// Assumes symbol-rate sampling!!!!
SoftVector *equalizeBurst(signalVector &rxBurst,
float TOA,
int samplesPerSymbol,
signalVector &w, // feedforward filter
signalVector &b) // feedback filter
{
delayVector(rxBurst,-TOA);
signalVector* postForwardFull = convolve(&rxBurst,&w,NULL,FULL_SPAN);
signalVector* postForward = new signalVector(rxBurst.size());
postForwardFull->segmentCopyTo(*postForward,w.size()-1,rxBurst.size());
delete postForwardFull;
signalVector::iterator dPtr = postForward->begin();
signalVector::iterator dBackPtr;
signalVector::iterator rotPtr = GMSKRotation->begin();
signalVector::iterator revRotPtr = GMSKReverseRotation->begin();
signalVector *DFEoutput = new signalVector(postForward->size());
signalVector::iterator DFEItr = DFEoutput->begin();
// NOTE: can insert the midamble and/or use midamble to estimate BER
for (; dPtr < postForward->end(); dPtr++) {
dBackPtr = dPtr-1;
signalVector::iterator bPtr = b.begin();
while ( (bPtr < b.end()) && (dBackPtr >= postForward->begin()) ) {
*dPtr = *dPtr + (*bPtr)*(*dBackPtr);
bPtr++;
dBackPtr--;
}
*dPtr = *dPtr * (*revRotPtr);
*DFEItr = *dPtr;
// make decision on symbol
*dPtr = (dPtr->real() > 0.0) ? 1.0 : -1.0;
//*DFEItr = *dPtr;
*dPtr = *dPtr * (*rotPtr);
DFEItr++;
rotPtr++;
revRotPtr++;
}
vectorSlicer(DFEoutput);
SoftVector *burstBits = new SoftVector(postForward->size());
SoftVector::iterator burstItr = burstBits->begin();
DFEItr = DFEoutput->begin();
for (; DFEItr < DFEoutput->end(); DFEItr++)
*burstItr++ = DFEItr->real();
delete postForward;
delete DFEoutput;
return burstBits;
}
// 1 is perfect and 0 means all the bits were 0.5
static float getEnergy(const SoftVector &vec,float &low)
{
int len = vec.size();
avg = 0; low = 1;
for (int i = 0; i < len; i++) {
float bit = vec[i];
float energy = 2*((bit < 0.5) ? (0.5-bit) : (bit-0.5));
if (energy < low) low = energy;
avg += energy/len;
}
}
void RACHL1Decoder::writeLowSide(const RxBurst& burst)
{
// The L1 FEC for the RACH is defined in GSM 05.03 4.6.
// Decode the burst.
const SoftVector e(burst.segment(49,36));
e.decode(mVCoder,mU);
// To check validity, we have 4 tail bits and 6 parity bits.
// False alarm rate for random inputs is 1/1024.
// Check the tail bits -- should all the zero.
if (mU.peekField(14,4)) {
countBadFrame();
return;
}
// Check the parity.
// The parity word is XOR'd with the BSIC. (GSM 05.03 4.6.)
unsigned sentParity = ~mU.peekField(8,6);
unsigned checkParity = mD.parity(mParity);
unsigned encodedBSIC = (sentParity ^ checkParity) & 0x03f;
if (encodedBSIC != gBTS.BSIC()) {
countBadFrame();
return;
}
// We got a valid RACH burst.
// The "payload" is an 8-bit field, "RA", defined in GSM 04.08 9.1.8.
// The channel assignment procedure is in GSM 04.08 3.3.1.1.3.
// It requires knowledge of the RA value and the burst receive time.
// The RACH L2 is so thin that we don't even need code for it.
// Just pass the required information directly to the control layer.
countGoodFrame();
mD.LSB8MSB();
unsigned RA = mD.peekField(0,8);
OBJLOG(INFO) <<"RACHL1Decoder received RA=" << RA << " at time " << burst.time()
<< " with RSSI=" << burst.RSSI() << " timingError=" << burst.timingError();
Control::AccessGrantResponder(RA,burst.time(),burst.RSSI(),burst.timingError());
}
SoftVector *demodulateBurst(signalVector &rxBurst,
const signalVector &gsmPulse,
int samplesPerSymbol,
complex channel,
float TOA)
{
scaleVector(rxBurst,((complex) 1.0)/channel);
delayVector(rxBurst,-TOA);
signalVector *shapedBurst = &rxBurst;
// shift up by a quarter of a frequency
// ignore starting phase, since spec allows for discontinuous phase
GMSKReverseRotate(*shapedBurst);
// run through slicer
if (samplesPerSymbol > 1) {
signalVector *decShapedBurst = decimateVector(*shapedBurst,samplesPerSymbol);
shapedBurst = decShapedBurst;
}
LOG(DEEPDEBUG) << "shapedBurst: " << *shapedBurst;
vectorSlicer(shapedBurst);
SoftVector *burstBits = new SoftVector(shapedBurst->size());
SoftVector::iterator burstItr = burstBits->begin();
signalVector::iterator shapedItr = shapedBurst->begin();
for (; shapedItr < shapedBurst->end(); shapedItr++)
*burstItr++ = shapedItr->real();
if (samplesPerSymbol > 1) delete shapedBurst;
return burstBits;
}
// Do the reverse encoding on usf, and return the reversed usf,
// ie, the returned usf is byte-swapped.
static int decodeUSF(SoftVector &mC)
{
// TODO: Make this more robust.
// Update: No dont bother, should always be zero anyway.
return (mC.bit(0)<<2) | (mC.bit(6)<<1) | mC.bit(4);
}
void ViterbiTCH_AFS7_4::decode(const SoftVector &in, BitVector& target)
{
ViterbiTCH_AFS7_4 &decoder = *this;
const size_t sz = in.size() - 12;
const unsigned deferral = decoder.deferral();
const size_t ctsz = sz + deferral*decoder.iRate();
assert(sz == decoder.iRate()*target.size());
// Build a "history" array where each element contains the full history.
uint32_t history[ctsz];
{
BitVector bits = in.sliced();
uint32_t accum = 0;
for (size_t i=0; i<sz; i++) {
accum = (accum<<1) | bits.bit(i);
history[i] = accum;
}
// Repeat last bit at the end.
for (size_t i=sz; i<ctsz; i++) {
accum = (accum<<1) | (accum & 0x01);
history[i] = accum;
}
}
// Precompute metric tables.
float matchCostTable[ctsz];
float mismatchCostTable[ctsz];
{
const float *dp = in.begin();
for (size_t i=0; i<sz; i++) {
// pVal is the probability that a bit is correct.
// ipVal is the probability that a bit is incorrect.
float pVal = dp[i];
if (pVal>0.5F) pVal = 1.0F-pVal;
float ipVal = 1.0F-pVal;
// This is a cheap approximation to an ideal cost function.
if (pVal<0.01F) pVal = 0.01;
if (ipVal<0.01F) ipVal = 0.01;
matchCostTable[i] = 0.25F/ipVal;
mismatchCostTable[i] = 0.25F/pVal;
}
// pad end of table with unknowns
for (size_t i=sz; i<ctsz; i++) {
matchCostTable[i] = 0.5F;
mismatchCostTable[i] = 0.5F;
}
}
{
decoder.initializeStates();
// Each sample of history[] carries its history.
// So we only have to process every iRate-th sample.
const unsigned step = decoder.iRate();
// input pointer
const uint32_t *ip = history + step - 1;
// output pointers
char *op = target.begin();
const char *const opt = target.end();
// table pointers
const float* match = matchCostTable;
const float* mismatch = mismatchCostTable;
size_t oCount = 0;
while (op<opt) {
// Viterbi algorithm
assert(match-matchCostTable<(int)(sizeof(matchCostTable)/sizeof(matchCostTable[0])-1));
assert(mismatch-mismatchCostTable<(int)(sizeof(mismatchCostTable)/sizeof(mismatchCostTable[0])-1));
const ViterbiTCH_AFS7_4::vCand &minCost = decoder.step(*ip, match, mismatch);
ip += step;
match += step;
mismatch += step;
// output
if (oCount>=deferral) *op++ = (minCost.iState >> deferral)&0x01;
oCount++;
}
}
}