Carrier Recovery Algorithm at Low SNR for QPSK Modulation Scheme

Abstract

In wireless communication system, when the transmitted signal travels through the medium, it gets distorted by various impairments that may be caused either by the channel (medium) or due to imperfection of communication elements. This introduces timing delay (Symbol delay), frequency and phase offset to the received signal. And whenever the environment is mobile, this introduces Doppler frequency offset to the transmitted signal. For optimum detection of information signal, it is required that modulator and demodulator should be perfectly synchronized. That means, both modulator and demodulator must know the exact symbol rate and the exact symbol phase, and both also must know the exact carrier frequency and phase. The synchronization algorithm should be the most robust and reliable element in the receiver design process, ensuring that data detection can continue over deteriorating channels, such as those with large carrier offsets etc. So at the receiver, for estimation of carrier frequency and phase, carrier recovery synchronization structure is required and for measurement of symbol timing, symbol timing synchronization is required. In this thesis, the simulation of carrier recovery algorithm for mobile environment under low SNR is done. At low SNR, the PLL and Costas loop shows poor performance. So this thesis presents improved performance by using adaptive loop filter bandwidth.