Role of recursive convolutional codes in turbocodes

Recently proposed ‘turbo codes’ yield very good performance in combination with simple iterative decoding strategies. They can be thought of as parallel concatenated coding schemes employing systematic convolutional codes as constituent codes linked by an interleaver. A distinctive feature of turbo codes is that they use, as constituent codes, recursive convolutional codes. Through the application of simple analytical considerations reinforced by a complete example, we show that the presence of a recursive code is crucial to the performance of the concatenated code.