Further WRAN Self-coexistence Considerations March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 IEEE P802.22 Wireless RANs Date: 2008-06-25 Further WRAN Self-coexistence Considerations 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 patcom@iee.org. > Gerald Chouinard, CRC Gerald Chouinard

WRAN Self-coexistence Considerations June 2008 WRAN Self-coexistence Considerations MAC self coexistence scheme PHY co-existence mechanism Spectrum Etiquette Different TV channel selection Interference-free scheduling Adaptive on-demand channel contention Frame allocation signalled by the superframe control header (SCH) Dynamic resource renting/offering Gerald Chouinard, CRC

WRAN Self-coexistence Considerations (cont’d) March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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 adaptative channel capacity allocation among co-channel WRAN cells. Gerald Chouinard, CRC Gerald Chouinard

RF environment resulting from overlapping co-channel WRAN cells March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 RF environment resulting from overlapping co-channel WRAN cells Number at each CPE location indicates the differential between the signals received from the two BSs including the CPE antenna discrimination Desired BS 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 Undesired BS Possible scenarios for license-exempt operation Gerald Chouinard, CRC Gerald Chouinard

RF environment resulting from overlapping co-channel WRAN cells March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 RF environment resulting from overlapping co-channel WRAN cells Number at each CPE location indicates the differential between the signals received from the two BSs including the CPE antenna discrimination Desired BS Positive signal differential Near 0 dB signal differential Grey area inside the contour is where normal demodulation (QPSK, rate: 1/2) will fail 31 km Undesired BS 31 km Negative signal differential Overlapping cells with worst case of negative signal differential Possible scenarios for license-exempt operation Gerald Chouinard, CRC Gerald Chouinard

RF environment resulting from overlapping co-channel WRAN cells March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 RF environment resulting from overlapping co-channel WRAN cells Number at each CPE location indicates the differential between the signals received from the two BSs including the CPE antenna discrimination Positive signal differential Desired BS Near 0 dB signal differential 7 km Undesired BS Negative signal differential Grey area inside the contour is where normal demodulation (QPSK, rate: 1/2) will fail Largely overlapping cells 31 km Possible scenarios for license-exempt operation Gerald Chouinard, CRC Gerald Chouinard

Conditions of operation in a co-channel self-coexistence situation March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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 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 will exist in license-exempt operation Gerald Chouinard, CRC Gerald Chouinard

March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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 carries 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. Gerald Chouinard, CRC Gerald Chouinard

March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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: 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 robust capability in multipath environment) Gerald Chouinard, CRC Gerald Chouinard

SFN operation for the superframe header in self-coexistence situation March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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: Will the short training sequence for the superframe preamble be adequate for such harsh multipath channel? Will a second preamble be needed with a long training sequence before the SCH?) Gerald Chouinard, CRC Gerald Chouinard

March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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? Gerald Chouinard, CRC Gerald Chouinard

March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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. Gerald Chouinard, CRC Gerald Chouinard

802.22 WRAN superframe and frame structure for the coexistence mode June 2008 802.22 WRAN superframe and frame structure for the coexistence mode Time buffer Frame preamble SCH Superframe preamble 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 Gerald Chouinard, CRC

Inter-frame capacity allocation and receiver AGC consideration March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 Inter-frame capacity allocation and receiver AGC consideration 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 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 AGC to block interfering frames destined to other BSs Gerald Chouinard, CRC Gerald Chouinard

Inter-frame capacity allocation March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 Inter-frame capacity allocation The Superframe Control Header (SCH) will carry information about: the frame allocation to various WRAN cells the location of the inter-frame and intra-frame quiet periods, and the location of the Self-Coexistence Windows (SCW) The frame allocation could be signalled by a 16-bit pattern per overlapping base station: 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. Gerald Chouinard, CRC Gerald Chouinard

Inter-frame capacity allocation (Cont’d) March 2008 doc.: IEEE 802.22-08/0xxxr0 June 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 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. Gerald Chouinard, CRC Gerald Chouinard

Inter-frame capacity allocation (Cont’d) Example June 2008 Inter-frame capacity allocation (Cont’d) Example Gerald Chouinard, CRC

Self-coexistence capacity allocation scheme March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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 will 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 cells 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 Gerald Chouinard, CRC Gerald Chouinard

Means to identify CPEs in overlap areas March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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 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 be asked to measure the RSSI on the channel 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 BS implementations and physical environment to include changes in signal propagation) Gerald Chouinard, CRC Gerald Chouinard

Means to identify CPEs in overlap areas (cont’d) June 2008 Means to identify CPEs in overlap areas (cont’d) Table 281 — Sensing registers at the CPE Original table in the 802.22 Draft 1.0 Gerald Chouinard, CRC

Means to identify CPEs in overlap areas (cont’d) June 2008 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 Gerald Chouinard, CRC

Coexistence table at the base station June 2008 Coexistence table at the base station RSSI measured from nearby BSs operating on channel N (dBm) and collected at the BS 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 for only one BS that is receivable at each CPE The resulting RSSI that will be reported and tabulated at the BS will be used to confirm that the CPE would suffer from frame collision (CPE is in an overlapped area) Gerald Chouinard, CRC

Frame incompatibility table at the BS June 2008 Frame incompatibility table at the BS BS1 BS2 BS3 BS4 BS5 … CPE1 CPE4 CPE7 - X CPE5 CPE8 This Table will be developed at each BS and include all CPEs associated with this BS (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 for the two BSs involved. Gerald Chouinard, CRC

Self-coexistence capacity allocation scheme June 2008 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 Based on this information, all BSs will develop a common frame assignment that will be signalled by the upcoming SCH for the coming frame. Note: The signalling among BSs will need to transmit sufficient infromation within the given time frame (a superframe) to make sure that all the algorithms at all BSs converge on the same frame scheduling. Gerald Chouinard, CRC

Self-coexistence capacity allocation scheme June 2008 Self-coexistence capacity allocation scheme Self-coexistence capacity allocation algorithm at each BS Capacity loading Interference-free scheduling Adaptive on-demand channel contention Dynamic resource renting/offering Frame allocation for the upcoming superframe transmitted by the 16-bit pattern for each BS QoS requirement Frame requirement and associated incompatibility flags Scheduling of non-concurrent probing frames 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 Gerald Chouinard, CRC

Exchange of information among BSs for self-coexistence June 2008 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 always converge to a common frame allocation for the upcoming superframe Transmission mechanism: Coexistence Beacon Protocol (CBP) burst Will it be sufficient? Will it be safe enough? Gerald Chouinard, CRC

Bi-directional burst collision at nearby CPEs March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 Bi-directional burst collision at nearby CPEs (Need to avoid interference from a transmitting CPE to a nearby receiving CPE) 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: 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 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

March 2008 doc.: IEEE 802.22-08/0xxxr0 June 2008 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 precisely under which conditions the mode of operation will switch between “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 STS appropriate for the superframe preamble to train for a channel with large excess delay active echoes?) (Is the LTS sufficient to synchronize the frames? 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 Gerald Chouinard, CRC Gerald Chouinard

June 2008 Conclusions (cont’d) The PHY section needs to be updated to include a new superframe structure with the required preambles and SCH to operate in a self-coexistence environment 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 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 Gerald Chouinard, CRC