Digital Am Radio System Synchronization Technology Research

The AM broadcast uses MW and SW radio channels which are bandwidth-limited and time-varying multipath channels. In order to make full use of frequency resource and enhance the immunity to time-varying multipath interference and fading, DRM standard adopts OFDM/QAM modulation technique. Therefore, the receiver must be precisely synchronized accordingly to realize correlation demodulation. On the basis of DRM standard, this paper mainly investigates OFDM synchronization methods, analyses the impact of the synchronization error on the system performance, and comes up with a synchronization algorithm in light of DRM frame and propagation characteristics.Firstly, this paper describes the essential features of DRM, gives an introduction to its key techniques, testifies that the AM broadcast channel is weak extended channel, and brings forward its own channel model. Then, it introduces the basic principle of OFDM and its base band model, andanalyses the OFDM's propagation characteristics under slow time-varying multipath channel. Afterwards, it introduces several typical synchronization methods of OFDM system, gives a comparison of their strong points and shortcomings, analyses the impact of the synchronization error on the system performance, points out that we should select an appropriate synchronization method according to different channel characteristics and signal formats. Finally, this paper brings forward a time and frequency synchronization algorithm which is suitable to DRM system. Then, according to the characteristics of wireless broadcast channels, this paper selects three channel models of DRM standard to simulate the performance of this synchronization algorithm with Mento Carlo simulation method, including frequency acquisition performance, time acquisition performance, frequency offset tracking performance and the bit error ratio under three channels suggested in DRM standard. We draw the conclusion by analyzing the simulation results: When the channel model is typical MW (channel 2) and typical SW channel (channel 3), the synchronization algorithm can get a good bit error ratio under the robust mode B; when the channel model is bad SW (channel 4), the robust mode B is no longer suitable. And if we want to get a satisfying receiving performance, the robust model C or D should be used.