The Ber Performance Analysis Of LDPC Codes Applying On OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) is a very attractive technique to reduce the multi-path disturbs and to achieve the high-bit-rate data transmission required for the recent mobile communications. The OFDM system divides the wide signal bandwidth into many narrowband subchannels that are transmitted in parallel. Using this technology we can conquer the multi-path fading influence, however, when some subchannels are in deep fading, the information will be wrongly transmitted. To improve the error rate performance of OFDM, forward-error correction coding is essential. Aiming at this goal, I apply LDPC codes which have strong correcting performance on OFDM systems. This system can correct the error bits, improve the BER performance. Through the simulation results, the system can achieve good performance whether in AWGN channel or multi-path Rayleigh channel. Below is the main work in this dissertation:First, the conception, the encoding and decoding methods of LDPC codes are stated. Since LDPC codes are linear block codes, the LDPC encoding is the classical encoding method of linear block codes. However this method is more complex, so an improved method is proposed so as the hot cyclic codes and quasi-cyclic codes. The decoding algorithm is the core part of LDPC codes. It states the iterative decoding thought through tanner graph. The information can be transmited between bit nodes and check nodes, ultimately decoding the bits correctly. This method can not only reduce the complexity of LDPC decoding, but also be near to Shannon Limit infinitely when the length of LDPC codes are long infinitely.Then the principle and core technologies of OFDM systems are introduced. How OFDM conquers the multi-path fading is described and the mathematics model is given. In the core technologies, we can use Guard interval, Cyclic Prefix to eliminate ICI. I also investigate some other important technology: Channel estimation, Synchronous technology, the control of Peak-to-average power ratio.Finally, I investigate the advantages of LDPC codes applying on OFDM systems. This model includes LDPC encoding, an inverse fast Fourier transform (IFFT), insert the CP, then transmite through the AWGN channel or multi-path fading channel. At the receiver, the information should first cut the CP, then FFT. The recived OFDM symbols generated by the FFT are demodulated at the demodulator. The demodulated bits are decoded with each LDPC encoded block and data bits are restored. Using the simulation to realize this system, it can reduce the BER value definitely. Through designing the vary parameters, I analyze how to improve the error correct performance using the related parameters as shown in SNR-BER graphs.