Energy efficiency of MIMO and Cooperative MIMO Techniques in Sensor Networks

We consider radio applications in sensor networks where the nodes operate on batteries so that energy consumption must be minimized while satisfying given throughput and delay requirements. In this context, we analyze the best modulation and transmission strategy to minimize the total energy consumption required to send a given number of bits. The total energy consumption includes both the transmission energy and the circuit energy consumption. We first consider MIMO systems based on Alamouti diversity schemes, which have good spectral efficiency but also more circuitry that consumes energy. We then extend our energy-efficiency analysis of MIMO systems to individual single-antenna nodes that cooperate to form multipleantenna transmitters or receivers. By transmitting and/or receiving information jointly, we show that tremendous energy saving is possible for transmission distances larger than a given threshold, even when we take into account the local energy cost necessary for joint information transmission and reception. We also show that over some distance ranges, cooperative MIMO transmission and reception can simultaneously achieve both energy savings and delay reduction.
Recent hardware advances allow more signal processing functionality to be integrated into a single chip. It is believed that soon it will be possible to integrate an RF transceiver, A/D and D/A converters, baseband processors, and other application interfaces into one device that is as small as a piece of grain and can be used as a fully-functional wireless sensor node. Such wireless nodes typically operate with small batteries for which replacement, when possible, is very difficult and expensive. Thus, in many scenarios, the wireless nodes must operate without battery replacement for many years. Consequently, minimizing the energy consumption is a very important design consideration and energy-efficient transmission schemes must be used for the data transfer in sensor networks. Multi-antenna systems have been studied intensively in recent years due to their potential to dramatically increase the channel capacity in fading channels . It has been shown that Multi-Input Multi-Output (MIMO) systems can support higher data rates under the same transmit power budget and bit-error-rate performance requirements as a Single-Input Single-Output (SISO) system. An alternative view is that for the same throughput requirement, MIMO systems require less transmission energy than SISO systems. However, direct application of multi-antenna techniques to sensor networks is impractical due to the limited physical size of a sensor node which typically can only support a single antenna. Fortunately, if we allow individual single-antenna nodes to cooperate on information transmission and/or reception, a cooperative MIMO system can be constructed such that energy-efficient MIMO schemes can be deployed.

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