System Level Performance Analysis of Advanced Antenna ... - Centers

The Universal Mobile Telecommunications System (UMTS) has already started to be
deployed in many countries. After some time, the traffic volume in these networks is expected
to further push the demand for spectral efficient solutions in order to increase the capacity
offered by the radio access systems. Among these solutions, advanced antenna concepts are
considered to be an attractive technology because they can provide outstanding capacity
and/or coverage gains by means of increased protection against fast fading, thermal noise and
multiple access interference.
Most studies on antenna arrays (AAs) conducted so far have been oriented to link level
investigations. However, less attention has been paid to the system level implications
associated with the deployment of this technology. In this study, the capacity gain from
beamforming AAs and transmit and/or receive (Tx/Rx) diversity techniques in UMTS is
assessed at system level, with special emphasis on radio resource management considerations.
In order to support the use of beamforming AAs at the Node-B, conventional power
based admission control (AC) criteria are extended to a directional power based AC
algorithm. This algorithm captures the available capacity gain from spatial filtering while
maintaining the network stability in both up and downlink (DL), regardless of the spatial
distribution of the interference. Uplink simulations with four-element AAs show a capacity
gain of approximately 200% for power controlled dedicated channels (DCHs). In DL, the
capacity gain is around 150% for the case without channelisation code restrictions. However,
when one scrambling code per cell is used, channelisation code shortage is a serious
limitation. Different factors resulting in higher channelisation code shortage, such as low time
dispersion, low link activity factor or large soft handover (SHO) overhead, are investigated.
To solve this problem, a solution is analysed, where the cell is split into spatially isolated
scrambling code regions. With four-element AAs, this solution has a marginal penalty of
4−8% due to the lack of orthogonality between signals with different scrambling codes.
The next step is to analyse the use of dual antenna Rake (2Rake) receivers at the UE.
The analysis is conducted for circuit switched DL connections over power controlled DCHs.
The achievable capacity gain and the potential channelisation code shortage are analysed for
different power delay profiles, block error rate (BLER) targets and SHO configurations. For
example, in Pedestrian A, with a BLER target of 10%, no SHO and no channelisation code
restrictions, the capacity gain is 167%. However, channelisation code shortage decreases the
achievable capacity with 36%. The dependency of the capacity gain upon the penetration rate
of the UEs with 2Rake receivers is analysed theoretically and simulation results verify the
outcome of such study. In addition, the radio resource management implications of having
2Rake receivers implemented at the UE are discussed.
As a third step, the High Speed Downlink Packet Access (HSPDA) concept of UMTS is
considered. In HSDPA, fast packet scheduling (PS) is feasible, so that the system can track
the instantaneous variations of the channel quality of all the UEs and therefore provide multiuser
diversity. This diversity mechanism, together with other sources of diversity embedded
in HSDPA, affects the capability of the system to benefit from Tx/Rx diversity techniques.
The HSDPA cell capacity with different Tx/Rx diversity techniques is assessed under three
PS algorithms: Round Robin (RR), Proportional Fair (PF) and Fair Throughput (FT). The
benefit is measured in terms of HSDPA cell capacity gain and/or increased coverage. For
example, in Pedestrian A at 3 kmph, the HSDPA cell capacity gain from closed loop transmit
diversity at the Node-B combined with 2Rake at the UE equals 201%, 123% and 38% for FT,
RR and PF, respectively. However, for RR under the same conditions, the average throughput gain
for UEs with G=-4 dB is 320%, which involves a coverage gain. The impact of both the UE speed
and the frequency selectivity of the radio channel on the HSDPA cell capacity are also addressed.
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