OFDM Baseband Receiver Design for Wireless Communications

5

Synchronization

Synchronization is a major task in wireless communication receivers. Normally, the phase and frequency of the carrier signal as well as those of the digital clock signal must be retrieved and maintained at all times to ensure reliable receiver operation.

5.1 Introduction

The synchronization issue is inevitable in all signal transmission systems. In digital transmission, though the bit streams are inherently discrete-time signals, all physical media, such as radio channels or transmission lines, are continuous-time in nature. In wireless communications, most physical transmission media are inefficient in transmitting baseband signals. Consequently, the digital baseband transmitted signal has to be converted to a continuous-time waveform and then modulated by a higher-frequency carrier signal. After the modulated signal passes through the physical media, several inverse processing procedures, including sampling/digitization and demodulation, are applied. Figure 5.1 illustrates a simplified diagram of a wireless communication system.

In wireless communication receivers, coherent demodulation needs to make use of a local oscillation (LO) signal that has exactly the same carrier frequency and phase as the carrier signal contained in the received signal. Furthermore, accurate sampling clock frequency and phase allow the demodulator to recover the transmitted digital data more effectively. Unfortunately, the receiver is unsynchronized with the transmitter most of the ...