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

Adaptive antennas in UMTS
Diversity techniques rely on the statistical independence between the AA elements and
reduce the likelihood of deep fades [36]. When they are used for reception, or in conjunction
with a priori knowledge of the radio channel at the transmitter [37], they are additionally able
to provide an average signal-to-interference-plus noise ratio (SINR) gain. When the AS of the
radio channel is large, low correlation between the antenna elements can be achieved without
excessive antenna separation [35], which makes the use of diversity techniques feasible. Low
correlation between antenna elements can be also obtained by using polarisation diversity.
In this chapter, the different AA techniques that are allowed in the Release 5 of the
UMTS specifications are described. The new proposals currently under discussion in the 3 rd
Generation Partnership Project (3GPP) (see e.g. [38]) are excluded, since their performance is
not assessed in this Ph.D. thesis.
The possibility of deploying AAs at both the Node-B and the UE is also discussed in
this chapter, as a manner to enable multiple-input-multiple-output (MIMO) operation in
UMTS. Note that the combination of AA techniques already allowed in UMTS opens for the
use of MIMO systems that aim at improving the SINR of a certain link conveying a single
data stream, without conducting spatial multiplexing of parallel data streams. This approach is
often referred to as diversity MIMO. There are also proposals in the open literature to utilise
the MIMO systems so that several parallel spatial multiplexed data streams are transmitted
between two transceivers equipped with AAs [39], leading to the so-called information
MIMO approach. Some of these proposals are being currently discussed within 3GPP [38].
The chapter is organised as follows. Section 2.2 describes different manners in which
AAs can be used at the Node-B in UMTS. Note that such description is based on the Rake
receiver with maximal ratio combining (MRC) of the Rake finger outputs as a basic structure
for dealing with multipath propagation environments. In Section 2.3, a similar discussion is
shown for the case where multiple antennas are implemented at the UE side. The deployment
of AAs at both the transmitter and the receiver in order to enable MIMO operation in UMTS
is addressed in Section 2.4. Concluding remarks are given in Section 2.5.
2.2 Antenna arrays at the Node-B
2.2.1 Uplink case − signal reception with AAs
Figure 2.1 depicts a general Rake receiver with M antenna elements deployed for uplink
(UL) signal reception at the Node-B. It is assumed that an independent antenna combining
operation is conducted for each Rake finger, followed by MRC of the N Rake finger outputs.
In this section, different options for the antenna combining are discussed. Since the AA is
operated at the receiver side, it is possible to estimate the radio channel and the spatial
covariance matrix of the interference, which opens for the use of advanced combining
schemes that make use of all this information. As will be seen, the receiver structure can be
simplified for some of the described options for antenna combining.
2.2.1.1 Optimum combining
The weights that maximise the SINR of the received signal were derived by Wiener
[14], and constitute a solution that is very often referred to as optimum combining. This
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