The model considered in this note has been referred to by Haight (1958), Kingman (1961) and Flatto and McKean (1977) as two queues in parallel. Customers choose the shorter of the two queues which are otherwise independent. This system is known to be inferior to a single queue feeding the two servers, but how much? Some elementary considerations provide a fresh perspective on this awkward boundary-value problem. A procedure is proposed for the solution in the context of finite waiting-room size and some comparisons are made with the single-queue system and an independent two-queue system.

The autostrada queueing problem

A special complex sequence called a complex quadratic sequence is proposed as the frequency-spreading sequence for the multicarrier spread-spectrum (MC-SS) modulation scheme. The resulting MC-SS signal has constant envelope in both time and frequency domains. This feature provides flexibility for the MC-SS system to mitigate the effects of both partial-band and pulse jamming. For partial-band jamming, the MC-SS system with frequency domain detection and combining can outperform direct sequence spread spectrum. For pulse jamming, the MC-SS system with time-domain detection and combining can outperform other MC-SS systems. The proposed MC-SS signal has a constant envelope, thus, reducing linearity requirements on the power amplifier. A simple jammer state estimation scheme for both partial-band and pulse-jamming channels is also proposed. Furthermore, multiaccess with the MC-SS system is possible by assigning users different cyclic-shifts of the complex quadratic sequence.

Multicarrier spread spectrum system with constant envelope: antijamming, jamming estimation, multiuser access

A special complex sequence, called a complex quadratic sequence, is proposed as the frequency-spreading sequence for multicarrier spread spectrum (MC-SS) modulation scheme. The resulting MC-SS signal has constant envelope in both time and frequency domains. This feature provides flexibility for the MC-SS system to mitigate the effects of both partial-bond jamming and pulse jamming. For partial-band jamming, the MC-SS system with frequency domain detection and combining can outperform DS/SS. for pulse jamming, the MC-SS system with time-domain detection and combining can outperform MC-SS systems. The proposed MC-SS signal has a constant envelope thus reducing linearity requirements on the power amplifier. A simple jammer state estimation scheme for both partial-band and pulse jamming channel is also proposed and analyzed. Furthermore multi-access with the MC-SS system is possible by assigning users with different cyclic-shifts of the complex quadratic sequence. Simulation results are provided to verify our analysis.

Anti-jamming performance of multi-carrier spread spectrum with constant envelope

We propose a OFDM-CDMA system that employs polyphase codes to support variable spreading factors. A systematic approach for constructing the polyphase code sequences of variable spreading factors is developed. Polyphase codes exhibit better auto- and cross-correlation properties than Hadamard codes. When employed in OFDM-CDMA systems, polyphase codes result in certain structured multiple-access interference (MAI) caused by multipath. Analytical and numerical results show that OFDM-CDMA systems employing polyphase codes have better PAPR performance than those using Hadamard codes. The BER performance of the OFDM-CDMA system using polyphase codes is evaluated by numerical results and compared to that of the OFDM-CDMA system using Hadamard codes with and without clipping.

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Polyphase codes for uplink OFDM-CDMA systems

Novel orthogonal polyphase codes with constant envelope design for OFDM-CDMA system are proposed in the paper. The proposed codes can support large number of users and variable spreading factor like Hadamard codes. Proposed orthogonal polyphase codes not only have better periodic auto- and cross correlations than Hadamard codes but also have constant envelope properties which is preferred for uplink/reverse link transmission. New transmitter and receiver architecture and MMSE receiver are proposed for the OFDM-CDMA system using the proposed polyphase codes. Simulation results show that OFDM-CDMA system using proposed polyphase codes has better PAPR and BER performance than OFDM-CDMA system using Hadamard codes and OFDMA system

Variable spreading factor orthogonal polyphase codes for constant envelope OFDM-CDMA system

A classification of methods for generating discrete Fourier transform pairs is given, followed by a table of 29 pairs. Many of these are new, whereas some have been collected from various literature sources. We have tried to make the table interesting rather than comprehensive. The generalization of the Gaussian sums is a good example. Hundreds of additional nonobvious finite identities can be deduced by using the Rayleigh-Parseval formula and convolutions.

Brian Conolly

Papers

A Table of Discrete Fourier Transform Pairs