gr::fec::code::cc_decoder Class Reference

Convolutional Code Decoding class. More...

#include <gnuradio/fec/cc_decoder.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_decoder
virtual void	generic_work (void *inbuffer, void *outbuffer)=0
int	unique_id ()
std::string	alias ()
	generic_decoder (void)
	generic_decoder (std::string name)
virtual	~generic_decoder ()
virtual int	get_input_size ()=0
virtual int	get_output_size ()=0
virtual int	get_history ()
virtual float	get_shift ()
virtual int	get_input_item_size ()
virtual int	get_output_item_size ()
virtual const char *	get_input_conversion ()
virtual const char *	get_output_conversion ()

Static Public Member Functions
static generic_decoder::sptr	make (int frame_size, int k, int rate, std::vector< int > polys, int start_state=0, int end_state=-1, cc_mode_t mode=CC_STREAMING, bool padded=false)

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

Detailed Description

Convolutional Code Decoding class.

This class performs convolutional decoding via the Viterbi algorithm. While it is set up to take variable values for K, rate, and the polynomials, currently, the block is only capable of handling the following settings:

• K = 7
• rate = 1/2 (given as 2 to the constructor)
• polynomials = [109, 79]

This is the well-known convolutional part of the Voyager code implemented in the CCSDS encoder.

The intent of having this FECAPI code classes fully parameterizable is to eventually allow it to take on generic settings, much like the cc_encoder class where the CCSDS settings would be a highly-optimized version of this.

The decoder 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. This mode is the only mode for this decoder that has a history requirement because it requires rate*(K-1) bits more to finish the decoding properly. This mode does not work with any deployments that do not allow history.

• 'CC_TERMINATED': is a mode designed for packet-based systems. This mode adds rate*(k-1) bits to the output as a way to help flush the decoder.

• 'CC_TAILBITING': is another packet-based method. Instead of adding bits onto the end of the packet, this mode will continue the code between the payloads of packets by pre-initializing the state of the new packet based on the state of the last packet for (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, Polynomials=[109, 79]. This is the Voyager code from NASA:

• 109: b(1101101) –> 1 + x + x^3 + x^4 + x^6
• 79: b(1001111) –> 1 + x^3 + x^4 + x^5 + x^6

Member Function Documentation

static generic_decoder::sptr gr::fec::code::cc_decoder::make ( int  frame_size, int  k, int  rate, std::vector< int >  polys, int  start_state = 0, int  end_state = -1, cc_mode_t  mode = CC_STREAMING, bool  padded = false  )

static

Build a convolutional code decoding FEC API object.

Parameters

frame_size	Number of bits per frame. If using in the tagged stream style, this is the maximum allowable number of bits per frame.
k	Constraint length (K) of the encoder.
rate	Inverse of the coder's rate (rate=2 means 2 output bits per 1 input).
polys	Vector of polynomials as integers.
start_state	Initialization state of the shift register.
end_state	Ending state of the shift register.
mode	cc_mode_t mode of the encoding.
padded	true if the encoded frame is padded to the nearest byte.

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

pure virtual

Returns the coding rate of this encoder.

Implements gr::fec::generic_decoder.

virtual bool gr::fec::code::cc_decoder::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_decoder.

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

• cc_decoder.h