通信原理英文版7.ppt

Chapter 7 Synchronization 7.1 Introduction Problems should be solved by synchronization: Carrier synchronization Bit synchronization Group synchronization Network synchronization The price for solving the synchronization problems: 7.2 Carrier Synchronization method 7.2.1 Pilot Insertion Method Example: 2PSK signal At the transmitter: to insert an orthogonal pilot The highest frequency in the spectrum of m(t) is fm . At the receiver: The pilot component is filtered out by a narrowband filter in the receiver, and is phase shifted ?/2; then it is multiplied with the received signal. Assume the received signal is still expressed by s0(t), then the product is The frequency component 2?0 is removed by the lowpass filter, then the original modulating signal m(t) is restored. If the pilot is not an orthogonal one, then a D.C. component will be added onto the output. Block diagram 7.2.2 Direct extraction method Square method Principle For the signals without carrier component, e.g., the 2PSK signal, assume the recived signal s(t) is： where m(t) is the modulating signal with no D.C. component. After the received signal is squared, we obtain The 2f0 component in the above equation is filtered out by a narrowband filter, and its frequency is divided by 2, then the component with the carrier frequency f0 is obtained. The block diagram is as follows: Problem existed: phase ambiguity. PLL instead of narrowband filter Block diagram Advantage: The output signal has better stability, and the incoming signal may be the extensively used discontinuous signal. Costas loop method Block diagram Principle Assume the received signal is a suppressed carrier double-sideband signal s(t), and the local carrier voltages are where ? is the phase difference between the signal and the local carrier. The incoming signal s(t) is multiplied by the local carrier, and we obtain After lowpass filtering, they are resp