1. Interference-free scheduling
Month Year
doc.: IEEE yy/xxxxr0
February 2008
Interference-free scheduling
IEEE P Wireless RANs Date:
Authors:
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Dave Cavalcanti, Philips
John Doe, Some Company

2. Month Year
doc.: IEEE yy/xxxxr0
February 2008
Abstract
This presentation gives an overview of the Interference-free scheduling mechanism for WRANs self-coexistence.
Dave Cavalcanti, Philips
John Doe, Some Company

3. Self-Coexistence The general problem:
Month Year
doc.: IEEE yy/xxxxr0
February 2008
Self-Coexistence
The general problem:
Depending on the channel availability, WRANs may have to share the same TV channel
How to avoid interference (collisions) in overlapping areas by adjusting the bandwidth allocation schedule?
TDMA Schedule
Dave Cavalcanti, Philips
John Doe, Some Company

4. Interference-free Scheduling
Month Year
doc.: IEEE yy/xxxxr0
February 2008
Interference-free Scheduling
Goal: Use information from neighboring cells to detect potential interference situations and adapt the bandwidth allocation schedule to solve the problems in the coexistence areas.
Interference-free scheduling is enabled by the CBP protocol
executed by CPEs, under BS control
CPEs transmit/receive CBP packets during SCW (Self-Coexistence Window)
BS schedules CBP transmissions/receptions
Dave Cavalcanti, Philips
John Doe, Some Company

5. Month Year
doc.: IEEE yy/xxxxr0
February 2008
How it works?
CPE collects information from potential interferers in the area, e.g.,
CBP packets from other WRANs
CBP packets carry information
About the cell (DS/US Boundary IE: information about contiguous blocks of DS or US allocations scheduled by the BS)
About a CPE’s reservations with its BS (Beacon IE: information about a particular CPE)
The CPE can use this information as follows:
The CPE reports the information to its BS, if requested (allowed) by the BS (the BS controls all the measurement requests/reports)
The CPE may also include constraint elements when requesting upstream bandwidth
For example, a CPE can specify: “Give me 100Kb of airtime, but not between T1 and T2”
The BS may explicitly ask the CPE for any constraints it might have regarding allocation (TRC-REQ message)
BS adapts its bandwidth allocation schedule if needed to avoid interference
Dave Cavalcanti, Philips
John Doe, Some Company

6. Month Year
doc.: IEEE yy/xxxxr0
February 2008
Example
BS A is operational and CPE A1 transmits CBP packets
BS B and CPE B1 starts operating and B1 receives CBP packets from A1
CPEs A1 and B1 reports to their BSs, this enables:
Network discovery
Spectrum etiquette
If no other channel is available, the BSs can use interference-free scheduling (BS B changes its schedule to CPE B1 to avoid interfering with CPE A1)
If interference-free scheduling is not effective (e.g. not enough resources)
BSs may go the next self-coexistence step: resource sharing (renting/offering, spectrum contention)
CPE B1
CPE A1
BS A
BS B
CPEs in the overlapping area would suffer from interference if no co-existence mechanism is in place.
Dave Cavalcanti, Philips
John Doe, Some Company

7. Month Year
doc.: IEEE yy/xxxxr0
February 2008
Applications
Interference-free scheduling provides the mechanisms to allow BS to solve localized interference problems and improve bandwidth efficiency
It provides more flexibility for maintaining QoS
It can be used in scenarios that do not require a more robust coexistence mode in which inter-frame coexistence techniques will be needed
It is not the solution for serious coexistence situations, e.g., when BSs are close to each other and there is a high signal differential
Dave Cavalcanti, Philips
John Doe, Some Company

8. Scheduling example February 2008 Month Year
doc.: IEEE yy/xxxxr0
February 2008
Scheduling example
BS 1
BS 2
CPE A
CPE B
CPE D
CPE C
Example of MAC frame shared by two cells.
Downstream
Upstream
BS 1 and BS 2 can adjust their schedule to avoid interference problems in the coexistence area
This is feasible if MAPs are not transmitted in every frame.
BS 1 -> D
BS1->B
SCW
cell 1:
Downstream
Upstream
MAPs
BS 2 -> C C A
SCW
cell 2:
Dave Cavalcanti, Philips
John Doe, Some Company

9. Practical considerations for Interference-free scheduling
Month Year
doc.: IEEE yy/xxxxr0
February 2008
Practical considerations for Interference-free scheduling
Downstream/upstream bandwidth allocations made by a BS to CPEs in coexistence areas should not change for a certain period of time
The BS should try to use the same allocation (time and frequency) for a CPE in the overlapping area
This guarantees that the information carried in CBP packets is valid for at least a minimum duration, thus allowing enough time to the recipients of the CBP packets to implement self-interference mitigation mechanism
The BS should maintain a stable bandwidth allocation schedule, at least for those CPEs in the overlapping area
Trade-off: the more dynamic the allocations, the higher the overhead (more inter-cell communication) to implement interference mitigation mechanisms
Dave Cavalcanti, Philips
John Doe, Some Company

10. Avoiding Collisions of the MAPs
Month Year
doc.: IEEE yy/xxxxr0
February 2008
Avoiding Collisions of the MAPs
The BSs could maintain the same allocation for 2 frames
MAPs could be transmitted every other frame
The MAP is valid for 2 frames
MAPs
MAPs
MAPs
MAPs
Dave Cavalcanti, Philips
John Doe, Some Company

11. Month Year
doc.: IEEE yy/xxxxr0
February 2008
Conclusions
Interference-free scheduling allows the BSs to avoid interference by adapting the bandwidth allocations
Especially useful to mitigate localized interference for CPEs in overlapping areas
No additional complexity, since required information can be included in CBP packets and other existing management messages (e.g. BW request, measurement reports)
Actual scheduling algorithm used is implementation dependent, but for the WRANs shall support
Discovery of neighboring WRANs through transmissions/receptions of CBP packets
This enables the detection of potential interference situations in overlapping areas
Dave Cavalcanti, Philips
John Doe, Some Company

12. References 802.22 Working Document draft 0.4.7
Month Year
doc.: IEEE yy/xxxxr0
February 2008
References
Working Document draft 0.4.7
Overview of CBP (doc 136r0, March 2007)
Dave Cavalcanti, Philips
John Doe, Some Company