Improved forward link error correction for CDMA systems with space time transmit diversity

Abstmct-The channel code decoder in a CDMA mobile not accounted for the non-stationary in-cell interference performs channel state estimation to determine the reli-that occuTs on a CDMA fonvard link using concatenated WalshjPN sequence spreading. CDMA forward link chan-ability of each received encoded symbol. This reliability information is typically an estimate of received signal envelope and interference pius noise miance. Most papers t h t ne1 code performance studies often assume a stationary in-investigate CDMA forward link c h-d code Perform-terference and noise process [3], [4]. Channel code studies assume that all interference and noise can be modeled as a that do consider non-stationary CDMA interference have single, stationary Gaussian process. This assumption leads to inaccurate state since i n-e e ~ ~ interfer-only considered the self-interference experienced by a sin-ence is non-stationary on the CDMA forward link. This ele multi-code CDMA user or the interference on a flat ferenee. Simulations are used to illustrate how this scheme improves convolutional and turbo code performance. consider the non-stationary interference experienced on a frequency selective CDMA downlink where concatenated WalshjPN sequence spreading is used. Channel state estimation accounting for the interference in an STTD system I. INTRODUCTION In order to decode a turbo or convolutional channel encoded sequence received over the wireless channel, the channel code decoder must estimate the received amplitude of each encoded symbol and the variance of the interference plus noise corrupting each symbol 111, [2]. This is referred to as channel state estimation. This paper presents a channel state estimation procedure that improves turbo and convolutional code performance on the CDMA forward link by accounting for the non-stationary nature of in-cell interference. This technique is applied to systems using single antenna transmission and Space Time Transmit Diversity (STTD) on a frequency selective channel. It is frequently assumed that all noise and interference on the CDMA downlink can be modeled as a single, stationary Gaussian process at the Rake receiver input [3], [4], [5]. This is a reasonable model for out-of-cell interference and thermal noise. However, in-cell interference travels through the same multipath channel as the desired signal and is partially canceled in the mobile Rake receiver due to the forward link Walsh/PN sequence concatenated spreading scheme. The result is a non-stationary Gaussian in-cell interference process at the mobile Rake receiver output with a variance that fluctuates with changes in the multipath channel. The CDMA forward link model presented in …