nce, CK Therefore, the resulting coded sequence is the result of com- bining the information sequence of bit (systematic bits, S) with the two sequences of parity bits (P1 and P2). Hence, the overall code rate is 1/3 even though the rate of each encoder is 1/2, because their outputs are combined with input data bits (see Figure 3.13). Finally, a trellis termination operation is performed to force the encoders back to a zero initial state after coding a transport block. Once all the information bits are encoded, the tail bits from the shift register feedback in each encoder are pad- ded after the encoded information bits. The tail bits are used to terminate each con- stituent encoder while the other constituent encoder is disabled (its correspondent switch in position 2). In order to reduce the complexity in both turbo encoder and decoder, a qua- dratic polynomial permutation (QPP) interleaver is used instead of release-6 turbo interleaver (3GPP 36.212). This kind of interleaver (Takeshita 2006, 1249) holds the maximum contention-free property. That is a great advantage from a hard- ware implementation perspective because contention in interleaver memory access is avoided and, therefore, it is possible to parallelize decoding with a single extrin- sic memory. The reason for this interleaving process is that the performances of © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3 GPP LTE Radio and Cellular Technology Systematic bits, S 1st constituent encoder Information bits Parity bits 1, P1 CK (1) 2nd constituent encoder Interlaving Parity bits 2, P2 Bits for trellis tail calculation Figure 3.13 Turbo code coding scheme proposed for LTE. turbo and convolutional codes are improved when the errors introduced by the radio channel are statistically independent because they are random error correct- ing codes. 3.2.4.3 Rate Matching The purpose of the rate matching process is to adjust the coded block size to the allocated physical resources. The first step in this process consists of an interleav- ing of the 