proposition 2 for orthogonal codes                                          However, there can be multiple pairs in the code that result
are difficult to minimize directly. We resort to minimizing a                                     in the same ζ, and they may differ dramatically in dij,m , for
simple but useful approximate bound of them. In the following                                    m = 1, . . . , M . Take the cyclic group code with M = 4
we present the derivation of transmit power control for the                                      and L = 16 for example, given by V4,16 = {Dk =
two codes separately for the ease of presentation, though the                                    diag(ejkπ/8 , ej3kπ/8 , ej5kπ/8 , ej7kπ/8 ), k = 0, . . . , 15} [27,
approaches are very similar.                                                                     Table I]. The data matrix pair (D0 , D1 ) is a dominant error
                                                                                                 pair and it has d1 = 0.1522, d2 = 1.2346, d3 = 2.7654 and
                                                                                                 d4 = 3.8478. On the other hand, the pair (D0 , D3 ) is also
A. Power Control for Cyclic Group Codes                                                          a dominant error pair but it has d1 = 1.2346, d2 = 3.8478,
                                                                                                 d3 = 0.1522, and d4 = 2.7654. The sets of power allocation
 The pairwise decision variable in (14) for the conventional                                     coefficients minimizing the two pairwise error probabilities
DD can be rewritten as                                                                           are obviously different. To overcome this problem, we take the
                                                                                                 mean of dij,m over all dominant error pairs for each m, denote
                      zij = 2ReTr{Y[k]H EY[k − 1]}                                    (30)             ¯
                                                                                                 it as dm , and replace all dij,m with it in the PEP bound (32).

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TAO: EFFECTS OF NON-IDENTICAL RAYLEIGH FADING ON DIFFERENTIAL UNITARY SPACE-TIME MODULATION                                                                                                        1365


                                                                                                                             0

Then, by using the monotonic property of logarithm function,                                                                10
                                                                                                                                                                          PEP with conv. decoder (17)
the power allocation problem can be formulated as                                                                                                                         PEP with ML decoder (17)
                                                                                                                                                                          PEP for i.i.d
                                                                                                                             −1
                                                                                                                                                                          Asymptotic PEP bound (21)
                                N     M
                                                σ 2 dm γ0
                                                    ¯                                                                       10                                            Pairwise error floor (24)
           max                        log 1 + εm mn                               . (33)
         M
         m   εm =M                                  8




                                                                                               Pairwise Error Probability
                              n=1 m=1
                                                                                                                             −2

   In the case of N = 1, we obtain the water-filling based                                                                   10
                                                                                                                                                                              ρ=0.99
closed-form expression for the optimal power control as [28]
                                                           +                                                                 −3
                                              8                                                                             10
                        εm =        μ−        ¯m 0
                                            2 d γ
                                                               ,                      (34)
                                           σm
where (x)+ = max{0, x}, and the Lagrange multiplier μ can                                                                    −4
                                                                                                                            10
                                                                                                                                                      ρ=1
                                  M
be determined by the constraint m=1 εm = M .
  If there are N > 1 number of receive antennas, closed-
                                                                                                                             −5
form expressions of optimal power coefficients are difficult to                                                               10
                                                                                                                                 0   5      10     15         20        25       30        35       40
find. Here we propose a suboptimal approach. Applying the                                                                                          total transmit SNR γ [dB]
                                                                                                                                                                     0

inequality [29, eq.(25)]
                                                                                              Fig. 1. PEP performance of differential cyclic group code V2,4 over the
                        N                                                                     dominant error pair at M = 2 transmit and N = 1 receive antenna.
                             (1 + xi ) ≥ (1 + xgm )N ,
                       i=1
                                                                                              Correspondingly, a water-filling based sub-optimal power con-
                              xi )1/N , we can reformulate (33) as
                        N
where xgm = (           i=1                                                                   trol that minimizes the bound is given by
                                M                         2    ¯                                                                                                                +
                                                         σm,gm dm γ0                                                                                         2P
             max                      log 1 + εm                              ,                                                          εm =    μ−                                 .             (36)
           M
           m   εm =M                                         8                                                                                                 2
                                                                                                                                                      cos2 (α)σm,gm γ0
                                m=1

         2                  2
where σm,gm = ( n=1 σmn )1/N . Hence, the solution in (34)
                            N
                                                                                                                                           VI. N UMERICAL R ESULTS
                               2       2
still applies after replacing σm with σm,gm , and is given by
                                                                                                 In this section we present some numerical examples to con-
                                                               +                              firm our analytical findings in previous sections. We first verify
                                              8                                               the error probability analysis using a system with M = 2
                     εm =           μ−     2    ¯                  .                  (35)
                                          σm,gm dm γ0                                         transmit antennas and N = 1 or N = 2 receive antennas. Then
                                                                                              we demonstrate the performance of the proposed transmit
   In summary, the proposed transmit power control aims
                                                                                              power allocation in a system with M = 4 transmit antennas
to minimize the Chernoff bound of an approximate PEP of
                                                                                              and N = 1 receive antenna.
dominant error pairs in the constellation. It has a water-filling
                                                                                                 In our first set of examples, we assume equal fade rates on
structure, and hence inherits the two distinguishing properties
                                                                                              all transmit-receive antenna pairs and illustrate the effects of
of water-filling principle. First, when the total transmit power
                                                                                              non-identical channel gain distribution. The unequal average
is low, the transmit antennas with smaller geometric mean
                                                                                              channel gains are generated using the Kronecker model [16].
of average channel gains should be turned off. Second, when                                                                                                2
                                                                                              In specific, the M N × M N diagonal matrix Δ with σmn
the total transmit power is high enough, the power tends to
                                                                                              on the [(n − 1)M + m]-th diagonal is decomposed as Δ =
be equally distributed among all the antennas. The second
                                                                                              ΔT ⊗ ΔR , where ΔT and ΔR are, respectively, the M × M
property is consistent with the finding from Section IV that
                                                                                              and N × N diagonal matrices inducing non-identical fading
equal power allocation is asymptotically optimal.
                                                                                              parameters at the transmitter and receiver. The sum of the
                                                                                              average channel gains is normalized so that Tr{ΔT } = M
B. Power Control for Orthogonal Codes                                                         and Tr{ΔR } = N . In the system with two transmit antennas,
                                                                                              we specify ΔT = diag( 1 , 9 ). For one receive antenna, the
                                                                                                                         5 5
   The transmit power allocation for DUSTM with orthogonal                                                                           2               2
                                                                                              average channel gains are given by σ1 = 1/5 and σ2 = 9/5.
codes is similar to that for DUSTM with cyclic group codes.                                                                                     1 9
                                                                                              For two receive antennas, we let ΔR = diag( 5 , 5 ), and the
The decision phasor zp (α) (27) can also be expressed as                                                                                    2            2
                                                                                              set of average channel gains is given by {σ11 = 1/25, σ12 =
(31), except that E should be defined as E = cos αΦH +    p                                      2             2
                                                                                              σ21 = 9/25, σ22 = 81/25}.
j sin αΨH . Using the orthogonal code structure, we can easily
         p                                                                                       In Figs. 1-3, we show the performance of differential cyclic
show that the distribution of zp (α) can be approximated
                                                                                              group codes. The exemplary cyclic group code for M = 2
as Gaussian with mean 2 cos(α)Es Σ1/2 H 2 and variance
                              √
                                P                                                             and L = 4 at rate 1-bit/s/Hz [27, Table I], denoted as V2,4 , is
           1/2    2
4Es N0 Σ H . Thus, we obtain the Chernoff bound of the                                        chosen. Figs. 1 and 2 show the analytical PEP of the dominant
approximate BEP for differential OSTBC as                                                     error pair (D0 , D1 ) using one and two receive antennas,
                         N      M                                          −1                 respectively. The exact PEP results over i.i.d channels are also
                     1            cos2 (α)     2                                              plotted for reference, which are obtained using (20) by letting
      Pb (α)                   1+          εm σmn γ0                              .
                                                                                                2
                     2 n=1 m=1      2P                                                        σmn = 1, ∀m, n. Several useful observations can be made

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1366                                                                                                                                              IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 57, NO. 5, MAY 2009


                               0                                                                                                                   0
                              10                                                                                                                  10

                                                                               PEP with conv. decoder (17)
                               −1                                                                                                                  −1
                              10                                               PEP with ML decoder (17)                                           10
                                                                               PEP for i.i.d
                                                                               Asymptotic PEP bound (21)
                               −2                                              Pairwise error floor (24)                                           −2
                              10                                                                                                                  10
 Pairwise Error Probability




                                                                                                                          Bit Error Probability
                               −3                                                                                                                  −3
                              10                                                                                                                  10


                               −4                                                                                                                  −4
                              10                                                                                                                  10


                               −5                                                                                                                  −5
                              10                                                                     ρ=0.99                                       10

                                                                                                                                                           Analytical BEP for i.n.i.d (28)
                               −6
                                                            ρ=1                                                                                    −6      Simulated BEP for i.n.i.d
                              10                                                                                                                  10
                                                                                                                                                           Asymptotic bound for i.n.i.d (29)
                                                                                                                                                           BEP for i.s.i.d [16]
                               −7                                                                                                                  −7
                                                                                                                                                           BEP for i.i.d [30]
                              10                                                                                                                  10
                                   0       5        10         15         20        25       30        35          40                                  0        5            10               15           20   25   30
                                                              total transmit SNR γ [dB]                                                                                           total transmit SNR γ [dB]
                                                                                 0                                                                                                                   0



Fig. 2. PEP performance of differential cyclic group code V2,4 over the                                                  Fig. 4. BEP performance of differential OSTBC with QPSK modulation at
dominant error pair at M = 2 transmit and N = 2 receive antennas.                                                        M = 2 transmit and N = 2 receive antennas in static fading.

                               0
                              10
                                                                               Union bound with conv. decoder
                                                                               Simulated BkEP with conv. decoder
                                                                                                                            The overall BkEP performance of this cyclic group code
                               −1
                                                                               Union bound with ML decoder               obtained via simulation is shown in Fig. 3 and compared
                              10                                               Simulated BkEP with ML decoder
                                                                                                                         with the BkEP union bound obtained analytically using (12).
                                                                                                                         The block-wise time-varying fading channel is generated
                               −2
                              10                                                                                         using Jakes model with autocorrection function 2Rmn [k] =
 Block Error Probability




                                                                                                                           2
                                                                                                                         σmn J0 (2πfd Ts M k), where J0 (·) is the zeroth order Bessel
                               −3
                              10
                                                                                                                         function of the first kind and fd Ts is the normalized Doppler
                                                                                                                         frequency. In our simulation we set fd Ts = 0.02, which results
                                                                                                                         in ρ = 0.98427. It is observed that the analytical BkEP union
                                                                                                                         bound serves as a tight upper bound on the actual BkEP with
                               −4
                              10

                                                                                                                         both conventional and ML detectors. This further validates our
                               −5
                              10                                                                                         theoretical analysis on the exact PEP in Proposition 1.
                                                                                                                            The BEP results of differential OSTBC over the i.n.i.d
                               −6
                                                                                                                         channel with two transmit and two receive antennas are
                              10
                                   0       5        10         15         20        25       30        35          40    depicted in Fig. 4. The orthogonal code for two transmit
                                                              total transmit SNR γ [dB]
                                                                                 0
                                                                                                                         antennas with P = 2 and QPSK modulation at rate 2-
Fig. 3. BkEP performance of differential cyclic group code V2,4 at M = 2
                                                                                                                         bit/s/Hz is used. The analytical BEPs are from (28) and are
transmit and N = 2 receive antennas in time-varying fading with fd Ts =                                                  validated by simulations. For the i.s.i.d channel, the non-
0.02 (ρ = 0.98427).                                                                                                      identical fading occurs at the receiver side only with ΔT = I2
                                                                                                                         and ΔR = diag( 1 , 9 ). Its exact BEP curve is obtained from
                                                                                                                                            5 5
                                                                                                                         [16, eq.(27)]. The exact BEP for i.i.d channels is from [30]. As
from the two figures. First, the pairwise error performance                                                               expected, the i.n.i.d channel yields the worst performance and
achieved using the conventional DD is almost the same as that                                                            the best performance is achieved over i.i.d channels. Note that
achieved by the ML detector at all SNR when ρ = 1 as well                                                                this conclusion only holds when the sum of average channel
as at low SNR in time-varying fading with ρ = 0.99. Second,                                                              gains is the same.
the ML detector considerably reduces the pairwise error floor                                                                Next, we illustrate the effects of unequal channel fluctuation
in fast fading compared with the conventional detector. In                                                               rates among different signalling branches on the error floors
particular, the pairwise error floor of the ML detector with                                                              as γ0 → ∞. Fig. 5 shows the irreducible dominant PEP of
two receive antennas is two order of magnitude lower than                                                                the differential cyclic group code V2,4 in the system with two
that of the conventional detector. Moreover, the error floors                                                             transmit antennas and one receive antenna. It is observed that
approach those in i.i.d channels and match very well with                                                                under the same averaged fading correlation coefficient, i.e.,
the flat lines predicted by the analytical result in (24). This                                                           ρ = (ρ1 + ρ2 )/2, the error floor reduces as the difference on
observation confirms our analytical finding from Corollary 2.                                                              the fade rates between the two antennas increases.
From the figures we also observe that the simple asymptotic                                                                  In all the above figures, equal power allocation is assumed.
PEP bound (21) is very tight when γ0 is large enough. Finally,                                                           We now illustrate in Figs. 6 and 7 the performance of the
compared with i.i.d channels, the non-identical channel gain                                                             proposed transmit power allocation in a system with four
distribution degrades the PEP performance. This confirms the                                                              transmit antennas and one receive antenna. An exponentially
analytical finding from Corollary 1.                                                                                      decaying average channel gain profile is used and character-

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TAO: EFFECTS OF NON-IDENTICAL RAYLEIGH FADING ON DIFFERENTIAL UNITARY SPACE-TIME MODULATION                                                                                                                                                  1367


                                           −1                                                                                                                            0
                                          10                                                                                                                            10
                                                                                                        ρ =ρ =ρ
                                                                                                         1    2
                                                                                                                                                                                                                             Equal PA, δ = 3
                                                               conv. detector                           ρ = ρ − 0.004, ρ = ρ + 0.004                                                                                         Proposed PA, δ = 3
                                                                                                         1                2
                                                                                                                                                                                                                             Equal PA, δ = 1
                                                                                                        ρ = ρ − 0.008, ρ = ρ + 0.008
                                                                                                         1                2                                              −1
                                                                                                                                                                        10                                                   Proposed PA, δ = 1
 Irreducible Pairwise Error Probability




                                           −2
                                          10




                                                                                                                                                Bit Error Probability
                                                                                                                                                                         −2
                                                                                                                                                                        10
                                                       ML detector


                                                                                                                                                                         −3
                                                                                                                                                                        10
                                           −3
                                          10


                                                                                                                                                                         −4
                                                                                                                                                                        10



                                           −4                                                                                                                            −5
                                          10                                                                                                                            10
                                                       0.93          0.94         0.95         0.96          0.97         0.98       0.99                                    0   5     10         15         20         25         30             35
                                                                            Fading correlation coefficient ρ                                                                                total transmit SNR γ [dB]
                                                                                                                                                                                                               0


Fig. 5. Dominant pairwise error floor of differential cyclic group code V2,4
                                                                                                                                               Fig. 7. BEP performance of differential OSTBC with transmit power control
at M = 2 transmit and N = 1 receive antennas
                                                                                                                                               at M = 4 transmit and N = 1 receive antenna in static fading.
                                               0
                                          10
                                                                                                                     Equal PA, δ = 3
                                                                                                                     Proposed PA, δ = 3        power allocation is more significant than in the cyclic group
                                                                                                                     Equal PA, δ = 1
                                          10
                                               −1                                                                    Proposed PA, δ = 1        code case. This is because the power allocation for orthogonal
                                                                                                                                               codes aims at minimizing the bound of the overall bit error
                                                                                                                                               probability directly, whereas the one for cyclic group codes is
 Block Error Probability




                                          10
                                               −2                                                                                              obtained only through minimizing the bound of the dominant
                                                                                                                                               pairwise error probability with certain approximations.

                                          10
                                               −3
                                                                                                                                                                                        VII. C ONCLUSION
                                                                                                                                                  The effects of non-identical fading statistics in MIMO chan-
                                          10
                                               −4                                                                                              nels on the performance of DUSTM were investigated. Con-
                                                                                                                                               trary to the detectors for the traditional i.i.d fading model, we
                                                                                                                                               found that the ML differential detector of DUSTM generally
                                          10
                                               −5                                                                                              requires joint optimization of the current data matrix and the
                                                   0    5        10             15         20        25
                                                                               total transmit SNR γ [dB]
                                                                                                                    30        35          40
                                                                                                                                               previously transmitted signal matrix. However, for DUSTM
                                                                                                    0
                                                                                                                                               with cyclic group design, the ML detector is much simplified
Fig. 6. BkEP performance of differential cyclic group code V4,16 with                                                                          and is similar to the conventional detector but applies fading
transmit power control at M = 4 transmit and N = 1 receive antenna in                                                                          statistics-dependent weights. Based on the analysis of exact
static fading.
                                                                                                                                               and asymptotic error probability for both cyclic group codes
                                                                                                                                               and orthogonal codes, we obtained several useful findings.
                                                                                                                                               Along with numerical results, we conclude that while the ML
           2
ized by σm = e−δ(m−1) , for 1 ≤ m ≤ 4, in which δ ≥ 0 is                                                                                       detector can significantly reduce the error floor over rapidly
the decay factor.                                                                                                                              time-varying fading channels, the conventional detector is
   Fig 6 shows the simulated BkEP of the differential cyclic                                                                                   near-optimal at all SNR in static fading and low SNR in time-
group code V4,16 with M = 4 and L = 16 at rate 1-bit/s/Hz                                                                                      varying fading. In addition, the non-identical channel gain
[27, Table I]. The conventional detector is employed. It is seen                                                                               distribution degrades the error performance compared with
that the proposed power allocation (34) cannot outperform                                                                                      the identical distribution for a same total received SNR. To
equal power allocation when the average channel gains are                                                                                      exploit the non-identical fading parameters at the transmitter,
only slightly unbalanced with δ = 1. On the other hand,                                                                                        we also presented a water-filling based transmit power control.
for highly unbalanced average channel gains with δ = 3,                                                                                        It was shown to provide considerable improvement in error
the water-filling based power allocation can save 2 ∼ 3 dB                                                                                      probability at low to moderate SNR region when the average
total transmit power at a given BkEP around 10−2 . But the                                                                                     channel gains are highly unbalanced. At sufficiently high SNR,
gain diminishes as the target BkEP reduces. This observation                                                                                   equal power allocation is still optimal.
confirms our analytical finding in Section IV that equal power
allocation is asymptotically optimal.
   The BEP performance of the differential orthogonal code                                                                                                                                 A PPENDIX A
with M = 4, P = 3 and QPSK modulation at rate 1.5-bit/s/Hz                                                                                                                           P ROOF OF P ROPOSITION 1
based on the analysis (28) is presented in Fig. 7. We see that                                                                                   By applying the result in [31], the characteristic function
the gain of the proposed power allocation (36) over equal                                                                                      (CF) of the pairwise decision variable zij in the quadratic

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1368                                                                                                  IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 57, NO. 5, MAY 2009



form of Gaussian vectors (14) can be written as [26]                                             symbols in the data matrix D, nor the previously transmitted
                                                                    1                            signal matrix S−1 . They are:
       φzij (s)        = E e−szij =                  N
                                                                                       (
                                                                                       . 37)
                                                     n=1   det(I + sΛn Ωij,n )                                             γmn cos(α + θp )
                                                                                                                                √
                                                                                                          λn,i      =
Substituting (3) and (15) (or (16)) into (37), we obtain                                                                          P
                                                                                                                                   2
                                                                                                                                 γmn cos2 (α + θp )
                        N      M                                               2
                                                                                       −1                                   ±                       + 2γmn + 1 (43)
                                             1   1    s    1                                                                            P
φzij (s) =                           amn       +   −     −                                   ,
                       n=1 m=1
                                             4 amn   bmn   2                                     where λn,i < 0 for 1 ≤ i ≤ M and λn,i > 0 for M + 1 ≤
                                                           (38)                                  i ≤ 2M . Substituting (43) into (42) and after some algebra,
where amn and bmn are given in (18) and (19), respectively.                                      we obtain the closed-form expression of Pb (α) in Proposition
After inverting the Laplace transform, we express Pe,ij defined                                   2.
in (13) as [24, Appendix A]
                                         1       j∞+η
                                                           φzij (s)                                                                 R EFERENCES
                       Pe,ij     =                                  ds,                (39)
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with a change of variables, we obtain                                                                 2002.
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 Pe,ij =                                                                                              2002.
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                                   M
                                   m=1
                                               N
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                                                                                                      channels,” in Proc. IEEE GLOBECOM’06, 2006.
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(27) is given by                                                                                      time block codes in Nakagami fading channels with non-identical m-
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  φzp (s)         = E e−sZp (α) =                    N       K
                                                                                         .            block codes over semi-identical MIMO fading channels,” IEEE Trans.
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where {λn,i }K are the distinct eigenvalues of Λn Ωp with
              i=1                                                                                     approach,” IEEE Trans. Inform. Theory, vol. 47, no. 4, pp. 1650–1656,
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obtained as [24]                                                                                      time modulation,” IEEE Trans. Commun., vol. 50, no. 5, pp. 768–777,
                                                                                                      May 2002.
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         Pb (α) = −                    Res             , s=−      ,                    (42)
                                                 s           λn,i                                     time modulation,” IEEE Trans. Commun., vol. 51, no. 7, pp. 1093–1101,
                            λn,i <0                                                                   July 2003.
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where the residues are evaluated at the positive poles of                                             of differential space-time modulation,” IEEE Trans. Inform. Theory,
φzp (s)/s, that is −1/λn,i with λn,i being negative. Using a                                          vol. 49, no. 10, pp. 2727–2734, Oct. 2003.
similar approach as in the proof [25, Corollary 1], we can                                       [21] C. Gao, A. M. Haimovich, and D. Lao, “Multiple-symbol differential
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th information symbol sp = ejθp , and do not rely on the other                                        Aug. 2006.


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TAO: EFFECTS OF NON-IDENTICAL RAYLEIGH FADING ON DIFFERENTIAL UNITARY SPACE-TIME MODULATION                                                                            1369



[22] P. Pun and P. Ho, “Fano multiple-symbol differential detectors for dif-                                           Meixia Tao (S’00-M’04) received the B.S. degree
     ferential unitary space-time modulation,” IEEE Trans. Comun., vol. 55,                                            in Electronic Engineering from Fudan University,
     no. 3, pp. 540–550, Mar. 2007.                                                                                    Shanghai, China, in 1999, and the Ph.D. degree in
[23] R. A. Horn and C. R. Johnson, Matrix Analysis. New York: Cambridge                                                Electrical and Electronic Engineering from Hong
     University Press, 1985.                                                                                           Kong University of Science & Technology in 2003.
[24] M. Brehler and M. K. Varanasi, “Asymptotic error probability analysis                                                She is currently an Associate Professor at the De-
     of quadratic receivers in Rayleigh-fading channels with applications to                                           partment of Electronic Engineering, Shanghai Jiao
     a unified analysis of coherent and noncoherent space-time receivers,”                                              Tong University, China. From Aug. 2003 to Aug.
     IEEE Trans. Inform. Theory, vol. 47, no. 6, pp. 2383–2399, Sept. 2001.                                            2004, she was a Member of Professional Staff in
[25] X. Cai and G. B. Giannakis, “Differential space-time modulation with                                              the Wireless Access Group at Hong Kong Applied
     eigen-beamforming for correlated MIMO fading channels,” IEEE Trans.                                               Science & Technology Research Institute Co. Ltd,
     Signal Processing, vol. 54, pp. 1279–1288, 2006.                                         where she worked on the design of wireless local area networks. From
[26] J. G. Proakis, Digital Communications, 4th ed. New York: McGraw-Hill,                    Aug 2004 to Dec. 2007, she was with the Department of Electrical and
     2001.                                                                                    Computer Engineering at National University of Singapore as an Assistant
[27] A. Shokrollahi, B. Hassibi, B. M. Hochwald, and W. Sweldens, “Rep-                       Professor. Her research interests include MIMO techniques, channel coding
     resentation theory for high-rate multiple-antenna code design,” IEEE                     and modulation, dynamic resource allocation in wireless networks, and
     Trans. Inform. Theory, vol. 47, no. 6, pp. 2335–2367, Sept. 2001.                        cooperative communications.
[28] S. Boyd and L. Vandenberghe, Convex Optimization. Cambridge, UK:                            Dr. Tao is an Editor of the IEEE T RANSACTIONS ON W IRELESS C OM -
     Cambridge University Press, 2004.                                                        MUNICATIONS . She served as Track Co-Chair for IEEE ICCCN’07 held in
[29] M. K. Byun and B. G. Lee, “New bounds of pairwise error probability                      August 2007 at Hawaii, USA, and IEEE ICCCAS’07 held in July 2007 at
     for space-time codes in rayleigh fading channels,” IEEE Trans. Com-                      Fukuoka, Japan. She also served as a Technical Program Committee member
     mun., vol. 55, no. 8, pp. 1484–1493, Aug. 2007.                                          for various conferences, including IEEE ICC (2006, 2007, 2008), IEEE
[30] T. P. Soh, P. Y. Kam, and C. S. Ng, “Bit error probability for                           WCNC (2007, 2008), IEEE GLOBECOM (2007), and IEEE VTC (2006-Fall,
     orthogonal space-time block codes with differential detection,” IEEE                     2008-Spring).
     Trans. Commun., vol. 53, no. 11, pp. 1795–1798, Nov. 2005.
[31] G. L. Turin, “The characteristic function of hermitian quadratic forms
     in complex normal variables,” Biometrika, vol. 47, pp. 199–201, 1960.




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