GNU Radio Manual and C++ API Reference
The Free & Open Software Radio Ecosystem
 All Classes Namespaces Files Functions Variables Typedefs Enumerations Enumerator Friends Macros Modules Pages
gr::fec::code::cc_encoder Class Referenceabstract

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
- 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 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

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

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 

Build a convolutional code encoding FEC API object.

frame_sizeNumber of bits per frame. If using in the tagged stream style, this is the maximum allowable number of bits per frame.
kConstraint length (K) of the encoder.
rateInverse of the coder's rate (rate=2 means 2 output bits per 1 input).
polysVector of polynomials as integers.
start_stateInitialization state of the shift register.
modecc_mode_t mode of the encoding.
paddedtrue if the encoded frame should be padded to the nearest byte.
virtual double gr::fec::code::cc_encoder::rate ( )
pure virtual

Returns the coding rate of this encoder.

Implements gr::fec::generic_encoder.

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.

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