Severina, D. and Lo Cigno, R. (2003) CDMA Radio Network Planning: A Comparison of Different Interference Evaluation Methods. UNSPECIFIED. (Unpublished)

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Abstract

The target of this work is the comparison of different approximation thechniques for the evaluation of the level of interference, and the consequent errors on data frames transmitted on the channel, generated by the sum of interfering signals. The interference, also known as MAI (Multiple Access Interference), is caused by presence of other users in the same cell of the reference user or in neighboring cells. The system considered is a generic spread-spectrum communication system, whose spreading technique can be described as CDMA (Code Division Multiple Access), with of without rake receivers, with a direcr or indirect spreading technique. Although the analysis techniques considered are extremely "broad" in their application, mainly as a consequence of their high level of abstraction and approximation, all the examples and numerical evaluation are done in the context of the UTRAN (UMTS-Terrestrial Radio Access NEtwork) interface, which uses a DS-CDMA (Direct Spreading CDMA}) technique with a superposition of spreading and scrambling codes. The analisy of behaviour of DS-CDMA system is done with different approximation: SGA, IGA, SEIGA and Fourier Series Expansion. The SGA (Standard Gaussian Approximation) is the most diffused approximation: it considers one useful signal while all the others are considered as white Gaussian noise over the band in exam. This algorithm is based on Central Limit Theorem: a sum of a large number of independent and identically distributed (iid) variables is a Gaussian variable. Is a rough approximation, but it allows easy and fast computation. The IGA (Improved Gaussian Approximation) is based on Central Limit Theorem too. In this case the MAI is a Gaussian variable conditionally to users states, defined by delay and phase. The SEIGA (Simplified Expression of Improved Gaussian Approximation) is similar to IGA and introduces simplification in the integral computation using Taylor Series and differences rather than derivatives. The Fourier Series Expansion is not based on Central Limit Theorem and does not use Gaussian Approximation for the MAI. In this case the hypothesis of a big number of users in a cell is not necessary and the approximation allows to study multi-service systems. The algorithms that use Gaussian approximation can not be used for multi-service systems because the hypothesis of iid variable can not be satisfied. These approximations allow to define parameters as SIR (Signal to Interference Ratio) and BER (Bit Error Rate}) to estimate performances of communication on up-link channel. For each of these approximations two different channel models are considered: no-fading (AWGN, Additive White Gaussian Noise) one and fading one.For each of these channel models three different classes of power control are considered: perfect one, imperfect one and absent one.

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