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2 4 6 8 10 12 10 −5 10 −4 10 −3 10 −2 10 −1 10 0 SNR per antenna [dB] BER rec.#1, q = 0 rec.#1, q = 4 rec.#1, q = 8 rec.#2 (b) Figure 3.19: (a) Complementary CDF of channel delay spread (τ rms ) for different po- sitions of the measurement route. (b) BER performance versus average E b /N 0 for the measured channel at position Rx19, 16-QAM transmis- sion. rate-1/2 convolutional code. The OFDM symbol duration was set to T S = 4µs and the length of the cyclic prefix to T c = 0.8µs. Figure 3.19 (a) shows the complementary cumulative probability density function (CDF) of the root-mean-squared channel delay spread for different positions of the measurement route. As observed from Fig. 3.19 (a), the delay spreads at position Rx19 are significantly large and exceed in some cases the length of the cyclic prefix of the MIMO-OFDM system, compared to other locations. The BER performance of the proposed turbo receiver at location Rx19, referred to as ’rec. #1’ for different values of the FD window length q is shown in Fig. 3.19 (b). As a reference the BER performance of the conventional MIMO-OFDM receiver (q = 0) and the non-banded SC-MMSE turbo receiver, referred to as ’rec. #2’, are also shown. It can be seen that with q = 8, the performance of the banded SC- MMSE turbo equalizer is very close to the non-banded turbo equalizer. Thus, the proposed technique works with properly chosen q values even in environments with relatively large channel delay spreads. Hybrid MMSE Turbo Equalization The SC-MMSE turbo detector in [3], [6] achieves excellent performance in frequency- selective spatially-uncorrelated Rayleigh multiple-access fading channels. However, the performance of the detector drops significantly in scenarios where the users’ channels are spatially correlated [9]. In [9], a hybrid turbo detection approach based on group-wise SC-MMSE filtering combined with MAP signal detection for OFDM multi-user systems has been proposed. The technique in [9], referred to as Hy SC- MMSE-MAP turbo detection, offers a design flexibility in terms of complexity in computation and robustness against spatial correlation of the users’ channels. The Hy SC-MMSE-MAP detector separates the U users into H disjoint groups {G 1 , G 2 , ..., G H } , such that each group contains G h integers corresponding to user |