TG4a Review of proposed UWB-IR Modulation Schemes

1
Project IEEE P802.15 Working Group for Wireless
Personal Area Networks (WPANs) Submission Title
TG4a MAC Protocol Enhancement Proposal Date
Submitted July 15th, 2005 Source Gian Mario
Maggio (STMicroelectronics), Philippe Rouzet
(STMicroelectronics) Contact Gian Mario
Maggio Voice 41-22-929-6917, E-Mail
maggio_at_ieee.org Abstract 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.
2
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)

3
Outline

• 802.15.4 MAC overview
• CSMA or not CSMA?
• MAC enhancements
• - Interference management
• - Ranging procedures
• Proposals
• (a) DCCP-MAC
• (b) (UWB)2-MAC

4
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

5
802.15.4 MAC Device Classes

• 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

6
802.15.4 MAC Star Topology
PAN Coordinator
Master/slave
Communications flow
Full function device
Reduced function device
7
802.15.4 MAC Peer-Peer Topology
Point to point
Cluster tree
Full function device
Communications flow
8
802.15.4 MAC Addressing

• All devices have IEEE addresses
• Short addresses can be allocated
• Addressing modes
• Network device identifier (star)
• Source/destination identifier (peer-peer)

9
802.15.4 MAC Frame Structure

• 4 Types of MAC Frames
• Data Frame
• Beacon Frame
• Acknowledgment Frame
• MAC Command Frame

10
802.15.4 MAC SuperFrame Structure
GTS 2
GTS 1
Contention Access Period
Contention Free Period
15ms 2n where 0 ? n ? 14
Transmitted by network coordinator. Contains
network information, frame structure and
notification of pending node messages.
Network beacon
Beacon extension period
Space reserved for beacon growth due to pending
node messages
Contention period
Access by any node using CSMA-CA
Guaranteed Time Slot
Reserved for nodes requiring guaranteed bandwidth
n 0.
11
802.15.4 MAC Traffic Types

• 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)

12
802.15.4 MAC Data Service
Recipient MAC
Originator MAC
MCPS-DATA.request
Channel access
Data frame
Originator
Recipient
Acknowledgement (if requested)
MCPS-DATA.indication
MCPS-DATA.confirm
13
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

14
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

15
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

16
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

17
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

18
(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

19
(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

20
(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

21
Example Achievable rates with several
interferers with/without exclusion protocol
Allow Interference
Mutual Exclusion
distance to interferer
22
Interference vs. Mutual Exclusion

• Interference should be allowed except when source
  is inside an exclusion region around a
  destination D1

23
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

24
(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
R1
R1
k data bits
k/R1 coded bits
25
(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

26
(A.3) Private MAC and TH Sequences

• Time hopping sequences (THS) are generated by a
  pseudo-random number generator
• Example linear congruential generator
  x(n1) 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

27
(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

28
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

29
Proposal (B)
(UWB)2 Uncoordinated, Wireless, Baseborn MAC for
UWB-LDR communication networks
New in (UWB)2 ranging support, enabling
position-based protocols and applications
30
(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

31

• Key assumptions

Design Choices
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)
No Carrier Sensing pure Aloha (with TH coding)
TH-CDMA Shared TH code available to all
devices Dedicated data code unique for each
transmitter
Need for broadcast packets
Time Hopping Impulse Radio with GHz BW
32
(B) Transmission and Ranging Procedure
Tx

• 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

LE
LC
DATA
ACK
Rx
33

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

34
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.
35
References

1. 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.