04-02-2016, 03:46 PM
Abstract
A well–known receiver strategy for a linearly modulated signal transmitted over a frequency–selective channel is channel equalization. Recently it was proposed to employ a minimum mean–squared error (MMSE) channel equalizer for the downlink of CDMA. In this paper, we introduce a new receiver concept using MMSE channel equalization as a first stage and MMSE decision–feedback equalization (DFE) utilizing soft feedback from the decoding unit as a second stage. Both schemes are compared for the downlink of CDMA. It turns out, that after channel decoding we gain about 1 dB compared to conventional MMSE channel equalization.
INTRODUCTION
The air interface of the third-generation mobile radio system UMTS (Universal Mobile Telecommunications System) is based on wideband code–division multiple access (WCDMA). UMTS uses orthogonal spreading sequences which are time– varying due to the embedded scrambling code. These time–varying spreading sequences cause severe problems when applying multiuser detection, e.g. symbol–level minimum mean–squared error (MMSE) multiuser equalization according to [1], because the receiver filter has to be recalculated at each symbol interval. To overcome this requirement, it was proposed in [2], [3] to apply linear MMSE channel equalization [4] with finite impulse response (FIR) filters at chip level on the CDMA downlink followed by a simple correlation with the spreading sequence of the desired user. Obviously complexity can be significantly reduced by doing so, because the receiver filter performing linear channel equalization
only has to be recalculated when the channel has changed noticeably.