Turbo coding performance in OFDM packet transmission

In transmission of the packets using OFDM (orthogonal frequency division multiplexing) system recommended by IEEE.802.11 there is a constraint on the interleaving depth due to the maximum allowable packet size and processing delay requirement. This results in a non-ideal interleaving which effectively limits the maximum achievable diversity from the channel. In such a channel, fading occurs in block wise manner, i.e., all transmitted symbols of the same block sense the same fade. Also, there is considerable correlation among the fade blocks. We call such a channel as correlated block fading (CBF) channel. With the excellent performance of turbo codes over AWGN, and ideally interleaved fading channels in mind, we tried to get similarly good performance by using turbo codes in OFDM packet transmission. We observed that the system severely suffers from the block fading behavior of the channel. A performance analysis is presented for CBF systems, and simulations for the OFDM system using rate 1/2 convolutional codes and turbo codes are performed. While the turbo code has no gain over the convolutional code with K=7 in BER (bit error rate) performance, it brings about 1.3 dB gain in PER (packet error rate) performance which is more important in packet transmission.