Fast Fading MIMO Broadcast Channel with Rate Limited Feedback: Degrees of Freedom Analysis

In this thesis, we study MIMO broadcast channel with rate limited feedback. For fast fading MIMO broadcast channel, it is well known that channel state information at receiver (CSIR) is not sufficient to provide significant increase of the throughput, namely, degrees of freedom gain. The transmitter needs the channel state information to carry out interference mitigation techniques such as beamforming to improve capacity. Typically, channel state state information at transmitter (CSIT) is obtained from feedback. Since rate-limited feedback creates delay and inaccuracy in CSIT, it is of fundamental interest to understand how such delayed imperfect CSIT can improve system performance. First, we consider independent and identically distributed (i.i.d.) fast fading channel. A scheme based on network coding ideas, where the receivers feedback quantized channel gains through rate-limited feedback links and the transmitter uses these channel gains to reconstruct interference and sends the combined interference to create network coding opportunities. However, the feedback requirement of this scheme grows very large as the number of users increases, and hence it does not utilize feedback efficiently. We propose a novel scheme in which the receivers feedback quantized interference through rate-limited feedback links instead of channel gains. This method can utilize the feedback resource more efficiently. We show that our proposed scheme and feedback method can provide more degrees of freedom gain than baseline scheme. After the analysis in i.i.d. fast fading channel, we extend our scheme to temporally correlated channel. Transmitter can estimate current CSIT from the feedback. We propose our transmission scheme which is suitable for our feedback-interference method. Our proposed feedback method and transmission scheme still has better performance than the best-known schemes in the literature.