Ultra-Wideband for Multiple Access Communications

MULTIPLE ACCESS TECHNOLOGIES FOR B3G WIRELESS COMMUNICATIONS Ultra-Wideband for Multiple Access Communications Robert C. Qiu, Tennessee Tech University Huaping Liu, Oregon State University Xuemin (Sherman) Shen, University of Waterloo ABSTRACT two laws, while narrowband communications has shifted to the last two laws. Ultra-wideband wireless communications Although often considered a recent break- techniques have many merits, including an through in wireless communications, UWB has extremely simple radio that inherently leads to actually experienced well over 40 years of tech- low-cost design, large processing gain for robust nological development. The physical cornerstone operations in the presence of narrowband inter- for understanding UWB pulse propagation was ference, covert operations, and fine time reso- established by Sommerfeld a century ago (1901) lution for accurate position sensing. However, when he attacked the diffraction of a time there are a number of challenges in UWB domain pulse by a perfectly conducting wedge. receiver design, such as capturing multipath In fact, one may reasonably argue that UWB energy, intersymbol interference especially in a actually had its origins in the spark gap transmis- non-line-of-sight environment, and the need for sion design of Marconi and Hertz in the late high-sampling-rate analog-to-digital converters. 1890s. In other words, the first wireless commu- In this article we provide a comprehensive nications system was based on UWB. Due to review of UWB multiple access and modulation technical limitations, narrowband communica- schemes, and their comparison with narrow- tions was preferred to UWB. Much like spread band radios. We also outline issues with UWB spectrum or code-division multiple access signal reception and detection, and explore var- (CDMA), UWB followed a path with early sys- ious suboptimal low-complexity receiving tems designed for military covert radar and com- schemes. munications. After accelerating development since 1994 when some of the research activities INTRODUCTION were unclassified, UWB picked up its momen- tum after the Federal Communications Commis- Modern communication theory was originated sion (FCC) Notice of Inquiry in 1998. Interest in from the attempts of communication engineers UWB was sparked when the FCC issued a who wanted to understand what they were doing Report and Order in February 2002 allowing its in the most general terms. The limits of digital commercial deployment with a given spectral wireless communications systems depend pri- mask requirement for both indoor and outdoor marily on four basic laws and their underlying applications. theories, attributed to Maxwell and Hertz, Shan- The main limiting factor of UWB wireless non, Moore, and Metcalfe. The first two laws systems is power spectral density rather than are laws of nature, while the last two are laws of bandwidth. Thus, the initially targeted applica- behavior. The order is in the sequence of their tion was short-range (< 10 m) high-data-rate (> discovery and importance. As the field of wire- 100 Mb/s) within IEEE 802.15.3a. Due to imple- less communications has matured, emphasis and mentation limitations, systems with moderate immediate relevance have shifted gradually range (100–300 m) and low data rates (less than downward in the list. Without an appreciation a few megabits per second) have received signifi- for Maxwell and Hertz’s theories, there would be cant attention since late 2003 within IEEE no controlled wireless propagation of electro- 802.15.4a. In addition, UWB applications should magnetic waves. Without an understanding of also include safety/health monitoring, personnel Shannon’s theories, efficient use of the spectrum security, logistics, industrial inventory control, through sophisticated signal processing could not industrial process control and maintenance, have been achieved. Ultra-wideband (UWB) is home sensing, control, and media delivery. In experiencing this shift, probably from the first this article our goal is to provide a comprehen- 80 0163-6804/05/$20.00 © 2005 IEEE IEEE Communications Magazine • February 2005 sive review of UWB multiple access and modulation schemes, and their comparison with narrow- Relative time shift band radios. We will also outline issues in UWB ‘1’ signal reception and detection, and explore vari- Time reference ous suboptimal low-complexity receiving ‘0’ schemes. The rest of the article is organized as follows. We describe the basics of pulsed UWB modulation schemes. Multiple access approaches employing pulsed UWB technologies are pre- (a) sented. UWB receiver designs and challenges are discussed, followed by conclusions. Pulse repetition interval ‘1’ ‘0’ MODULATION SCHEMES Based on FCC regulation, pulsed UWB is a form of bandpass communications. The transmitter sends pulses with a bandwidth (10 dB) that is at least 500 MHz and edge frequencies Pulse width within 3.1–10.6 GHz [1]. (b) MULTIBAND DESIGN The major reasons for the standards body (IEEE ‘1’ ‘0’ 802.15.3a) to adopt a multiband scheme are: • Spectrum flexibility/agility: regulatory regimes may lack large contiguous spectrum allocation; spectrum agility may ease coexistence with existing services. • Energy collected per RAKE finger scales (c) with longer pulse widths used, which prefers fewer RAKE fingers. n Figure 1. UWB modulation options: a) binary PPM; b) binary PAM; c) on- • Reduced bandwidth after downconversion off keying. mixing reduces receiver power consumption and linearity requirements. • A fully digital solution for signal processing munication system. The multiband orthogonal is more feasible than a single-band solution frequency-division multiplexing (OFDM) system for the same occupied bandwidth. transmits information on each subband. This • Transmitter pulse shaping is made easier; technique has nice properties including the abili- longer pulses are easier to synthesize and ty to efficiently capture multipath energy with a less distorted by integrated circuit (IC) single RF chain. The drawback is that the trans- package and antenna properties. mitter is slightly more complex because it • It is capable of utilizing frequency-division requires an inverse fast Fourier transform multiple access (FDMA) for severe near-far (IFFT), and the peak-to-average ratio may be scenarios. higher than that of pulse-based multiband approaches. It seems to be very challenging for The Multiband Pulsed Scheme — The main both the pulse-based and OFDM-based multi- disadvantage of narrow time domain pulses is that band solutions to meet the target costs imposed building radio frequency (RF) and analog circuits by the market. as well as high-speed analog-to-digital converters (ADCs) to process signals of extremely wide MODULATION OPTIONS bandwidth is challenging, and usually results in In choosing modulation schemes for UWB sys- high power consumption [2]. Collection of suffi- tems, one must consider a number of aspects cient energy in dense multipath environments such as data rate, transceiver complexity, spec- requires a large number of RAKE fingers. The tral characteristics, robustness against narrow- pulsed multiband approach can eliminate the dis- band interference, intersymbol interference, and advantages associated with large front-end pro- error performance. For pulsed UWB systems, cessing bandwidth by dividing the spectrum into the widely used forms of modulation schemes several subbands. The advantage of this approach include pulse amplitude modulation (PAM), on- is that the information can be processed over off keying (OOK), and pulse position modula- much smaller bandwidth, thereby reducing the tion (PPM). To satisfy the FCC spectral mask, design complexity and power consumption, lower- passband pulses are used to transmit information ing cost, and improving spectrum flexibility and in pulsed UWB systems. Although these pass- worldwide compliance. However, it is difficult to band pulses could be obtained by modulating a collect significant multipath energy using a single baseband pulse using a sinusoidal carrier signal, RF chain. In addition, there are very stringent the term binary phase-shift keying (BPSK) is still frequency switching time requirements (< 100 ps) somewhat imprecise in the context of pulsed at both transmitter and receiver. UWB signaling as the carrier-modulated baseband pulse is usually treated as a single entity, The Multiband OFDM Scheme — Multipath the UWB pulse shape. energy collection is an important issue as it is a The three modulation schemes mentioned major factor that determines the range of a com- above are illustrated in Fig. 1. For a single-user IEEE Communications Magazine • February 2005 81 system with binary PPM signaling, bit 1 is rep- minimize the probability of collisions due to Because of the resented by a pulse without any delay and bit 0 multiple access. In TH UWB, each frame is sub- by a pulse with delay relative to the time refer- divided into Nh chips of duration Tc. Each user random polarities of ence. A major factor governing the perfor- (indexed by k) is assigned a unique pseudo-ran- the information mance of this system, like any other PPM-based dom time shift pattern, {hk,n}, 0 ≤ hk,n < Nh, symbols, the PAM system, is the set of time shifts used to repre- called a TH sequence, which provides an addi- sent different symbols. The most commonly tional time shift to each pulse in the pulse train. scheme inherently used PPM scheme is the orthogonal signaling The nth pulse undergoes an additional shift of

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