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

Introduction
classified into two main groups: linear detectors and subtractive interference cancellation
detectors. Linear detectors ([4], [5]) apply a linear transformation to the soft output of the
conventional detector in order to reduce the multiple access interference seen by each user.
Subtractive interference cancellation detectors generate estimates of the interference and
subtract it out of the received signals. They can be classified into successive [6] and parallel
[7] interference cancellation detectors.
In general, multi-user detection techniques involve extra complexity in the receiver, and
many times the nature of many of the proposed algorithms makes it impossible to use them
for the downlink (DL) case, due to the fact that the system does not provide the receiver with
all the necessary pieces of information. However, in the uplink (UL) case, all the needed
information can be obtained, which makes these techniques feasible.
Another possibility to increase the spectral efficiency of the system is the application of
adaptive antenna arrays (AAs) [8], which are considered to be an attractive technology
because they can provide capacity and/or coverage gains in both UL and DL by means of
increased protection against fast fading, thermal noise and multiple access interference.
The use of AAs also involves extra complexity in the system, i.e. extra antennas, extra
cabling, extra amplifiers and extra management complexity at the base station, as well as
serious implementation challenges at the terminal side. Moreover, the performance
enhancement that can be provided by AAs is not attractive for all environments. For example,
in the case of conventional beam steering towards the average direction of arrival of each UE,
the performance gain is expected to be low when the azimuth spread of the radio channel at
the base station is high. Additionally, the deployment of AAs (if clearly visible as a set of
antennas) at the sites could be perceived by the population as an increase in the radiated signal
power, which the subsequent increase of the public health concerns that have been already
raised during the recent years about the potential effects of cellular radio networks. Among
others, these issues could be one reason why adaptive AAs have not been massively deployed
so far.
Regarding the question about whether it is better to use deeper sectorisation or adaptive
AAs with conventional beam steering, it has to be pointed out that adaptive AAs can be used
to perform deeper sectorisation if desired, offering more flexibility in the system
configuration. This possibility is discussed and analysed in [9], where the different
alternatives are assessed. The result of this study suggests that the deployment adaptive AAs
within one cell offers better performance than the option in which deeper sectorisation is
conducted.
Compared with the deployment of extra sites, the use of adaptive AAs (in an
environment where their performance is attractive) should be less expensive, due to the fact
that no extra sites have to be acquired. In addition, adaptive AAs can be used for capacity or
coverage enhancement. Thus, in the initial roll-out phase, adaptive AAs can be used for
coverage enhancement so that the number of necessary sites is reduced. Later, when the
traffic demand starts to grow, more sites can be installed and adaptive AAs can start to be
exploited for capacity enhancement rather than for increasing the coverage range.
This Ph.D. thesis is focussed on the system level performance of adaptive AAs as a
capacity enhancing technique. In this context, the expression adaptive AAs includes both
beamforming and diversity techniques, which can be used for either signal transmission or
reception. As already seen, there is a practical cost associated to the use of this technology.
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