Advances in MIMO Techniques for Mobile Communications—A Survey <s> MIMO over High-Speed Mobile Channels <s> In this letter, a new orthogonal frequency-division multiplexing (OFDM) technique for multiple-input multiple output (MIMO) channels is proposed to reduce interchannel interference (ICI) caused by high-speed mobiles in cellular environments. After analyzing the ICI caused by high-speed mobile channels using a simple curve fitting technique, the weighting factor for group transmission is optimized. Then, a new MIMO-OFDM technique, based on the weighting factor optimization, is proposed for reducing ICI caused by time-varying channels. Performances of the proposed technique are verified by using the I-METRA channel, proposed for an MIMO channel to 3GPP, and a MIMO-OFDM simulator designed for macrocellular mobile communication. It is shown by computer simulation that the proposed MIMO-OFDM technique is effective in reducing ICI and noise as well as in obtaining diversity gain even under highly-correlated fast fading channels, compared with the conventional MIMO-OFDM schemes. <s> BIB001 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> MIMO over High-Speed Mobile Channels <s> We investigate space-frequency block coding for OFDM systems with multiple transmit antennas, where coding is applied in the frequency domain (OFDM carriers) rather than in the time domain (OFDM symbols). In particular we consider Alamouti's code, which was shown to be the optimum block code for two transmit antennas and time domain coding. We show that the standard decoding algorithm results in significant performance degradation depending on the frequency-selective nature of the transmission channels, such that a low coherence bandwidth results in a huge degradation. The optimum decoding algorithm that alleviates this problem is the maximum-likelihood decoder for joint symbol detection. We present a performance analysis for the investigated space-frequency decoders in terms of the achievable BER results. Furthermore we compare space-time and space-frequency coding and discuss the respective advantages and drawbacks of the different decoding algorithms in terms of their complexity. It should be noted that for the space-time approach we introduce the so-called matched-filter receiver, which shows significantly lower complexity compared to the maximum-likelihood decoder known from literature. The HIPERMAN system serves as an example OFDM system for quantitative comparisons. <s> BIB002
Open-loop MIMO diversity techniques like STC and space frequency coding (SFC) are appropriate choices for high-speed mobile channels that vary rapidly with time. In such scenarios, maintaining a reliable link becomes the foremost priority rather than maximizing system throughput. High-speed mobile channels undergo fast fading which may cause time variation of the fading channel within an OFDM symbol period. This results in the loss of subchannel orthogonality and leads to interchannel interference (ICI) due to the distribution of leakage signals over other OFDM subcarriers. The error floor associated with ICI increases with the speed of the mobile terminal BIB001 . An improved MIMO-OFDM technique for high-speed mobile access in cellular environments is proposed in BIB001 . This technique reduces ICI and provides diversity gain as well as noise averaging even for highly correlated channels. ICI is reduced by transmitting weighted data on adjacent subcarriers. The weights are selected such that the mean ICI power is minimized. The adopted weight selection procedure however results in suboptimal weights. Diversity gain in BIB001 is achieved by using space-frequency block coding (SFBC) which is based on Alamouti code but the coding is applied in frequency domain i.e. to OFDM subcarriers rather than to OFDM symbols in time domain BIB002 . Figure 23 shows the data assignment scheme for the 2 u 1 SFBC-OFDM system, without the weighting factors. The transmit data is assigned to subcarrier groups each consisting of two adjacent subcarriers, as shown in the figure. Instead of SFBC, other diversity techniques such as STBC, space-frequency trellis coding (SFTC), maximal- ratio receive combining (MRRC) etc. can also be used. The proposed technique operates without CSI at the transmitter and does not require any pilot signals for channel tracking. However, it is suitable for OFDM systems with subcarrier group spacing less than the channel coherence bandwidth because the channel coefficients are assumed to be identical for adjacent subcarriers. Figure 24 shows the simulated BER performance of the proposed SFBC-OFDM scheme in comparison with conventional SFBC MIMO-OFDM schemes for 2 u 1 antenna configuration using I-METRA MIMO channel model Case A for downlink transmission with mobile speed of 250 km/h. Case A corresponds to a frequencyflat Rayleigh fading channel with uncorrelated antennas. 25 MHz of downlink channel bandwidth is used at 2 GHz with 2048 OFDM subcarriers. Performance results for QPSK and 16-QAM are provided. Figure 25 shows the performance comparison using 