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Top1. Introduction
It is predicted that the number of M2M devices (M2MD) will massively increase in the near future. Today, there are around 4 billion such devices, while in 2022 the number of M2MDs around the world is predicted to be 50 billion (R. Lu, 2011). M2M communication is expected have a huge commercial market and will be deployed on a large scale because of the number of attractive use-cases and the advantages it can bring. It is expected that M2MDs will dominate several markets, including smart home, retail, automotive and smart cities (R. Lu, 2011). The huge number of M2M devices face many challenges, especially with spectrum scarcity, standards inflexibility, security, and energy efficiency. These challenges need effective solutions before mass deployment of this technology by 2020.
The concept of CR was proposed by Mitola and Maguire (Mitola, 1999) and is a promising technology to solve the so-called spectrum scarcity problem. The main idea of CR is that the secondary user (SU) can sense and exploit licensed spectrum opportunistically and adapt its radio parameter to communicate over the spectrum of interest without affecting or interfering with the primary user (PU) (Mitola, 1999). CR can bring various advantages for M2M communications, including opportunistic spectrum usage and energy efficiency (Adnan, 2015). CR has proved to be green (R. Lu, 2011), as the transmission power levels in secondary networks can be adjusted based on the operating environment, without interfering with the main network or causing interference with other neighbour networks. Generally, M2MDs are expected to be low-cost and low-power and are designed to work for many years without battery replacement (Niyato, 2011). A number of M2M applications can benefit from the new functionality that can be achieved by the combination of cognitive radio and M2M communications, such as smart grid, healthcare, and car parking (Yao J, 2013). Energy efficiency in CM2M has recently gained attention in the published literature (Yao J, 2011). The current schemes for improving energy efficiency in CM2M networks mainly focus on spectrum discovery schemes such as the non-cooperative, cooperative and time-division energy-efficient schemes (Fadlullah et al, 2011). Further, the previous work addressed optimal power allocation to improve quality of service and energy efficiency in a CM2M network (Illanko, 2011). To the best of the authors’ knowledge, no previous work has considered spectrum handoffs and the wait/switch tradeoff in a CM2M network with multiple SU’s (i.e. CM2MD). In addition, no previous work has considered the scenario with multiple CM2MDs and addressed the collision among CM2MDs. In this paper, we propose an energy efficient mechanism for CM2M network by optimising spectrum sensing and switching mechanisms. The proposed mechanism guarantees sensing reliability and users’ throughput constraints simultaneously. In order to optimise overall energy consumption, our mechanism ensures that spectrum handoff is not used excessively. Instead, CM2MD may sometimes choose to stop transmission and stay in a sleep state on their current channel for a specific period of time, causing more delay and lower average throughput.