CARRIER-LESS HIGH BIT RATE DATA TRANSMISSION: ULTRA WIDE BAND TECHNOLOGY

Transcription

1 CARRIER-LESS HIGH BIT RATE DATA TRANSMISSION: ULTRA WIDE BAND TECHNOLOGY Manoj Choudhary Gaurav Sharma Samsung India Software Operations Samsung India Software Operations #67, Infantry Road, Bangalore #67, Infantry Road, Bangalore ABSTRACT This paper contains a tutorial review of the Ultra Wide Band (UWB) technology. UWB technology has traditionally been used in Radar tracking, military applications etc. However, the recent decision of FCC allowing its commercial use has drawn the attention of research community and communication equipment makers towards the potential benefits of UWB technology. The paper discusses the technology, history thereof, potential, and use of UWB technology in the area of communications. 1. INTRODUCTION Ultra Wide Band (UWB) is a simple and cheap method for distributing high-bandwidth wireless data at up to a kilometer in range[1]. A UWB device works by emitting a series of short, lowpowered electrical pulses (billionth of a second apart) which are not directed at one particular frequency in the radio spectrum but across the entire spectrum, across all frequencies at once. It s like turning on a bright bulb without the lampshade!! As the name suggests, UWB uses very wide frequency band, much wider than the current wideband technologies like Universal Mobile Telecommunication Systems (UMTS), CDMA based systems, IEEE x etc. The bandwidth of the UWB system, as defined by FCC, is typically more than 25% of the central frequency or more than 1.5 GHz [2]. Typical continuous sine wave transmission technology, where information is embedded in the phase, frequency, amplitude of the wave, is reaching their peak point in terms of the data capacity and the number of users supported. Conventional narrowband and wideband systems use RF carriers to move the signal in the frequency domain from baseband to the actual carrier frequency where the system is allowed to operate. On the other hand, UWB implementations can directly modulate an impulse that has a very sharp rise and fall time, thus resulting in a waveform that occupies several GHz of bandwidth. Because of this, UWB systems are also referred to as Impulse Radio. UWB uses the same spectrum that is currently being used by conventional telecommunication devices. Ordinarily, this would jam other wireless devices, mobile phones and radios. However, UWB emits its pulses at a predetermined rate which can only be picked up by a receiver that is tuned to that exact pulse sequence. Therefore, the receiver has to know exactly when to listen in order to make out the data transmissions. This makes UWB very secure and immune to outside jamming rather like an invisible Morse Code. Because it is sent on all frequencies, from high to very low, UWB can pass straight through objects like the sea or layers of rock, and be used in radar applications. Not only that, UWB pulses can be set at any random interval, meaning the number of devices, it could be embedded in, is virtually limitless. UWB has features which make it very attractive for indoor wireless networking, tracking, and imaging applications, including multipath

2 interference, ranging & positioning, low-cost chipset architecture, and low transmit power. Very less was heard about UWB technology as its usage was predominantly limited to military arena, apart from stray applications such as in Radar tracking. However, with the recent FCC approval for the commercialization of this technology, UWB technology is catching the eye of many high bandwidth industrial players. This paper is organized in 6 sections. We briefly discuss the history of UWB technology in Section 2. In Section 3, technical aspects of the UWB technology are reviewed. Some light is thrown on the potential use of UWB technology in Section 4, while Section 5 contains a brief comparison of the UWB technology with existing wideband technologies. We conclude the paper in Section 6. nature of the UWB emission, it could potentially interfere with other licensed band users. FCC rule place a limit on the emission of the radiated power in the unlicensed band. These emissions are defined in terms of microvolt/meter. The emitted power from a radiator is given by following formula [5] 2 2 P = Eo 4π R /η (1) Here Eo is the electric field strength in terms of Volt/meter. R is the distance from the source of energy and η is the characteristic impedance of vacuum having a value of 377 ohms. The proponents of UWB have claimed that UWB s emission level is well under the norms set by FCC (Fig. 1). 2. BRIEF HISTORY OF UWB TECHNOLOGY UWB technology has its origin in the development of time-domain (impulse response) techniques for the characterization of linear, time-invariant microwave structures [3,4]. The advent of time-domain sampling oscilloscope by Hewlett Packard and development of subnanosecond pulse generation techniques provided much needed impetus to the growth of UWB technology, however its usage was limited to low cost high resolution radar to specific communications systems such as having low probability of detection and low interference potential. This was mainly due to the reason that research was performed under classified US government programs. A major concern regarding UWB emission is the potential interference it can cause to other wireless systems working in the same band, like GPS, Airplane signaling, etc. There are many factors that define UWB interference on devices, viz., modulation scheme, transmit power level, propagation loss, pulse repetition frequency employed by the UWB systems, etc. In February 2002, first step was taken by the FCC towards legal selling of the UWB products in commercial market. Due to the wideband Fig. 1: Emission level of UWB transmission [6] 3. UWB TECHNOLOGY In UWB systems, a transmitter emits sequences of impulses that are detected by a corresponding receiver whose front-end amplifiers are synchronised and time-gated to the transmitted pulse sequences. Data information that is to be sent is modulated onto certain parameters of the transmitted impulse. Such parameters may include the pulse position, amplitude or orientation. The receiver's front-end amplifiers are enabled for only very short time durations. Therefore the receiver is able to reject most unwanted signals. If enhancements to the received signal-to-noise ratio is required, the transmitter can use pulse repetition to send each information bit several

3 times. The receiver then integrates the received signal over several time durations to build up the received signal power. In UWB systems, each transmit and receive pair is active only for a very short period of time. It is possible to envisage many transmit-receive pairs, each with its own unique pulse sequences in time operating within the same area without causing mutual interference. To eliminate discrete spectral lines arising from the transmission of fixed pulse sequences, pseudorandom codes are used to cause a dithering effect and make the final emitted spectrum more noise-like. Essentially, UWB systems have power constraint as compared to typical narrowband systems which have bandwidth constraint. This characteristic makes the design of UWB systems quite different from that of the existing technologies. On a happier note, UWB system design is quite simplified as it eliminate RF processing. Some key features and applications of the UWB technology are summarized below: 3.1. Modulation Scheme UWB is unique that it achieves data transmission without using RF. In other words, UWB does not require analog modulation scheme. Instead, UWB requires pulse modulation technique to module data bit stream. Stream of pulse is transmitted over a large bandwidth with low power. In the case of pulse position modulation, a "1" may cause the transmitted pulse to be slightly advanced in time, whereas a "0" may cause a slight retardation in pulse position. Various traditional pulse modulation schemes like Pulse Amplitude modulation (PAM), Pulse Position Modulation (PPM), On-Off Keying, M- Ary PAM, BPSK can be applicable for UWB. Some propriety modulation scheme like Spectral keying [6] has also been proposed for UWB Processing Gain Processing gain is defined as the ratio of the transmitted bandwidth to the signal bandwidth. Processing gain of UWB systems is much more than the existing wideband technology based on the CDMA (WCDMA and CDMA-2000 based) Robustness to Multi Path UWB pulse are very small in duration, their pulse duration is much smaller than the delay spread of channel. This fine time resolution enables the receiver to combine individual multipath components Ranging Small time duration of the UWB pulse is helpful in the development of the precise measurement devices. This is also the reason that initially UWB technology was limited to military Radar applications Low Cost System Design A typical carrier based technology requires base band and RF processing elements. Architecturally, UWB system cost less than the carrier base technology as the need of RF processing is eliminated. UWB systems are truly pulse modulation based system which does not require various complex RF/IF stages, like IF/RF conversion, local oscillator, mixer, Phase lock loop, etc. This simplicity, in turn, leads to low complexity in terms of money and design as compared to other broadband systems. A typical design for UWB system is shown in Fig. 2. Fig. 2: Schematic of a simple UWB system Power reduction is another advantage of the UWB systems. UWB devices consume only several tens of microwatt of power when transmitting data [1], providing a better battery life. A comparison of transmitted power required for UWB systems and the competitor Bluetooth system is given in [5]. Multipath

4 diversity can also be used for better power performance using RAKE type receiver design. However, there are still some design challenges for the UWB systems. These issues attribute to very small pulse width and the timing constraint of the UWB systems. Major concerns for designers include the design of matched filter at such large bandwidth, timing accuracy of the pulse, antenna design, etc Privacy UWB is inherently secure: low power, wide band signals offer thermal noise like characteristics. These signals are difficult to intercept by unauthorized access. As it is necessary for a receiver to have prior knowledge of the timing and code sequences of the UWB transmitter to effect detection and decoding, it is very difficult for another person to eavesdrop or intercept UWB transmissions Multiple Access Issues Multiple access technology refers to the manner in which resources are shared between different users. In UWB, multiple access is mainly provided by time-hopping schemes. UWB systems, because of their very high bandwidth, are targeting audio and video traffic having different QoS requirements at higher data rates. Thus, MAC issues become very critical for efficient deployment of the system. Various traditional and propriety schemes are applicable to UWB MAC. To name a few, Time Hopping, TDMA, CDMA are some potential candidates. 4. POTENTIAL USE OF UWB TECHNOLOGY UWB has shown promise for many commercial applications, including wireless communications within buildings and the locating of objects on the other side of walls or other barriers. To use UWB for range-finding applications, the receiver determines the time delay for the signal to get from the transmitter to the receiver and works out the range by multiplying the measured time delay by the speed of light, which is a known constant. To use UWB for radar applications, the receiver extracts information from the reflected signal to derive certain useful characteristics about the target. In all these applications, if the amplitude of the transmitted impulses is kept sufficiently low, it may be possible to keep its frequency spectrum below the ambient radio frequency noise floor and thus operate the system in stealth mode. A market report forecasting the UWB component market has been published by Advanced Strategies for Integrated Solutions, Inc. [7]. The publication forecasts that the UWB market will grow at a compound annual growth rate of 285% over the next 5 years to 274 million units worldwide. The main reason for the forecasted growth, is that UWB technology does not require the same level of infrastructure investment as that required by competing technologies such as Bluetooth or IEEE x. It is expected that UWB applications will grow in areas such as Medical Imaging, Collision Avoidance, and DSL etc. Table 1 lists some of the applications of UWB in military and commercial environment. Military/Government Applications - Tactical handheld and network LPI/D Radios - Non-LoS LPI/D Groundwave communications -LPI/D Altimeter Obstacle Avoidance Radar - Precision Geolocation Systems Commercial Applications - High speed (> 20 Mbps) LAN/WAN - Commercial aviation altimeter and obstacle avoidance radar - Industrial RF monitoring systems Table 1: Some applications of UWB technology FCC has released standards that establish three different device categories: - Imaging systems, such as ground penetrating radars (GPR) and medical imaging systems

5 - Vehicular radar systems, and - Communication and measurement systems More information about the UWB standards and application groups can be found in [8,9]. 5. A COMPARISON OF UWB WITH THE EXISTING WIDEBAND TECHNOLOGIES In this section, we briefly compare UWB with some of the existing technologies: (a) UWB vs IEEE x: IEEE x standard is surely the most challenging competitor to UWB technology as they both target the same market. UWB range is limited as compared to IEEE x for same data rate UWB has more potential to support high data rate traffic than IEEE x, as discussed in the paper. (b) UWB Vs DSSS (Direct Sequence Spread Spectrum): Coherent-UWB has a low probability of interception and detection than DSSS[10]. UWB system has a clear cost advantage over DSSS. This gain is significant at higher operating band. At very high data, it s very difficult to achieve any processing gain with DSSS. This limitation is due to system reliability and cost constraint. In contrast, UWB system bandwidth does not depend upon the underlying data rate. So, there has been considerable interest in UWB for very high data rate applications such as real time video. UWB has a good multipath resistance because of very small pulse width. DSSS also offer very good multipath protection, however bandwidth constraints for most commercial system put a limit on this gain. 6. CONCLUSION The Ultra Wide Band (UWB) technology and its potential benefits, especially with reference to applications in wireless communications arena has been discussed in this review paper. We also briefly compare UWB technology with competing technologies. Due to recent developments in high-speed switching technology, UWB is becoming more attractive for low-cost consumer communications applications. The authors feel that the UWB technology, with the recent approval from FCC for commercial use, may well be the potential next wave in the wireless communications area. REFERENCES [1] Gemma Paulo. "The Promise of Ultra Wideband: Early UWB Market-Makers, Reed Electronics Group.. [2] FCC Notice of Proposed Rule Making, Revision of Part 15 of the Commission s Rules Regarding Ultra Wideband Transmission Systems, ET-Docket [3] Robert J. F. Recent Applications of Ultra Wideband Radar and Communication Systems, MultiSpectral Solution Inc. ( [4] C. Leonard Bennett, G. F. Ross, Time- Domain Electromagnetics and Its Applications, Proceedings of the IEEE, Vol 66, No 3, March [5] Jeff F, Evan G, Srinivasa S, David L, Ultra- Wideband Technology for Short or Medium Range Wireless Communications, Intel Architecture Lab. [6] General Atomic Advance Wireless Group: gy/uwb.htm [7] [8] me.html [9] Ultra Wideband Working Group: [10] Robert J. Fontana. On Range-Bandwidth per Joule for Ultra Wideband and Spread Spectrum Waveform, MultiSpectral Solution Inc. (