Multiuser diversity orthogonal frequency division multiple access systems

Abstract: Multiuser diversity can be used to significantly increase system throughput in wireless communication systems. The idea is to schedule users when they experience good channel conditions and let them wait when the channels are weak. In this thesis, several aspects on multiuser diversity OFDMA systems are investigated. An adaptive reduced feedback scheme for multiuser diversity OFDMA is proposed. It significantly reduces the total feedback overhead while maintaining a multiuser diversity gain. The scheme uses clusters of sub-carriers as feedback units and only feeds back information about the fading peaks. Furthermore, an opportunistic beamforming scheme for clustered OFDM is presented and evaluated. A key aspect of the opportunistic beamforming scheme is that it increases the frequency fading of users with relatively flat channels, which increases the likelihood of being scheduled. Scheduling is an important aspect of multiuser diversity. A modified proportional fair scheduler is proposed in this thesis. It incorporates user individual target bit-rates and delays and a tunable fairness level. These features makes the scheduler more attractive for future mixed service wireless systems. The use of the feedback information in the opportunistic beamforming process is discussed and evaluated. This extra information can help to increase the performance of unfairly treated users in the system. Several aspects of the proposed system are evaluated by means of simulation, using the 3GPP spatial channel model. In the simulations, the clustered beamforming performs better than three comparison systems. The modified proportional fair scheduler manages to divide the resources according to the user targets, while at the same time exploiting the multiuser diversity as well as the standard proportional fair algorithm. The thesis also includes results on coded packet error rate estimation from a channel realization by means of a two dimensional table. This can be useful in large network simulations as well as in designing adaptive modulation schemes.