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Robustness of Uncoordinated MAC in channelimpaired Low Data Rate UWB networks

Luca De Nardis, Guerino Giancola, Maria-Gabriella Di BenedettoUniversity of Rome La Sapienza

UWB4SN Workshop, EPFL, Lausanne, November 4 2005

• 802.15.4– Ultra-low complexity, ultra-low power, ultra-low cost– Low Data Rate transfers (sensors, automation,

interactive toys): 20 – 250 Kb/s– Personal and Local Area Network: at least 10 meters

range

Motivation of the work

• 802.15.4– Ultra-low complexity, ultra-low power, ultra-low cost– Low Data Rate transfers (sensors, automation,

interactive toys): 20 – 250 Kb/s– Personal and Local Area Network: at least 10 meters

range• 802.15.4a

– Ultra-low complexity, ultra-low power, ultra-low cost– Low Data Rate transfers (sensors, automation,

interactive toys): 10 – 250 Kb/s– Local Area Network: 30 meters range (extended to

100s of meters by relaying)– Location and tracking capability


General research goal

• To design MAC and routing strategies tailored forlow data rate networks composed of ImpulseRadio (IR) Ultra Wide Band wireless devices,capable of:– taking advantage of the specific characteristics of IR-

UWB– achieving a high energy efficiency in order to

• extend network lifetime• reduce power emissions and help coexistence


UWB-specific MAC design

• Ultra Wide Band based on time hopping CDMA andimpulse radio is characterized by:– Low probability of pulse collision– Accurate ranging (theoretical limit far below 1 cm)

• Medium Access Control can therefore:– Exploit high resistance to Multi User Interference in Low

Data Rate (LDR) applications– Define procedures for bidirectional distance estimation

(UWB)2: Uncoordinated, Wireless, Baseborn medium access for UWBDi Benedetto, M.-G., L. De Nardis, M. Junk, and G. Giancola, “(UWB)2: Uncoordinated, Wireless,Baseborn medium access for UWB communication networks,” Springer Mobile Networks andApplications, Special issue on WLAN Optimization at the MAC and Network Levels, vol. 10, issue5, pp. 663-674, October 2005


(UWB)2 MAC: Design choices

Key assumptions Design Choices

TH-CDMA:Common signaling code available

to all devices+

Dedicated data code unique foreach transmitter

No Carrier Sensing: pure Aloha(with TH coding)

Synchronization is achieved on apacket-by-packet basisSimple Synchronization Hardware

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

average rate ≤ 20 Kb/s)

Time Hopping Impulse Radio withGHz BW

Need for broadcast packets


(UWB)2 Key features

• (UWB)2 is a Hybrid multi-channel MACprotocol– Each channel is identified with a Time

hopping code– Control packets are transmitted on a Common

channel, i.e. using a Common TH-code knownto all terminals

– DATA packets are transmitted on dedicatedchannels identified by Transmitter-unique THcodes, and the agreement on the code to beused for a DATA packet is the result of ahandshake performed on the Common code


Step 1: Tx node sends a LinkEstablishment (LE) PDU to Rxusing the Common TH code. TheLE PDU contains:

•IDs of TX and RX•the Tx TH Code•Sync trailer + CRC field

Step 2: Rx node replies witha Link Confirmation (LC) PDUand switches to the Tx THCode

Tx

RxLE

Sync Trailer Rx Node ID Tx Node ID TH-Code

TH-Flag

LE PDU

CRC

Tx

RxLC

LC PDU

Sync Trailer Rx Node ID Tx Node ID CRC

The Tx procedure is composed of 4 steps:

(UWB)2 MAC: Transmission andRanging procedure


Step 3: Tx node sends theDATA PDU on the dedicated,transmitter-specific TH codecommunicated in the LE PDU

Step 4: Rx node sendsan ACK packet whenrequired by the Txnode (not sent forPDUs transferringbroadcast information)

Tx

RxDATA

Sync Trailer Rx Node ID Tx Node ID PDUNumber NPACKETS PAYLOAD

DATA PDU

CRC

ACK PDU

Sync Trailer Rx Node ID Tx Node ID DATA PacketStatus

Tx

RxACK

(UWB)2 MAC: Transmission andRanging procedure


t0+ ! + " + ! + "

t2=t

0+ ! + " + !

Tx Rx

t0

t0 + !

t1=t

0+ ! + "

Time Time

t3=t

0+ ! + " + ! + "+ !

• The LE -> LC -> DATA exchange allows both Tx and Rx terminals todetermine their distance:

2 0

2TxRx

t td c c!

" "#= =

3 1

2RxTx

t td c c!

" "#= =

DATA

LE

LC

(UWB)2 MAC: Transmission and Rangingprocedure (2/2)


Ranging information management

• Each terminal builds a database for distanceinformation:

22.1267.232.........

21.3542.45425.6273.571

Timestamp (s)Distance (m)Neighbor ID

Example of ranging database

• The database can be used by positioning protocolsfor building a network map


(UWB)2 and 802.15.4a- impact on PHY -

• No Carrier Sensing required• The Aloha approach can be applied in the case

of simple terminals adopting only one TH-codeor no TH-coding at all

• But when capability of switching betweendifferent Time Hopping codes is available, thecode dimension is used in the protocol

• Ranging capability at the PHY is required– It can work with any of the proposals for ranging now

under consideration in 802.15.4a


(UWB)2 and 802.15.4a- 802.15.4 MAC 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 channelaccess strategy (see below); all topologiesdefined in 802.15.4 can be adopted withoutmodifications

Possible in (UWB)2, as long as a slotted time axis isadopted (guaranteed slots can be defined, as in802.15.4)

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

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

No relevant differences in (UWB)2


• Performance of (UWB)2 was analyzed as a function of:– Number of users– User data rates

• Both Pure Aloha and Slotted Aloha were analyzed– Slotted Aloha could improve performance in centralized network topologies,

where the coordinator can provide slot synchronization with low overhead• Assumptions:

– Impulse Radio (IR) UWB terminals with TH-coding– Packet length L = 2000 bit + 100 bit for synchronization– 802.15.4a path loss model and channel impulse response in four different

channel scenarios:• CM1 - indoor propagation in residential environments with LOS• CM2 - indoor propagation in residential environments with NLOS• CM5 - outdoor propagation with LOS• CM6 - outdoor propagation with NLOS

– Specific MUI model for IR UWB, taking into account delay spread due tomultipath

– No correction coding was considered: all bits in a packet must be correct, for apacket to be correct

(UWB)2 MAC: Performance analysis


(UWB)2 MAC: Results

Impact of number of terminals and scenario on throughput:


(UWB)2 MAC: Results

Impact of number of terminals and scenario on delay:


(UWB)2 MAC: Results

Impact of user data rate and scenarios on throughput:


(UWB)2 MAC: Results

Impact of user data rate and scenarios on delay:


Ongoing work

– Harmonization of MACPDU format with 802.15.4– Validation of the MUI model– Analysis of the impact of selection of TH-codes

(or no TH-coding) on performance– Comparison with alternative solutions for the

same scenarios


Acknowledgment

• P.U.L.S.E.R.S. (Pervasive Ultra-wideband Low Spectral EnergyRadio Systems)

• Integrated EU Project launched in January 2004 within theEuropean IST 6th Framework (project n. IST-506897)

• Topic at UoR: advanced MAC design for low data rate UWBnetworks

• Ultra Wide Band Ranging and Positioning inRadio Communication Systems• research contract between the INFOCOM Department

of University of Rome La Sapienza and STMicroelectronics Italy, started in July 2004

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