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Overloading Scheme for Cellular DS-CDMA using Quasi-Orthogonal Sequences and Iterative Interference Cancellation Receiver

Authors: Preetam Kumar, Saswat Chakrabarti

Abstract:

Overloading is a technique to accommodate more number of users than the spreading factor N. This is a bandwidth efficient scheme to increase the number users in a fixed bandwidth. One of the efficient schemes to overload a CDMA system is to use two sets of orthogonal signal waveforms (O/O). The first set is assigned to the N users and the second set is assigned to the additional M users. An iterative interference cancellation technique is used to cancel interference between the two sets of users. In this paper, the performance of an overloading scheme in which the first N users are assigned Walsh-Hadamard orthogonal codes and extra users are assigned the same WH codes but overlaid by a fixed (quasi) bent sequence [11] is evaluated. This particular scheme is called Quasi- Orthogonal Sequence (QOS) O/O scheme, which is a part of cdma2000 standard [12] to provide overloading in the downlink using single user detector. QOS scheme are balance O/O scheme, where the correlation between any set-1 and set-2 users are equalized. The allowable overload of this scheme is investigated in the uplink on an AWGN and Rayleigh fading channels, so that the uncoded performance with iterative multistage interference cancellation detector remains close to the single user bound. It is shown that this scheme provides 19% and 11% overloading with SDIC technique for N= 16 and 64 respectively, with an SNR degradation of less than 0.35 dB as compared to single user bound at a BER of 0.00001. But on a Rayleigh fading channel, the channel overloading is 45% (29 extra users) at a BER of 0.0005, with an SNR degradation of about 1 dB as compared to single user performance for N=64. This is a significant amount of channel overloading on a Rayleigh fading channel.

Keywords: DS-CDMA, Iterative Interference CancellationOrthogonal codes, Overloading.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1083425

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References:


[1] F. Adachi, M. Sawahashi and H. Suda, " Wideband DS-CDMA for next-generation mobile communication systems, " IEEE Commun. Mag., vol.36, pp. 56-59, September 1998
[2] H. Sari, F. Vanhaverbeke, and M. Moeneclaey, " Multiple access using two sets of orthogonal signal waveforms," IEEE Commun. Lett., vol. 4, no. 1, pp. 4-6, Jan. 2000.
[3] S. Verdu, Multi - user Detection. Cambridge University Press, 1998.
[4] J. A. F. Ross and D. P. Taylor, "Vector assignment scheme for M+N users in N-dimensional global additive channel," Electronics. Letter, vol. 28, August 1992.
[5] R. E. Learned, A. S. Willisky and D. M. Boroson, "Low complexity joint detection for oversaturated multiple access communications," IEEE Trans. Signal Processing, vol. 45, pp. 113-122, January 1997.
[6] F. Vanhaverbeke, M. Moeneclaey and H. Sari, "DS/CDMA with two sets of orthogonal sequences and iterative - detection," IEEE Commun. Lett., vol. 4, pp. 289-291, Sept. 2000.
[7] F. Vanhaverbeke and M. Moeneclaey, "Critical load of oversaturated systems with multistage successive interfernce cancellation", Proceedings of IEEE VTC, vol. 4, pp. 2663-2666, Apr. 2003.
[8] P. Kumar, M. Ramesh, and S. Chakrabarti, "Performance evaluation of orthogonal/scrambled-orthogonal overloaded DS-CDMA system", Proceedings of IFIP International Conference on Wireless Communications and Networks (WOCN), pp. 1-5, July 2007, Singapore
[9] D. Djonin and V. K. Bhargava, "New results on low complexity detectors for oversaturated CDMA systems", Proceedings of Globecom 2001, pp. 846-850, Nov. 2001
[10] P. Kumar and S. Chakrabarti, "A New Overloading Scheme for DSCDMA System", Proceedings of National Conference on Communicatiosn, pp. 285-288, Jan.- 2007, IIT Kanpur
[11] K. Yang, Y.K. Kim and P. V. Kumar, "Quasi-orthogonal Sequences for Code-division Multiple-Access Systems", lEEE Trans. Inform. Theory, vol. 46, pp. 982-993, May 2000.
[12] Physical Layer Standard for cdma2000 Spread Spectrum Systems, Realse B, TIA/EIA 3GPP2 C.S0002-B, Jan. 16, 2001
[13] J. G. Proakis, Digital Communications, McGraw-Hill, New York, 4th edition, 2001