TURBO Receiver Of TURBO CODED OFDM System

In B3G systems, the bandwidth will be 20 MHz or even more than 100MHz, and the modulation order will also increase. MIMO technology is the main solution of increasing data rate. In order to improve the reliability of the system, error-correcting coding is an indispensable technology. In such a future mobile communication system, how to achieve the channel capacity of MIMO becomes a very important question. The present research show that Turbo Receiving is an effective solution to this question. In addition, there will be frequency selective fading in the future broadband MIMO wireless communication systems, and the traditional high-performance equalizer will become extremely complicated. As an effective multi-carrier modulation technology, OFDM can overcome frequency selective fading and reduce ISI. Therefore, applying it to broadband MIMO wireless communication systems is an effective method to improve bandwidth efficiency and reduce the complexity of receiver. Based on these reasons, Turbo Receiver of a Space-Time Block-Coded TCOFDM System has been researched. In this paper, the algorithms of detecting and Turbo decoding are discussed respectively for BPSK and QAM modulated TCOFDM systems at first. In QAM modulated TCOFDM systems, the output of detecting part is soft information, therefore the Turbo decoding algorithm need to be modified. By modifying the MAP algorithm, a SISO MAP Turbo decoding algorithm has been suggested in this paper. Then, the theory of decoding turbo code is applied to the detection of signals by jointing the equalization and decoding. This constructs a information feedback system and realizes Turbo Equalization. The iterative equalization algorithm of MAP and linear MMSE are discussed in detail. The lower-complexity MMSE equalization algorithm has eventually been chosen as the simulation algorithm because that the complexity of MAP will increase exponentially with the modulation order and the memory length of ISI channel. The characteristics of OFDM's cyclic prefix reduce the complexity of MMSE equalizer further.AS a MIMO method, Space-Time Block-Code is applied to the TCOFDM system. Space-Time Block-Code will increase the space diversity gains of the system further, and Turbo Equalization exploits the space-multipath diversity gains, the joint of these two method will improve performance remarkably.According to the theory of iteration, the channel estimator, equalizer and decoder are jointed at last to construct a complete information feedback system, and this will realize the Turbo Receiving of the system. Channel estimators are usually divided into two kind: decision-feedback estimator and pilot-aided estimator. Pilot-aided estimator is chosen in this paper because of the requirements of the complexity of receiver and system performance. The channel estimating algorithms are usually two kind: LS and MMSE. Because the LS algorithm is independent of channel statistical characteristics and has low complexity, LS channel estimating algorithm and DFT interpolating algorithm are chosen. The channel estimator refines the estimation through using the feedback from the decoder, and this realizes the iterative channel estimation and constructs the Turbo Receiver of TCOFDM systems.The Turbo Receiver of TCOFDM systems is realized in this paper by applying the theory of iteration and MIMO, and this not only increases the space diversity gains, but also exploits the space-multipath diversity gains of the system, therefore the performance of the system is improved greatly.