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());
}
