Investigating the relation between optimum guard interval and channel delay spread for a MC-CDMA system

This paper demonstrates a novel approach to determining the optimum guard interval for a multicarrier code division multiple access (MC-CDMA) system. Analytical expressions for useful and interference power are derived as a basis for comparison. From these, an expression for the signal-to-noise ratio of a detected bit is derived and used to determine the optimum guard interval for a given channel profile and system parameters. In contrast to other works, we use channel models based on actual measurements and we highlight important differences from theoretical models to support our approach. From our results, we propose an empirical rule for optimum guard intervals given prevailing channel parameters. We show that the optimum guard interval can be selected as the delay window that includes 95% and 99% multipath power for Es/N0 = 10 dB and Es/N0 = 20 dB, respectively. In our case, the optimum guard interval was between 2 trms and 4 trms for Es/N0 = 10 dB and between 3 trms and 6.4 trms for Es/N0 = 20 dB.

Anahtar Kelimeler:

Optimum guard interval, multicarrier code division multiple access, outdoor radio channel, root-mean-square delay spread, Delay window

Investigating the relation between optimum guard interval and channel delay spread for a MC-CDMA system

Keywords:

Optimum guard interval, multicarrier code division multiple access, outdoor radio channel, root-mean-square delay spread, Delay window,

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those between the optimum guard interval and the rms delay spread, and they indicate that a 95% or 99% delay window better represents the optimum guard intervals for Es/N0= 10 dB and Es/N0= 20 dB, respectively. Based on our results, we propose the empirical rule that the guard interval may be set to the delay window containing 95% and 99% of multipath power for Es/N0= 10 dB and Es/N0= 20 dB, respectively, if it is to avoid ISI and be optimum in the sense of maximizing the SNR.
This compares with the 2-ray equal-power proŞle, where the ratio of TG,opt/ τrmsmust take on a value slightly greater than 2 if it is to hold more than 90% of the multipath energy, so that it accommodates the second arriving component that has an excess delay of 2 τrms. In this case, it will hold 100% of the multipath energy. Our results for TG,optversus τ
(Section 3.3) all had an optimum guard interval that was greater than 2 τrmsfor both Es/N0= 10 dB and Es/N0= 20 dB, and we conclude that none of our data had a 2-ray equal-power proŞle. Although the 2-ray equal-power proŞle can be used in the optimization of the number of subcarriers, it is not suitable for the optimization of the guard interval. 5. Conclusion
In this paper, an analytical expression for calculating SNR was derived and used as a measure of performance to determine the optimum guard interval for MC-CDMA systems. The guard interval giving the maximum SNR was determined to be the optimum guard interval TG,opt.
The results showed that the optimum guard interval varied between 2 τrmsand 4 τrmsfor Es/N0= 10 dB and between 3 τrmsand 6.4 τrmsfor Es/N0= 20 dB. The characteristics of the PDPs aﬀected TG,opt as well as the rms delay spread value. For a high SNR per symbol, such as 20 dB of Es/N0, exponentially decaying proŞles were found to have worst-case guard intervals.
An empirical rule based on actual data was derived for the optimum guard interval and delay window for Es/N0= 10 dB and Es/N0= 20 dB. The delay windows with approximately 95% and 99% of multipath energy were found to be a good choice for guard intervals for Es/N0= 10 dB and Es/N0= 20 dB.
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