A configuration structure for the CSMA transmit algorithm.

One of RAIL's schemes for polite spectrum access is an implementation of a Carrier-Sense Multiple Access (CSMA) algorithm based on IEEE 802.15.4 (unslotted). In pseudo-code it works like this, showing relevant event notifications:

// Return true to transmit packet, false to not transmit packet.
bool performCsma(const RAIL_CsmaConfig_t *csmaConfig)
{
  bool isFixedBackoff = ((csmaConfig->csmaMinBoExp == 0)
                         && (csmaConfig->csmaMaxBoExp == 0));
  int backoffExp = csmaConfig->csmaMinBoExp; // Initial backoff exponent
  int backoffMultiplier = 1; // Assume fixed backoff
  int try;

  // Special-case tries == 0 to transmit immediately without backoff+CCA
  if (csmaConfig->csmaTries == 0) {
    return true;
  }

  // Start overall timeout if specified:
  if (csmaConfig->csmaTimeout > 0) {
    StartAbortTimer(csmaConfig->csmaTimeout, RAIL_EVENT_TX_CHANNEL_BUSY);
    // If timeout occurs, abort and signal the indicated event.
  }

  for (try = 0; try < csmaConfig->csmaTries; try++) {
    if (try > 0) {
      signalEvent(RAIL_EVENT_TX_CCA_RETRY);
    }
    // Determine the backoff multipler for this try:
    if (isFixedBackoff) {
      // backoffMultiplier already set to 1 for fixed backoff
    } else {
      // Start with the backoff exponent for this try:
      if (try > 0) {
        backoffExp++;
        if (backoffExp > csmaConfig->csmaMaxBoExp) {
          backoffExp = csmaConfig->csmaMaxBoExp;
        }
      }
      // Pick random multiplier between 0 and 2^backoffExp - 1 inclusive:
      backoffMultiplier = pickRandomInteger(0, (1 << backoffExp) - 1);
    }
    // Perform the backoff:
    delayMicroseconds(backoffMultiplier * csmaConfig->ccaBackoff);
    // Perform the Clear-Channel Assessment (CCA):
    // Channel is considered busy if radio is actively receiving or
    // transmitting, or the average energy detected across duration
    // is above the threshold.
    signalEvent(RAIL_EVENT_TX_START_CCA);
    if (performCca(csmaConfig->ccaDuration, csmaConfig->ccaThreshold)) {
      // CCA (and CSMA) success: Transmit after RX-to-TX turnaround
      StopAbortTimer();
      signalEvent(RAIL_EVENT_TX_CHANNEL_CLEAR);
      return true;
    } else {
      // CCA failed: loop to try again, or exit if out of tries
    }
  }
  // Overall CSMA failure: Don't transmit
  StopAbortTimer();
  signalEvent(RAIL_EVENT_TX_CHANNEL_BUSY);
  return false;
}