Precoded OFDM Systems for Multiple Antenna Transmissions

The OFDM system is receiving growing attention as an important transceiver for wireless communications. It is well-known that OFDM systems su er from the problems of spectral nulls. In general, a power error correction code is needed for OFDM system to perform well in the presence of spectral nulls. Several other methods have been proposed to solve the spectral problem. The first method is apply an
antipodal paraunitary(APU) matrix to OFDM system. By adding APU precoder, the noise power between all subchannels will be equalized. It was known that a MMSE receiver for APU precoded OFDM system has a much better performance than the OFDM system. The second solution is to use multiple antennas. The probability that all antennas experience a deep fading at the same frequency is very small. If multiple antennas are available at the receiver, one can apply maximum ratio combining(MRC) technique. Its disadvantage is that multiple receive antennas are needed. If multiple antennas are available at the transmitter, one can use space-time coding such as the Alamouti scheme.

In the rst part of this thesis, we will apply linear precoder to MIMO-OFDM system. Both space-time coded and space-frequency coded OFDM systems will be studied. Both zero-forcing(ZF) and minimum mean square error(MMSE)receivers are derived, and we will compare their performances. Moreover, the ISI-e ect if exists can be mitigated by adding the precoder.

In the second part of this thesis, we consider an SISO-OFDM system where an OFDM block symbol contains several smaller repetitive blocks of information bearing symbols. At the receiver, we will receive multiple copies of corrupted versions of the same symbols. Maximum ratio combining(MRC) technique is employed to combine these received samples to obtain a high SNR output. Such a system will be called
MRC-OFDM system. We can also apply APU precoder to the MRC-OFDM system to further improve the performance. Experiments show that under the conditions of equal transmission power and equal transmission rate, MRC-OFDM system has a
much better BER performance than OFDM system.