Ultra-Wideband (UWB 1): Promises and Challenges

1. Ultra-Wideband (UWB 1): Promises and Challenges

2. Fundamentals Fundamentals of short/medium range wireless communication 1 digital transmission systems fading channels diversity Fundamentals of short/medium range wireless communication 2 MIMO wireless space-time processing Fundamentals of short/medium range wireless communication 3 OFDM Systems I: OFDM based broadband access WLAN1I: IEEE 802.11g, a WLAN 2: IEEE 802.11n WMAN: (mobile) WiMAX Systems II: Wireless short range access technolgies and systems Vehicular Networks UWB 1: Promises and Challenges UWB 2: Physical Layer Options and Receiver Structures Wireless Body Area Network case study UWB BAN channel measurements modem design alternatives ultra low power issues The IEEE 802.15x familiy of Wireless Personal Area Networks (WPAN): Bluetooth, ZigBee, UWB Systems III: RF identification (RFID) and sensor networks RFID 1 RFID 2 Outline of Course

3. Outline • Introduction, Definitions, and Regulations • Applications • Capacity • Fundamental considerations • Impulse Radio • Pulse Position Modulation (PPM) • UWB Channel • Localization via UWB

4. Introduction, Definitions, and Regulations

5. What is UWB? • Nothing else, but signaling with a very high (fractional) bandwidth (BW > 500 MHz or fractional BW >0.2) • Narrowband example • GSM: center frequency 900 MHz and BW of 0.2 MHz • Wideband example • WLAN: center frequency 2400 MHz and BW of 20 MHz • UWB example • Center frequency 3500 MHz and BW of 500 MHz

6. Large Fractional (and Absolute) Bandwidth • UWB is a form of extremely wide spread spectrum where RF power is spread over several GHz • Wider than any narrowband/wideband system by orders of magnitude • Power seen by a narrowband system is a fraction of the total • UWB signals can be designed to look like imperceptible random noise to conventional radios • Possible problem: aggregated power of numerous UWB devices • e.g.: sensor networks Narrowband (30kHz) Wideband CDMA (5 MHz) Part 15 Limit UWB (Several GHz) Frequency

7. Brief History • 1960ies: work in time-domain electromagnetics • Describe transient behavior of system with impulse response • Terms: baseband, carrier-free or impulse technology • Later (1989) called Ultra-Wideband technology • Bennett & Ross 1978: Application of UWB in radar and communications • Military applications • Ground penetrating impulse radar • Pioneering work in 1997: • R.A. Scholtz, M.Z. Win, “Impulse Radio”,Printed inWireless Communications, TDMA vs CDMA, Kluwer Academic Publisher, London, pp. 245-263

8. 3.1 10.6 1.99 GPS Band 0.96 1.61 Regulation in USA UWB Emission Limit for indoor systems Feb 14, 2002 FCC (Federal Communications Commission) Communication allowed in frequency band 3.1-10.6 GHz with: 1. EIRP spectral density -41.3 dBm / MHz (as Part 15 limit for unintentional emission) EIRP spectral density = 74nW / MHz EIRPmax=0.55mW using 7.5 GHz BW 2. Peak EIRP spectral density 0 dBm / 50 MHz EIRP: Effective Isotropically Radiated Power

9. Power Measurement and squarer ^2 • Spectrum analyzer of FCC for compliance tests • Average power limit • -41.3 dBm / MHz averaged over 1 ms (Tav) • BW of Gf0(f) = 1 MHz centered around max. emission • Peak power limit • 0 dBm / 50 MHz and peak value • BW of Gf0(f) = 50 MHz centered around max. emission

10. ISM, GPS and Mobile Radio – Victim Systems GSM2/ UMTS dbm/MHz DECT

11. Promises and Potentials • Unlicensed operation • Capacity • Possibility of achieving high throughput, robustness to narrowband interferers • High multiple access capacity • Fading robustness • Wideband nature of the signal reduces time varying amplitude fluctuations (fading) • Multipath resolution • Position location capability • Short impulse (wideband signal) allows for accurate delay estimates, which can be used for accurate position location • Flexibility • Can easily and dynamically trade-off throughput for distance, making the technology compatible for a large number of applications • Low Power and Low Cost • Can directly modulate a baseband pulse…no need for mixers or local oscillators • Low power and low complexity receivers like energy detector • Low transmit power level • Promising for mobile applications (low cost/low battery usage)

12. Applications

13. Overview • Communications • Short range communication links • Data & voice & control links • Wireless LAN • Radar • Ground penetrating radars • Through-wall radars • Intelligent Sensors • Collision avoidance • RF Tags • Others • Imaging and ranging • Geo-location • Wireless door openers • Medical applications • radio stethoscope

14. Wireless Low Data Rate (LDR) Multimedia (Source: Pulsers)

15. Wireless High Data Rate (HDR) Multimedia (Source: Pulsers) See article: “Wireless USB erlangt Marktreife” at http://www.heise.de/newsticker/meldung/74664 from 24.06.2006. “Auf der vom 20. bis 22. Juni 2006 im kalifornischen San Jose abgehaltenen "Wireless USB Developers Conference" der WiMedia Alliance stellten die Mitglieder aus der Halbleiter- und Computerindustrie funktionsfähige Wireless-USB-Chips und -Anwendungen vor, unter anderem eine drahtlose Video-Übertragung, Hubs und Dongles sowie Treiber für Windows XP und Vista.”

16. Personal and Body Area Networks (Source: Pulsers)

17. Medical Applications (Source: Pulsers)

18. Positioning Systems (Source: IBM) LT: location tracking

19. Sensor Networks (Source: Pulsers)

20. Capacity

21. Fundamentals • Capacity of a continuous AWGN channel • Received power P and one-sided bandwidth B • Thermal noise energy: • Signal-to-noise ratio (SNR): • Low SNR approximation: • Power limited regime: increasing B has little impact on capacity • High SNR approximation: • Bandwidth limited regime: increasing B linearly increases capacity

22. Dependency on Bandwidth and Receive Power Power limited region SNR = 0.53 dB SNR = 9 dB SNR = 4 dB SNR = -4.47 dB Bandwidth limited region

23. Impulse Radio Pulse Position Modulation (PPM)

24. Impulse Radio • Very low duty cycle (Tf / Tp > 100) • One pulse transmitted per frame (Tf ) • Nfframes per symbol time Tb=NfTf • Example binary PAM:

25. Spectral Peaks [SPM Nov 04] • UWB systems typically use many pulse repetitions (100s) to represent each data symbol. • The power spectral density with binary PAM is proportional to the energy spectrum of the bit waveform • A train of Nf uniformly spaced pulses exhibits spectral peaks • For multiple access equispaced pulses could lead to catastrophic collisions • Time-hopping is one possible solution… Binary PAM with Gaussian pulse and Tp= 0.4 ns. Nf= 64, Tf= 100 ns

26. Time Hopping • Within each frame time, the pulse is pseudo-randomly positioned in time. • Smoothes the spectrum • Allows for multiple access • cjis a PN sequence, Tcis the chip time, usually in the range of Tp Binary PAM with Gaussian pulse and Tp= 0.4 ns. Nf= 64, Tf= 100 ns, Tc= 2 ns (i.e. 50 time hopping slots per frame)

27. Tc • transmitting 0 t Tf Tb d d d d s(t) Tc • transmitting 1 t Tf Tb Time Hopping and Pulse Position Modulation Cardinality of code = Nc Cardinality of modulation alphabet = M s(t) g(t) PPM transmitter has implementation advantage in comparison to PAM, because pulse amplitude is not modulated

28. UWB Channel

29. Multipath Resolution

30. Channel Amplitude Delay Profile Ray Tracing Deterministic channel impulse response for two reflections [France Télécom R&D] Maximum Excess Delay

31. Channel Amplitude Delay Profile Ray Tracing Deterministic channel impulse response for three reflections and one diffraction ("Beugung") [France Télécom R&D]

32. Different Propagation Effects • Large scale effects: • Deterministic path loss • Free space signal propagation described by Friis law: • Shadowing • Path loss Rmn varies due to signal blockage • Modeled randomly (log-normal), i.e., ln(Rmn) is a Gaussian random variable • Small scale effects: • Constructive and destructive interference of multiple signal paths • Spatial scale of carrier wavelength => frequency dependent • Modeled as random linear time-varying impulse response • Time variation due to moving TX, RX and environment (reflectors, scatterers)

33. Measured Channel Impulse Response in 3 – 6 GHz • Power delay profile • Multiple signal paths • Magnitude of transfer function • Frequency selectivity equivalent path loss (from delay)

34. UWB Channel Model • Indoor Channels: >80% of envisioned applications • Between 100 and 400 multipath components • Maximum excess delay up to 100ns • Exponentially decaying multipath components in clusters • multiple scattering Power delay profile of indoor channel with 1GHz bandwidth Maximum excess delay ~ 60 ns

35. Peculiarities of UWB Channels • Very large channel bandwidth from 500MHz up to 7.5GHz • Very accurate multipath resolution • Transmit pulse energy spread over all multipath components • Important to collect energy from as many multipath components as possible • Each path sees independent channel realization • Robustness to fading due to multipath diversity • Optimal: RAKE receiver • Coherent, i.e., estimation of channel impulse response is necessary. • Suboptimal: Energy detector • Non-coherent, low complexity

36. Localization via UWB based on: H. Arslan, Z. N. Chen, M.G. di Benedetto: Ultra Wideband Wireless Communication, Wiley 2006

37. Localization via UWB • UWB IR is good candidate for short-range and low-rate communication networks • Nodes operating on battery (autonomy) • High precision ranging capapility • Derivation of location information from radio signals between target node and reference nodes • Positioning systems categorized into • Direction of arrival (angle) • Signal-strength • Time-based approaches

38. Angle of Arrival • Measure angle of arrival of target node signal at different reference nodes • Antenna arrays required • AoA measurement based on phase (time) difference of received wave-front at different antennas • In 2 dimensional space, 2 reference nodes are enough

39. Cramer-Rao Lower Bound (CRLB) • Lower bound on the variance of an unbiased estimator • For a uniform linear antenna array with N antennas, each with distance d • For  = n (n {…,-1,0,1,…}) the CRLB diverges • Two dimensional antenna grid necessary • Not suited for UWB positioning • More antennas => higher complexity and increased costs • Large number of multipaths => multidimensional search for maximum likelihood AoA estimation

40. Received Signal Strength • Path loss model relates distance between two nodes to energy loss • Distance estimates from 3 reference nodes are required for triangulation in 2 dimensional space

41. Cramer-Rao Lower Bound (CRLB) • Dependence on channel characteristics • Very sensitive to the estimation of channel parameters • E.g. path loss exponent (np), variance of log-normal shadowing (sh2), i.e. • UWB signal characteristic (huge bandwidth) is not exploited

42. Time-Based Approaches I • Measurements of the propagation delay between nodes • Two nodes (A and B) with common clock • Node A sends time-stamped signal • Node B receives delayed version and can estimate time of arrival (ToA) and also the distance by correlation with a template signal • Single path and additive white Gaussian noise (AWGN) channel the CRLB is given by • Beff is the root mean square signal bandwidth for signal s(t) with Fourier transform S(f) • UWB very beneficial here! • However, node synchronization is an important assumption • Accuracy of clocks plays an important role

43. Time-Based Approaches II • N reference nodes (positions pi) with ToA estimates i do positioning via least square minimization • The weights wi reflect reliability of ToA estimates • Method becomes optimal, if ToA measurements are modeled as true ToAs plus independent Gaussian noise samples • Main sources of error in realistic environment • Multipath propagation • Non-line-of-sight propagation (direct path is blocked) • Interference from other nodes or coexisting systems