NDLI: Multiuser asynchronous SS/CDMA communication using dovetail constructed families of complex-valued spreading sequences

Content Provider	IEEE Xplore Digital Library
Author	Haspel, M. Kalet, I. Weiss, B.
Copyright Year	2002
Description	Author affiliation: Stochastikos Solutions, Jerusalem, Israel (Haspel, M.)
Abstract	This paper takes as point of departure a novel dovetail technique, previously developed, for constructing families of spreading sequences mutually orthogonal in the asynchronous sense. Any two members of the family have periodic cross-correlation values which are identically zero for all values of the shift index, obviating the need for mutual synchronization. Other methods achieve orthogonality between members of a family of sequences at isolated values of shift or delay index, requiring synchronization between the sequences in order to maintain orthogonality, a difficult or impossible task for many users possibly in motion. In light of advances in technology we permit these sequences to be complex-valued with no uniformity constraint on the sequence element absolute values. We have examined communications using these families of sequences, it being well recognized that multiaccess interference (MAI) in SS/CDMA systems is a strongly limiting factor on the performance of these systems. We assume the performance is interference, rather than noise, limited. In order to facilitate comparisons with state of the art CDMA systems, for example IMT-2000, which use spreading factors of 4 512, we give various results for values of sequence length N, equal to 64, 128 and 256. Families of sequences, of size q, are constructed utilizing the dovetail construction guaranteeing that the real and imaginary parts of the periodic cross-correlations between pairs of sequences are identically zero for all shifts. The technique uses random pairwise interchanges, the so-called exchange shuffle, to generate the sequences. The q-1 cochannel interferers, CCIs, determine the probability of error in the communications. Even and odd cross-correlations are used in the simulations and in accordance with the prevailing assumption, the binary information bits form a sequence of independent, identically distributed, i.i.d., random variables each occurring with probability 1/2 . A constant random phase is associated with each cochannel (CCI) signal along with a randomly drawn shift index accounting for the asynchronous nature of the communications. For the majority of the results the voice activity factor (VAF) is 1, assuming all users transmit all the time, although voice communications are active only about 3/8 of the time. A brief glimpse of the effects of VAF < 1 is given to note the effects of lower activity. The simulations indicated that for equal energy sequences with N=256, in a typical family of size 64 the probability of error is of order of magnitude 10/sup -3/ while for a typical family of size 32 the probability of error is of order of magnitude 10/sup -6/. The latter value must have some uncertainty due to the very infrequent occurrence of errors. We noted from the results for N=256 and N=64 that q/N/sup 1/2 / = 2 yields a probability of error of order 10/sup -6/ and q/N/sup 1/2 / = 4 yields a probability of error of order 10/sup -3/. For Gaussian behavior one expects the same probability of error for equal values of (q-1) 1/2 /N 1/2 . An extension of the basic scheme is investigated where information is transmitted over both the real and imaginary channels, an attempt to increase the number of users. Results of this dual channel scheme are given and analyzed in the context of the single channel system and the presence of the signature sequence in the other channel. The PDF of the CCI is modeled and is found through simulations along with the probability of error. An assumption of Gaussian modeling, despite questionable independence, is applied. This is done through the variance as found by simulation with resulting probability of error. In addition a commonly applied Gaussian derived expression for probability of error is compared. For the cases taken it is found that the PDF is different from Gaussian and the probability of error as found by simulations is lower than predicted by both Gaussian based models. The results indicate that one
Starting Page	598
Ending Page	602
File Size	412490
Page Count	5
File Format	PDF
ISBN	0780376277
DOI	10.1109/ISSSTA.2002.1048612
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2002-09-02
Publisher Place	Czech Republic
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Multiaccess communication Radiofrequency interference Predictive models Delay Isolation technology Interference constraints Multiple access interference Random variables Uncertainty Image sequence analysis
Content Type	Text
Resource Type	Article

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