Efficient utilization of channel state information in modern wireless communication systems

The modern view considers channel 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. This dissertation focuses on efficient designs of wireless systems which, depending on the different availability of CSI, exploit channel fading in various ways.;We first study how the transmitter Multiple-Input Multiple-Output (MIMO) beamforming can have a dramatic impact on the receiver channel estimation in an Orthogonal Frequency Division Multiplex (OFDM) system, if it is not designed carefully. A beamforming method is proposed which helps improve the receiver channel estimation performance without degrading any benefit of a conventional beamformer, and several relevant problems are addressed.;We then shift the attention to communication over slow fading channels and study several communication strategies. It is shown that multi-level coding, which was previously studied mainly due to its ability to achieve the constrained channel capacity, is especially powerful in slow fading channels. The key observation is that different bit positions in a constellation typically have different error protection capabilities. This is especially beneficial in a slow fading channel, as it allows for a gradual performance degradation with the decreasing receive SNR.;In the third part of this dissertation, we study how to exploit the ARQ feedback in a scalar slow fading channel. Traditional use of ARQ is merely an indicator for possible retransmission. This work takes a different view and exploits the implicit CSI embedded in the ARQ feedback. It is shown that using Hybrid-ARQ (HARQ) can significantly improve the average rate performance, provided that the rate assignment between different ARQ rounds is carefully done. The average rate of several HARQ schemes is optimized and compared. Optimal power allocation is studied. Comparison of ARQ feedback with one-shot CSI feedback is also made.;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 how to design practical 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 analyzed. 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.