YES
NO
13
NO
YES
NO
14
Figure 6 The procedure of proposed Rate Adaptation scheme
In this paper, the objective is how to obtain the minimal energy consumption while achieving the quality of service for data transmission in WBAN. To achieve the design objective, we propose a new rate adaptation scheme for improving the wireless link utilization. Based on the wireless channel conditions in WBAN, the proposed rate adaptation scheme decides the data transmission mode. Details of the proposed rate adaptation are described in Figure 6. We add additional signal-strength-based mechanism supporting the minimal energy consumption based on the WBAN. To provide maximal throughput in the signal-strength-based mechanism, we propose a method to calculate the optimal RSSI reference according to the previous RSSI record which use the same modulation scheme every time for switching to the higher modulation scheme. The method to obtain the optimal RSSI reference is to accomplish the effective throughput by calculating the average RSSI between the previous RSSI record stored in the used buffer due to the same
15
modulation scheme used and the same success transmission result and the RSSI record which under the same modulation scheme used but got the failed transmission result. Hence, we can obtain the minimal energy consumption from the maximal effective throughput.
However, the most important goal of the rate adaptation scheme is getting higher link utilization by adjusting the transmission mode. With this point of view, the signal-strength-based approach is better than the statistics-based one. But the signal-strength-based approach, for example RBAR scheme, which performs rate adaptation at the receiver instead of at the sender has some limitations. This is mainly because of that it requires the receivers to measure SNR, which may be difficult to realize in low cost WBAN devices. It is not trivial to obtain a reliable estimate of the SNR of a link. Therefore, the most important element of SNR when selecting a rate is the received signal strength (RSS). In practice, wireless nodes need to rely on the “Received Signal Strength Indicator “ (RSSI) value as a measure for the RSS. This quantity can be estimated at the receiver using the RSSI, but must be known at transmitter where the transmission rate is selected. In WBAN, the transmitting WSN can estimate the path loss and channel behavior relatively by keeping track of the RSS measured from the packet sent by the CPN. As long as the CPN used a fixed transmission power level for all its transmissions, the changes in the RSS should be indicative of the changes in the path loss and channel behavior.
Furthermore, we find that the RSS has multiple relationship in average with the packet success rate by our simulation result. In our simulation result, the most common human behavior including walking and sleeping appears that the RSSI range has the multiple stability mapping. Stability is the probability of the success packet transmission. Figure 7 shows the channel measurement in the sleeping situation, human wears the transmitting WSN device in the left chest and the receiving CPN
16
device behind the human body, and then shows the stability and RSSI relationships in Figure 8. As shown in Figure 8, most of the RSSI range has the multiple stability mapping. The multiple stability mapping will cause the original RBAR scheme suffer many uncertainly effect such as the variation in mapped packet success rate, and the transmitter WSN will unable to select the transmission mode needed accurately.
Figure 7 The channel measurement in the sleeping situation
Figure 8 The stability and RSSI relationship in the sleeping situation
17
Therefore, based on the multiple stability mapping, our rate adaptation mechanism overcomes these problem by calculating the average RSSI value as the reference for switching the modulation scheme. The basic idea of our proposed scheme is that the transmitter WSN adapts the transmission rate depending on the RSSI calculated from the buffer which used to store the RSSI value. Changes in the RSSI indicate that the conditions in the wireless link between CPN and WSN are changing, and it might be necessary to adapt the transmission rate accordingly. Besides, for using lower modulation scheme like BPSK in the worse channel condition, once the transmission result appears failed after the continuous success result while using QPSK, our proposed mechanism will force the transmitter WSN to switch the lower modulation scheme. In order to switch to the higher modulation scheme like QPSK in the better channel condition, the rate adaptation mechanism make the transmitter WSN to follow the RSSI reference calculated in the used buffer and decide whether adapts the transmission rate. Morever, in order to catch up with the fluctuates in WBAN channel condition, we use the constructive attempt to switch the transmission mode.
Hence, the PHY rate adaptation can be made when the RSSI measured from the received packet passes the average RSSI value calculated from the used buffer. On the other side, we also can judge the timing of getting into the worse channel condition, and the rate adaptation mechanism will get higher link utilization.
18