Efficient Utilization of Channel State Information in Modern Wireless Communication Systems

Cong Shen
Advisor: Michael Fitz

Monday, September 14, 2009 at 2:00pm-4:00pm
Engr IV Room 57-124

Abstract:
The modern view considers fading as a source of randomization and system design should exploit instead of compensate for such randomness. It is well-known that the knowledge of channel state information (CSI) plays a fundamental role in exploiting fading. In this talk, we focus on communication over slow fading channels and study several communication strategies which, depending on the availability of CSI, exploit channel fading in different ways. We first demonstrate that even if only the receiver knows the channel, opportunistic communication can still be achieved with the broadcast superposition codes. We show that multi-level coding (MLC), which was previously studied mainly due to its ability to achieve the constrained channel capacity, is especially powerful in slow fading channels. In the second part of the talk, we consider the scenario where the implicit CSI embedded in the ARQ feedback is exploited as partial transmit CSI. The ARQ feedback is in a sequential and incremental manner, which differs from most studies where quantized CSI is fed back in a one-shot fashion before the data transmission. An "aggressive transmission" concept is proposed to best utilize the ARQ feedback. Several related design problems are also discussed. The idea of exploiting ARQ feedback to obtain partial transmitter CSI is further extended to the multiple-antenna system. The focus is on both the fundamental performance limit and practical design of space-time code to achieve this limit. Optimal average rate of existing HARQ protocols is first analyzed, and then the joint design of linear space-time block code and ARQ feedback is studied. Two different performance metrics, mutual information and decoding error probability, are studied. Design criterion for each metric is proposed, and existing codes are evaluated. Finally, we study the impact of CSI on a multi-user cognitive radio network. The availability of receiver CSI is the key to the proposed Opportunistic Spatial Orthogonalization (OSO) scheme, which allows the existence of secondary users and hence increases the system throughput, even if the primary user occupies all the frequency bands all the time. The key idea is to utilize the multi-user diversity effect to opportunistically align multi-user interference at primary user's receiver. Both SIMO and full MIMO systems are studied, and in the latter case the OSO scheme can be interpreted as "riding the peaks" over the eigen-channels, and ill-conditioned MIMO channel, which is traditionally viewed as detrimental, is shown to be beneficial with respect to the sum throughput.

Biography:
Cong Shen received the B.E. and M.S. degrees, in 2002 and 2004 respectively, from the Department of Electronic Engineering, Tsinghua University, Beijing, China. He is currently working towards the Ph.D. degree in the Electrical Engineering Department, University of California, Los Angeles (UCLA). His research interest is on general communication theory with emphasis on wireless communications.