Cross-layer Aspects in OFMDA Systems : Feedback, Scheduling and Beamforming

This thesis mainly studies the downlink of a wireless multiuser system, where the transmitter has limited knowledge about the communication channels of the users. Key techniques to improve the performance of such systems are, for instance, multiple antennas, multiuser diversity and orthogonal frequency-division multiple access (OFDMA). Common for these techniques is that, to exploit them fully, a cross-layer approach has to be adopted. This means that the scheduling and the signal designs are done jointly and based on parameters from several communication layers. 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 of OFDMA systems that exploit multiuser diversity are investigated. An adaptive reduced feedback scheme for 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 feeds back information about the fading peaks only. It adapts to the number of users so that less feedback per user is required if there are many users in the system. With such a selective feedback scheme, there is a risk that the scheduler has no instantaneous channel quality information for some parts of the spectrum. Better uses for these sub-carriers are investigated. In addition, an alternative based on the the channel quality feedback of some uniformly spaced sub-carriers is proposed. The scheduler estimates the channel quality on the other sub-carriers. Channel-aware scheduling is necessary in order to exploit multiuser diversity. A modified proportional fair (PF) scheduler is proposed. It incorporates individual target bit-rates and delays and a tunable fairness level. An opportunistic beamforming scheme for clustered OFDMA is presented and evaluated. A key aspect of the opportunistic beamforming scheme is that it induces artificial frequency selectivity for users with relatively flat channels. Several aspects of the proposed system are evaluated by means of simulations. In the simulations, the clustered beamforming with the modified PF scheduler performs better than three comparison systems. The modified PF 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 PF algorithm. In many scenarios, the largest gains from having multiple antennas at the base-