Channel estimation and signal detection for wireless relay.

摘要

Wireless relay plays a critical role in realizing ubiquitous and reliable wireless communications. With wireless relay, signal coverage can be extended and transmission reliability can be enhanced. In practice, a wireless relay station (RS) can be utilized to boost the signal strength in a coverage hole, such as in thick buildings or underground tunnels, or to improve the quality-of-service (QoS) under adverse channel conditions, such as on high-speed trains. The application of wireless relay, especially the amplify-and-forward (AF) relay, entails advanced signal processing techniques to be implemented at both the RS and the destination station (DS). This thesis focuses on developing novel channel estimation and signal detection techniques to improve the performance of a wireless relay system.;Our work starts from signal processing in a two-hop multi-input-multi-output (MIMO) AF relay system consisting of a source station (SS), a DS, and an RS that simply amplifies and forwards its received signal to the DS without any further processing. Since noise is amplified and forwarded from the RS to the DS together with the signal, the overall noise at the DS is colored. To improve signal detection at the DS, we first propose a blind algorithm to estimate the noise correlation based on the statistics of the broadband channel when orthogonal frequency division multiplexing (OFDM) modulation is employed. Since no pilots are inserted at the RS, estimation of the backward and the forward relay channels over the SS-RS and the RS-DS hops becomes a challenging issue. To deal with this problem, we propose to estimate the two cascaded relay channels based on a predefined amplifying matrix sequence at the RS and the corresponding overall channel sequence obtained at the DS through conventional channel estimation algorithms. We derive rules to design low-complexity amplifying matrices to guarantee successful relay channel estimation at the DS. Both the blind noise correlation estimation and the indirect relay channel estimation effectively improve the overall performance of a two-hop MIMO AF relay system.;For a channel-reuse-relay station (CRRS) to work properly, the co-channel cross-talk interference from the transmit to the receive antennas must be cancelled. To that end, the coupling channel between the transmit and the receive antennas needs to be estimated first. While the conventional coupling channel estimator requires the RS to transmit dedicated pilots, we propose to utilize the random forwarded signals of the RS as pilots for coupling channel estimation. Without making any modification to the signal structure in the physical layer and causing any in-band interference at the DS, the proposed cross-talk canceller achieves significant performance improvements over the conventional one.;When wireless relay is deployed on a high-speed train to improve the QoS provided to passengers, Doppler compensation techniques need to be implemented at the RS to shield mobile terminals from adverse channel conditions between the base station (BS) and the train. This is especially necessary when OFDM modulation is applied because the time-varying channel causes inter-subchannel interference (ICI). Since typically a line-of-sight (LOS) path exists between the BS and the train, there is strong correlation in and between the desired signal and the ICI. In light of this, we develop the Wiener filtering in the downlink and the transmit preprocessing in the uplink to mitigate ICI by utilizing such correlation. To mitigate ICI in the absence of a LOS path, we further develop a general reduced-rate OFDM transmission scheme to trade spectral efficiency for ICI self-cancellation in a high-mobility environment. By transmit and receive preprocessing, we transform the original N-subcarrier OFDM system into an equivalent K-subcarrier one with significantly reduced ICI. With the preprocessing coefficients optimized based on the statistics of the time-varying channel, the developed reduced-rate OFDM transmission achieves significant performance improvements over the existing ICI self-cancellation schemes.