Subject : Wireless Communication
Survey of Equalization Techniques
Introduction:
Equalization techniques can be subdivided into two general categories
- Linear equalization.
- Nonlinear equalization.
Types of Equalization Techniques: These categories are determined from how the output of an adaptive equalizer is used for subsequent control (feedback) of the equalizer.
Figure 13.2 |
The decision maker determines the value of the digital data bit being received and applies a slicing or thresholding operation (a nonlinear operation) in order to determine the value of d(t).
- Linear Equalizer: If d(t) is not used in the feedback path to adapt the equalizer, the equalization is linear.
- Non Linear Equalizer :if d(t) is fed back to change the subsequent outputs of the equalizer, the equalization is nonlinear.
Linear transversal equalizer (LTE):
- The most common equalizer structure is a linear transversal equalizer (LTE).
- A linear transversal filter is made up of tapped delay lines, with the tappings spaced a symbol period (Ts).As shown in above fig13.2
- The simplest LTE uses only feedforward taps, and the transfer function of the equalizer filter is a polynomial in Z^{-1}. This filter has many zeroes but poles only at z = 0, and is called a finite impulse response (FIR) filter, or simply a transversal filter.
- If the equalizer has both feedforward and feedback taps, its transfer function is a rational function of Z^{-1} , and is called an infinite impulse response (IIR) filter with poles and zeros.
- Since IIR filters tend to be unstable when used in channels where the strongest pulse arrives after an echo pulse (i.e., leading echoes), they are rarely used.
Classification Of Equalization Technique(see figure 13.3)
Figure 13.3 |