Low Energy Efficient Wireless Communication Network Design

Energy efficient wireless communication network design is an important and challenging problem. It is important because mobile units operate on batteries with energy supply. It is challenging because there are many different issues that must be dealt with when designing a low energy wireless communication system (such as amplifier design, coding, and modulation design), and these issues are coupled with one another. Furthermore, the design and operation of each component of a wireless communication system present trade-offs between performance and energy consumption. Therefore, the challenge is to exploit the coupling among the various components of a wireless communication system, and understand trade-offs between performance and energy consumption in each individual component, in order to come up with an overall integrated system design that has optimal performance and achieves low energy (power). The key observation is that constraining the energy of a node imposes a coupling among the design layers that cannot be ignored in performing system optimization. In addition, the coupling between layers requires simulation in order to accurately determine the performance. The purpose of this power is to present a methodology for the design, simulation and optimization of wireless communication networks for maximum performance with an energy constraint.

Before we proceed, we illustrate, through simple examples, a couple of issues that need to be addressed. To highlight the trade-offs between performance and energy consumption at individual components, consider the design and operation of an amplifier. The amplifier boosts the power of the desired signal so that the antenna can radiate sufficient power for reliable communications. However, typical power amplifiers have maximum efficiency in converting DC power into RF power when the amplifier is driven into saturation. In this region of operation, the amplifier voltage transfer function is nonlinear. Because of this non linearity, the amplifier generates unwanted signals (so called intermodulation products) in the band of the desired signal and in adjacent bands. When the amplifier drive level is reduced significantly (large back off) the amplifier voltage transfer characteristic becomes approximately linear. In this case it does not generate intermodulation products. However, with large back off the amplifier is not able to efficiently convert DC power into RF power. Thus, there is considerable wasting of power at low drive levels, but at high drive levels more interfering signal are generated.

To highlight the coupling among the design of individual components of a wireless system, consider packet routing in a wireless network that contain no base station (i.e. an ad hoc network). For simplicity consider a network with nodes A, B and C shown in figure. If Node A wants to transmit a message to Node C, it has two options. Transmit with power sufficient to reach Node C in a single transmission, or transmit first from A to B with smaller power, and then B to C. since the received signal power typically decays with distance as d4, there is significantly smaller power loss due to propagation in the second option because d^4ac>d^4ab+d^4bc.however even though Node A transmits with smaller output power, it does not necessarily proportionally decreases the amount of actually consumed because of the amplifier's effect discussed above.

Furthermore, besides the energy required for packet transmission, there are energy requirements for packet reception and information decoding. The probability of packet error reception that is achieved depends on energy allocated to the receiver. Consequently, there is a coupling among amplifier design, coding and modulation design, and decoding design as well as routing protocol.