The Application Research Of OFDM In The Short-wave Data MODEM

Short-wave communication is one of communication means found earliest by human. It's also an effective method for lowest-cost and long-distance radio communication with small transmitting power and low equipment cost. In particular, it has non-destroyed relay system ----- ionosphere. Now, it has been widely used in government, military, weather, business and so on for the transmission of voice, text, images, data and other information.At present, single-carrier transmission rate can be up to 9.6kbps in the short-wave communications.Due to the time-varying channel and multi-path transmission, it's very hard to further improve the transmission rate. Orthogonal frequency division multiplexing technology not only has high spectrum efficiency, but also has ability to turn frequency-selective fading channel into flat- fading one, that is with strong anti-ISI, and can modulate sub-carrier and distribute the power flexibly according to the channel condition. So it has been widely used in many areas.In this paper, we first study the characteristics of short-wave channel, then bring in OFDM technology to improve the rate of short-wave communication. This article firstly introduced a variety of technologies involved in the system, then focused on solution about how to reduce PAPR, how to achieve synchronization and channel estimation and so on.First, we introduce two new simple methods to lower PAPR: one named peak-deduct, the other named improved SLM. The peak-deduct method picks up the peak that over threshold as peak noise. The correction function obtained through making the peak noise FFT/IFFT transform, is used to amend the original signal. Compared with amplitude-clipping, this method saves computation and has no out-band interference. The improved SLM method contains two kinds: inserting virtual sequence and recycling energy expansion.The former insert different ancillary code on the not-effective subcarriers, the one with lowest PAPR is selected for transmission. At the receivie end, the auxiliary information is discarded directly to restore the original data. The latter expands the symbol energy regularly on the original ones. Because the regularity hides the side information,so it saves bandwidth and avoids to receive error side information that resulting in the received signal be demodulated inaccurately.Secondly, we in detail analyze the source of synchronization error and the synchronization methods of OFDM system. After that, respectively, introduce the realization method of symbol timing and frequency offset estimation algorithm, which use cyclic prefix and the PN sequence.Thirdly, we study the channel estimation and its realization of OFDM system, including the design of pilot patterns, interpolation, and DFT channel estimation and so on.Finally, we design the parameters following research task, and give the system block diagram and the main program. The all algorithm simulated in this article are based on these parameters.