doc.: ieee 15-05-0409-00-004a tg4a july 15, 2005 gian mario maggio & philippe rouzet (stm)slide...

July 15, 2005

Gian Mario Maggio & Philippe Rouzet (STM)Slide 1

doc.: IEEE 15-05-0409-00-004a

TG4a

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

Submission Title: TG4a MAC Protocol Enhancement ProposalDate Submitted: July 15th, 2005Source: Gian Mario Maggio (STMicroelectronics), Philippe Rouzet

(STMicroelectronics)Contact: Gian Mario MaggioVoice: +41-22-929-6917, E-Mail: [email protected]: Preliminary proposal for potential MAC protocol enhancements in

conjunction with UWB-IR PHY layer, including support for ranging. Purpose: To provide a basis for further discussion on MAC protocol enhancements

(w.r.t. 802.15.4) keeping into account UWB-PHY features.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.

July 15, 2005


doc.: IEEE 15-05-0409-00-004a

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MAC Protocol Enhancementsfor 802.15.4a (UWB-PHY)

List of Contributors:

- G.M. Maggio, P. Rouzet (STMicroelectronics)- J.-Y. Le Boudec, R. Merz, B. Radunovic, J. Widmer (EPFL)- M.G. Di Benedetto, L. De Nardis (U. di Roma)

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Outline

• 802.15.4 MAC overview

• CSMA or not CSMA?

• MAC enhancements: - Interference management

- Ranging procedures

• Proposals: (a) DCCP-MAC

(b) (UWB)2-MAC

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802.15.4 MAC: Characteristics

• Short-range operation

• Star or Peer-to-Peer operation

• Support for low latency devices

• CSMA-CA channel access

• Dynamic device addressing

• Fully handshaked protocol

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• Full function device (FFD)– Any topology– Network coordinator capable– Talks to any other device

• Reduced function device (RFD)– Limited to star topology– Cannot become a network coordinator– Talks only to a network coordinator– Very simple implementation

802.15.4 MAC: Device Classes

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Full function device

Reduced function device

Communications flow

Master/slave

PANCoordinator

802.15.4 MAC: Star Topology

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Full function device Communications flow

Point to point Cluster tree

802.15.4 MAC: Peer-Peer Topology

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• All devices have IEEE addresses• Short addresses can be allocated• Addressing modes:

– Network + device identifier (star)– Source/destination identifier (peer-peer)

802.15.4 MAC: Addressing

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802.15.4 MAC: Frame Structure

Payload

PH

Y L

ayer

MA

CLa

yer

MAC Header(MHR)

MAC Footer(MFR)

MAC Protocol Data Unit (MPDU)

MAC Service Data Unit(MSDU)

PHY Header(PHR)

Synch. Header(SHR)

PHY Service Data Unit (PSDU)

4 Types of MAC Frames:

• Data Frame

• Beacon Frame

• Acknowledgment Frame

• MAC Command Frame

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15ms * 2n

where 0 n 14

Network beacon

Contention period

Beacon extensionperiod

Transmitted by network coordinator. Contains network information,frame structure and notification of pending node messages.

Space reserved for beacon growth due to pending node messages

Access by any node using CSMA-CA

GTS 2 GTS 1

GuaranteedTime Slot

Reserved for nodes requiring guaranteed bandwidth [n = 0].

802.15.4 MAC: SuperFrame Structure

Contention Access Period

Contention Free Period

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• Periodic data– Application defined rate (e.g. sensors)

• Intermittent data– Application/external stimulus defined rate

(e.g. light switch)• Repetitive low-latency data

– Allocation of time slots (e.g. mouse)

802.15.4 MAC: Traffic Types

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OriginatorMAC

RecipientMAC

MCPS-DATA.request

Data frame

MCPS-DATA.confirmMCPS-DATA.indication

Acknowledgement(if requested)

Channelaccess

802.15.4 MAC: Data ServiceO

rigin

ator

Recipient

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15.4a MAC Enhancements: Goal

• Design a MAC strategy tailored for low data-rate networks composed of Impulse Radio (IR) UWB wireless devices

• Innovative features of MAC proposals– Take advantage of the impulsive nature UWB-IR

transmission (quasi-orthogonal TH codes rare “collisions”, not always destructive)

– Support ranging procedures

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CSMA or not CSMA?

• CSMA (Carrier Sensing Multiple Access) is not suitable for UWB-IR signals– UWB-IR: CSMA is basically equivalent to signal

acquisition (with worst-case unknown sequence)

• Note: Contention scheme cannot be ignored completely if a node can only do one thing at a time Mutual exclusion

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Preliminary Study (1/2)

• System model assumptions: – variable (FEC) coding rate– no multi-user detection– flexible power allocations, with peak (voltage) and average (battery)

constraints– random channel states (fading, mobility)– arbitrary schedule (i.e. mutual exclusion in the time domain)– arbitrary routing (possibly multi-path) – protocol overhead of exclusion not accounted for

Numerically solve for proportional fairness

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Preliminary Study (2/2)

• Finding 1: Optimal power control is ON/OFF– send/do not send, but when sending always use max power

• Finding 2: Allow interference– interference is small or negligible because interference

mitigation protects from strong interferers (near-far scenarios)

– It is more profitable to allow interference than to try to implement a mutual exclusion protocol

• Finding 3: Adapt coding rate to channel condition– Adapt to random or time-varying channel– Variations may be due to (residual) interference

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General Approach• Random access protocol (without CSMA)• Synch. is per source-destination pair• THS is generated by a pseudo-random

number generator seeded with the MAC address of the destination

– Proposal A): DCCP-MAC - Dynamic Channel Coding + “Private” MAC

– Proposal B): (UWB)2-MAC - Uncoordinated, Wireless, Baseborn UWB MAC

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(A) DCCP: Introduction

• State-of-the-Art: PHY and MAC are separated– PHY provides a «channel»– The goal of MAC is then «Mutual Exclusion»

• TDMA (GSM), CSMA( WiFi) or combinations (Bluetooth, IEEE 802.15.3)

Notable Exception– CDMA: allows interference requires power control

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(A) DCCP Approach

• MAC for UWB-IR PHY layer:A.1) Interference Mitigation: Detect and cancel the

impact of interfering pulses that have a significantly higher energy than the signal received from the sender

A.2) Dynamic Channel Coding: Continuously adapts the coding rate, packet per packet, to variable channel conditions and interference (backward compatible)

A.3) Private MAC: Resolves contention for the same destination

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(A.1) Interference Mitigation• We assume interference mitigation is

implemented

• Idea: transform interference in erasures– if received energy at demodulator is high,

declare an erasure and ignore the sample(Ex: high = larger than 5 * average output level)

– may be due to collision or noise

kills interfering pulses, but also some valid pulses when noise is high

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Mutual Exclusion

Allow Interference

distance to interferer

Example: Achievable rates with several interferers with/without exclusion protocol

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Interference vs. Mutual Exclusion• Interference should be allowed except when source is inside an

“exclusion region” around a destination D1

D2

S1 D1

S2

D2

S1 D1

S2

S1 and S2 should send

simultaneously and adapt

rates

S1 and S2 should not

send simultaneously

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Proposal (A): DCC + Private MAC

• Our findings indicate that the MAC protocol can be simple:– Send when you want to send– Adapt coding rate to the channel and to interference level Solved by Dynamic Channel Coding (DCC)

• It remains to solve the exclusion problem due to nodes being able to do only « one thing at a time » – a node cannot both send and receive at the same time– a node can receive only from one source Solved by “Private MAC”

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(A.2) DCC with Incremental Redundancy Codes

• A family of codes that cover rates from 1 to 1/32• No penalty for sending incremental bits later

encoder decoder

k data bits R1 k/R1 coded bitsR1

R2 R1 k/R2 - k/R1 bits

incremental redundancy

k data bits R1 k/R2 coded bitsR2

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(A.2) DCC: Source Keeps Track of Best Rate Estimate

• Goal: use the most economical code– set for every packet

– avoid hard failure

• Source keeps estimate of code to use with a safety margin

• Rate is adapted by an adaptation protocol at the MAC layer – no channel estimation required

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(A.3) «Private MAC» and TH Sequences

• Time hopping sequences (THS) are generated by a pseudo-random number generator– Example: linear congruential generator

x(n+1) = a x(n) mod b

where b = 231 -1 and a =16'807

– Seed x(0) is MAC address of destination (in principle, except for ACKs)

• THS is used to generate signal acquisition preamble

• THSs are not perfectly orthogonal, but probability of collision is small– Even two sources using the same THS are unlikely to collide

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(A.3) Private MAC

• Combination of invitation and detection by sender

• Source estimates failure and backs off; S' waits for either ACK or Idle

Concurrent sources do not collide!

• Two THSs per node (Dr, Dt): Dr for transmissions to D, Dt for transmissions from D

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Simulations: No Collapse for Many Users• We implemented the DCCP-MAC in ns2 (PHY to support

interference/collision during transmission)• Performance comparison with:

– mutual exclusion (TDMA, Random Access); power control

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(UWB)2: Uncoordinated, Wireless, BasebornMAC for UWB-LDR communication networks

Proposal (B)

New in (UWB)^2: ranging support, enabling position-based protocols and applications

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(B) (UWB)2 Key features

• (UWB)2 is a Hybrid multi-channel MAC protocol– Each channel is identified with a Time Hopping code– Control packets are transmitted on a shared channel,

i.e. using a common TH-code known to all terminals– Data packets are transmitted on dedicated channels

identified by Transmitter-unique TH codes, and the agreement on the code to be used for a data packet is the result of a handshake performed on the shared code

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Key assumptions Design Choices

TH-CDMA:Shared TH code available to all

devices+

Dedicated data code unique for each transmitter

No Carrier Sensing: pure Aloha

(with TH coding)

Synchronization is achieved on a packet-by-packet basis

Simple Synchronization Hardware

Low Data Rate and rare packets(peak rate 1 Mb/s,

average rate 20 Kb/s)

Time Hopping Impulse Radio with GHz BW

Need for broadcast packets

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Example of Tx procedure:• Step 1: Tx node sends a Link Establishment (LE)

packet to Rx using the Common TH code. The LE packet contains

– IDs of TX and RX– the Tx TH Code

• Step 2: Rx node replies with a Link Confirmation (LC) packet and switches to the Tx TH Code

• Step 3: Tx node sends the DATA packet• Step 4: Rx node sends an ACK packet

Tx

Rx

LELCDATA

Sync Trailer Rx Node ID Tx Node ID

x bits 16 bits16 bits 1 bit 16 bits

TH-Code

TH-Flag


x bits 16 bits16 bits



PDUNumber

8 bits 8 bits

NPACKETS PAYLOAD

M bits



DATA PacketStatus

4 bits

ACK

(B) Transmission and Ranging Procedure

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• The LE LC DATA exchange allows both Tx and Rx terminals to determine their distance:

t0+ + +

t2=t0+ + +

Tx Rxt0

t0 +

t1=t0+ +

Time Time t3=t0+ + +

2 0

2TxRx

t td c c

3 1

2RxTx

t td c c

DATA

LE

LC

July 15, 2005


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MAC: 802.15.4 vs. (UWB)^2• Data rates of 250 kb/s, 40 kb/s

and 20 kb/s

• Star or Peer-to-Peer operation

• Support for low latency

devices

• CSMA-CA channel access

• Fully handshaked protocol for

transfer reliability

• Low power consumption

Possible in (UWB)^2

Possible in (UWB)^2, with different channel access strategy (see below); all topologies defined in 802.15.4 can be adopted without modifications

Possible in (UWB)^2, as long as a slotted time axis is adopted (guaranteed slots can be defined, as in 802.15.4)

Replaced by Aloha in (UWB)^2:- Pure Aloha in Peer-to-Peer operations- Pure/Slotted Aloha in Star operations (where a slotted time axis can be provided by the Network coordinator)

Same for (UWB)^2 (optional acknowledgment is already in the protocol, as in 802.15.4)

Potentially improved in (UWB)^2, since in low bit rate scenarios Aloha can be adopted, without need for beacons to define the time axis, thus saving power.

July 15, 2005


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References1. R. Merz, J. Widmer, J. Y. Le Boudec, B. Radunovic

"A Joint PHY/MAC Architecture for Low-Radiated Power TH-UWB Wireless Ad-Hoc Networks“ In Wireless Communications and Mobile Computing Journal, Special Issue on Ultrawideband (UWB) Communications, to appear, also at: http://lcawww.epfl.ch/Publications/Merz/MerzWLBR05.pdf

2. M.-G. Di Benedetto, L. De Nardis, M. Junk, G. Giancola, "(UWB)^2: Uncoordinated, Wireless, Baseborn, medium access control for UWB communication networks," to appear in Mobile Networks and Applications special issue on WLAN Optimization at the MAC and Network Levels ( 3° quarter 2005).

3. L. De Nardis and M.-G. Di Benedetto, “Joint communications, ranging, and positioning in low bit rate Ultra Wide Band networks,” IEEE INFOCOM 2005 Student Workshop, March 14 2005, Miami, Florida, U.S.A.

4. L. De Nardis, G. Giancola, M.-G. Di Benedetto, "Power-Aware Design of MAC and Routing for UWB Networks", in Proceedings of the IEEE Global Telecommunications Conference (Globecom), 2004, 19 November - 3 December 2004.

doc.: ieee 15-05-0409-00-004a tg4a july 15, 2005 gian mario maggio & philippe rouzet (stm)slide...

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