UWB for Sensor Networks:

Transcription

1 IEEE-UBC Symposium on future wireless systems March 10 th 2006, Vancouver UWB for Sensor Networks: The 15.4a standard Andreas F. Molisch Mitsubishi Electric Research Labs, and also at Department of Electroscience, Lund University

2 CONTENTS Introduction and Motivation UWB overview Sensor networks and applications The PHY layer of 15.4a Modulation and waveforms Coding and spreading Band plans and data rates The MAC layer of 15.4a Multiple access Ranging Summary Slide 2

3 Basic Features (I) Uses same spectrum as existing services 1. Information spread over wide spectrum; low power spectral density 2. Very low power Small interference looks like noise to other systems a (100MHz) UWB (7.5 GHz) Part 15 Limit 3.1GHz GHz 10.6GHz Frequency Slide 3

4 Basic Features (II) High location resolution Short pulses allow accurate measurement of echo time Low power (battery!) Simple and cheap transceiver structures possible Inherent security Slide 4

5 Large Absolute Bandwidth High resistance to fading Fine delay resolution; impulse response resolved into many delay-bins Fading within each delay-bin is smaller Sum of all bins have even less fading Mechanisms for suppression of interference spreading gain: depends on data rate exclusion of frequency range with heaviest interference Impulse response Power bin 1 bin 2 > < < 2ns bin 3 bin 4 Sum of powers Slide 5 time

6 Large Relative Bandwidth Good wall and floor penetration (for some frequency ranges) Low-frequency components can go through material Implementation Problems: - antennas, - Amplifiers - Wideband Frontend Electronics - Frequency Agility (FH) Advantages: - pure baseband system - no RF devices Slide 6

7 Key Features of Ad-hoc Sensor Networks - Automatic network formation and dynamic self configuration source - Scalable (expandable in size) - Simple cost effective protocols (route discovery, route recovery, broadcast, multicast etc) - Low power consumption (users expect battery to last months to years!) Before broadcast source D D D Before multicast After broadcast After multicast Slide 7

8 Sensor Networks Environmental Monitoring SENSOR NETWORKS Application Network Infrastructure Comm. Comm. Sensor Sensor node Sensor node Sensor Environment Slide 8

9 CONTENTS Introduction and Motivation UWB overview Sensor networks and applications The PHY layer of 15.4a Modulation and waveforms Coding and spreading Band plans and data rates The MAC layer of 15.4a Multiple access Ranging Summary Slide 9

10 Basic Philosophy Allow detection with Coherent receivers (high-complexity, high-performance) Noncoherent receivers (low-complexity, low-performance) in the same network Separate spreading structure for Preamble (acquisition, ranging, channel estimation) Payload data Data rates: 1 Mbit/s (mandatory) 0.1, 4, 24 (optional) Slide 10

11 Packet Structure Preamble Header Payload SYNCSYNC/CE SFD SYNC: Synchronization Field SFD: Start Frame Delimiter Field CE: Channel Estimation Field S i S i S i S i S i S i -S i 0 -S i 0 -S i 0 -S i 0 Copyright IEEE Periodic structure of preamble enables synchronization, Is also used for ranging Slide 11

12 Modulation for Preamble S i =PBTS (Perfect Balanced Ternary Sequences) The code can be selected from length-31 or length-127 PTBS Length-31 ternary codes: Index ID Sequence 1 S S S 3 S Copyright IEEE 5 S S Adaptive pulse repetition frequency: to be able to adjust to different delay spreads Slide 12

13 Auto-correlation & Cross-correlation (L = 31) 15 Coherent Receiver: Periodic Autocorrelation Function Non-Coherent Receiver: Periodic Autocorrelation Function Coherent Receiver: Periodic Cross-correlation Function 4 Non-Coherent Receiver: Periodic Cross-correlation Function Copyright IEEE Slide 13

14 Modulation Format for Data Transmission of bursts of pulses Modulation: combination of PPM and BPSK Spreading: Time hopping of bursts Polarity scrambling of pulses Important time constants (typical): Chip duration: 2ns Burst duration: 32ns Burst hopping duration: 32ns PPM position duration: 502ns Guard time: 256ns Slide 14

15 C O H. 00 N O N C O H. 0 - Modulation S PPM bit (seen by coherent and non coherent receiver) BPSK bit (seen by coherent receiver only) S S PPI burst symbol duration -S Slide 15 Copyright IEEE

16 Spreading Bursts of 16 pulses Time hopping of the pulse bursts Polarity scrambling of the pulse bursts Purpose: Multiple-access capability - Polarity scrambling for coherent receivers - TH for noncoherent receivers Spectral smoothing Slide 16

17 Spreading - Polarity scrambling S = Slide 17

18 Spreading burst hopping C O H. N O N C O H. possible positions obtained through scrambling Guard time for channel delay spread (260ns) S S S S S S -S -S -S -S S S S S S -S -S -S Note: S value is also changed at each symbol Copyright IEEE Slide 18

19 Coding Concatenated coding Outer code: Reed-Solomon Inner code: systematic convolutional code How does the inner code work: Systematic bits are mapped onto position of bursts -> can be detected with noncoherent receiver Redundant bits are mapped into polarity of pulses -> can be detected with coherent receiver Noncoherent receiver: sees only RS-encoded stream Coherent receiver: Concatenated decoding, inner code can feed soft or hard decisions to RS Decision of inner code based on position and polarity Slide 19

20 Coding - details Reed Solomon Primitive polynomial: Generator polynomial: 6 1+ x + x g 7 i ( x) = ( x + α ) i= 0 Slide 20 Copyright IEEE

21 Coding: How to Decode Parity / Data [p d] Tx Signal Rx Signal [x 1 x 2 ] Correlator Ouput 0 0 s 0 s + w 1 w 2 C 1 = +x 1.s H 1 0 -s 0 -s + w 1 w 2 C 2 = -x 1.s H s w 1 s + w 2 C 3 = +x 2.s H s w 1 -s + w 2 C 4 = -x 2.s H Soft decision rule: soft_bit(bit) = max(c k s where bit is 0) - max(c k s where bit is 1) soft_bit(p) = max(c1,c3) - max(c2,c4) = C 1 + C3 soft_bit(d) = max(c1,c2) - max(c3,c4) = C 1 - C 3 Slide 21 Copyright IEEE

22 Bandplan Low frequency band All frequencies derived from single LO (13 or 19 MHz) f GHz High frequency band PSD dbm/mhz f GHz Slide 22 Copyright IEEE

23 Advanced Pulseshapes Purposes: Better spectral properties Increased number of piconets Simpler implementation All advanced pulseshapes are optional Simple pulseshape is mandatory 1. Weighted linear combination of pulses 2. Chirp pulses (CoU) 3. Continuous Spectrum (CS) pulses 4. Chaotic pulses Slide 23

24 Linear Combination of Pulses Solution: linear combination of delayed, weighted pulses Adaptive determination of weight and delay Number of pulses and delay range restricted Can adjust to interferers at different distances (required nulldepth) and frequencies Weight/delay adaptation in two-step procedure - Initialization as solution to quadratic optimization problem (closed-form) - Refinement by back-propagating neural network Matched filter at receiver good spectrum helps coexistence and interference suppression Slide 24

25 CONTENTS Introduction and Motivation UWB overview Sensor networks and applications The PHY layer of 15.4a Modulation and waveforms Coding and spreading Band plans and data rates The MAC layer of 15.4a Multiple access Ranging Summary Slide 25

26 Multiple Access ALOHA Send out packets without listening to channel Reasons: Devices have low duty cycle (1%) -> ALOHA is efficient Random scrambling and time hopping during payload transmission -> carrier sense becomes extremely difficult Optional modifications: Option 1: device senses for preamble on the air, then waits for maximum packet length Option 2: preamble-type signal is time-multiplexed into the data packet, allowing devices to determine whether channel is occupied Slide 26

27 Ranging for 15.4a Distinguishing feature: first standard with explicit requirement Ranging should be accurate within 1m Possible ranging technologies: Received signal strength Direction of arrival Time of arrival Find arrival time of first multipath component Slide 27

28 Difficulty in Distance Measurement Accurate TOA detection under multipath, multi-user interference and white noise is challenging Transmitted signal Interference (signal from other users) Strongest Path noise Actual TOA Leading Edge Received signal Slide 28

29 Difficulty in Distance Measurement What should be the right threshold level? Depends on noise level and maximum signal strength How far back from the strongest we should search for the first arriving signal? Risk: mistake a noise peak for a first component Search window Strongest Path threshold Search-back direction (Received signal) Slide 29

30 Searchback Scheme Searchback starts from the peak sample, and searches back till the leading edge based on a threshold If there are multiple clusters (as in certain UWB channels), they have to be dealt with appropriate algorithms The waveform used in preamble also impacts the clustering of the samples, as well as location of the peak sample Slide 30

31 Private versus Public Ranging Copyright IEEE For many applications, location of devices should be disclosed only to authorized inquiries Possible attacks: Snooper attack: hostile device listens to ranging exchanges Impostor attacks: - hostile device sends range request, finds out range - hostile device gives answer, provides wrong range to inquirer Jamming attack: hostile device jams during transmission of ranging signal Slide 31

32 Private Ranging Two-step ranging: Authentication and Ranging Turn around time is randomly manipulated to add more security - Snoopers will receive worthless or no information - Impostors will be avoided Ranging waveform is chosen pseudorandomly - Prevents snooper from getting information at all - Prevents jamming Copyright IEEE Slide 32

33 Authentication Step Conventional authentication + ranging waveform settings are conveyed to the target in the encrypted payload of the authentication packet Authentication Packet Structure Ranging Step Ranging symbol (modified) is repeated No need for header or payload Ranging Packet Structure Slide 33

34 Summary and conclusion UWB is well suited for sensor networks Low power Large number of nodes possible Robust Precise location possible IEEE a gives UWB for mixed sensor networks Spreading: bursts of pulses with randomized polarity + burst hopping Modulation: BPSK + burst-ppm, combined with concatenated coding structure Allows simple detection with noncoherent receivers and great performance improvements with coherent receivers Ranging: Based on round-trip time of arrival Private ranging Slide 34

35 Acknowledgements Thanks to Dr. Jin Zhang Dr. Kent Wittenburg Dr. Phil Orlik Dr. Zafer Sahinoglu Sinan Gezici Prof. Vince Poor Dr. Makoto Miyake Johan Karedal Dr. Dajana Cassioli Prof. Francesco Vatalaro Dr. Gianmario Maggio (a number of figures in this presentation are from one of his IEEE contributions, reproduced with his kind permission) The IEEE a group Slide 35