The final version of the wireless sensing system was fabricated using ZigBee based RF communication. XBee Pro Series 1 RF module, which works in point to point fashion, was considered for experimentation. The XBee chips transmit 2.4 GHz signals with a baud rate of 115200 maximum. Both the coordinator and a receiver node are interface to a C8051F020 microcontroller. And the coordinator is connected to the PC via microcontroller. The PC is on a software program which is able to track the sending and receiving of data. This software logs the received data and analyses the lost packets, delay packets, and the corrupted packets of data. In this experiment 100 bytes of data were sent one at a time. After sending each byte I gave 5000ms to the receiving ZigBee to reply back. If the receiving ZigBee does not reply back within that time that byte is counted as “not-received”. If the receiving ZigBee sends a wrong byte then that byte is counted as “not-received”. If the receiving ZigBee sends the correct byte that byte is treated as “valid”. I kept the count of how many “valid” bytes after sending the 100th byte. The experimental setup is shown in figure 4.7.
Figure 4.7: XBee communication reliability test setup
A spectrum analyser was used to check the received signal strength. The longest distance I could find was 35 meters. At that distance, the reliability of the communication was near perfect. So we tried placing microwave oven near one of the XBee. It looks like the reliability of the XBee series 1 reduces drastically when there are 2.4 GHz background noises. The possible explanation for the trend I noticed in this experiment is that, due to the presence of the obstacles in the path, when the distance gets longer the amount of obstacles in the path increases too. So as the distance gets longer the XBee signals have higher chance of colliding different objects and get reflected and lose its energy. So as the distance gets longer the reliability drops as shown in Table 4.2. The other thing that was noticed is the Wi-Fi noise comes as a burst and during that burst the entire communication of the XBee’s becomes unreliable.
The calculation of the communication reliability is done using the formula in equation (4.1).
…..(4.1)
Table 4.2: Communication Success Rate - Indoor-(No clear line of sight)
Distance Reliability 2 100 4 100 6.5 99.67 8 99.67 10 99.33 12 99.33 14 99.33 16 98.33 18 96.33 20 96 22 95 24 80.25 26 91.67
From the table 4.2, it can be noticed that at the distance of 24m the reliability showed an uncharacteristically lower value. The reason being, one of the XBee receiver is placed near a Wi-Fi hot spot which has interfered with the XBee communication signal. The figure 4.8 is the graph plotted based on the values in table 4.2.
Figure 4.8: XBee – Distance Vs. Reliability
When there are obstacles present in the path of the XBee modules the reliability drops when the distance gets longer. At short distances (up to 10m) the rate of change of reliability is very small and the reliability is 100%. At longer distances the rate of the reliability drops is very high. It almost looks like an exponential growth for the rate of the reliability drop at longer distances. Based on the valves cumulative reliability
difference is calculated in table 4.3 and a graph is plotted (figure 4.9).
Table 4.3: Reliability Drop vs. Distance
Distance (m) Cumulative Difference
2 0 4 0 6.5 0.33 8 0.33 10 0.33 12 0.67 14 0.67 16 1.67 18 3.67 20 4 22 5 26 8.33
Figure 4.9: Reliability vs. Distance of XBee
In another experiment this reliability test was done in direct line-of-sight and there were not any considerable obstacles present in the path. The longest distance I could find was 35 meters. Even at that distance, the reliability of the communication was near perfect. So I tried placing microwave oven near one of the XBee module.
x Reliability at 30m when microwave interference is not present 100% x Reliability at 30m when microwave interference is present 66.15%
When it is clear line of sight and no real background noises are present, the reliability of 100% up to 30 meters. It looks like the reliability of the XBee modules (series 1) reduces drastically when there are 2.4 GHz background noises present and when there are obstacles presents. The possible explanation for the trend that came across in the prior experiment is that, due to the presence of the obstacles in the path, when the
distance gets longer the amount of obstacles in the path increases too. So as the distance
gets longer the XBee signals have higher chance of colliding different objects and get reflected and lose its energy. So as the distance gets longer the reliability drops. The other thing that was noticed is that Wi-Fi noise comes as a burst and during that burst
0 1 2 3 4 5 6 7 8 9 0 5 10 15 20 25 30 Cumul ati ve D if fe rence Distance (m)
the entire communication of the XBee becomes non-existent. In this experiment, it is observed that if there is no obstacles present, XBee signal strength can carry even up to 30m with near perfect reliability. Based on this experiment and also from literature survey [111, 112], the channels of the XBee are changed, a considerable improvement in the reliability in communication was noticed. Obtained results from this experiment enabled us to determine precision of the distance and communication reliability evaluation in sensors communication.
Finally implementing XBee Pro Series 2 RF chips, which communicate in mesh network, was considered. An improvement in the reliability of the communication was noticed with these modules.