Adaptive MIMO-OFDM using OSTBC with imperfect CQI feedback

In this paper, closed form expressions are derived for the average data rate and uncoded bit error rate of a multi-user multiple input multiple output (MIMO) orthogonal frequency division multiple access (OFDMA) system, where two transmission modes are considered, both transmitting the data of the scheduled user using orthogonal space-time block coding (OSTBC) at the transmitter and maximum ratio combining (MRC) at the receiver to exploit spatial diversity. Applying the adaptive transmission mode, an adaptive subcarrier allocation and modulation is performed based on channel quality information (CQI) fed back digitised over a feedback channel to the transmitter. As CQI the resulting instantaneous signal-to-noise ratio (SNR) at the output of the maximum ratio combiner of the different subcarriers is applied to exploit multi-user diversity. The CQI available at the transmitter side is assumed to be imperfect due to time delay, estimation and feedback errors. Applying the non-adaptive transmission mode, no instantaneous CQI is required. A transmission scheme exploiting frequency diversity in addition to the spatial diversity using OSTBC and MRC is employed. The data rates are compared in order to identify the optimal transmission mode depending on the grade of CQI imperfectness, where a given target bit error rate has to be fullfilled.