Pre-coding for MIMO Broadcast Channels Using Dirty Paper Coding Techniques

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Date

2011-07

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Addis Ababa University

Abstract

Wireless communication systems with multiple antennas are the focus of many applications nowadays due to the higher throughput and/or more robust performance than single antenna communication. One variety of these systems is the multi-user system in which many users share the same wireless environment. In this thesis, communication strategies that use dirty paper coding (DPC) techniques to form independent spatial streams to the Multiple Input Multiple Output broadcast channel (MIMO BC) are studied. Unlike previous studies that assume ideal channel conditions, the performance of DPC algorithms under frequency selective channels having spatially correlated path gains is investigated. These algorithms are found to be equally applicable to frequency selective channels if orthogonal frequency division multiplexing (OFDM) is employed with 1.5 dB power loss due to correlation for realistic correlation values. The second part of the thesis considers issues that arise in practical implementation of the DPC algorithms. The effect of imperfect channel estimates at the transmitter and the problem of scheduling users for transmission are considered. For time division duplex (TDD) systems, we show that errors in channel estimation result in performance degradation that can almost completely be eliminated above a SNR of 25 dB. For frequency division duplex (FDD) systems, it is shown that delay in estimation feedback results in severe performance degradation that becomes unacceptable above some delay depending on the algorithm used. User scheduling is formulated as user selection for single carrier systems. We adapt an algorithm proposed for linear pre-coding techniques and show there is an increase in sum rate in using this algorithm over uniform scheduling of users. For OFDM systems, applying the user selection algorithm proposed for single carrier systems to each subcarrier results in increased sum rate with lower fairness index compared to uniform scheduling. Allowing users to select their strongest subcarriers, on the other hand, is shown to give lower sum rate and improved fairness compared with the sum rate maximizing algorithm. Key words: Dirty Paper Coding, MIMO Broadcast Channel, Pre-coding

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Keywords

Dirty Paper Coding, Mimo Broadcast Channel, Pre-coding

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