Adaptive Opportunistic Beamforming in Ricean Fading Channels - Adaptive Signal Processing in Wireless Communications - page 197

7.5 Conclusions

In the context of multiuser diversity, fading is seen as a resource that can be exploited,
not an impediment that must be combated. In a multiuser system where every user has
an independent fading channel, transmitting data only to the user with the best chan-
nel condition not only increases the throughput of the system but also increases the
throughput of every user in the long term on average. The multiuser diversity technique
has been adopted in practical systems, such as the downlink of the IS-856 system.
In order to achieve the higher multiuser diversity gain, the dynamic range of the
channel fluctuation should be larger and the variation rate of the channel should be
faster. When the underlying physical channels have small fluctuation and change slowly,
opportunistic beamforming artificially induces the channel fluctuation by using some
pseudorandom weight coefficients at the base station. By doing this, higher multiuser
diversity can be achieved. However, this opportunistic beamforming requires a large
number of users, especially with many transmit antennas at the base station.
Adaptive opportunistic beamforming can solve this problem very well in Ricean fad-
ing channels. The adaptive opportunistic beamforming algorithm generates the weight
coefficients not randomly, but intelligently by estimating the DOAs of the users. That
is, each beam is generated only in the direction where users really exist. This enables
adaptive opportunistic beamforming to achieve excellent performance even with a small
number of users.

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