doc.: ieee 802.22-08/0137r3 submission june 2008 gerald chouinard, crcslide 1 further wran...

June 2008

Gerald Chouinard, CRC

Slide 1

doc.: IEEE 802.22-08/0137r3

Submission

Further WRAN Self-coexistence Considerations(Comments #131, #149, #150 and #664)

IEEE P802.22 Wireless RANs Date: 2008-07-17

Name Company Address Phone email

Gerald Chouinard CRC 3701 Carling Avenue, Ottawa, Ontario, Canada K2H 8S2

(613) 998-2500 [email protected]

Authors:

Notice: This document has been prepared to assist IEEE 802.22. 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 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 IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s 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 802.22.

Patent Policy and Procedures: The contributor is familiar with the IEEE 802 Patent Policy and Procedures http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf including the statement "IEEE standards may include the known use of patent(s), including patent applications, provided the IEEE receives assurance from the patent holder or applicant with respect to patents essential for compliance with both mandatory and optional portions of the standard." Early disclosure to the Working Group of patent information that might be relevant to the standard is essential to reduce the possibility for delays in the development process and increase the likelihood that the draft publication will be approved for publication. Please notify the Chair Carl R. Stevenson as early as possible, in written or electronic form, if patented technology (or technology under patent application) might be incorporated into a draft standard being developed within the IEEE 802.22 Working Group. If you have questions, contact the IEEE Patent Committee Administrator at [email protected].>

http://standards.ieee.org/guides/bylaws/sb-bylaws.pdf

mailto:[email protected]

mailto:[email protected]

June 2008


Slide 2

doc.: IEEE 802.22-08/0137r3

Submission

WRAN Self-coexistence considerations

Spectrum Etiquette

Interference-freescheduling

Adaptive on-demand channel contention

Dynamic resource renting/offering

Different TV channel selection

Frame allocation signalledby the superframe control

header (SCH)

MAC self coexistence schemes PHY co-existence mechanisms

June 2008


Slide 3

doc.: IEEE 802.22-08/0137r3

Submission

WRAN Self-coexistence considerations (cont’d)

• The 802.22 WRAN standard needs to allow for coexistence of overlapping co-channel cells with proper capacity sharing to avoid abrupt system failure caused by ‘self-interference’

• Self-coexistence algorithms have been proposed for the MAC layer but the PHY layer has not been fully developed to operate in such burst collision environment

• There will be a need to define a self-coexistence PHY mode to operate in the context of overlapping co-channel cells in addition to the current normal PHY mode expected to be used where WRAN self-interference does not exist

• This new self-coexistence mode will need to be as efficient as possible while allowing for adaptive channel capacity allocation among co-channel WRAN cells.

June 2008


Slide 4

doc.: IEEE 802.22-08/0137r3

Submission

RF environment resulting from overlappingco-channel WRAN cells

31 km

52 kmAdjacent

non-interfering cells

For flat terrain, 100 W, 75 m BS and ITU-R P.1546 propagation

model = minimum BS-to-BS separation

is 52 km

Number at each CPE location indicates the

differential between the signals received from the two BSs including

the CPE antenna discrimination

Desired BS

Undesired BS

Possible scenarios for license-exempt

operation

June 2008


Slide 5

doc.: IEEE 802.22-08/0137r3

Submission


31 km

31 km

Grey area inside the contour is where normal

demodulation (QPSK, rate: 1/2) will fail

Overlapping cells with worst case

of negative signal differential

Positive signal differential

Negative signal differential

Near 0 dB signal differential




Desired BS

Undesired BS


operation

June 2008


Slide 6

doc.: IEEE 802.22-08/0137r3

Submission


31 km

7 km

Grey area inside the contour is where normal

demodulation (QPSK, rate: 1/2) will fail

Largely overlapping

cells

Positive signal differential

Negative signal differential

Near 0 dB signal differential




Desired BS

Undesired BS


operation

June 2008


Slide 7

doc.: IEEE 802.22-08/0137r3

Submission

Conditions of operation in a co-channel self-coexistence situation

• WRAN frame structure needs to be synchronized among co-channel overlapping WRAN cells (and adjacent channel overlapping cells)– Need for concurrent quiet periods for on-channel and adjacent

channel sensing of incumbents– Need for concurrent self-coexistence windows (SCW) scheduled at

the end of some frames for inter-cell signalling• TDM is required for the frame transmission in co-

channel overlapped areas to alleviate large negative signal differentials that may exist in license-exempt operation (example: >-50 dB, see slide #5)

June 2008


Slide 8

doc.: IEEE 802.22-08/0137r3

Submission

Conditions of operation in a co-channel self-coexistence situation (cont’d)

• Common superframe control header information needs to be received by all CPEs belonging to the co-channel overlapped WRAN cells to carry:– the capacity allocation, on a frame basis, among WRAN cells– the scheduling of the inter-frame and intra-frame quiet periods– the scheduling of the self-coexistence windows for information

exchange• The transmission of the superframe header should be done

concurrently for efficiency sake since it will carry the same information for all co-channel overlapping cells

• Such concurrent transmission of the common superframe header by all overlapping BSs will allow CPEs in negative signal differential areas to still receive it.– The CPE will decode the largest received signal from the closest BS

even if it is not from the BS with which it is associated.

June 2008


Slide 9

doc.: IEEE 802.22-08/0137r3

Submission

Conditions of operation in a co-channel self-coexistence situation (cont’d)

• In the grey areas where more than one superframe header burst will be received within less than 6 dB signal level differential, decoding of one burst over the others with QPSK rate: 1/2 will not be possible– repeat 4 of the superframe control header information will reduce

this range to almost 0 dB but an hysteresis is needed for stability • Two methods exist to alleviate this problem:

1. Use of a more robust modulation for the superframe header preamble and SCH to allow demodulation under collision conditions (assuming a worst case where demodulation takes place at a CPE where three superframe header bursts are received wih equal power at the edge of the contour, one burst can be demodulated over the other ones if demodulation can be done with SINR≥ -3.7 dB)

2. Use of Single-Frequency-Network (SFN) operation for the superframe header in self-coexistence situation (taking advantage of the OFDM inherent robust capability in multipath environment)

June 2008


Slide 10

doc.: IEEE 802.22-08/0137r3

Submission

SFN operation for the superframe header in self-coexistence situation

• The superframe header bursts will be received at the CPEs as the same burst but displaced in time due to the different propagation distances to the various BSs– this will appear to the CPE as long excess delay active multipaths– the cyclic prefic used for the superframe header burst is:

• Cyclic prefix = 1/4 useful symbol = 74.667 ms => 22.4 km– Signals arriving at the CPE from BSs with distance differentials

of less than 22.4 km will fall within the cyclic prefix– Since all base stations will broadcast the superframe header at

about the same power, the signal received from a BS 22.4 km further away than the other BSs will arrive at a lower amplitude and will unlikely affect the decoding of the superframe header

Note: The short training sequence in the superframe preamble as well as the long training sequence in the frame preamble will be needed for

synchronization, AGC and channel training before the SCH.

June 2008


Slide 11

doc.: IEEE 802.22-08/0137r3

Submission

SFN operation for the superframe header in self-coexistence situation (cont’d)

• Frequency and time synchronization will be acquired from the superframe preamble to decode the SCH but the timing information will not be usable for the following frame synchronization due to the various propagation delays from the different BSs

• Timing information will need to be acquired by CPEs from the frame preamble received from the BS to which it is associated

Note: Will the long training sequence preamble be sufficient for proper synchronization and channel training or an STS preamble will be needed as well in front of each frame in this coexistence mode?

June 2008


Slide 12

doc.: IEEE 802.22-08/0137r3

Submission

SFN operation for the superframe header in self-coexistence situation (cont’d)

• A buffer will be needed before the superframe header burst to absorb the time difference in the arrival of the last symbol from the possibly distant BS to which the CPE is associated and the arrival of the broadcast superframe header from a nearby BS.– In order to allow for operation of a CPE with a BS at a distance of up

to 100 km, the time buffer before the superframe header will need to be some 333 usec. The RTG will therefore need to be augmented by a fraction of a symbol.

• Similarly, a time buffer to absorb the ‘active echoes’ produced by the single-frequency-network of BSs broadcasting the superframe header will be needed following the superframe header.– A fraction of a symbol period (say 150 usec) would be sufficient to

absorb the still sizeable active echoes that would be coming from distant BSs while trying to capture the first frame from a close-in BS.

June 2008


Slide 13

doc.: IEEE 802.22-08/0137r3

Submission

802.22 WRAN superframe and frame

structure for the coexistence mode

frame 0

Superframe n-1 Superframe n Superframe n+1

SuperframePreamble

SCH

. . .

FramePreamble

160 ms

frame 1

10 ms

frame 15

DS sub-frame TTG

RTG

US sub-frame(smallest US burst portion on a given subchannel= 7 symbols)

26 to 42 symbols corresponding to bandwidths from 6 MHz to 8 MHz and cyclic prefixes from 1/ 4 to 1/ 32

Fram

e Pr

eam

ble

FCH

DS-M

AP

Burst

1DCD

Burst

2 time b

uffer

time b

uffer

Self-

coex

isten

ce w

indo

w (4

or 5

sym

bols

when

sche

dule

d)

Burst 1

60 s

ubch

anne

ls

Burst 2

Burst 3more than 7 OFDMA symbols

Burst

Burst n

BurstBu

rst m

Ranging/ BW request/ UCS notification

Burst

Burst

Bursts

Burst

s

... Time...

US-M

APUS

-MAP

UCD

10 ms 10 ms

FramePreamble

FramePreamble

Superframe header transmitted in SFN by all BSs

One symbol buffer including RTG needed before and a

fraction after the SFN superframe header to absorb propagation time differential

Timebuffer

Frame preamble SCHSuperframe

preambleTimebuffer

June 2008


Slide 14

doc.: IEEE 802.22-08/0137r3

Submission

Inter-frame capacity allocationand receiver AGC consideration

• Special AGC requirements at the CPE receiver to deal with widely varying frame amplitudes– AGC to keep track of superframe header amplitude– AGC to keep track of wanted downstream subframe amplitude– AGC to block interfering frames destined to other CPEs

• Special AGC requirements at the BS receiver to deal with widely varying frame amplitude– AGC to keep track of wanted upstream subframe amplitude (i.e.,

upstream bursts from associated CPEs)– AGC to block interfering upstream subframes destined to other

BSs (i.e., upstream bursts from non-associated CPEs)

June 2008


Slide 15

doc.: IEEE 802.22-08/0137r3

Submission

Inter-frame capacity allocation• The Superframe Control Header (SCH) will carry

information about:1) the frame allocation to the various ovelapping WRAN cells2) the location of the inter-frame and intra-frame quiet periods, and3) the location of the Self-Coexistence Windows (SCW)

• The frame allocation could be signalled by a 16-bit pattern per overlapping WRAN cell:– Since a superframe contains 16 frames, the number of overlapping

WRAN cells in one area that could be accommodated if a minimum of one frame per superframe is to be assigned for each BS is 16

– In practice, less than 16 cells are likely to overlap in one area– Current SCH MAC message allows for up to 12 overlapping BSs

(12x16 bits= 24 bytes)

Note: The MAC addresses of the BSs involved would be transmitted to CPEs as part of the BS advertising payload rather than being repeated in every SCH since they would not change often.

June 2008


Slide 16

doc.: IEEE 802.22-08/0137r3

Submission

Inter-frame capacity allocation (Cont’d)• Minimum pace for frame multiplexing:

– To keep time sync at the CPE: minimum one per superframe (?)– To provide QoS: every two frames but not practical =>

compromise on QoS will be needed in self-coexistence situation: “best effort”

• The output of the MAC coexistence algorithms will be the assignment of the 16 frames in a superframe to the local co-channel overlapping BSs according to:– their respective capacity loading requirement– special QoS requirements for real-time applications– whether the CPEs to which the information in a given frame is

addressed will suffer from frame collision and from which BS (i.e., CPEs are located in overlapped areas) to avoid concurrent frame transmissions in these cases.

June 2008


Slide 17

doc.: IEEE 802.22-08/0137r3

Submission

A, B, C, D: WRAN Base Stations

.....

.....

.....

.....

.....

.....

.....

.....

C

D

B

Aa

ab

bd

bc

A

B

C

D

a a a a a

b

c

d

ab

c

d

c c c

d d d

bdb bc b b

c

d

Superframe header: carrying frame scheduling(common to all overlapping cells)

Downstream sub-frame

Upstream sub-frame

Self-coexistence window

a, b, c, d: CPEs located in their own WRAN cell

ab, bc, bd: CPEs located in cell overlap areas

Inter-frame capacity allocation (Cont’d) Example

June 2008


Slide 18

doc.: IEEE 802.22-08/0137r3

Submission

Self-coexistence capacity allocation scheme• The capacity allocation scheme will include frames that can

be transmitted concurrently by non-overlapping WRAN cells as well as cells that are overlapping if the traffic carried during these frames address CPEs outside the overlap areas.– Collisions would occur in the overlap areas but no CPE would be

listening in those areas since it is not addressed to them.• When the traffic is directed to CPEs in an overlapped area,

only one of the overlapped WRAN BSs will be able to transmit during a given frame to avoid frame collisions at the CPE in the downstream and at the BSs in the upstream.

• It is very important to identify the CPEs in the overlapped areas to maximize the transmission capacity– A method to precisely quantify the state of WRAN cells’ overlap at each

CPE is proposed. This will be more accurate than a geolocation-based method.

June 2008


Slide 19

doc.: IEEE 802.22-08/0137r3

Submission

Means to identify CPEs in overlap areas• The frame capacity allocation algorithm will need to allocate

frames that are allocated to only one BS at a time on a periodic basis so that all the CPEs in the area can measure the RSSI level during that frame to assess the extent of potential collision between the different BS transmissions at these CPEs.

• The CPEs associated with the BS transmitting the frame would operate as usual (receiving and transmitting information).

• The CPEs associated with the other BSs will measure the RSSI on their WRAN receive path during that frame and record it locally.

• Their respective BSs will then request that this RSSI information be transmitted to them with the appropriate MAC message. (A higher priority message could also be initiated by the CPE to signal a change of status.)

• The rate at which these ‘probing’ frames will be sent will depend on the expected rate of change of the WRAN environment (both in new BS implementations and change in physical environment to include changes in signal propagation)

June 2008


Slide 20

doc.: IEEE 802.22-08/0137r3

Submission

Means to identify CPEs in overlap areas (cont’d)

Channel number … 25 26 27 28 29 30 … Time of last sensing Time of last positive Sensing path RSSI WRAN path RSSI Signal type WRAN service advertisement

Sensing path RSSI under WRAN

Signal type under WRAN

Channel occupancy from BS (CHO-UPD)

Max EIRP at CPE (SME-MLME-DB-RESPONSE) (6.11.7)

Table 281 — Sensing registers at the CPE

Original table in the 802.22 Draft 1.0

June 2008


Slide 21

doc.: IEEE 802.22-08/0137r3

Submission

Channel number … 25 26 27 28 29 30 … Time of last sensing Time of last positive Sensing path RSSI WRAN path RSSI Signal type WRAN service 1 advertisement

WRAN path RSSI WRAN service 2 advertisement

WRAN path RSSI … WRAN service N advertisement

WRAN path RSSI Sensing path RSSI under WRAN

Signal type under WRAN

Channel occupancy from BS (CHO-UPD)

Max EIRP at CPE (SME-MLME-DB-RESPONSE) (6.11.7)

Means to identify CPEs in overlap areas (cont’d)Table 281 — Sensing registers at the CPE

Additional information to be

acquired at the CPE for self-coexistence

purposes

RSSI measured on non-concurrent

frame transmissions on the same channel

June 2008


Slide 22

doc.: IEEE 802.22-08/0137r3

Submission

Coexistence table at the base station

Location WRAN service

A

WRAN service

B

WRAN service

C

WRAN service

D

WRAN service

E

…

CPE-1 -80 -90 -- -- --

CPE-2 -75 -85 -- -- --

CPE-3 -90 -54 -- -- --

CPE-4 -82 -- -- -95 --

CPE-5 -87 -- -- -83 --

…

RSSI measured from nearby BSs operating on channel N and collected at the BS (dBm)

• The resulting RSSI will be reported and tabulated at the BS and used to quantify at what level that CPE would suffer from frame collision:

– difference between the desired RSSI and the interfering RSSI– this difference is compared to the SNR required for the level of modulation used

for transmission to and from the CPE => possible collision? => in overlap area?– CPE in overlap area is determined on actual signal levels rather than geolocation

June 2008


Slide 23

doc.: IEEE 802.22-08/0137r3

Submission

Frame incompatibility table at the BS

• This Table will be developed at each BS and include all CPEs associated with it (e.g., BS3) that will be affected by transmissions from other specific BSs.– An incompatible CPE is added to the Table if the difference between the

RSSI of the desired BS and that of the undesired BS is less than x dB depending on the modulation used for the communication, see TPC Table.

• When BS3 needs to communicate with one of these CPEs, an incompatibility flag will be transmitted with the frame allocation request exchanged among BSs so that concurrent frame transmission is avoided by the frame allocation algorithm used at all overlapping BSs for the two BSs involved.

BS1 BS2 BS3 BS4 BS5 …

BS3 CPE1CPE4CPE7

- (void) CPE5CPE8

CPE5CPE7CPE8

June 2008


Slide 24

doc.: IEEE 802.22-08/0137r3

Submission

Self-coexistence capacity allocation scheme• Frame capacity will be assigned among overlapping cells

based on a distributed algorithm operating at each BS• Information on the capacity loading of each BS and

special QoS requirements will be transmitted among BSs• Each frame request from each overlapping BS will be

accompanied with incompatibility flags depending on the CPEs served so that concurrent frame transmissions are avoided by the frame allocation algorithm for the specific BSs involved (Better frame scheduling would be possible if all CPE capacity requests were centrally known but this would create privacy concern)

• Based on this information, all BSs will develop a common frame assignment that will be signalled by the upcoming SCH broadcast by all BSs for the coming frame.

Note: The signalling among BSs will need to transmit sufficient information within the given time frame (a superframe) to make sure that all the

algorithms at all BSs converge on the same frame scheduling.

June 2008


Slide 25

doc.: IEEE 802.22-08/0137r3

Submission

Self-coexistence capacity allocation scheme

Capacity loading for each BS

QoS requirement per service for each BS

Frame requirementper BS and associated incompatibility flags

Frame scheduling for the upcoming

superframe transmitted by the 16-bit pattern for

each BS

Information to be exchanged among all overlapped BSs in

preparation for the next frame capacity allocation

Information to be transmitted by all

overlapped BSs in their superframe header

Better frame scheduling would be possible if information about traffic to specific CPEs were to be shared

but this bring privacy concern.

Interference-free

schedulingAdaptive on-demand channel contentionDynamic resource renting/offering

Scheduling ofnon-concurrentprobing frames

Self-coexistence capacity allocation

algorithm at each BS

Interference-free





Interference-free





Inter-BS communication using

the CBP bursts

June 2008


Slide 26

doc.: IEEE 802.22-08/0137r3

Submission

Exchange of information among BSs for self-coexistence

• A sufficient number of Self-Coexistence Windows (SCW) will need to be allocated per superframe to exchange the information necessary to make sure that the distributed capacity allocation algorithms in each BS always converge to a common frame allocation for the upcoming superframe

• Transmission mechanism:

PreambleSuperframe

ControlHeader

Symbol

(Optional)

CBP MACPDU

Coexistence Beacon Protocol (CBP) burst

Will sufficient information be exchanged among BSs within a superframe to make all BS

algorithms to converge on the same frame scheduling for the

next superframe?

Will the transmission be reliable enough considering possible obstructions etc.?Transmission of the SCH

content of the next superframe may be needed to confirm that

it is common to all BSs.Not needed if convergence at each

BS is reliable enough.

June 2008


Slide 27

doc.: IEEE 802.22-08/0137r3

Submission

A, B, C, D: WRAN Base Stations

.....

.....

.....

.....

.....

.....

.....

.....

C

D

B

Aa

ab

bd

bc

A

B

C

D

a a a a a

b

c

d

ab

c

d

c c c

d d d

bdb bc b b

c

d

Superframe header: carrying frame scheduling(common to all overlapping cells)

Downstream sub-frame

Upstream sub-frame

Self-coexistence window

a, b, c, d: CPEs located in their own WRAN cell

ab, bc, bd: CPEs located in cell overlap areas

Inter-frame capacity allocationExample

June 2008


Slide 28

doc.: IEEE 802.22-08/0137r3

Submission

Bi-directional burst collision at nearby CPEs

• Downstream/upstream apportionnement should not need to be common to all overlapping cells to avoid such bi-directional hits

• If TTG is not common, downstream and upstream bursts may collide unless proper burst scheduling is provided at the BS– Need to identify which CPEs in different cells are close enough to

create upstream/downstream interference: need for local CBP burst probing, monitoring, recording and update sent to each associated BS

– Downstream bursts addressing CPEs potentially creating upstream/downstream interference should be scheduled early in the downstream subframe to allow sufficient flexibility for the other BS to use as wide an upstream subframe as possible without collision

– A minimum width for the downstream subframe should be preserved to give sufficient room for all the BSs involved to schedule all the downstream bursts going to these affected CPEs

(Need to avoid interference from a transmitting CPE to a nearby receiving CPE)

Note: No need to identify and exchange actual scheduling information between WRAN cells.

June 2008


Slide 29

doc.: IEEE 802.22-08/0137r3

Submission

Conclusions• The 802.22 standard needs to allow for two modes of

operation– Normal mode to keep overhead to minimum– Self-coexistence mode in case of co-channel overlapping cells (to

secure smooth transition to shared capacity among WRAN cells)• Work is needed to define under which conditions the mode of

operation will switch between “normal operation” and “self-coexistence operation” (identify presence of CPEs in overlapped areas)

• Need to transmit the superframe header in SFN mode– need to include necessary buffer before and after the superframe

header (composed of STS preamble, LTS preamble and the SCH)• Distributed frame scheduling among BSs will need to operate

fast with its inter-BS communications taking place in the previous superframe so that the new schedule can be broadcast in the upcoming superframe header

June 2008


Slide 30

doc.: IEEE 802.22-08/0137r3

Submission

Conclusions (cont’d)• The PHY section needs to be updated to include the new

superframe structure with time buffers before and after, and time buffers between frames allocated to different BSs

• The MAC section needs to be revised to add to the “normal operation” superframe header a “self-coexistence operation” superframe header to be transmitted simultaneously by all overlapping BSs

• The MAC section needs to be modified for the right information to be transmitted by the CPB bursts

• At least one feasible distributed frame allocation algorithm needs to be demonstrated for feasibility and documented in an annex to the 802.22 Standard

• At least one example of a feasible exchange of information among a group of overlapping BSs using the CBP burst needs to be demonstrated and documented in an annex to the 802.22 Standard

doc.: ieee 802.22-08/0137r3 submission june 2008 gerald chouinard, crcslide 1 further wran...

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