Adaptive Antenna Array for IMT-2000

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     The adaptive antenna array is one of the advanced techniques which could be
     implemented in the IMT-2000 mobile telecommunications systems to achieve high
     system capacity. In this paper, an integrated adaptive antenna array system for
     W-CDMA, which consists of both the uplink and the downlink beam-formers in the
     baseband, is described. The uplink beam-former is based on the concept of “finger
     beam-former” in which each dominant component of the multi-path signal is allocated to
     a dedicated beam, and the normalised least mean square algorithm (NLMS) is used as
     the beam-forming algorithm. In the meanwhile, a low complexity but high performance
     algorithm known as the iterative beam steering (IBS) is applied to the uplink signal to
     form a steering beam for the downlink. Simulation results show that, compared with
     using the conventional sectorial antenna, about four times as much system capacity
     can be achieved by employing a four-element adaptive antenna array. In addition, the
     effectiveness of the antenna array is demonstrated by experimental results.

         Y. J. Guo                              Marío A. Bedoya-Martinez                      Jahangir E. Austin

         He has been with Fujitsu Europe        He joined Fujitsu Europe Telecom              He has now been with Advanced
         Telecom R & D Centre, U. K., working   R&D Centre (UK), where he has been            Wireless Communications Section of
         on advanced BTS techniques for         working on R&D of Second-and                  Fujitsu Europe Telecom R & D Centre
         IMT-2000.                              Third-Generation mobile communi-              Ltd.
                                                cation systems. Currently he is a             Principal Engineer responsible for the
                                                Principal Engineer in Advanced                hardware development of Second-
                                                Wireless Communications Section.              and Third-Generation mobile tele-
                                                                                              communication systems.

66                                                                                   FUJITSU.51, 1, pp.66-72 ( 01,2000)
                                                        Adaptive Antenna Array for IMT-2000

1.    Introduction                                       HDR user in a sector, which is equivalent to many
      Currently, the mobile telecommunications in-       co-located speech users, the number of channels
dustry is working on the 3         generation mobile     available to other users will be greatly decreased.
telecommunications systems known as IMT-2000                  Instead of having all the users in a sector
(International Mobile Telecommunications 2000).          share the same antenna beam, the adaptive an-
IMT-2000 is designed to provide wireless access          tenna array is aimed to generate an optimal
to the global telecommunication infrastructure           antenna beam pattern for each user. By creating
through both satellite and terrestrial systems,          a narrow beam pointed at the wanted user and a
serving fixed and mobile users in public and pri-        null in the direction of the HDR user, the total
vate networks.        One of the most promising          MAI experienced by the former can be effectively
members of the IMT-2000 family is based on the           reduced. As a result, the system can accommo-
wideband code division multiple access (W-CDMA)          date a much larger number of HDR and common
technique jointly developed in Japan and Europe.1)       users. In addition, employing the adaptive an-
Compared with the 2nd generation mobile commu-           tenna array can also reduce the outage at the cell
nications systems like GSM or PDC, the W-CDMA            edge and within buildings, and extend cell range.
is expected to offer much higher system capacity         When used in the uplink (from mobiles to a base
and to accommodate larger number of high data            station), it can reduce the required mobile termi-
rate (HDR) applications such as mobile multime-          nal transmit power and hence increase the battery
dia and mobile computing.                                life in terms of both talk time and standby time.
      In a W-CDMA system, different users are al-             It is envisaged that many W-CDMA systems
located with different codes and all the active user     will be based on the six-sector site configuration
signals are present all the time. For any given          in order to guarantee high initial capacity, and
active user, the interference caused by other ac-        adaptive antenna arrays will be introduced in the
tive users can be effectively rejected by virtue of      second phase of the system deployment when traf-
the so-called processing gain. When the number           fic congestion caused by HDR users becomes
of active users in a cell is very large, however, the    severe. To this end, a four-element linear adap-
total interference caused by many users to any           tive antenna array with an inter-element spacing
particular user, which is known as multiple ac-          of one wavelength of the operating frequency is
cess interference (MAI), becomes so severe that          studied for W-CDMA base stations using six sec-
the signal quality at the receiver degrades to an        tors in macro-cellular environments. The NLMS
unacceptable level. A conventional solution to this      and IBS beam-forming algorithms are used for the
problem is the sectorisation technique, which is         uplink and the downlink (from a base station to
to divide each cell into several sectors (normally       mobiles) respectively, but the same wide-band
three or six) and group the active users accord-         antenna elements are employed for the two FDD
ingly. Because different sectorial antennas are          (frequency division duplex) sub-bands. The two
used in different sectors and each antenna beam          algorithms have four salient features: fast
is concentrated in its own sector, users in differ-      convergence, low complexity, stability and ease of
ent sectors do not interfere with each other             implementation. Compared with using one sec-
strongly and the total MAI experienced by each           torial antenna for each sector, about four times of
active user is significantly reduced. The problem        as much system capacity can be achieved using
with the conventional sectorisation techniques is        the proposed technique.
that each sectorial beam is designed to support
all the users in the sector so the beam may not be       2.   System configuration
optimal for any individual one. When there is a               The proposed adaptive antenna array system

FUJITSU.51, 1, (01,2000)                                                                                 67
 Adaptive Antenna Array for IMT-2000

consists of both the uplink and the downlink beam-      where there are a number of finger beam-
formers in the base band. The uplink beam-former        formers, each equipped with a dedicated NLMS
employs the NLMS algorithm with fading com-             algorithm and a channel estimator, and the MF
pensation. The downlink beam is synthesised             blocks refer to correlators. Also, a combination of
based on the angle of arrival (AoA) information         the pilot signal and tentative decisions is taken
and the downlink data rates required by mobile          as the common reference signal. It is noted that
users, with the former being derived from the up-       signals at the output of the correlators are norm-
link signal using a new algorithm called iterative      alised to the mean signal intensity of each
beam steering (IBS). The combination of the two         individual path before being fed into the beam-
beam-forming algorithms which are suited for two        former. Furthermore, the reference signal is phase
links provides an effective means of improving the      compensated before being subtracted from the
system capacity of an IMT-2000 system.                  output of the beam-former. These two functions
                                                        are used to separate the fading compensation from
2.1 Uplink beam-forming                                 the angular tracking of mobiles, thus improving
        The uplink beam-former employs the so-          the performance of the beam-former.
called “finger beam-former” configuration, in
which a set of “finger beams” are formed to track       2.2 Downlink beam-forming
different signal components of each active mobile             Downlink beam-forming is inherently differ-
          The number of fingers allocated to a user     ent from the uplink beam-forming. For the uplink,
is determined by the number of signal components        an optimum beam can be formed for each individ-
caused by the multipath in the channel, and the         ual mobile to maximise the SINR experienced by
beam of each finger is locked to each path. A dy-       the corresponding receiver at the base station. For
namic finger beam-former allocation scheme will         the downlink, however, the quality of the signal
be employed in the proposed system. In other            received by each mobile depends on not only the
words, a base station has a number of finger beam-      beam patterns formed for the mobile and the as-
formers as a shared resource and up to four finger      sociated transmit power, but also the beam
beam-formers will be activated by each active user.     patterns formed for other mobiles and the associ-
Compared with the “common beam-former” which            ated powers. In theory, therefore, all the beams
employs only one common beam for all the                should be jointly optimised in association with the
multipath components, the finger beam-former            transmit powers in order to achieve maximum
configuration offers much better performance            SINR at each mobile and minimum interference
when the angular spread of the mobile signal is
large, as a narrower finger beam can be produced
to point at the AoA of each path of the wanted
user signal. Another advantage of the finger             MF

beam-former configuration is that it can be oper-
ated at the symbol rate instead of the chip rate,                                                       MRC
                                                                                               Ch Est   Combiner
so the power consumption can be reduced and              MF
the algorithm can be implemented using re-                                                                   Tentative
                                                         MF                                                  Decision
programmable signal processing devices, which                      Normalisation
offers flexibility in fine-tuning and updating the                                 NLMS        -
algorithm at a later stage.
        A generic architecture of the proposed uplink   Figure 1
beam-former using NLMS is shown in Figure 1,            NLMS based uplink beam-former.

68                                                                                        FUJITSU.51, 1, (01,2000)
                                                       Adaptive Antenna Array for IMT-2000

to other users. Unfortunately, such a global opti-      tive antennas, a large number of multi-user sim-
misation approach is not feasible in practice due       ulations                                      have     been    conducted       using            the
to both the excessive signal processing complexi-       parameters specified in the 3GPP document.                                                      In
ty and the inaccuracy of downlink AoA estimation.       the simulations, it is assumed that all the users
Therefore, the following practical beam-forming         are randomly located according to an uniform dis-
approach is proposed. First, based on the signal        tribution in each sector, and the multi-path
received from the uplink, the IBS algorithm is          components of each user signal are spatially dis-
used to estimate the AoA’s of all the users. Sec-       tributed according to a Gaussian distribution with
ond, based on the AoA’s of each wanted user, a          2.5º standard deviation. The power control mech-
common steering beam is synthesised for it. It          anism is modelled as a Gaussian process with a
should be pointed out that although the propaga-        2.3 dB standard deviation. The ITU Vehicular
tion mechanism for the uplink is similar to that        B model is used for all the radio channels and the
for the downlink, the angular positions of the dom-     Doppler frequency is assumed to be 80 Hz. The
inant paths are generally different in the two cases    processing gain is chosen as 128, which corre-
because of the frequency difference in the FDD          sponds to a 32 kb/s raw data rate, and a
scheme. Therefore, a dedicated common beam for          convolutional coding scheme with R = 1/3 and
each user is required in the downlink to cover the      K = 9 is used. Previous study showed that the
spatial spread of the multi-path signals. As an         inter-element spacing is an important parameter
illustration to the concept, a generic diagram of       in the design of an adaptive antenna array.4) In
the downlink beam-former is shown in Figure 2.          the proposed system, this parameter is chosen as
       Compared with the uplink beam-former             one wavelength of the operating frequency to
shown in Figure 1, the configuration of the down-       achieve high capacity.
link beam-former appears to be much simpler,                                                 Figure 3 shows the required Eb/No per an-
which is due to the fact that major signal process-     tenna branch for different number of simultaneous
ing is performed by the AoA estimator.                  speech channels which the adaptive antenna ar-
                                                        ray can support in one sector to achieve the
3.     Simulation results                               targeted bit error ratio (BER) of 10-3, where a prac-
       In order to obtain a quantitative capacity       tical antenna pattern with -3 dB tapering at the
measure of the W-CDMA system employing adap-            sector boundaries is used. To account for discon-

                                                         No of speech channels per sector

                                                                                                                                  Adaptive antenna
                                                                                            300                                   2 Branch diversity
                                                                                                                                  One antenna

                Code                                                                        200
                                            Weights                                         150

                                   Beam synthesis                                           100

                                              AoA                                           50

                                    AoA estimator                                            0
                                                                                                 -2   0    2     4     6    8    10    12     14        16
                                                                                                          Required receive Eb/No per branch
                                     Uplink signal
                                                         Figure 3
 Figure 2                                                Capacity comparison of using one antenna, two branch
 Illustration of downlink beam-former.                   diversity and adaptive antenna in uplink.

FUJITSU.51, 1, (01,2000)                                                                                                                                69
          Adaptive Antenna Array for IMT-2000

                       18                                                                                   0
Tx power saving (dB)

                                                                                  Signal intensity (dB)
                       8                                                                                  -15
                       2                                                                                                                    beam pattern
                                                                                                          -25                               wanted user
                       0                                                                                                                    HDR user
                            0   10    20    30    40    50    60    70    80
                                Sector load (Number of speech channels)                                     -40   -30   -20     -10     0      10     20   30   40
                                                                                                                              Angle of arrival (deg.)
Figure 4
Mobile transmit power saving in different loading condi-
tions when using adaptive antenna array at base station                          Figure 5
compared with using single antenna.                                              Formed beam pattern.

tinuous transmission of voice signals, a voice ac-                              range of the uplink could be extended by about
tivity factor of 0.5 has been assumed.                                    For   2.5 times under the same load. The mobile trans-
comparison, simulation results obtained using one                               mit power saving achieved in different loading
antenna and two-branch diversity are also shown                                 conditions when using the adaptive antenna at
in the figure. It is seen that when only one anten-                             the base station is shown in Figure 4.
na is used, the maximum number of speech                                                                  Another major advantage of using adaptive
channels which the system can support is about                                  antennas is to accommodate HDR users.4) The
82. The system capacity is increased to about 157                               adaptive antenna can not only support much
when two-branch diversity is employed. Using the                                greater number of simultaneous HDR users by
adaptive antenna array, the maximum number of                                   virtue of a narrow beam, but also reduce the strong
simultaneous speech channels the system can                                     interference caused by HDR users by nulling, thus
support reaches about 314. When the inter-cell                                  increasing the overall system capacity. To illus-
and inter-sector interference is considered, it is                              trate this point, the following scenario has been
found that the average system capacity becomes                                  studied. Assume that there are a 0.5 Mb/s HDR
smaller than that shown in Figure 3, but the ca-                                user located at -10º and a speech user located at
pacity achieved by using the adaptive antenna is                                10º with 32 kb/s data rate. To the speech user, the
still approximately four times of that achieved                                 HDR user is equivalent to a group of 30 co-located
using one antenna.                                                              speech users interfering with it. When the adap-
                           Figure 3 also reveals the advantage of using         tive antenna array is used, an optimum beam
adaptive antennas for saving mobile transmit                                    pattern can be so formed that the interference
power. It is observed that in a system loaded with                              from the HDR user can be reduced to a negligible
80 speech users, using single antenna at the base                               level. The beam pattern formed for the speech
station requires 15.8 dB Eb/No to achieve a BER                                 user is shown in Figure 5 and it is observed that
of 10-3, whereas with the adaptive antenna array                                a deep null is formed in the direction of the HDR
only 0 dB Eb/No per branch is required to achieve                               user and the main beam is also shaped.
the same BER. This implies that the transmit                                                              Figure 6 shows the BER for the speech user
power of the mobile terminals can be reduced by                                 achieved by using the adaptive antenna and also
15.8 dB, thus resulting in many fold increase in                                included is the BER achievable for a single user
battery life. Alternatively, the communication                                  when there is no multi-user interference. It can

70                                                                                                                                        FUJITSU.51, 1, (01,2000)
                                                                                                       Adaptive Antenna Array for IMT-2000

                                                                           1 User + 1 High data
                                                                           Rate interferer
                                                                           1 user


                                          -4            -3          -2            -1              0
                                                                                                              Figure 8
 Figure 6                                                                                                     Prototype model of adaptive antenna array.
 BER for speech user shown in Figure 5.

                                     500                                                                improves the system capacity by about four times.
                                                    1 Antenna element
                                     450                                                                Compared with Figure 3, it is seen that the down-
  No of speech channels per sector

                                                    Adaptive antenna
                                     400                                                                link capacity for an isolated sector is greater than
                                     350                                                                that of the uplink, which is due to the ortho-
                                                                                                        gonality of the downlink channel codes.1) When
                                                                                                        inter-cell interference and the spectral occupancy
                                                                                                        of the common control channels are considered, 3)
                                                                                                        however, it is expected that the downlink capaci-
                                      50                                                                ty will be actually smaller than that of the uplink.
                                            4   6     8   10 12 14 16 18                20     22
                                                       Required Eb/No at receiver
                                                                                                        4.   Prototype model
                                                                                                             Figure 8 shows a photograph of an experi-
 Figure 7                                                                                               mental base station model where the proposed
 Capacity comparison of using one antenna and adap-
 tive antenna array in downlink.                                                                        4-element adaptive antenna array is implement-
                                                                                                        ed. Amongst various experiments, the generated
                                                                                                        antenna beam pattern as well as improvement of
be seen that, because of the use of the adaptive                                                        BER performance is being evaluated. Figure 9
antenna array, the Eb/No required for the speech                                                        shows generated directivity characteristics of the
user to achieve the targeted BER of 10                                                        in the    antenna array and Figure 10 shows the improve-
presence of a HDR user is more or less the same                                                         ment of BER performance by the antenna array
as that for the single user case. In practice, this                                                     in presence of an equal power interferer.
means that more active users can be accommo-
dated in the sector and/or the power required for                                                       5.   Conclusions
the speech user can be reduced.                                                                              The architecture and beam-forming algo-
                                      Figure 7 shows the capacity improvement                           rithms of an adaptive antenna array are described.
when the proposed adaptive antenna array is                                                             The adaptive antenna consists of an uplink beam-
employed in the downlink. It is observed again                                                          former and a downlink beam-former, and is
that employing the four-element antenna array                                                           designed to operate in an existing six-sector site

FUJITSU.51, 1, (01,2000)                                                                                                                                   71
 Adaptive Antenna Array for IMT-2000

                                     0                                                                      to improve system capacity. The uplink employs
                                         1 path                                    Measured
                                         rayleigh fading 80 Hz                                              the finger beam-former configuration and the
      Relative signal power (dB)


                                                                                                            NLMS algorithm is used. The IBS algorithm is
                                                                                                            operated on the uplink signal to form a steering
                                                                                                            beam for the downlink. Simulation and experi-
                                   -20                                                                      mental results demonstrate the effectiveness of
                                                                                                            using the adaptive antenna array to increase sys-
                                                                                                            tem capacity by a factor of four and to support
                                     -50 -40 -30 -20 -10 0 10 20 30                      40      50         HDR users in W-CDMA.
                                                Direction of arrival (deg.)

  Figure 9                                                                                                  References
  Directivity characteristics of 4-element antenna array.                                                   1)   F. Adachi, M. Sawahashi, and H. Suda:
                                                                                                                 Wideband DS-CDMA for Next Generation
                                                                                                                 Mobile Communications Systems. IEEE
                                                                 1 path static channel without FEC
                                                                        Desired user AoA = 0 deg.                Communications Magazine., pp.56-69, Sept.
               1.E-02                                                                                       2)   A. J. Paulraj and C. B. Papadias: Space-Time

               1.E-03                                                                                            Processing for Wireless Communications.
               1.E-04                                                                                            IEEE Signal Processing Magazine., pp.49-83,
                                                                                                                 Nov. 1997.
                                                                                                            3)   Technical Specification, V2, Working Group 1
                    -125                        -120      -115       -110     -105      -100          -95        (WG1), Technical Specification Group (TSG),
                                               Received power of desired signal (dBm)                            Radio Access Network (RAN), 3 rd Generation
                                                      in one antenna element
                                                       Single element, equal power interferer
                                                                                                                 Partnership Project (3GPP), September 1999.
                                                       Beamformer, equal power interferer                   4)   Y. J. Guo, M. Davies. M. Zarri, S. Vadgama,
                                                       AoA = 0 deg.
                                                       Beamformer, equal power interferer                        and E. Fukuda: Improving the System
                                                       AoA = 0 deg.
                                                       Beamformer without interferer                             Capacity of UMTS Using Digital Beamform-
                                                                                                                 er. Proceedings of European Wireless’99,
Figure 10
BER performance improvement.                                                                                     Munich, Germany, Oct. 1999.

72                                                                                                                                       FUJITSU.51, 1, (01,2000)

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Description: International Mobile Telecom System-2000, the third generation mobile communication system as early as 1985 by the International Telecommunication Union (ITU) proposed, then known as the future public land mobile telecommunications system (FPLMTS), 1996 was renamed as IMT-2000 (International Mobile Communications - 2000), which means that the system in the 2000MHz band, the maximum service speed of up to 2000kb / s, is expected to be commercially available in around 2000. Since 1997, the second generation of the great success of mobile communication system, the user's rapid growth and the limited system capacity and limited conflict between the business is becoming clear that the standardization of third generation mobile communication work in real terms since 1997 to enter stage.