1. Issues for Coexistence in 2.4 GHz ISM
December 2006
doc.: IEEE /1899r0
December 2006
Issues for Coexistence in 2.4 GHz ISM
Date:
Authors:
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james woodyatt / Apple Computer, Inc.
james woodyatt / Apple Computer, Inc.

2. December 2006
doc.: IEEE /1899r0
December 2006
Abstract
40 MHz TX in the 2.4 GHz ISM considered harmful. There are no practical mechanisms for protecting Bluetooth and other IEEE 802 non-dot11 networks. Hidden-node and power-save operations pose intractable problems. Current proposals are worse than “nuke and pave over” as a solution.
james woodyatt / Apple Computer, Inc.
james woodyatt / Apple Computer, Inc.

3. December 2006
Scope of the Problem
Coexistence with different networks requires different approaches.
802.11b/g stations in overlapping BSS.
Overlapping non networks, e.g. Bluetooth.
Passive protection mechanisms can overcompensate for interference from other microwave emitters.
Microwave ovens, for example.
User experience may provide incentive to abuse.
Moral hazards for equipment vendors and network operators.
May exacerbate rather than mitigate coexistence problems.
james woodyatt / Apple Computer, Inc.

4. December 2006
Separable Concerns
How do HT stations know about other users in the secondary channel?
What do HT stations do when they learn of other users of the secondary channel?
How do other users of the secondary channel perform in the presence of any proposed coexistence mechanism as opposed to with non-HT stations?
james woodyatt / Apple Computer, Inc.

5. Detecting Users of the Secondary Channel
December 2006
Detecting Users of the Secondary Channel
Unsolved problems abound.
A Bluetooth WPAN twenty meters away from the AP is too far away to hear short-range transmissions. Only local stations will switch to 20 MHz, while all the others will stay at 40 MHz.
A configurable threshhold may cause stations to ignore other users, e.g. a typical VoIP call falls below proposed minimum activity threshold.
Cooperation between co-located interfaces is no solution.
A properly power-managed station is dozing most of the time and will not listen for secondary channel activity.
james woodyatt / Apple Computer, Inc.

6. What Happens When Users Collide?
December 2006
What Happens When Users Collide?
The proposed solutions are worse than the problem.
Constraining to 20 MHz for configurable detection interval causes bizarre user experience.
Long periods of constrained channel width may appear like broken rate adaptation. Power cycling the AP appears to “fix” it (until the next time secondary channel activity is detected.)
Performance sensitive vendors may set activity threshold high enough that 20 MHz fallback never happens.
No requirement that HT stations check the secondary channel before resuming 40 MHz transmissions.
james woodyatt / Apple Computer, Inc.

7. How Other Users Respond…
December 2006
How Other Users Respond…
HT stations will not detect most Bluetooth networks.
No 20 MHz fallback, except maybe at co-located interfaces, so they will have no protection. Informal experiments show Bluetooth is unusable in the presence of 40 MHz transmission.
Power-managed stations will not detect overlapping b/g BSS stations.
They will come out of power-save mode to transmit 40 MHz and step all over the overlapping BSS in the secondary channel.
james woodyatt / Apple Computer, Inc.

8. December 2006
Conclusions
The existing family of compromise proposals are ineffective, and an ineffective coexistence mechanism is worse than none at all.
An effective coexistence mechanism is highly unlikely to achieve sufficient support from the membership.
…THEREFORE…
Removing 40 MHz transmissions from 2.4 GHz ISM band is the only practical compromise.
james woodyatt / Apple Computer, Inc.

9. December 2006
What Can Be Done?
Require very low fixed activity thresholds and short detection intervals to 1) minimize false appearance of broken rate adaption, and 2) remove moral hazard in setting activity threshold too high.
More aggressive rules about detection, e.g. require stations to detect Bluetooth explicitly.
…AND…
A mechanism for distributing information about detection of users in the secondary channel to all the stations in a BSS.
james woodyatt / Apple Computer, Inc.