Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

MC-CDMA 77broadcasting, WLAN, and WLL systems, since very simple decoding algorithms basedon the Viterbi algorithm exist that can achieve a soft decision decoding gain. Moreover,convolutional codes are used as component codes for Turbo codes, which have becomepart of 3 G mobile radio standards. A detailed channel coding description is given inChapter 4.Many of the convolutional codes that have been developed for increasing the reliabilityin the transmission of information are effective when errors caused by the channel arestatistically independent. Signal fading due to time-variant multi-path propagation oftencauses the signal to fall below the noise level, thus resulting in a large number of errorscalled burst errors. An efficient method for dealing with burst error channels is to interleavethe coded bits in such a way that the bursty channel is transformed into a channel withindependent errors. Thus, a code designed for independent errors or short bursts can beused. Code bit interleaving has become an extremely useful technique in 2 G and 3 Gdigital cellular systems, and can, for example, be realized as a block, diagonal, or randominterleaver.A block diagram of channel encoding and user-specific spreading in an MC-CDMAtransmitter assigned to user k is shown in Figure 2-10. The block diagram is the samefor up- and downlinks. The input sequence of the convolutional encoder is representedby the source bit vectora (k) = (a (k)0 ,a(k) 1 ,...,a(k) L a −1 )T (2.74)of length L a . The code word is the discrete time convolution of a (k) with the impulseresponse of the convolutional encoder. The memory M c of the code determines the complexityof the convolutional decoder, given by 2 M cdifferent memory realizations, alsocalled states, for binary convolutional codes. The output of the channel encoder is a codedbit sequence of length L b , which is represented by the coded bit vectorb (k) = (b (k)0 ,b(k) 1 ,...,b(k) L b −1 )T . (2.75)a (k)channelencoderb (k)interleaverb ~ (k)symbolmapperd (k)spreaderc (k)s (k)(a) channel encoding and user-specific spreadingrdetectorv (k)symboldemapperw ~ (k)deinterleaverw (k)reliabilityestimatorl (k)channeldecodera^(k)(b) single-user detection and soft decision channel decodingrdetector &symbol demapperwith LLR outputdeinterleaverl (k)channeldecodera^(k)(c) multiuser detection and soft decision channel decodingFigure 2-10Channel encoding and decoding in MC-CDMA systems