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Techniques to control noise and fadingPowerPoint Presentation

Techniques to control noise and fading

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Techniques to control noise and fading

- Noise and fading are the primary sources of distortion in communication channels
- Techniques to reduce noise and fading are usually implemented at the receiver
- The most common mechanism is to have a receiver filter that can cancel the effects of noise and fading, at least partially
- Digital technology has made it possible to have adaptive filters

Principle of Equalization

- Equalization is the process of compensation at the receiver, to reduce noise effects
- The channel is treated as a filter with transfer function
- Equalization is the process of creating a filter with an inverse transfer function of the channel
- Since the channel is a varying filter, equalizer filter also has to change accordingly, hence the term adaptive.

Equalization Model-Signal detection

Carrier

Transmitter

Channel

Receiver

Front End

IF Stage

Message signal x(t)

Detector

Detected signal y(t)

Equalization model-Correction

Reconstructed

Signal

nb(t)

Decision

Maker

Equalizer

+

Equivalent

Noise

Equalizer System EquationsDetected signaly(t) = x(t) * f(t) + nb(t)=> Y(f) = X(f) F(f) + Nb(f)Output of the Equalizer ^ d(t) = y(t) * heq(t)

Equalizer System EquationsDesired output ^ D(f) = Y(f) Heq(f) = X(f) => Heq(f) X(f) F(f) = X(f)=> Heq(f) F(f) = 1Heq(f) = 1/ F(f) => Inverse filter

Equalizer Operating Modes

- Training
- Tracking

Training and Tracking functions

- The Training sequence is a known pseudo-random signal or a fixed bit pattern sent by the transmitter. The user data is sent immediately after the training sequence
- The equalizer uses training sequence to adjust its frequency response Heq (f) and is optimally ready for data sequence
- Adjustment goes on dynamically, it is adaptable equalizer

Digital Equalizer equations

- In discrete form, we sample signals at interval of ‘T’ seconds : t = k T;
- The output of Equalizer is:

Error minimization

- The adaptive algorithm is controlled by the error signal,

The equalizer weights are varied until convergence is reached.

Types of equalizers

- Linear Equalizers.
- Non Linear Equalizers.

Diversity techniques

- Powerful communications receiver technique that provides wireless link improvement at relatively low cost.
- Unlike equalization, diversity requires no training overhead.

Principle of diversity

- Small Scale fading causes deep and rapid amplitude fluctuations as mobile moves over a very small distances.

…Principle of diversity

- If we space 2 antennas at 0.5 m, one may receive a null while the other receives a strong signal. By selecting the best signal at all times, a receiver can mitigate or reduce small-scale fading. This concept is Antenna Diversity.

Diversity Improvement

- Consider a fading channel (Rayleigh)
Input s(t) Output r(t)

- Input-output relation
r (t) = (t) e -j q(t) s (t) + n (t)

- Average value of signal to noise ratio
___

SNR = = (Eb / No) 2 (t)

Channel

Average SNR Improvement Using Diversity

- p.d.f., p(γi) = (1 / ) e – γi /
where (γi 0 ) and γi = instantaneous SNR

Probability [γiγ]

- M diversity branches,
Probability [γi>γ]

Average Snr Improvement Using Diversity

- Average SNR improvement using selection Diversity,

- Example : Assume that 5 antennas are used to provide space diversity. If average SNR is 20 dB, determine the probability that the SNR will be 10 dB. Compare this with the case of a single receiver.
Solution :

= 20 dB => 100.

Threshold γ = 10 dB = 10.

…Example

Prob[γi>γ] = 1 – (1 – e – γ/ )M

For M = 5,

Prob= 1 – (1 – e – 0.1)5 = 0.9999

For M = 1(No Diversity),

Prob= 1 – (1 – e – 0.1)= 0.905

Maximal Ratio Combining (MRC)

- MRC uses each of the M branches in co-phased and weighted manner such that highest achievable SNR is available. If each branch has gain Gi,
rM = total signal envelope

=

…Maximal Ratio Combining (MRC)

… assuming each branch has some average noise power N, total noise power NT applied to the detector is,

EXAMPLE : Repeat earlier problem for MRC case

…Example

e-0.1

Types of diversity

- Space Diversity
- Either at the mobile or base station.
- At base station, separation on order of several tens of wavelength are required.

- Polarization Diversity
- Orthogonal Polarization to exploit diversity

…Types of diversity

- Frequency Diversity :
- More than one carrier frequency is used

- Time Diversity :
- Information is sent at time spacings
- Greater than the coherence time of Channel, so multiple repetitions can be resolved

Practical diversity receiver – rake receiver

- CDMA system uses RAKE Receiver to improve the signal to noise ratio at the receiver.
- Generally CDMA systems don’t require equalization due to multi-path resolution.

Block Diagram Of Rake Receiver

α1

M1 M2 M3α2

r(t) αM Z’ Z

Correlator 1

()dt

Correlator 2

Σ

Correlator M

>

<

m’(t)

Principle Of Operation

- M Correlators – Correlator 1 is synchronized to strongest multi-path M1. The correlator 2 is synchronized to next strongest multipath M2 and so on.
- The weights 1 , 2 ,……,M are based on SNR from each correlator output. ( is proportional to SNR of correlator.)
- M Z’ = M ZM
m =1

…Principle Of Operation

- Demodulation and bit decisions are then based on the weighted Outputs of M Correlators.

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