## 1 Introduction
The potential mHealth application scenarios are numerous, including the active management of diseases such as diabetes (e.g., by measuring blood sugar levels and controlling the insulin dosage accordingly), the support for independent aging to the elderly (e.g., by tracking their medication intake and their activity level) and the monitoring of personal fitness activities to improve health and well-being (e.g., by logging health and fitness indicators during workouts) [6]. As the sensing technology advances, future mHealth applications are expected to emerge [7,8], such as cancer detection (through the monitoring of cancer cells), management of heart condition (through the monitoring of biochemical markers) and vision enhancement (through implanted retina prosthesis chips), to name a few. Overall, mHealth can offer significant benefits for both patients and healthcare providers, ensuring enhanced quality, efficiency, flexibility and cost reductions in healthcare delivery. The M2M paradigm in the context of mHealth involves the use of appropriate sensor devices on patients. The medical sensors, placed in the vicinity of, or inside, the human body, are usually interconnected through a short-range wireless technology, thus forming a Wireless Body Area Network (WBAN). An M2M-enabled gateway node collects all the sensory data from the WBAN and forwards them to a remote online server, where processing and integration with medical-related software applications take place. The connection of the gateway to the Internet is generally based on long-range communication access technologies for Wireless Local/Metropolitan Area Networks (WLANs/WMANs). The M2M technology paves the way for new possibilities for mHealth applications, by enabling the remote monitoring of vital signals [9], the early detection of critical conditions and the remote control of certain medical treatments [6]. The collection of large amount of data and their timely delivery to the healthcare providers in an unobtrusive way for the patient and without the need for human interaction can significantly facilitate the management of chronic diseases and speed up the diagnosis of critical conditions. In the recent years, the research community has been motivated by the diversity of applications, the promising benefits and the potential market opportunities of mHealth M2M solutions. The main technological challenges for M2M communications, the most representative usage models and the status of global standardization efforts are discussed in [10]. Focusing on the emerging M2M technologies for mHealth applications, a technical discussion on the communication network design is given in [11], but generally most of the related work adopts a high-level approach. In [12], some interesting challenges from the network perspective are identified, while interoperability issues and recent standardization efforts are presented in [13]. On a different level, a lot of research activity has been focused on the body area domain, on the design of medical sensor devices [14] and on the main advances and challenges in the field of WBANs [15, 16, 17, 18]. However, recent important work in the rapidly growing field of WBAN communication technologies have not been included in the aforementioned works, while the end-to-end holistic M2M approach is barely considered. In this context, this paper aims to provide a comprehensive survey on M2M systems for mHealth applications from a wireless communication perspective.