Ultra-wideband Systems: Review

Utra-wideband Systems: Review Wiaiporn Lee Department of Eectrica Engineering, Facuty of Engineering, King Mongkut s University of Technoogy North Bangkok, Bangkok, Thaiand, 10800 E-mai: wiaiponrs@kmutnb.ac.th Abstract Nowadays, the demand for higher capacity, faster service, and more secure wireess connections increases, new enhanced technoogies have to find their pace in the overcrowded and scarce radio frequency (RF) spectrum. This is because every radio technoogy aocates a specific part of the spectrum; for exampe, the signas for TVs, radios, ce phones, and so on are sent on different frequencies to avoid interference to each other. As a resut, the constraints on the avaiabiity of the RF spectrum become stricter with the introduction of new radio services. Utrawideband (UWB) technoogy offers a promising soution to the RF spectrum drought by aowing new services to coexist with current radio systems with minima or no interference. This coexistence brings the advantage of avoiding the expensive spectrum icensing fees that providers of a other radio services must pay. This artice reviews the some basic knowedge of UWB systems incuding definition, transmitter, channe mode and receiver of UWB systems. Moreover, we discuss the iterature of three interesting topics, incuding puse shape design, timing jitter and ocaization of UWB systems. Keywords Impuse-radio, Utra-wideband, Transmitter, Receiver, Channe mode 1. INTRODUCTION Since the Utra-wideband (UWB) technoogy has been approved by the Federa Communications Commission (FCC) in 2002 [1], it becomes one of the fast growing research topics. At the current stage, UWB technoogy is the promising candidate for the physica ayer of the upcoming short range wireess network. The UWB radio is conventionay based on a very short puse, which occupies a very broad bandwidth. Unike conventiona wireess communications systems that are carrier based, UWB systems transmit the information without transating it to a higher carrier frequency. This carrieress technique wi greaty reduce the compexity and cost of the transceiver. Therefore, the some basic knowedge of UWB is important to known for using to study and design the UWB research fieds. In this artice, we introduce the some basic knowedge of UWB systems. UWB systems can be cassified into three important parts. The first part is UWB transmitter. This part describes the UWB moduation techniques and the mutipe access methods. Furthermore, the advantage and disadvantage of each moduation technique are compared. The second part is UWB channe mode. The channe mode based on IEEE802.15 that derived from the Saeh-Vaenzuea mode is considered. The fina part is UWB receiver. In this paper, we are summary two weknown UWB receivers that are the RAKE and the transmitted-reference (TR) receiver. Additionay, we are summary the three interesting UWB research fieds, namey, the puse shape design, the timing jitter probem and the ocaization techniques. We organize the rest of this paper as foows. In section 2, history of UWB systems is presented. We show the UWB reguations and definition of UWB systems in section 3 and 4, respectivey. In section 5, UWB transmitters are described. The UWB moduation techniques and the mutipe access schemes are presented in section 6 and 7, respectivey. Section 8 shows the UWB channe mode. In section 9, the UWB receiver incuding the RAKE receiver and TR receiver are aso reviewed. Moreover, we are summary the interesting UWB research fieds in section 10. Concuding remarks are in section 11. 2. HISTORY OF UWB SYSTEMS In fact, UWB technoogy is not a new; it was originay discovered by Gugiemo Marconi in 1901 to transmit Morse code sequences across the Atantic Ocean using spark gap radio transmitters. However, the benefit of a arge bandwidth and the capabiity of impementing muti-user systems were never considered at that time. Approximatey fifty years after Marconi, Harmuth at Cathoic University of America, Ross and Robbins at Sperry Rand Corporation, and Van Etten at the United States Air Force or USAF, Rome Air Deveopment Center were some of the pioneers [2-3] who started the modern UWB communications from time domain eectromagnetic systems in the eary 1960s. They a referred to the systems as baseband radio. During the same period, Los Aamos, an engineer at Lawrence Livermore Nationa Laboratories (LLNL and LANL), and other engineers esewhere performed some of the origina researches on puse transmitters, receivers, and antennas. However, a major breakthrough in the UWB communications occurred as a resut of the deveopment of the samping oscioscope by both Tektronix and Hewett-Packard in the 1960s. These samping circuits provided a method to dispay and integrate UWB signas. Moreover, this research provided the simpe circuits necessary for subnanosecond on baseband puse

generation. Cook and Bernfed summarized Sperry Rand Corporation s deveopments in puse compression, matched fitering, and correation techniques in their book pubished in 1967. In 1972, Robbins invented a sensitive baseband puse receiver as a repacement for the samping oscioscope. This invention ed to the first patented design of the UWB communication systems by Ross at the Sperry Rand Corporation. By the eary 1970s, the basic designs for UWB radar and communication systems were deveoped with advances in eectronic component technoogy. In 1974, Morey at the Geophysica Survey Systems Corporation commerciaized the first ground-penetrating radar based on UWB. McEwan at LLNL deveoped the Micropower Impuse Radar (MIR) that provided compact, inexpensive and ow power UWB systems for the first time in 1994. Around 1989, the United States Department of Defense (DOD or DoD) created the term utra wideband to describe communication via the transmission and reception of impuses. The U.S. government has been and continues to be a major supporter of UWB research. The Federa Communications Commission (FCC) effort to authorize the use of UWB systems [3] spurred a great amount of interest and fear of UWB technoogy. In response to the uncertainty of how UWB systems and existing services coud operate together, the U.S. government has sponsored severa UWB interference studies In 1993, Robert Schotz at the University of Southern Caifornia presented a mutipe access technique for UWB systems [4]. This technique aocates each user a unique spreading code that determines specific instances in time when the user is aowed to transmit. With a viabe mutipe access scheme, UWB systems become capabe of supporting many fieds ike radar, point-to-point communications and wireess networks. For about the wireess network fieds, a number of researchers in the ate 1990s and eary 2000s began investigations on UWB propagation in detais. These propagation studies and the channe modes deveoped from the measurement resuts were cuminated in a number of notabe pubications (Cassioi, Win, Schotz, Moisch [5-7] and Foerster [8]). 3. UWB REGULATIONS In February 2002, the FCC amended the Part 15 rues to aow operation of devices incorporating UWB technoogy in the frequency band of 3.1-10.6 GHz [3]. Devices must operate within this 7.5 GHz of unicensed spectrum and be designed to coexist with other aocated radio systems in an uncontroed environment. The FCC rues ensure that UWB emission eves are exceedingy sma, with very ow power spectrum density (PSD). The Equivaent Isotropicay Radiated Power (EIRP) imit is - 41 dbm/mhz. The tota emitted power over severa gigahertz of bandwidth is a fraction of a miiwatt. The spectrum mask of the UWB signa is shown in Figure 1. 4. DEFINITION OF UWB SYSTEMS UWB is used to refer to signa occupy fractiona bandwidth greater than 20 % of the center frequency or more than 500 MHz of bandwidth in the 7.5 GHz band of spectrum between 3.1 GHz and 10.6 GHz. On the contrary, narrowband signas have fractiona bandwidth ess than 1% of the center frequency. Figure 1 FCC spectra masks for indoor communications. The fractiona bandwidth (FB) is defined as FB 2 fh fl f H f where f H and f L are the upper and ower frequencies, respectivey, measured at -10 db beow the peak emission point as shown in Figure 2. L Figure 2 Narrowband versus UWB systems. The UWB signas can be categorized into two main groups: singe band or impuse radio UWB (IRUWB) [5-8] and muti-band or muticarrier UWB (MC-UWB) [9]. The IR-UWB systems are based on a very short puse, which occupies a very broad bandwidth. Unike conventiona wireess communications systems, those are carrier based; UWB systems transmit information without transating it to a higher carrier frequency. This carrieress technique wi greaty reduce the compexity and cost of the transceiver. On the other hand, the MC-UWB systems were based on muticarrier communications. The muticarrier techniques were firsty used in the ate 1950s and the eary 1960s for higher data rate High frequency (HF) miitary communications. Since that time, (1)

Binary data stream 001010 Bits to symbo Symbo stream Logic - Symbo-to-puse mapper - Timing circuitry, etc. Puse generator Ampifier Figure 3 A genera UWB transmitter bock diagram. orthogona frequency division mutipexing (OFDM) has emerged as a specia case of muticarrier moduation. OFDM is a muticarrier moduation with the minimum carrier spacing [10-11]. OFDM subcarriers are separated apart by the reciproca of the symbo duration. Spectra of each subcarrier are mutuay overapping. Therefore, higher spectra efficiency, compared with the conventiona singe carrier transmission, can be obtained from OFDM. Even though, OFDM techniques of muticarrier moduation can aso satisfy the wide bandwidth requirement, the impuse based UWB systems remain the strong competence due to its simpicity. Therefore, this artice primariy focuses on impuse moduation. 5. UWB TRANSMITTERS Figure 3 shows a bock diagram of genera UWB transmitters. The UWB transmitter part can be divided into three important bocks [12]. The first bock is bits to symbos. This bock maps bits received from the binary data stream to symbos by using moduation and mutipe access techniques. The second bock incudes many parts such as symbo-to-puse mapper, timing circuitry, etc. The symbos from the first bock are then mapped to an anaog puse shape. Puse shapes are generated by a puse generator that is the ast bock. Precise timing circuitry which is cruciay requires sending puses out at intervas. If PPM is empoyed the timing must be even more precise, usuay ess than on puse width. Puses can then be optionay ampified before being passed to the transmitter. However, in genera, a arge gain is typicay not needed to meet power spectra requirements and many are omitted. 6. UWB MODULATION SCHEMES In this section, we review the conventiona UWB moduation methods and discuss the advantages or disadvantages of each technique. For UWB systems, the data moduation is typicay done using puse-moduation techniques in the time domain. The choice of moduation method can affect a number of design parameters in UWB system s deveopment such as data rate, robustness to interference and noise and transceiver compexity that directy impacts the overa size and cost of the systems. 6.1 On-Off Keying The on-off keying (OOK) is the simpest technique of the UWB moduation. The transmission of a puse represents a data bit 1 and its absence represents a data bit 0. Figure 4 shows an exampe of the OOK moduation technique in the UWB communication systems. 1 0 1 0 Figure 4 On-off keying moduation. The genera signa mode, s(t), for an OOK moduated signa can be represented by N n (2) n1 s t b P t nt where N is the maximum number of transmitted bits, b n [0,1] represents the n th data bit, P(t) is the UWB puse and T is the puse repetition period. The main advantages of the OOK moduation are simpicity and ow impementation cost. The OOK transmitter is quite uncompicated. This technique can use a simpe RF switch that turns on and off to represent data. This way, the OOK moduation aows the transmitter to ide whie transmitting a bit 0 and thus save power. However, this moduation scheme has severa disadvantages in UWB systems. The OOK moduation is highy sensitive to noise and interference: an unwanted signa can be detected as a fase data bit 1. Moreover, the difficut task of UWB synchronization becomes even more chaenging for the OOK moduation if a stream of zeros is transmitted. Therefore, the OOK technique is not a popuar moduation technique for UWB systems.

Figure 6 Discrete spectra ines of periodic puses. 6.2 Puse-Ampitude Moduation The puse-ampitude moduation (PAM) encodes the data bits based on different eves of power (ampitude) in short-duration puses. In this moduation technique, a puse with higher ampitude represents a data bit 1 and a puse with ower ampitude represents a data bit 0. Figure 5 shows an exampe of the PAM for the UWB communications. 1 0 1 0 Figure 5 Puse ampitude moduation. The genera signa mode for the PAM signas is given by where b n N bn (3) n1 s t A P t nt A is the specific power eve for each user s data bits, N is the maximum number of transmitted bits, P(t) is the UWB puse, b n [0,1] represents the n th data bit, and T is the puse repetition period. As shown in (3), the PAM and the OOK signa modes are very simiar, except for the existence of the ampitude parameter, A, in the PAM mode, which represents the different ampitudes considered for a UWB puse data transmission. The advantage of the PAM moduation is aso simpicity. It is simpe because the PAM generation requires puses with ony one poarity to represent data. On the contrary, the first disadvantage of the PAM moduation is noise immunity. Athough PAM puses are ess sensitive to noise than the OOK moduated puses when OOK has ong zero sequences, attenuation in wireess channes can convert them to the OOK case. Furthermore, because of the periodicity of transmitted puses, some discrete ines wi be presented on the PSD of the PAM puses. These discrete ines can cause harmfu interference to other narrowband and wideband signas sharing the frequency spectrum with UWB systems. Figure 6 iustrates such discrete spectra ines on the PSD of periodic puses. b n The discrete power spectra ines can interfere with the conventiona radio services and the UWB signas. Therefore, it s quite important to avoid the discrete spectra ines. One method to overcome these spectra ines is to dither the transmitted signa by adding a random offset to each puse and removing the common spectra components [12]. However, the random offset of this technique is unknown at the receiver, making it extremey difficut to acquire and track the transmitted the UWB signa. Another method with simiar random properties, but using a known sequence, is to use pseudorandom noise (PN) codes. For more information on this technique, we wi describe it at the end of the next subsection. 6.3 Puse-Position Moduation The puse-position moduation (PPM) is pseudorandomy encoded based on the position of the transmitted puse trains by shifting the puses in a predefined window in time. Compared to the OOK and the PAM puses, the PPM signas are more immune to fase detection due to channe noise. This is because the puses that represent the data bits in the PPM have the same ampitude, so the probabiity of detecting a fase data bit is reduced. A version of the PPM represents a data bit 0 by not shifting with respect to a specific reference point in time; it represents a data bit 1 by a puse advancing the same reference point as shown in Figure 7. 0 1 Figure 7 Puse-position moduation. The genera signa mode for the PPM signas is given by N n (4) n1 s t P t nt b

Figure 8 Smooth spectrum of PN time offsets. where is the moduation index that provides a time shift to represent digita bit, N is the maximum number of transmitted bits, P(t) is the UWB puse, b n [0,1] represents the n th data bit, and T is the puse repetition period. The disadvantage of the PPM scheme is its sensitivity to timing synchronization. Because data bits are recovered excusivey based on their exact position in time, timing uncertainties, such as jitter and drift, can degrade their performance significanty. For instance, timing uncertainties can cause synchronization errors that resut in increased MAI in mutipe-access channes. Further, the strict timing synchronization of narrow UWB puses prior to the correation process in the PPM receivers requires very fast (on the order of gigahertz) anaog-to-digita converters (ADCs). Moreover, mutipath distortions can stretch the puses and cause them to overap; thus detection becomes chaenging of the puse positions in the PPM systems. For the discrete spectra ines probem, another method to overcome this probem is to use PN codes to add an offset to the PPM signa [12]. Since these codes are known and easiy reproducibe at the receiver, the probem for the receiver becomes mosty acquisition of the signa, but tracking makes it much easier. Moreover, the use of a PN time shift has other benefits besides just reducing the spectra ines. Since the PN code is a channe code, it can be used as a mutipe access method to separate users in a simiar manner to the code division mutipe access (CDMA) scheme. By shifting each puse at a pseudorandom time interva the puses appear to be white background noise to users with a different PN code. Furthermore, the use of the PN code makes data transmission more secure in a hostie environment. The impact of the PN time offsets on energy distribution in the frequency domain is iustrated in Figure 8. 6.4 Biphase Moduation The biphase moduation (BPM) empoys the poarity of the puse changes to represent digita data bits. Figure 9 demonstrates the biphase moduation for the UWB puses. 1 0 1 0 Figure 9 Biphase moduation. The genera signa mode for the biphase moduation is given by N n (5) n1 s t b P t nt where N is the maximum number of transmitted bits, P(t) is the UWB puse, b n [0,1] represents the nth data bit, and T is the puse repetition period. The first advantage of the biphase moduation is ess susceptibe to distortion because the difference between the two puse eves is twice the puse ampitude. Another advantage of biphase moduation is that the change in poarity can remove the discrete spectra ines in the puse s PSD, because changing the poarity of puses produces a zero mean [13]. However, for a stream of data, accurate timing between the two transmitters is of great importance. Figure 10 TH-PPM Moduation Exampes. Figure 11 DS-BPAM Moduation Exampe.

6.5 Summary of UWB Moduation Methods In this subsection, we concude the discussion of the moduation methods for the UWB communications with Tabe I which summarizes the advantages and disadvantages of each of the moduation methods [12]. Tabe I. Advantages and disadvantages of the various moduation methods. Moduation methods OOK PAM PPM BPM Advantages Simpicity Simpicity Simpicity Simpicity, efficiency Disadvantages Binary ony, noise immunity Noise immunity Fine time resoution needed Binary ony 7. UWB MU L T I P L E A C C E S S T E C H N I Q U E S In this section, we describe the typica mutipe-access methods for the UWB communications systems. The mutipe-access techniques are needed to perform the channeization for mutipe users because severa users transmit information simutaneousy and independenty over a shared channe. For the impuse-radio UWB, two common mutipe access (MA) techniques are empoyed such as time-hopping (TH) technique and directsequence (DS) technique. Both of the methods are typicay appied to the moduation schemes previousy discussed. First of a, the TH technique can be appied to a of the moduation schemes, where each user is assigned a time- hopping sequence. This sequence reduces coisions in the communication system by assigning each user a unique time shift pattern. Each receiver can detect a signa during its own unique hopping pattern, mitigating interference. The mathematica representation for the k th user s transmit signa is given as [14]: k t k k y s t jt f c j Tc b j N s (6) j where s(t) is the transmitted baseband puse waveform, T f is the puse repetition time, c (k) j is the time-hopping sequence, T c is the duration of the time deay bins, b (k) j is the data sequence, N s is the number of puses in any given binary symbo and δ is the moduation index. An exampe of puse trains for binary data bit 1 and 0 is demonstrated in Figure 10, in which the first puse train is transmitting binary bit 1, and the second puse train is sending binary bit 0. The ony difference between the two is that a puses in puse train 0 are deayed a itte bit comparing to the puse in puse train 1. The direct sequence (DS) is the other form of MA commony used with the impuse-radio UWB, athough it is typicay imited to OOK, Binary PAM and biphase moduation schemes. The idea is to moduate an antipoda pseudo-random noise (PN) sequence, which is unique at the time of communication. Therefore, a minima amount of interference occurs with other users as they are assigned with different PN codes that have good autocorreation and cross- correation properties. The transmitted DS-UWB waveform is defined as [15]: y N 1 r k k k t bi an st itr ntc i n0 where s(t) is the transmitted baseband puse waveform, (k) N r is the spread spectrum processing gain, b j is the moduated data symbos for the k th user, b (k) n is the k th user spreading chips, T r is the bit period and T c is the chip period. An exampe of the direct sequence binary ampitude moduation (DS-BPAM) is demonstrated in Figure 11, where data bit 1 is spread into a binary sequence of 010100011001. 8. UWB CHANNEL MODELS Based on the custering phenomenon observed in severa channe measurements, the IEEE 802.15 [16] task group adopted an UWB channe mode derived from the Saeh- Vaenzuea mode [17] with a coupe of sight modifications. Specificay, the mutipath mode is described by the discrete time impuse response beow: h L K t X k, t T k, 0 k 0 (7) (8) where α k, is the path gain coefficient of the k th path within the th custer, T f is the arriva time of the first path of the th custer, τ k, is the deay of the k th path within the th custer reative to the first path arrive time T and X is the og-norma shadowing coefficient. Ceary, we have τ 0, = 0 by definition. To proceed, we further define the foowing two arriva rates: Λ is custer arriva rate and λ is ray arriva rate, i.e., the arriva rate of path within a custer Now, the distribution of the T f and τ k, can be characterized by T 1 pt T 1 e T, 0 (9) k, k, e 1 p, k 0 (10) k, k 1, i,e., the inter-custer arriva time is exponentiay distributed with rate Λ and the inter-ray arriva time is exponentiay distributed with rate λ. The channe gain coefficient {α k, } is defined as foows: where k (11) k, p k, k, p, is equay probabe of ±1 to account for the signa inversion due to refections, ζ refects the th fading associated with the custer and k, corresponds to the fading associated with the k th ray of the th custer. With the mean energy of the first path of the first custer being denoted by, the mean 0 energy of k, is given by [16]:

2 2 k e T, k, k, 0 e (12) where is the custer decay factor and γ is the ray decay factor. The distribution of the path gain magnitude α k, is assumed to be og-normay distributed: simuated using the Saeh-Veenzuea mode. The simuated channe impuse responses for CM1, CM2, CM3 and CM4 are shown in Figures 12(a), 12(b), 13(a) and 13(b), respectivey. 20og 2 2 ~,, 10 k, k, 1 2 N (13) where 1 is the standard deviation of the custer ognorma fading (in db) and is the standard deviation 2 of the ray og-norma fading (in db). From (12) and (13), we obtain: 2 2 10n 0 10T 10 k, n10 1 2, (14) k n10 10 Finay, with a standard deviation of in db, the ognorma shadowing term X is characterized x by 20og x 2 10 X ~ N 0, (13) Since we can capture the tota mutipath energy in term X, the tota energy contained in the terms {α k, } is normaized to unity for each reaization. Figure 13 Impuse response of (a) CM3 (b) CM4. Figure 12 Impuse response of (a) CM1 (b) CM2. In IEEE 802.15 working group, the UWB channe is further cassified into four modes. Channe mode 1 (CM1) represents Line-Of-Sight (LOS) and distance from 0 to 4 m UWB channe, whie channe mode 2 (CM2) represents Non-Line-Of-Sight (NLOS) and distance from 0 to 4 m UWB channe. Distance from 4 m to 10 m and NLOS UWB channe is modeed as CM3 and distance over 10 m NLOS UWB channes are a cassified into the extreme mode CM4. The simuation parameters setting for a the four channe modes are isted in Tabe II [16]. With the exact channe parameters isted in Tabe II, the UWB channes for a the four channe scenarios are 9. UWB RECEIVERS A genera UWB receiver bock diagram is shown in Figure 14. The receiver performs the inverse operation of the transmitter to recover the data. As shown in the Figure 14, the first bock of the receiver incudes many parts such as acquisition, tracking, demapping puse shapes-to-symbos, etc. This bock performs the functions of detection or acquisition to ocate the required puses amongst the other signas and then to continue tracking these puses to compensate for any mismatch between the cocks of the transmitter and the receiver. The second bock is puse generation that generates the tempate puse for the use of detecting the received signas of a first bock. The ast bock is a symbo to bits. This bock converses an anaog puse shape to the binary data stream. The most common UWB receiver design incudes threshod/energy detectors, correation detectors, RAKE and transmit-reference (TR) receivers. The threshod/energy detectors are simpe to impement. However, this receiver is a trade-off between the simpicity and the performance. The correation receiver is a matched fiter system and it can provide the optimum detection SNR if the tempate wave- form exacty matches the time and shape of the incoming waveform. However, this receiver has high compexity from the match fiter structure. RAKE receivers are a bank of correators. Each finger of the RAKE is synchronized to a

Ampifier Logic - Acquisition - Tracking - Demapping puse shape to symbo Symbo stream Bits to symbo Binary data stream 001010 Puse generator Figure 14 A genera UWB receiver bock diagram. mutipath component. Most of eary receiver researches focused on RAKE type of receivers. Recenty due to the difficuty and compexity from stringent timing synchronization requirement and energy capture of mutipaths, suboptima TR (aso caed autocorreation) receivers attract significant attentions. Next we wi ook into the ideas of the RAKE and the TR receiver structures, and compare their advantages and disadvantages. 9.1. RAKE Receivers For AWGN channes, the typicay optimum receiver is a correator (i.e. match fiter) receiver. The oca receiver generated tempate waveform is perfecty synchronized and correated with the incoming puse train, which is ony distorted by AWGN noise. For impuse-radio UWB systems, the UWB transmissions can resove many paths and are thus rich in mutipath diversity. RAKE receivers can be used to expoit the diversity by constructivey combining the separabe received mutipath components. It consists sub-receivers each of which deays to tune into the individua mutipath components. Each branch of the RAKE receivers is a correator (match fiter) that coects coherent received signa energy independenty. The output of each finger is combined in order to make the most use of the different transmission characteristics of each transmission path. This can be very we resut in higher signa- to-noise ratio (SNR, aso known as Eb/N0) in a mutipath environment. Figure 15 shows the receiver bock diagram, which con- sists of L correators/fingers to coect the received signa energy from the excess deays p L p 0 1 L p strongest paths having. The th correator, 0,1,2,, L 1,, is to correate the received signa p with the receiver ocay generated reference signa deayed by. The output of the correators can be ineary combined in different ways to form the decision variabe. The maxima ratio combining (MRC) approach provides optima performance, with the prerequisite of accurate channe information at the receiver. As accurate channe information are not avaiabe, equa gains combining (EGC) and some other methods coud be seected. 9.2. Transmit-reference Receivers From the previous subsection, we know that the coherent RAKE receivers offer optima performance that rey on enough fingers to accuratey capture a or a significant part of resovabe mutipath components (MPCs) [18-19]. In a puse based UWB systems, the number of resovabe paths coud reach tens to over a hundred in typica indoor propagation environments [20], which impose technica hurdes as we as impementation difficuties. In order to capture a

Synchronization Channe Estimation Timing Contro AGC Rake Receiver Finger 0 Rake Receiver Finger 1 Decoder Rake Receiver Finger Lp -1 Figure 15 Bock diagram of a typica RAKE receiver structure. considerabe portion of the signa energy scattered in mutipath components, a conventiona RAKE-based digita receiver not ony has to sampe and operate at a minimum of hundreds of MHz to even muti-ghz cock rates, but aso requires an impracticay arge number of RAKE fingers. In addition, reaizing optima RAKE reception performance requires accurate channe and timing knowedge, which is quite chaenging to obtain as the number of resovabe paths grows. The received puse shapes of resovabe mutipath are distorted differenty due to diffraction, which make it suboptima to use ine-ofsight signa waveform as the correation tempate in RAKE reception. Since these issues are unique to the UWB pused radios, an optima RAKE receiver design becomes either ineffective or very compicated. For these reasons, TR receivers (aso caed autocorre ation receivers) have drawn significant attention in recent years [21-22]. As a suboptima, owcompexity aternative, TR receivers offer a better mutipath capture capabiity at much ower hardware compexity than RAKE receivers. TR encodes the data in the phase difference of the two puses of a puse pair. The first puse in that pair does not carry information, but serves as a reference puse; the second puse is moduated by the data and is referred to as the data puse. The two puses are separated by a fixed deay. It can be easiy shown that the receiver can demoduate this signa by simpy mutipying the received signa with a deayed version of itsef. The simpe TR transceiver structure is shown by the bock diagram in Figure 16. In a sow fading environment, TR coects mutipath energy efficienty without requiring mutipath tracking or channe estimation. Anaog autocorreation aso aeviates the burden on A/D converters, thus owering the power consumption by interface circuits in the UWB regime. However, TR autocorreators entai severa drawbacks and usuay show worse performance than coherent RAKE receivers: the use of reference puses increases transmission overhead and reduces data rate, which resuts in reduced transmission power efficiency; the bit error rate (BER) performance is imited by the noise term in the reference signa [22]. Finay, the performance of TR receivers reies on the impementation of accurate anaog deay ines which can save and deay the reference waveforms for up to tens of nanoseconds. This is sti a big chaenge to current circuit technoogy. 10. CHALLENGES FOR ULTRA-WIDEBAND Whie UWB has many reasons to make it an exciting and usefu technoogy for future wireess communications and many other appications, it aso has some chaenges which must be overcome for it to become a popuar and ubiquitous technoogy. Perhaps the most obvious one to date has been reguatory probem. Wireess communications have aways been reguated to avoid interference between different users whose spectrum. Since UWB occupies such a wide bandwidth, there are many users whose spectrum wi be affected and need to be convinced that UWB wi not cause undue interference to their existing services. In many cases t h e s e users have paid to have excusive use of the spectrum. One soution for soving this probem is puse design techniques. Apart from the puse design probem, UWB systems are aso very sensitive to a timing jitter. Timing jitter resuts from the presence of non-idea samping cocks in practica receivers. This distortion affects the correation of signas at the receiver and thus the signa detection abiity of UWB systems. Since UWB puse duration is very short (in the range of a nanosecond), ony sma timing misaignment can severey affect the performance of a correation detector. Finay, how to appy the other appication combine with UWB systems especiay the wireess networks. The IR-UWB system which is characterized by the transmission of extremey short duration puses (very ow duty-cyce) can enabe very accurate ranging and ocation appications. The UWB appications are popuar in the wireess network appication. UWB based wireess networks ranging and positioning techniques can achieve centimeter-eve precision as we as high data rate, ow power and compexity. Therefore, we review many researches about the wireess ocation network.

Transmitter Receiver AGC RF puse generator 0 Td Td Moduating bits (binary) Channe Figure 16 Bock diagram of a transmit-reference system. 10.1. Puse Shap Design The conventiona puse, Gaussian 2 nd derivative puse, is widey adopted in the investigation of UWB appications due to mathematica convenience and ease of generation [14, 23]. Unfortunatey, this puse is not fexibe enough to conform the FCC spectra masks. Thus, the Gaussian mono- cyce puse must be modified and fitered to meet the FCC requirements [24-27]. One of the techniques to modified Gaussian monocyce is the method based on the inear combination of a number of Gaussian derivative puses to form on singe puse that are introduced in [28-29]. In [29], W. Gao, R. Venkatesan and C. Li generated puse using the combination of Gaussian derivative puses from the 1 st to 15 th ones. The power spectra density (PSD) of this technique conforms the FCC spectra masks. However, this combination makes the impementation of the puses very compicated. In the iterature, the puse design techniques are categorized according to two criteria for a puse design. Most of those works [30-50] consider a s the methods of obtaining puse that meets the power spectra constraint of FCC masks. The other portion [51-53] devotes to the design of puse that provides timing jitter toerance. However, the ater group spans ony sma number of works concerning with the optima puse design. The puse generation agorithms fitting the FCC masks can be grouped into three different techniques. Firsty, the most straight-forward method probaby is the digita generation from samped frequency response [30-31]. The impementation of this technique is prohibitivey imited by the need of extremey high samping rate digita to an anaog (D/A) converter. Secondy, the method is based on inear combination of orthogona puse sets. The orthogona puse famiies are ranged from Proate Spheroida, Waveet to Modified Hermite Puse [32-45]. For the Proate Spheroida schemes, A. B. Parr, B. L. Cho and Z. Ding presented a puse design agorithm for acquiring a set of orthogona UWB impuse with imited time duration subject to a pre-seected frequency mask [32] in 2003. In addition, L. Yin and Z. Hongbo [33] proposed an optimized puse design by using the Approximate Proate Spheroida Wave Functions (APSWF) in 2005. Their puses meet the power spectra constrain and simpe mathematica expression. Unfortunatey, the mutua orthogonaity of the puses, which are generated using the proate spheroida function, are not preserved with the distortion of channe and the characteristics of antennas. Thus, this puse design agorithm does not provide the orthogonaity for received puses which have notabe impact on the performance of the correation receiver [32]. Therefore, the new orthogona puse based on waveet technique is proposed in [34-35] for a UWB puse design that can maintain orthogonaity at a receiver. Moreover, other inear combination puse techniques such as Nyquist criteria or term of B-spines are proposed in [44-45]. These works offer a design of puses that meet the FCC spectra masks and have timeimit signas for no intersymbo interference (ISI). A popuar technique of the inear combination puse is based on the modified Hermite poynomias (HP) that are proposed in [36-43]. Hermite puses are orthogona to one another; this orthogonaity suggests that variety of designs making use of simutaneous puse transmission can be considered. Furthermore, these waveforms are we confined in both time and frequency, ensuring that a puses have the same duration, which reates to achievabe data rate, whie maximizing frequency efficiency. Therefore, many works investigate the orthogona UWB puse using this method. In 2003, G. T. F. de Abreu, C. J. Mitche and R. Kohno [36] presented orthogona puse of transmit waveform that are constructed as the combination of eementary Hermites with weighting coefficients derived by empoying the Gram-Schmidt factorization. They originay estabished orthogona puse in UWB systems, but their puses did not consider the condition of power spectra density imitation. In 2005, W. Hu and G. Zheng [37] proposed M-ary biorthogona moduation based on orthogona Hermite Puse Shapes whose frequency response met the FCC standard. For the TH- PPM UWB system, J. G. Xing, Z. H. Bo and C. Wei [38] presented the design of a cass of puses that were

based on Hermite functions. However, the high order of these puses woud be more susceptibe to timing jitter. In 2007, X. L. Wu, X. J. Sha and N. T. Zhang [42] suggested a modified Hermite function based puse shaping agorithm on the TH-PPM UWB system. The puse duration presented in this study is shorter than orthogona Hermite puse in [38]. Unfortunatey, frequency shifting and bandpass fiters are required for the HP of order 0 or 1 and higher order HP, to satisfy the FCC spectra masks. High order HP puses are susceptibe to timing jitter and noise, and they aso need bandpass fiters to fit their PSD into the FCC mask [54]. The puse design techniques, based on famous orthogona puse sets, gain from the rich knowedge of those puse sets. However, these techniques are aso difficut to impement due to the need of hundreds of puse generators. This probem can even more severe if the puse generator creates drifts or offsets during the generation process. Some authors suggested the digita impementation of such orthogona puse transformation [55]. However, the digita impementation of ow duty cyce signas eads to the expensive hardware as in the case of D/A technique. The ast technique, the digita FIR fiter soution, differs from the previous two methods because it does not aim to directy generate the desired puse. On the other hand, it synthesizes the desired puse by fitering the conventiona input puse, such as monocyce which was proposed in [46-50]. In 2003, X. Luo, L. Yang and G. B. Giannakis [48] proposed digita FIR fiter design based on the Parks-McCean (PM) agorithm for shaping UWB puses under mask-fitting requirements [3]. The PM design utiizes the bandwidth (BW) and power aowed by the FCC spectra masks, and it can dynamicay avoid narrow-band interference. However, tria-and-error may be required to find suitabe vaues for the impicit parameters in a PM design incuding the edge toerances of the pass- and stop-bands, and the frequency weighting of the approximation error. In 2006, X. Wu, Z. Tian, T. N. Davidson and G. B. Giannakis [49] introduced a gobay optima puse design method based on semidefinite programming (SDP) to maximize the power utiization efficiency whie compying with the spectra mask. The digita FIR fiter technique was proposed to avoid the extremey high samping rate impementation of the direct impuse optimization method. In addition, the direct puse optimization aso suffers from numerica instabiity because of the compicated optimization of very arge number of variabes. The optimization of FIR fiter invoves much smaer number of variabes thus it is more attractive in the impementation perspective. 10.2. Timing Jitter Probem For the prior research on the performance of timing jitter, it can be separated into two forms - as uncorreated (white timing jitter) and correated (coored timing jitter). The performance of UWB systems under white timing jitter has been extensivey studied by many works [56-63]. In 2002, W. M. Loveace and J. K. Townsend [56] considered the effects of timing jitter performance of binary and orthogona 4-ary PPM UWB communications. Moreover, they aso investigated the tracking error performance on UWB systems. I. Guvenc and H. Arsan [57] presented UWB systems performance by evauating the degradation of the signato-noise ratio due to timing jitter. They present the BER performance of various moduation options for UWB systems such as PAM, PPM, OOK and BPSK. Moreover, they consider another practica condition, incuding mutipath, mutipe access interference (MAI) and narrowband interference (NBI). In 2004, L. Mucchi, D. Marabissi, M. Ranadi, E. De Re, and R. Fantacci [58] compared the sensitivity to synchronization errors of the different mutipe access techniques between the TH-PPM and the DS. Their resuts show that the TH technique overcomes DS when timing jitter occurs. In 2005, C. S. Sum, M. A. Rahman, S. Sasaki and J. Zhou [60] presented the impact of timing jitter on a DS-UWB system in AWGN and mutipath channe. Investigation is performed in different Rake receivers (a-rake and seective-rake) and two types of timing jitter (uniformy and Gaussian distributed). In 2005, Z. Tian and G. B. Giannakis [61], [62] presented the BER sensitivity to epoch timing offset under different operating conditions, incuding frequencyfat fading channes, dense mutipath fading channes, mutipe access with and without time hopping, and various receiver types incuding siding correators and RAKE combiners. In 2006, N. V. Kokkais, P. T. Mathiopouos, G. K. Karagiannidis, and C. S. Koukouris [63] aso studied the performance of UWB under timing jitter. They consider the BER performance of M-ary PPM which is not the same as that reported in [56] in the presence of MUI. In addition, our work [64] investigated the effect of puse shapes to timing jitter toerance. The simuation resuts confirm that different puses are the resut of different eve of immunity to timing jitter. The puses having reativey fat autocorreation function tend to be ess sensitive to timing jitter. On the other hand, the performances of UWB systems under coored timing jitter have been studied as foow. In 2004, P. B. Hor, C. C. Ko and W. Zhi [65-66] presented the BER performance of the puse UWB system in the presence of coored timing jitter. They found that coored jitter degrades the BER performance more than white jitter and they are proposed a new jitter compensation scheme to improve the BER performance under coored jitter. In 2006, U. Onunkwo, Y. Li and A. Swami [67] investigated the impact of timing jitter on OFDM-based UWB systems and derived an exact expression for the ICI power due to timing jitter. For mathematica simpification, however, many works investigated timing jitter that has a norma distribution [56], [58], [63]. Thus, in this dissertation, we wi use timing jitter modeed by the norma distributed random process. Besides performance evauation of timing jitter, the prior works in an attempt to aeviate the effect from timing jitter have been studied. These proposed methods can be categorized in two approaches: improve the timing

synchronization accuracy and improve timing jitter invoving the puse shape. The first scheme which is techniques for improving the timing synchronization accuracy is proposed in [68-73]. In 2005, W. Zhang, Z. Bai, H. Shen, W. Liu and K. S. Kwak [68] proposed a virtua received waveform as integration of time-shifted version of conventiona received waveform and the distribution of timing jitter. However, they neither ceary state the motivation of their virtua received waveform design nor show method in obtaining timing jitter distribution. The on- off keying moduation scheme is considered by Q. Li and W. S. Wong in [69]. A receiver is proposed to over-samping the received waveform and the optimum threshod is determined by a version of Kiefer-Wofowitz agorithm. The technique, proposed in [69], is difficut to appy to other popuar moduation systems such as PPM or PAM. In 2006, S. Gezici, Z. Sahinogu, H. Kobayashi and H. V. Poor proposed mutiuser UWB systems that use mutipe puse waveforms [73]. For the conventiona system that uses singe puse waveform, puse with fast decaying autocorreation function is desired in order to prevent interframe interference. However, such an autocorreation function aso resuts in a considerabe de- crease in the desired signa part of the receiver output in the presence of timing jitter [74]. In 2007, R. Merz, C. Botteron and P. A. Farine [75] estimated the BER performance for a UWB impuse radio in an AWGN transmission channe and with Gaussian jitter. They investigated the infuence of the jitter on the received signa by assuming that the received puses are combined to increase the SNR. Though, the timing synchronization accuracy is imited by the advancement of both agorithms and equipments. It seems to be impossibe to perfecty synchronize the timing in the near future. The other approach that proposed by G. T. F. de Abreu and R. Kohno in 2005 to improve the robustness against timing jitter invoves the puse shape optimization [51]. In [51], the authors expoited the autocorreation properties of the modified Hermite puses to generate a puse. Their works inherenty ead to a reativey fat puse. The puse, obtained from the agorithm in [51], is a singe poarized puse with arge time duration. Such puse has narrow bandwidth, which does not efficienty utiize the aocated spectrum and vioates the wide continuous bandwidth requirement. Moreover, Y. Chen and J. Chen and T. Lv [52] proposed a nove technique that coupes High Order Monocyce (HOM) with biphase moduation (BPM) to achieve timing jitter-robust in UWB systems. However, the proposed technique finding high order monocyce puse tends to engthen the puse duration, thereby reducing the data rate and system capacity as a resut of [25]. In 2007, a new method for constructing UWB puse based on Daubechies waveets is proposed by L. Xin, A. B. Premkumar and A. S. Madhukumar [53]. The constructed puse not ony meets the FCC masks, but aso provides good performance in the presence of timing jitter. However, the proposed puse of this previous study has utiized ow spectra mask and the high order tends to engthen the puse duration. This resut of engthening the puse duration is the same as that yieded by using the technique in [52]. In 2008, W. Lee, S. Kunaruttanapruk and S. Jitapunku [76] jointy consider two crucia probems such as puse transmission power and timing jitter toerance. They proposed a nove technique in designing the optimum puse shape for UWB systems under the presence of timing jitter. The proposed puse attains the adequate power to survive the noise foor and simutaneousy provides good robustness to timing jitter. It aso meets the power spectra mask restriction as prescribed by the FCC for the indoor UWB systems. Moreover, the impementation of the proposed puse and the essentia parameters of the proposed optimization agorithm are aso investigated. 10.3. Locaization Technique The we-known ocaization techniques such as Ange of Arriva (AOA), Received Signa Strength Indication (RSSI) and Time of Arriva (TOA) are considered in many iteratures [77-81]. An AOA-based positioning technique invoves measuring anges of the target node seen by reference nodes, which are done by means of antenna arrays. However, the AOA scheme is not suited to UWB positioning because this method use of antenna arrays that increase the system cost. Moreover, an AOA technique aso difficut to find the accuracy ange estimation due to the number of scattering from objects in the environment may be very arge owing to the arge bandwidth of a UWB signa. For RSSI technique, different wireess technoogies, ike WiFi and ZigBee are normay use the RSSI for finding the indoor ocaization. However, the main probem of RSSI estimation is that position errors are neary 1 meter, which is unacceptabe for wireess network ocaization. The IR-UWB system which is characterized by the transmission of extremey short duration puses (very ow duty-cyce) can enabe very accurate ranging and ocation appications. This high time resoution nature of the UWB signa makes TOA estimation method a good candidate for positioning estimation in UWB communications. We seected TOA method because it can be shown that it achieves the best accuracy reated with Cramer-Rao ower bound (CRLB) [77]. The different transceiver types such as the SR and ED are common used in TOA estimation techniques for IR-UWB systems. In [78-79], they consider the tradeoffs between SR- and ED-based transceiver architectures by using the raised cosine puse. They found that when the system has a sufficient samping rate and no time mismatch, the SR performance is better than the ED approach. If ony ower samping rates are possibe, the SR performance wi be quicky decreased because it can not be abe to coect sufficient energy. On the other hand, if the symbo energy is spread over more puses, the ED schemes suffer from degraded SNR than SR approach. [80] presents the performance of TOA estimation with different signa waveforms such as direct sequence impuse radio (DS-IR),

transmitted reference impuse radio (TR-IR), time hopping impuse radio (TH-IR) and M-ary ternary orthogona keying impuse radio (MTOK-IR) by using non-coherent receiver, the energy detection. In [81] considers the different Gaussian derivative puses on ranging and positioning based on the estimation of TOA with IR-UWB signa. Their simuation resuts show that the higher order Gaussian puse is advised to be expoited in order to improve the accuracy of range and position estimation. 11. CONCLUSION In this paper, we review the some basic knowedge of UWB systems, namey, UWB transmitter, UWB channe mode and UWB receiver. At UWB transmitter, we show the conventiona moduation scheme and the mutipe access techniques of UWB systems. We summarized the advantages and disadvantages of the various moduation methods. The UWB channe mode in S-V mode is aso discuss. At UWB receiver, two we-known receiver types e.g., RAKE and Transmitted-reference are reviewed. 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