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Wireless World Research Forum (WWRF)

White paper Ultra Wideband: Technology and Future perspectives1 V3.0, March 2005

Editor: Ben Allen, King’s College London, UK [email protected]

Co-editors: Tony Brown, University of Manchester, UK Katja Schwieger, Ernesto Zimmermann, University of Dresden, Germany Wasim Malik, David Edwards, University of Oxford, UK Laurent Ouvry, LETI, France Ian Oppermann, University of Oulu, Finland Abstract Ultra Wide Band (UWB) communications offers a radically different approach to wireless communication compared to conventional narrow band systems. Global interest in the technology is huge. This white paper reports on the state of the art of UWB wireless technology and highlights key application areas, technological challenges, higher layer protocol issues, spectrum operating zones and future drivers. The majority of the discussion focuses on the state of the art of UWB technology as it is today and near future.

Contents 1. 2. 3. 4. 5. 6. 7. 8. 9.

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Introduction Applications Focus on Technology UWB MAC Considerations Spectrum Landing Zones What Ever Next? Summary References List of Contributors

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The work towards and contained in this white paper has been derived independently from that previously published in the WWRF white paper “Pervasive Ultra-Wideband Low Spectral Energy Radio System (PULSERS)”, November 2002. 1


1.

INTRODUCTION

Ultra Wide Band (UWB) communications offers a radically different approach to wireless communication compared to conventional narrow band systems. Global interest in the technology is huge. Some estimates predict the UWB market will be larger than the existing wireless LAN and Bluetooth markets combined by year 2007 [1]. This is due to the capability of these license exempt wide bandwidth wireless systems to yield low cost, short range, extremely high capacity wireless communications links. The actual achievable data rate naturally depends on the particular technology and propagation conditions; figure 1 shows typically quoted data rates for UWB links, with 500 Mbps at 2 m range, 110Mbps at 10m used as conservative figures. The use of UWB has already been deregulated in the USA, Singapore is set to follow shortly with Japan, China and elsewhere not far behind. The European position on deregulation, at the time of writing, is unclear although substantive work is being undertaken by CEPT and others [2] to produce a co-ordinated approach across Europe. However Europe has considerable activity in UWB. In addition to ETSI developing regulations, the EU, as part of its IST (Information Society and Technologies) initiative, has funded a number of projects including UCAN (UWB Concepts of Ad-Hoc Networks), PULSERS (Pervasive Ultra-Wideband Low spectral Energy Radio Systems) and ULTRAWAVES (ULTRA Wiideband Audio Video Entertainment System). The publication of this white paper is therefore highly opportune, bringing into focus the application and implementation of this exciting new technology.

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UWB Data Throughput 1000 900 800 700 600 500 400 300 200 100 0 0

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Figure 1. UWB data throughput- typically quoted system performance 1.1

Setting the Scene

The fundamental concept behind UWB communications, that of using a low peak power which is spread over a wide (typically an octave or more) bandwidth is hardly new; indeed the very earliest experiments of Hertz used a spark gap generator in effect producing UWB signals! Historically, wide bandwidth radio has used extremely narrow impulses (typically between 0.2 and 0.5 nanosecond pulse width) to provide the communications link. These approaches are hence known as ‘impulse radio’ (IR) or ‘carrier free’ systems since a conventional carrier frequency is not present. More recently multiple sub-band systems have been developed using more conventional narrow band techniques occupying a number of sub bands, which together utilise the available UWB spectrum. Even though not all modern UWB systems employ impulse transmission, it is 2

Wireless World Research Forum (WWRF) appropriate to consider a ‘birth date’ of the technology in the early 1960’s [3] when time domain electromagnetics was being developed for circuit characterisation using short duration impulses. During the 1970’s and early 1980’s the emphasis was on using wide band, low power, impulse signals in radar and other defence related applications. The low spectral power density makes these systems attractive to the military due to their inherent low probability of intercept. Equally, the broadband nature of the signal is effective in allowing radar analysis of complex environments such as that encountered in certain ground probing radar. More recently, UWB position location is being developed to determine the location, and tracking, of moving objects within an indoor space to an accuracy of a few centimetres or less. The application of UWB to ubiquitous commercial communication scenarios was not seriously considered in these early years due principally to the cost of implementation and an unclear market need. By the late 1980’s digital technology had improved to a point that the commercial practicality of low power wide bandwidth communications could be clearly demonstrated. The growth throughout the late 1990’s of mobile multimedia communications gave a clear market imperative for high data rate wireless communications. Current wireless LAN technologies are capable of up to 54Mbps up to distances of 150m (e.g. IEEE 802.11, also known as Wi-Fi). Equally Bluetooth (FHSS) provides a short range 2Mbps data rate. All these systems are inherently bandwidth limited. While improvements in technology may be anticipated, the inherent narrow band nature of the systems will limit capacity. Comparison with the 500Mbps 2over 2 to 4m range of UWB systems shows why this technology is generating such interest. The low spectral power density of UWB technology in principle allows UWB to coexist with existing systems (with some caveats) and hence eases de-regulation of their use, opening the way for UWB to become the short range wireless technology of choice for many systems. As an example of application growth, a real time UWB link between a handheld camcorder and plasma television was demonstrated in May 2004 [4]. The attractions of UWB in the high data rate communications environment can be summarised as the potential to deliver ultra high speed data transmission (potentially 1Gbps over short distances [5]; some experts even expect data rates of 10 Gbps in 2006), coexisting with existing electrical systems (due to the extremely low power spectrum density) with low power consumption using a low cost one-chip implementation. UWB equipment can also be used in lower rate applications such as indoor location determination of people and assets, or other sensor network related applications. In this case, the available channel capacity is used to service a large number of lower data rate devices. Currently, global growth is inhibited by the lack of a clear operational standard. Equally important, the timetable for deregulation of such systems in some countries is unclear; a key point here being the interoperability (or otherwise) of UWB with existing users. The issue here is achieving agreement on the spectral emission mask necessary to avoid undesirable interference levels to and from UWB wireless systems. It is possible that different regions of the globe may require different emission standards. 1.2

UWB Communication Technologies

Modern UWB radio includes both impulse and multi-band solutions. As noted above, the adoption of the FCC emission regulations has provoked a number of differing technological solutions all capable of meeting the FCC emission mask and hence using the UWB frequency ranges. For impulse radio, information is carried in a set of narrow duration pulses of electromagnetic energy. Approximately, the bandwidth required is inversely proportional to the pulse width- so for example a 1ns pulse has a bandwidth of 1GHz (Note this is only approximately true due to the actual shape of the transmitted pulse in practice). The centre frequency depends on the ‘zero crossing rate’ of the waves making up the pulse. This is the same type of pulse structure as could be used in high resolution radar for example. In UWB communications, information can by carried by consideration of various properties of subsequent pulses. 2

As of March 2004, at least a1Gbps over a 2 metres appears technologically practical 3


In multi-band solutions, the allowable UWB spectra is split into a number of sub-bands allowing the use of a modified form of narrow band techniques within each sub-band. An important example of this is the proposed use of OFDM within the ultra wide band context. Examples of these classes of UWB systems include: •

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1.3

Proposed OFDM Pulsed Multiband Approach. In this approach, the full UWB spectrum is split into a number of sub-bands each of 528MHz width. Each 528MHz channel comprises 128 carriers modulated using QPSK on OFDM tones. The composite signal occupies the 528MHz channel for approximately 300ns before switching to another channel. In this way seven time-frequency hopping OFDM signals occupy approximately 3.7 GHz of bandwidth, 14 would occupy 7.4GHz, etc. Direct Sequence (DS-UWB). Derived directly from secure military communications, a direct sequence spread spectrum can be designed to occupy 1.5GHz at the lower part of the UWB spectrum and 3.7GHz at the high edge [6]. By using a combination of both bands with MBOK modulation (Multi-level Bi-Orthogonal Keying) and QPSK yields a data rate of 448Mbps Time Modulated UWB (TM-UWB). This uses extremely short pulses (less than one nanosecond) with a variable pulse-to-pulse interval. The interval variation is measured to produce information flow across the link, including the required information plus a channel code. A single bit of information may be spread over multiple pulse pairs and coherently added in the receiver. Since TM-UWB is based on accurate timing, it is well suited to both communications and distance determination. Systems are commercially available demonstrating bit rates of approximately 10Mbps at 40m. Regulations and Standards

A major landmark in the development of UWB has been the publication in early 2002 of the Federal Communication Commission (FCC) Order and Report deregulating the use of UWB systems in the USA. The FCC has defined a UWB system as having bandwidths greater than 20% of the centre frequency, measured at points 10 dB down from the peak level, or RF bandwidths greater than 500 MHz, whichever is smaller. There are a number of key points to the related emission regulations (US 47 CFR Part 15(f)). Firstly, to avoid inadvertent jamming of existing systems such as GPS satellite signals, the lowest band edge for UWB for communication purposes is set at 3.1GHz, with the highest frequencies 10.6GHz. Within this operational band, emission must be below –43 dBm/MHz EIRP- a limit the FCC have stated to be conservative. Importantly, the FCC deregulation does not specify nor imply any particular implementation technology. There are currently at least five identifiable UWB technologies that have been designed to comply with the FCC legal limits (as indicated in the previous section). Each has is proponents and detractors, each has its own technical benefits and restrictions and none are mutually compatible! It should also be noted that the FCC has deregulated UWB technology for other applications as well as communications- for example various types of imaging and vehicular applications. The defined allowable spectrum is different for different applications. While this deregulation has obviously had a major impact on the USA’s development of UWB, other countries are also progressing the regulatory framework. Of particular note is Singapore, which has established an aggressive schedule for deregulation. Experimental licenses have been granted for UWB with an EIRP limit 6dB higher than the FCC specification. These will be assessed before the final deregulation standard is formalised. Japan has granted experimental licenses for the demonstration of UWB with deregulation stated for a 2004/5 timeframe. The ITU is preparing reports in a similar timeframe. The legal situation in Europe has generally yet to be clarified. There is, however, very substantial activity. Of considerable concern has been the system co-existence issue of potentially a large number of UWB users operating with existing licensed spectrum users. It may be that the 4

Wireless World Research Forum (WWRF) European deregulation will modify the spectral mask of UWB around fixed link operational frequencies (approximately 3 to 5 GHz ) with a 20dB reduction over the FCC mask in these areas. With this as a background, UWB has generated an enormous interest within various standard forums. A UWB physical layer is under development within the IEEE 802 LAN/WAN forum, group 3a of IEEE P802.15 where UWB has been considered for an alternative radio physical layer for the emerging Wireless Personal Area Network (W-PAN ) standard. This is seen as a key standard and has been the subject of much controversy over competing technology such that, at the time of writing this standard has not yet been decided. Competing alternative technologies are very different. Each technology can provide the targets data rates of at least 110 Mbps to 480 Mbps over a personal area network distance from to 2 to 10 m. Which ever standard is eventually adopted, UWB systems can significantly out-perform competing wireless communications systems operating in license-free spectrum by virtue of the large available bandwidth despite the significantly lower EIRP levels. They also have unique and interesting propagation characteristics. UWB systems can also provide opportunity for distance and position finding information. As such these are likely to be considered as possible physical layer definition by the IEEE 802.15.4 a group. This is also under consideration, with longer range (up to 100m) applications in mind, and includes wireless sensor networks with high power efficiency requirements. 1.4

Future Challenges

Once the standards and regulation positions have been resolved, there is little doubt that deployment and growth in the use of UWB will be rapid. One area of future interest is to increase the data achievable rate at longer ranges (10m to 30m). As would be expected, this is limited both by the total energy in a particular UWB pulse structure and by propagation channel characteristics. For indoor application any system can be dominated by multipath considerations. As an approximate guide for UWB, a propagation characteristic inversely proportional to the distance cubed is often quoted3 [8]. The long delay spread caused by the multipath can have both positive and negative implications. On the positive side, the multipath arrivals will undergo less amplitude fluctuations (fading) since there will be fewer reflections that cause destructive/constructive interference within the resolution time of the received pulse. However the multipath also causes the channel to be dispersive – that is the output received signal is significantly different to the input pulse. Figure 2 illustrates the received signal for a 2ns input pulse width in a complex, non line of sight indoor environment.

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The validity of these models in cases where communication is not line of sight (for example through a office partition) is still under investigation 5


Figure 2. Effect of a complex indoor environment on impulse transmission over 30m range, nonline of sight conditions To produce higher data rates at these longer ranges will require much greater understanding of the propagation characteristics. For example, the antenna characteristic (itself a dispersive structure) couples strongly to the propagation problem. It may be that better control of the antenna characteristics (though still commensurate with low cost implementation) could give significant performance enhancement. UWB based indoor networks capable of extremely high data rates are clearly technologically feasible. However, optimum routing and network configuration is still a research topic, complicated by the unique propagation characteristics of UWB. One can foresee the UWB based indoor networks being evolved over the next few years. Whether future regulators will allow further extension of the UWB band will depend, at least in part, on the results of the various system coexistence studies currently being performed. With the use of spectrum ‘notching’ techniques around critical areas extension of bandwidth is a real possibility and, providing crucial technologies such as the antenna can be solved, should yield still further increase in short range data rates. It remains to be seen just how the propagation channel will limit this with range. UWB technology will improve with time. Advances in transmitter and receiver design, including the antenna, will improve efficiency and performance. In this regard commercial UWB technology is still in its infancy- the future looks exciting.

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Wireless World Research Forum (WWRF) 1.5 Structure The remainder of this white paper elaborates on the above discussion and is structured as follows. Section 2 describes perceived applications of UWB technology for communications. Section 3 then focuses on technological aspects of UWB. Medium Access Control (MAC) for UWB are discussed in section 4. Section 5 describes spectrum issues that relate to the transmission of UWB signals. Some future perceptions of UWB technology are presented in section 6, and the white paper is finally summarised in section 7.

2.

APPLICATIONS

Future wireless networks are envisaged to provide users with their desired information anywhere and at any time, by working seamlessly with other communication infrastructure. Example high rate applications in this context comprise access to the information backbone and distribution of multimedia content in home and public access environments as well as the replacement of wire based networks in single-office/small business environments. The inherent features of Ultra Wideband (UWB) make this technology a promising candidate for the above mentioned scenarios. Moreover UWB is resilient against multipath fading, making it perfectly suitable for indoor environments. Coexistence with other wireless systems and robustness against jamming (or, in this context, interference from other spectrum users) helps to ensure easy deployment. Last but not least, the high-resolution position location capabilities enable new applications such as location aware content provision in offices and public areas in general. The above mentioned notion of UWB implies the provision of high data rates. Conversely, the distinct potential of UWB in low data rate applications where location estimation may be desired is owed to its inherent high temporal resolution due to the large signal bandwidth. This enables precise location determination and tracking. Based on those capabilities, new paradigms in protocol design, location aware computing, location determination etc. are feasible. This paves the way for innovative low data rate applications, e.g. the identification of asset and their exact positions are useful in logistics. Wireless Body Area Networks WBAN) are yet another feasible application, facilitating medical supervision, which may improve the quality of life of patients. Moreover, the potential for transceiver simplicity yields low cost, low power devices. This allows for a long lifetime of battery operated nodes making large sensor networks for (indoor/outdoor) surveillance, smart homes and security applications viable [12]. Such low power implementation is considered feasible with careful signal and transceiver architecture design. The origins of modern UWB radio technology date back to the late 1960s [13]. Since then its application range has been extended tremendously. At the beginning radar technology pioneered a basic understanding of non-sinusoidal (pulse) signals and the development of working systems. Later, GPR (Ground Penetration Radar) became popular opening the way for detecting hidden objects. Applications never thought of before became possible: discovery of underground water resources, landmine detection, detecting cracks in rocks – a life-saving feature in mining, etc. Extending the idea of discovering invisible targets, nowadays radar technology is further developed for imaging systems employed in security systems and for biomedical imaging applications. This technique can reveal hidden objects or subjects, not perceptible with the human eye [14]. Wireless communications using UWB follows a completely different technical approach compared to traditional radar, even though in both applications large-bandwidth signals are used. In radar, the transmitted signal is known and the communication channel is unknown, i.e., radar can be interpreted as estimating the channel and trying to extract distinct features of a channel. On the other hand, in communications, the transmitted signal is unknown and has to be estimated using at least certain knowledge of the channel. Thus, the extension of UWB to wireless communications is a completely new approach, yet just utilizing known concepts. 7


Considering already established wireless communications systems, such as narrowband and optical communications, UWB technology needs to verify that it can offer benefits that other systems cannot provide. Optical communications for example usually requires line-of-sight for efficient operation, whilst narrowband radio communications systems would require an extremely high transmit power to operate at the envisaged high data rates that UWB can obtain. In the future an abundant deployment of devices communicating with high bandwidth signals will become common, penetrating our daily life at home, in industry and in logistics – just to mention a few example areas. The remainder of this section focuses on communication based applications, which forms the context of this paper.

2.1. Low data rate applications Here, impulse radio is considered as one technology accomplishing UWB operation. With careful signal and architecture design, the transmitter can be kept much simpler than with conventional narrowband systems, permitting extreme low energy consumption and thus long-live batteryoperated devices, which are mainly used in low data rate networks with low duty cycles. Nevertheless, the receiver design remains the major challenge for IR-based systems. As the number of resolvable multipath components is much higher than in narrowband signals, traditional Rake-receivers are too complex to be implemented in low energy devices. Energy detection receivers are a promising approach to build simple receivers [56]. Energy management schemes may alleviate the strict energy bounds imposed by batteries [12]. Surveillance of areas difficult to access by humans can be achieved by the deployment of sensor networks [15]. Collecting difficult-to-gather data might lead to new insights and research topics in other research areas. The inherent noise-like behaviour of UWB systems makes robust security systems highly feasible. They are not only difficult to detect, but also excel in jamming resistance. These characteristics are essential, not only for traditional security alarm systems, but also for Wireless Body Area Networks (WBANs), which are envisaged for medical supervision. Due to the simple transceiver architecture and the thereby expected low costs of transceivers, the number of devices to be employed can be over dimensioned. This allows for highly redundant data sources, pushing new algorithms and paradigms in data aggregation, coding and transmission reliability. With this approach, a certain percentage of nodes may fail (due to device failure, bad transmission conditions etc.) without affecting the functioning of the system as such. Deliberately designing devices with higher failure probability will again lower the cost of a single device. For complementing smart homes, actuators can be controlled by a central operator, making human intervention unnecessary. Even though short range, low data rate communications using alternative PHY concepts (i.e., UWB) are currently discussed in IEEE 802.15.4a working group [16], a lot of research is necessary to actually bring those systems into our daily life in a large scale, where user acceptance and applications are central to large scale deployment as well as technological research. The very distinct potential of UWB in low rate applications is owed to its inherent temporal resolution due to large bandwidth, enabling positioning with previously unattained precision, tracking, and distance measuring techniques, as well as accommodating high node densities due to the large operating bandwidth [12]. Many routing protocols are known which reduce controlling overhead using location information [17]. GPS (Global Positioning System) is often unavailable in indoor environments, hence, innovative methods for location determination are highly desired. Even in areas with good GPS reception, GPS transceivers may turn out to be too complex and too energy-hungry for long-life battery operated devices. Today’s indoor solutions use either infrared or ultrasonic approaches. The former requires line-of-sight-propagation which can not be guaranteed, 8

Wireless World Research Forum (WWRF) and the latter has the disadvantage of propagating with limited penetration. Simple UWB radio technology may fill this gap between demand and physical constraints, and is currently under development [18]. With proper position information, new paradigms in location aware protocol design and computing come into play opening new opportunities for ad-hoc network design. By reducing protocol overhead, this will again decrease the energy costs for data transmission. For industrial needs, e.g. in the automotive field, distance measuring systems are yet another example for the deployment of UWB systems as logistics will also profit from highly precise location determination.

2.2. High data rate applications High data rate applications of UWB wireless technology have initially drawn much attention, since many of the applications are suited to the consumer market. Hence, commercial interest in technology development, standards and regulation has, and still is very high. The very definition of ultra wideband [18] – a bandwidth exceeding 500 MHz (for carrier frequencies above 2.4 GHz) and an extremely low power spectral density (75nW/MHz, according to FCC rules), along with inherent features such as operation in unlicensed frequency bands and resilience against multi path fading make UWB the perfect candidate technology for these kinds of scenarios. The problem of designing transceivers with reasonable complexity, also suitable for handheld devices, is one of the main challenges for high-rate applications. Robustness against jamming is also very important, as a large number of electrical devices emitting narrowband noise are usually found in home and office environments, as well as interfering signals from other wireless services operating in section of the UWB bandwidth. Within the context of high data rate, the main application areas include: • Internet Access and Multimedia Services: Regardless of the envisioned environment (home, office, hot spot), very high data rates (> 1 Gbit/s) have to be provided – either due to high peak data rates (download activity, streaming video), high numbers of users (lounges, cafés, etc.), or both. Due to their high algorithmic complexity, conventional narrowband systems with high spectral efficiency may not be applicable for low cost and low power devices (e.g. for small handheld devices). Using very large bandwidths at lower spectral efficiency and employing simple transceivers, UWB is an attractive solution for such applications. • Wireless peripheral interfaces: A growing number of devices (laptop, mobile phone, PDA, headset, etc.) is employed by users to organize themselves in their daily life. The interconnectivity of all these devices is increasingly important as a common data basis (contact information, calendars, emails, documents) is held redundantly and in parallel on several devices. Users expect the required data synchronization and exchange to happen conveniently or even automatically. Standardized wireless interconnection is highly desirable to replace cables and proprietary plugs [19][20]. It has to be emphasized, however, that wireless solutions in this context will be attractive mainly for battery-powered devices without the need for an external power supply. • Location based services: In the context of high rate data applications, location based service provisioning is an increasingly important topic. To supply the user with the information he/she currently needs, at any place and any time (e.g. location aware services in museums or at exhibitions), the users’ position has to be accurately measured. Especially in indoor environments, current solutions (such as GPS) cannot fulfil this demand. Here, UWB techniques may be used to accommodate positioning techniques and data transmission in a single system. 9


2.3 Home networking and home electronics One of the most promising commercial application areas for UWB technology is wireless connectivity of different home electronic systems. It is thought that many electronics manufacturers are investigating UWB as the wireless means to connect together devices such as televisions, DVD players, camcorders, and audio systems, which would remove some of the wiring clutter in the living room. This is particularly important when we consider the bit rate needed for high-definition television that is in excess of 30Mbps over a distance of at least a few meters. An example of a possible home-networking setup using high-speed wireless data transfer of UWB is shown in figure 3 [21].

Figure 3. Home Networking Setup Using UWB Of course, UWB wireless connections to and from personal computers (i.e., wireless USB) are also another possible consumer market area, with products expected in the next few years. A recent proposal is to use UWB as the wireless link in a ubiquitous ``homelink'', which consists of an amalgamation of wired and wireless technologies [22]. The wired technology proposed by the authors is based on the IEEE 1394 standard. This is an attempt to effectively integrate entertainment, consumer communications and computing within the home environment. The reason for the choice of IEEE 1394 is that it provides an isochronous mode, in which data are guaranteed to be delivered within a certain time frame after transmission has started. Bandwidth is reserved in advance, which gives a constant transmission speed. This is important for real time applications, such as video broadcasts, to ensure that there is no break in the movie or television program for the viewer. Some possible services and required data rates are shown in Table 1. Service Digital Video DVD, TV Audio Internet PC Other

Data Rate (Mbps) 32 2-16 1.5 >10 32