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FIGURE 42.53 Turbo encoder and decoder.<br />

codes are considered [2]. Turbo decoding (shown in Fig. 42.53) is a collaborative structure of softinput/soft-output<br />

(SISO) decoders with the inclusion of interleaver memories between decoders to scatter<br />

burst errors [2]. Either soft-output viterbi algorithm (SOVA) [7] or maximum a posteriori (MAP) [1]<br />

can be used as SISO decoders. Within a turbo decoder, the two decoders can operate on the same or<br />

different codes. Turbo codes have been shown to provide coding performance to within 0.7 dB of the<br />

Shannon limit (after a number of iterations).<br />

The log MAP algorithm can be implemented in a manner very similar to the standard Viterbi algorithm.<br />

The most important difference between the algorithms is the use of a correction factor on the “new path<br />

metric” value (the alpha, beta, and log-likelihood ratio values in Log MAP). This correction factor<br />

depends on the difference between the values being compared in the add-compare-select butterfly (as<br />

shown in Fig. 42.52). This means that the Viterbi acceleration units, that implement this add-compareselect<br />

operation, need to be modified. Turbo coding is one member of a large class of iterative decoding<br />

algorithms. Recently low density parity check codes (LDPC) that have gained renewed attention as<br />

another important class, which are potentially more easily translated to efficient implementations.<br />

Other trends seem to place an even larger burden on the DSP processor. The Japanese i-Mode system<br />

includes e-mail, web browsing, banking, location finding in combination with the car navigation system,<br />

etc. Next generation phones will need to support video and image processing, and so on. Applications<br />

and upgrades will be downloadable from the Internet.<br />

But at the same time, consumers are used to longer talk times (a couple of hours) and very long<br />

standby times (days or weeks). Thus, they will not accept a reduction of talk time nor standby time in<br />

exchange for more features. This means that these increased services have to be delivered with the same<br />

power budget because the battery size is not expected to grow nor is the battery technology expected to<br />

improve substantially.<br />

DSP Architecture<br />

The fundamental property of a DSP processor is that it uses a Harvard or modified Harvard architecture<br />

instead of a von Neumann architecture. This difference is illustrated in Fig. 42.54.<br />

A von Neumann architecture has one unified address space, i.e., data and program, are assigned to<br />

the same memory space. In a Harvard architecture, the data memory map is split from the program<br />

memory map. This means that the address busses and data busses are doubled. Together with specialized<br />

address calculation units, this will increase the memory bandwidth available for signal processing applications.<br />

This concept will be illustrated by the implementation of a simple FIR filter. The basic equation<br />

for an N tap FIR equation is the following:<br />

© 2002 by CRC Press LLC<br />

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