doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 1 Project: IEEE P 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: , 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 ) keeping into account UWB-PHY features. Notice:This document has been prepared to assist the IEEE P 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 P

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 2 MAC Protocol Enhancements for a (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)

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 3 Outline MAC overview CSMA or not CSMA? MAC enhancements: - Interference management - Ranging procedures Proposals: (a) DCCP-MAC (b) (UWB)2-MAC

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 5 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 MAC: Device Classes

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 6 Full function device Reduced function device Communications flow Master/slave PAN Coordinator MAC: Star Topology

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 7 Full function deviceCommunications flow Point to point Cluster tree MAC: Peer-Peer Topology

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 8 All devices have IEEE addresses Short addresses can be allocated Addressing modes: –Network + device identifier (star) –Source/destination identifier (peer-peer) MAC: Addressing

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide MAC: Frame Structure 4 Types of MAC Frames: Data Frame Beacon Frame Acknowledgment Frame MAC Command Frame

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 10 15ms * 2 n where 0  n  14 Network beacon Contention period Beacon extension period 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 2GTS 1 Guaranteed Time Slot Reserved for nodes requiring guaranteed bandwidth [n = 0] MAC: SuperFrame Structure Contention Access Period Contention Free Period

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 11 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) MAC: Traffic Types

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 12 Originator MAC Recipient MAC MCPS-DATA.request Data frame MCPS-DATA.confirm MCPS-DATA.indication Acknowledgement (if requested) Channel access MAC: Data Service Originator Recipient

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide a 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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 )  Notable Exception –CDMA: allows interference  requires power control

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 21 Mutual Exclusion Allow Interference distance to interferer Example: Achievable rates with several interferers with/without exclusion protocol

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 22 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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”

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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 encoderdecoder k data bits R1R1 k/R 1 coded bits R1R1 R 2  R 1 k/R 2 - k/R 1 bits incremental redundancy k data bits R1R1 k/R 2 coded bits R2R2

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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(n+1) = a x(n) mod b where b = 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 29 (UWB)2: Uncoordinated, Wireless, Baseborn MAC for UWB-LDR communication networks Proposal (B) New in (UWB)^2: ranging support, enabling position-based protocols and applications

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 31 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

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 32 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 LE LC DATA Sync TrailerRx Node IDTx Node ID x bits16 bits 1 bit 16 bits TH-Code TH-Flag Sync Trailer Rx Node ID Tx Node ID x bits 16 bits Sync Trailer Rx Node ID Tx Node ID x bits16 bits PDU Number 8 bits N PACKETS PAYLOAD M bits Sync Trailer Rx Node ID Tx Node ID x bits 16 bits DATA Packet Status 4 bits ACK (B) Transmission and Ranging Procedure

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 33 The LE  LC  DATA exchange allows both Tx and Rx terminals to determine their distance: DATA LE LC

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 34 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 channel access strategy (see below); all topologies defined in 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 ) 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 ) 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.

doc.: IEEE a TG4a July 15, 2005 Gian Mario Maggio & Philippe Rouzet (STM)Slide 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: 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 , 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.