UNIT 1 Coherent and non coherent communication Answers

Part A
1. Differentiate Coherent and Non Coherent Receivers
Ans : inCoherent detection requires carrier phase recovery at the receiver and hence, circuits to perform phase estimation . Sources of carrier-phase mismatch at the receiver: inPropagationtalking causes carrier-phase offset in the received signal. inThe oscillators at the receiver which generate the carrier signal, are not usually phased locked to the transmitted carrier. 
coherent detection: Huge need for a reference in phase with the received carrier
inLess complexity compared to incoherent detection at the price of higher error rate.

Coherent ( synchronous ) detection: in coherent detection , the local carrier generated at the receiver in phase locked with the carrier at transmitter
Non coherent ( envelope ) detection : this type of detection does not need receiver carrier to be phase locked with transmitter carrier
2. Define Rayleigh Channel 
ans :

• Rayleigh channel is a communications channel having a fading envelope in the form of the Rayleigh Probability Density Function.
  
  Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission medium (also called a communications channel) will vary randomly, or fade, according to a Rayleigh distribution — the radial component of the sum of two uncorrelated Gaussian random variables.
  
  3. Minimum distance for decoding for an optimum waveform reciever
  ans : 
  
  4.Define Optimum M- FSK Receiver
  The tones are symmetrically spaced around the carrier fc which can be easily modeled by  
   r(t) = sqr root ( 2P) cos (2 *pi *fc + 2 * pi ( K/T ) t + θ )  n (t)   
  . the optimum reciever is M - array non coherent receiver where various signals are the corresponding sinusoids 
  
  5 . Define Sub Optimum M-FSK Receiver