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A Comparison of ZigBee/802.15.4 vs. Ultra-Wideband Transmission Techniques for Wireless Fire and Intrusion Detectors

Recently, wireless sensor networking has been becoming a disruptive technology in such fields as industrial control and monitoring, building automation and security, automotive sensing. The motivation for use of wireless technology includes the reduction of installation cost, the avoidance of the “last meter connectivity problem” and intelligent maintenance. Many types of wireless sensors, for example fire and intrusion detectors, are based on proprietary solutions, which are uncomfortable for the customer because of lack of product interoperability and vendor dependence. On the other hand, the lack of standardized technologies that can address the requirements both at the application level and from the communication point of view has become the hurdle of the widespread of wireless sensor systems. The above constraints suggest the necessity of an open global standard to enable the cost-effective, reliable wireless monitoring and control products. The recently finished IEEE 802.15.4 standard and the corresponding undergoing Zigbee standard represent the achievements of the efforts addressing these kinds of requirements. Zigbee/802.15.4 operates at 868/915MHz, or 2.4GHz and belongs to classical narrowband transmission systems. An entire opposite transmission approach is based on extremely low power signals with ultra-wide bandwidth (UWB). UWB belongs to the few wireless key emerging technologies and is under intense discussion for various areas of applications, e.g. highest data rates in the order of several hundred MBit/s for indoor communications, ranging and positioning with accuracy in the sub-centimeter range, and recently also for low power and low data rate applications. Aim of this contribution is to compare narrowband vs. ultrawideband transmission with special emphasis on wireless fire and intrusion detector networks. Introduction The Zigbee/802.15.4 standards, with such features as ultra low complexity, ultra low power and low bit rate, are ideally suited for wireless fire and intrusion detectors. Numerous wireless fire and intrusion detectors are based on proprietary solutions, which are uncomfortable for the customer because of lack of product interoperability and vendor dependence. From this point of view ZigBee seems to be the right choice in order to serve future market demands. On the other hand, in early 2004 the Institute of Electrical and Electronics Engineers (IEEE) 802.15 Working Group voted to form a new study group to investigate amendment to 802.15.4 for an alternative physical layer with additional features like precision ranging/location capability, high aggregate throughput, scalable data rates, and extremely low power consumption. All the features can be principally achieved with Ultra-Wideband Technology. The aim of this contribution is to shed some light on the pros and cons of Zigbee vs. UWB with emphasis on their application for wireless fire and intrusion detectors. For example, because ZigBee is a conventional narrowband technique operating with a carrier frequency of either 2.4 GHz (Global), 915 MHz (North American) or 868 MHz (Europe) it suffers from other narrowband interferences as well as the known fading phenomenon. In contrast, UWB may not only mitigate narrowband interferers simply by notch filtering or other advanced signal processing techniques, but also mitigates fading by non-destructive impulsive transmission. For both approaches, the very low duty cycle guarantees low power consumption and co-existence, but the lack of severe fading in UWB transmission may result in a reduced path loss and consequently in a larger coverage. However, according to current American regularization, UWB systems are allowed to operate mainly in the frequency range between 3.1 GHz and 10.6 GHz with a maximum power spectral density of -41.25 dBm/MHz, resulting in a maximum transmit power of less than 0dBm. Compared to Zigbee, this higher frequency range causes additional attenuation by walls and other obstacles and therefore limits the coverage gain. In turn, due to long spreading sequences, UWB devices can principally be designed to work below the noise floor so that jamming becomes extremely difficult. Such a property is beneficial especially for intrusion detectors. Note also the pulse duration in the sub-nanosecond range providing a fine time resolution and enabling precise ranging with centimeter resolution. All these facts open the question whether UWB represents a valuable alternative to standardized narrowband transmission systems. The following section summarizes the properties of UWB whereas the next section deals about Zigbee/802.15.4. Then, the individual link budgets are calculated and compared for different carrier frequencies, typical data rates in wireless fire and intrusion detectors, and bandwidths in order to allow a quantitative comparison. A conclusion and an outlook finalize this contribution. The Ultra-Wideband Approach Ultra-wideband systems are considered as systems that are operating with a large absolute bandwidth of more than 500 MHz. Using a large bandwidth in UWB systems provide us with several possibilities and advantages compared to other narrowband systems. First, the spectral density of the transmit signal can be made rather small and consequently the probability of interference to narrowband systems is greatly reduced. Immunity to narrowband interference, mitigating fading, high data rate and a better propagation in different environments are among other advantages of UWB systems [1]. In February 2002, the American Federal Communication Commissions (FCC) issued its first report and order on UWB technology, thereby providing regulations to support deployment of UWB radio systems. After that, a great deal of efforts and money has been invested in finding solutions for implementation of this technology in practice. Modulation The most popular current modulation schemes are Pulse-Position Modulation (PPM), Pulse-Amplitude Modulation (PAM) and Orthogonal Frequency Division Multiplex (OFDM), whereas the extensions Direct Sequence (DS) and Time Hopping (TH) enable multiple accesses to the common transmission medium. PPM encodes the data through different positions in time and PAM encodes the data through signal amplitudes. TH assigns to multiple different codes, where the codes in turn represent a particular pattern in a series of pulses per transmitted bit, i.e. the transmitted signal in time hopping is represented by