Adaptive Opportunistic Beamforming in Ricean Fading Channels - Adaptive Signal Processing in Wireless Communications - page 185

7.4 Adaptive Opportunistic Beamforming

in Ricean Channels

In section 7.3, it was indicated that the throughput of opportunistic beamforming con-
verged to that of coherent transmit beamforming, when the number of users was suf-
ficiently large. However, the number of users required grows exponentially with the
number of antennas. For example, in Figure 7.4 , the throughput is approximately 65% of
coherent transmit beamforming even with one hundred users. In order to improve the
throughput up to 90% of coherent transmit beamforming, the number of users must be
increased up to about four hundred, which may not be a practical number of users in a
cell. With a realistic number of users, the performance of opportunistic beamforming
falls much lower than that of coherent transmit beamforming. This problem can be
solved by the adaptive opportunistic beamforming proposed in [17] over Ricean chan-
nels. This section is devoted to presenting the adaptive opportunistic beamforming
technique and its performance. In this section, we will refer to opportunistic beamform-
ing in [ 13 ] as conventional opportunistic beamforming in order to distinguish it from
adaptive opportunistic beamforming.

7.4.1 Adaptive Opportunistic Beamforming

The downlink of a single cell is considered, where the number of users is M and each
user has a single antenna. The base station is equipped with a linear antenna array with
N elements. The fading channel is a Ricean channel modeled by (7.10). The parameter θ k
is related to the DOA Θ k of the k t h user as follows:

= 2

π df

0 cosΘ ,

θ

k

(7.12)

k

c

where c is the speed of propagation of the plane wave, f 0 is the carrier frequency of the
transmitted signal, and d is the spacing between two antenna elements [4].
It is assumed that a mini-slot exists at the beginning of each time slot. Through the
mini-slot, a known pilot signal s k ( t ) is transmitted from every antenna after being multi-
plied by a weight coefficient w n ( t ). When the transmission power is uniformly allocated
to the N antenna elements, w n ( t ) is given by

= 1

wt

n ()

exp( jn φ ( )). t

(7.13)

N

The signal r k ( t ) received by the k t h user during the mini-slot of time slot t is given by



rt h tst () = () () + η , () t (7.14)

k k k

k

where η k ( t ) ~ C N (0,ρ 2 ). The overall equivalent channel ˜ k ( t ) for user k is given by