tailieunhanh - Adaptive thu phát không dây P4

In this chapter, the conceptof Adaptive Quadrature AmplitudeModulation (AQAM) is introduced, whereby the modulation mode is adapted at the transmitter on a burst by burst basis. This adaptation is implemented based on thereceiver’s perceived channel qualityand its main BER. motivation is to maximise the transmission throughput at a given target In our investigations AQAM is initially applied in a narrow-band environment in conjunction with power control, where the transmitted power is only varied near the modem mode switching thresholds of the AQAM scheme. Subsequently, AQAM is investigated in a wideband channel in conjunction with a DE-assisted receiver | Adaptive Wireless Tranceivers L. Hanzo . Wong . Yee Copyright 2002 John Wiley Sons Ltd ISBNs 0-470-84689-5 Hardback 0-470-84776-X Electronic Chapter Adaptive Modulation In this chapter the concept of Adaptive Quadrature Amplitude Modulation AQAM is introduced whereby the modulation mode is adapted at the transmitter on a burst by burst basis. This adaptation is implemented based on the receiver s perceived channel quality and its main motivation is to maximise the transmission throughput at a given target BER. In our investigations AQAM is initially applied in a narrow-band environment in conjunction with power control where the transmitted power is only varied near the modem mode switching thresholds of the AQAM scheme. Subsequently AQAM is investigated in a wideband channel in conjunction with a DFE-assisted receiver. In this context a new channel quality metric is proposed in order to control the choice of AQAM modes. Let us now commence with a brief overview of the AQAM scheme. Adaptive Modulation for Narrow-Band Fading Channels A brief overview of the principles of AQAM in a narrow-band Rayleigh fading channel environment is given here. In a narrow-band channel as a result of its rapid fading the short term SNR can be severely degraded especially if the channel exhibits a deep fade. The general philosophy of AQAM is to employ a higher-order modulation mode when the channel quality is favourable in order to increase the transmission throughput and conversely a more robust lower-order modulation mode is invoked when the channel quality is low. This is achieved at a constant symbol-rate regardless of the modulation mode selected and hence at a constant bandwidth requirement. Therefore the impact of AQAM mode switching on the system s design remains as low as possible. The concept of invoking AQAM is hence to a certain extent analogous to employing power control schemes which are typically used to combat the effects of pathloss and slow fading. However .