Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks

Transcription

1 Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks J. Keith Townsend William M. Lovelace, Jon R. Ward, Robert J. Ulman N.C. State University, Raleigh, NC N.C. A&T State University, Greensboro, NC The Johns Hopkins Applied Physics Laboratory, Laurel, MD U.S. Army Research Office, Research Triangle Park, NC Supported ARO contract number DAAD Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.1/24

2 Outline of Presentation Problem statement and motivation UWB impulse radio (IR) characteristics Chip discrimination Chip discrimination in multipath channels Adaptive rate control ALOHA multiple access protocol applied to IR Results Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.2/24

3 Motivation: Ad Hoc IR Communications Problems: Large Near-Far power disparity High power interferers have significant affect on performance Minimal centralized control of network Keep transmitted pulse density at a minimum Maximize network packet throughput Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.3/24

4 Motivation: Ad Hoc IR Communications Solutions: "Chip discrimination" and "adaptive rate control" Present an algorithm for packet transmission that combines chip discrimination and rate adaptation with ALOHA Techniques utilize locally obtainable information at the receiver Adaptive rate control achieves close to the optimal throughput with lower transmitted pulse densities Less interference => less transmitted power Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.4/24

5 Impulse Radio Physical Layer: Pulses 1 Ideal Transmitted Normalized Pulse 1 Ideal Received Normalized Pulse Normalized Amplitude time(nsec) Normalized Amplitude time(nsec) Pulse width less than 1 ns 2 GHz bandwidth, low power spectral density (Watts/Hz) Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.5/24

6 Impulse Radio Physical Layer Timing packet time Tf 1bit pulse Ns pulses bit time time Relationship between pulse, bit, and packet timing Low duty cycle transmission Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.6/24

7 Ad Hoc Network Scenerio T/R Collocated & Uncoordinated System Tx Rx T/R T/R Distant Transponders T/R T/R T/R Issues: Near-Far problem", no base stations, minimal centralize control Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.7/24

8 Chip Discrimination: Overview T c 2 T 3 T c c t Receiver knows hopping code of desired signal 3 interferers: can collide with small probability Correlator sum overwhelmed by high power interferer since threshold exceeded, results discarded for this chip Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.8/24

9 RAKE Receiver with Chip Discrimination r(t) v(t φ (0) x x 0 ) v(t φ (0) 1 ) T p dt T p dt Z 0 Z 1 x x Chip Discriminator α 0 α 1 Z chip Z bit Detector ˆb v(t φ (0) x L 1 ) T p dt Z L 1 x α L 1 Each RAKE finger demodulates one resolvable path of the multipath channel Apply chip discrimination to each path Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.9/24

10 Adaptive Rate Control: Overview Estimation Interval Desired Packet High Power Interferers time Desired link packet (blue), interference estimation interval (green) uses same chip discrimination logic as before Estimates number of high power interfering packets (red) Adjust chip/bit rate based on interferer count to maximize throughput Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.10/24

11 Adaptive Rate Control: Rate Table Provides optimal chip/bit rate for packet transmission based on amount of interference Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.11/24

12 Adaptive Rate Control: Rate Table Previous work: Assumed the number of interferers,, known at bit 1" (ideal) Practical approach used here: Algorithm counts the number of interferers present during the observation interval Uses rate table to set optimal chip/bit rate Rate table developed by trials Achieves near optimal throughput with large reduction in pulse density Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.12/24

13 ALOHA for Impulse Radio ALOHA: Transmit packet listen for collision within max roundtrip time Unlike conventional" ALOHA, here we assume SOFT COLLISIONS Packet success determined for all degrees of overlap Exploits low duty cycle nature of impulse radio Consider for cases with and without channel load sensing (CLS) Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.13/24

14 System Model and Analysis 1#! "t#µ(k 1 )"t! "t! "t k-1 k k+1 µ(k 1 )"t µ(k 1 )"t Model transition from to Markov process Transition probabilities: Poisson arrival rate Departure rate, interferers as a packets/sec is the number of 1#! "t#µ(k 1 )"t 1#µ(k 1 )"t interfering packets present at bit one! "t! "t Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.14/24

15 System Model and Analysis: CLS Protocol 1#! "t#µ(k 1 )"t! "t! "t 1#µ(k 1 )"t k-1 k $-1 µ(k 1 )"t µ(k 1 )"t Transmitter does not send a packet if interferers or greater are detected Same Markov model, except limit to interferers Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.15/24

16 Results: Chip discrimination Hard Limiting Receiver Front End (fixed chip rate) Chip Discrimination Receiver (fixed chip rate) Comparison ALOHA and CLS Protocol Variable rate algorithm performance Comparison between assumed and estimated interference level Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.16/24

17 Chip Discrimination 10 0 No Discrimination BER 10-6 Chip Discrimination 9dB # Collocated Transponders 1000 far interferers, 9 db threshold, 40 db I/S Greatly reduces effects of high power interferers Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.17/24

18 Hard Limiting Receiver n=16 n=32 n=8 n=4 ThroughputS Offered Load G Bit rate adjusted to maintain packet duration = 4 ms, 5000 bits binary PPM signaling, pulse width = 1.2 ns Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.18/24

19 Chip Discrimination Receiver Simulation n=16 n=32 n=8 n=4 ThroughputS Offered Load G Assume limiting case where all pulse collisions result in blanking Same packet parameters: 4 ms, 5000 bits per packet, binary PPM Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.19/24

20 Chip Discrimination: CLS Protocol 7000 ThroughputS Tf= 25ns Tf=100ns Tf= 50ns Tf=10ns Offered Load G Longer packets better with improved distribution of colliding pulses Benefit of soft collisions evident Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.20/24

21 Adaptive Rate Adapt chip rate based on dynamically ALL interferers also select chip rate based on interference detected at bit one of the packet Packet duration variable, fixed number of bits/packet (5 kbits) Consider 2 cases: chip discrimination known, and estimated using Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.21/24

22 Performance Comparison Variable Rate ALOHAn=16 CLSP!=17 Var K1 known Var Pulse Est. ThroughputS OferedLoadG Fixed rate Adaptive rate with known chips/bit CLSP optimal nearly optimal Interference estimation close to optimal performance Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.22/24

23 Reduction in Pulse Density PulseDensity(x 10 6 ) n=16 Var K1 known Var Pulse Est. ReductioninPulseDensityOvern=16:(%) Offered Load G Offered Load G Variable rate results in reduced total transmitted pulse density compared to a fixed, 16 chip/bit case Less overall interference, lower average transmitted power Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.23/24

24 Conclusions Presented MAC layer technique for peer-to-peer, UWB impulse radio Combines chip discrimination, rate adaptation, and unslotted ALOHA Addresses the problem of interference from high power interferers Operation based on locally derived information at the receiver Achieves nearly theoretical throughput at reduced pulse densities Ultra-Wideband Impulse Radio for Tactical Ad Hoc Communication Networks p.24/24