Limitations in Spectral Efficiency of a Rate-Adaptive MIMO System Utilizing Pilot-Aided Channel Prediction Bengt Holter, Geir E. Øien, Kjell J. Hole, and Henrik Holm Norwegian University of Science and Technology Department of Telecommunications O.S.Bragstads plass 2B, N-7491, Trondheim, Norway Abstract— A performance analysis of an adaptive coded mod- ulation (ACM) system operating on a multiple-input multiple- output (MIMO) channel with uncorrelated Rayleigh fading subchannels is presented. Rate adaptation is based on period- ically transmitted channel state information (CSI) back to the transmitter, providing information about the channel signal-to- noise ratio (CSNR) as predicted by the receiver. Transmit diversity is utilized by employing space-time block coding (STBC) at the transmitter. I. I NTRODUCTION Adaptive coded modulation (ACM) is a promising data transmission scheme for simultaneously achieving high spec- tral efficiency and a low bit error rate (BER) on wireless and mobile channels [1]. In [2], a method for assessing performance merits of an ACM system has been employed to evaluate the average spectral efficiency (ASE) of a rate- adaptive coding scheme utilizing any set of multi-dimensional trellis codes originally designed for additive white Gaussian noise (AWGN) channels. The analysis is based on a single- input multiple-output (SIMO) flat-fading channel with statis- tically independent and identically distributed (i.i.d.) Rayleigh fading subchannels. Perfect coherent detection is assumed and maximum ratio combining (MRC) is employed to maximize the overall received channel signal-to-noise ratio (CSNR). Motivated by the fact that many base stations already are equipped with multiple antennas, the results in [2] are extended to encompass a multiple-input multiple-output (MIMO) flat-fading channel. In a MIMO system, the benefit of transmitting from multiple antennas may be utilized to improve either the diversity order or the information rate of the system. These two transmission strategies are commonly denoted as MIMO diversity and spatial multiplexing, respec- tively [3]. In this study, a MIMO diversity system is analysed, since it represents a natural extension of the approach in [2]. Transmit diversity is realized by utilizing space-time block coding (STBC) at the transmitter [4], [5]. The rest of this paper is organized as follows. In Section II, the system model is presented. The MIMO channel model is presented in Section III, and in Section IV, formulas to determine the average BER and ASE of a rate-adaptive MIMO diversity system is presented. Simulation results are presented in Section V and conclusions are given in Section VI. II. SYSTEM MODEL Adaptive coded modulator Pilot symbol insertion Space-time block coding 1 n T . . . Information bits CSI (from receiver) Fig. 1. Transmitter STBC combining scheme and adaptive decoding Decoded informationbits 1 n R . . . Channel predictor CSI Optimal Wiener estimators ....... Buffer Buffer Fig. 2. Receiver The rate-adaptive MIMO diversity system considered is depicted in Figure 1 (transmitter) and Figure 2 (receiver). The number of transmit and receive antennas is denoted by and , respectively. The adaptive coded modula- tor/demodulator contains transmitter-receiver pairs, indexed by . Transmitter has a spectral efficiency of information bits/s/Hz, such that , and is designed to provide BER BER (a desired target BER) on an AWGN channel with CSNR . The codes are based on quadrature amplitude modulation (QAM) signal constellations with different number of symbols , where is some positive integer. Rate adaptation is performed by splitting the CSNR range into fading regions (bins) and letting the transmitter respond according to the overall (total) CSNR as predicted by the receiver. When the predicted CSNR falls within the fading region , the associated channel state information (CSI), i.e. the fading region index , is sent back to the transmitter. The transmitter then adapts its transmission rate according to the predicted quality of the channel by transmitting with a signal constellation realizing a spectral efficiency of . If the