to confirm that the most significant bit should have been a 1. X1X2XXXXXXXXXXXX 10100110 C1 =X 00011011 C2=X4X50 X8 11011000 C3=XX20X X5 01100101 C4=X2@X3@XX@XX Error correction step 2 1st and 3rd bits of the Sampling Matrix recomputed code word reversed by flipping the Transmitted code word incorrectly received bit X1 X1X2X3X4X5XXXXXX 10100101 C2=0@0@0@1 = 1 1000000 = Error correction step 1 Example input word C4=0@1@1@1 = 1 1st and 3rd bits of the recomputed code word differ from the received Recomputed code word XXXXXXXXXXXXXXXX code word C100101@0 = 0 00100101 C2=0 X = 1 C3=0@0@O@O = 0 Incorrect decoded word Figure 14-3: LDPC Computation and Error Correction Leading Edge Wireless Networking Technologies Turbo codes are another form of error correction code in which two parity checks are performed on the data block, one on the straight data and one on a known permutation of the data. In the receiver, two decoders use the two parity check blocks to compute the most likely sequence of transmitted bits. If the two decoders come up with different results, they exchange information and re-compute the most likely transmitted bit sequence, iterating until the two results are identical. The advantage of turbo codes is that they achieve very effective error recovery while keeping the coding rate close to 1, while the disadvantages are computational complexity and latency - in view of the iterative decoding process. Multi-Channel Code Division Multiple Access (MC-CDMA) As described in the Section "Code Division Multiple Access, p. 94", CDMA assigns an orthogonal Walsh-Hadamard code to each receiver and uses this as a chipping code to spread the input data stream. The orthogonality of chipping codes ensures that each receiver is only able to decode symbols encoded with that user's unique code. In MC-CDMA (Figure 14-4) each chip is transmitted in parallel using the same number of carriers as there are chips in the chipping code. In this case the orthogonality of codes allows multiple reuse of the same set of OFDM subcarri