Section 20.2. Communication Energy Model

20.2. Communication Energy Model

IEEE standardsas 802.11a, b, and g provide a wide range of data rates: 54, 48, 36, 24, 18, 12, 9, and 6 Mb/s. This range reflects the trade-off between the transmission range and data rate intrinsic in a wireless communication channel. An accurate energy model is crucial for the development of energy-efficient clustering and routing protocols. The energy consumption, E , for all components of the transceiver in watts is summarized as

Equation 20.1

where is the distance-independent term that accounts for the overhead of the radio electronics and digital processing, and ·‰d ⁿ , is the distance-dependent term, in which · represents the amplifier inefficiency factor, ‰ is the free-space path loss, d is the distance, and n is the environmental factor. Based on an environmental condition, n can be a number between 2 and 4, and · specifies the inefficiency of the transmitter when generating maximum power ‰d ⁿ at the antenna. Clearly, the distance-dependent portion of total energy consumption depends on the real-world transceiver parameters, , · , and the path attenuation ‰d ⁿ . If the value of overshadows ·‰d ⁿ , the reduction in the transmission distance through the use of multihop communication is not effective.

In theory, the maximum efficiency of a power amplifier is 48.4 percent. However, practical implementations show that the power-amplifier efficiency is less than 40 percent. Therefore, is calculated assuming that · = 1/0.4 = 2.5. Using Equation (20.1), we can express the energy consumption of a transmitter and a receiver, E _T and E _R , respectively, by

Equation 20.2

and

Equation 20.3

where _T and _R are the distance-dependent terms for the transmitter and the receiver, respectively. Although maximum output power and total power consumption are provided in the manufacturer's data sheet, can be calculated the following formula:

Equation 20.4

Example.

Table 20.1 shows values of E _T and E _R based on a manufacturer's data sheet and and ·‰d ⁿ calculated for a selected chipset. Although path-attenuation energy increases exponentially by the transmission distance, the data illustrates that the static power consumption, , dominates the path loss. Clearly, this causes total power consumption to remain constant as the transmission distance increases. Standards 802.11a, 802.11b, and 802.11g have multirate capabilities. Although, sensor nodes in general generate data in low rates, they can transmit the information using wireless high-speed modulation and techniques.

Table 20.1. Energy consumption parameters

IEEE Standard	Max. Output Power, ‰d n (dBm)	Total Power Consumption (W)	(W)	· x ‰d n (W)
802.11a	+14	1.85 ( E TX ) 1.20 ( E RX )	2.987	0.0625
802.11b	+21	1.75 ( E TX ) 1.29 ( E RX )	2.727	0.3125
802.11g	+14	1.82 ( E TX ) 1.40 ( E RX )	3.157	0.0625

Table 20.2 shows the expected data rate for the 802.11g wireless technology. Although exploiting the multirate capabilities of wireless standards has never been proposed for sensor networks, this technique can decrease the transmission energy for smaller distances by switching to higher data rates and keeping the transceiver on for a shorter period of time. In this case, the energy in terms of Joule/bit reduces discretely as transmission distance shrinks:

Equation 20.5

Table 20.2. Expected data rate of IEEE 802.11g technology

Rate (Mb/s)	Maximum Range	Rate (Mb/s)	Maximum Range
1	100.00 m	18	51.00 m
2	76.50 m	24	41.25 m
6	64.50 m	36	36.00 m
9	57.00 m	48	23.10 m
12	54.00 m	54	18.75 m

where R is the rate in bits/sec. Figure 20.4 shows energy consumption using 802.11g technology at the constant rate of 1 Mb/s and the same technology with the multirate extension. Owing to large values of compared to the maximum output power, singlerate communication energy consumption remains constant as the transmission distance increases, whereas the communication energy consumption for multirate transmission decreases for shorter transmission ranges. However, this scenario does not follow the model of ‰d ⁿ . Meanwhile, the multirate communication necessitates the presence of a robust rate-selection protocol.

Figure 20.4. Energy consumption versus transmission distance for single-rate and multirate communication using 802.11g technology

Multi-Hop Communication Efficiency

Considering the impact of real-world radio parameters and multirate communication, we should reevaluate the effectiveness of multihop communications. Since a multirate communication reduces energy consumption for shorter distances by switching to higher data rates, multihop communication can conserve energy. The traditional objective of multihop communication is to divide the transmission distance into a number of hops, m, and to relatively conserve energy, considering Equation (20.3), by means of

Equation 20.6

However, if the division of transmission distance happens when the maximum range is less than 18.75 m for standard 802.11g, the data rate remains constant, and total energy consumption multiplies by the number of hops. Since sensor networks deal with two-or even three-dimensional spaces, multihop efficiency depends on the network scale and density.

Example.

Figure 20.5 shows an organization in which sensor nodes A, B, C, D, and E are placed d meters apart and tend to send their data packets to the cluster head (CH). Note that d is an application-dependent parameter and can be chosen based on the sensor's characteristics. Assume that standard 802.11g technology is used in an environment in which sensors are placed on average no more than 10 meters apart. Compare nodes' energy consumptions, using Figure 20.5. Figure 20.5. Cluster-head distances from sensor nodes A, B, C, D, and E in a two-dimensional model

Solution.

With the choice of 10 meters for d in the 802.11g charts , if node B tries to use node A as a relay node and then sends data to the cluster head, the total energy of the chosen two-hop path is larger than the direct-transmission energy obtained as Equation 20.7 Also, for nodes C and D, there is no multihop path that can lead to better energy consumption than the direct communication path: Equation 20.8 and Equation 20.9 But if node E first sends the data to the intermediate node D, total energy consumption will be less than the direct communication path: Equation 20.10 Node E is 41.23 meters away from the cluster head. This shows that for nodes more than 41.23 meters apart, direct transmission is no longer the best-possible communication method.

Example.

Continuing the previous example using 802.11g technology, set up an environment representing one cluster. The dimension of the field is 50 m x 50 m, and 25 nodes are randomly dispersed in the field. Compare the energy consumption of direct and multihop communication inside the cluster.

Solution.

At this point, we assume that the cluster head is chosen randomly among the sensors. (The details of cluster-head selection algorithms explained in Section 20.3.) Figure 20.6 shows the energy consumption of a direct, minimum-energy two-hop and minimum-energy three-hop path, based on the distance between nodes in the cluster and the cluster head. For 802.11g technology, the direct transmission is the optimum choice for ranges less than 37 meters, which is almost the same as the result from analytical calculations (41 m). However, for ranges greater than 37 meters, the minimum-energy two-hop path can lead to significantly lower energy consumption.

Figure 20.6. Communication energy versus distance from cluster head for 802.11g technology