Small-Scale Fading Characteristics in Cellular Networks in Ghana

Introduction

Small-scale fading (SSF) is concerned with the rapid fluctuations of the signal over short time periods or over short distances. Multiple versions of the signals arriving at the receiver at different times are subjected to constructive and destructive interference which leads to fading. According to Rappaport (1996) small-scale fading manifests itself in three ways:

• •

  Variation of the signal strength over short distances or short time intervals

• •

  Random frequency modulation on the signal due to Doppler shift and

• •

  Time dispersion (echoes) as result of multipath propagation delay

Small-scale fading occurs in the urban areas due to the height of the mobile antenna which is far below the height of the surrounding buildings. In addition to this, there is no line of sight (LOS) path to the base station. In case where there is LOS, multipath propagation still occurs due to reflection from the ground and buildings. SSF is the most severe problem encountered in the design of mobile communication networks. Depending on the relation between the signal parameters such as bandwidth, symbols duration and the channel parameters as root mean square delay spread and the Doppler spread, different transmitted signals will undergo different types of fading. There are four types of small-scale fading. They are flat fading, frequency-selective fading, fast fading and slow fading. Small-scale fading, depending on multipath time delay spread, can be flat fading and frequency-selective. In flat fading the bandwidth (BW) of the signal is less than the coherence bandwidth of the channel. The channel delay spread is less than the symbol duration. This contrasts with frequency-selective fading which occurs when coherence bandwidth falls below signal bandwidth. Moreover, small-scale fading based on Doppler spread can either be fast or slow fading. In fast fading, the Doppler spread time is high, and the coherence time is less than the symbol duration while channel variations are faster than baseband signal variations, The opposite situation results in slow fading (Sklar, 1997).