Non linear equalization

1. Equalization Techniques
AANCHAL KUMARI
17001UEL01
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2. INTRODUCTION:
 The Mobile communication system requires signal processing techniques
which improves the link performance in mobile radio environments.
 There are three most popular techniques:
1) Equalization
2) Diversity
3) Channel coding
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Equalization:
 If the modulation bandwidth exceeds the coherence bandwidth of the
radio channel, ISI occurs and modulation pulses are spread in time.
 It is used for compensating Inter Symbol Interference.
 Equalizers must be adaptive as the channel is unknown and time varying.

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Diversity:
 Diversity technique used to compensate for fading channel impairments.
 Types of Diversity Techniques:
 Antenna Diversity
 Frequency Diversity
 Time Diversity
 Polarization Diversity
 Angle Diversity
 Code Diversity
 Antenna Diversity is implemented by using two or more receiving antennas.

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 While Equalization is used to counter the effects of ISI, diversity is
usually employed to reduce the depth and duration of the fades
experienced by a receiver in flat fading channel.
 These techniques can be employed at both base station and
mobile receivers.
 Spatial Diversity is the most widely used diversity technique.

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Channel coding:
 Used to Improve mobile communication link performance by adding
redundant data bits in the transmitted message.
 The coded message is then modulated for transmission in the wireless channel.
 coding can be considered to be a post detection technique.
 Types of channel codes:
 block codes
 convolutional codes.
 Channel coding is generally treated independently from the type of modulation
used.

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Classification of Equalizers:

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Linear Transversal Equalizer:
 LTE is the most common Equalizer structure.
 LTE is made up of tapped delay lines, with the taps spaced a symbol period
(Ts) .
 The transfer function can be written as a function of the delay
operator −𝑗𝜔𝑇𝑠 𝑜𝑟 𝑍^-1 .

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Two Types of LTE:
Finite impulse response (FIR) filter:
 Transfer function is a polynomial in Z^-1.
 Has many zeroes but poles only at z = 0.
Infinite impulse response (IIR) filter:
 Has both feedforward and feedback taps.
 Transfer function is a rational function of Z^-1 with poles and zeros.
 Tend to be unstable when used in channels where the strongest pulse arrives
after an echo pulse.

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Advantages of Lattice Equalizer:
 Numerical stability
 Faster coverage time
 Its unique structure allows dynamic assignment of the most effective length
of the lattice equalizer.
 When the channel becomes more dispersive, the length of the equalizer
can be increased by the algorithm without stopping the operation of the
equalizer.
 Only a fractions of stages is used if the channel is not very time dispersive.
 The main disadvantage of the lattice equalizer is its complex structure.

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