# A Signaling-Free Underwater Code Division Multiple Access Scheme
## 6 Performance Evaluation  
As comparing schemes, we considered whether or not a signaling procedure is present, and whether or not it is CDMA-based, and we chose from three schemes: \(p\)-persistent CSMA, MACA-U [12], and CDMA, using a four-way handshake of RIPT protocol [21]. Table 2 categorizes the schemes. In the CDMA of RIPT protocol, the sink broadcasts ready-to-receive (RTR) packets to invite sensor nodes that are willing to transmit data packets. Having gathered the responses (RTP: Ready-to-participate) from the sensor nodes, the sink lists the sensor nodes that are served within CDMA capacity and replies to them with an ORDER packet. Reading the ORDER packets, the sensor nodes that are included in the list transmit their data packets. The operation of CDMA of RIPT protocol is illustrated in Figure 3. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \(n\) & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline \(E\Big{[}P_{X\gets Y}^{rcvp}\Big{]}\) & & & & & 67 & & \\ (dB re 1 \(\upmu\)pa) & & & & & & & \\ \hline \(n\cdot E\Big{[}P_{X\gets Y}^{rcvp}\Big{]}\) & 67 & 70 & 71 & 73 & 74 & 75 \\ (dB re 1 \(\upmu\)pa) & & & & & & & \\ \hline \(\Big{|}P_{X}^{rcvp}-n\cdot E\Big{[}P_{X\gets Y}^{rcvp}\Big{]}\) & 73 & 72 & 70 & 68 &
\begin{tabular}{c} 62 \\ (minimum) \\ \end{tabular} & 65 \\ (dB re 1 \(\upmu\)pa) & & & & & & \\ \hline \(N_{e}\) & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parametric values generated during traffic estimation for the example of Figure 2. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Signaling-Free** & **Signaling-Based** \\ \hline
**CDMA** & Proposed scheme & CDMA of RIPT [21] \\ \hline
**Non-CDMA** & \(p\)-persistent CSMA & MACA-U [22] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Categorization of proposed and comparing scheme. A data packet is generated at each node to follow the Poisson process at the rate of \(\lambda\) (packet/sec). To cope with traffic congestion, all schemes are assumed to employ the binary exponential random back-off specified in the IEEE 802.11 standard [23]. Packet losses are only caused by collisions, not by physical channel deterioration such as noise and multipath fading. Table 3 summarizes the system parameters used in the computer simulation. The data rate in Table 3 corresponds to the multiplexed one, i.e., in the case of CDMA with a capacity of four, each channel has a data rate of 2400 bps. We averaged the results of 1000 times iteration for each parameter to derive the result. During packet generation, the sensor node measures receiving power based on the estimated number of nodes currently transmitting packets to the sink. Because the transmission power of a sensor node is controlled not to extend far beyond the sink, in the case where the estimating node is located at the opposite side from the transmitting nodes, the received power may not be adequate for correct estimation. However, even with such a difficult condition in the simulation, the successful traffic estimation ratio is maintained above 95% for almost all cases of offered load as shown in Figure 4.  
Figure 3: Operation of CDMA of RIPT protocol. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Data rate & 9600 bps [24] \\ Data packet size & 2000 bits \\ Control packet size & 200 bits \\ CDMA system capacity (\(C\)) & 4 \\ Spreading gain & 6 dB \\ Number of sensor nodes & 20 \\ Radius of topology (\(R\)) & 5 km \\ Number of transmitting node & 20 \\ Acoustic speed & 1500 m/s \\ Bandwidth & 20 kHz \\ Center frequency & 15 kHz \\ Transmission probability of CSMA (\(p\)) & 0.5 \\ Simulation time & 3600 s \\ \hline \hline \end{tabular}
\end{table} 
Table 3: System parameters and values. Figure 5 shows the throughput varying the offered load per node. Throughput is defined as the number of data bits successfully received at the sink per second from all sensor nodes. The throughput of the proposed system is shown to be much higher than the comparing schemes for almost cases of the offered load.