Synchronous Simultaneously Operating Pico-nets

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Synchronous Simultaneously Operating Pico nets] Date Submitted: [September 2003] Source: [Yossi Erlich] Company [Infineon Technologies] Address [P.O.Box 8631, Poleg Industrial Area, Netanya 42504, Israel] Voice:[+972-9-8924100], FAX: [+972-9-8658756], E-Mail:[Yossi.Erlich@infineon.com] Re: [] Abstract: [A synchronization mechanism is presented. This synchronization improves the performance under simultaneously operation pico-nets scenarios (SOP). The proposed solution has some drawbacks. However we think that it is worth considering the approach since the currently inspected methods do not sufficiently treat the SOP problem] Purpose: [Suggest a solution to the SOP problem] 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. Erlich, Infineon Technologies

2. Synchronous Simultaneously OperatingPico nets A MB-OFDM Extension Erlich, Infineon Technologies

3. Do We Give Up Dense Utilization? • SOP, Specifically the “near-far” scenario, is a major unsolved 802.15.3a issue Erlich, Infineon Technologies

4. Principles • We suggest a MB-OFDM extension that enables 3 or 4 SOP, with very low dint/dref without performance degradation • The OFDM symbols, transmitted by SOP, do not overlap in the T-F space • Rough time synchronization among neighbor UWB • Low-level synchronization mechanism that requires - no inter pico-nets management communication and is (almost) independent on the MAC layer • No substantial additional complexity (cost) Erlich, Infineon Technologies

5. T-F OFDM Allocation3-SOP Alternative • OFDM symbol duration T=312.5nSec • 242.4 nSec - OFDM info length • 60.6 nSec - Cyclic prefix (or zero pad) • 9.5 nSec - Fast-Hopping time • Transmit N consecutive OFDM symbols within each band (N=4) • For 3-bands devices – Slow hopping - Half PRF • For 6-bands devices (advanced modems) – Fast hopping • Same hopping order for all channels • Inter channel guard time Tg=N·T = 1.25uSec (for N=4) • The actual guard time is Tg + 9.5nSec = 1.26uSec Erlich, Infineon Technologies

6. T-F OFDM Allocation4-SOP Alternative • Maintain the same guard time Tg=N·T = 1.25uSec for (N=4) • Double the transmission time within each band (transmit 2·N consecutive OFDM symbols) • Same duty cycle per band • Same line code Erlich, Infineon Technologies

7. Synchronization Signals • Every Tsync≈50uSec (an integer number of the transmission period Tp) the communication halts (~4uSec+Tg) for a Sync signal • The Sync signal is composed of 6 transmissions for the 3-bands case, and 12 for the 6-bands case • Each symbol is a sequence (303nSec) with good autocorrelation properties • The drawn example is plotted for 3-SOP Erlich, Infineon Technologies

8. Synchronization Mechanism • Every device selects (per Tsync=50uSec) whether it transmits or receives the Sync signal (selection policy will be presented) • If the earliest detected Sync was received before the receiver’s local timer, the device advances the local timer accordingly • The timing correction is never done within burst reception/transmission •  Result: • Every device periodically advances its local timer according to the fastest device in the neighborhood • The fastest clock dictates synchronous transmission time scale Erlich, Infineon Technologies

9. Sync Transmission Policy • Inter-device successful synchronization happens when the faster transmits and the slower receives • Define four sequences (one sequence per channel): S0 = [1, 1, 1, 0, 0, 0]; S1 = [1, 1, 0, 0]; S2 = [1, 0]; S3 = [1, 1, 0]; For the 3-channels case use S0, S1, and S2 • A PNC at channel ‘n’ transmits at Sync #m if Sn[modulus(m,n)]=1 • A non-PNC device at channel ‘n’ transmits at Sync #m if Sn[modulus(m,n)]=0 • Within a pico-net, the Sync indices are synchronized  Whenever the PNC transmits the non-PNC receive (and vice versa) • Every pair of nodes at neighbor channels, synchronize at least every 7 Sync intervals • Every pair of nodes within the same pico-net, synchronize at least every 7 Sync intervals • This method was designed to limit synchronization error due to clock drift effects Erlich, Infineon Technologies

10. Synchronization Error • The synchronization error is a result of 3 effects: • Drift (Skewed clock of all transmitters ‘seen’ by a single receiver) • In steady state < 13*40PPM*Tsync = 26nSec • Under acceleration (to be explained) < 13*240PPM*Tsync = 156nSec • Propagation delay (Sync signals and data signals) • Worst case < 4*15m/c = 200nSec • Near-Far (Distant Sync signals masked by near transmitters) • Detection error < 3*303nSec = 909nSec • Total effect (under acceleration) < 1.265uSec, which is by 5.5nSec longer than 4T+9.5nSec • Devices position scenarios where this upper bound is reached are very rare Erlich, Infineon Technologies

11. Devices Clusters • Define a ‘Cluster’ by the set of devices sharing the same synchronized time scale • All devices that ‘see’ each other – are considered as a part of the same cluster • Particularly all devices within a pico net belong to the same cluster • Initializing device • Search Sync signals • No Sync signal No Cluster  Be the first • Otherwise (Detected Sync signal)  • Join the cluster’s Sync signals (Denote this cluster “Primary Cluster”) • Use Sync Tx. policy Snew = [1, 0, 0, 0, 0, 0, 0]; (transmit Sync every 7 Tsync intervals) • Potentially, merge the cluster with one or more other clusters (“Secondary Clusters”) Erlich, Infineon Technologies

12. Clusters Merging (by a new device) • For the Sync signal transmission, the merging device uses a timer which is 200 PPM faster then its free running clock (advance by 10nSec every Sync signal) • The slowest drift with respect to any other cluster would be 160PPM, which means that after at most Tmerge=0.31Sec (50uSec/160PPM) the whole Tsync interval was swept • Therefore it is guaranteed that all “Secondary Clusters” joined the Primary cluster • The “Drift Effect” on the synchronization within the acceleration period was increased from 26nSec (13*240PPM*50uSec) to 156nSec (13*240PPM*50uSec) Erlich, Infineon Technologies

13. Periodic Clusters Merging Attempts (1) • Consider a case where, due to a device movement, inter-cluster interference shows up • Devices within a well-covered area (e.g. plugged repeaters) are protected from such merging requirements • Periodic merging attempts are done by randomizing clock acceleration incidents • When one cluster is ‘accelerating’ and the other cluster is not, for a complete Tmerge=0.31Sec period, then successful merging is guaranteed Erlich, Infineon Technologies

14. Periodic Clusters Merging Attempts (2)(Random Process) • Within each cluster, there is a single device (dynamically selected) that randomizes acceleration incidents • After M(n)·Tmerge time since the end of each acceleration, the accelerating device initiates another acceleration • {M(n)} are randomized IID with probability ½ between {1.5, 5} (given as a distribution example) • An accelerating device that senses Sync acceleration that was not initiated by itself, leaves duty • All other devices (potentially only PNC), monitor Sync accelerations • A device that senses no acceleration for a certain time (e.g. 6·Tmerge) : • Initiates an immediate acceleration • Becomes an accelerator (starts the random process) • It is guaranteed that a set of devices that simultaneously become accelerators is decimated exponentially in time, and finally a single accelerator remains (each acceleration is expected to half the set population) (*) Tmerge = 0.31Sec Erlich, Infineon Technologies

15. Periodic Clusters Merging Attempts (3) • The figure shows the waiting-time statistics from clusters interaction until they successfully merge • At 80% of cases, within less then 6∙Tmerge=1.9Sec • At 90% of cases, within less then 10∙Tmerge=3.1Sec • At 95% of cases, within less then 15.4∙Tmerge=4.8Sec • At 99% of cases, within less then 26∙Tmerge=8.1Sec • The duration is short with respect to device movements Erlich, Infineon Technologies

16. Work to be Done • The presented work is incomplete • Full simulations should be carried out • We should design parameters such as: • Sync signal length • Extend the Sync signal  Increase sensitivity  Merge before interruption • Sync signal interval (Tsync=50uSec?) • Increase Sync rate  Accelerate merging • Per-band transmission duration (N=4?) • Merging procedure • And more… • We should explore many issues such as: • Effects on other networks • Multiple merging devices • Big clusters • And more… Erlich, Infineon Technologies

17. Disadvantages • Long transmission time within each band: 1.25uSec for 3-SOP instead of 303uSec - 615nSec (currently proposed) • Longer effect on narrowband systems • UWB: Requires longer interleaving for maintaining frequency diversity • The N=4 duration is designed for rare worst case positioning. Some shortening may be done by simulations analysis • System complication • We think that the complication is small, judging against the solved SOP problem. It’s more appealing to take the asynchronous solution – Is this the right solution? • The 1/6 duty cycle • The achievable rate with QPSK is limited (the 480Mbps requires higher constellation)  Complexity penalty • 3dB higher effect on narrowband system • Co-channel Interference • Alien pico-net devices which re-use the same channel interfere more severely Erlich, Infineon Technologies

18. Suggestion • We encourage the WG members to join us exploring this approach • We support the MB-OFDM and consider it as the best asynchronous solution • We suggest that the WG considers synchronous solution AFTER the MB-OFDM proposal is accepted Erlich, Infineon Technologies

19. ? Erlich, Infineon Technologies

20. The Drift Effect (1)(Back Slide) • Define four sequences (one sequence per channel): S0 = [1, 1, 1, 0, 0, 0]; S1 = [1, 1, 0, 0]; S2 = [1, 0]; S3 = [1, 1, 0]; For the 3-channels case use S0, S1, and S2 • A PNC at channel ‘n’ transmits at Sync #m if Sn[modulus(m,n)]=1 • A non-PNC device at channel ‘n’ transmits at Sync #m if Sn[modulus(m,n)]=0 • Within a pico-net, the Sync indices are synchronized  Whenever the PNC transmits the non-PNC receive (and vice versa) • Consider two devices from different channels, n1 and n2. Define D(n1,n2) as the maximal number of synchronization intervals* between two successful incidents {a device from channel #n1 transmits and a device from channel #n2 receives}. • For the selected sequences: • ’Neighbor’ channels satisfy: • D(0,1)≤7 D(1,2)≤4 D(2,3)≤6 D(3,0)≤6 • Other channel pairs satisfy: • D(0,2)≤6 D(1,3)≤9 (*) The synchronization interval is Tsync=~50uSec Erlich, Infineon Technologies

21. The Drift Effect (2)(Back Slide) • For 40 PPM maximal relative drift and for Tsync=50uSec: • The maximal relative drift between two devices from different channels: • D(n1,n2)≤7 (neighbor channels)  Drift effect ≤ 14nSec • D(n1,n2)≤9 (distant channels)  Drift effect ≤ 18nSec • For the 3-channels case the maximal relative drift between two devices from different channels: • D(n1,n2)≤7  Drift effect ≤ 14nSec • For two devices within the same pico-net (channel #n), Define D(n,n) as the maximal number of elapsing Sync intervals from transmission of Sync signal by the faster device, until the next transmitted Sync massage is received (directly/indirectly) by the slower device • D(n,n)≤7  Drift effect ≤ 14nSec Note: The worst case is when the two devices are non-PNC devices within channel #0 • For any device, consider two devices that are within detection range, one within the same pico net of the receiving device, and the other uses a neighbor channel (interfering). Claim: The drift effect between the two nodes is upper bounded by 13 synchronization intervals: Drift effect ≤ 40PPM•13•Tsync =26nSec This claim can be proven by simply showing that the synchronization time is upper bounded by D(n1,n1)+D(n1,n2)-1, where ‘n1’ is the index of the receiver’s channel, and n2 is the other channel. • For robustness under Sync signal miss-detection - double drift effect could be assumed Erlich, Infineon Technologies

22. The Propagation Delay Effect(Back Slide) • Consider the receiver at device “A” • Consider any interfering transmitter from other channels (other pico nets) • Directly communicates Sync signals with “A” • “Int”’s local time is between –(dint-A/c) to +(din-At/c) with respect to “A” ’s local time • “Int”’s signal is received at “A” between 0 to +2dint-A/c with respect to “A”’s local time • Consider any co-cannel transmitter (named “B”) from the same pico net • “B” is synchronized with the PNC, and the PNC is synchronized with “A” • “B”’s time is between –(dPNC-B+dA-PNC)/c to +(dPNC-B+dA-PNC)/c with respect to “A” • “B”’s signal is received at “A” between –(dPNC-B+dA-PNC-dA-B)/c to +(dPNC-B+dA-PNC-dA-B) with respect to “A”’s local time • The total difference between the two signals as seen at device “A”: • E = 2(dPNC-B+dA-PNC-dA-B+dint-A)/c • For dPNC-B, dA-PNC, dint-A ≤ d = 15m  E ≤ 4d/c = 200nSec Erlich, Infineon Technologies

23. The Near-Far Effect(Back Slide) • Sync signals from near transmitters masks Sync signals from far transmitters • The worst case is a delayed detection error of 303nSec • A synchronization between two transmitters seen by a third device may involve up to 3 such detection errors  Near-Far effect < 3•303nSec=909nSec Erlich, Infineon Technologies

24. Clusters Merging (by a new device)(Back Slide) Sync signals’ interference to nodes in secondary clusters • Within the merging process some devices at the “Secondary Clusters” experience interferences • Less then once per 7*Tsync=350uSec • For 3-bands devices - • A merging Sync signal either affects 2 non-consecutive OFDM symbols, or a single OFDM symbol, or none. Such a Sync signal is transmitted at a 1/7 rate • For N=4, the noise effect is at 2/3 rate  once every 525uSec • For N=5, the noise effect is at 4/5 rate  once every 437uSec Erlich, Infineon Technologies