Energy Consumption Model

The purpose of this study is to understand the effect of a hierarchical architecture has on energy consumption rather than to perform power management or energy conservation techniques. However, quantifying the energy consumption of a wireless network device under various traffic conditions is more challenging than it actually first appears, because the energy performance is effected by the external radio environment, spectral efficiency and various components that make up wireless interface such as its baseband signal processing circuit, power amplifier and circuit switch with each such parameters affecting this efficiency energy metric very differently [6].

It was highlighted in [6], [51], and [94] that the power amplifier uses a significant portion of the energy used in a Macro-cell base station. The high amount of power is used to reach highly shadowed and high path loss destinations. The measurements conducted in [95] on network interface card (NIC) concluded that the RF power level should be at its lowest since it reduces the power consumption of power amplifier and it was also noted in [96] that the battery life is inversely proportional to the transmit power. Therefore to maximise the battery life (or total fixed amount of energy consumed in the case of externally powered nodes), each node has to reduce transmit power to a fixed level, which in some circumstances may result in a lower, but more energy efficient data rate per unit bandwidth. For these reasons, the scope of the energy consumption model of a device is limited to the transmission/reception components of wireless devices and combining all the various electronic components into a single module. The proposed radiated energy consumption model will be highly affected by the external radio environment unlike that of [20], [21].

The period in which a node spends time for transmission or reception ts of a file/data

is dependent upon its length fh to be transmitted or received and the channel capacity

which is mapped on to the SINR. Therefore the energy consumed by a node during transmission et and reception er for a period of ts:

5.13

5.14

Where Ptxi is the required transmitted power such that the receiver can successfully

receive the transmission and is dependent upon the required radiated transmitted power .; remains constant upon concurrent transmission and reception by a cluster head as unlike macro base station in which the power consumption scales with the number of connected users in a given time, the affect is negligible in smaller cells [97]. pe is the power consumed by all the electronic components in a wireless network

interface card and thus it is also when a device is not transmitting, i.e. power consumed during idle. As mentioned earlier, it was noted by [70] and [71] that the power consumed by IEEE 802.11 network interface during reception is only slightly greater than during an idle period, this is because during idle is continuously scanning the radio environment [98], [99].

As in [20] and [51], it is assumed that there is a linear relationship between transmitted power consumption Ptxi and the required transmitted uplink radiated

power . Achieving such efficiency and linearity below the saturated point of the power amplifier is still ongoing research by RF power amplifier designers [100]. The baseline measurements conducted by [95] on commercially available IEEE 802.11 wireless network card is used to understand the relationship between the radiated power and the power consumed during transmission. Their result is shown in Figure 5-8 which indicated that although the uplink radiated power and transmission power consumption Ptxi is not exactly linear, a reasonable approximation can still be

made using simply least squares regression.

Figure 5-8: The relationship between transmission power consumption of IEEE 802.11 and the actual radiated power.

Nodes must be awake throughout te to be able to receive files from its neighbours, this

is very energy inefficient at low offered traffic. In [20], although the authors noted that a node has to remain awake or idle during the steady state phase, it is assumed that energy is only consumed during transmission and reception only. Such assumptions however may not be applicable to higher powered devices such as network interface for IEEE 802.11b as the energy consumed during idle period is comparable to that during reception [70], [71]. Therefore the total energy consumed by the nodes in the network Et after a period of te assuming that the nodes are in idle

mode when it is not transmitting or receiving can be summarised as:

∑ (∑

)

5.16

Where n is the total number of nodes in the network, is the number of times node i transmit within te period, note that only cluster heads can receive a file.

Equation (5.16) assumes that nodes are awake or idle throughout te, which is the worst

case scenario in terms of energy efficiency during very low offered traffic. Power savings can be achieved by effectively turning off or forcing nodes into sleep mode when they are not transmitting or relaying files from their neighbours [101]. The effectiveness of such power management has been discussed in an earlier chapter. Such as approach however introduces a new set of problems such as how long does a node should remain turned off or in sleep mode and under such an event, is it more

energy efficient for cluster members to transmit to neighbouring nodes that are in idle period or transmit directly to HBS?.

Equation (5.17) presents the total energy consumption of nodes in the network in ‘active mode’ (transmission and reception), otherwise they are turned off or in a sleep mode. Since sleep mode can consume up to 100 times less energy than transmission mode it is therefore considered negligible [72]. Such an energy saving technique however assumes an ideal scenario as it would require the cluster heads to accurately predict and anticipate the occurrence of uplink transmissions such that it can serve its respective cluster members.

∑ (∑ ∑

)

5.17

Where nr is the number of time node i receive a file within te period.

The energy model as described above considers the energy consumed during files lost due to dropped transmissions. Such a model enabled us to understand energy consumption behaviour of a network under various traffic load, network planning and transmitted power.