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SPUC::MOD_QPSK

SPUC::MOD_QPSK Class Reference
[Digital Modulation]

QPSK-modulator class. More...

#include <modulator.h>

List of all members.

Public Member Functions

 MOD_QPSK ()
 Class Constructor.
virtual ~MOD_QPSK ()
 Destructor.
void modulate_bits (const bvec &bits, cvec &out)
 Modulation of bits.
cvec modulate_bits (const bvec &bits)
 Modulation of bits.
void demodulate_bits (const cvec &signal, bvec &out)
 Demodulation of bits.
bvec demodulate_bits (const cvec &signal)
 Demodulation of bits.
void demodulate_soft_bits (const cvec &rx_symbols, double N0, vec &soft_bits)
 Soft demodulator for AWGN channels.
void demodulate_soft_bits (const cvec &rx_symbols, const cvec &channel, double N0, vec &soft_bits)
 Soft demodulator for the Rayleigh fading channel.

Detailed Description

QPSK-modulator class.

Symbol numbering is counter clockwise starting with $(1,1)/\sqrt{2}$ as symbol 0. The bits are Gray coded onto symbols. The energy is normalized to one.

Example of use: 

  MOD_QPSK qpsk;
  bvec bits = "0 0 0 1 1 0 1 1";
  cvec symbols = qpsk.modulate_bits(bits);

This class can also perform soft demodulation, calculating the log-MAP estimate of the individual bits. To use the soft demodulate member functions the received symbols shall equal

\[ r_k = c_k \times s_k + n_k, \]

where $c_k$ is the complex channel gain, $s_k$ is the transmitted QPSK symbols, and $n_k$ is the AWGN of the channel (with variance $N_0/2$ in both the real and the imaginary valued components).

The input samples to the soft demodulate functions should be $r_k$ . It is also assumed that the channel estimates are perfect when calculating the soft bits.

When these member functions are used together with MAP-based turbo decoding algoritms then the channel reliability factor $L_c$ of the turbo decoder shall be set to 1. The output from these member functions can also be used by a Viterbi decoder.

Constructor & Destructor Documentation

SPUC::MOD_QPSK::MOD_QPSK (   )  [inline]
 

Class Constructor.

virtual SPUC::MOD_QPSK::~MOD_QPSK (   )  [inline, virtual]
 

Destructor.

Member Function Documentation

bvec MOD_QPSK::demodulate_bits (  const cvec &  signal  ) 
 

Demodulation of bits.

void MOD_QPSK::demodulate_bits (  const cvec &  signal,
bvec &  out
) 
 

Demodulation of bits.

void MOD_QPSK::demodulate_soft_bits (  const cvec &  rx_symbols,
const cvec &  channel,
double  N0,
vec &  soft_bits
) 
 

Soft demodulator for the Rayleigh fading channel.

This function calculates the log-MAP estimate assuming equally likely bits transmitted

\[ \log \left( \frac{\Pr(b_0=0|r)}{\Pr(b_0=1|r)} \right) = \frac{4 \Re \{r c^{*} \} }{N_0} \]

\[ \log \left( \frac{\Pr(b_1=0|r)}{\Pr(b_1=1|r)} \right) = \frac{4 \Im \{r c^{*} \} }{N_0} \]

It is assumed that what is received is the complex-valued model: $r = c s + n$ .

Parameters:
rx_symbols The received noisy constellation symbols, $r$ (complex)
channel The channel coefficients, $c$ (complex)
N0 The single sided spectral density of the AWGN noise, $n$
soft_bits The soft bits calculated using the expression above

void MOD_QPSK::demodulate_soft_bits (  const cvec &  rx_symbols,
double  N0,
vec &  soft_bits
) 
 

Soft demodulator for AWGN channels.

This function calculates the log-MAP estimates assuming equally likely bits transmitted

\[ \log \left( \frac{\Pr(b_0=0|r)}{\Pr(b_0=1|r)} \right) = \frac{4 \Re \{r\} }{N_0} \]

\[ \log \left( \frac{\Pr(b_1=0|r)}{\Pr(b_1=1|r)} \right) = \frac{4 \Im \{r\} }{N_0} \]

It is assumed that what is received is $r = s + n$ . $s$ is the QPSK symbol and the mapping between symbols and bits is Gray-coded.

Parameters:
rx_symbols The received noisy constellation symbols, $r$ (real)
N0 The single sided spectral density of the AWGN noise, $n$
soft_bits The soft bits calculated using the expression above

cvec MOD_QPSK::modulate_bits (  const bvec &  bits  ) 
 

Modulation of bits.

void MOD_QPSK::modulate_bits (  const bvec &  bits,
cvec &  out
) 
 

Modulation of bits.

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