Overloading Scheme for Cellular DS-CDMA using Quasi-Orthogonal Sequences and Iterative Interference Cancellation Receiver
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.
[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
[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
@article{"International Journal of Electrical, Electronic and Communication Sciences:63674", author = "Preetam Kumar and Saswat Chakrabarti", title = "Overloading Scheme for Cellular DS-CDMA using Quasi-Orthogonal Sequences and Iterative Interference Cancellation Receiver", 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.", volume = "4", number = "1", pages = "211-6", }