1. 1 Time Synchronization Methods for Femtocell Document Number: IEEE C802.16m-08/1323r1 Date Submitted: 2008-11-05 Source: Yung-Ting Lee, Hua-Chiang Yin E-mail : yungting.lee@coiler.com.tw Coiler Shiann-Tsong Sheu, Chih-Cheng Yang National Central University (NCU) Whai-En Chen National Ilan University (NIU) Kanchei (Ken) Loa,Yi-Hsueh Tsai, Chun-Yen Hsu,Youn-Tai Lee Institute for Information Industry (III) Venue: IEEE 802.16m-08/040-Call for Comments and Contributions on Project 802.16m System Description Document (SDD), on the topic of TGm SDD Femtocells Base Contribution: IEEE 802.16m-08/003r5 Purpose: For discussion and approval by IEEE 802.16m Working Group Notice: This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the Source(s) field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEEs name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEEs sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and.http://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/guides/opman/sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2. Time Synchronization Requirements The WiMAX Forum Requirements for WiMAX Femtocell Systems V4.0 document lists two requirements: –The WiMAX Femtocell SHALL be synchronized with the rest of the network. –The WiMAX Network Architecture SHALL NOT mandate the use of GPS (or other satellite-based system) for synchronization of Femtocells.

3. Analysis on Time Synchronization Requirements in Presence of Femto BSs In Section 7.4 of SDR (IEEE 802.16m-07/002r6), it defines the cell coverage shall up to 5km under the optimized performance. Moreover, the coverage of a femtocell is around tens meters. So, the time difference between the DL signals from macro BS and femto BS arriving at the MS may exceed the CP. –For example, the cell ranges of macro BS and femto BS are 5km and 100m, respectively. Then t1-t2 ~=(16.67-0.33)us = 16.37us > CP (102.857/9 = 11.43us) as shown in the figure below In order to resolve the above problem, the signals sent from macro BS and femto BS should arrive at the MS at almost the same time (<CP/4~=2us) t1 t3 t2 Macro BS Femto BS MS

4. Possible Synchronization Methods Three existing time synchronization methods: –GPS It is an optional for femto BS It may not be applicable for indoor femto BS –IEEE 1588 over backhaul link The limitation is the backhaul link should have the features of symmetric delay and small jitter –Ranging with Macrocell via wireless link In C802.16m-08_1090, it has proposed the time synchronization of femto BS can be achieved by periodically listening to the preamble of its neighboring macro BS However, the femtobs may not be able to receive the signals sent from macro BS in some cases In addition to the GPS, IEEE 1588, and direct ranging with macro BS, we propose that the femto BS may synchronize with the network by referring to the timing information of the neighboring femto BS(s) or MS(s) who have already synchronized with the network

5. Usage Scenario of Proposed Over-the- air Time Synchronization For example, –Femto BSs A and C can synchronize with network via macro BS directly. –Femto BS B can not synchronize with network via macro BS direclty. Instead, it can synchronize with network via femto BS A or MS1 when femto BS A and MS1 have already synchronized with macro BS x Macro BS x Femto BS A x Femto BS B x Femto BS C MS1 : Coverage

6. Over-the-air time synchronization methods Three over-the-air time synchronization methods could be adopted: –Direct time synchronization (DTS) If the femto BS is able to perform ranging with macro BS directly –Reverse time synchronization (RTS) If there is any the neighboring MS who has synchronized with network MS-assisted time synchronization –Indirect time synchronization (ITS) If there is any neighboring femto BS who has synchronized with network Femto BS-assisted time synchronization

7. Direct time synchronization (DTS) Broadcast preamble Macro BS Femto BS A MS Receive preamble Send ranging code Time delayed by propagation Send RNG Rsp with time adjustment T Time advance to compensate propagation delay Time advance Broadcast preamble Broadcast preamble Receive preamble Time delay = T sync t1~=T T t2 << T x Macro BS x Femto BS A Overlapped area = interfered area Femto BS A is inside the coverage of Macro BS MS Macro BS Femto BS MS

8. Reverse time synchronization (RTS) MS-assisted time synchronization Broadcast preamble Macro BS Sync.MS Femto BS B Femto BS B requests network to assist time sync. via backhaul link Send RNG Req. with time delay T-t Command MS to range with femto BS B Broadcast preamble Time delayed T by propagation Broadcast preamble Time delayed t by propagation Send RNG code Send RNG Rsp with time adjustment t Time delay=T-t Receive preamble Broadcast preamble sync T t x Macro BS x Femto BS B Femto BS B is outside the coverage of Macro BS Sync. MS Macro BS Sync. MS Femto BS B

9. Indirect time synchronization (ITS) Femto BS-assisted time synchronization MacroBS Sync. Femto BS A MS Broadcast preamble Broadcast preamble Receive preamble Time delay = T sync Femto BS B Time delayed t by propagation Send RNG code Ranging Rsp with time advanced t and time delay T Receive preamble Broadcast preamble Broadcast preamble Receive preamble Time delay = T Time delay = T-t Broadcast preamble t1~=T t4 T t t2 x Macro BS x Femto BS A x Femto BS B Femto BS A is inside the coverage of Macro BS Femto BS B is outside the coverage of Macro BS, but is inside the coverage of femto BS A MS Macro BS MS Femto BS B Femto BS A

10. Proposed Text [Add the following section 17.x Time Synchronization Function] 17.x Time Synchronization Function The femto BS shall be synchronized with the rest of the network. The synchronization may be achieved by GPS, backhaul link, macrocell, synchronized neighboring MS, synchronized neighboring femtocell, or combinations of them..