The Theory Of OFDM And Its Application Research In Shortwave Communication

Shortwave communication is widely used in wireless communication area as for its advantages of long distance transmission, easily spanned and convenient to move. In modern shortwave communication systems, the channels are mostly frequency-selective, multipath transmission causes the coherent attenuation and symbol interference. Because the bandwidth of shortwave channel is very limited, it restricts the communication capability and data transmission rate to a certain extent.OFDM (Orthogonal Frequency Division Multiplexing) is the most frequency-efficient transmission system known so far;meanwhile, it has the advantages of fast transmission rate and the strong ability of anti-multipath. It combines the digital modulation, digital signal processing and multi-carrier transmission technique together, which makes it much more competitive in frequency usability, power usability and system complexity. It is one of the most potential high speed communication techniques of the new generation. Just for these reasons, the paper advances to apply OFDM to shortwave communication system.The paper analyzes the features of shortwave channel, and the method of Matlab simulation is adopted, simulates and discusses the Rayleigh fading, time delay spread and inter-symbol interference caused by multipath effect. According to shortwave channel model, the paper advanced a suit of comparatively intact shortwave OFDM system design scheme. Comparatively comprehensively simulate and analyze the technique of over-sampling, modulation, channel coding, symbol interleave and so on. The performance of shortwave communication mainly rests on the design of channel compensation. The paper discusses the system performance improved by channel estimation as a main point. Analyze and compare the performance of LS, MMSE, LMMSE and their modified algorithm. Generally speaking, MMSE based channel estimation technique and its modified technique have a preferably estimation effect. Simulation shows that the modified LS algorithm could indeed suppress noise and consequently increase the signal to noise ratio, especially under low signal to noise ratio, the modified LS algorithm has a better performance compared to the original LSalgorithm. By text and sound file transmission instances, compare and discuss the performance difference between QAM modulation and OFDM modulation, validate that OFDM- a multi-carrier transmission technique has the feature that is much fitter for transmitting in multipath channel than QAM- a single carrier transmission technique. The paper also introduces the improved technique of OFDM- Frequency Domain Reciprocal Modulation (FDRM) for the first time. It has the crude feature of anti-multipath interference. The paper simulates and compares the performance of FDRM with OFDM, and found that FDRM has a better performance than OFDM. As for rapid flat fading channel, FDRM could work well.