A Thesis for the Degree of Master LS Codes Aided Channel Estimation for MIMO-OFDM Systems in Multipath Channels

High-data rate techniques in communication systems have gained considerable attraction in recent years. Strong candidates for these communication systems are Multiple Input Multiple Output (MIMO) systems, Orthogonal Frequency Division Multiplexing (OFDM) system and MIMO-OFDM systems. MIMO systems have shown their potential by improving the diversity gain with scheme as Space-Time Block Codes (STBC) and increasing the capacity gain significantly with scheme as Bell Lab Layered Space-Time Architecture (BLAST). However, it is difficult to get the both sides at the same time since transmit diversity comes at the cost of throughput and vice versa. Recently, Double Space Time Transmit Diversity (DSTTD) was proposed, which is a combination of STBC and BLAST to increase the diversity gain and data rates simultaneously. OFDM system is also one of the most promising techniques for the high data rate wireless communications in complex multipath fading environment. OFDM system is a special form of multi-carrier transmission where all the subcarriers are orthogonal each other. In OFDM system, the high data rate input stream is divided into many lowrate streams and transmitted in parallel. i MIMO-OFDM systems have received considerable attentions in order to combat the multipath fading and increase system capacity. To take the advantages of the MIMO-OFDM system, the reliable channel estimation techniques are needed. For the single-antenna OFDM system, various channel estimation techniques such as least square (LS) and minimum mean square error (MMSE) estimators have been studied. However, these techniques are inadequate for channel estimation of MIMO-OFDM systems, since other transmission antennas act as interference. Smart codes have been studied previously by many researchers. LS codes, one of the smart codes based on a Golay complementary pairs (GCP), have the zero correlation zone (ZCZ) or Interference Free Window (IFW). Utilizing the ZCZ or IFW, it is proposed that the multi-path and multi antennas interference can be tremendously reduced. In this paper, we propose two channel estimation techniques for MIMO-OFDM system by using LS codes as training sequences. The first one is the channel estimation method using two uncorrelated LS codes in IFW. We simultaneously transmit two uncorrelated LS codes through the two transmission antennas in 2 1 × system. At the receiver, we estimate the channel with space time transmit diversity (STTD) blocks. The second proposed one is a channel estimation method using four minimally correlated LS codes in IFW. We simultaneously transmit four LS codes through the four transmission antennas in system. At the receiver, we can estimate the channel with conventional DSTTD blocks. Because of the IFW between LS codes, MIMOOFDM systems can show much smaller interference and consequently have better performance than traditional channel estimation schemes. 4 1 ×