Turbo encoder/decoder Class. More...
#include <itpp/comm/turbo.h>
Public Member Functions  
Turbo_Codec (void)  
Class constructor.  
virtual  ~Turbo_Codec (void) 
Class destructor.  
void  set_parameters (ivec gen1, ivec gen2, int constraint_length, const ivec &interleaver_sequence, int in_iterations=8, const std::string &in_metric="LOGMAX", double in_logmax_scale_factor=1.0, bool in_adaptive_stop=false, LLR_calc_unit lcalc=LLR_calc_unit()) 
Set parameters for the turbo encoder/decoder.  
void  set_interleaver (const ivec &interleaver_sequence) 
Set a new internal interleaver sequence for the turbo encoder/decoder.  
void  set_metric (std::string in_metric="LOGMAX", double in_logmax_scale_factor=1.0, LLR_calc_unit lcalc=LLR_calc_unit()) 
Set the decoder metric.  
void  set_iterations (int in_iterations=8) 
Sets the number of decoding iterations. Default value is 8.  
void  set_adaptive_stop (bool in_adaptive_stop=true) 
Use and adaptive number of iterations.  
void  set_awgn_channel_parameters (double in_Ec, double in_N0) 
Set parameters for decoding on an AWGN channel.  
void  set_scaling_factor (double in_Lc) 
Set scaling factor for decoding on e.g. Rayleigh fading channels.  
void  encode (const bvec &input, bvec &output) 
Encoder function.  
virtual void  decode (const vec &received_signal, bvec &decoded_bits, const bvec &true_bits="0") 
Decoder function.  
virtual void  decode (const vec &received_signal, bvec &decoded_bits, ivec &nrof_used_iterations, const bvec &true_bits="0") 
Decoder function.  
void  encode_block (const bvec &input, bvec &in1, bvec &in2, bmat &parity1, bmat &parity2) 
Encode a single block.  
virtual void  decode_block (const vec &rec_syst1, const vec &rec_syst2, const mat &rec_parity1, const mat &rec_parity2, bmat &decoded_bits_i, int &nrof_used_iterations_i, const bvec &true_bits="0") 
Decode a single block.  
int  get_Ncoded () const 
Get number of coded bits.  
int  get_Nuncoded () const 
Get number of uncoded bits.  
Protected Member Functions  
void  decode_n3 (const vec &received_signal, bvec &decoded_bits, ivec &nrof_used_iterations, const bvec &true_bits="0") 
Special decoder function for R = 1/3 i.e. two parity bits for each systematic bit.  
Static Protected Member Functions  
static std::string  string_from_metric (const Metric &m) 
Protected Attributes  
int  interleaver_size 
int  Ncoded 
int  Nuncoded 
int  m_tail 
int  n1 
int  n2 
int  n_tot 
int  iterations 
double  Ec 
double  N0 
double  Lc 
double  R 
double  logmax_scale_factor 
bool  adaptive_stop 
Metric  metric 
bvec  decoded_bits_previous_iteration 
Rec_Syst_Conv_Code  rscc1 
Rec_Syst_Conv_Code  rscc2 
Sequence_Interleaver< bin >  bit_interleaver 
Sequence_Interleaver< double >  float_interleaver 
Related Functions  
(Note that these are not member functions.)  
ITPP_EXPORT ivec  wcdma_turbo_interleaver_sequence (int interleaver_size) 
Generates the interleaver sequence for the internal turbo encoder interleaver used in WCDMA.  
ITPP_EXPORT ivec  lte_turbo_interleaver_sequence (int interleaver_size) 
Generates the interleaver sequence for the internal turbo encoder interleaver used in LTE.  
Turbo encoder/decoder Class.
To set up the turbo encoder used in e.g. WCDMA the following code can be used (assuming a code block size of 320 bits):
void itpp::Turbo_Codec::set_parameters  (  ivec  gen1, 
ivec  gen2,  
int  constraint_length,  
const ivec &  interleaver_sequence,  
int  in_iterations = 8 , 

const std::string &  in_metric = "LOGMAX" , 

double  in_logmax_scale_factor = 1.0 , 

bool  in_adaptive_stop = false , 

LLR_calc_unit  lcalc = LLR_calc_unit() 

) 
Set parameters for the turbo encoder/decoder.
gen1  A vector with n1 elements containing the generator polynomials for the first constituent encoder 
gen2  A vector with n2 elements containing the generator polynomials for the second constituent encoder 
constraint_length  The constraint length of the two constituent encoders 
interleaver_sequence  An ivec defining the internal turbo interleaver. 
in_iterations  The number of decoding iterations. Default value is 8. 
in_metric  Determines the decoder metric: "MAP", LOGMAP", "LOGMAX", or "TABLE". The default is "LOGMAX". 
in_logmax_scale_factor  The extrinsic information from each constituent decoder is to optimistic when LOGMAX decoding is used. This parameter allows for a downscaling of the extrinsic information that will be passed on to the next decoder. The default value is 1.0. This parameter is ignored for other metrics than "LOGMAX". 
in_adaptive_stop  If this parameter is true, then the iterations will stop if the decoding results after one full iteration equals the previous iteration. Default value is false. 
lcalc  This parameter can be used to provide a specific LLR_calc_unit which defines the resolution in the tablelookup if decoding with the metric "TABLE" is used. 
LLR_calc_unit
Definition at line 55 of file turbo.cpp.
References it_assert, it_error, itpp::LOGMAP, itpp::Rec_Syst_Conv_Code::set_generator_polynomials(), itpp::Sequence_Interleaver< T >::set_interleaver_depth(), itpp::Sequence_Interleaver< T >::set_interleaver_sequence(), and itpp::Rec_Syst_Conv_Code::set_llrcalc().
Referenced by itpp::Punctured_Turbo_Codec::set_parameters().
void itpp::Turbo_Codec::set_interleaver  (  const ivec &  interleaver_sequence  ) 
Set a new internal interleaver sequence for the turbo encoder/decoder.
By changing the interleaver sequence it is possible to change the code word size of the turbo codec. Note that you still must use the set_parameters function first to set the other parameters of the turbo codec.
Definition at line 115 of file turbo.cpp.
References itpp::Sequence_Interleaver< T >::set_interleaver_depth(), and itpp::Sequence_Interleaver< T >::set_interleaver_sequence().
void itpp::Turbo_Codec::set_metric  (  std::string  in_metric = "LOGMAX" , 
double  in_logmax_scale_factor = 1.0 , 

LLR_calc_unit  lcalc = LLR_calc_unit() 

) 
Set the decoder metric.
in_metric  Determines the decoder metric: "MAP", LOGMAP", "LOGMAX", or "TABLE". The default is "LOGMAX". 
in_logmax_scale_factor  The extrinsic information from each constituent decoder is to optimistic when LOGMAX decoding is used. This parameter allows for a downscaling of the extrinsic information that will be passed on to the next decoder. The default value is 1.0. This parameter is ignored for other metrics than "LOGMAX". 
lcalc  This parameter can be used to provide a specific LLR_calc_unit which defines the resolution in the tablelookup if decoding with the metric "TABLE" is used. 
Definition at line 130 of file turbo.cpp.
References it_error, itpp::LOGMAP, and itpp::Rec_Syst_Conv_Code::set_llrcalc().
void itpp::Turbo_Codec::set_adaptive_stop  (  bool  in_adaptive_stop = true  ) 
Use and adaptive number of iterations.
When the adaptive stop criterion is used the iterations will stop before the maximum number of iterations is executed if the decoding results after one full iteration equals the previous iteration. Default value is true
.
void itpp::Turbo_Codec::set_awgn_channel_parameters  (  double  in_Ec, 
double  in_N0  
) 
Set parameters for decoding on an AWGN channel.
in_Ec  The received energy per channel symbol (i.e. per soft input bit) 
in_N0  The single sided spectral density of the AWGN noise 
Definition at line 165 of file turbo.cpp.
References itpp::sqrt().
void itpp::Turbo_Codec::set_scaling_factor  (  double  in_Lc  ) 
Set scaling factor for decoding on e.g. Rayleigh fading channels.
Setting the correct value of the channel reliability function is important for MAP decoder algorithms. However, if the LogMAX decoding algorithm is used, then the value of Lc is not important. Assuming that the received soft values from the channel equal
where is the (complex valued) channel gain, is the transmitted symbol equal to , and is the AWGN (complex valued) noise with total variance of the real plus imaginary part equal to . The input to the turbo decoder shall then be
where is the conjugate of the channel estimate. Assuming that the channel estimate is perfect, the channel reliability factor shall be set to
in_Lc  the channel reliability factor of the channel. 
void itpp::Turbo_Codec::encode  (  const bvec &  input, 
bvec &  output  
) 
Encoder function.
This function can encode several consecutive coding blocks. The output is organized as follows:
In the above expression is the nth systematic bit and is the nth bit from the kth encoder polynomial of the lth constituent encoder. A tail of both systematic and parity bits is added after each coding block to force both encoder to the zero state. The tail of each encoder is structured as follows (using encoder one as an example):
The tailbits from the first encoder are placed before the tailbits from the second encoder.
input  The input bits to the encoder. Must contain an integer number of code blocks 
output  The encoded bits including two tail, one from each constituent encoder, after each coding block. 
Definition at line 178 of file turbo.cpp.
References encode_block().
Referenced by itpp::Punctured_Turbo_Codec::encode().

virtual 
Decoder function.
This function can decode several consecutive coding blocks that were encoded with the encode member function
received_signal  The vector of received bits 
decoded_bits  A vector of decoded bits 
true_bits  If this input vector is provided then the iterations will stop as soon as the decoded bits equals the true_bits . Note that this feature can not be used if the in_adaptive_stop parameter in the setup function is set to true . 
Reimplemented in itpp::Punctured_Turbo_Codec.
Definition at line 226 of file turbo.cpp.
Referenced by itpp::Punctured_Turbo_Codec::decode().

virtual 
Decoder function.
This function can decode several consecutive coding blocks that were encoded with the encode member function
received_signal  The vector of received bits 
decoded_bits  A vector of decoded bits 
nrof_used_iterations  Returns the number of used iterations for each code block. 
true_bits  If this input vector is provided then the iterations will stop as soon as the decoded bits equals the true_bits . Note that this feature can not be used if the in_adaptive_stop parameter in the setup function is set to true . 
Reimplemented in itpp::Punctured_Turbo_Codec.
Definition at line 232 of file turbo.cpp.
References decode_block(), decode_n3(), and it_assert.
void itpp::Turbo_Codec::encode_block  (  const bvec &  input, 
bvec &  in1,  
bvec &  in2,  
bmat &  parity1,  
bmat &  parity2  
) 
Encode a single block.
This function is useful if rate matching is to be applied after the turbo encoder. The size of in1 and in2 equals the size of input plus the tail bits. Tailbits are appended ate the end of in1 and in2. The number of rows in parity1 and parity2 equals the lengths of in1 and in2 respectively. The number of columns of parity1 and parity2 equals the number of parity bits from the first and the second constituent encoders respectively.
input  The input bits to the encoder. Must contain a single code block 
in1  The input bits to the first constituent encoder with a tail added at the end 
in2  The input bits to the second constituent encoder (i.e. the interleaved data bits) with a tail added at the end 
parity1  The parity bits from the first constituent encoder (including parity bits for the first tail) 
parity2  The parity bits from the second constituent encoder (including parity bits for the second tail) 
Definition at line 326 of file turbo.cpp.
References itpp::concat(), itpp::Rec_Syst_Conv_Code::encode_tail(), itpp::Sequence_Interleaver< T >::interleave(), and it_assert.
Referenced by encode().

virtual 
Decode a single block.
This function can decode a single coding blocks that was encoded with the encode_block member function. In order to speed up the decoding process it is possible to input the correct data bits. If this information is provided the decoder can stop iterating as soon as the decoded bits match the correct data bits. This simulation trick can greatly speed up the simulation time for high SNR cases when only a few iterations are required. If errors still exist after the maximum number of iterations have been performed, the decoding process stops.
The matrix decoded_bits_i contains the result from decoding iteration i on row i. Even though both rec_syst1 and rec_syst2 are given as inputs, the systematic bits in rec_syst2 will in most cases be punctured and only the tailbits at the end of the vector rec_syst2 will have values different from zero.
rec_syst1  The received input bits to the first constituent decoder with a tail added at the end 
rec_syst2  The received input bits to the second constituent decoder with a tail added at the end 
rec_parity1  The received parity bits for the first constituent decoder (including parity bits for the first tail) 
rec_parity2  The received parity bits for the second constituent decoder (including parity bits for the second tail) 
decoded_bits_i  Contains the result from decoding iteration i on row i. 
nrof_used_iterations_i  Returns the number of iterations used for decoding of this block. 
true_bits  Optional input parameter. If given, the iterations will stop as soon as the decoded bits match the true bits. 
Definition at line 361 of file turbo.cpp.
References itpp::concat(), itpp::Sequence_Interleaver< T >::deinterleave(), itpp::Sequence_Interleaver< T >::interleave(), it_assert, it_error, itpp::Vec< Num_T >::left(), itpp::Rec_Syst_Conv_Code::log_decode(), itpp::LOGMAP, itpp::Rec_Syst_Conv_Code::map_decode(), itpp::Vec< Num_T >::right(), and itpp::zeros().
Referenced by decode().

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