Effect
- Enhance the correlation among symbols so as to recover the signal when interference occurs
- Provides better error correction at receiver, but brings increment of the delay
Channel Types
- No Coding
- Convolutional Coding (1/2, 1/3)
- Turbo Coding (1/3)
UTRAN employs two FEC schemes: convolutional codes and turbo codes. The idea is to add redundancy to the transmitted bit stream, so that occasional bit errors can be corrected in the receiving entity.
The first is convolution that is used for anti-interference. Through the technology, many redundant bits will be inserted in original information. When error code is caused by interference, the redundant bits can be used to recover the original information.
Convolutional codes are typically used when the timing constraints are tight. The coded data must contain enough redundant information to make it possible to correct some of the detected errors without asking for repeats.
Turbo codes are found to be very efficient because they can perform close to the theoretical limit set by the Shannon’s Law. Their efficiency is best with high data rate services, but poor on low rate services. At higher bit rates, turbo coding is more efficient than convolutional coding.
In WCDMA network, both Convolution code and Turbo code are used. Convolution code applies to voice service while Turbo code applies to high rate data service.
Note that both block codes and channel codes are used in the UTRAN. The idea behind this arrangement is that the channel decoder (either a convolutional or turbo decoder) tries to correct as many errors as possible, and then the block decoder (CRC check) offers its judgment on whether the resulting information is good enough to be used in the higher layers.