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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: TG4a MAC Protocol Enhancement Proposal Date Submitted: July 15th, 2005 Source: Gian Mario Maggio (STMicroelectronics), Philippe Rouzet (STMicroelectronics) Contact: Gian Mario Maggio

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

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  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@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. Gian Mario Maggio & Philippe Rouzet (STM)

  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) Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  6. 802.15.4 MAC: Star Topology PAN Coordinator Master/slave Communications flow Full function device Reduced function device Gian Mario Maggio & Philippe Rouzet (STM)

  7. 802.15.4 MAC: Peer-Peer Topology Cluster tree Point to point Full function device Communications flow Gian Mario Maggio & Philippe Rouzet (STM)

  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) Gian Mario Maggio & Philippe Rouzet (STM)

  9. 802.15.4 MAC: Frame Structure • 4 Types of MAC Frames: • Data Frame • Beacon Frame • Acknowledgment Frame • MAC Command Frame Gian Mario Maggio & Philippe Rouzet (STM)

  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]. Gian Mario Maggio & Philippe Rouzet (STM)

  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) Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  21. Example: Achievable rates with several interferers with/without exclusion protocol Allow Interference Mutual Exclusion distance to interferer Gian Mario Maggio & Philippe Rouzet (STM)

  22. S1 and S2 should send simultaneously and adapt rates S1 D1 S2 D2 S1 and S2 should not send simultaneously S1 D1 S2 D2 Interference vs. Mutual Exclusion • Interference should be allowed except when source is inside an “exclusion region” around a destination D1 Gian Mario Maggio & Philippe Rouzet (STM)

  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” Gian Mario Maggio & Philippe Rouzet (STM)

  24. incremental redundancy R2 R1 k/R2 - k/R1 bits R1 R2 k data bits k/R2 coded bits (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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  26. (A.3) «Private MAC» and TH Sequences • Time hopping sequences (THS) are generated by a pseudo-random number generator • Example: linear congruential generatorx(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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  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 Gian Mario Maggio & Philippe Rouzet (STM)

  31. Design Choices Key assumptions 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 Gian Mario Maggio & Philippe Rouzet (STM)

  32. DATA Packet Status Tx Node ID Rx Node ID Sync Trailer TH-Flag x bits 16 bits 16 bits 4 bits PDU N PAYLOAD Tx Node ID Rx Node ID Sync Trailer Sync Trailer Rx Node ID Tx Node ID TH-Code Tx Node ID Number PACKETS Rx Node ID Sync Trailer x bits 16 bits 16 bits 8 bits 8 bits M bits x bits 16 bits 16 bits 1 bit 16 bits x bits 16 bits 16 bits (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 Gian Mario Maggio & Philippe Rouzet (STM)

  33. LE LC DATA • The LE  LC  DATA exchange allows both Tx and Rx terminals to determine their distance: Gian Mario Maggio & Philippe Rouzet (STM)

  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. Gian Mario Maggio & Philippe Rouzet (STM)

  35. References • 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 • 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). • 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. • 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. Gian Mario Maggio & Philippe Rouzet (STM)

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