1. Interference Management for TVWS Networks
Month Year
doc.: IEEE yy/xxxxr0
Interference Management for TVWS Networks
Date: July 15, 2011
Authors:
Name
Company
Address
Phone
Padam Kafle
Nokia
6021 Connection Drive, Irving, TX, 75039
Mika Kasslin
Itämerenkatu 11-13, Helsinki, Finland
Prabodh Varshney
John Doe, Some Company

2. Contents Abstract Comments Discussion
The Interference Management Problem
Over-the-DS Signaling to enable Coexistence
RLSS for Interference Management
Summary

3. doc.: IEEE 802.15-<doc#>
<month year>
Abstract
There are many comments asking to specify the coexistence mechanisms to avoid interference between overlapping BSS in TVWS band. This presentation discusses the problems and proposed solutions for the following CIDs on coexistence:
CIDs: 8, 13, 270, 500, 733, 753, 797, 811 and 973.
>
Page 3

4. Comments > doc.: IEEE 802.15-<doc#> <month year> Page 4
CID
Clause Number(C)
Page(C)
Line(C)
Comment
Proposed Change 8
Need a mechanism for a portable BSS to detect presence of co-channel fixed devices so that it can move into a different channel. This is because there is a 16dB EIRP difference between the fixed and portable. So fixed APs may never detect a portable devices in the co-channel and thereby cause interference to the portable device BSS.
Make transmission of unsolicited TPC report element mandatory for fixed TVWS devices. Also see 11/0265r0 for suggested text. 13
17.3 & 19.1 66
Coexistence of 20 MHz with clocked down versions i.e. 5MHz and 10MHz cannot be achieved since no portion of a 20MHz PPDU is decodable by a 5 or 10MHz device and vice versa. Similar issue arises with co-existence of 5MHz and 10MHz modes.
Design PHY layer to enable co-existence of 5,10,20MHz operation modes.
270
19.1.1 61
What is the impact on OBSS operation when there are BSSs with different BW sharing some spectrum? Do slot times line up? Are slot times the same? If either condition is not true, does this increase the rate of inter-BSS collisions? Or are the different modes of operation restricted by regulatory domain so that overlapping BSSs of dissimilar width will not arise in practice?
Clarify.
>
Page 4

5. Comments .. > doc.: IEEE 802.15-<doc#> <month year>
CID
Clause Number(C)
Page(C)
Line(C)
Comment
Proposed Change
500
10.af1 50 20
The channel power management currently provides mechanisms for advertising available channels or changes to channel(s) or constrained maximum transmit power level(s). However, the problem of potential interference among multiple overlapping BSSs while operating on the allocated channel and transmit power levels are not taken in to account.
Given the greater coverage range in TVWS band to more likely incur interferences, options for interference management among multiple overlapping networks should be specified in the specification.
733
General
there is no specification for how the 5 and 10 MHz mode would coexist in a same TVWS channel, since the preambles cannot interoperate
add detailed coexistence mechanisms
797 E
123 1
The channelization defined creates fractional overlap between 5, 10, 20, and 40 MHz channels. OBSS interference will be a mess and is unacceptable. This is similar to 2.4GHz. This must be fixed.
Either change the channelization to eliminate the fractional overlap, or add scanning and OBSS avoidance rules from 11n in 2.4 GHz.
811 66 36
802.11af is intended for increased range. However with that comes increased OBSS interference.
Recommend studying interference mitigation schemes between OBSS
>
Page 5

6. Comments .. > doc.: IEEE 802.15-<doc#> <month year>
CID
Clause Number(C)
Page(C)
Line(C)
Comment
Proposed Change
753 79 38
"The slot time shall follow for 5 GHz and TVWS bands and for 2.4 GHz bands." According to the above sentence, BSSs using different channel width will have different slot times. In the overlapping BSSs case this will have negative impact on the BSS with narrower bandwidth in terms of medium access. For example, when a BSS using 5 MHz wide channel (5 MHz BSS) is overlapping with a BSS using 10 MHz wide channel (10 MHz BSS), the 10 MHz BSS will have more chance to access the medium than the 5 MHz BSS since the slot time of the 10 MHz BSS (13 usec) is much shorter than the slot time of the 5 MHz channel (21 usec). This will result in unfair medium access because contention backoff time will be much shorter for the 10 MHz BSS than the 5 MHz BSS even if both BSSs choose the same backoff counter value.
Provide a coexistence mechanism for overlapping BSSs with different channel bandwidths.
973
The rules for 5/10Mhz Coexistence in TVWS bands are not clear and seems not specified. Can some BSSs operate on 5 while some other BSSs operate on 10MHz channel overlapped channels?
Specify the rules for 10/5MHz Coexistence in TVWS band.
>
Page 6

7. doc.: IEEE 802.15-<doc#>
<month year>
Discussion
The larger cell coverage in TVWS band exacerbates the problem of overlapping BSS with hidden node STAs. In order to improve the utilization of limited available spectrum in TVWS band, enhanced coexistence mechanisms are required
There are many possible scenarios for which, the legacy mechanisms currently available in (e.g. CSMA/CA, RTS/CTS etc) are not sufficient
Interference avoidance among overlapping WLAN networks from same or different operator’s service area becomes important
How high-power BSS and low-power BSS can operate in same area?
How to manage different channel widths in overlapping BSS ?
Two solutions are discussed in this presentation:
Over-the-DS message exchange with remote request broker (RRB) for coexistence
Interference management among multiple APs connected to a common RLSS
>
Page 7

8. The Interference Management Problem
doc.: IEEE <doc#>
<month year>
The Interference Management Problem
There are two main problems:
When the transmit power levels in two overlapping BSS are significantly different (4 W Fixed vs 100 mW/40 mW Personal/Portable operation), the conventional RTS/CTS protection can not solve the interference from a hidden STA.
The larger coverage range in TVWS makes it more difficult problem than in the legacy WLAN bands (limited channels but increased interference area)
>
Page 8

9. The Interference Management Problem ..
doc.: IEEE <doc#>
<month year>
The Interference Management Problem ..
Overlapping BSS with different channel bandwidths will be common due to limited TV channels in many locations.
One BSS with 5 MHz bandwidth may be adjacent with another which uses 10 MHz channel overlapping to it. However, OBSS scanning and coexistence notifications like in n can not work due to non-interoperable preambles
>
Page 9

10. Over-the-DS message exchange for coexistence
doc.: IEEE <doc#>
<month year>
Over-the-DS message exchange for coexistence
To effectively manage overlapping networks, stations in two overlapping BSS should be able to exchange coexistence messages
802.11r has fast BSS transition (FT) protocol already specified which utilizes over-the-DS message exchange using the remote request broker (RRB) function
>
Page 10

11. Over-the-DS message exchange ..
doc.: IEEE <doc#>
<month year>
Over-the-DS message exchange ..
The RRB mechanisms can be extended for coexistence purposes. Another possible use of RRB is for establishing communication link from a Fixed/Mode II device not having its direct internet connection to another Fixed/Mode II
The RRB function of AP allows message passing between two BSS
The STA of BSS-A transmits FT action frame to its AP
The AP of BSS-A generates a Remote Request message with the payload containing a FT action frame and forwards it to neighbouring AP of BSS-B.
The message payload inside Remote Request/Response is delivered with encapsulation (Ethertype 89-0d).
Two new FT action frame types are proposed to support remote coexistence request/response transmissions over the DS:
Request can be for aligning the operational channel widths (for Problem 2)
Request to switch the operating channel (for Problem 1 or 2)
Request for sharing the medium time (for Problem 1)
>
Page 11

12. Over-the-DS message exchange ..
doc.: IEEE <doc#>
<month year>
Over-the-DS message exchange ..
Example- low power/high power overlap:
A STA in LP_BSS notifies its AP to indicate presence of signal from HP_BSS (frames from STA Type = Fixed, or identifying signal from Fixed device), and HP_OBSS_Present flag is set
A STA in LP_BSS transmits FT action frame for Remote Coexistence Request to be forwarded to the AP of HP_BSS. The request could be for:
Request to switch the operating channel
or change the channel width,
or Request for a medium time for its use
The AP of HP_BSS responds with the Remote Coexistence Response action frame with indication of result (success or denied). For the case of medium request, the AP transmits a CTS_to_Self frame with NAV set to the requested period, and transmits FT action frame in the remote response to the AP of the LP_BSS.
>
Page 12

13. Over-the-DS message exchange ..
doc.: IEEE <doc#>
<month year>
Over-the-DS message exchange ..
What is required?
FT Action frames for Remote Coexistence Request and Response
Remote Coexistence Request Frame
Category
FT Action
STA Address
TargetAP Address
Request Type
Channel Protection Parameters
Channel Width Alignment Info
Medium Reservation Parameters
1 octet
6 octet
6 octets
optional
Remote Coexistence Response Frame
Category
FT Action
STA Address
TargetAP Address
Response Result code
1 octet
6 octet
>
Page 13

14. Network Coexistence Support through RLSS
doc.: IEEE <doc#>
<month year>
Network Coexistence Support through RLSS
A registered location secure server (RLSS) entity has been specified in current af specification:
RLSS is an advertisement server to support RLQP using inter-working capability of u. RLSS can be implemented inside an AP or can reside outside
It can provide the following functions for white space operation:
proxy to external geo-location database
a network management service node that facilitates the operational controls
In many deployments, networks of different coverage range and capacity (e.g. low data rate coverage for parking lots/campus, and high-data rate indoor service) may be managed through a common RLSS
How the RLSS can provide interference management (IM) service?
Additional information about neighbourhood during network initiation by an AP
Notification of any changes in operational environment (changes in frequency channel, bandwidth, power, mobility)
>
Page 14

15. Coexistence Support through RLSS ..
doc.: IEEE <doc#>
<month year>
Coexistence Support through RLSS ..
Once an AP (Mode II or Fixed) obtains the list of authorized frequencies and transmit power levels for the network initiation, additional information about other networks around its area becomes important for choosing its actual operational parameters (e.g. channel, bandwidth, power)
Assumption: multiple APs or master mode capable devices are served by the same RLSS within one operator’s (i.e., service provider’s) domain
>
Page 15

16. Coexistence Support through RLSS ..
doc.: IEEE <doc#>
<month year>
Coexistence Support through RLSS ..
For setting up a network, an AP first obtains list of available channels at its location by performing channel availability query. It can then request additional IM information from the RLSS for other networks in its neighbourhood served by the same RLSS
IM Information Request from a Master mode capable TVBD to RLSS:
Device ID, geo-location, device class (max. transmit power, spectrum mask etc.)
Request Type:
0 for Neighbourhood information only
1 for Request for operational parameters (for actively managed nodes)
Optionally indicate the radius of its influence
IM Information Response from RLSS:
The response message contains current operational parameters of potentially interfering networks (BSSID, used frequency channels, used channel bandwidths, used transmit power levels, relative distances, device classes)
Recommended operational parameters if Request Type is 1
>
Page 16

17. Coexistence Support through RLSS ..
doc.: IEEE <doc#>
<month year>
Coexistence Support through RLSS ..
How RLSS can provide the relevant information?
From the known information on geo-location and device class of each TVBD, RLSS internally identifies the possible interfering networks within its service zone within a proximity region
An exclusion zone may be calculated based on possible overlapping in list of available channels, and propagation ranges of other BSS based on types of devices and their current transmit power levels
Actual algorithm for deriving the information could be implementation specific
After receiving the IM Information Response from RLSS, the AP can select its operating frequency, bandwidth and transmit power to minimize interference to itself, as well as, possible interference to its neighbouring networks
What is needed in the specification?
RLQP element for IM Information Request and Response for use by the master capable devices
The RLQP element can be transmitted over the air using GAS frames for devices not having direct connection to Internet
For master devices connected to RLSS, only the message content/format is defined
>
Page 17

18. doc.: IEEE 802.15-<doc#>
<month year>
Summary
The interference management among multiple BSS is more challenging in TVWS band due to larger coverage range
Message exchange through remote request broker mechanism among neighbouring APs can provide coexistence support between networks of dissimilar power levels or bandwidths
RLSS can also provide additional information and control mechanisms for coexistence among multiple BSS operated by same service provider
>
Page 18

19. doc.: IEEE 802.15-<doc#>
<month year>
Straw-polls
Do you support the use of RRB communication mechanism between neighbouring BSSs to address some of coexistence comments?
Yes: 4
No: 2
Abstain: 7
Do you support the use of RLSS to allow exchange of neighbourhood knowledge to address some of coexistence related comments?
Yes: 7
Abstain: 5
>
Page 19