Adaptive modulation is applied in conjunction with a decision-feedback equalizer (DFE) in order to mitigate the effects of the slowly varying wide-band multipath Rayleigh fading channel in a noise-limited environment. An upper-bound mean bit-error rate and bits per symbol performance is introduced for this scheme by utilizing the pseudo signal-to-noise ratio at the output of the DFE in order to switch the modulation schemes on a burst-by-burst basis. Index Terms—AQAM, QAM, quadrature amplitude modulation, wide-band adaptive modulation

Hanzo, L.

Wong, C.H.

English

Southampton (e-Prints Soton)

368 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 48, NO. 3, MARCH 2000Fig. 1. Schematic of the joint AQAM and equalization scheme.Fig. 2. The impulse response of a COST 207 TU channel.between the transmitter and the receiver. The error propagationof the DFE will degrade the upper-bound performance. How-ever by using AQAM, the mean BER is lowered which then de-creasestheprobabilityoferrorpropagation.Therefore,theerrorpropagation is neglected to simplify the analysis.III. PSEUDO-SNR OF THE DFEWe will begin our justification of using the pseudo-SNR as aswitchingcriterionbypresentingtheequationsusedtocalculatethe pseudo-SNR as follows [8]:wanted signal powerresidual ISI power + effective noise power(3)where and is the transmitted signalat time which was assumed uncorrelated.For the pseudo-SNR to be viable as a switching criterion,it has to be able to estimate the mean BER for fixed modula-tion schemes and consequently for AQAM schemes. However,since it is difficult to characterize the probability density func-tion (pdf) of the residual interference, the residual interferencewas approximated as additional noise at the output of the DFE.This was also applied by Monsen [10] who asserted that theinterference characteristics do not significantly alter the meanBERperformance.Toverifythis,severalexperimentswerecon-ductedusingfixedmodulationschemesinafadingmultipathen-vironment, where the mean BER performance for each of theseschemes, was numerically calculated as follows:(4)where MODN can be for binary phase-shift keying (BPSK),4-QAM, 16-QAM, or 64-QAM. is the pdf of the in-stantaneous pseudo-SNR for a given average channel SNR .In order to obtain , the instantaneous values ofwere calculated using (3). Subsequently, the discretised pdf wasconstructed and inserted into (4). The integration was approx-imated by the trapezoidal rule with the limits set to the lowestand highest values of the instantaneous pseudo-SNR. The nota-tion is the individual theoretical QAM Gaussian BERperformance used in our AQAM system [9].The results shown in Fig. 3 displayed close correspondencebetween the numerical and simulated performance for each in-dividual modulation scheme. Consequently, the utilization ofas a modulation switching criterion in the proposed jointscheme is justified.IV. NUMERICAL AVERAGE UPPER-BOUND PERFORMANCEThe numerical method suggested by Torrance [3] is modifiedfor this joint adaptive scheme by using the pseudo-SNR to yieldthe corresponding numerical mean BER performance:BPSKQAMQAMQAM(5)

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Upper-Bound Performance of a Wide-Band Adaptive Modem

In this contribution, adaptive modulation is applied in conjunction with a Decision Feedback Equalizer (DFE) in order to mitigate the effects of the slowly varying wideband multi-path Rayleigh fading channel. The upper-bound mean BER and Bits Per Symbol (BPS) performance of this scheme is determined by utilizing the pseudo-SNR at the output of the DFE, in order to switch the modulation schemes on a burst-by-burst basis. The performances of each individual modulation schemes and their amalgamated adaptive scheme were compared in terms of information throughput for a low- and a high- BER arrangement. The results indicate both SNR gains of approximately 1 - 3dB and 7 - 9dB, for the high- and low- BER arrangement, respectively using the proposed burst-by-burst adaptive wideband scheme in a noise limited environment

Upper-Bound Performance of a Wideband Burst-by-Burst Adaptive Modem

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.37.1937

Upper-Bound Performance of a Wide-Band Adaptive Modem

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