Receiver Baseband Digital Communication System Design And Implementation

This paper focuses on design and implementation of a baseband receiver in a digital communication system. The modulation mode of the signal in this system is DBPSK, which is widely used in low-orbit satellite communications.The baseband receiver in the system needs to demodulate the DBPSK signal.In satellite communications, Doppler frequency offset may has the same order of magnitude as the data rate of the baseband signal, so the Doppler frequency offset may become a primary factor in the channel conditions.In order to ensure the reliability and stability of the communication link in the data transmission, we have to estimate the Doppler frequency offset, and amend it. Because of the channel propagation delay and clock skew between transceivers, the sampling may not happened at the best sampling time, which may result in large deviation between sampled data and actual data. In order to avoid or reduce inter-symbol interference, the clock in the receiver must be synchronized with the clock in the transmitter. The receiver adopts Gardner algorithm for symbol timing recovery. So we can get the values of the signal at the optimal sampling points, then that will reduce the error rate.In order to ensure a reliable transmission of digital signals, reduce the additive channel interference and the multiplicative interference, error correction coding techniques are usually used in the actual communication system. Error correction coding increases a certain amount of redundant information, which can improve the reliability of information transmission. In this system, the transmitter uses RS encoding, convolutional encoding and block interleaving cascade of error correction coding. Therefore, the baseband receiver has to finish the de-interleaving, convolutional decoding, and RS decoding, correspondingly. Meanwhile, the power control is another important aspect in the satellite communication system, in order to achieve power control, so in the baseband receiver, we have to estimate SNR of the received signal.This paper discusses several algorithms of SNR estimation, and after that we select a simple and effective SNR estimation algorithm in the implementation.