ns-3 manual: 12.3 The WifiChannel and WifiPhy models

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12.3 The WifiChannel and WifiPhy models

The WifiChannel subclass can be used to connect together a set of ns3::WifiNetDevice network interfaces. The class ns3::WifiPhy is the object within the WifiNetDevice that receives bits from the channel. A WifiChannel contains a ns3::PropagationLossModel and a ns3::PropagationDelayModel which can be overridden by the WifiChannel::SetPropagationLossModel and the WifiChannel::SetPropagationDelayModel methods. By default, no propagation models are set.

The WifiPhy models an 802.11a channel, in terms of frequency, modulation, and bit rates, and interacts with the PropagationLossModel and PropagationDelayModel found in the channel.

This section summarizes the description of the BER calculations found in the yans paper taking into account the Forward Error Correction present in 802.11a and describes the algorithm we implemented to decide whether or not a packet can be successfully received. See "Yet Another Network Simulator" for more details.

The PHY layer can be in one of three states:

TX: the PHY is currently transmitting a signal on behalf of its associated MAC
RX: the PHY is synchronized on a signal and is waiting until it has received its last bit to forward it to the MAC.
IDLE: the PHY is not in the TX or RX states.

When the first bit of a new packet is received while the PHY is not IDLE (that is, it is already synchronized on the reception of another earlier packet or it is sending data itself), the received packet is dropped. Otherwise, if the PHY is IDLE, we calculate the received energy of the first bit of this new signal and compare it against our Energy Detection threshold (as defined by the Clear Channel Assessment function mode 1). If the energy of the packet k is higher, then the PHY moves to RX state and schedules an event when the last bit of the packet is expected to be received. Otherwise, the PHY stays in IDLE state and drops the packet.

The energy of the received signal is assumed to be zero outside of the reception interval of packet k and is calculated from the transmission power with a path-loss propagation model in the reception interval. where the path loss exponent, n, is chosen equal to 3, the reference distance, d_0 is choosen equal to 1.0m and the reference energy is based based on a Friis propagation model.

When the last bit of the packet upon which the PHY is synchronized is received, we need to calculate the probability that the packet is received with any error to decide whether or not the packet on which we were synchronized could be successfully received or not: a random number is drawn from a uniform distribution and is compared against the probability of error.

To evaluate the probability of error, we start from the piecewise linear functions shown in Figure fig:snir and calculate the SNIR function.

figures/snir

Figure 12.2: SNIR function over time

From the SNIR function we can derive bit error rates for BPSK and QAM modulations. Then, for each interval l where BER is constant, we define the upper bound of a probability that an error is present in the chunk of bits located in the interval l for packet k. If we assume an AWGN channel, binary convolutional coding (which is the case in 802.11a) and hard-decision Viterbi decoding, the error rate is thus derived, and the packet error probability for packet k can be computed..