1. WRAN Self-coexistence Considerations
March 2008
doc.: IEEE /0xxxr0
March 2008
IEEE P Wireless RANs Date:
WRAN Self-coexistence Considerations
Authors:
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Gerald Chouinard, CRC
Gerald Chouinard

2. Self-coexistence Capacity Allocation Methods
March 2008
doc.: IEEE /0xxxr0
March 2008
Self-coexistence Capacity Allocation Methods
The 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
Following an analysis of pros and cons (#08/99r0), one method is preferred to share the channel capacity among overlapping co-channel cells:
inter-frame capacity allocation: frame-by-frame basis
Gerald Chouinard, CRC
Gerald Chouinard

3. RF environment resulting from overlapping co-channel WRAN cells
March 2008
doc.: IEEE /0xxxr0
March 2008
RF environment resulting from overlapping co-channel WRAN cells
For flat terrain, 100 W, 75 m BS and ITU-R P.1546 propagation model = minimum BS-to-BS separation is 52 km
Adjacent non-interfering cells
52 km
31 km
Gerald Chouinard, CRC
Gerald Chouinard

4. RF environment resulting from overlapping co-channel WRAN cells
March 2008
doc.: IEEE /0xxxr0
March 2008
RF environment resulting from overlapping co-channel WRAN cells
Positive signal differential
Grey area inside the contour is where normal demodulation (QPSK, rate: 1/2) will fail
31 km
31 km
Negative signal differential
Overlapping cells with worst case of negative signal differential
Gerald Chouinard, CRC
Gerald Chouinard

5. RF environment resulting from overlapping co-channel WRAN cells
March 2008
doc.: IEEE /0xxxr0
March 2008
RF environment resulting from overlapping co-channel WRAN cells
Positive signal differential
7 km
Negative signal differential
Grey area inside the contour is where normal demodulation (QPSK, rate: 1/2) will fail
Greatly overlapping cells
31 km
Gerald Chouinard, CRC
Gerald Chouinard

6. Inter-frame capacity allocation
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation
WRAN frame structure needs to be synchronized among co-channel overlapping WRAN cells (and adjacent channel cells)
Need for concurrent quiet periods for on-channel and adjacent channel sensing
Concurrent self-coexistence windows (SCW) will be scheduled at the end of some frames for inter-cell signalling
TDM is required for the frame transmission in overlapped areas to alleviate large negative signal differentials
However, the transmission of the superframe header cannot be done in TDM mode
Common superframe header carrying the same information needs to be broadcast in synchronous mode by all overlapping BSs to allow CPEs in negative signal differential areas to still receive it
Gerald Chouinard, CRC
Gerald Chouinard

7. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation (Cont’d)
In the grey areas where more than one superframe header burst will be received within a 6 dB range, decoding of one burst over the others with QPSK rate: 1/2 will not be possible
There are two possible methods to alleviate this problem:
Use of a more robust modulation for the superframe header preamble and SCH to allow demodulation under collision conditions (assuming a worse case where demodulation takes place at a CPE where three equal power superframe header bursts are received at the edge of the contour, one burst can be demodulated over the other ones if demodulation can be done in SINR≥ -3.7 dB)
Use of Single Frequency Network (SFN) operation for the superframe header in self-coexistence situation (taking advantage of the OFDM inherent capability in multipath environment)
Gerald Chouinard, CRC
Gerald Chouinard

8. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation (Cont’d)
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 base stations => resulting in long excess delay active multipaths
Cyclic prefic used for the superframe header burst is:
Cyclic prefix = 1/4 useful symbol = 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 and will be constructive (will the short time sequence for the superframe preamble be adequate for this harsh multipath channel?)
If all base stations broadcast the superframe header at 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
Gerald Chouinard, CRC
Gerald Chouinard

9. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation (Cont’d)
The Superframe Control Header (SCH) needs to carry the information about:
the frame allocation to various WRAN cells
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 base station is 16
In practice, much less than 16 cells will be overlapping in one area
Current SCH MAC message would allow for up to 12 overlapping BSs
the location of the inter-frame and intra-frame quiet periods, and
the location of the Self-coexistence windows (SCW)
The MAC section needs to be revised to include a “normal operation” SCH transmitted by each BS and a “self-coexistence operation” SCH to be transmitted by all overlapping BSs
Gerald Chouinard, CRC
Gerald Chouinard

10. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation (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 BSs
Timing information will need to be acquired by CPEs from the frame preamble received from the BS to which it is associated
A buffer of one symbol before the superframe header burst and one symbol after will be needed before starting the frame transmission to absorb the propagation path differential for up to 100 km
Gerald Chouinard, CRC
Gerald Chouinard

11. 802.22 WRAN superframe and frame structure
March 2008
WRAN superframe and frame structure
Time buffer
Frame preamble
SCH
Superframe preamble
Superframe header transmitted in SFN by all BSs
One symbol buffer needed before and after the SFN superframe header to absorb up to 100 km path differential
Gerald Chouinard, CRC

12. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
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
The output of the MAC coexistence algorithms will be the assignment of the 16 frames in a superframe to the local BSs according to:
their respective throughput requirement, and
whether the CPEs to which the information in the current frame is addressed are located in overlapped areas or not (allows concurrent frame transmissions or not)
Gerald Chouinard, CRC
Gerald Chouinard

13. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation (Cont’d)
Special AGC requirements at the CPE receiver to deal with widely varying frame amplitude
AGC to keep track of superframe header amplitude
AGC to keep track of wanted downstream subframe amplitude
AGC to block interfering frames
Special AGC requirements at the BS receiver to deal with widely varying frame amplitude
AGC to keep track of wanted upstream subframe amplitude
Gerald Chouinard, CRC
Gerald Chouinard

14. Inter-frame capacity allocation (Cont’d)
March 2008
doc.: IEEE /0xxxr0
March 2008
Inter-frame capacity allocation (Cont’d)
The standard needs to allow for two modes of operation
Normal mode to keep overhead to minimum
SFN mode in case of co-channel overlapping cells (self-coexistence)
Work is needed to define under which conditions the mode of operation will switch between “normal operation” and “self-coexistence operation”
Distributed frame scheduling among BSs will need to operate fast with its inter-BS communications taking place in the previous superframe so that the schedule can be broadcast in the superframe header
Special scheduling will be needed from time to time for all non-concurrent frame transmissions for RSSI measurement to identify CPEs located in overlapped areas
Gerald Chouinard, CRC
Gerald Chouinard

15. Coexistence capacity allocation Example
March 2008
Coexistence capacity allocation Example
Gerald Chouinard, CRC

16. Coexistence among WRAN systems
March 2008
Coexistence among WRAN systems
Inter-cell communication mechanism to keep BSs aware of the other nearby WRAN cell operation:
Coexistence beacon
Transmitted during the self-coexistence windows by the BS and/or some designated CPEs
Monitored by BSs and other CPEs from same and different cells
Coexistence Beacon Protocol (CBP) burst
Gerald Chouinard, CRC

17. Coexistence among WRAN systems (Cont’d)
March 2008
Coexistence among WRAN systems (Cont’d)
Gerald Chouinard, CRC

18. Coexistence among WRAN systems (Cont’d)
March 2008
Coexistence among WRAN systems (Cont’d)
Table 281 — Sensing registers at the CPE
Original table in the Draft 1.0
Gerald Chouinard, CRC

19. Coexistence among WRAN systems (Cont’d)
March 2008
Coexistence among WRAN systems (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
Gerald Chouinard, CRC

20. Coexistence table at the base station
March 2008
Coexistence table at the base station
RSSI measured from nearby BSs (dBm)
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
-54
CPE-4
-82
-95
CPE-5
-87
-83
Each RSSI is to be measured during a frame that is scheduled to be active at only one BS that is receivable at the CPE
The resulting RSSI that will be reported and tabulated at the BS will be used to establish that the CPE is in an overlapped area and thus to schedule non-concurrent frames from these BSs
Gerald Chouinard, CRC

21. March 2008
doc.: IEEE /0xxxr0
March 2008
To avoid interference from a transmitting CPE to a nearby receiving CPE
Downstream/upstream apportionnement will not need to be common to all overlapping cells
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: CBP bursts
Downstream bursts addressing CPEs potentially creating upstream/downstream interference should be scheduled first in the downstream subframe to allow enough flexibility for the other BS to use as wide an upstream subframe as possible
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
Gerald Chouinard, CRC
Gerald Chouinard

22. Conclusions Coexistence with other services in the band
March 2008
Conclusions
Coexistence with other services in the band
Synchronized quiet periods are needed for the WRAN systems to allow sensing of incumbents in their operating channel
Width of quiet periods will be dictated by wireless microphone beacon sensing:
Intra-frame quiet period: 5.1 ms to capture the beacon sync burst
Inter-frame quiet period: up to 16 frames (one superframe) to capture the beacon payload including the signature and certificate
Is the Shorst Time Sequence appropriate for the superframe preamble to train for a channel with such large excess delay active echoes?
Gerald Chouinard, CRC

23. March 2008
Conclusion
Inter-frame capacity allocation seems to be the only reasonable alternative for accommodating coexistence at the PHY layer
Need to define two modes of operation: normal operation and self-coexistence operation
Need to transmit the superframe header in SFN mode
need to re-define the modulation and coding of the superframe header (preamble and FCH) (Is the Short Time Sequence appropriate for the superframe preamble to train for a channel with such large excess delay active echoes?)
Need to determine minimum CPE time re-sync refresh rate for minimum frame allocation to BSs (one per superframe?)
Need to review the SCH MAC section to re-define the payload requirement
Need to define distributed self-coexistence frame scheduling and the required inte-BS communications
Gerald Chouinard, CRC