gr::fec::code::cc_encoder Class Reference

Convolutional Code Encoding class. More...

#include <gnuradio/fec/cc_encoder.h>

Public Member Functions
virtual bool	set_frame_size (unsigned int frame_size)=0
virtual double	rate ()=0
Public Member Functions inherited from gr::fec::generic_encoder
virtual void	generic_work (void *in_buffer, void *out_buffer)=0
int	unique_id ()
std::string	alias ()
virtual int	get_input_size ()=0
virtual int	get_output_size ()=0
virtual const char *	get_input_conversion ()
virtual const char *	get_output_conversion ()
	generic_encoder (void)
	generic_encoder (std::string name)
virtual	~generic_encoder ()

Static Public Member Functions
static generic_encoder::sptr	make (int frame_size, int k, int rate, std::vector< int > polys, int start_state=0, cc_mode_t mode=CC_STREAMING, bool padded=false)

Additional Inherited Members
Public Types inherited from gr::fec::generic_encoder
typedef boost::shared_ptr< generic_encoder >	sptr
Public Attributes inherited from gr::fec::generic_encoder
int	my_id
std::string	d_name
Static Public Attributes inherited from gr::fec::generic_encoder
static int	base_unique_id
Protected Attributes inherited from gr::fec::generic_encoder
gr::logger_ptr	d_logger

Detailed Description

Convolutional Code Encoding class.

This class performs convolutional encoding for unpacked bits for frames of a constant length. This class is general in its application of the convolutional encoding and allows us to specify the constraint length, the coding rate, and the polynomials used in the coding process.

The parameter k sets the constraint length directly. We set the coding rate by setting rate to R given a desired rate of 1/R. That is, for a rate 1/2 coder, we would set rate to 2. And the polynomial is specified as a vector of integers, where each integer represents the coding polynomial for a different arm of the code. The number of polynomials given must be the same as the value rate.

The encoding object holds a shift register that takes in each bit from the input stream and then ANDs the shift register with each polynomial, and places the parity of the result into the output stream. The output stream is therefore also unpacked bits.

The encoder is set up with a number of bits per frame in the constructor. When not being used in a tagged stream mode, this encoder will only process frames of the length provided here. If used in a tagged stream block, this setting becomes the maximum allowable frame size that the block may process.

The mode is a cc_mode_t that specifies how the convolutional encoder will behave and under what conditions.

• 'CC_STREAMING': mode expects an uninterrupted flow of samples into the encoder, and the output stream is continually encoded.

• 'CC_TERMINATED': is a mode designed for packet-based systems. This mode flushes the encoder with K-1 bits which adds rate*(K-1) bits to the output. This improves the protection of the last bits of a block and helps the decoder.

• 'CC_TAILBITING': is another packet-based method. Instead of adding bits onto the end of a packet (as with 'CC_TERMINATED'), this mode will pre-initialize the state of the encoder with a packet’s last (k-1) bits.

• 'CC_TRUNCATED': a truncated code always resets the registers to the start_state between frames.

A common convolutional encoder uses K=7, Rate=1/2, and the polynomials

• 1 + x^2 + x^3 + x^5 + x^6
• 1 + x + x^2 + x^3 + x^6 This is the Voyager code from NASA.

Another encoder class is provided with gr-fec called the gr::fec::code::ccsds_encoder, which implements the above code that is more highly optimized for just those specific settings.

Member Function Documentation

make()

static generic_encoder::sptr gr::fec::code::cc_encoder::make ( int  frame_size, int  k, int  rate, std::vector< int >  polys, int  start_state = 0, cc_mode_t  mode = CC_STREAMING, bool  padded = false  )

static

Build a convolutional code encoding FEC API object.

Parameters

frame_size	Number of bits per frame; must be > 1. If using in the tagged stream style, this is the maximum allowable number of bits per frame.
k	Constraint length (K) of the encoder; must be in the range [2, 31]. K = 1 implies a code without memory which does not make sense; upper limit is due the way the polynomials of the code are passed in polys.
rate	Inverse of the coder's rate; must be > 1. (rate=2 means 2 output bits per 1 input).
polys	Vector of polynomials as integers. The least significant bit (LSB) denotes the coefficient of exponent zero of the coding polynomial. The position of the most significant set bit (zero based counting) is K-1. Note: this representation is reversed compared to the common representation as found in most books and references. The common representation puts the coefficient of the highest exponent into the LSB and the coefficient of exponent zero is the highest set bit. Example: The common binary representation of the Voyager code polynomials (see above) is 1011011 and 1111001; the octal representation is 133 and 171. For this block, the binary representation must be reversed: 1101101 and 1001111; octal this is 155 and 117; decimal this is 109 and 79. Some standards (e.g. CCSDS 131.0-B-3) require the inversion of some outputs. This is supported by providing the negative value of the polynomial, e.g. -109.
start_state	Initialization state of the shift register; must be in range [0, 2^(K-1)-1] where K is the constraint length. The bits in start_state are also used to flush the encoder in mode 'CC_TERMINATED'. Note: Most books and references use a shift register shifting from left to right. This implementation, however, shifts from right to left. This means that the start state must be reversed. (The different shift direction is also the reason why the polynomials must be reversed as described above.)
mode	cc_mode_t mode of the encoding.
padded	true if the encoded frame should be padded to the nearest byte.

rate()

virtual double gr::fec::code::cc_encoder::rate ( )

pure virtual

Returns the coding rate of this encoder.

Implements gr::fec::generic_encoder.

set_frame_size()

virtual bool gr::fec::code::cc_encoder::set_frame_size ( unsigned int  frame_size )

pure virtual

Sets the uncoded frame size to frame_size. If frame_size is greater than the value given to the constructor, the frame size will be capped by that initial value and this function will return false. Otherwise, it returns true.

Implements gr::fec::generic_encoder.

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The documentation for this class was generated from the following file:

• cc_encoder.h