Maximum Likelihood Blind Channel Estimation for Space-Time Coding Systems
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Maximum Likelihood Blind Channel Estimation for Space-Time Coding Systems Hakan A. C¸ırpan Department of Electrical Engineering, Istanbul University, Avcilar, 34850 Istanbul, Turkey Email: [email protected]
Erdal Panayırcı Department of Electronic Engineering, IS¸IK University, Maslak, 80670 Istanbul, Turkey Email: [email protected]
Erdinc C¸ekli Department of Electrical Engineering, Istanbul University, Avcilar, 34850 Istanbul, Turkey Email: [email protected] Received 30 May 2001 and in revised form 7 March 2002 Sophisticated signal processing techniques have to be developed for capacity enhancement of future wireless communication systems. In recent years, space-time coding is proposed to provide significant capacity gains over the traditional communication systems in fading wireless channels. Space-time codes are obtained by combining channel coding, modulation, transmit diversity, and optional receive diversity in order to provide diversity at the receiver and coding gain without sacrificing the bandwidth. In this paper, we consider the problem of blind estimation of space-time coded signals along with the channel parameters. Both conditional and unconditional maximum likelihood approaches are developed and iterative solutions are proposed. The conditional maximum likelihood algorithm is based on iterative least squares with projection whereas the unconditional maximum likelihood approach is developed by means of finite state Markov process modelling. The performance analysis issues of the proposed methods are studied. Finally, some simulation results are presented. Keywords and phrases: blind channel estimation, conditional and unconditional maximum likelihood.
1.
INTRODUCTION
The rapid growth in demand for a wide range of wireless services is a major driving force to provide high-data rate and high quality wireless access over fading channels [1]. However, wireless transmission is limited by available radio spectrum and impaired by path loss, interference from other users and fading caused by destructive addition of multipath. Therefore, several physical layer related techniques have to be developed for future wireless systems to use the frequency resources as efficiently as possible. One approach that shows real promise for substantial capacity enhancement is the use of diversity techniques [2]. Diversity techniques basically reduce the impact of fading due to multipath transmission and improve interference tolerance which in turn can be traded for increase capacity of the system. In recent years, the use of antenna array at the base station for transmit diversity has become increasingly popular, since it is difficult to deploy more than one or two antennas at the portable unit. Transmit diversity techniques make several replicas of the signal
available to the receiver with the hope that at least some of them are not severally attenuated. Moreover, the methods of transmitter diversity combined with channel coding have been employed at the transmitter, which is referred to as space-time coding,
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