Turbo Encoder Basics: Convolutional Turbo Codes Explained

This page covers Convolutional Turbo Code (CTC) encoder basics with a rate 1/3 example, a Forward Error Correction (FEC) technique. Turbo code is a popular FEC technique used in next-generation wireless communication systems such as Mobile WiMAX and LTE.

Let’s explore the CTC technique. The figure below depicts a rate 1/3 CTC encoder used in Mobile WiMAX (IEEE 802.16e) OFDMA physical layer.

As shown in Figure 1, the CTC encoder is placed between the randomizer and the puncturing/interleaving stages. The CTC encoder itself is composed of a CTC interleaver and a constituent encoder, as depicted in Figure 2.

The encoding process involves three main steps:

Step 1: Input bits A and B are passed directly to the output.

Step 2: Input bits A and B are passed through a Constituent encoder, which produces outputs Y1 and W1. The Constituent encoder is explained in more detail below.

Step 3: Input bits A and B are passed through a CTC Interleaver and then through another Constituent encoder, producing outputs Y2 and W2.

The Constituent encoder is composed of three shift registers: S1, S2, and S3. All three step outputs mentioned above are self-explanatory and illustrated in Figure 3 (which, unfortunately, is not provided in the original text).

Constituent Encoder details

The original article mentioned that the Constituent Encoder details are present in figure 3. Since, figure 3 is unavailable, I will leave it at this.