Subcarrier Availability in Downlink OFDM Systems with Imperfect Carrier Synchronization in Deep Fading Noisy Doppler Channels

Litifa NOOR, Alagan ANPALAGAN, Sithamparanathan KANDEEPAN

.

DOI: 10.4236/ijcns.2009.28090                

Abstract

Multicarrier systems such as orthogonal frequency division (OFDM) are considered as a promising candidate for wireless networks that support high data rate communication. In this article, we investigate the perform-ance of a multiuser OFDM system under imperfect synchronization. Analytical results indicate that the SNR degrades as the average power of the channel impairments such as AWGN, carrier frequency offset due to Doppler frequency and fading gain is increased. The SNR degradation leads to imperfect synchronization and hence decreases the total number of subcarriers available for allocation. Monte Carlo analysis shows up to 22% loss in the number of allocatable subcarriers can be expected under a specific imperfect synchroniza-tion condition as compared to perfect synchronization. We utilize empirical modelling to characterize the available number of subcarriers as a Poisson random variable. In addition, we determine the percentage de-crease in the total number of allocatable subcarriers under varying channel parameters. The results indicate 19% decrease in the number of available subcarriers as average AWGN power is increased by 10dB; 44% decrease as the Doppler frequency is varied from 10Hz to 100Hz; and 56% decrease as the fading gain is varied from 0dB to -30dB.

Keywords

OFDM, Subcarrier Availability, Synchronization

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	D. H. J. Sun and J. SauVola, “Features in future: 4g vi-sions from a technical perspective,” IEEE GLOCOM, Vol. 6, pp. 3533–3537, November 2001.
[2]	L. J. Cimini, “Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplex-ing,” IEEE Transaction on Communications, pp. 665–675, July 1995.
[3]	J. A. C. Bingham, “Multicarrier modulation for data transmission: An idea whose time has come,” IEEE Com- munication Magazine, pp. 5–14, 1990.
[4]	M. v. B. T. Pollet and M. Moeneclaey, “Ber sensitivity of ofdm systems to carrier frequency offset and wiener phase noise,” IEEE Transaction on Communications, Vol. 43, pp. 191–193, 1995.
[5]	M. Luise and R. Reggiannini, “Carrier frequency acquisi-tion and tracking for OFDM systems,” IEEE Transaction on Communications, Vol. 44, 1996.
[6]	P. H. Moose, “A technique for orthogonal frequency division multiplexing frequency offset correction,” IEEE Transaction on Communications, Vol. 42, pp. 2908–2913, 1994.
[7]	P. B. J. van de Beek and M. Sandell, “Ml estimation of timing and frequency offset in ofdm systems,” IEEE Transaction on Signal Procesings, Vol. 45, pp. 1800– 1805, 1997.
[8]	S. K. N. Lashkarian, “Globally optimum ml estimation of timing and frequency offset in ofdm systems,” IEEE In-ternational Conference on Communications, Vol. 2, pp. 1044–1048, 2000.
[9]	K. L. R. M. C. Y. Wong, and R. S. Cheng, “Multiuser OFDM with adaptive subcarrier, bit, and power alloca-tion,” IEEE Journal, Selected Areas in Communications, Vol. 17, 1999.
[10]	J. Jang and K. B. Lee, “Transmit power adaptation for multiuser OFDM systems,” IEEE Journal, Selected Areas in Communications, Vol. 21, 2003.
[11]	B. E. Z. Shen and J. G. Andrews, “Optimal power alloca-tion in multiuser OFDM systems,” IEEE Global Tele-communications Conference, Vol. 1, 2003.
[12]	J. C. W. Rhee, “Increased in capacity of multiuser ofdm system using dynamic subchannel allocation,” IEEE 51st, Vehicular Technology Conference, Vol. 2, 2000.
[13]	S. Y. C. Suh and Y. Cho, “Dynamic subchannel and bit allocation in multiuser OFDM with a priority user,” IEEE Eighth International Symposium, Spread Spectrum Tech-niques and Applications, pp. 919–923, 2004.
[14]	P. Song and L. Cai, “Multi-user subcarrier allocation with minimum rate request for downlink OFDM packet trans-mission,” IEEE 59th Vehicular Technology Conference, Vol. 4, 2004.
[15]	D. Rife and R. Boostyn, “Single-tone parameter estima-tion from discrete-time observations,” IEEE Transaction on Information Theory, Vol. 5, 1974.
[16]	S. Kandeepan, “Synchronisation techniques for digital modems,” PhD thesis, University of Technology, Sydney, July 2003.
[17]	S. Kandeepan and S. Reisenfeld, “Performance analysis of a correlator based maximum likelihood frequency es-timator,” SPCOM, pp. 169–173, 2004.
[18]	S. Ross, “Introduction to probability models,” Academic Press, 2003.