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itpp::Turbo_Codec Class Reference
[Forward Error Correcting Codes]

Turbo encoder/decoder Class. More...

#include <itpp/comm/turbo.h>

List of all members.

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, 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.
long get_Ncoded ()
 Get number of coded bits.
long get_Nuncoded ()
 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.

Protected Attributes

long interleaver_size
long Ncoded
long 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
std::string metric
bvec decoded_bits_previous_iteration
Rec_Syst_Conv_Code rscc1
Rec_Syst_Conv_Code rscc2
Sequence_Interleaver< binbit_interleaver
Sequence_Interleaver< double > float_interleaver

Related Functions

(Note that these are not member functions.)

ivec wcdma_turbo_interleaver_sequence (int interleaver_size)
 Generates the interleaver sequence for the internal turbo encoder interleaver used in WCDMA.


Detailed Description

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):

    Turbo_Codec turbo;
    ivec gen(2);
    gen(0) = 013; gen(1) = 015;
    int constraint_length = 4;
    ivec interleaver_sequence = wcdma_turbo_interleaver_sequence( 320 );
    turbo.set_parameters(gen, gen, constraint_length, interleaver_sequence);

Definition at line 57 of file turbo.h.


Constructor & Destructor Documentation

itpp::Turbo_Codec::Turbo_Codec ( void   )  [inline]

Class constructor.

Definition at line 61 of file turbo.h.

virtual itpp::Turbo_Codec::~Turbo_Codec ( void   )  [inline, virtual]

Class destructor.

Definition at line 64 of file turbo.h.


Member Function Documentation

void itpp::Turbo_Codec::set_parameters ( ivec  gen1,
ivec  gen2,
int  constraint_length,
const ivec &  interleaver_sequence,
int  in_iterations = 8,
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.

Parameters:
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 down-scaling 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 table-lookup if decoding with the metric "TABLE" is used.

Definition at line 38 of file turbo.cpp.

References adaptive_stop, bit_interleaver, float_interleaver, interleaver_size, it_assert, it_error, iterations, Lc, logmax_scale_factor, m_tail, metric, n1, n2, n_tot, Ncoded, Nuncoded, rscc1, rscc2, 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().

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 94 of file turbo.cpp.

References bit_interleaver, float_interleaver, interleaver_size, m_tail, n1, n2, n_tot, Ncoded, Nuncoded, 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.

Parameters:
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 down-scaling 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 table-lookup if decoding with the metric "TABLE" is used.

Definition at line 109 of file turbo.cpp.

References it_error, logmax_scale_factor, metric, rscc1, rscc2, and itpp::Rec_Syst_Conv_Code::set_llrcalc().

void itpp::Turbo_Codec::set_iterations ( int  in_iterations = 8  ) 

Sets the number of decoding iterations. Default value is 8.

Definition at line 130 of file turbo.cpp.

References iterations.

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.

Definition at line 135 of file turbo.cpp.

References adaptive_stop.

void itpp::Turbo_Codec::set_awgn_channel_parameters ( double  in_Ec,
double  in_N0 
)

Set parameters for decoding on an AWGN channel.

Parameters:
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 140 of file turbo.cpp.

References Ec, Lc, N0, and 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 Log-MAX decoding algorithm is used, then the value of Lc is not important. Assuming that the received soft values $r_k$ from the channel equal

\[ r_k = h_k c_k + w_k \]

where $h_k$ is the (complex valued) channel gain, $c_k$ is the transmitted symbol equal to $\{-\sqrt{E_c},+\sqrt{E_c}\}$, and $w_k$ is the AWGN (complex valued) noise with total variance of the real plus imaginary part equal to $N_0$. The input to the turbo decoder shall then be

\[ z_k = \hat{h}_k^{*} r_k \]

where $\hat{h}_k^{*}$ is the conjugate of the channel estimate. Assuming that the channel estimate is perfect, the channel reliability factor shall be set to

\[ L_c = 4\sqrt{E_c} / {N_0} \]

Parameters:
in_Lc the channel reliability factor of the channel.

Definition at line 147 of file turbo.cpp.

References Lc.

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:

\[ s(1), p_{1,1}(1), p_{1,2}(1), \ldots , p_{1,n_1}(1), p_{2,1}(1), p_{2,2}(1), \ldots , p_{2,n_2}(1), s(2), \ldots \]

In the above expression $s(n)$ is the n-th systematic bit and $p_{l,k}(n)$ is the n-th bit from the k-th encoder polynomial of the l-th 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):

\[ t_1(1), pt_{1,1}(1), pt_{1,2}(1), \ldots , pt_{1,n_1}(1), \ldots pt_{1,n_1}(m) \]

The tailbits from the first encoder are placed before the tailbits from the second encoder.

Parameters:
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 153 of file turbo.cpp.

References encode_block(), m_tail, n1, n2, Ncoded, and Nuncoded.

void itpp::Turbo_Codec::decode ( const vec &  received_signal,
bvec &  decoded_bits,
const bvec &  true_bits = "0" 
) [virtual]

Decoder function.

This function can decode several consecutive coding blocks that were encoded with the encode member function

Parameters:
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.

Definition at line 198 of file turbo.cpp.

void itpp::Turbo_Codec::decode ( const vec &  received_signal,
bvec &  decoded_bits,
ivec &  nrof_used_iterations,
const bvec &  true_bits = "0" 
) [virtual]

Decoder function.

This function can decode several consecutive coding blocks that were encoded with the encode member function

Parameters:
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.

Definition at line 204 of file turbo.cpp.

References decode_block(), decode_n3(), it_assert, iterations, Lc, m_tail, metric, n1, n2, Ncoded, and Nuncoded.

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.

Parameters:
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 291 of file turbo.cpp.

References bit_interleaver, itpp::concat(), itpp::Rec_Syst_Conv_Code::encode_tail(), itpp::Sequence_Interleaver< T >::interleave(), it_assert, m_tail, n1, n2, Nuncoded, rscc1, and rscc2.

Referenced by encode().

void itpp::Turbo_Codec::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" 
) [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.

Note:
This decoding function assumes that the input is scaled as +-2*SNR + noise. This means that the channel reliability factor Lc must be equal to 1.0. No additional scaling is performed by this function.
Parameters:
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 321 of file turbo.cpp.

References adaptive_stop, itpp::concat(), itpp::Sequence_Interleaver< T >::deinterleave(), float_interleaver, itpp::Sequence_Interleaver< T >::interleave(), interleaver_size, it_assert, it_error, iterations, itpp::Vec< Num_T >::left(), itpp::Rec_Syst_Conv_Code::log_decode(), logmax_scale_factor, m_tail, itpp::Rec_Syst_Conv_Code::map_decode(), metric, Nuncoded, itpp::Vec< Num_T >::right(), rscc1, rscc2, and itpp::zeros().

Referenced by decode().

long itpp::Turbo_Codec::get_Ncoded (  )  [inline]

Get number of coded bits.

Definition at line 259 of file turbo.h.

References Ncoded.

long itpp::Turbo_Codec::get_Nuncoded (  )  [inline]

Get number of uncoded bits.

Definition at line 262 of file turbo.h.

References Nuncoded.

void itpp::Turbo_Codec::decode_n3 ( const vec &  received_signal,
bvec &  decoded_bits,
ivec &  nrof_used_iterations,
const bvec &  true_bits = "0" 
) [protected]

Special decoder function for R = 1/3 i.e. two parity bits for each systematic bit.

Definition at line 440 of file turbo.cpp.

References adaptive_stop, decoded_bits_previous_iteration, itpp::Sequence_Interleaver< T >::deinterleave(), float_interleaver, itpp::Sequence_Interleaver< T >::interleave(), it_assert, iterations, Lc, itpp::Vec< Num_T >::left(), itpp::Rec_Syst_Conv_Code::log_decode_n2(), logmax_scale_factor, m_tail, metric, Ncoded, Nuncoded, itpp::Vec< Num_T >::replace_mid(), rscc1, and rscc2.

Referenced by decode().


Friends And Related Function Documentation

ivec wcdma_turbo_interleaver_sequence ( int  interleaver_size  )  [related]

Generates the interleaver sequence for the internal turbo encoder interleaver used in WCDMA.

Definition at line 569 of file turbo.cpp.


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