Beacon Design for BAN Superframe

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title:[Beacon design of BAN superframe] Date Submitted: [September, 2008] Source:[Bin Zhen, Huan-bang Li, Changle Li and Ryuji Kohno] Company [National Institute of Information and Communications Technology (NICT)] Contact:Bin Zhen Voice: [+81 46 847 5445, E-Mail: zhen.bin@nict.go.jp] Abstract: [This document describes the beacon design issue in TG6.] Purpose: [To help discussion in IEEE 802.15.6.] 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. Zhen, Li, Li and Kohno

2. Beacon design of BAN superframe Bin Zhen, Huan-bang Li, Changle Li and Ryuji Kohno National Institute of Information and Communications Technology (NICT) Zhen, Li, Li and Kohno

3. Review • When and how to wakeup an inactive device with the least power consumption? • Dynamic duty cycle of BAN devices • Both BAN coordinator and device have limited battery budget • Battery is more precious for device • Coordinator can also go to sleep • MAC layer clock • BAN superframe as time unit of the semi-synchronized piconet • Wakeup point in a “BAN day” and distributed listening to wakeup point to balance power consumption and packet latency • Slot scalability • BAN superframe is easy for QoS support Zhen, Li, Li and Kohno

4. Clock maintenance • Clock drift • Typical clock accuracy is ±40ppm • Relative clock drift between coordinator and device doubles the value • Total clock offset is proportional with clock accuracy and sleep time Sleep time Clock accuracy: drift in unit time Zhen, Li, Li and Kohno

5. Beacon in BAN superframe • The device’s clock can be maintained by beacon listening at the wakeup points • An active BAN superframe is initialized by a beacon with more than one equally sized slot • The beacon consists of preamble and data • Preamble portion is used for bit-wise synchronization • Data part include header, payload and trailer Active BAN superframe Inactive BAN superframe Preamble Data Zhen, Li, Li and Kohno

6. Beacon listening • Except bit-wise synchronization, the beacon carries slot information of superframe • Where is the start or end of a slot? • How to cooperate beacon transmit/listen and maintain the slot information in an energy efficient way? • Beacon start at the Beginning of slot (B-B) • Beacon end at the End of slot (B-E) Scheduled time Listen time Tx time time Relative clock offset Zhen, Li, Li and Kohno

7. Beacon starts at the beginning of slot • It is the device’s duty to consider the clock offset • Beacons start at the beginning of slot, like 802.15.4 and Bluetooth • Device must listen before the scheduled time • Slot is described by the start of beacon Wakeup point BAN superframe beacon …… Slot border Device A: Device B: Start to listen Zhen, Li, Li and Kohno

8. Beacon start at the beginning of slot (cont.) • Pros • Constant beacon preamble transmission and power saving of coordinator • Simple slot border detection • Cons • More power consumption of low duty cycle devices since device must listen before the beacon and assume the worst clock offset Zhen, Li, Li and Kohno

9. Beacon end at the end of slot • It is the coordinator’s duty to consider clock offset • Coordinator shall transmit before the scheduled time • But the beacon shall be end at the slot border • Device wakeup per its clock • Slot is described by the end of beacon BAN superframe Wakeup point Start of beacon …… Slot border All devices Start to listen Zhen, Li, Li and Kohno

10. Beacon end at the end of slot (cont.) • Pros • Constant listening time and power saving of device • Cons • Coordinator must prepare the worst clock offset • Variable start time of beacon, which means variable preamble in beacon • More power consumption of coordinator Zhen, Li, Li and Kohno

11. Dynamic beacon payload • Slot border can be determined by the end of preamble plus a constant of time delay • When real payload is less than the constant number, the left time to slot edge is known. • When real payload is more than the constant number, the beacon can cross to the next slot. The slot edge can be determined by the part of beacon payload. Constant duration Extra-preamble Known duration Normal preamble portion Slot border Beacon frame Slot border Data portion Zhen, Li, Li and Kohno

12. Where are the differences? Slot border in BAN superframe Preamble Data To combat clock offset, extra Rv of beacon preamble must be 2Γ B-B Beacon listen Extra transmission Extra listening Frame delimiter To combat clock offset, extra Tx/Rv of beacon preamble must beΓ B-E Beacon listen time Slot border Beacon frame Slot border Normal preamble portion Extra Tx/Rv Data portion Zhen, Li, Li and Kohno

13. Where are the difference? (cont.) Zhen, Li, Li and Kohno

14. Clock offset estimation • Clock drift in a unit time can be measured and partially compensated to enable longer inactive period • Devices use its own clock to count the duration between two beacons, or • clock drift can be measured during synchronization timestamp Known duration per coordinator’s clock: Tc …… Wakeup point Wakeup point Measured duration per device’s clock: Td Zhen, Li, Li and Kohno

15. Clock drift compensation • Clock estimation accuracy can be ±10ppm • Clock drift compensation can be implemented by control the VCO of clock circuit Zhen, Li, Li and Kohno

16. Conclusions • Two beacon designs in the BAN superframe • Beacon starts at the beginning of a slot • Beacon ends at the end of a slot • B-E can save device’s power and maintain slot structure of BAN superframe • To enable TDMA based channel access Zhen, Li, Li and Kohno