Performance analysis of a QAM multitone-on-FM system at and above the FM threshold in a non-frequency-selective slowly fading environment

by Chow, Jocelyn

Data trafic has already exceeded voice &c on wireline networks. A similar trend is expected for wireless networks. Currently, there is widespread deployrnent of Cellular Digital Packet Data (CDPD) networks in the United States of Amenca. In spite of this, due to a lack of nationwide coverage and capacity Iimitations during peak tdic loads, the demand for wireless data communication brought about the extension of the CDPD network through circuit switched CDPD (CS CDPD) to achieve nationwide coverage. In this thesis, we examine the application of a QAM multicarrier modulation scheme for CS CDPD systems and compare the performance of this modulation scheme to the QAM single tone modulation scheme currently used in modems of CS CDPD systems. The system studied is a multicarrier-on-FM system at regions of SM2 at and above the FM threshold region of a Limiter-Discriminator receiver. The channel model of the system is a non-frequency selective and slow Rayleigh fading channel. A closed-fonn expression for a lower bound on the bandwidth efficiency of a comprehensive model of the system is derived. The solution for the optimal power distribution which maximizes the bandwidth efficiency for a11values of SNR is found. Cochannel interference (CCI) is also considered in this thesis since when celI footprints are small, CC1 is often the dominant impairment It is shown in general that, CC1 for angular modulated systems in slow Rayleigh fading is a Gaussian interference. The performance analysis of the multitone QAM-on-FM system with CC1 is similar to the performance analysis of the system corrupted by thermal noise only. Furthemore, thermal noise and CC1 can be treated as a single Gaussian impairment with a power spectral density derived fiom the individual power spectrai density of each Gaussian impairment. The key error mechanisms of the comprehensive model of the system are identified to be the composite Gaussian disturbance (thermal noise plus CCI) and fading. A simplified model of the system is derived £komthe comprehensive model. Performance results comparing the multicanieron-FM system to a single-tone-on-FM system show that the bandwidth efficiency increases as the number of tones is increased for a given value of SNR. Also, using multitone signakg reduces the required value of SNR for reliable data transmission ffom 25 dB for one carrier dom to 6.9 dB for four carriers. The lirnit in performance as represented by a continuos tone mode1 of the system is derived. It is proved that the bandwidth efficiency of the finite tones complex model converges to that of the continuous tone simplified model in the limit as the number of tones approaches infinity. It is also shown that as the number of tones approaches infinity, the FM modulation index becomes unbounded. This indicates that the effect of IF bannlimiting on the multitoneon-FM signal must be considered.