standards are designed with fixed transmission time interval (TTI) specifications. For example 3G mobile communications systems primarily use a 10 ms TTI. All user data and Broadband Telecommunications Technologies and Management 1.0E+00 No coding 1.0E-01 1.0E-02 1.0E-03 Shannon limit 1.0E-04 1.0E-05 Eb/No (dB) Figure 6.22 BER and BLER for an LDPC (4599,4227) code. Source: Ref. [6] control information must be organised SO it can be transmitted in an integer number of TTIs. At times the FEC encoder output can be designed to perfectly fit one or more TTIs. At other times the number of bits may be slightly larger or smaller than the payload size of a TTI. Since the TTI needs to be a specified number of bits, a process of 'rate matching' needs to be carried out. This can be done by adding some extra bits to an encoder output through repeating some of the bits. This process is known as 'bit repetition'. Alternatively some bits are deleted from an encoder output in order that the TTI may be perfectly filled. This process is known as 'puncturing'. Repetition of bits adds some extra energy to the data block and improves its correct detection probability. Puncturing on the other hand removes some energy and there- fore degrades the performance. Note that because of the mathematical relation between the codewords correct detection is still possible even if some output bits are missing. Example 6.7 A control channel in a 3G standard has a transmission time interval of 10 ms and can carry 90 bits. The control information block is 27 bits long. Either of the standard specified encoders shown in Figure 6.18 may be used. What degree of 'repetition' or 'puncturing' should be used? Answer If the rate 1/2 encoder is used, the number of output bits will be: (27+8) X 70 as 8 tail bits are needed to return the encoder to its initial all 0 state. Therefore 20 output bits need to be repeated to fill the transmission time interval. If the rate 1/3 encoder is used, the number of output bits will be: (27+8) = 105. Therefore 15 output bits