tailieunhanh - CDMA truy cập và chuyển mạch P9

Nonlinear Amplification of Synchronous CDMA Overview As we have described in Chapter 3, the SS/CDMA uses orthogonal CDMA for both uplink and downlink transmission. On board the satellite, each CDMA channel is routed to a destination downlink beam by the Code Division Switch (CDS). All channels in the same output port of the CDS are combined and then amplified with a Traveling Wave Tube (TWT) amplifier for downlink transmission. | CDMA Access and Switching For Terrestrial and Satellite Networks Diakoumis Gerakoulis Evaggelos Geraniotis Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-49184-5 Hardback 0-470-84169-9 Electronic 9 Nonlinear Amplification of Synchronous CDMA Overview As we have described in Chapter 3 the SS CDMA uses orthogonal CDMA for both uplink and downlink transmission. On board the satellite each CDMA channel is routed to a destination downlink beam by the Code Division Switch CDS . All channels in the same output port of the CDS are combined and then amplified with a Traveling WVvw Tbbe TWT amplifier for downlikk transmission. Due to the large number of users in the system the amplitude of the combined signal has a large variance which makes its amplification difficult since it may drive the TWT into saturation. This phenomenon also appears in terrestrial wireless systems if the downlink transmission at the base stations is CDMA. Nonlinear distortions in satellites may also result for other reasons such as drastically physical changes in the environment for example temperature variations and vibration noise. The causes of these distortions are sometimes predictable such as the significant temperature variations in satellites or they might be unpredictable such as the variable aging of local oscillators in harsh environments. These hardware distortions appearingin the forms of phase noise spurious phase modulation frequency offset filter amplitude and phase ripple data asymmetry and modulator gain imbalance in the transmitter as well as nonlinear amplitude and phase distortions in the power amplifier typically reduce the system performance from a few tenths of dB to as large as ten dB. Among all of them the nonlinear distortions existing in the power amplifiers contribute to the degradation of the system. While analyses of CDMA systems are based on the assumptions that signal waveforms are ideally linearly retransmitted over the power amplifiers it has been known that the