A Digital AM Broadcasting System Compatible With The Analog AM Broadcasting System

In this thesis a digital AM broadcasting system compatible with the existing analog system is proposed based on the DRM standard, and according to the practical instance of our country. Transmitters and receivers of the existing broadcast system need not to be changed and can still be used in this scheme, so it is easy and simple to implement.In our country the input audio of existing AM transmitters is still in analog version, while in the DRM standard, its amplitude and phase of the carrier are controlled by the baseband signal, so we must change the existing equipments. However, our scheme described in this article does not need to rebuild the existing devices. In transmitting end, COFDM baseband signal is sent directly to its audio interface of the existing transmitter, and in receiving end, digital COFDM baseband signal is derived from the audio interface of common radio to the digital processing unit. Then analog signal can be gained and sent to another audio interface as output.The bandwidth of medium wave and shortwave is 9kHz or 10kHz. Amplitude and phase signal of DRM system are sent simultaneously. So the actual bandwidth of the signal is 9kHz/10kHz. We only send real part of the signal to the audio interface, so the actual bandwith is just only 4.5kHz/5kHz. How to guarante the transmission rate and quality? This paper answers this problem. Some key problems are researched emphatically, such as the amelioration of the MSC signal constellationgs and mapping ,the application of 64-DAPSK+OFDM ,the gaining method of the real Amplitude signal the wrap demodulation of the Amplitude signal , the demodulation of the OFDM baseband signal and so on. All these problems are tested in Matlab simulation platform.In this paper we introduce the background of digital AM broadcasting at first, and then indicate the system's characteristics and its wide market foreground .Secondly we analyse the OFDM system and DRM standard. According to these we discuss some key problems of our country's digital AM system, and test their performance and give the relevant analysis.