robustness of uncoordinated mac in channel impaired low ...ica · •integrated eu project launched...
TRANSCRIPT
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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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
UWB4SN Workshop, EPFL, Lausanne, November 4 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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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)
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
• 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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(UWB)2 MAC: Results
Impact of number of terminals and scenario on throughput:
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(UWB)2 MAC: Results
Impact of number of terminals and scenario on delay:
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(UWB)2 MAC: Results
Impact of user data rate and scenarios on throughput:
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
(UWB)2 MAC: Results
Impact of user data rate and scenarios on delay:
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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
UWB4SN Workshop, EPFL, Lausanne, November 4 2005
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