## 2 Related Work  
In the past few years, MIMO has established itself as a reliable and energy conserving technology for wireless networks. Recently, MIMO has been proposed in conjunction with sensor networks to improve energy conservation, network throughput and reliability in fading channels [9]. In [9], the authors propose a multiple-input multiple-output (MIMO) technique where multiple nodes within a cluster cooperate in signal transmission and reception. A cross-layer design is used for efficient routing and minimal energy consumption and delay. For cooperative MIMO scheme, routing criterion is set based on an equivalent single-input single-output (SISO) system, where each cooperating cluster is treated as a super node. In [5], authors present a detailed analysis of the dissipated power during a sensor node's operation. Authors extend the work presented in [4] where they analyze the best modulation and transmission strategy to minimize total energy consumption required to send a given number of bits. It is shown that traditional belief of MIMO systems being more energy-efficient than SISO systems in Rayleigh fading channels may be misleading when both the transmission energy and the circuit energy consumption are considered. The paper demonstrate that in short-range applications, especially when the data rate and the modulation scheme are fixed, SISO systems may outperform MIMO systems as far as energy efficiency is concerned. Nevertheless, if constellation size is optimized then MIMO outperforms SISO even for short distances. In [5] the authors have implemented a simple cooperative node selection algorithm to achieve higher energy gains in the MIMO approach, and examined how their algorithm affects the calculated thresholds. Moreover, authors have reached expressions to estimate threshold values regarding the channel conditions, the distance between source and destination nodes, and the network density, which determine the areas where the MIMO structure is more energy-efficient. The authors show that if channel conditions are unfavorable, multihop SISO approach proves to be more energy-efficient. They further argue that as the network density increases, gains are achieved due to multihop transmission. However, for large networks, single hop MIMO may not be practical. This work does not cover clustering aspect in sensor networks. Clustering is important for scalability and offers a reduced communication flow which is important for saving energy. In [3], authors propose a distributed MIMO-adaptive energy-efficient clustering/routing scheme, coined cooperative MIMO (CMIMO), which aims at reducing energy consumption in multihop WSNs. In CMIMO, each cluster has two cluster heads, which are responsible for routing traffic between clusters (_i.e._, inter-cluster communications). CMIMO has the ability to adapt the transmission mode and transmission power on a per-packet basis. The transmission mode can be one of four transmit/receive configurations: 1 \(\times\) 1 (SISO), 2 \(\times\) 1 (MISO), 1 \(\times\) 2 (SIMO), and 2 \(\times\) 2 (MIMO). It employs a dynamic clustering approach with two clusterheads in a cluster. There is a master CH (MCH) which gathers data from the sensory nodes in the cluster while the second one is a slave CH (SCH). The two CHs operate as a cooperative MIMO node for inter-cluster communications. The operation of CMIMO has three main phases: cluster formation, intra-cluster communications, and inter-cluster communications with cooperative MIMO capabilities. In this scheme, reclustering affects the whole network, and not only the cluster to which the MCH belongs. The new selection of clusterheads affects the functions of the nodes, as an MCH may become an SCH or a non-CH node. Similarly, an SCH may also become an MCH. Thischange in nodes' functions necessitates reclustering the whole network. Reclustering in CMIMO is an energy intensive task which in worst case may require the whole network to get reclustered. Further, there are no bounds on the number of cluster pairs in the network.