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

Sammanfattning: 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 basei station come from space-division multiple access (SDMA). In the downlink, this means that data is transmitted to several users simultaneously by using several beams. Opportunistic space-division OFDMA is proposed and evaluated. An enhancement that exploits temporal channel correlation is able to boost the throughput significantly. SDMA based on subspace packings is proposed and evaluated. A set of beamforming matrices (a subspace packing) is made a priori available at the base-station and at all users. In each block, one of the matrices is used for multi-beam transmission. The users pick and feed back the index of one preferred column (beam) from one of the matrices, and the corresponding SINR, which includes all potential inter-beam interference. This enables scheduling of spatially compatible users and accurate rate adaptation, with relatively little feedback. Three different subspace packings are considered and evaluated with simulations. For the i.i.d. Rayleigh fading channel, Grassmannian subspace packings were the best choice. Moreover, a method to further reduce the feedback for large packings is proposed and evaluated. It is based on the arrangement of beams in a graph and the feedback of a neighbor index. Numerical results show that the feedback can be significantly reduced with only small performance losses, even for relatively fast fading channels.