Minimum probability of error for asynchronous Gaussian multiple-access channels
IEEE Transactions on Information Theory
Microwave Mobile Communications
Microwave Mobile Communications
IEEE Journal on Selected Areas in Communications
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This paper investigates the sensitivity to systemimperfections of a reference symbol assisted multi-stagesuccessive interference cancelling (RAMSIC)receiver. Reverse link of a CDMA system with binaryantipodal modulation and coherent detection isconsidered. Performance of systems using eitherbiphase and quadriphase spreading is compared underdifferent operating conditions. Analysis of aconventional matched filter receiver operating on anAWGN channel reveals that when the number of users issmall (such that the multiple access interferencecannot be accurately modelled as Gaussian),quadriphase spreading has a significant advantage overbiphase spreading. This advantage, however, disappearswhen the number of users per sector is large (of theorder necessary for the multiple access interferenceto be considered Gaussian). Results for the RAMSICreceiver with quadriphase spreading, on the otherhand, show that for hexagonal cell geometry with pathloss exponent of 4 and without any forward errorcorrection coding, the traffic capacity is between1.17 and 1.67 times that of the IS-95. These numbersrepresent a significant increase over those obtainedwith biphase spreading. Further investigation withnonidealized cell geometries and other path lossexponents also shows substantial capacity improvementover that of conventional correlator receivers.Performance losses due to nonideal transmitter poweramplifier gating, imperfect power control andsynchronization errors in the RAKE receiver are alsodetermined. The results for biphase spreading showthat for path loss exponent of 4, imperfect amplifiergating causes relatively minor decrease in the trafficcapacity, while no such effect is observed for pathloss exponents of 2 and 3. As expected, relaxing ofpower control for both biphase and quadriphasespreading has a similar capacity reducing effect. Inspite of these two effects the resultant capacity isstill significantly higher than that with theconventional matched filter receiver. Capacityincrease with quadriphase over biphase spreading isbetween 1.4 and 2.0 times. Chip synchronization errorsof the order to be expected in a properly designedconventional CDMA system have only minimal effecton performance. Therefore, we conclude thatconventional synchronization algorithms should performadequately with successive interference cancellingreceivers considered in the paper.