Some Considerations Of Performance And Capacity Of Voice Over IP High Bit Rate Wireless Reverse Links

Abstract

Evaluation of the performance and capacity of the wireless reverse links, that support concurrent voice over IP and packet data services, is important for spectral efficiency considerations. For a given traffic mix, this capacity is the maximum Erlangs and data throughput at a given packet error rate, voice packet expected wait time and probability of wait time outage. The voice packet wait times are traffic induced and due to retransmissions caused by high packet error rate. In this dissertation the error rate of high bit rate DS-SS BPSK RAKE receivers and traffic induced expected packet wait time for voice over IP reverse links, that support one or two bursts at acceptable packet error rate, are determined.In high bit rate DS-SS BPSK mobile radio reverse links with dual space diversity RAKE demodulators the frequency selective channel causes intersymbol interference. For many years the standard Gaussian approximation error rate has been used, and for high bit rates it's accuracy is not known. We compare the Gaussian approximation to the error rate derived from the total probability theorem, Chernoff and Prabhu bounds with true statistics of intersymbol interference. For signal-to-noise ratio's (SNR's) greater than 6 dB (cases of interest), we show that the Gaussian approximation is a much looser upper bound than our bounds. The Prabhu bound which can be tailored to different intersymbol interference conditions is the tightest bound. We show that the RAKE receiver combats incoherent intersymbol interference while the immunity it offers decreases for SNR's greater than 10 dB. Finally, a dual space diversity RAKE provides an 8 dB space diversity gain. We have also derived the bandwidth occupancy in a rigorous way for the first time by developing a method to compute the spectral density. This density is used to determine operating SNR by deriving received signal power and averaging it over the channel. The fractional containment bandwidth with two Gold codes is smaller than that with a PN sequence. The spectral density has no discrete lines; while it is a function of the signature coefficients and the chip Fourier transform. Within the bandwidth the spectral density of a set of frequencies is 15 dB greater than at other frequencies and this set varies from signature to signature. These methods can be used to select signatures that minimize adjacent and co-channel interference.For voice over IP wireless reverse links with a G/D/1 (G/D/2) queue, where the radio channel supports one (two) radio burst(s) or server(s) at any time with acceptable packet error rate, traffic induced expected packet wait times are obtained by deriving the modified Little's-multinomial analytical approximation for the first time. For high utilization factors, which is the operating point of interest, correlation between interarrival times results in high wait times. For the G/D/1 queue our method provides a better estimate of expected packet times than the M/D/1 queue approximation at high utilization factors. For the G/D/2 queue our method provides a much better estimate than the Kingman upper bound approximation at high utilization factors. An upper bound on the probability of outage derived for the first time using the Steffensen inequality is a better estimate than that obtained by the Markov inequality. The dual burst wireless reverse link provides a capacity gain of 2.16 over the single burst case at a given packet expected wait time threshold. Using our bound at a 2% probability of outage and 60 ms wait time threshold, the voice over IP users supported by the G/D/1 and G/D/2 queue is 26 and 59 respectively.Methods presented here can be used to assess radio packet wait times that can be used for Erlang capacity determination and end-to-end delay budgets. The methods can be extended to G/D/K queues.