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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: IEEE802.15.3: Guard Time in the 802.15.3 MAC. Date Submitted: 19 September, 2001 Source: Dr. William Shvodian Company: XtremeSpectrum

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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: IEEE802.15.3: Guard Time in the 802.15.3 MAC. Date Submitted: 19 September, 2001 Source: Dr. William Shvodian Company: XtremeSpectrum Address: 8133 Leesburg Pike, Suite 700, Vienna, Virginia 22182 Voice: +1.703.269-3047, FAX: +1.703.269.3092, E-Mail: bshvodian@xtremespectrum.com Re: [] Abstract: This proposal describes the use of Guard Time for the 802.15.3 MAC. Purpose: To provide an improvement to the current version of the 802.15.3 MAC 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. <William Shvodian>, <XtremeSpectrum>

  2. Overview • This presentation describes the use of Guard Time for 802.15.3 • It also describes Guard Time modifications for static GTS slots <William Shvodian>, <XtremeSpectrum>

  3. Guard Time • In a TDMA system, Guard Time is needed between slots to keep the transmissions from two stations colliding. • Each station synchronizes to the beacon, but clock inaccuracies result in drift • The maximum drift between the ideal time and a station’s clock is: Beacon CAP GTS0 GTS1 GTS2 GTS3 GTS4 Beacon <William Shvodian>, <XtremeSpectrum>

  4. Guard Time Position Beacon CAP GTS0 GTS1 GTS2 GTS3 GTS4 Beacon • Guard time is time between adjacent slots • Guard time can be defined as being at • the beginning of the slot • the end of the slot • half at the beginning of the slot and half at the end of the slot. • For Simplification, the basic Guard Time for each slot in 802.15.3 will be at the end of the slot. The slot time in the CTA will denote the beginning of a GTS. Guard time occurs at the end of the slot, before the start of the next GTS. Guard Guard G G <William Shvodian>, <XtremeSpectrum>

  5. Worst Case Timing (Transmission) drift drift Ideal GTS n position Ideal GTS n+1 position Late Station’s Estimate of GTS position Early Station’s Estimate of GTS position SIFS Guard Time • The above picture illustrates the maximum offset between two stations. The station with slot n has drifted the maximum time to the right (late), while the station assigned to slot n+1 has drifted the maximum amount to the left (early). • In order to avoid transmission collisions or violating SIFS, the guard time must be calculated to be two times the maximum drift plus SIFS <William Shvodian>, <XtremeSpectrum>

  6. Worst Case Timing (receive) Early Station Transmits Ideal GTS n position Late Station’s Estimate of GTS position Guard Time Guard Time Late Station’s Receive Window • Since a station doesn’t know whether it is early or late, it must start listening one full Guard Time before where it estimates the start of the GTS slot to be and it must listen until the time that it estimates the start of the next GTS slot to be - it must listen during the time that it estimates to be Guard Time - on both sides of the GTS slot. <William Shvodian>, <XtremeSpectrum>

  7. Constant vs. Variable Guard Time Beacon CAP GTS0 GTS1 GTS2 GTS3 GTS4 Beacon • The minimum amount of guard time is a function of the amount of time from the reference event to the event of interest • More guard time is actually needed at the end of a superframe than the beginning because it is farther from the beacon, which provides the reference timing. For simplicity, the same guard time will be used for the entire superframe in 802.15.3. <William Shvodian>, <XtremeSpectrum>

  8. Guard Time for static GTS slots Beacon CAP GTS0 GTS1 GTS2 GTS3 GTS4 Beacon • Static GTS slots allow a station to transmit in a superframe when it did not receive the beacon correctly. • If a device with static GTS slots receives the Beacon header correctly, it can synchronize to the beacon header • A device with static GTS slots does not need to correctly process the CTAs in the beacon. • If a station with static GTS slots does not correctly receive the beacon header, additional guard time is needed. • Dynamically changing the guard time to account for missed beacons requires adding guard time at the start of a GTS and at the end of the GTS and increases complexity. • Fixed guard time can be used to account for the worst case – maximum missed beacons. This is simpler and will be recommended for 802.15.3 <William Shvodian>, <XtremeSpectrum>

  9. Guard Time Calculation when Static GTS allowed • Guard time is calculated by the PNC • Guard Time is a new field in the Piconet Synchronization Parameters Information Element. <William Shvodian>, <XtremeSpectrum>

  10. Guard Time Overhead (function of superframe duration) • Setting Guard Time based on superframe duration yields reasonable overhead. <William Shvodian>, <XtremeSpectrum>

  11. Guard Time Overhead (fixed to 142s regardless of superframe duration) • Setting Guard Time to worst case (142s) regardless of superframe duration causes unreasonable overhead for short superframes. <William Shvodian>, <XtremeSpectrum>

  12. Guard Time relationship to Channel Time Requests and CTAs • Station requests channel time which includes one SIFS per frame (and per ACK if applicable) • PNC adds drift part of guard time, but not SIFS. This simplifies calculations for the requesting device and for the PNC if the PNV combines or divides GTS slots. Channel Time Request Frame 1 Ack Frame2 Ack Ack drift SIFS SIFS SIFS SIFS Frame 3 SIFS SIFS Guard Time Slot n Start Time in CTA n Slot n+1 Start Time in CTA n+1 <William Shvodian>, <XtremeSpectrum>

  13. End of transmission • A transmitting device shall ensure that transmission will end one guard time before the start of the next GTS slot based on the transmitting stations time reference. • If Immediate ACK is used, the transmitting device shall not transmit a frame unless there is sufficient time for the frame, a SIFS, the ACK and the Guard Time Frame+SIFS+ACK+Guard Time Frame 1 Ack Frame2 Ack Ack SIFS SIFS SIFS SIFS Frame 3 SIFS SIFS Guard Time Slot n Start Time in CTA n Slot n+1 Start Time in CTA n+1 <William Shvodian>, <XtremeSpectrum>

  14. Guard Time for 802.15.3 • Fixed Guard time for all GTS slots broadcast in the Beacon • New Field in the synchronization Information Element • GuardTime <William Shvodian>, <XtremeSpectrum>

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