Wireless Channel Digital Transmission System Design And Analysis

In wireless communication, increasing bandwidth efficiency to transfer more data in the band-limited channel always is a task deserve for attention. We will discuss the construction of a digital transmission system with high bandwidth efficiency and good error rate performance. in the condition of 25kHz wireless channel. One way to construct this system base on single-carrier modulation technique. To increase the bandwidth efficiency, such a system scheme use the MSK modulation method combining with a premodulation filter. Several important factors like the bandwidth compression efficiency, ISI caused by filter and the affection on system performance that arise from filtering should be considered. According to the simulation result, the bandwidth efficiency brought by different pemodulation filters and the ISI cause by them are analyzed and compared. Then the system error rate performance is also analyzed in the article. As we have introduced a multilevel decision method with discriminator detection to improve the error rate performance, the deducing and analyzing for this method are present in this article. The theoretical error rate and the Monte Carlo simulation result are showed in figure to compare the system error rate performance affected by different premodulation filter. Considering the bandwidth efficiency, the error rate performance and the complexity for realization together, we construct a digital transmission system with single-carrier modulation technique using the optimum premodulation filter. Then the system parameters and performance indexes are prensent.Another scheme to construct such a transmission system in 25kHz channel is the using of OFDM technique. With QPSK modulation method employed in our OFDM system, there are several important parameters relating to bandwidth efficiency should be consider carefully. They are the number of subcarrier, the length of cycle prefix and the parameters of pulse shaping filer. The analyse of the simulation result show that system performance (including bandwidth efficiency and errorrate performance) vary with the different combination of these parameters. Choosing an optimum combination of these parameters accord to the comparison of simulation result, we can construct an optimum OFDM system which employ a peak-amplitude-suppression method in addition. This peak-amplitude-suppression method won't cause power spectrum density expanding. The threshold level for suppression is determined by simulation result. The optimum parameters for this system and the performance indexes are presented too.A comparison between the single-carrier system and the OFDM system is also made in this article. A summarization is present at the last chapter.