doc.: ieee 802.15-02/382r1 submission september 2002 matthew welborn, xtreme spectrum, inc.slide 1...

September 2002

Matthew Welborn, Xtreme Spectrum, Inc.Slide 1

doc.: IEEE 802.15-02/382r1

Submission

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)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:[[email protected]]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.

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Multiple Access Options for UWB WPANs

Matt Welborn, John McCorkle, Tim Miller, and Jerry LynchXtremeSpectrum, Inc.

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Outline

• The challenges for UWB multiplexing• Multiplexing technologies for UWB networks

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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?

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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

ContentionAccessPeriodB

eaco

n Contention Free Period

1 2 7 86 93 4 5

Management Time SlotGuaranteed Time Slot Unassigned Time

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Multiple Access Between WPANs --- Not Within a WPAN

• Multiple access interference (MAI) between piconets will be dealt with through PHY-layer multiplexing techniques

PNCPNC

PNCPNC

PNCPNC

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Key Factors Affecting Choice OfIntra-Piconet and Inter-Piconet Structure

• Take advantage of a priori knowledge 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

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Multiplexing Technologies for UWB WPANS

• Options considered here: – FDM, TDM (two flavors), CDM

• Assumptions:– FCC regulations– SG3a-type applications

• High data rates: 110, 200, 480+. In the context of a pulsed UWB system, this means PRF is likely large (100+ MHz ?), so (average) pulse interval would be < 10 ns (binary mod.)

• Low power consumption, low cost• Minimum ~4 piconets• Operation with 802.15.3 MAC

September 2002


doc.: IEEE 802.15-02/382r1

Submission

FDM Has Major Problems for UWB• 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)

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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

1

2

lh

lh

cf ff

fffBB

September 2002


doc.: IEEE 802.15-02/382r1

Submission

The Effect of Fractional Bandwidth on Small-scale Fading

• Simplified, two-ray propagation model, with variable differential delay less than the minimum resolvable time based on absolute bandwidth

• Receiver sees the sum of the pulses, interference effect depends on the differential delay

• Severity of fading depends on the fractional signal bandwidth

Variable Delay

Receiver

UWBSignal

September 2002


doc.: IEEE 802.15-02/382r1

Submission

UWB Multipath Interferometry Has Less ScintillationThan Equal Resolution Narrowband

• Two pulses with equal absolute bandwidth, different fractional bandwidth

• Lower fractional bandwidth leads to more severe worst-case fading

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Chief Advantage of UWB – Robustness in Multipath(Worst Multipath Null Is Minimized With High Relative Bandwidth)

• Benefits of UWB fall as relative bandwidth drops

Relative Bandwidth at –10 dBp

Wor

st C

ase

Nul

l Due

to M

ultip

ath

(dB

)

0.6 0.8 1 1.2 1.4 1.625

20

15

10

5

0

Opposite-Phase

Equal-Phase

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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

Piconet APiconet A Piconet BPiconet B Piconet APiconet ATimeTime

TimeTime

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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%)

RxRx

TxTx

TxTx

RxRx

September 2002


doc.: IEEE 802.15-02/382r1

Submission

• 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

• Required synchronization times?• Suitability to RF IC process technology

– Low duty-cycle leads to higher peak-to-average power

Interleaved TDM Has Serious Drawbacks

Collision = Detection ErrorCollision = Detection Error

September 2002


doc.: IEEE 802.15-02/382r1

Submission

Course Grain Time Division MultiplexingHas Significant Problems

• Already a fall-back mode in 802.15.3 MAC protocol

• Desired Features– Contention-free

• Deterministic QoS• Low Overhead

– Contention Based (CSMA)

• Uncoordinated

• Significant challenges:– 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

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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 more Gaussian-like for sufficiently long (>~8) codes– Interleaved TDM does not look Gaussian at high data rates (PRF=Baud)– Relevant MAI measurement point is the output of the RX matched filter

Single Pulse

Direct Sequence

ReceiveWaveform

Correlating Waveform(Matched Filter)

Matched FilterOutput

September 2002


doc.: IEEE 802.15-02/382r1

Submission

MAI Properties of CDM Are Well Understood And Allow Highest Aggregate Data Rates

September 2002


doc.: IEEE 802.15-02/382r1

Submission

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 for 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.

doc.: ieee 802.15-02/382r1 submission september 2002 matthew welborn, xtreme spectrum, inc.slide 1...

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