# Virtual Sensing Directional Hub MAC (VSDH-MAC) Protocol with Power Control
## 2 Medium Access Control Protocol
#### VSDH-MAC Channel Access Algorithm
### Simulation Setup

To evaluate the performance of the proposed protocol, simulations have been performed using Riverbed Modeler (formerly known as OPNET) [22]. In all simulations, we consider only free space propagation as an illustrative example. We chose to use four antennas as a reasonably practical number to illustrate the performance of a multi-antenna hub. Fewer antennas could be used with litter effort. However, if a significant increase in the number of antennas were required, the issue of beam overlap may become a significant problem. Some overlap is necessary as it is not possible to design antennas with ideal cutoff at the beam edges, but as described in [5], beam overlap is a significant factor in limiting the throughput performance. As the sector angle decreases with increasing numbers of antennas, the degree of overlap must be reduced by the same amount to maintain the same performance per antenna. We suspect this will create some practical difficulties in antenna design and alignment. The transmission parameters are shown in Table 1. Note that SIFS and BPSK, in Table 1 stand for short interframe space and binary phase shift keying, respectively. The simulator uses the SINR to determine the bit error rate (BER). This BER value is used to determine if each individual bit is received in error, assuming randomly distributed errors. A uniformly distributed random number between zero and one is generated randomly. This number is compared with the BER threshold (obtained from

Figure 1: An example of centralized wireless sensor networks (WSNs) topology. a look up table of SINR vs BER for a given modulation scheme), and one or more bit errors will result in a discarded packet. Packets with errors are rejected by the protocol. The data packets are generated according to a Poisson process with a rate (\(G\)), which is referred as the channel offered load or traffic load. The Poisson arrival process gives an exponentially distributed inter-arrival time of the data packet generation. ### Directional Antennas

In order to demonstrate the effect of antenna pattern on performance, simulations were performed with two real antenna patterns. Antenna 1 (Ant 1) is based on a 3 element Yagi design and the second antenna (Ant 2) is based on the low cost antenna from [23], as demonstrated in Figure 2. In addition, we consider an ideal sectored antenna with uniform gain over a 90\({}^{\circ}\) sector and zero elsewhere as commonly used in other studies. Detailed discussion on SIR limit analysis can be found in [5]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Frequency band & 2.4 GHz \\ Channel bit rate & 250 kbit/s \\ RTS, CTS, ACK length & 8 bits \\ Data length & 1024 bits \\ Number of Hub Antenna (\(M\)) & 4 \\ Maximum Transmission Power & 0.052 W \\ Node Received Power & 0.059 W \\ Node Sleep Power & 0003 mW \\ Digital modulation & BPSK \\ CW\_min & 31 \\ CW\_max & 1023 \\ SIFS & 10 us \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Transmission parameters. Figure 2: Polar plot of antenna gain pattern for Ant 1 and Ant 2 with its SIR (signal-to-interference ratio) limit angles, where \(\theta_{A}=\theta_{A1}+\theta_{A2}\). ### Energy Consumption Calculation

This section provides the average energy consumption analysis of the proposed VSDH-MAC protocol.