Published in IET Wireless Sensor Systems Received on 5th December 2010 Revised on 23rd February 2011 doi: 10.1049/iet-wss.2010.0094 ISSN 2043-6386 Wireless sensor communication system based on direct-sum source coder H.T. Nguyen 1 T.A. Ramstad 1 I. Balasingham 2,3 1 Department of Electronics and Telecommunications, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway 2 Department of Electronics and Telecommunications, NTNU, NO-7491 Trondheim, Norway and Interventional Center, Oslo University Hospital, NO-0027 Oslo, Norway 3 Institute of Clinical Medicine, University of Oslo, NO-0317 Oslo, Norway E-mail: hieu.nguyen@iet.ntnu.no Abstract: The authors propose a low delay and low complexity communication system for wireless sensor networks based on a so- called direct-sum source coder. The source encoder converts a continuous-amplitude signal into an n-tuple binary representation directly by a scalar quantisation. The authors derive the mean-square error distortion of the proposed scheme from which the power allocation strategy can be optimally designed for each of the quantisation bits. The authors also consider the performance of a conventional system where the Lloyd–Max source coder/decoder is employed. The simulation and analytical results show that the proposed system offers superior performance compared with the conventional system with low-system complexity and low delay. Furthermore, this investigation reveals that there is no performance gain obtained from the optimal power allocation over the equal gain allocation for low signal-to-noise ratio, when no channel state information is available at the transmitter side. Based on this observation, a well-structured form of power allocation for the whole SNR range is achieved. 1 Introduction 1.1 Motivations In this paper, we propose a communication system based on a simple source encoder for wireless sensor networks. We focus on a scenario where no error-correcting coding is used because of the complexity constraint in wireless sensor networks. Wireless sensor networks are expected to have several potential and attractive applications. The application of wireless sensor networks in medical treatment and healthcare, so-called medical sensor networks, has received great interest recently. See [1] and references therein. In medical wireless sensor networks, medical sensors are placed inside and/or on a patient’s body to continuously track the patient’s physiological states, so that it can help to improve the mobility for both the patient and the hospital staff. Medical wireless sensor networks can also be utilised as the in-home healthcare and emergency response system [1]. From an engineering perspective, wireless sensor networks are generally subject to stringent resource constraints: low hardware complexity and low power consumption, etc. For battery-operated sensor networks, the energy consumption is one of the most important design parameters since replacing batteries may be difficult or impossible in many applications. The works on energy efficiency for wireless sensor networks can be found in [2–5]. For a particular in-body wireless sensor network, the sensor resides inside a pill that patients can swallow, thus facilitating to capture videos of her/his internal parts [1]. The miniaturisation is the toughest design constraint, hence the electronic device is restricted in transmitting power, and its circuit complexity must be maintained at as low level as possible [1, 6]. Another obstacle of the medical in- body wireless sensor is that the radio wave propagating via the medium of the human body tissues suffers severe attenuation [7]. Therefore the requirement on energy- efficient communication systems for medical wireless sensor networks is critically demanding in coupling with the low complexity requirement. Motivated from the above observations, in this paper we propose a communication system for wireless sensor networks with a low complexity and delay source encoder, called direct-sum source coder. The proposed source coder does not use either codebook or entropy coding as in many conventional communication systems. The analytical and simulation results have shown that the proposed communication system performs better than the conventional one provided that the output sequence’s characteristic of the source coder is taken into consideration. We will present the detailed descriptions of the proposed communication system and its performance in later sections. 1.2 Related works The source encoder for the continuous-amplitude and time- discrete signal class such as sampled speech and images usually consists of two components. The first component is 96 IET Wirel. Sens. Syst., 2011, Vol. 1, Iss. 2, pp. 96–104 & The Institution of Engineering and Technology 2011 doi: 10.1049/iet-wss.2010.0094 www.ietdl.org