GPS L2C Signal Acquisition Algorithms for Resource-Limited Applications in Challenging Environments

					                                                              (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                            Vol. 9, No. 9, September 2011

          GPS L2C Signal Acquisition Algorithms for
         Resource-Limited Applications in Challenging
                        Environments
                                                          Nesreen I Ziedan
                                        Computer and Systems Engineering Department
                                          Faculty of Engineering, Zagazig University
                                                       Zagazig, Egypt
                                                        ziedan@ieee.org



   Abstract—Many emerging indoor and wireless applications                 and CL, that are multiplexed chip-by-chip, i.e. a chip of the
require the positioning capabilities of GPS. GPS signals, however,         CM code is transmitted followed by a chip of the CL code.
suffer from attenuations when they penetrate natural or man-               The chipping rate of each code is 511.5 KHz. The CM code
made obstacles. Conventional GPS receivers are designed to
detect signals when they have a clear view of the sky, but they            has a length of 10230 chips; it repeats every 20 ms, and it is
fail to detect weak signals. This paper introduces novel algo-             modulated by a 50 Hz data message. The data and the CM code
rithms to detect the new GPS L2C civilian signal in challenging            are synchronized such that each data bit has exactly one code.
environments. The signal structure is utilized in the design to            The CL code is 75 times longer than the CM code (767,250
achieve high sensitivity with reduced processing and memory                chips), and it is data-less. Performance evaluations for the L2C
requirements to accommodate the capabilities of resource-limited
applications, like wireless devices.                                       signal were presented in [3] [4].
   The L2C signal consists of a medium length data-modulated                  GPS signals suffer from attenuation if their paths are ob-
code (CM) and a long length dataless code (CL). The CM code                structed by natural or man-made objects- such as trees or
is acquired using long coherent and incoherent integrations to             buildings. Conventional GPS receivers can detect signals if
increase the acquisition sensitivity. The correlation is calculated        their carrier to noise ratio, C/N0 , is over 35 dB-Hz, but they
in the frequency domain using an FFT-based approach. A bit
synchronization method is implemented to avoid acquisition                 fail to detect weaker signals. Special algorithms are needed
degradation due to correlating over the unknown bit bound-                 to acquire and track weak signals. Many devices that are
aries. The carrier parameters are refined using a Viterbi-based             prone to receiving weak signals, like cell phones, have limited
algorithm. The CL code is acquired by searching only a small               resources. So, the processing and memory requirements must
number of delays, using a circular correlation based approach.             be considered when designing such algorithms.
The algorithms’ computational complexities are analyzed. The
performances are demonstrated using simulated L2C GPS signals                 The acquisition goal is to find the visible satellites, the
with carrier to noise ratio down to 10 dB-Hz, and TCXO clocks.             code delay, τ , and the Doppler shift, fd . A search for a
                                                                           satellite is done by locally generating its code and using it
  Index Terms—GPS, L2C, Acquisition, Weak Signal, Indoor,                  in a 2-dimensional search on τ and fd . The received signal is
Viterbi                                                                    correlated with different versions of a code-modulated local
                                                                           signal, each version is compensated by one possible τ -fd
                                                                           combination. The codes’ properties cause the correlated signals
                       I. I NTRODUCTION                                    to generate a clear peak only if their codes are the same
   The Block IIR-M GPS satellite series started the transmis-              and their code delays and Doppler shifts are close enough.
sion of a new and more robust civil signal on the L2 carrier               A positive acquisition is concluded if a correlation exceeds a
frequency- the signal is known as L2C. The first satellite in the           predefined threshold.
series was launched in September 2005, and by August 2009,                    The conventional hardware approach [5] [6] searches for
the eighth and final IIR-M satellite was launched. The L2C                  a satellite at each possible code delay and Doppler shift
signal [1] [2] has different structure and enhanced properties             sequentially. Circular correlation [7] [8] uses Fast Fourier
over the GPS L1 C/A signal. The L2C codes and the C/A                      Transform (FFT) methods. It calculates the correlation at all
code have a chipping rate of 1.023 MHz. The C/A signal is                  the delays at once, for each Doppler shift. Double Block
modulated by a 1023-chip code, and a 50 Hz data message.                   Zero Padding (DBZP) [7] [9] [10] calculates the correlations
The code repeats every 1 ms, and each data bit has exactly                 in the frequency domain, and uses only one version of the
20 codes. While the L2C signal consists of two codes, CM                   replica code. It requires less processing, but it suffers from




                                                                      42                              http://sites.google.com/site/ijcsis/
                                                                                                      ISSN 1947-5500
                                                           (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                         Vol. 9, No. 9, September 2011
limitations when working with weak signals. This is because             is 20 ms, so the phase difference between the start and the end
it does not consider the Doppler effect on the code length. The         of one code could be relatively large. Using 20-ms correlated
Doppler shift changes the speed of the code, so the code length         signals directly will not provide high accuracy estimation for
either shrinks or expands based on the Doppler shift’s polarity.        the carrier parameters. This problem is handled in this paper
This effect can be ignored with small integration lengths,              by dividing the code into small length segments, calculating
but it will cause acquisition failure with long integration             the correlation for each segment separately, and then using
lengths. The problem is that correlating a fixed length local            the correlated segments to find fine estimates for the carrier
code with a changing length received code will cause the                parameters.
delay to continuously change with respect to the local code.               The acquisition of the long length CL code is done using
As the integration length increases, the signal power will              a minimized search approach, MS-CL. It uses the estimates
continue to accumulate at different delays. This will prevent           of the CM-ABS and FAVA-L2 to acquire the CL code by
the correct delay from accumulating enough power to exceed              searching only 75 possible delays. A method is introduced
the acquisition threshold. This limitation is circumvented in           to calculate the coherent integration in smaller steps to avoid
a modified version of DBZP, called MDBZP, which was                      processing large number of samples at once.
introduced in [11]. The MDBZP divides the whole Doppler
range into a small number of ranges. The correlations in each                                 II. S IGNAL M ODEL
range are calculated using a version of the replica code that              The received L2C signal is down converted to an interme-
is compensated, in length, by the Doppler shift located in the          diate frequency (IF), fIF , and sampled at a rate of fs . The
middle of that range.                                                   signal model for one satellite is
   A joint acquisition algorithm of the CM and CL codes was
introduced in [12]. An assisted acquisition of the CL code was            rL2C (tδ ) = A {d(tδ,τ ) CM0 (tδ,τ ) + C0L (tδ,τ )}
presented in [13]. An FFT-based approach was introduced in               cos θnδ + θ0 + 2 π (fIF + fd0 ) tδ + π α t2 + n(tδ ),
                                                                                                                   δ                       (1)
[14] to acquire the CM and CL codes. A method called XFAST
was introduced in [15] to acquire the long P(Y) code of the             where tδ is the sampling time. tδ,τ                =      (tδ −
L1 signal. This method was extended in [16] to acquire the              τ ) {1 + (fd0 + α tδ /2)/fL2} is the sampling time taking into
long CL code; the extended method was called hyper-codes.               account the Doppler effect on the code length. τ is the code
For resource-limited devices, the problem with acquiring weak           delay. fd0 is the initial Doppler shift. α is the Doppler rate.
signals using the CL code is the high processing and memory             fL2 is the L2 carrier frequency. A is the signal amplitude,
required to correlate and search the 767,250-chip code.                 which is normalized to drive the noise variance to 1 as in
   This paper introduces acquisition and fine acquisition al-            [5], i.e. A = 4 C/N0 Ts , Ts = 1/fs . d is the navigation
gorithms for the new L2C signal to work under weak signal               data. θ0 is the initial phase. θnδ is the phase noise at tδ ; it is
conditions. The algorithms utilize the L2C signal structure to          composed of the total phase and frequency clock disturbances.
achieve high sensitivity and reduced processing and memory              n is a white Gaussian noise (WGN) with zero mean and
requirements. Three algorithms are introduced to work sequen-           unit variance. The two codes are modeled such that CM0 is
tially to first acquire the medium-length CM signal, then refine          a chip-by-chip combination of the CM code and zeros, and
the estimates of the carrier parameters, and then acquire the           C0L is a chip-by-chip combination of zeros and the CL code.
long-length CL code. A computational complexity analysis for
the algorithms is provided.                                                 III. CM ACQUISITION AND B IT S YNCHRONIZATION
   The acquisition of the CM code is done using a new                                        (CM-ABS)
version of the MDBZP designed to fit the CM code structure                  The CM-ABS calculates the correlation in the frequency
and deal with the fact that each CM code is modulated                   domain. The separation between the code delays is taken as the
by one data bit, which has an unknown value. The new                    sampling time. The number of possible code delays is defined
algorithm- called CM Acquisition and Bit Synchronization                as Nτ . The algorithm produces Doppler bins with frequency
(CM-ABS)- implements a bit synchronization method within                separation of fres = 1/TI , where TI is the coherent integration
the acquisition to avoid correlating over bit boundaries. The           length. The number of Doppler bins, Nfd , depends on the
correlations are calculated in the frequency domain, and the            Doppler shift range, ±fdcov , where Nfd = 2 fdcov TI . The
Doppler effect on the code is considered. Long coherent and             values of the Doppler bins, defined as fdv , can be calculated
incoherent integrations are used, without requiring assisting           as
                                                                                             Nfd
information from outside sources- like wireless networks. The                   fdv = v −         − 1 fres , v = 1, . . . , Nfd .    (2)
likely data bit combination is estimated over each coherent                                   2
integration interval, and used to remove the data signs.                The samples in each TI ms, of the received signal and the
   The fine acquisition algorithm is based on the Viterbi                replica code, are divided into Nfd blocks. The size of each
Algorithm (VA) [17] [18] [19], which is an optimal dynamic              block is Sblock = fs TI /Nfd samples.
programing technique. The new algorithm is called Fine Ac-                 Coherent and incoherent integrations are used. The coherent
quisition VA-based for L2, or FAVA-L2. The CM code duration             integration length, TI , can be multiple, Nt , of one data bit



                                                                   43                               http://sites.google.com/site/ijcsis/
                                                                                                    ISSN 1947-5500
                                                           (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                         Vol. 9, No. 9, September 2011
length, Tdms . The incoherent integration accumulates L results
of TI -ms coherent integrations to get the total integration,
L TI ms. The coherent integration is obtained by first gen-
erating a TI /Nfd -ms partial coherent integration, and then
generating the TI -ms coherent integration. The partial coherent
integrations, at the Nτ possible code delays, are generated in
Nstep steps. In each step, the partial coherent integrations are
generated at a number of adjacent code delays equal to one
block size, Sblock . So, Nstep = Nτ /Sblock . The arrangement
of the blocks of the replica code and the received signal
relative to each other determines which Sblock partial coherent
integrations are generated in each step. Each Sblock partial
coherent integrations are generated by applying circular corre-
lation (FFT/IFFT) between each two corresponding blocks of
the replica code and the received signal.
   Each CM code period has exactly one data bit, which has an
unknown value. To avoid correlating over bit boundaries, a bit
synchronization method is implemented within the acquisition.
The idea of the algorithm is to search for the start of the code
in the received signal by fixing the samples of the replica code
and moving forward the samples of the received signal, until
the start of the received code coincides with the start of the          Fig. 1.   Illustration of the CM-ABS algorithm.
replica code. This is done instead of searching directly for the
code delay in the received signal by allowing the correlation
to be calculated using samples from two adjacent received               the ith range, and Nfdi as the number of Doppler bins in the
codes, which could have data bits with different polarities             ith range. The indexes of the first Doppler bin and the last
that will result in correlation loss. The aforementioned idea           Doppler bin, respectively, in each range are
is implemented by arranging the replica code’s blocks to start
                                                                                                          i−1
at the beginning of the code and fixing that arrangement at all
the Nstep steps, and shifting the received signal’s blocks in                                     λsi =         Nfdj + 1,                       (4)
                                                                                                          j=1
each step. The received signal’s blocks are arranged such that
the first block contains the code delays at which the partial                                                    i
coherent integrations are generated. After each step, the first                                      λei =           Nfdi .                      (5)
block is discarded, the remaining blocks are shifted forward,                                               j=1
and an additional block is added at the end of the blocks.              The following items are repeated L times, once for each
Nfd + Nsteps − 1 blocks are needed to find the Nτ partial                coherent integration:
coherent integrations.                                                    1. Nrange versions of the replica code are generated. Each
   The process of generating Nτ partial coherent integrations           version is compensated, in length, by one of the fmidi fre-
is repeated L times, once for each TI -ms integrations. In each         quencies. The un-compensated length of each version is TI
repetition, the last (Nstep − 1) blocks used in the generation          ms. The model for each version is
of the previous coherent integrations are the same as the                                                            fmidi
first (Nstep − 1) blocks used in the generation of the current                 CLM0D i (tδ , fmidi ) = CLM0 tδ 1 +             . (6)
                                                                                                                      fL2
coherent integrations. This is because the generation of two
consecutive coherent integrations, for the same possible delay,         Where, CLM0D i is the ith replica code version. CLM0 is
should use two consecutive TI -ms lengths of the signal. Since          an un-compensated code, which consists of the CM code
circular correlation involves calculating FFT for each block            multiplexed chip-by-chip with zeros. The samples of each
of the received signal, then FFT for each of the (Nstep − 1)            version is divided into Nfd blocks, each block has a size
overlapping blocks is calculated only once. The algorithm is            of Sblock samples. From the blocks of the ith version, only
illustrated in Fig. 1.                                                  Nfdi blocks are preserved, and the others are discarded. The
   The algorithm’s implementation details are as follows. The           preserved blocks, of the ith version, are those located at offsets
received signal in (1) is converted to baseband to produce              from λsi to λei . All the preserved blocks are arranged together,
                                                                        where their order is maintained, i.e. blocks coming from the
             rc (tδ ) = r(tδ ) exp {− j 2 π fIF tδ } .      (3)
                                                                        ith replica code are located at offsets from λsi to λei . Each
The whole Doppler range is divided into a small number,                 block is padded with Sblock zeros at its end. The complex
Nrange , of ranges. Define fmidi as the middle frequency of              conjugate of the FFT of each block is calculated. Assume



                                                                   44                                    http://sites.google.com/site/ijcsis/
                                                                                                         ISSN 1947-5500
                                                                      (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                    Vol. 9, No. 9, September 2011




Fig. 2.   Example illustrating the local code’s blocks arrangement.



a simple example where Nrange = 2 and Nfd = 4. Fig. 2
shows the blocks arrangement. The 1st and 2nd replica codes’
blocks are defined by (A1, A2, A3, A4) and (B1, B2, B3, B4),
respectively. Here, Nfd1 = Nfd2 = 2, and λs1 = 1, λe1 = 2,
λs2 = 3, λe2 = 4, so A1, A2, B3, and B4 are preserved, while
A3, A4, B1, and B2 are discarded. The preserved blocks are
then padded with zeros and the processing proceeds.                              Fig. 3.   Example illustrating the received signal’s blocks arrangement.
   2. Processing the received samples depends on whether this
is the first coherent integration or not. Thus, there are two
approaches as follows:                                                           integrations for Sblock delays. They are repeated Nstep times
   2.a. If this is the first coherent integration, a size of                      to calculate the Nτ partial coherent integrations. The index of
(Nfd + Nstep ) Sblock samples of the received signal is divided                  the first used received signal’s block is increased by one in
into (Nfd + Nstep ) blocks. Each two adjacent blocks are                         each step to simulate the shifting process, which is explained
combined into one block to produce (Nfd + Nstep − 1)                             earlier in this section.
overlapping blocks, with a size of 2 Sblock . FFT is calculated                     3.a. At the mth step, of the Nstep steps, the blocks at indexes
                                                                                                  r
for each block. The last (Nstep − 1) blocks are preserved in a                   from mr to (mr + Nfd − 1), which are generated in item 2,
matrix MF F T temp to be used in the next coherent integration.                  are multiplied block-wise by the blocks generated in item 1.
Returning to our example where Nfd = 4, assume that                              IFFT is calculated for the multiplication result. This produces
Nstep = 3. Fig. 3-a illustrates the blocks arrangement. The                      Nfd blocks of size 2 Sblock points.
received samples are divided into 7 blocks, defined as R1 to                         3.b. The first Sblock points from each produced block are
R7. Each 2 adjacent blocks are combined to form 6 blocks.                        preserved, while the rest are discarded. The preserved points
After FFT is calculated, the 2 blocks marked as (R5, R6) and                     are arranged to form a matrix of size Nfd xSblock . The ith
(R6, R7) are preserved in MF F T temp .                                          row contains the results of the ith block, and the j th column
   2.b. If this is not the first coherent integration, a size of                  contains the results at index j from each block. This matrix is
(Nfd + 1) Sblock samples of the received signal is divided into                  appended to a matrix, Mc , at row indexes from 1 to Nfd and at
(Nfd + 1) blocks. Each two adjacent blocks are combined                          column indexes from [(mr − 1) Sblock + 1] to [mr Sblock + 1].
into one block to produce Nfd overlapping blocks. FFT is                         At the end of the Nstep steps, Mc will have a size of Nfd xNτ .
calculated for each block. The blocks preserved in MF F T temp                      4. Each cell in the Mc matrix is generated using samples of
are added at the start of the Nfd blocks. The last (Nstep − 1)                   size TI /Nfd ms, where TI = Nt Tdms . Each Nfd /Nt rows are
blocks are preserved in MF F T temp , overwriting the previous                   generated using samples of size equal to one data bit interval,
preserved blocks. Returning to our example, Fig. 3-b illustrates                 Tdms . To remove the effect of the data bit signs, the Mc matrix
the blocks arrangement in the second coherent integration.                       is multiplied by the possible data bit combinations. Since the
Only 5 blocks are generated, where the first block will be                        purpose is to have data bits with the same sign over each
block R7 from the previous step. Again, each two adjacent                        coherent integration interval, only half the possible data bit
blocks are combined to form 4 blocks. After FFT is calculated,                   combinations are considered, i.e. 2Nt −1 possible combinations.
the 2 blocks preserved from the previous step, (R5, R6) and                      This will generate 2Nt −1 matrices. Define these matrices as
(R6, R7), are added at the start of the 4 blocks to complete                     McEi , where E = 1, . . . , 2Nt −1 .
the 6 blocks needed in the processing of this step. The 2                           5. The TI -ms coherent integrations are found by applying
blocks marked as (R9, R10) and (R10, R11) are preserved                          FFT to each column in the McEi matrices. Each cell (i, j), of
in MF F T temp .                                                                 the results, corresponds to the TI -ms coherent integration at
   3. Items 3.a and 3.b are done to generate the partial coherent                the ith Doppler shift and the j th code delay.



                                                                            45                                    http://sites.google.com/site/ijcsis/
                                                                                                                  ISSN 1947-5500
                                                                    (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                  Vol. 9, No. 9, September 2011
   6. To allow the signal power to accumulate at the correct                        IV. F INE ACQUISITION BASED ON T HE VA (FAVA-L2)
delay, a compensation is done to account for the difference in                      The model for the local signal is
the Doppler effect on the code length between a middle range
                                                                                                                          ˆ     ˆ
                                                                                                                          fd0 + α tδ /2
fmidi and each frequency in that range. Since each row in                            IQLM0 (tδ ) = CLM0       tδ    1+
McEi corresponds to a possible Doppler shift, it is circularly                                                                fL2
shifted by a number of samples, Nsv , which depends on the                                    ˆ              ˆ
                                                                                        exp j θ0 + 2 π fIF + fd0            tδ + π α t2
                                                                                                                                   ˆ δ        ,      (7)
difference between fmidi and the frequency value, fdv , and
the coherent integration length TI and its index, l, within the                            ˆ
                                                                                  where fd0 is the estimated initial Doppler shift, α is theˆ
total incoherent integration. Where                                                                               ˆ
                                                                                  estimated Doppler rate, and θ0 is the estimated initial phase.
                                                     |fdv − fmidi |               The CM code duration is 20 ms, so the phase difference
Nsv = sign(fdv − fmidi )round (l − 1)TI fs                          .             between the start and the end of one code could be relatively
                                                          fL1
                                                                                  large. Thus, using 20-ms correlated signals in a fine acquisition
   7. Only one of the 2Nt −1 matrices is preserved and the                        algorithm will not provide high accuracy estimation for the
others are discarded. The preserved matrix corresponds to the                     carrier parameters. This problem is overcome by dividing the
likely data bit combination, which is estimated by a method                       code into NcF rag segments, calculating the correlation for each
we developed in [11], chapter 3, and is briefly described below.                   segment separately, and then using the correlated segments to
   8. The preserved matrix is added incoherently to the previ-                    find the fine estimates. Considering the Doppler effect, the
ous total incoherent integration.                                                 estimated code length is defined as Tdm in the mth code
   After calculating the final total incoherent integration, the                   period. The integration is calculated over several codes, Ndα .
cell that contains the maximum power will correspond to the                       Hence, there will be NcF rag Ndα correlated segments. Each
                         ˆ                        ˆ
estimated code delay, τ , and Doppler shift, fd , given that the                  correlated segment is expressed as
power exceeds the acquisition threshold. After an acquisition                                                              Ti
is concluded, the carrier parameters are refined using the fine                          yi = A di R(τei ) sinc     fei + αe      Ti
                                                                                                                           2
acquisition algorithm presented in the next section.
                                                                                                               Ti          T2
   Estimation of the likely data bit combination: One or                               exp j θei + 2 π fei + 2 π αe i              + ny i ,     (8)
more of the following approaches can be used.                                                                   2           6
   1. Let IcE and QcE define the real and imaginary parts of the                   where Ti is the segment length. nyi is a noise term. R(.) is the
lth coherent integration after the multiplication by a possible                   autocorrelation function. τei is the code delay error. θei and
data combination, E. Define Pl,E as the incoherent integration                     fei are the phase and Doppler shift errors, respectively, at the
after including that coherent integration, Pl,E (τu , fdv ) =                     start of the ith segment. αe is the Doppler rate error. fei =
Pl−1 (τu , fdv ) + IcE (τu , fdv )2 + QcE (τu , fdv )2 l . Each Pl,E              fei−1 + αe Ti + Wfd , i . θei = θei−1 + 2 π fei Ti + Wθ, i . Wθ, i
is a matrix of size Nτ xNfd . The matrix that contains the cell                   and Wfd , i are the clock phase and frequency disturbances.
that has the maximum power, out of all the matrices, is chosen                       FAVA-L2 works in three stages to find fine estimates for
as Pl , and the others are discarded. The problem with applying                   the phase, Doppler shift and rate. First, a Doppler rate error,
this approach directly with weak signals is that the cell that                    αfe , is estimated under the assumption that θei and fei
has the maximum power could be a noise cell.                                      are zeros. Second, both αe and fe0 are estimated, where
   2. The likely data combination is estimated separately for                     αe = αfe − (3/2) (fe0 /Tα ) − Eα . Tα is the total data intervals
each cell. The 2Nt −1 matrices are compared cell wise, i.e. cells                 used to obtain αfe , and Eα is an estimation error. Third, the
at the same index in each matrix are compared. The maximum                        phase error is estimated. This method is similar to a method
from each index forms the new integration. At a delay τu and                      we introduced in [11], chapter 4, for the C/A L1 signal.
Doppler fdv the new integration is                                                This section focuses on describing the modifications and new
                                                                                  approaches that are developed for the L2C signal.
     Pl (τu , fdv ) =                                                                αfe is obtained as follows. Define Nα as the number of
     max Pl1 (τu , fdv ), Pl2 (τu , fdv ), . . . , Pl2Ntk −1 (τu , fdv ) .        possible Doppler rate errors, and αt as a possible error. The
                                                                                  segments that belong to the mth code, defined in (8), are
There will be no loss due to a wrong combination in the correct                   counter rotated by each αt , and then added, so
cell, but the noise power might increase.                                                                    NcF rag m
   3. The likely data bit combination is estimated once for each                          Sm,αt    =                         yi
code delay. The data combination is the one that generates the                                           i=NcF rag (m−1)+1
maximum power among all the Doppler shifts, in the 2Ntk −1
                                                                                                                                1
matrices, at each code delay.                                                                            exp −j π αt       Tt2 + Ti2          ,      (9)
                                                                                                                             i
                                                                                                                                3
   The algorithm can start with the 2nd or the 3rd approach,
                                                                                                   m−1             i−1
then switch to the 1st one after several integrations. This is                    where Tti = n=1 Tdn + k=NcF rag (m−1)+1 Tk . The data
to increase the sensitivity and reduce the degradation due to a                   bit values are jointly estimated with αfe . The estimation can
wrong data bit combination.                                                       be expressed by



                                                                             46                               http://sites.google.com/site/ijcsis/
                                                                                                              ISSN 1947-5500
                                                                             (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                           Vol. 9, No. 9, September 2011
                                                                                         increase by NEα . The estimation is done iteratively, starting
                                  Ndα
                                                                       2
                                                                                         with the whole frequency range and with large frequency
 αfe = arg         min                   ˆ             ¯¯
                                         dm |Sm,αt | − A dm                , (10)        separation. After each iteration, an estimated ft is obtained
                   ¯ N
             αt , {dm } dα                                                               and used as the middle value of a reduced frequency range.
                       m=1        m=1
                                                                                                                                   ˆ
                                                                                         After the last iteration, the estimate of fe0 is concluded, and the
       ˆ
where dm is an estimated value for the mth bit, which is                                 corresponding αft ,Eα is taken as αe . Following that, a similar
                                                                                                                               ˆ
                                                    ¯
obtained as the sign of the real part of Sm,αt . A is the                                approach is used to estimate θe0 . Following the CM-ABS and
                       ¯m is a possible data bit value (±1).
expected signal level. d                                                                 FAVA-L2, the CL code is acquired using the MS-CL.
The estimation process is based on the VA. Define,
                           Ndα
                                                               2                         V. M INIMIZED S EARCH       FOR T HE CL C ODE ACQUISITION
         ΛNdα , αt , p =          ˆ              ¯
                                  dm |Sm, αt | − A dm, p           ,         (11)                                    (MS-CL)
                           m=1
                                                                                            The CL and CM codes are synchronized such that the start
where p defines a possible data sequence, p = 1, . . . , 2Ndα .                           of the CL code coincides with the start of the CM code. Since
dm, p is the mth data in the pth sequence. Each combination                              the CL code is 75 times longer than the CM code, and the CM
of p and αt defines a path in a trellis graph. The algorithm                              code is acquired, then there are only 75 possible code delays
operates recursively to find the path that generates the mini-                            to search for the start of the CL code. The local replica is
mum ΛNdα , αt , p . Define Γm, αt , pm as the optimal ΛNdα , αt , p
in the mth step, for αt . Since there are Nα of αt , there are
Nα of Γm, αt , pm . Since an optimal path consists of optimal                                                                   ˆ     ˆ
                                                                                                                                fd0 + α tδ /2
                                                                                             IQL0L (tδ ) = CL0L      tδ   1+
sub-paths, it can be shown that                                                                                                     fL2
 Γm+1,αt ,pm+1 = Γm,αt ,pm                                                                          ˆ              ˆ
                                                                                              exp j θ0 + 2 π fIF + fd0             tδ + π α t2
                                                                                                                                          ˆ δ       .       (12)
       ˆ                 ¯      ˆ                 ¯
+ min (dm+1 |Sm+1,αt | − A)2 , (dm+1 |Sm+1,αt | + A)2 .                                  Long coherent integration can be used, but the error in the
After each recursive step, only the Nα paths that correspond to                          estimated Doppler shift, fe , will put a limit on its length, TI ,
Γm, αt , pm are retained, while the other paths are discarded. At                        where TI ≤ 1/fe . The coherent integration is generated in
the last step, αt that corresponds to the minimum cumulative                             steps, with a length of Tn ≤ TI , to avoid processing large
path, among the Nα optimal paths, is taken as the estimated                              number of samples at once. The number of steps needed to
αfe .                                                                                    obtain the total coherent integration is Lcoh = ⌈TI /Tn ⌉. The
   The accuracy of the estimated αfe depends on the separation                           coherent integration in the last step could be less than Tn .
between the αt values. If the range of the possible αfe is                               In addition, LCL coherent integrations are incoherently added
large, then FAVA-L2 is operated iteratively to avoid increasing                          to obtain a longer incoherent integration. The total number
the processing and memory requirements. The first iteration                               of steps to get the total incoherent integration is Ntotal =
covers the whole αfe range, and uses large separation between                            LCL Lcoh .
the αt ’s. After each iteration, an estimated αfe is obtained                               The integrations are calculated for each possible code delay
and used as the middle value of a reduced range. In each                                 and placed in two vectors, H and P , which hold, respectively,
iteration, Nα possible values are considered, but the separation                         the total coherent and incoherent integrations. U and V are
between the αt ’s is decreased. The process continues until the                          counters initialized to zero. Tk is the total length of the
separation between αt ’s is within the desired resolution.                               received signal that is used up to the start of the step k,
   Following the estimation of αfe , fe0 and αe are estimated                            k = 1, . . . , Ntotal ; Tk is initialized to zero. The MS-CL works
using a similar approach. Define Nf req as the number of                                  as follows:
possible frequencies, and ft as a possible frequency value.                                 1. 75 versions of the CL replica code are used. Each version,
The counter rotation of yi is                                                            CL0Li , is initialized to start at one of the possible code delays,
                                                                                         where i = 1, . . . , 75.
                     NcF rag m
                                                                                            2. The estimates of phase and Doppler shift are propagated
                                                                                                                                                 ˆ
                                                                                         to each Tn ms. Define the propagated values as θk and fdk .    ˆ
  Sm,ft ,Eα =                           yi
                                                                                            3. The Tn length, taking into account the Doppler effect, is
                i=NcF rag (m−1)+1
                                                                                         calculated as
                             Ti                   Tt2   T2
exp −j2π        ft Tti +           + αft ,Eα         i
                                                       + i                  ,                                             fL2
                             2                     2     6                                            Tnk = Tn                         .                    (13)
                                                                                                                       ˆ + αTn /1000/2
                                                                                                                 fL2 + fdk ˆ
                                   3       ft
             αft ,Eα = αfe −             Ndα
                                                      − Eα .                                4. 75 versions of the local signal in (12) are generated, each
                                   2            Tdm
                                         m=1                                             version uses one of the CL0Li code versions. Each one starts
Eα models the error in the estimated αfe . It can be set to a few                        at a time Tk and spans Tnk ms of the CL code. Define each
values, NEα , (e.g. 0,1,-1), and thus the number of paths will                           version as IQL0Li .



                                                                                    47                               http://sites.google.com/site/ijcsis/
                                                                                                                     ISSN 1947-5500
                                                               (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                             Vol. 9, No. 9, September 2011
   5. Each IQL0Li is correlated with the received signal,                     In item 7, if the 1st approach is used to find the likely data
starting at Tk . Define each result as Yi .                                 combination, each matrix is added incoherently to the previous
   6. Yi is added coherently to the previous total coherent                total incoherent integration. The number of computations is
integration, H(i) = H(i) + Yi . The counters are updated,
                                                                                              CN C = 2Nt −1 Nτ Nfd .                         (15)
U = U + 1, V = V + 1.
   7. If the desired coherent integration length is reached, i.e.          Only the matrix that corresponds to the likely data combination
U = Lcoh , then the contents of H are added to the previous                is kept and the other matrices are discarded. Finding the
total incoherent accumulation, P (i) = P (i) + ℜ{H(i)}2 +                  maximum can be done by one of the sorting methods described
ℑ{H(i)}2 , where ℜ and ℑ define the real and imaginary parts,               in [20], chapter 2. Thus, the number of comparisons needed
respectively. Following that, H and U are set to zero.                     is
   8. If V < Ntotal , Tk+1 is set as Tk + Tnk , and then steps                              CCM1 = 2Nt −1 Nτ Nfd − 1.                 (16)
(2)–(7) are repeated.
                                                                           If the 2nd approach is used to estimate the likely data combi-
   9. If V = Ntotal , the CL code delay is concluded from the
                                                                           nation, the number of comparisons is
P (i) that has the maximum power.
   To reduce processing as the algorithm progresses, the un-                                  CCM2 = 2Nt −1 Nτ Nfd .                         (17)
likely code delays can be eliminated. Those will be the delays
                                                                             In item 8, the real and imaginary parts of each cell are
that generate the minimum P (i). The unlikely code delays can
                                                                           squared and added together. This requires
be eliminated every Nelim steps, where Nelim is set based on
the C/N0 , which can be estimated after the fine acquisition.                           CIQ = 3 2Nt −1 Nτ Nfd computations.                   (18)
Another method is to eliminate the delays at indexes imin if
P (imax )/P (imin ) > pelim . Where, imax is the index of the              B. FAVA-L2
delay that generates the maximum power at the current step,
                                                                             The operations include generating correlated signals,
and pelim is a predefined ratio.
                                                                           counter rotating the correlated signals by the possible carrier
                                                                           parameters, and choosing the optimal path. The number of
      VI. C OMPUTATIONAL C OMPLEXITY A NALYSIS                             computations can be found directly.
A. CM-ABS
                                                                           C. MS-CL
   The following are repeated L times to get the total inco-
                                                                              The operations include generating 75 versions of the CL
herent integration. In item 1, FFT is calculated for Nfd local
                                                                           local code, multiplying the samples of the received signal
code’s blocks. In item 2.a, FFT is calculated for (Nfd +Nstep −
                                                                           and the local one, adding them together to form the coherent
1) received signal’s blocks, while in item 2.b, FFT is calculated
                                                                           integration, and adding the coherent integration to the total
for Nfd received signal’s blocks. In item 3.a, each two corre-
                                                                           incoherent integration. The total number of computations can
sponding blocks of the received signal and the local one are
                                                                           be shown to be
multiplied, then IFFT is calculated; this is repeated Nstep Nfd
times to get the coherent integration at all the code delays.                           CCL ≈ 75 LCL Lcoh [4 Tn fs + 4] .                    (19)
Each FFT or IFFT operation requires (2 Sblock ) log2 (2 Sblock )
computations. The number of computations to get the first                                 VII. S IMULATION AND R ESULTS
coherent integration (items 1, 2.a and 3) is                                  The algorithms are demonstrated using simulated GPS L2C
                                                                           signals. The CM code is modulated by ±1 data with 50 Hz
 CBP1    =    [2 Nfd + Nstep − 1 + Nstep Nfd ]
                                                                           rate and 0.5 probability of data transition. fs = 3500 kHz.
              [2 Sblock log2 (2 Sblock )] + [2 Sblock Nfd Nstep ] .        fL2 = 1227.6 MHz. The initial phase is modeled as a uni-
                                                                           formly distributed random variable (UDRV) between (−π, π).
For the rest of the coherent integrations (items 1, 2.b and 3),
                                                                           A Doppler shift range between (−5, 5) kHz is assumed. The
this number is
                                                                           oscillator phase and frequency noises are modeled with normal
  CBPl    =    [2 Nfd + Nstep Nfd ] [2 Sblock log2 (2 Sblock )]            random walks; the model is similar to the one in [21], and the
                                                                           variances are derived as in [22]. A temperature compensated
          +    [2 Sblock Nfd Nstep ] .
                                                                           crystal oscillator (TCXO) is simulated. The values of the phase
  The matrix Mc will have a size of Nfd xNτ . In item 4,                   and frequency random walk intensities are Sf = 5 · 10−21 s,
2Nt −1 versions of Mc are generated, each one corresponds to               and Sg = 5.9 · 10−20 s−1 , respectively.
a possible data bit combination. In item 5, FFT is calculated                 The CM-ABS algorithm is tested using very low C/N0 ,
for each column of the 2Nt −1 matrices. The number of                      10 and 15 dB-Hz, to demonstrate its ability. A coherent
computations in items 4 and 5 is                                           integration length of 80 ms is used. For the 15 dB-Hz signal,
                                                                           a total of 30 incoherent accumulations are calculated. Fig. 4
              CF F T = 2Nt −1 Nτ Nfd log2 (Nfd ).              (14)        shows the power versus the code delay. The acquired signal



                                                                      48                              http://sites.google.com/site/ijcsis/
                                                                                                      ISSN 1947-5500
                                                                  (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                Vol. 9, No. 9, September 2011
                                                                                                   6


              130                                                                                  5

              120
                                                                                                   4




                                                                                       σα Hz/s
              110
   Σ I2 +Q2




                                                                                                   3
              100




                                                                                              e
               90                                                                                  2

               80
                                                                                                   1
               70
                                                                                                   0
               60                                                                                  10        12            14         16        18        20        22        24
                                                                                                                                 C/N0 dB−Hz
                0     0.5         1          1.5          2
                        Code Delay (chips)             x 10
                                                           4
                                                                              Fig. 6. Standard deviation of Doppler rate estimation error vs. C/N0 using
                                                                              the FAVA-L2 algorithm, with TCXO clock.
Fig. 4. Power vs. code delay of the acquisition of the CM signal, with
C/N0 = 15 dB-Hz, TCXO clock, TI = 80 ms, and 30 incoherent                                         10
accumulations.
                                                                                                       9

                                                                                                       8

              450                                                                                      7

                                                                                                       6




                                                                                      σf Hz
                                                                                                       5




                                                                                             e
                                                                                                       4
              400
   Σ I2 +Q2




                                                                                                       3

                                                                                                       2

                                                                                                       1
              350
                                                                                                       0
                                                                                                       10        12        14         16        18        20        22        24
                                                                                                                                 C/N0 dB−Hz

                                                                              Fig. 7. Standard deviation of Doppler shift estimation error vs. C/N0 using
              300                                                             the FAVA-L2 algorithm, with TCXO clock.
                 0    0.5          1          1.5          2
                        Code Delay (chips)              x 10
                                                              4



Fig. 5. Power vs. code delay of the acquisition of the CM signal, with        coherent integration of 100 ms is used, with 4 incoherent
C/N0 = 10 dB-Hz, TCXO clock, TI = 80 ms, and 145 incoherent                   accumulations. Fig. 8 shows the power versus the 75 possible
accumulations.
                                                                              code delays of the acquisition result. The algorithm correctly
                                                                              estimated the CL code delay at the 31st delay, which generated
                                                                              a power of 52. The maximum noise power was 23.
had a power of 135, while the maximum noise power was
100. For the 10 dB-Hz signal, a total of 145 incoherent
                                                                                                  50
accumulations are calculated. Fig. 5 shows the power versus
                                                                                                  45
the code delay. The acquired signal had a power of 453, while
                                                                                                  40
the maximum noise power was 402.
                                                                                                  35
   The FAVA-L2 algorithm is tested using C/N0 between 10
                                                                                                  30
and 24 dB-Hz and TCXO clocks. For this test, the Doppler
                                                                                          Power




shift and rate errors are modeled as UDRV’s in the ranges of                                      25

(-50, 50) Hz and (-20, 20) Hz/s, respectively. The algorithm                                      20

is run for 1000 trials; each trial used 6 seconds of data. The                                    15

standard deviation (SD) of the estimation error is calculated.                                    10

Fig. 6 shows the SD of the Doppler rate estimation error versus                                   5

C/N0 . The SD was about 5.5 Hz/s at 10 dB-Hz, and was about                                                 10        20         30        40        50        60        70
0.5 Hz/s at 24 dB-Hz. Fig. 7 shows the SD of the Doppler shift                                                                   Code Delays

estimation error versus C/N0 . The SD was about 8.5 Hz at
                                                                              Fig. 8. Power vs. the 75 possible code delays of the acquisition of the CL
10 dB-Hz, and was about 2 Hz at 24 dB-Hz.                                     signal, with C/N0 = 15 dB-Hz, TCXO clock, TI = 100 ms, and a total of
   The MS-CL algorithm is tested using 15 dB-Hz signal. A                     4 incoherent accumulations.




                                                                         49                                                     http://sites.google.com/site/ijcsis/
                                                                                                                                ISSN 1947-5500
                                                                      (IJCSIS) International Journal of Computer Science and Information Security,
                                                                                                                    Vol. 9, No. 9, September 2011
            VIII. S UMMARY AND C ONCLUSIONS                                          [5] B. Parkinson and J. Spilker, Global Positioning System: Theory and
                                                                                         Applications. AIAA, 1996.
   Acquisition and fine acquisition algorithms were presented                         [6] P. Misra and P. Enge, Global Positioning System: Signals, Measurements,
in this paper for the new L2C GPS signal. The algorithms                                 and Performance. Ganga-Jumuna Press, December 1 2001.
were designed to work with weak signals, without requiring                           [7] J. B. Y. Tsui, Fundamentals of Global Positioning System Receivers: A
                                                                                         Software Approach, Second Edition. Wiley-Interscience, 2004.
assisting information from wireless or cellular networks. The                        [8] D. J. R. V. Nee and A. J. R. M. Coenen, “New Fast GPS Code-
paper focused on implementing techniques to increase sensitiv-                           Acquisition Technique Using FFT,” IEEE Electronics Letters, vol. 27,
ity and reduce processing and memory requirements to enable                              no. 2, January 17, 1991.
                                                                                     [9] D. M. Lin and J. B. Y. Tsui, “Comparison of Acquisition Methods for
the implementation of the algorithms on devices with limited                             Software GPS Receiver,” in ION GPS, Salt Lake City, UT, September
resources, like wireless devices.                                                        19–22, 2000, pp. 2385–2390.
   Three algorithms were developed to work sequentially to                          [10] D. M. Lin, J. B. Y. Tsui, and D. Howell, “Direct P(Y)- Code Acquisition
                                                                                         Algorithm for Software GPS Receivers,” in ION GPS, Nashville, TN,
acquire the CM and CL codes and to provide fine estimates for                             September 14–17, 1999.
the carrier parameters. The CM-ABS was designed to acquire                          [11] N. I. Ziedan, GNSS Receivers for Weak Signals.            Artech House,
the CM code and implemented a bit synchronization method                                 Norwood, MA, July, 2006.
                                                                                    [12] C. Yang, “Joint Acquisition of CM and CL Codes for GPS L2 Civil
to avoid correlating over bit boundaries. Long coherent and                              (L2C) Signals,” in ION AM, Cambridge, MA, June 27–29 2005, pp.
incoherent integrations were used. The Doppler effect on the                             553–562.
code length was handled by correctly aligning the coherent                          [13] G. S. Ayaz, T. Pany, and B. Eissfeller, “Performance of Assisted
                                                                                         Acquisition of the L2CL Code in a Multi-Frequency Software Receiver,”
integration before adding it to the incoherent accumulation.                             in ION GNSS, Fort Worth, TX, September 25–28 2007, pp. 1830–1838.
The FAVA-L2 was designed to provide fine estimates for the                           [14] M. L. Psiaki, “FFT-Based Acquisition of GPS L2 Civilian CM and CL
phase, Doppler shift and rate. It was based on the optimal                               Signals,” in ION GNSS 04, Long Beach, CA, September 21–24 2004,
                                                                                         pp. 457–473.
Viterbi algorithm. The difference between the total phase at                        [15] C. Yang, J. Vasquez, and J. Chaffee, “Fast Direct P(Y)-Code Acquisition
the start and end of the CM code could be relatively large.                              Using XFAST,” in ION GPS 1999, Nashville, TN, September 14–17
So, the CM code was divided into small segments, and the                                 1999, pp. 317–324.
                                                                                    [16] A. R. A. Moghaddam, R. Watson, G. Lachapelle, and J. Nielsen, “Ex-
correlation for each segment was calculated separately. The                              ploiting the Orthogonality of L2C Code Delays for a Fast Acquisition,”
carrier parameters were propagated to each segment’s time and                            in ION GNSS 06, Fort Worth, TX, September 26–29 2006, pp. 1233–
used to counter rotate each segment’s total phase before further                         1241.
                                                                                    [17] A. J. Viterbi, “Error Bounds for Convolutional Codes and an Asymptoti-
processing the segments. The MS-CL was designed to acquire                               cally Optimum Decoding Algorithm,” IEEE Transactions on Information
the CL code by searching only 75 possible delays. It used long                           Theory, vol. 13, no. 2, pp. 260–269, April 1967.
coherent and incoherent integrations. The integrations can be                       [18] G. D. Forney, “The Viterbi Algorithm,” Proceedings of the IEEE, vol. 61,
                                                                                         no. 3, pp. 268–278, March 1973.
calculated in smaller steps to avoid exhausting the available                       [19] R. Riccardo, P. Andreas, and T. Ching-Kae, “Per-survivor processing,”
resources.                                                                               Digital Signal Processing, vol. 3, no. 3, pp. 175–187, July 1993.
   The computational complexities of the algorithms were                            [20] T. Cormen, C. Leisersen, R. Rivest, and C. Stein, Introduction to
                                                                                         Algorithms, Second Edition. MIT Press, 2001.
analyzed. The analysis results can be used to determine the                         [21] R. G. Brown and P. Y. C. Hwang, Introduction to Random Signals and
maximum integration lengths that can be used, and the mini-                              Applied Kalman Filtering. J Wiley, 1992.
mum C/N0 that can be detected based on a device’s available                         [22] A. J. V. Dierendonck, J. B. McGraw, and R. G. Brown, “Relationship
                                                                                         Between Allan Variances and Kalman Filter Parameters,” in 16th PTTI
resources.                                                                               Application and Planning Meeting, NASA Goddard Space Flight Center,
   The algorithms were tested using C/N0 down to 10 dB-                                  November 27–29, 1984, pp. 273–293.
Hz and TCXO clocks. The results indicated the ability of the
algorithms to work efficiently with such low C/N0 .                                                             AUTHOR PROFILE

                                                                                    Nesreen I Ziedan received a Ph.D. degree in Electrical and Computer
                             R EFERENCES                                            Engineering from Purdue University, West Lafayette, IN, USA, in 2004. She
 [1] R. Fontana, W. Cheung, P. Novak, and T. Stansell, “The New L2 Civil            also received an M.S. degree in Control and Computer Engineering from
     Signal,” in ION GPS, Salt Lake City, UT, September 11–14 2001, pp.             Mansoura University in 2000, a Diploma in Computer Networks from the
     617 –631.                                                                      Information Technology Institute (ITI), Cairo, in 1998, and a B.S. degree
 [2] R. Fontana, T. Stansell, and W. Cheung, “The Modernized L2 Civil               in Electronics and Communications Engineering from Zagazig University in
     Signal,” GPS World, 2001, September 2001.                                      1997. Dr. Ziedan is an Assistant Professor at the Computer and Systems
 [3] M. Tran and C. Hegarty, “Performance Evaluations of the New GPS                Engineering Department, Faculty of Engineering, Zagazig University, Egypt.
     L5 and L2 Civil (L2C) Signals,” in ION NTM, Anaheim, CA, January               Dr. Ziedan has several U.S. Patents in GPS receivers design and processing,
     22–24 2003, pp. 521–535.                                                       and she is the author of a book entitled “GNSS Receivers for Weak Signals”,
 [4] M. Tran, “Performance Evaluations of the New GPS L5 and L2 Civil               which was published by Artech House, Norwood, MA, USA, in 2006.
     (L2C) Signals,” ION Journal of Navigation, vol. 51, no. 3, pp. 199–212,
     2004.




                                                                               50                                    http://sites.google.com/site/ijcsis/
                                                                                                                     ISSN 1947-5500

				
DOCUMENT INFO
Shared By:
Stats:
views:231
posted:10/12/2011
language:English
pages:9