A constrained mutual information based CQI measure for coded MIMO-CDMA systems

In this paper, we propose a novel channel quality indicator (CQI) measure for the space-time jointly encoded MIMO-CDMA systems in frequency selective channels. The CQI proposed here is derived from a so-called per-Walsh code joint detection structure consisting of a front-end linear filter followed by joint symbol detection across all streams. The linear filter is designed to convert the multipath channel into a single-path channel to restore orthogonality of Walsh codes and to avoid joint sequence detection. We derive a class of filter that maximizes the so-called constrained mutual information, and show that the conventional LMMSE and MVDR equalizers belong to this class. Similar to the notion of generalized SNR (GSNR), this constrained mutual information provides us with a CQI measure describing the MIMO link quality. Such a CQI measure is essential in providing a simple one-dimensional mapping for both link adaptation and link-to-system mapping for jointly encoded MIMO-CDMA system.