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Unformatted text preview: 4.5 Error Detection and Correction 273 Input signal Output signal Summer t t t t w w 1 w 2 w 3 w 4 Figure 4.12 A transversal filter, a commonly used structure for equalization. The output (equalized) signal is obtained by adding together suitably delayed versions of the input signal, with appropriate weights. a linear effect, and hence the effect of the channel on the pulse, due to dispersion, can be modeled by the response of a filter with transfer function H D (f ) . Hence, in principle, by using the inverse of this filter, say, H 1 D (f ) , as the equalization filter, this effect can be canceled completely at the receiver. This is what an equalization filter attempts to accomplish. The effect of an equalization filter is very similar to the effect of dispersion compensating fiber (DCF). The only difference is that in the case of DCF, the equal- ization is in the optical domain, whereas equalization is done electrically when using an equalization filter. As in the case of DCF, the equalization filter depends not only on the type of fiber used but also on the fiber length. A commonly used filter structure for equalization is shown in Figure 4.12. This filter structure is called a transversal filter. It is essentially a tapped delay line: the signal is delayed by various amounts and added together with individual weights. The choice of the weights, together with the delays, determines the transfer function of the equalization filter. The weights of the tapped delay line have to be adjusted to provide the best possible cancellation of the dispersion-induced pulse broadening. Electronic equalization involves a significant amount of processing that is difficult to do at higher bit rates, such as 10 Gb/s. Thus optical techniques for dispersion compensation, such as the use of DCF for chromatic dispersion compensation, are currently much more widely used compared to electronic equalization. 4.5 Error Detection and Correction An error-correcting code is a technique for reducing the bit error rate on a communi- caton channel. It involves transmitting additional bits, called redundancy, along with the data bits. These additional bits carry redundant information and are used by the receiver to correct most of the errors in the data bits. This method of reducing the 274 Modulation and Demodulation error rate by having the transmitter send redundant bits (using an error-correcting code) is called forward error correction (FEC). An alternative is for the transmitter to use a smaller amount of redundancy, which the receiver can use to detect the presence of an error, but there is insufficient redundancy to identify/correct the errors. This approach is used in telecommunica- tion systems based on SONET and SDH to monitor the bit error rate in the received signal. It is also widely used in data communication systems, where the receiver re- quests the transmitter to resend the data blocks that are detected to be in error. This technique is called automatic repeat request...
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- Spring '09