Distance Measurements using Ultra Wide-Band (UWB)

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Distance Measurements using Ultra Wide-Band (UWB)

  1. 1. Masters Thesis Distance Measurement Using Ultra Wideband Md. Iqbal Hossain Talat Karim Minhas Supervisor and Examiner: Thomas Lindh Stockholm February 2012 Masters in Computer Networks School of Technology and Health Kungliga Tekniska Högskolan  
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  3. 3. Acknowledgements  Praise to Almighty Allah, the origin of knowledge, who enables us toundertake and accomplish this thesis. We are very thankful to Allah, creatorof this universe whose guidance always remained with us at every moment ofour lives, especially during this work in the form of knowledge, courage andhopes. May Allah bless our Prophet Hazrat Muhammad (Peace be Upon Him)whose teachings show us right path in every darkness of our lives.Our special gratitude goes to our supervisor, Thomas Lind whose preciousguidance accompanied us during our research and study at Royal Institute ofTechnology (KTH). Also, I would like to thank Bo Åberg for his help andkind cooperation during our studies. We feel ourselves very much obliged toour parents, brothers and sisters whose prayers have enabled us to reach up tothis stage. The efforts of us, and inspirations of many, have led to a successfulcompletion of this final thesis.   3  
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  5. 5. AbstractUltra wideband (UWB) is vastly under consideration of research industry thatpromises high data rata, low power consumption and economic solution.UWB was in use of military since 1950’s. In 2002 Federal communicationcommission (FCC) approved the use of 3.1-10.6 GHz band for unlicensedUWB applications. UWB is a suitable choice for sensing and position objectsbecause of high bandwidth and fine time resolution.The goal of this work is to explore the UWB technology in context ofdistance measurement between two nodes. We have described thecharacterization; reliability and ranging precision of an impulse UWB basedtransceiver for both indoor and outdoor environments. This thesis discuss indetail about UWB technology. Chapter 1 discusses about UWB applications,regulation and bandwidth properties. Chapter 2 and 3 discuss about singleband and multi band modulation and detection techniques. Chapter 4 gives acomplete description how to measure position through ranging andpositioning parameters. Finally, to estimate the ranging and positioning, a twoway ranging algorithm based on TOA employed as part of this work isdescribed in detail in chapter 5. A theoretical analysis of impulse UWB radiofor wireless communication and ranging is provided employing the ShannonHartley theorem and Cramer-Rao lower bound (CRLB) method.   5  
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  7. 7. Contents Chapter 1…………………………………………………………………..12 1 Introduction……………………………………………………………13 1.1 Ultra Wideband……………………………………………..……13 1.2 UWB features.……………….…………………………………..16 1.3 UWB and Narrowband technologies…………………………….19 1.4 Applications of UWB……………………………………………21 1.5 UWB Regulations………………………………………………..24 1.6 Bandwidth property of UWB signals…………………………....26 1.6.1 Definition………………………………………………...26 1.6.2 Advantages of large relative bandwidth…………….......27 1.6.2.1 Processing gain potentiality…………………....27 1.6.2.2 Penetration of obstacles………………………..27 1.6.2.3 Propagation loss……………………………….28 1.7 Modulation techniques……………………………………………29 Chapter 2……………………………………………………………………31 2 Single band UWB modulation…………………………………………..32 2.1Modulation Techniques….……………………………………….32 2.1.1 Pulse Amplitude Modulation……………………………32 2.1.2 On-off Keying…………………………………………...33 2.1.3 Pulse Position Modulation………………………………33 2.1.4 Pulse Shape Modulation………………………………...35 2.1.5 Phase Shift Keying……………………………………...36 2.2 Multiple accesses in single band UWB…………………………37 2.2.1 Time-Hopping UWB……………………………………37 2.2.2 Direct-sequence UWB…………………………………..39 2.3 Detection Techniques…………………………………………....40 2.3.1 Correlation Receiver…………………………………….41 2.3.2 Rake Receiver……………………………………….......43   7  
  8. 8. Chapter 3……………………………………………………………………45 3 Multiband UWB Modulations…………………………………………46 3.1 Introduction………………………………………………………...46 3.2 Modulation Techniques…………………………………………….46 3.2.1 Impulse Radio MB-IR…………………………………….46 3.2.2 MB-OFDM……………………………………………. …47 3.3 Architecture of OFDM Transmitter………………………………..48 3.3.1 Channel Encoding…………………………………………50 3.3.2 Bit Interleaving……………………………………………51 3.3.3 Time and Frequency Domain Spreading………………….52 3.3.4 Subcarrier Constellation Mapping………………………...53 3.4 MB-OFDM Receiver Architecture………………………………...54 3.4.1 System Model……………………………………………..54 3.4.2 Channel Estimation………………………………………..56 3.4.3 Frequency Domain Channel Equalization………………...56 3.4.4 Channel Decoding…………………………………………58 Chapter 4…………………………………………………………………....60 4 Ultra Wideband Position Estimation………………………….……....61 4.1 Introduction to Position Estimation………………………………...61 4.2 Position Estimation Applications…………………………………..61 4.3 Ranging and positioning parameters.………………………………62 4.3.1 Received Signal Strength…………………………………63 4.3.2 Angle of arrival (AOA)…………………………………...64 4.3.3 Time of Arrival (TOA)……………………………………66 4.3.4 Time Difference of Arrival (TDOA)……………………...68 4.3.5 Round Trip Time (RTT)…………………………………..69 4.4 Position Estimation…………………………………………………70 4.4.1 Geometric and statistical Approach……………………….70 4.4.2 Mapping or fingerprinting…………………………………73 Chapter 5……………………………………………………………………74 5 UWB Distance Measurements..………………………………………..75 5.1 Introduction………………………………………………………...75   8  
  9. 9. 5.2 Ranging algorithm based on TOA……...…………………………..76 5.2.1 Signal aspects………..……………………………………78 5.2.2 Hardware aspects..………………………………………...80 5.3 Theoretical analysis of UWB distance measurements..……………82 5.4 Experimental ranging results………………………………………87 5.4.1 LOS area test results…….………………………………...88 5.4.2 Soft-NLOS area test results.………………………………90 5.4.3 Hard-NLOS area test results….…………………………...91 6 Conclusion………………………………………………………………92 7 References……………………………………………………………….94   9  
  10. 10. List of AcronymsAOA Angle of ArrivalAWGN Additive White Gaussian NoiseAP Access PointADC Analog-to-digital ConvertorBPSK Binary Phase Shift KeyingBPF Band-Pass FilterCP Cyclic PrefixCRLB Cramer-rao Lower BoundCSS Chirp Spread SpectrumCEPT Conference of Postal and Telecommunications AdministrationDAC Digital to Analog ConversionDLP Digital Light ProcessorDS Direct SequenceETSI European Telecommunications Standards InstituteFCC Federal Communication CommissionFFT Fast Fourier TransformGPS Global Positioning SystemIFFT Inverses Fast Fourier TransformISI Inter Symbol InterferenceIF Intermediate FrequencyLAN Local Area NetworksLNA Low-noise AmplifierLOS Line of Sight   10  
  11. 11. MB-OFDM Multiband orthogonal Frequency-division MultiplexingMMSE Minimum Mean-Square ErrorNLOS Non Line of SightOOK On-Off keyingPC Personal ComputerPG Processing GainPPM Pulse Position ModulationPSM Pulse Shape ModulationPAM Pulse Amplitude ModulationPR Pseudo RandomRF Radio FrequencyRSS Received Signal StrengthRTT Round Trip TimeRMSE Root-Mean-Square errorSNR Signal-to-Nose RatioTH-PPM Time Hopping Pulse Positioning ModulationTH Time HoppingTFC Time Frequency CodeTOA Time Of ArrivalTDOA Time Difference of ArrivalUWB Ultra WidebandWPAN Wireless Personal Area NetworkWi-Fi Wireless FidelityZMCSCG Zero-Mean Circularly Symmetric Complex GaussianZF Zero-Forcing Equalization   11  
  12. 12. Chapter 1   12  
  13. 13. 1. Introduction1.1 Ultra WidebandUltra-Wideband (UWB) is a high data rate, low power short-range wirelesstechnology that is generating a lot of interest in the research community andthe industry, as a high-speed alternative to existing wireless technologies suchas IEEE 802.11 WLAN, HomeRF, and HiperLANs [3]. Although, itconsidered as a recent technology in wireless communications, ultra-wideband (UWB) has actually experienced over 40 years ago. In fact, UWBhas its origin in the spark-gap transmission design of Marconi and Hertz inthe late 1890s. In other words, the first wireless communication system wasbased on UWB. Owing to technical limitations, narrowband communicationswere preferred to UWB. In the past 20 years, UWB was used for applicationssuch as radar, sensing, military communication and localization. A substantialchange occurred in February 2002, when the Federal CommunicationCommission (FCC) issued a report allowing the commercial and unlicenseddeployment of UWB with a given spectral mask for both indoor and outdoorapplications in the USA. This wide frequency allocation initiated a lot ofresearch activities from both industry and academia. In recent years, UWBtechnology has mostly focused on consumer electronics and wirelesscommunications. The United States Federal Communications Commission(FCC) uses the following two-part requirement to identify UWB emissions:• -10 dB fractional bandwidth greater than 0.20, or   13  
  14. 14. • -10 dB bandwidth equal to or greater than 500 MHz, regardless of thefractional bandwidth. The fractional bandwidth is based on the frequencylimits of the emission bandwidth using the formula [21], !! !!! 𝐵! = 2×                                                                                                                                            (1.1)   !! !!!Where, f! upper frequency, f! lower frequency.Ultra-Wideband (UWB) technology is loosely defined as any wirelesstransmission scheme that occupies a bandwidth of more than 25% of a centerfrequency, or more than 1.5GHz. A UWB system can also be determined bya duty cycle less than 0.5 %. Equation 1.2 illustrates the duty cycle of a UWBpulse. T! Stands for the symbol duration and T! for the UWB pulse width. ! Duty  Cycle  =  !! (1.2) !UWB characteristics can be analyzed according to the Shannon capacityformula. For an AWGN channel of bandwidth B, the maximum data that canbe transmitted can be expressed as                              𝐶 = 𝐵𝑙𝑜𝑔! 1 + 𝑆𝑁𝑅    𝑏𝑖𝑡/𝑠𝑒𝑐𝑜𝑛𝑑   (1.3)SNR is representing the signal-to-noise ratio. From this equation it is clear, ifwe increase the bandwidth of the system, the capacity of the channel willincrease. If we see in the context of UWB, the bandwidth is very high andvery low power is required for transmission. So we can gain a very highchannel capacity using UWB with lower power that can make batter lifelonger and reduce the interference with existing systems.   14  
  15. 15. UWB is a Radio Frequency (RF) technology that transmits binary data, usinglow energy and extremely short duration impulses or bursts (in the order ofpicoseconds) over a wide spectrum of frequencies. It delivers data over 15 to100 meters and does not require a dedicated radio frequency, so is also knownas carrier-free, impulse or base-band radio. UWB systems use carrier-free,meaning that data is not modulated on a continuous waveform with a specificcarrier frequency, as in narrowband and wideband technologies. Figure 1: FCC spectral mask for indoor UWB transmission   15  
  16. 16. 1.2 UWB featuresUWB technology has the following significant characteristics.High Data rateUWB can handle more bandwidth-intensive applications like streaming video,than either 802.11 or Bluetooth because it can send data at much faster rates.UWB technology has a data rate of roughly 100 Mbps, with speeds up to 500Mbps, This compares with maximum speeds of 11 Mbps for 802.11b (oftenreferred to as Wi-Fi) which is the technology currently used in most wirelessLANs; and 54 Mbps for 802.11a, which is Wi-Fi at 5MHz. Bluetooth has adata rate of about 1 Mbps [3]. Figure 2: Maximum range and data rate of different wireless technologies [7].   16  
  17. 17. Low power consumptionUWB transmits short impulses constantly instead of transmitting modulatedwaves continuously like most narrowband systems do. UWB chipsets do notrequire Radio Frequency (RF) to Intermediate Frequency (IF) conversion,local oscillators, mixers, and other filters. Due to low power consumption,battery-powered devices like cameras and cell phones can use in UWB [3].Interference ImmunityDue to low power and high frequency transmission, USB’s aggregateinterference is “undetected” by narrowband receivers. Its power spectraldensity is at or below narrowband thermal noise floor. This gives rise to thepotential that UWB systems can coexist with narrowband radio systemsoperating in the same spectrum without causing undue interference [3].High SecuritySince UWB systems operate below the noise floor, they are inherently covertand extremely difficult for unintended users to detect [3].Reasonable RangeIEEE 802.15.3a Study Group defined 10 meters as the minimum range atspeed 100Mbps However, UWB can go further. The Philips Company hasused its Digital Light Processor (DLP) technology in UWB device so it canoperate beyond 45 feet at 50 Mbps for four DVD screens [3].   17  
  18. 18. Low Complexity, Low CostThe most attractive of UWB’s advantages are of low system complexity andcost. Traditional carrier based technologies modulate and demodulatecomplex analog carrier waveforms. In UWB, Due to the absence of Carrier,the transceiver structure may be very simple. The techniques for generatingUWB signals have existed for more than three Decades. Recent advances insilicon process and switching speeds make UWB system as low-cost. Alsohome UWB wireless devices do not need transmitting power amplifier. Thisis a great advantage over narrowband architectures that require amplifierswith significant power back off to support high-order modulation waveformsfor high data rates [3].Large Channel CapacityThe capacity of a channel can be express as the amount of data bitstransmission/second. Since, UWB signals have several gigahertz ofbandwidth available that can produce very high data rate even ingigabits/second. The high data rate capability of UWB can be best understoodby examining the Shannon’s famous capacity equation: ! 𝐶 = 𝐵   log ! (1 + )                                                                                      (1.4)   !  Where C is the channel capacity in bits/second, B is the channel bandwidth inHz, S is the signal power and N is the noise power. This equation tells us thatthe capacity of a channel grows linearly with the bandwidth W, but onlylogarithmically with the signal power S. Since the UWB channel has an   18  
  19. 19. abundance of bandwidth, it can trade some of the bandwidth against reducedsignal power and interference from other sources. Thus, from Shannon’sequation we can see that UWB systems have a great potential for highcapacity wireless communications [7].Resistance to JammingThe UWB spectrum covers a huge range of frequencies. That’s why, UWBsignals are relatively resistant to jamming, because it is not possible to jamevery frequency in the UWB spectrum at a time. Therefore, there are a lot offrequency range available even in case of some frequencies are jammed.ScalabilityUWB systems are very flexible because their common architecture is softwarere-definable so that it can dynamically trade-off high-data throughput forrange [6].1.3 UWB and Narrowband technologiesWireless technologies are growing faster with great flexibility and mobility.Wireless technology reduces the use of cables and more easy to install.Currently there are four wireless technology protocols are working around theglobe. Bluetooth (over IEEE 802.15.1), ultra-wideband (UWB, over IEEE802.15.3), ZigBee (over IEEE 802.15.4) and Wi-Fi (over IEEE 802.11).These all protocols can be compared with each other on the basis of cost,   19  
  20. 20. complexity and power, so that it will be easier to select specific protocol fornetwork engineers for wireless network deployment. Following section givesa short overview of narrowband technologies [4].Bluetooth is a short-range radio system, designed for devices operatingwithin a short distance, for example to connect computer peripherals such asmouse, keyboard, printers. Blue tooth is used also in mobile handsets toconnect to mobiles for sharing music and files. Piconet and Scatternet are twotopologies used by Bluetooth for connection management. The maximumsignal rate is 1Mb/s with power from 0-10dbm.ZigBee also known as IEEE 802.15.4 is a low rate wireless technologyoperating in simple devices that consumes less power and in range of 10m.ZigBee is a mesh networking with long battery lifetime. The maximum signalrate is 256kb/s with power from (-25) - 0 dBm.Wireless Fidelity (Wi-Fi) is IEEE802.11a/b/g standards for local areanetworks. It allows users to use internet at broadband speed connect to anaccess point (AP). Wi-Fi operates in range of 100 meter. The maximum signalrate is 54Mb/s with power from 15 - 20 dBm. Figure 3 gives a graphical viewof power output of different technologies. UWB uses very less emitted poweraccording to all other technologies. The maximum signal rate is 110Mb/s.UWB is the suitable choice for WPAN due to low power, high speed and easyto use.   20  
  21. 21. Figure 3: Compression between different technologies1.4 Application of UWBWireless technology is playing now main role in our daily lives. In recentyears, demand of higher quality and faster delivery of data is increasing dayby day. The need of more speed and quality brought up many wirelesssolutions for short rang communication. The family of Wi-Fi standards(IEEE802.11), Zigbee (IEEE802.15.4) and the recent standard 802.15.3,which are used for wireless local area networks (WLAN) and wirelesspersonal area networks (WPAN), can’t meet the demands of applications thatneeds much higher data rate. UWB connection function as cable replacementwith date rate more than 100 Mbps. Applications of UWB can be categorizedin following section.   21  
  22. 22. Imaging SystemsUWB was firstly used by military purpose to identify the buried installations.In imaging system emission of UWB is used as illuminator similar to radarpulse. The receiver receives the signal and the output is processed usingcomplex time and frequency functions to differentiate between materials atvarying distance. The lower part of radio spectrum < 1 GHz have ability topenetrate the ground and solid surfaces. This property makes UWB a bestchoice for detection of buried objects and public security and protectionorganizations.UWB plays an important role in medical imagine and human body analysis.Now a day’s ultra wideband radars are used for heart treatment. All of innerbody parts of human being can be imaged by adjusting the emitting pulsepower [21].Radar SystemsIn early days military used UWB technology in radar system to detect theobject in high-density media like ground, ice and air targets. Research andstudies in this area found, radar can be used everywhere where we needsensing of moving objects. Radar systems can be installed in vehicle to avoidaccident during driving and parking. UWB radars can be used in guardingsystems as alarm sensors to detect unauthorized entrance into the territory.These radars can be used to find objects or peoples in collapsed buildings bydetecting the movement of person; but in case person is not moving, it can   22  
  23. 23. still be detected by heart beat and thorax beats. Police department can usesuch radars to find criminals hidden in shelters. These radars are able tomeasure the patient’s cardiac and breathing activity in hospitals as well as athome [21].Home NetworksIn a home environment, variety of devices are operating such as DVD players,HDTVs, STBs, Personal video recorders, MP3 players , digital cameras,camcorders and others. The current popular usage of home networking issharing date from PC to PC and from PCs to peripherals. Customers aredemanding multiplayer gaming and video distributions in home network.These all devices are connected using wires to share contents at high speed.UWB is a wire replacement technology provides high bandwidth more than100 Mbps. These all devices can be connected in a home network to sharemultimedia, printers, scanners and etc. UWB can connect a plasma display orHDTV to a DVD or STB without using any cable. UWB also enablesmultiple streaming to multiple devices simultaneously, that allows viewingsame or different content on multiples devices. For example, movie contentcan be shared on different display devices in different rooms [1] [3].The home networks are directly connected to a broadband through aresidential gateway. This approach is cost effective but is ineffective forwhole house coverage. Cables are installed to connect different devices withInternet in a home environment. With a right UWB solution Internet traffic   23  
  24. 24. from multiple users in a home can be routed to single broadband connection.UWB enable devices can be connected in an ad-hoc manner like Bluetooth toshare contents. For example a camera can be connected to a printer directly toprint pictures; MP3 player can be connected to another MP3 player andshared music.Sensor NetworksWireless sensor networks are an important area of communication. Sensornetworks have many applications, like building control, surveillance, medical,factory automation etc. Sensor networks are operated under many constraintssuch as energy consumption, communication performance and cost. In manyapplications sensor size is also considered to be smaller. UWB use pulsetransmission, with very low energy consumption. This property enables us todesign very simple transmitters and thus long time battery operated devices.These sensors can be used in locating hospitals, tracking and communicationsystems. These systems enable us to locate and track objects includingfacilities, equipment’s, nurses, doctors and patients in a hospital [2].Furthermore these systems can be used in factories to track equipment’s,employees and visitors.1.5 UWB RegulationNumber of wireless systems is working around the globe. Every systemoperates under a defined bandwidth range by FCC. Interference was also an   24  
  25. 25. issue to consider while allowing the UWB for commercial use. To avoidinterference with other systems, UWB have many restrictions to operateunder FCC approved UWB commercial license in March 2002. The approvedrange of band for UWB is in between 3.1-10.6 GHz for short-range indoorwireless networks. This limitation keeps UWB noise out of the sensitiveareas occupied by GPS, cellular phone and WLAN systems. This approvalgave companies to develop high-speed wireless solutions for home andconsumer electronics.The European Telecommunications Standards Institute (ETSI) and EuropeanConference of Postal and Telecommunications Administrations (CEPT) havebeen working closely to establish a legal framework for the deployment ofunlicensed UWB devices. Within ETSI, there are two TGs to develop UWBregulation and standards for the European Union. The ETSI TG31A isresponsible for identifying a spectrum requirement and developing radiostandards for short range devices using UWB technologies, while the ETSITG31B is responsible for developing standards and system referencedocuments for automotive UWB radar applications. Lastly, CEPT SE24 isresponsible for regulatory issues and spectrum management e.g., studyingspectrum sharing for < 6 GHz [6].The Japanese Ministry of Telecommunications has shown some interest inadopting UWB technology for communications applications. It plans throughits agencies to develop with industry an indoor UWB system to networkpersonal devices such as video cameras and computers. In August 2002, a   25  
  26. 26. UWB technology group was set up by the Communications Research Centreto work with industry on UWB research, development and standardizationthrough Japan’s Communications Research Laboratory [21].1.6 Bandwidth Property of UWB signals1.6.1 DefinitionBandwidth is the most important characteristic of UWB communicationsystems. The definition of ultra-wideband is a signal with greater than 25%relative bandwidth. UWB signals need large absolute bandwidths. Therelative bandwidth definition of UWB is stated as follows [9]: !! !!! !! !!! !                                            𝐵!"# = = 2.   ≈                                                                              (1.5)   !!"# !! !!! !!  Where, f! = upper band frequency f! = Lower band frequency W= Absolute bandwidth f! = Center frequencyAny signal will considered as a UWB signal that have a relative bandwidthproperty.           26  
  27. 27. 1.6.2 Advantages of large relative bandwidth  1.6.2.1 Processing Gain PotentialityThe ratio of the noise bandwidth at the front and end of the receiver is knownas processing gain (PG). Usually, this ratio is calculated as the ratio of thechannel symbol rate R ! ,  to the bit rate  R ! : !"#$%  !"#$%&$!  !" !! 𝑃𝐺 = =                                                                                                (1.6)   !"#$%  !"#$%&!  !"# !!UWB devices using large scale of bandwidth that’s why, most of theapplication desired high data rate and a margin of processing gain can beachieved simultaneously [9].1.6.2.2 Penetration of obstacles  In order to implement wider bandwidth, conventional narrowbandcommunications must use higher carrier frequencies. When frequencies ofthese signals increase, the propagation losses and bandwidth becomes larger.On the other hand, UWB signals can achieve high data rates with lower centerfrequencies.   ! 𝑓! = ⇒ 𝑓!! < 𝑓!!    for    𝐵!"#! > 𝐵!"#!                                                (1.7)   !!"#   27  
  28. 28.  It follows that UWB signals have the potential for greater penetration ofobstacles such as walls than do conventional signals while achieving the samedata rate. From FCC 2002 rules, if relative bandwidth is 3.55 GHz and theabsolute bandwidth is 900 MHz. If the actual data symbol rate is 100 MHz,then a conventional communications waveform can be designed with a centerfrequency of 3.15 GHz. In this case, the conventional signal will penetratematerials slightly better than the UWB signal. This example highlights thatthe material penetration advantage of UWB signals applies when they arepermitted to occupy the lower portions of the RF spectrum [9].    1.6.2.3 Propagation Loss  UWB signal can be used to estimate propagation loss can be estimate byUWB signal without incurring a significant error in the calculation ofreceived power: Let the signal spectrum be denoted Gs (f);                  Gs  (f)≈ Const. W                                                                                                  (1.8)  Propagation loss for UWB signals can be obtained using conventionalmethods and the nominal center frequency of the signal. ! !"#$%. !"#$%. 𝑝! ≈ 𝑐𝑜𝑛𝑠𝑡.× ! ! = = !                                  (1.9)   !! ! !! !! !! !!Where, 𝑝! = Recieve  power ! ! 𝑓! = 𝑓! − , 𝑓!" = 𝑓! + ! !   28  
  29. 29. 𝑓! = 𝑓! 𝑓! is the geometric mean of the lower and upper band-edgefrequencies.Thus the received power is estimated correctly using the geometric mean asthe nominal frequency [9].1.7 Modulation TechniquesEarly implementation of UWB communication systems was based ontransmission and reception of extremely short duration pulses (typically subnanosecond), referred to as impulse radio. Each impulse radio has a very widespectrum, which leads to the very low power levels permitted for UWBtransmission. These schemes transmit the information data in a carrier lessmodulation; where no up/down conversion of the transmitted signal isrequired at the transceiver. The time hopping pulse position modulation (TH-PPM) introduced in 1993 by Schultz and better formalized later by Win andSchultz.Until February 2002, the term UWB used only impulse radio modulation.According to the new UWB ruling of FCC from 2002, New frequencyspectrum from 3.1 to 10.6 GHz is allocated for unlicensed application.Furthermore, any communication system that has a bandwidth larger than 500MHz is considered as UWB. As a result, well known and more establishedwireless communication technologies (e.g., OFDM, DS-CDMA) can be usedfor UWB transmission [7].   29  
  30. 30. In last few years, UWB system design has experienced a shift from thetraditional single-band to a multiband design approach. Multiband consists individing the available UWB spectrum into several sub bands, each oneoccupying approximately 500 MHz. This bandwidth reduction relaxes therequirement on sampling rates of analog-to-digital converters (ADC),consequently enhancing digital processing capability. One example ofmultiband UWB is multiband orthogonal frequency-division multiplexing(MB-OFDM).   30  
  31. 31. Chapter 2   31  
  32. 32. 2 Single band UWB modulationSingle band UWB modulation is also called impulse radio modulation. This isbased on very short-time impulse of transmission radio, which are typicallythe derivative of Gaussian pulses. This type of transmission does not requirethe use of additional carrier modulation, as the pulse will propagate well inthe radio channel. The technique is a baseband signal approach. The mostcommon modulation technique in UWB is introduced in figure 4 [7].2.1 Modulation Techniques2.1.1 Pulse Amplitude ModulationPulse amplitude modulation (PAM) is implemented using two antipodalGaussian pulses as shown in Figure 4(a). The transmitted binary pulseamplitude modulated signal str (t) can be represented as, str(t)  =  dk  𝜔tr(t)                                                                                        (2.1)  Where 𝜔 tr(t) is the UWB pulse waveform, k represents the transmitted bit(“0” or “1”) and −1          𝑖𝑓  𝑘 = 0                                dk  =                                                                          (2.2)   +1            𝑖𝑓𝑘 = 1   32  
  33. 33. Is used for the antipodal representation of the transmitted bit k. Thetransmitted pulse is commonly the first derivative of the Gaussian pulsedefined as [12] !!! ! 𝜔tr(t)=-­‐ 𝑒 !!!                                                                        (2.3)   !!! !Where σ is related to the pulse length Tp by σ  =  Tp/2π.2.1.2 On-off KeyingThe second modulation scheme is the binary on-off keying (OOK) and isdepicted in Figure 4 (b). The waveform used for this modulation is defined asin (2.1) with [12] 0          𝑖𝑓  𝑘 = 0 dk  =                                                                        (2.4) +1            𝑖𝑓𝑘 = 1The difference between OOK and PAM is, no signal is transmitted in OOKwhile k=0.2.1.3 Pulse Position ModulationPulse position modulation (PPM) signal is encoded by the position and theinformation of the data bit to be transmitted with respect to a normal position.More precisely, while bit “0” is represented by a pulse originating at the timeinstant 0, bit “1” is shifted in time by the amount of δ from 0. Let us first   33  
  34. 34. assume that a single impulse carry the information corresponding to eachsymbol. The PPM signal can be represented as Figure 4: Single band (impulse radio) UWB modulation schemes [7]   34  
  35. 35. ! 𝑠!" (𝑡) = 𝜔!" !!!! 𝜔!" (𝑡   −  𝑘𝑇𝑠   −   𝑑! 𝛿)                                                          (2.5)  Where ω!" (t) = transmitted impulse radio δ = The time between two states of the PPM modulationThe value of δ may be chosen according to the autocorrelation characteristicsof the pulse. For instance, to implement a standard PPM with orthogonalsignals, the optimum value of δ (δopt) which results in zero auto correlationρ(𝛿!"# ) is such as: ! 𝜌 𝛿!"# = !! 𝜔!" 𝜏 𝜔!" 𝛿!"# + 𝜏 = 0                                                          (2.6)  Normally, the symbol is encoded by the integer 𝑑! (0 ≤ 𝑑! ≤ M) where M isthe number of states of the modulation. The total duration of the symbol is 𝑇! , 𝑤ℎ𝑖𝑐ℎ  𝑖𝑠  𝑓𝑖𝑥𝑒𝑑, and chosen greater than Mδ+TGI where TGI is a guardinterval inserted for inter symbol interference (ISI) mitigation. The binary (𝑀)transmission rate is thus equal to R =𝑙𝑜𝑔! 𝑇! .  Figure 4 (c) shows a two-state (binary) PPM where a data bit “1” is delayed by a fractional timeinterval δ whereas a data bit “0” is sent at the nominal time [7].2.1.4 Pulse Shape ModulationPulse shape modulation (PSM) is an alternative to PAM and PPMmodulations. As shown in Figure 4(d), in PSM the information data isencoded by different pulse shapes. This requires a suitable set of pulses for   35  
  36. 36. higher order modulations. The orthogonality of signals used in PSM usesorthogonal signal that’s why it has desirable property. This attribute permitsan easier detection at the receiver. The application of orthogonal signal setsalso enables multiple access techniques to be considered. This can be attainedby assigning a group of orthogonal pulses to each user, who uses the assignedset for PSM. The transmission will then be mutually orthogonal and differentuser signals will not interfere with each other [7].2.1.5 Phase Shift KeyingIn binary PSK (BPSK) or bi phase modulation, the binary data are carried inthe polarity of the pulse. The waveform used in BPSK defined in equation 2.1with [12], 1          𝑖𝑓  𝑘 = 1 dk  =                                                                    (2.7)   −1            𝑖𝑓𝑘 = 0A pulse has a positive polarity if k=1, whereas it has negative polarity if k=0.A BPSK signal shown in figure 5.Since the different pulse level is twice thepulse amplitude; the BPSK signal has better performance than OOK [12]. Figure 5: Binary PSK signal   36  
  37. 37. 2.2 Multiple accesses in single band UWBUntil now, we assumed that each symbol was transmitted by a single pulse.This continuous pulse transmission can lead to strong lines in the spectrum ofthe transmitted signal. In practical system, due to the very restrictive UWBpower limitations, UWB system shows a high sensitivity to interference fromexisting systems. The modulation schemes that are described above, theydon’t provide multiple access capability.To reduce the potential interference from UWB transmissions and providemultiple access capability, a randomizing technique is applied to thetransmitted signal. This makes the spectrum of the UWB signal more noise-like. The two main randomizing techniques used for single band UWBsystems are time hopping (TH) and direct-sequence (DS). The TH techniquedescribes the position of the transmitted UWB impulse in time and the DSapproach is based on continuous transmission of pulses composing a singledata bit. The DS-UWB scheme is similar to conventional DS spread-spectrumsystems where the chip waveform has a UWB spectrum [7].2.2.1 Time-Hopping UWB The multiple access and power limit considerations motivate the use of animproved UWB transmission scheme where each data symbol is encoded bythe transmission of multiple impulse radios shifted in time. A pseudo-random(PR) code determined the position of each pulse in Time-Hopping (TH)   37  
  38. 38. scheme. To increase the range of transmission, more energy is allocated to asymbol. Unique TH code distinguishes different users; they can transmit atthe same time. A TH-PPM signal format for 𝑗!! user can be written as, ! ! !! !! (!) (!) 𝑆!" 𝑡 = !!!! !!! 𝑤!" (𝑡 − 𝑘𝑇! − 𝑙𝑇! − 𝑐! 𝑇! )𝑑!                (2.8)   (!)Where d! is the k-th data bit of user j. 𝑁! is the number of impulsestransmitted for each symbol. The total symbol transmission time 𝑇! isdivided into 𝑁! frames of duration T! and each frame is itself sub-divided intoslots of duration   𝑇 𝑐 . Each frame contains one impulse in a position (!)determined by the PR. TH code sequence c! (unique for the j-th user) andThe symbol to be encoded shown in Figure 6 for TH-PPM binary modulation.TH spreading can be combined with PAM, PPM and PSM [7]. Figure 6: TH-PPM binary Modulation   38  
  39. 39. 2.2.2 Direct-sequence UWB DS-UWB employs sequences of UWB pulses. Each user is distinguished byits specific pseudo random sequence, which performs pseudo randominversions of the UWB pulse train. DS transmission is modulated by apseudo-random (or specific) binary sequence that serves to spread thewaveform spectrum; a correlator at the receiver evaluates the energy at thebinary sequence-defined frequencies and dispreads the signal prior todecoding it. Instead of transmitting bit per bit, such system will transmit asequence for every bit. Since the chip rate is higher than the bit rate, thebandwidth used has increased. W-CDMA has been accepted as the 3rdgeneration cellular standard. This system provides multiple-access, that is,many users can share the same bandwidth and each has its unique spreadingsequence [13].The DS-UWB spreading technique is combined with PAM, OOK, PSM andBPSK modulations. Since PPM is a time-hopping technique, it is not used forDS-UWB transmission. DS spreading approach for PAM and OOK signalformat for 𝑗!! user can be written as [14], ! ! !! !! (!) (!) 𝑆!" = 𝐴! !!!! !!! 𝑤!" (𝑡 − 𝑘𝑇! − 𝑙𝑇! )𝑐! 𝑑! (2.9) (!)Where d! is the 𝑘 !! data bit (!)c! is the 𝑙 !! chip of PR code   39  
  40. 40. Figure 7: Time domain representation of (a) TH-UWB and (b) DS-UWB spreadingtechniques. 𝑤!" (t) is the pulse waveform of duration 𝑇! 𝑇! is the chip length and 𝑁! is the number of pulse per data bit.J is user index. The PR sequence is {-1,+1} and bit length is 𝑇! = 𝑁! 𝑇!2.3 Detection TechniquesIn single band UWB systems, two widely used demodulators are correlationreceivers and Rake receivers.   40  
  41. 41. 2.3.1 Correlation ReceiverThe correlation receiver is the optimum receiver for binary TH-UWB signalsin additive white Gaussian noise (AWGN) channels. TH format is typicallybased on PPM. TH-PPM was the first physical layer proposed for UWBcommunications. We consider TH-PPM signal for correlation receiver [16].Let us consider that multiple access active in TH-PPM transmitter. Thereceived signal r(t) at the receiver is modeled as !! (!) r(t)= !!! 𝐴! 𝑆!"# (t-­‐ 𝜏! )+n(t)                                                            (2.10)   Figure 8: Correlation receiver block diagram for TH-PPM Signal [7]     41  
  42. 42. Where, N! is  number  of  users, A!   means the attenuation over the propagationpath of the received signal. τ! Represent the time asynchronies between clockof received signal and the receiver clock. n(t) is the additive receiver noise.The propagation channel modifies the shape of the transmitted impulse 𝑤!" (𝑡)to 𝑤!"# (𝑡). We consider the detection of the data from the first user, i.e., d(1).As shown in Fig. 8, the data detection process is performed by correlating thereceived signal with a template v(t) defined as, V(t)=𝑤!"# 𝑡 − 𝑤!"# (t-­‐𝛿)                                                              (2.11)  Where 𝑤!"# (t) and 𝑤!"# (t − 𝛿) represent a symbol with duration Ts. Thereceived signal in a time interval of duration 𝑇! = 𝑁! 𝑇! is given by !! !! 𝑟 𝑡 = !!! 𝑤!"# (𝑡 − 𝑇! − 𝑙𝑇! − 𝑐!! 𝑇! − 𝑑 1 𝛿 + 𝑛!"! (𝑡) (2.12)Where n!"! (t) is the multi-user interference and noise. It is assumed that the !receiver knows first transmitter’s TH sequence {c! }and the delay T! .Whenthe number of users is large, the approximate the interference-plus noisen!"! t is as a Gaussian random process. Although TH-PPM has interestingfeature, but the data rate is reduced by a factor of Ns. Modification introducedby the UWB channel on the shape of the transmitted signal is anotherdisadvantage. Thus, the receiver has to construct a template by using theshape of the received signal. The construction of an optimal template is animportant concern for practical PPM based systems. Besides, due toextremely short duration pulses employed, timing mismatches between the   42  
  43. 43. correlator template and the received signal can result in serious degradation inthe performance of TH-PPM systems. For this reason, accuratesynchronization is of great importance for UWB systems employing PPMmodulation.2.3.2 Rake ReceiverA typical Rake receiver is shown in Fig. 9. The idea of the RAKE receiver isto combine the energy of the different multipath components of a receivedpulse in order to improve the performance. It is composed of a number ofcorrelators followed by a linear combiner. Each correlator is synchronized toa multipath component and the results of all correlators are added. Finally, aDecision device decides which symbol was transmitted after analyzing theoutput of the adders.The Rake receiver takes advantage of multipath propagation by combining alarge number of different and independent replicas of the same transmittedpulse, in order to exploit the multipath diversity of the channel. In general,Rake receivers can support both TH and DS modulated systems Rakecorrelators also called as fingers. The major consideration in the design of aUWB Rake receiver is the number of paths to be combined, that’s why thecomplexity increases with the number of fingers [15].     43  
  44. 44. Figure 9: RAKE Receiver with J fingers   44  
  45. 45. Chapter 3   45  
  46. 46. 3 Multiband UWB modulation3.1 IntroductionIn single band system whole spectrum is used simultaneously and differenttechniques are implemented to provide multi band environment. Single bandis traditional approach to use the whole spectrum as one. In multiband UWB,the spectrum is divided into many sub bands of 500 MHz bandwidth. Thisapproach makes use of spectrum in more efficient way and reducesinterference with other system working in same environment. Differentmultiple access and modulation options can be applied to each sub-band.There are two kind of Multiband UWB modulation, Multiband impulse radio(MB-IR) and multiband OFDM (MB-OFDM). The following sectionselaborate both classes in sequence.3.2 Modulation Techniques3.2.1 Impulse Radio MB-IRIn MB-IR system, the allocated spectrum of UWB is divided in to non-overlapping small channels with a minimum bandwidth of 500 MHzModulations techniques from single band UWB, PAM, PPM or PSM areapplied to each sub-band. A pulse repetition scheme is used to avoid ISI.These systems are less complex to implement. The Rake receiver is used withfew fingers. The drawback of MB-IR system is that, for each sub-band a Rakereceiver is required separately.   46  
  47. 47. 3.2.2 MB-OFDMOrthogonal frequency division multiplexing is a mature technique is beingused in narrow band technologies. The data is transmitted on multiplecarriers. These carriers are spaced with precise frequencies. OFDM approachhas many good properties what makes OFDM a best choice. OFDM increaseSpectral efficiency, inherent ability to avoid RF interference. Multi-pathenergy is captured efficiently. The drawback of OFDM technique is that, thearchitecture of transmitter and receiver is complex as compared to MB-IR.Batra et al. in 2004 proposed a scheme to IEEE802.15.3a [22], [23] In thisproposal, spectrum of UWB is divided into non-overlapping sub-bands of 528MHz each and five band groups are defined within 3.1-10.6 ranges as shownin figure 10. Band Group #1 Band Group #2 Band Group #3 Band Group #4 Band Group #5 Band Band Band Band Band Band Band Band Band Band Band Band Band Band #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 3432 3960 4488 5016 5544 6072 6600 7128 7656 8184 8712 9240 9768 10296 f MHz MHz MHz MHz MHz MHz MHz MHz MHz MHz MHz MHz MHz MHzFigure 10: Division of the UWB spectrum from 3.1 to 10.6 GHz into band groupscontaining sub-bands of 528 MHz in MB-OFDM systems [24].The first four groups further have tree sub-bands while last group have onlytwo sub-bands. Transmission of data over first three groups is known asmandatory mode I. Information within each sub-band is transmitted using   47  
  48. 48. conventional coded OFDM modulation. The presence of time-frequency code(TFC) differentiates MB-OFDM from conventional OFDM system. Thepurpose of time-frequency code (TFC) is to provide a different carrierfrequency at each time-slot and also used to distinguish between multipleusers.Figure 11: Example of time-frequency coding for the multiband OFDM system in mode I,TFC = {1, 3, 2, 1, 3, 2, … }. 3.3 Architecture of OFDM TransmitterMB-OFDM transmitter architecture is complex to design as compared tosignal band OFDM and MB-IR. Sub-bands transmit information in a time-slot fashion. One sub-band at a time transmits information in a particulartime-slot. Binary date is inputted into transmitter that is encoded by non-recursive non-systematic convolution (NRNSC) code before interleaving.Bits interleaving is used to provide more diversity to transmit over multipathfading channels and reduce burst errors. QPSK symbols using Gray labeling   48  
  49. 49. was used in basic MB-OFDM proposal. Figure 12 presents the architecture ofOFDM transmitter. Figure 12: Transmitter architecture for the MB-OFDM system.In OFDM scheme, whole bandwidth is divided into channels each with 528MHz, each sub-bands contains 𝑁! = 128 subcarriers and each subcarrier isseparated by Δf= 4.125 [13]. Transmitter at each time-slot applies 128 pointinverse fast Fourier transform (IFFT) with a OFDM symbol duration of 𝑇!!" = 1/Δf = 242.42ns. Output signal is added with a cyclic prefix (CP)TCP=60.6 ns to avoid ISI and guard interval (GI) 𝑇!" =9.5ns. The purpose ofGI is to switch transmitter and receiver form one sub-band to next. Nextstage is for digital-to-analog conversion. OFDM signal with CP and GI ispassed by DAC, which produce an analog baseband OFDM signal. The   49  
  50. 50. duration of this symbol is the sum of all interval times. 𝑇!"# = 𝑇!!" + 𝑇!" + 𝑇!" =312.5 ns as in figure 12. This process can be mathematically expressedas, !! !! ! 𝑥! (𝑡)= !!! 𝑆! exp{𝑗2𝜋𝑘∆𝑓(𝑡 − 𝑇!" )} (3.1)This equation represents the baseband signal to be transmitted. Where t ∈[𝑇!" , 𝑇!!" + 𝑇!" ] and j = √−1. 𝑥! (t) Represent the copy of last OFDM symboland is zero at interval (𝑇!!" +𝑇!" ,  𝑇!"# ) corresponding to the GI duration. Thebaseband OFDM signal is filtered and generated RF signal that is sent totransmitting antenna. The transmitted signal can be defined by this equation.   !!"# !! 𝑟!" (𝑡  )=   !!! 𝑅𝑒  (𝑥! 𝑡 − 𝑛𝑇!"# exp  {𝑗2𝜋𝑓!! 𝑡})                          (3.2)     𝑁!"# Represent total number of symbols in the frame and 𝑓 ! represent the !carrier frequency over which signal is transmitted. 3.3.1 Channel EncodingSignals traveling through an environment are subject to fading. OFDMsignals are sent on different carriers, which may suffer fading, which cancause erroneous decisions. To overcome these kinds of fading effects, forwarderror correction coding scheme is used in MB-OFDM. Mother codes withdifferent coding rate are used. Puncturing procedure is used to generatedifferent coding rates from the rate R=1/3. In this process, some bits are   50  
  51. 51. omitted at transmitter and dummy bits are added on receiver side at the placeof omitted bits [25]. 3.3.2 Bit InterleavingBit interleaving is used when transmission is over multiple fading channels. Itprovides high diversity and is of two types [13]. Inter-Symbol interleaving:This permutes the bits across 6 consecutive OFDM symbols, enables the PHYto exploit frequency diversity within a band group. Intra-symbol toneinterleaving: This permutes the bits across the data subcarriers within oneOFDM symbol, exploits frequency diversity across subcarriers and providesrobustness against narrow-band interferers. In Intra-symbol tone interleaving 𝑁!"#$ blocks are made by grouping coded bits. 𝑁!"#$ represent number ofcoded bits per OFDM symbol. Coded bits are than ordered by Blockinterleaver of size Nbi= 6 * 𝑁!"#$ . If {U (i)} is sequence, {S(i)},(i=0…..6𝑁!"#$ -1) are input and output bits of interleaver than we haveequation representation as ! 𝑆 𝑖 = 𝑈{𝐹𝑙𝑜𝑜𝑟 + 6𝑀𝑜𝑑(𝑖, 𝑁!"#$ )}                                            (3.3)   !!"#$Floor is used which give largest integer value less or equal to its argumentsvalue and reminder is returned by Mod function. Blocks of  𝑁!"#$ bits areconstructed by interleaver output and organized by using a regular blockintra-symbol.   51  
  52. 52. 3.3.3 Spreading TechniquesBandwidth expansion can be achieved by two schemes.Frequency Domain SpreadingIn this spreading scheme, copy of information is sent on a single OFDMsymbol, information is sent twice. The subcarriers are divided in two half.Date is sent on first half of subcarriers and conjugate symmetric are sent onsecond half of subcarriers.Time Domain SpreadingIn this scheme, different frequency sub-bands is used to transmit same OFDMsymbol. This approach introduces inter-sub-band diversity as well asmaximizes frequency-diversity. Performance is also improved in the presenceof other non-coordinated devices. Data Modulation Code Rate Frequency Time Spreading Rates(Mbps) Spreading 53.3 QPSK 1/3 Yes 2 55 QPSK 11/32 Yes 2 80 QPSK ½ Yes 2 106.7 QPSK 1/3 No 2 110 QPSK 11/32 No 2 160 QPSK ½ No 2 200 QPSK 5/8 No 2   52  
  53. 53. 320 QPSK ½ No 1 400 QPSK 5/8 No 1 480 QPSK 3/4 No 1 Table 1: Rate-dependent parameters in multiband OFDM systems.Table1 shows different data rates achieved by combining the different channelcodes with time and frequency spreading. Both time and frequency techniquesare used for data rate below 80Mbps. For date rate 106.7 and 200 Mbps, timedomain spreading is used with gain 2. Date rate more than 200 Mbps exploitneither of these techniques and spreading gain is 1.3.3.4 Subcarrier Constellation MapQPSK constellation is used. After coding and interleaving process,two bits groups of binary date are made and then converted to oneof four complex points of QPSK constellation. Gray labeling is usedfor conversation [23].   53  
  54. 54. Q   QPSK   𝑏! 𝑏!     +1   01                                                                                                              11         -­‐1                                                                                                                                                                +1   I                                                                                                     00                              -­‐1                                                                            10         Figure 13. Gray mapping QPSK Constellation.3.4 MB-OFDM Receiver Architecture3.4.1 System ModelThe receiver proposed for MB-OFDM [17] is shown in Figure 14. As shownin figure, the channel estimation process and data detection are performedindependently. Let us consider a single-user MB-OFDM transmission with 𝑁!"#" =100 data subcarriers per sub-band, through a frequency selectivemultipath fading channel, described in discrete-time baseband equivalentform by the channel impulse response coefficients {ℎ! }!!! . Furthermore, we !!!assume that the cyclic prefix (CP) is longer than the maximum delay spread   54  
  55. 55. of the channel. After removing the CP and performing FFT at the receiver, thereceived OFDM symbol over a given sub-band can be written as [7][17] y  =  𝐻! 𝑠 + 𝑧                                                                        (3.4)  where (N!"#" × 1) vectors y and s denote the received and transmittedsymbols, respectively; the noise vector z is assumed to be a zero-meancircularly symmetric complex Gaussian (ZMCSCG) random vector and 𝐻! =diag(H) is the (N!"#" ×N!"#" ) diagonal channel matrix with diagonal elementsgiven by the vector H = [H! , … . , H!!"#" !! ]! , where !!!!"# !!! 𝐻! = !!! ℎ! 𝑒 !! (3.5)In MB-OFDM, the channel is assumed to be time invariant over thetransmission of one frame and changes to new independent values from oneframe to the next.Figure 14: The basic receiver architecture proposed for MB-OFDM in [17].   55  
  56. 56. 3.4.2 Channel EstimationIn order to estimate the channel, a MB-OFDM system sends some OFDMpilot symbols at the beginning of the information frame. Here, we considerthe estimation of the channel vector H with NP training symbols S!,! (i = 1,.....,  N! ). According to the model (20), the received signal for a given channeltraining interval is      𝑌! = 𝐻! 𝑆! + 𝑍!                                                                                                              (3.6)  Where each column of the (𝑁!"#" × 𝑁! ) matrix 𝑆! = [𝑆!,! , ..., 𝑆!,!! ]contains one OFDM pilot symbol. The entries of the noise matrix 𝑍! have thesame distribution as those of z [7].  3.4.3 Frequency Domain Channel EqualizationIn order to estimate the transmitted signal vector s from the received signalvector y, the effect of the channel must be mitigated. To this end, the MB-OFDM uses a frequency domain channel equalizer, as shown in FEQ block inFigure 14. It consists of a linear estimator as ŝ = 𝐺! 𝑦 (3.7)The two design criteria usually considered for the choice of the linear filter Gare,   56  
  57. 57. Zero-forcing equalization (ZF): ZF equalization uses the inverse of thechannel transfer function as the estimation filter. In other words, we have G ! = !!H! . Since in OFDM systems, under ideal conditions, the channel matrix H!is diagonal, the ZF estimate of the transmitted signal is obtainedindependently on each subcarrier as !                          ŝ!",! = 𝑦!      k=0,  …...,   𝑁!"#" − 1                                                (3.8)   !!Minimum mean-square error equalization (MMSE): To minimize themean-squared error between the transmitted signal and the output of theequalizer, applying the orthogonality principle, we obtain 𝐺 ! !!"# = (𝐻! 𝐻 ! ! + 𝜎! ⫿𝑁! )!! 𝐻 ! !                                                                          (3.9)   !Due to the diagonal structure of H! , equalization can again be done on asubcarrier basis as ∗ !! ŝ!!"#,! = !  𝑦!      k=0,.......,  𝑁!"#" − 1                            (3.10)   ⃓!! ⃓! !!!The main drawback of the ZF solution is that for small amplitudes of H! , theequalizer enhances the noise level in such a way that the signal-to-noise ratio(SNR) may go to zero on some subcarriers. The computation of the MMSEequalization matrix requires an estimate of the current noise level. Notice thatwhen the noise level is significant, the MMSE solution mitigates the noiseenhancement problem even when  H! ’s close to zero while for high SNRregime, the MMSE equalizer becomes equivalent to the ZF solution [7] [20].   57  
  58. 58. 3.4.4 Channel DecodingAfter frequency domain equalization and de-interleaving, the MB-OFDMusually uses a hard or soft Viterbi decoder in order to estimate the transmitteddata bits. The maximum-likelihood path is reconstructed by using Viterbialgorithm according to the input sequence. Bits information containingreliable estimates are received by soft decision decoder while only bits arereceived by hard decision decoder. A branch metric represents the distancebetween the bits pair received and “ideal” pairs (“00”, “01”, “10”, “11”) whilepath metric is representing the sum of metrics of all branches in the path.The role of distance is totally based on the decoder type. If we consider harddecision decoder, hamming distance is used while Euclidean distance is usedfor soft decision decoder. Path with minimal path metric is chosen as themaximum-likelihood path. Viterbi algorithm performs three calculations asfollows;1. Branch metric calculation performs distance calculation on the basis ofinput pair and the ideal pair (“00”, “01”, “10”, “11”).2. Path metric calculation is metric calculation for survivor path for everystate of encoder. Here survivor path is representing the minimum metric path.3. Traceback purpose is to store one bit information when a path is selectedout of two. Traceback doesn’t keep track of full information about the path[18], [19].   58  
  59. 59.       Branch  metric   Path  metric   Traceback   Decoded  Encoded   stream   calculation   calculation   stream   Figure 15: Viterbi decoder data flow   59  
  60. 60. Chapter 4   60  
  61. 61. 4. Ultra Wideband Position Estimation4. 1 Introduction to Position EstimationUltra wideband transmit small pulses with high bandwidth. UWB signal caneasily penetrate through walls and grounds. Due to low energy, highbandwidth, and fine temporal resolution characteristics, UWB is an idealcandidate technology for position and ranging applications. The processinvolves exchange of signals between nodes and measures the parameters toestimate the position or range. The process of measuring the distance betweentwo nodes is called ranging. The measured distance is than processed furthermore to estimate the position of the node is called positioning. The accuracyof measuring distance plays an important role for better position estimation.UWB is an ideal choice for application where more precise accuracy isdemanded. Different techniques are used to measure the parameters, likeTOA, AOA, TDOA, RTT and RSS. This section mainly discuss in detailabout the parameters measurement between the nodes4.2 Position Estimation ApplicationsUWB can be implemented in many areas to get more precise estimationsabout objects or nodes. It can be used in medical sector to monitor the patientconditions and measure the position of patient inside hospital. Additionally itcan be used to locate medical equipment inside hospital. UWB positionmeasurement can be used to provide information about military securitypersonals and identify their authorization. It can also be used by military to   61  
  62. 62. locate the weapons. Positioning can be used to locate employee, machineryand resources in a manufacturing plant, tracking of children’s, tracking theshipments with a precision of less than 1 inch.4.3 Ranging and positioning parametersProcess of ranging and position can be categorized as Direct Positioning ortwo-step positioning [37]. In direct positioning actual signal transmittedbetween nodes is processed to estimate the position of the node as showing infigure 16(a), while in two-step positioning parameters are measured and then,on the basis of measured parameters range or position is estimated. Thefollowing section elaborates in detail about the parameters estimationapproaches. Received               Position           Position         signal   Estimation   estimation     (a)      Parameters   Parame Ranging  /Position   Received               Position         Estimation   ter   Estimation   signal   estimation     Estimat   e   (b)   Figure 16. (a) Direct positioning (b) Two step positioning   62  
  63. 63. 4.3.1 Received Signal StrengthWhen a signal is transmitted in an open environment, it is affected by manyobstacles in its way to target node. The most common cause of signal loss isPath-loss. These obstructions causes decrease in signal energy. The distancecan be calculated by analyzing the power transmitted, attenuation and thepower received on target node. The relation between signal energy anddistance must be known. The energy or power of the signal decreases duringpropagation is known as Path-loss. The Path-loss can be shown by followingequation expressed in [37]. ! 𝑝 𝑑 = 𝑝 𝑑! − 10𝑛𝑙𝑜𝑔( )                                                              (4.1)   !!Where d represent the distance between source and destination, 𝑑! isreference distance while P(d) and P(𝑑! ) represent the signal strength receivedat d and 𝑑! . Additionally signal is affected by reflection, scattering anddiffraction that cause variation in RSS. Signal power is estimated by thefollowing equation for precise range estimation. ! ! ! 𝑝 𝑑 = 𝑟(𝑡, 𝑑) 𝑑𝑡                                                        (4.2)   ! !In this relation r(t,d) is representing the received signal at a distance d while Tis representing the integration interval. Shadowing is another factor thatcauses signal energy degradation and expressed in log scale by zero-meanGaussian random variable as. !                                                                𝑝(𝑑)~𝑁(𝑝 𝑑 , 𝜎!! )                                                                                  (4.3)     63  
  64. 64. From this discussion it is clear that precise measurement Path-loss andshadowing is highly important for precise range estimation. For accuracymeasurement of range obtained, Cramer-Rao Lower Bound approach is usedas shown in the following equation [35]. (!" !")!!! ! 𝑣𝑎𝑟 𝑑   ≥ (4.4) !"! 𝑑 is the representation of an unbiased estimate for distance d. If theshadowing affects are decreased, more accurate results can be are obtained inrange estimation .RSS method is not providing accurate range results becauseof dependency on the parameters.4.3.2 Angle of arrival (AOA)Angle of arrival is another approach used to measure the position ofdestination node on the basis of angle measured. For this approach, numbersof antenna are used in an array fashion. [27]. Antenna elements are receivingsignals at different times. According to space coordinate, the angle of straightline that connect target node with reference node is measured. As expressedin [37], if we arrange the antenna elements in the form of uniform linear array(ULA) as shown in figure 17, signal received in this configuration have timedifference of l sin α/c. If we consider these parameters, l is inter spacing,   64  
  65. 65. angle is α and c is the speed of light.Figure 17. Uniform Linear Array of antenna with different signals arrival with angle 𝛼.CRLM lower bound can be used to investigate the AOA estimates. !! 𝑣𝑎𝑟 ∝     ≥   (4.5) !  ! !"#  ! ! !! !!!!  !  !"#$In this equation α is representing AOA, c is representing the speed of light,SNR is representing signal-to-noise ratio for each element, l is representinginter-element spacing and β is representing the effective bandwidth. From theequation (4.5), it can be seen that, if SNR, bandwidth, antenna elements andinter-element spacing is increased, the accuracy of AOA estimation willincrease linearly as well.   65  
  66. 66. 4.3.3 Time of Arrival (TOA)This is the most widely used approach for positioning and ranging. The wholeidea behind the TOA approach is the measurement of propagation delaybetween sender and receiver. To obtain this, nodes must have a commonclock or share the timing information. In a simple way distance can bemeasured if we know the speed of signal traveling between source anddestination and total time taken from source to target or transmission timedelay [35]. 𝑑 = 𝑠𝑝𝑒𝑒𝑑 ∗ 𝑡𝑖𝑚𝑒                                                                                                  (4.6)  Where speed represent the speed of signal traveling between nodes, whiletime represents the total time spent by signal during transmission betweentransmitter and receiver. As a result we obtained d as distance between sourceand target. Speed here is constant value. This method is efficient indoordistance measurement.As mentioned about TOA measure the propagation delay in order to measurethe distance. TOA uses Matched filter or Correlator for various delayestimation. If we transmit a signal s(t) and target receive it as r(t) than we canrepresent mathematically as 𝑟 𝑡 = 𝑠 𝑡 − 𝜏 + 𝑛(𝑡)                                                                                                          (4.7)     66  
  67. 67. 𝜏 is the time of arrival, 𝑛(𝑡) Represents the noise. This received signal ismatched against different templates s 𝑡 − 𝜏 for various delays 𝜏  as 𝜏 !"# = arg 𝑚𝑎𝑥! 𝑟(𝑡)𝑠 𝑡 − 𝜏 𝑑𝑡 (4.8)If there is no noise than correlator output is maximized at τ=τ while in case ofnoise results can be erroneous. The process involves matching of transmittedsignal using MF receiver and measuring the instant with peak value whichresults in equation (4.8). These two approaches, Correlator and MF, are bestfor signal in (4.7) (Figure18 (a)). In a practical scenario signals are arrivingfrom multiple paths as represented in figure 18 (b) [36]. In this case, it isdifficult to obtain the required parameters about TOA. To overcome thisproblem and measure the accurate TOA in multipath, algorithms are used toidentify the first signal arrived instead of stronger signal peak [37]. Tomeasure the Accuracy of the estimation CRLB can be representedmathematically as [44], ! 𝑣𝑎𝑟(𝜏) ≥ (4.9) ! !! !"#!   67  
  68. 68. 11 Fig. 7 Figure 18. a) Single path received signal b) Multipath received signal.A) R ECEIVED SIGNAL IN A SINGLE - PATH CHANNEL . B ) R ECEIVED SIGNAL OVER A MULTIPATH CHANNEL . N OISE IS NOT SHOWN IN THE FIGURE . Where SNR is representing signal-to-noise ratio, τ is representing unbiasedsignal modelTOA estimate in (11): and β is the effective bandwidth. The TOA approach gives more accurate results if we increase SNR and effective bandwidth. Accurate 1 Var(ˆ) ≥ √ τ √ , (13) TOA measurement estimate position2π SNR β to the original. 2 more closewhere τ represents an unbiased TOA estimate, SNR is the signal-to-noise ratio, and β is the effective bandwidth ˆ[33], [34]. The CRLB expression in (13) implies that the accuracy of TOA estimation increases with SNR andeffective bandwidth. Time Difference of Arrival (TDOA) 4.3.4 Therefore, large bandwidths of UWB signals can facilitate very precise TOA measurements.As an example, for the second derivative of a Gaussian pulse [35] with a pulse width of 1 ns, the CRLB for the TDOA is similar to TOA approach. Difference of time is calculated on thestandard deviation of an unbiased range estimate (obtained by multiplying the TOA estimate by the speed of light) basis of signal arrival on synchronized reference nodes. It is important to keepis less than a centimeter at an SNR of 5 dB. synchronization among reference nodes to calculate TDOA [26]. The process 4) Time Difference of Arrival: Another position related parameter is the difference between the arrival times of involves first estimation of TOA on target node as well as reference nodestwo signals traveling between the target node and two reference nodes. This parameter, called time difference of and then applies subtraction and estimate the difference. As we can see, nodesarrival (TDOA), can be estimated unambiguously if there is synchronization among the reference nodes [23]. are properly synchronized, so the timing offset is same for TOA. The offset One way to estimate TDOA is to obtain TOA estimates related to the signals traveling between the target node  and two reference nodes, and then to obtain the difference between those two estimates. Since the reference 68   nodesare synchronized, the TOA estimates contain the same timing offset (due to the asynchronism between the targetnode and the reference nodes). Therefore, the offset terms cancel out as the TDOA estimate is obtained as thedifference between the TOA estimates [17].
  69. 69. term are cancelled when calculating difference of TOA and the result isTDOA.TDOA can also be calculated based on cross-correlations of the signals. Inthis approach, cross-correlation is applied between signals traveling betweensource and target. In this case delay is measured that is caused by largestvalue of cross-correlation. The mathematical model for delay is !                𝜏 !"# = arg 𝑚𝑎𝑥! 𝑟 (𝑡)𝑟! (𝑡 ! ! + 𝜏)𝑑𝑡 (4.10)Where T is representing the observation interval whiler! (t), for i=1,2, isrepresenting the signal transmission between target and 𝑖 !! reference node.4.3.5 Round Trip Time (RTT)RTT is simple handshake between source and target node. During handshaketransmission time from source to target and from target to source is measured.With RTT, the distance is calculated as follows: !!" !∆! ×!"##$ 𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒   𝑑 = (4.11) !Where t !"  is representing the total time spent by the signal during travel fromsource to destination plus destination to source, while ∆𝑡  is the processingtime, taken by the source and destination nodes to process the packet. Speedis a predetermined constant that represent the speed of signal. There is noneed to synchronize the clocks on both sender and receiver. One node is   69  
  70. 70. enough to calculate the time according to own local clock. Using RTT thenecessity of perfect synchronization is not anymore needed. [35].4.4 Position EstimationPosition estimation means, locating a node according to geometrical area.UWB is a perfect choice to measure the distance between nodes. In previoussection we have discussed different methods to measure the importantparameters to estimate the position of the node. After measuring the necessaryparameters, the next step is to analyze those parameters and estimate therange or position of the destination node. Different approaches are used toanalyze the parameters will be the main focus of the following section.To estimate the position of target node on the basis of already presentdatabase, following two schemes are used [27]. a) Geometric and statistical Approach b) Mapping or fingerprintingWe discuss these two approaches one by one in following section.4.4.1 Geometric and statistical ApproachThis approach uses the measured parameters and estimates the position ofnode on the basis of geometric relationships. If we consider the TOA or RSS,these two give the range between reference and target nodes that make acircle for possible node position. If we have three measurements so with the   70  
  71. 71. intersection of three circles, we can estimate the position of desired nodeusing trilateration method as showing in following figure 19. Figure 19: Position estimation via trilateration. Figure 20: Position estimation via triangulationIn other case of AOA, the main idea of using Triangulation method forposition estimation is shown in figure 20. When we measure parameters usingTDOA, it gives hyperbola for the position of desired node. If we have threereference nodes, considering one-reference node two TDOA measurementscan be determined. Finally intersection of two hyperbolas according to TDOA   71  
  72. 72. measurements, gives the desired node estimation as showing in the followingfigure 21[26]. Figure 21: Position estimation based TDOA measurement.Geometric method can be used in a noise free environment while in case ofnoise this method is not suitable. In real scenario parameters measurementconsist of noise that can cause the intersection of lines at multiple pointsinstead of one. So there is no insight which intersection point should bechosen to measure the position of desired node. Also if reference nodes areadded more, intersection occurrence will increase more. This approach is notefficient way to estimate the position [37].On the other hand statistical approach makes use of multiple position relatedparameters including noise or noise free as showing in the following model[27]. 𝑧! =   𝑓!    𝑥, 𝑦 +   𝜂! ,      𝑖 = 1, … … … , 𝑁! (4.12)N!  is number of parameters estimates and f!    x, y is the ith signal parameter,which is function of desired node position (x, y), and η! is noise. Statistical   72  

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