Resource allocation for the OFDMA downlink with limited feedback

In a downlink Orthogonal Frequency Division Multiple Access (OFDMA) cellular system, optimally allocating sub-channels across mobile users can require excessive feedback of channel state information (CSI). The acquisition of CSI at the transmitter (base station) and the receiver imposes a practical challenge for current wireless communication systems. In this thesis, we address limited rate feedback schemes for optimizing resource allocation in OFDMA downlink systems.;We begin by considering a limited feedback scheme, where each user is pre-assigned a threshold and feeds back one bit per sub-channel to indicate whether the channel gain is above the threshold or not. If more than one user requests the same sub-channel, the base station picks the user with the largest priority weight to transmit. We consider a class of distributions that includes most common fading models such as Rayleigh fading. We characterize the asymptotic behavior of the optimal thresholds and the growth of the weighted sum capacity as the number of users and sub-channels become large. We then compare the asymptotic capacity achieved by this one bit feedback scheme with the capacity when full CSI is available at the transmitter and show that the difference is bounded at most by a finite constant as the system size grows.;Next we consider an OFDMA model in which the feedback overhead is explicitly taken into account, given a fixed feedback rate and finite coherence time. The tradeoff between feedback rate and sum capacity is studied for two limited feedback schemes: a sequential scheme in which the users send compressed feedback bits over consecutive time slots, and a contention scheme in which users send their feedback via a random access protocol. For both schemes each feedback bit indicates a request for a group containing multiple sub-channels. We show that the sum capacity for both schemes with optimized sub-channel groups grows linearly with the number of sub-channels and that the associated constant increases as the log of the normalized feedback rate measured in bits per coherence time per sub-channel. We also compare the asymptotic performance of the two limited feedback schemes as a function of the feedback rate and load (users per sub-channel). The sequential scheme performs best with moderate to large feedback rates, or small loads, whereas the contention scheme performs best with small feedback rates or large loads.;Next, we extend the model to OFDMA systems with multiple antennas, which may require much larger amounts of feedback and overhead to exploit the available spatial dimensions. Again, our analysis explicitly accounts for the feedback overhead by assuming a time division duplex system in which all feedback and data transmission must occur in a coherence time. In the proposed scheme, the base-station sequentially receives feedback from the users and adaptively decides when to stop receiving additional feedback and begin data transmission. Each user feeds back their best codeword selected from a beam-forming codebook on each group of OFDM sub-channels, provided that the channel gain exceeds a given threshold. For a given feedback threshold, the optimal stopping rule used by the base station is derived. The dependence of the total throughput on the coherence time and feedback rate are characterized.;Finally, we consider a narrowband system with correlation across time and study methods for further reducing feedback overhead. In the previous models, the base station schedules only users that feed back CSI. However, with channel memory, users can be scheduled based on the previous feedback history, for example, a user can continue to transmit without additional feedback as long as the transmission does not fail. Given this option, the feedback frequency and feedback amount are optimized as a function of the rate at which the channel varies.