| Getting Started | |
Blocks in the Model
The model contains the following new blocks.
Convolutional Encoder
The Convolutional Encoder block encodes the signal from the Bernoulli Binary Generator. The example uses the industry standard rate 1/2 convolutional code, with constraint length 7, defined by the following diagram.
Convolutional Encoder Schematic Block Diagram
The encoder structure is described by a pair of binary numbers, having the same length as the code's constraint length, that specify the connections from the delay cells to modulo-2 addition nodes. The binary number for the upper addition node is 1111001. A 1 indicates that the bit in the corresponding delay cell (reading from left to right) is sent to the addition node, and a 0 indicates that the bit is not sent. The binary number for the lower addition node is 1011011. Converting these two binary numbers to octal gives the pair [171,133]. You can enter this pair into the block's mask by typing poly2trellis(7, [171 133]) in the field for Trellis Structure.
To learn more about the convolutional coding features of the Communications Blockset, see Convolutional Coding in the online Communications Blockset documentation.
Complex to Real-Imag
The Complex to Real-Imag block, labeled Re(u), receives the complex signal and outputs its real part. Since the output BPSK Modulator Baseband block has zero complex part, all of the signal is carried by the real part. You can set this option by selecting Real in the Output parameter field in the mask. It is not necessary to demodulate the signal, because the Viterbi Decoder block can accept unquantized inputs.
Viterbi Decoder
The Viterbi Decoder block decodes the signal using the Viterbi algorithm. The Decision Type parameter is set to Unquantized so that the block can accept real numbers from the Complex to Real-Imag block. The Traceback depth parameter, which is set to 96, is the number of branches in the trellis that the block uses to construct each traceback path. This produces a delay of 96 between the input and output of the block. For more information on delays, see Finding the Delay in the Model.
For an example of a convolutional coding model that uses soft-decision decoding, see the section Example: Soft-Decision Decoding in the online Communications Blockset documentation.
| | Building the Convolutional Code Model | Setting Parameters in the Convolutional Code Model | |
