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UBC Theses and Dissertations

Channel aware strategies in wireless communications Elkashlan, Maged


Channel-aware resource allocation is widely considered to be crucial for realizing high data rates in wireless networks. This thesis considers the resource allocation problem in a multiuser wireless network using link adaptation. Link Adaptation (LA), which loosely refers to changing transmission parameters over a link, such as modulation, coding rate, power, etc., in response to changing channel conditions is considered to be a powerful means of achieving higher efficiency or throughput in wireless networks. The adaptation of the transmission parameters is performed according to the predicted future quality of the channel, also referred to as the channel state (CS). The objectives of this thesis are threefold: devise new methods for the improvement of multicarrier code division multiple access (MC-CDMA) as an effective signaling scheme in correlated fading channels, the development of new channel aware algorithms with adaptive subchannel allocation, and the investigation of the statistics of general order selection in correlated Nakagami fading channels. A novel adaptive subcarrier allocation algorithm is developed for MC-CDMA to improve the overall bit error rate (BER) performance. The proposed method is suitable for use in correlated fading channels. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics, while simultaneously reducing the amount of interference caused to other users. This method examines the effect of equalizing the interference in each subcarrier group while maintaining a reduced correlation among the subcarrier fading processes for a group. Consequently, subcarriers are separated into non-contiguous groups to maximize frequency diversity and minimize multiple access interference (MAI). Previously proposed adaptive subcarrier allocation algorithms, are considered to be greedy algorithms. They are greedy in the sense that they consider every reservation request individually, and make the choice that looks best at the moment. Adaptive allocation using the simple greedy method gives an optimal solution for a single user system. Adaptive bit allocation using the greedy method cannot give an optimal solution for multiuser cases. It is possible that the sub-channels with the largest channel gain for one user are also the largest for another user. We present in this thesis a new class of dynamic resource allocation schemes that are based on all the users’ subband gains. Due to the time-varying nature of the wireless channel, dynamic resource allocation makes full use of multiuser diversity to achieve higher performance. In this thesis, we formulate the multiuser subcarrier allocation problem and propose an iterative algorithm to perform the subcarrier allocation. This method involves the ordering of subband gains. Thus, the analytical foundation is the theory of order statistics. The users are recursively assigned to that subcarrier which provides the highest possible signal-to-noise ratio (SNR). While ensuring that no more than one user is assigned to the same subband. Our objective is to find low-complexity schemes which can improve system capacity and throughput, and simultaneously minimizing the MAI. Finally, the cumulative distribution function (cdf) (and hence outage probability) of the r -th order branch SNR in correlated Nakagami- m fading is studied. The accuracy of a simple exchangeable approximation to reduce the computational load is examined. The effects of correlation on the error performance of channel aware systems that involve the ordering of channelgains in a correlated Nakagami fading environment is investigated. A useful analytical formula for the BER of the r-th order statistic of a set of arbitrarily correlated and not necessarily exchangeable diversity branch gains is described and shown to be applicable in the performance analysis of various diversity systems.

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