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Multiple Access Options for UWB WPANs

This submission discusses the motivation for UWB personal area communications, the challenges of UWB multiplexing, and the multiplexing technologies for UWB networks.

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Multiple Access Options for UWB WPANs

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Multiple Access Options for UWB WPANs] Date Submitted: [ 9 September, 2002] Source: [Matthew Welborn] Company [Xtreme Spectrum, Inc.] Address [8133 Leesburg Pike, Vienna VA 22180] Voice:[(703) 269-3052], FAX: [(703) 269-3092], E-Mail:[mwelborn@xtremespectrum.com] Re: [] Abstract: [Multiple Access Options for UWB WPANs, including motivation for UWB personal area communications, understanding the challenge for UWB multiplexing and multiplexing technologies for UWB networks] Purpose: [Provide information to the 802.15 Study Group 3a] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Matthew Welborn, Xtreme Spectrum, Inc.

  2. Multiple Access Options for UWB WPANs Matt Welborn, John McCorkle, Tim Miller, and Jerry Lynch XtremeSpectrum, Inc. Matthew Welborn, Xtreme Spectrum, Inc.

  3. Outline • Motivation for UWB personal area communications • Understanding the challenge for UWB multiplexing • Multiplexing technologies for UWB networks • FDM, TDM, CDM Matthew Welborn, Xtreme Spectrum, Inc.

  4. Motivation for Ultra-Wideband Communications • UWB is legal - Recent FCC Report and Order • Extremely wide bandwidth: 3.1 to 10.6 GHz • Very low power densities: 500mv/m/MHz @ 3m • Key app for UWB is battery-powered high-rate digital comm’s • 100+ Mbps, ten meter range • UWB as a PHY technology for WPANs • Native support for multimedia and QoS traffic • UWB has significant advantages for the applications identified in the CFA Matthew Welborn, Xtreme Spectrum, Inc.

  5. Motivation for Ultra-Wideband Communications • UWB provides advantages relative to other technologies • Robust high QoS in multipath environment • Low complexity/low cost implementations • Extremely high data rates relative to other unlicensed technologies What is the best multi-user multiplexing technology to leverage UWB’s strengths in today’s (and tomorrow’s) networks? Matthew Welborn, Xtreme Spectrum, Inc.

  6. Contention Access Period Contention Free Period Beacon 1 2 3 4 5 6 7 8 9 Guaranteed Time Slot Management Time Slot Unassigned Time Multiple Access Between WPANs --- Not Within a WPAN • Within a WPAN, piconet coordinator (PNC) prevents interference through MAC-layer assignment of transmission slots to other devices (DEVs) using time-division multiple access Matthew Welborn, Xtreme Spectrum, Inc.

  7. Multiple Access Between WPANs --- Not Within a WPAN • Multiple access interference (MAI) between piconets will be dealt with through PHY-layer multiplexing techniques PNC PNC PNC Matthew Welborn, Xtreme Spectrum, Inc.

  8. Key Factors Affecting Choice OfIntra-Piconet and Inter-Piconet Structure • Take advantage of a priori knowledge of within piconet • TDMA is best fit • Coordination is easy • Enables very efficient use of data bandwidth – high aggregate data rates • High QoS – Guaranteed rates between nodes – good for video • Enables effective yet simple power management – scheduled wakeup times • Goals for multiplexing between piconets • No coordination • Security • Synchronization timing • Maintains independence • Scaling to high aggregate data rates Matthew Welborn, Xtreme Spectrum, Inc.

  9. Compromises the benefits of UWB propagation Complex to realize Multiple pulse generators (different center frequencies) Poor channel isolation without long time sidelobes Reduced TX power Example case of 3 bands leads to minimum of 5 dB reduction in TX power relative to single-band system Fixed Allocation is Inefficient Fixed allocation of multiple bands means inferior performance, even when no other piconets are present (e.g. other channels are empty) FDM Has Major Problems for UWB Matthew Welborn, Xtreme Spectrum, Inc.

  10. FDM Loses the Benefits UWB Brings • The fractional bandwidth of the signal: • Fractional BW utility is based on physics of wave interaction(i.e. Scattering and loss phenomenology) • The interaction of UWB emissions with the environment enables utility that cannot be obtained with narrowband • Objects in the environment reflect RF signals preferentially in narrow frequency bands according to their size • Minimizing multipath by minimizing the number of objects reflecting the signal (i.e. objects < ¼ ) • Capability to penetrate at high data rates and high resolution • Skin effect • Fractional BW minimizes small-scale fading (scintillation) or ambiguous multipath Matthew Welborn, Xtreme Spectrum, Inc.

  11. 1 0.5 1 1 1 2 3 4 0.5 0.5 0.5 0 0 0 0 - 0.5 - 0.5 - 0.5 - 0.5 - 1 - 1 - 1 - 1 - 1 - 0.5 0 0.5 1 - 1 - 0.5 0 0.5 1 - 1 - 0.5 0 0.5 1 - 1 - 0.5 0 0.5 1 1 8 0.5 0 - 0.5 - 1 - 1 - 0.5 0 0.5 1 1 12 0.5 0 - 0.5 - 1 - 1 - 0.5 0 0.5 1 1 20 0.5 0 - 0.5 - 1 - 1 - 0.5 0 0.5 1 First 20 Derivatives of a GaussianShow Progression of UWB to Narrowband • Like modulated sine wave at higher derivatives • Allows study of scaling of relative bandwidth Matthew Welborn, Xtreme Spectrum, Inc.

  12. Gaussian Modulated Cosine 1 0 1 -6 -4 -2 0 2 4 6 8 10 12 In-Phase Case Out-of-Phase Case u 2 1.5 1 0.5 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 8 UWB Multipath Interferometry Has Much Less ScintillationThan Equal Resolution Narrowband Matthew Welborn, Xtreme Spectrum, Inc.

  13. 0 Opposite-Phase 5 Equal-Phase 10 Worst Case Null Due to Multipath (dB) 15 20 25 0.6 0.8 1 1.2 1.4 1.6 Chief Advantage of UWB – Robustness in Multipath(Worst Multipath Null Is Minimized With High Relative Bandwidth) • Benefits of UWB fall as relative bandwidth drops • For sub-optimal modulation (e.g. PPM) the curves move down • Argues that UWB definition and designs be > 80% at -10dB points Relative Bandwidth at –10 dBp Matthew Welborn, Xtreme Spectrum, Inc.

  14. Time Piconet A Piconet A Piconet B Time Time Division Multiplexing Has Several Cases • Course-grained TDM • Channel is multiplexed at packet-length granularity (or larger) • Contention Free Case • Contention Based Case (e.g. CSMA) • Fine-grained or “interleaved” TDM • Channel is multiplexed at pulse-length granularity • Synchronous Case • Asynchronous Case Matthew Welborn, Xtreme Spectrum, Inc.

  15. Tx Rx Rx Tx Interleaved TDM Cases • Synchronous case • Requires precise synchronization between piconets • Correct offset must be found based on receiver time scale • Correct offset is a function of propagation distance • Asynchronous case (e.g. “time-hopping”) • Transmitters use pseudo-random timing to alleviate requirements for synchronization • Occasional collisions result, minimal if duty cycle is low (<1%) Matthew Welborn, Xtreme Spectrum, Inc.

  16. Interleaved TDM Has Serious Drawbacks • Multipath and propagation delays compromise orthogonality • Multipath looks like more users • Duty cycle limitations prevent scaling to higher data rates • Higher density of pulses leads to more frequent collisions and high error rates • Long synchronization times required • Not suitable to RF IC process technology • Low duty-cycle leads to higher peak-to-average power Collision = Detection Error Matthew Welborn, Xtreme Spectrum, Inc.

  17. Already a fall-back mode in 802.15.3 MAC protocol Desired Features Contention Free Deterministic QoS Low Overhead Contention Based (CSMA) Uncoordinated Significant Problems Contention Free Requires coordination between piconets Requires a common time base (synchronization) Results in decreased per-piconet capacity- doesn’t scale to high aggregate data rates. Contention Based (e.g. CSMA) Collision overhead Results in decreased per-piconet capacity- doesn’t scale to high aggregate data rates. Error prone in wireless environments (e.g. hidden-node) Limited QoS ability Course Grain Time Division MultiplexingHas Significant Problems Matthew Welborn, Xtreme Spectrum, Inc.

  18. Code Division Multiplexing Is A Good Choice ForUWB Personal Area Networks • CDM uses code design techniques to support multi-piconets • Codes chosen to provide • Low cross-correlation • Spike autocorrelation • Spectrum control • Compatible with low voltage RF IC process technology-- Provides low peak-to-average power • Provides superior MAI performance – next slide • Provides the same fundamental ranging capability as TDM • Similar to many pulse coded radar systems Matthew Welborn, Xtreme Spectrum, Inc.

  19. Receive Waveform Correlating Waveform (Matched Filter) Matched Filter Output Single Pulse Direct Sequence MAI Properties of CDM Are Well Understood And Allow Highest Aggregate Data Rates • CDM naturally operates at higher pulse rates • Significant advantages of multiple pulses-per-symbol • Key is that CDM leads to scalability of aggregate data rate because MAI looks Gaussian for sufficiently long (>~8) codes • Interleaved TDM does not look Gaussian at high data rates • Relevant MAI measurement point is the output of the RX matched filter Matthew Welborn, Xtreme Spectrum, Inc.

  20. Summary of Multiplexing Options for WPANs • FDM between piconets is a poor choice • Spectral overlap • Power reduction • Trades away the fundamental advantages of UWB (e.g. robustness in multipath etc.) • Not enough bandwidth in the FCC rules to support more than one true “UWB” channel. • TDM between piconets is poor choice • Requires coordination • Does not scale well to higher aggregate data rates • CSMA is not generally suitable for all apps • Poor QOS – not suitable for streaming video • CDM techniques are a particularly good match for UWB • Allows multiple piconets to be relatively independent • Produces the highest aggregate data rate – combats MAI in high multipath • Compatible with high QoS, video streaming capable MACs • For example: TDMA-based IEEE 802.15.3 • Matches high-speed low-voltage RF IC technology • Low peak-to-average pulse trains that use high chipping rates. Matthew Welborn, Xtreme Spectrum, Inc.

  21. Multi-user Perspectives in UWB Communications Networks Presented by: Matt Welborn mwelborn@XtremeSpectrum.com (703) 269-3052 www.xtremespectrum.com Matthew Welborn, Xtreme Spectrum, Inc.

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