Research On MIMO-OFDM Technique

With the increasing of mobile users and the demands which are transforming from the pure voice service to multimedia service, the limited frequency spectrum resources are running short. In the future the transmission rate in wireless multimedia communications will reach 100Mbit/s. So promotion the utilization ratio of frequency spectrum and increasing the channel capacity is becoming a work which offers many challenges. So will it do in the future. It spurs us to develop highly efficient coding, modulation and signal processing techniques to improve the wireless frequency spectrum efficiency. An effective approach to increase the data rate over wireless channels is to use multiple transmit and receive antennae. A MIMO system provides multiple independent transmission channels, thus, under certain conditions, leading to a channel capacity that increases linearly with the number of antenna elements. Space-time coding technique is a coding technique which is applied in MIMO system. It correlates to multiple antennae and signals transmitted in different periods in temporal and spatial domains, which enable the receiver can overcome fading over MIMO channel and reduce error code transmitted. Comparing with the system which does not space-time code, space-time coding can obtain transmit diversity and increase channel capacity without frequency bandwidth loss. It is shown in research that space-time coding is a feasible and effective approach that reaches or be close to channel capacity in MIMO system. Space-time codes are mainly designed for flat fading channels; however, most of channels in nature are frequency-selective fading channels. So frequency-selective fading channel must be transformed into flat fading channel. OFDM is just the technique. OFDM is a multiple carrier's modulation technique. Its basic theory is that the whole channel is divided into multiple sub channels, which are orthogonal and overlapped in frequency domain, which mitigates the effects of delay spread effectively over frequency-selective channels. So a system which combines space-time coding technique and OFDM to develop their potential fully is becoming a hot issue in communications. Firstly, the basic principle of space-time coding is discussed and its design criteria for quasi-static flat fading channels are given. Furthermore, improved design criteria are presented. Then the encoding and decoding theories, features and performances of STBC and STTC are detailed in all aspects. Especially, the capacity of orthogonal STBC is researched for rayleigh fading channel. A simplified algorithm that looks for MRC combinatorial in receiving terminal by formula calculation is proposed in decoding algorithms of STBC. In chapter 4, the combination space-time coding and OFDM techniques is detailed. A realizable scheme is proposed which effectively combines STBC and OFDM over frequency-selective fading channel. The system can obtain the maximum diversity gains which are the product of the number of transmit antennae, receive antennae and multi-paths. In the end, the further research and its development in future wireless communications are prospected.