Channel and delay estimation algorithms for wireless communication systems

Abstract

This thesis addresses the problem of channel and propagation delay estimation in wireless communication systems. Channel estimation and equalization compensate for channel distortions. Consequently, transmitted data may be reliably recovered. A feasible communication link, in both single user and multi-user communications, requires synchronization between the transmitter and the receiver. Traditional channel estimation and synchronization methods use training data, therefore decreasing the effective data rates. More efficient methods which use smaller amounts of training data are of great interest. In particular blind equalization methods, as well as receiver based synchronization methods enable higher effective data rates.

In Global System for Mobile Communications (GSM) more than 22 % of the transmitted signal is used for channel estimation and synchronization purposes. If blind equalization methods could be applied in GSM, this part of the signal could be used for transmitting information bits. Blind channel identifiability problems in GSM are investigated in this thesis. The performance of several blind equalization methods is also evaluated, for both the Gaussian Minimum Shift Keying (GMSK) modulation and for the 8-Phase Shift Keying (8-PSK) modulation proposed for the future evolution of the GSM, Enhance Data Rates for Global Evolution (EDGE). Blind equalization methods are feasible for GSM in low mobility scenarios.

The uplink (mobile to base station (BS) link) in direct-sequence code division multiple access (DS-CDMA) wireless networks is asynchronous. A DS-CDMA receiver has to simultaneously estimate channel impulse responses (CIR) and propagation delays for the active users. Commercial CDMA based systems use long spreading codes, with the period much longer than the symbol period. In this thesis, a novel uplink multi-user adaptive receiver is developed for long-code DS-CDMA. It is also capable of tracking time variations of the channels. Multiple antennas are considered at the receiver end, taking advantage of the signal to noise ratio (SNR) gain and the antenna diversity gain. A specific system model is developed, characterized by a channel matrix which also includes the effect of the propagation delays. Estimating the channel matrix leads to the implicit estimation of the propagation delays. Algorithms for the explicit estimation of the propagation delays are also derived. The proposed receiver structures are capable of estimating and tracking the impulse responses of the channels and synchronizing the active users by using low complexity adaptive techniques.

This thesis also addresses the problem of channel estimation and time synchronization in Orthogonal Frequency Division Multiplex (OFDM) systems. OFDM is robust with regard to frequency selective channels but is very sensitive to time and frequency synchronization errors. A novel low-complexity iterative method is developed for channel and time-offset estimation in OFDM by using a system model specific to fixed wireless links, e.g. wireless local area networks (WLAN) IEEE 802.11 standard.