Joint optimal relay location and power allocation for ultra-wideband-based wireless body area networks

Ding et al. EURASIP Journal on Wireless Communications and Networking Joint optimal relay location and power allocation for ultra-wideband-based wireless body area networks Jie Ding 0 Eryk Dutkiewicz 0 Xiaojing Huang 1 0 Department of Engineering, Macquarie University , Balaclava Road, North Ryde, NSW, 2109 , Australia 1 University of Technology , 15 Broadway, Ultimo, NSW, 2007 , Australia In this paper, we study the joint optimal relay location and power allocation problem for single-relay-assisted ultra-wideband (UWB)-based wireless body area networks (WBANs). Specifically, to optimize spectral efficiency (SE) for single-relay cooperative communication in UWB-based WBANs, we seek the relay with the optimal location together with the corresponding optimal power allocation. With proposed relay-location-based network models, the SE maximization problems are mathematically formulated by considering three practical scenarios, namely, along-torso scenario, around-torso scenario, and in-body scenario. Taking into account realistic power considerations for each scenario, the optimal relay location and power allocation are jointly derived and analyzed. Numerical results show the necessity of utilization of relay node for the spectral and energy-efficient transmission in UWB-based WBANs and demonstrate the effectiveness of the proposed scheme in particular for the around-torso and in-body scenarios. With the joint optimal relay location and power allocation, the proposed scheme is able to prolong the network lifetime and extend the transmission range in WBANs significantly compared to direct transmission. Body area networks; Spectral efficiency; Optimal relay location; Implant sensor node - Introduction With the decreasing size and increasing capability of electronic devices, the wireless body area network (WBAN) is an enabling technology for pervasive healthcare by using several small and portable sensors on/in the human body [1,2]. For WBANs, the IEEE 802.15.6 standard has specified impulse radio ultra-wideband (IR-UWB) as its physical layer technology, owing to its simple electronics, high data rate capacity, and low power consumption, which is less likely to affect human tissues and cause interference to other medical equipments [3]. In healthcare applications, WBANs may be employed to monitor the vital signs of a patient, where spectral efficiency (SE) and reliability of emergency signal transmission are vitally important for the patients life. However, the achievable SE of direct transmission in UWB-based WBANs is often unsatisfactory because of the propagation blockage from the body torso as well as the limited transmit power due to the UWB regulatory limitations. Thus, SE is one of the most critical concerns in WBANs [4]. On the other hand, relay-assisted cooperative transmission has drawn considerable attention in wireless networks which can improve the information rate and link reliability effectively [5]. Considerable studies have been conducted in wireless sensor networks (WSNs) regarding the SE of the relayassisted communications [6-8]. In [6], power allocation between a source and relay was optimized to maximize the SE in single-relay-based cooperative networks. In [7], joint relay selection and power allocation strategy for multiuser amplify-and-forward (AF) networks were studied to maximize users SEs. In [8], power allocation and relay selection schemes were proposed to achieve the maximum SE and minimum outage probability for multiple relay-assisted cooperative networks. These studies reveal that cooperative transmission is an effective way that can greatly increase the SE in WSNs. Unlike WSNs, UWB-based WBANs have some unique properties such as analog transmission, distinct channel characteristics, and limited network size, where the signal strength in a WBAN is mostly affected by the physical location of the nodes in relation to each other as well as the human body [9]. As a result, the aforementioned existing schemes and results on the SE in WSNs may be inadequate if they are applied to UWB-based WBANs directly. This fact motivated us to study the SE of relay-assisted cooperative transmission in UWB-based WBANs. For cooperative transmission, some related studies have been conducted in WBANs [10-16]. Particularly, channel modeling and system diversity were analyzed in [10]. In [11], the energy efficiency of cooperative transmission was investigated with constrained outage probability. Cooperative scheduling schemes were proposed to decrease inter-BAN interference and increase packet reception rate of intra-BAN communications in [12]. In [13] and [14], packet error rate performance evaluation of two-hop links against the direct link was presented over narrow-band channels. In [15], the energy efficiency of cooperative transmission was considered from a relay selection perspective for UWB-based WBANs and the energy-efficient performance was analyzed with different relay regions. In [16], the reliability and energy efficiency of two-hop cooperative communication was assessed theoretically in terms of outage probability and bit error rate for narrow-band medical services. Although these studies have demonstrated that cooperative communication can be effectively implemented in WBANs, the SE of cooperative communication is still an open issue for UWB-based WBANs. Furthermore, to the best of our knowledge, research on the SE of cooperative communication related to the optimal relay location (RL) and power allocation (PA) in UWB-based WBANs has been rarely conducted, especially for scenarios involving both onbody to on-bodyand in-body to on-body propagation links. To this end, this work aims to solve the joint optimal RL and PA problem to optimize the SE in single-relay-assisted UWB-based WBANs. Three practical transmission scenarios are investigated herein, which are along-torso scenario, around-torso scenario, and in-body scenario, respectively. Each scenario refers to a specific physical location between source and destination nodes in relation to each other. For each scenario, we seek the relay with the optimal location to achieve the maximum SE, together with the corresponding optimal power allocation. More precisely, generic relay-location-based network models are proposed for UWB-based WBANs firstly. Taking into account realistic power considerations for each scenario, the SE optimization problem is then mathematically formulated and the optimal RL and PA are jointly derived to achieve the maximum SE. The analysis on the optimal RL and PA is given accordingly. Numerical results show the necessity of utilization of relay node for the spectral and energy-efficient transmission in UWB-based WBANs and reveal that the relay location is an influential parameter in WBANs. By utilizing an on-body relay node with the joint optimal relay location and power allocation, the transmission range in WBANs can be extended effectively and the power consumption can (...truncated)