Channel Estimation for Relay-Based M2M Two-Way Communications Using Expectation-Maximization

The growing popularity of machine-to-machine (M2M) communications in wireless networks is driving the need to update the corresponding receiver technology based on the characteristics of M2M. In this paper, an expectation-maximization-based maximum likelihood cascaded channel estimation method is developed for relay-based M2M two-way communications. As the closed-form solution of maximum likelihood channel estimation does not exist, and the superimposed signal structure at the receiver is conducive to the expectation-maximization application, the expectation-maximization algorithm is utilized to provide the maximum likelihood solution in the presence of unobserved data through stable iterations. Even in the absence of the training sequence, the cascaded channel estimates are obtained through the expectation-maximization iterations. The Bayesian Cramér-Rao lower bounds are derived under random parameters for the channel estimation, and the simulation demonstrates the validity of the proposed studies. 1. Introduction Machine-to-machine (M2M) communication is expected to be one of the major derivers of cellular networks [1, 2] and has become one of the focuses in 3GPP [3]. M2M technology allows machine devices to communicate directly with each other without human intervention, which has been attracting more and more interests for their wide applications in many popular wireless communication systems, such as mobile ad hoc networks [1–3]. Wireless M2M networks supporting M2M-enabled machine devices are pivotal to the success of M2M. Sensor nodes in wireless M2M networks could be connected by a wide range of wireless network technologies, for example, cellular networks. Cooperation is crucial in wireless networks as it can greatly attribute to ensuring connectivity, reliability, and performance. Relaying is promising in a wide variety of network types for the cooperation purpose. According to the recent IEEE 802.16p proposals, a wireless M2M device may act as an aggregation point and communicate data packets on behalf of the other M2M devices, which may have a poor communication link to the network. Relay-based schemes have been proposed to improve the link reliability for devices with weak links [4–7]. As a basic model of the relay-based M2M two-way communications, the three-node cooperation model, which is also referred to as the two-way relay channel, where two source nodes exchange information via a relay node using amplify-and-forward (AF) or decode-and-forward (DF) strategy, has attracted a great deal of research interest because of its improved