A Study Of Turbo Codes Based On OFDM-CDMA System

For the instability of the wireless channel and remotion of the terminal, the performance of the wireless channel is not good as the one of the wire channel, which is not fit for transmitting high-speed date. This requestes better correct channel code and effective modulation. In this paper, the two technic of Orthogonal Frequency Division Multiplexing (OFDM) and Code Division Multiplex Access (CDMA) are combined to a system of OFDM-CDMA. And Turbo code is adopted to code and decode, because the performance of the Turbo decoding nearly achieves the limitation of Shannon code theory. So far, any scheme of the code can not be compared to Turbo code, so the appearance of Turbo code arouses extensive attention and becomes a break through in code theory field. So CDMA, OFDM and Turbo code are combined to COFDM-CDMA. One side, this can satisfy multiuser share the same frequency resource, on the other side, it can overcome ISI and improve the bit error performance by reducing coding rate of subchannel, thereby improve the performance of BER. So the emergence of COFDM-CDMA will be a hotspot in communication area after CDMA.The background, the signficance and the present research status of Turbo code based on OFDM-CDMA system are introduced firstly in the chapter 1-introduction part of the thesis.In Chapter 2, the key technic of Turbo code based on OFDM-CDMA system is described. First, the configuration, basic theory and interleaver of the Turbo coder and decoder are introduced. Then the technic of extending frequency, Walsh orthogonal code, the technic of Rake are analyzed. Moreover we argue the realizing process of OFDM on the model of accepting and sending by using FFT/IFFT transform. And the advantage and disadvantage of OFDM is given. To say strictly, it is not suitable to call OFDM as a system for it is just implemented technology, so-called OFDM system is noting more but a short name of the communication system that carry on OFDM as its core technology. Then the composing of the OFDM system and correlated technic are introduced. This can be ready for later Matlab-Simlink of system. By the foundation of profoundly analyzing the key technic of OFDM and CDMA, a model of Turbo code based on OFDM-CDMA is advanced in chapter 3. This model is consisted of three function modules: channel code, extending code and multicarrier modulation. By great computer simulations, we search a suitable group of simulation parameter to acquire better decoding performance.In chapter 4 under channel of AGWN and multipath, the decoding performance of this model is simulated by Matlab-Simlink. The simulation curve shows that this model has the better performance. Then we change the value of three parameters: interleaver length, alternater times and coding rate to see the infection on BER (Bit Error Rate). We make some conclusion as follows:(1)The performance of Turbo code through OFDM-CDMA system has improved. Under perfect channel model, the results of the simulation show that when decoded with Log-Map algorithm, the BER performance of Turbo code under this model is improved about 0.3dB than which without adopting this model(2)The BER will be reduced along with the increase of alternater time. But with enhancement of SNR (Signal Noise Ratio), the BER of different alternater time will be close and incline to a point. This shows that code progain from enhancing alternater time will decline and go to smoothness under high SNR.(3)The BER will reduce along with the enhancement of interleaver length. When SNR is very low, the performance of BER is not obviously improved by enhancing interleaver length. (4)Turbo code without puncture has obvious effect on improving correct capability. Under the same SNR, this can be reduced and acquire 1dB code progain. But 1/3 rate will reduce the transmission efficiency, and be not fit for transmitting high-speed data. Although 1/2 rate is much lower, it can provide higher transmitting rate.Except above factor, decoding algorithm is one of fators influencing the performance. At present, popular algorithm mostly has two genuses: one is Log-map algorithm, which has better performance but higher complexity; another is SOVA, which has the advandge of calculate simply but poor performance.In chapter 5 decoding algorithm is discussed. And the amelioration algorithm is given. Mathematical analysis and simulation results are provided to show the validity of the amelioration algorithm. It is improved 0.2dB.In Chapter 6 study summing-up is provided, some research emphases and direction that the thesis still needs in the future are pointed out.In conclusion, this paper researches on the Turbo code based on the OFDM-CDMA system. This model is designed and realized with Matlab-Simlink and acquire good decoding performance. Then the problems in decoding algorithm are improved and get a good effect. For the sake of the time, this system is not perfect, and the author will improve it during afterward works.