Research On The Key Technologies Of Modulation And Coding Of The Next Generation Wireless Communication Systems

Wireless communications have been unprecedentedly prosperous motivated by mobile communication in the recent 20 years. With the commercial deployment of the 3rd generation mobile communication systems and the development of the 4th generation mobile communication technology, wireless communications will continue to be the most prosperous area in the communication field. The rapid development of wireless communications in wireless access of Internet and multimedia application fields has raised very high requirements to the quality and speed of wireless transmission. Thus how to provide high-speed and high-quality data transmission using efficient signal processing and channel coding technique remains to be the hot spot in the field of modern communication technology.After extensive study and survey on the key technologies of modulation and coding of the next generation communications, this dissertation discusses in detail the key technologies of low density parity check (LDPC) codes, low density lattice codes (LDLC) and Orthogonal Frequency Division Multiplexing (OFDM) system. The main contributions of this dissertation are as follows.(1) The concept of recoverable variable set and recoverable variable are proposed in the decoding of LDPC codes under binary erasure channel (BEC). When the peeling decoder is stopped, the decoder tries to get the recoverable variable and makes the process continue. To get the recoverable variable efficiently, the concepts of loop recoverable set and loop recoverable variable are proposed and the loop recoverable variable can be located rapidly and efficiently using the loop location technique, which is used in the design of LDPC codes. Simulation results show that the performance of peeling decoder is improved.(2) In the decoding process of LDLC, the most complex step is the processing of messages at the check nodes. After deeply analysis on the principle of iterative decoding used in low density lattice codes (LDLC), an efficient implementation scheme is proposed for the process of convolution, stretching and periodic extension steps at the check nodes. The decoding process is implemented by simulation and the result shows the capacity-achieving performance of LDLC and that the time for decoding using the new scheme is shortened effectively.(3) In the original paper that presented LDLC, a method for constructing girth 6 LDLC is proposed. A new size-4 loop may be created when one size-4 loop is removed in the method so it needs execute detection and removal step repeatedly and the time for the construction is long. With candidate sequence’s cyclic shift and randomization, a new method for constructing LDLC is proposed. Simulation results show that the time for constructing the LDLC of girth 6 using this method is short than the scheme proposed in the original paper, and the performance is improved when using the girth-8 LDLC constructed by this method.(4) A method for reducing the peak to average power ratio (PAPR) of OFDM system is proposed based on constellation mapping selection, and a low complexity search algorithm using trellis factor search for the method is proposed. Irregular repeat accumulate (IRA) codes is a capacity-achieving coding scheme, and the combination of IRA and OFDM can improve the performance of communication systems. For the IRA-OFDM system, a method using balance sequence located in variable nodes to lower the PAPR is proposed. The values of balance sequence are determined by feedback decoding which using aperiodic autocorrelation function as the metric. Simulation results show that both of the proposed schemes are not only effective in reducing the PAPR, but also with low complexity and good practical value.