Types of Small-Scale Fading
Depending on the relation between the signal parameters (such as bandwidth, symbol period, etc.) and the channel parameters (such as rms delay spread and Doppler spread), different transmitted signals will undergo different types of fading. Multipath delay spread leads to time dispersion and frequency selective fading; Doppler spread leads to frequency dispersion and time selective fading. The two propagation mechanisms are independent of one another.
Fading Effects Due to Multipath time Delay Spread
If the mobile radio channel has a constant gain and linear phase response over a bandwidth
which is greater than the bandwidth of the transmitted signal, then the received signal will undergo flat fading.Also known as amplitude varying channels and narrowband channels.Here, Bs<<Bc & Ts>>σtwhere;Bs=BW of signal,Bc=BW of channel,Ts=reciprocal bandwidth( symbol period),&σt=Delay spread
Characteristics of a flat fading channel:
If the channel gain changes over time,a change of amplitude occurs in the received signalr(t).
r (t) varies in gain, but the spectrum of the transmission is reserved.
Frequency Selective Fading:If the channel possesses a constant-gain and linear phase response over a bandwidth that is smaller than the bandwidth of transmitted signal, then the channel creates frequency selective fading on the received signal, are also known as wideband channels
Figure 2.5Frequency selective fading channel characteristics.
Here, Bs>Bc & Ts<σt where ; Bs= BW of signal, Bc= BW of channel, Ts= reciprocal bandwidth ( symbol period),& σt= Delay spread
Fading Effects Due to Doppler Spread:
- In a fast fading channel, the channel impulse response changes rapidly within the symbol duration.
- The coherence time of the channel is smaller than the symbol period of the transmitted signal.This causes frequency dispersionHere, TS>TC &
- BS<BD Where; Bs= BW of signal, Bc= BW of channel TS= symbol period &TC =coherence time
- In a slow fading channel, the channel impulse response changes at a ratemuch slower than the transmitted baseband signal s(t)..
- The coherence time of the channel is greater than the symbol period of the transmitted signal.This causes frequency dispersionHere, TS<<TC &
- BS>>BD Where; Bs= BW of signal, Bc= BW of channel TS= symbol period &TC =coherence time