October 2007
doc.: IEEE 802.22-07/xxxxr0
IEEE P802.22
Wireless RANs
General Superframe Structure and General Frame Structure
Date: 2007-11-02
Author(s):
Name
Company
Wendong Hu
STMicroelectronics
Gerald Chouinard
CRC
Address
1060 East Brokaw Road,
San Jose, CA 95131
3701 Carling Avenue,
Ottawa, Ontario, Canada
Phone
email
1-408-467-8410
wendong.hu@st.com
1-613-998-2500
Gerald.Chouinard@crc.ca
Abstract
1.1
The document contains revised text for sub-clause 6.3 – General Superframe Structure, and subclause
6.4 – General Frame Structure of the IEEE 802.22 draft standard.
It contains changes agreed over the MAC teleconference call held on 24 October down to Figure 11.
It also contains changes agreed during the MAC teleconference call held on 31 October.
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@ieee.org>.
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October 2007
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6.3 General Superframe Structure
The superframe structure employed in the MAC is depicted in 0, where it can be seen that it is comprised of three main parts:



A PHY preamble – see section 8
A Superframe Control Header (SCH) – see section 6.5.1
16 MAC frames – see section 6.4
At the beginning of every superframe, the BS shall transmit a special preamble and SCH (with a known modulation/coding)
on the operating TV channel. Any CPE tuned to that TV channel must synchronize and receive the SCH in order to obtain the
information it needs to establish communication with the BS. During the lifetime of a superframe, 16 MAC frames are
transmitted. During each MAC frame, the BS has the responsibility to manage the upstream and downstream operations,
which may include ordinary data communication, measurement activities, coexistence procedures, and so on.
The superframe shall start with a superframe preamble, followed by the first frame preamble, the superframe control header
(SCH), a frame control header (FCH), a DS-MAP and a US-MAP, a DCD and a UCD if present, and finally the first frame
payload. The first frame payload shall be reduced by two symbols to keep the frame size consistent.
...
Superframe n-1
Superframe n
frame 0
Superframe
Preamble
Frame
Preamble
SCH
frame 1
Frame
Preamble
...
Superframe n+1
...
Time
frame m
Frame
Preamble
Figure 8 —General superframe structure
6.4 General Frame Structure
The top-down TDD frame structure employed in the MAC is illustrated in Figure 9, whileFigure 10 depicts another view of
the frame structure which is comprised of a series of time slots that may be allocated for either downstream or upstream
traffic. The bursts depicted in Figure 9 may be transmitted across several logical subchannels as defined by the PHY. This is
shown in 0, which depicts how a frame may be transmitted (in time and frequency) by the PHY layer.
[Note: a new definition was added to the section 3 as follows:
3.39 Time slot: minimum unit of time allocation at the MAC layer]
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October 2007
...
doc.: IEEE 802.22-07/xxxxr0
frame n-1
frame n
DS subframe
DSMAP
FCH
U
D
C
USMAP
DS burst 1
D
C
D
Time
US subframe
Initialization
slots
DS PHY PDU
Preamble
...
frame n+1
DS burst 2
...
BW request
slots
UCS
Notification
Slots
US PHY PDU
(CPE m)
DS burst x
BCH
MAC
Broadcast
MAC PDU 1
...
US PHY PDU
(CPE p)
Selfcoexistence
window
US burst
... MAC PDU k
Pad
PDU
MAC PDU 1
... MAC PDU y
Pad
MAC
Header
MAC
Header
MAC Payload
MAC Payload
CRC
CRC
Figure 9—MAC Frame structure
As illustrated in 0 and 0, a frame is comprised of two parts: a DS subframe and a US subframe. The boundary between these
two segments is adaptive. Thus, the downstream and upstream relative capacity can be controlled. The downstream subframe
shall consist of only one PHY PDU. An upstream subframe may consist of contention intervals scheduled for:
- initialization (e.g., initial ranging, geolocation between BS and CPE),
- bandwidth request,
- UCS (Urgent Coexistence Situation) notification,
- self-coexistence window.
It may also contain scheduled upstream PHY PDUs, each transmitted from different CPEs. At the MAC layer, the frame is
constitute of an integer number of time slots as shown in Figure 11
...
frame n-1
PS 0
frame n
Time slot
frame n+1
Adaptive
...
Time
PS N-1
N PS
Downstream
Subframe
Upstream
Subframe
Figure 10 —The slotted structure of a MAC frame. The boundary between upstream and downstream is
adaptive.
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October 2007
...
doc.: IEEE 802.22-07/xxxxr0
frame n-1
frame n
...
frame n+1
Time
10 ms
US-MAP
Ranging/BW request/UCS notification
Burst
Burst
60 subchannels
time buffer
Burst m
Bursts
Burst 2
Burst
time buffer
DS-MAP
Bursts
US-MAP
Frame Preamble
more than 7 OFDMA symbols
Burst 3
(4 or 5 symbols when scheduled)
Burst 2
Self-coexistence window
Burst 1
Burst 1
UCD
DCD
FCH
26 to 42 symbols corresponding to bandwidths from 6 MHz to 8 MHz and cyclic prefixes from 1/4 to 1/32
Burst n
US sub-frame
RTG
DS sub-frame
TTG
Burst
(smallest US burst portion on a given subchannel= 7 symbols)
Figure 11 – Example of aTime/frequency structure of a MAC frame (with only key zones
Figure 11 shows an example of the two-dimensional (time/frequency) structure of the MAC frame which will consist of an
integer number of fixed size OFDM slots, each of which shall consist of one OFDM symbol and one sub-channel (i.e., 1
OFDM slot = 1 symbol x 1 sub-channel). To help understand Figure 11, the MAC packets are assumed to be structured in a
linear TDM manner (see Figure 9) while the PHY packets are arranged in a two-dimensional time/frequency domain (symbol
in the horizontal direction, logical sub-channels in the vertical direction). For the FCH, the DS/US-MAP, the DCD and UCD,
and for the downstream payload the MAC information is first layed vertically by sub-channels, then stepped horizontally in
the time direction.
The MAC data elements from Figure 9, starting from the FCH and including the first broadcast burst, are entered into the
second OFDM symbol, as shown in Figure 11, in the increasing order of logical subchannels until all logical subchannels are
occupied. Then, the subsequent data elements, if they have not all been mapped, are placed in the same order on the
following OFDM symbols. The balance of the last OFDM symbols is then padded with zeros.
The MAC data elements that are contained in upstream bursts are mapped to the US sub-frame in a different order as shown
in Figure 11. They are first mapped OFDM symbol by OFDM symbol, in the same logical subchannel. Once a logical
subchannel has been filled to the maximum capacity, the balance of the MAC data elements shall be mapped to the next
logical sub-channel, in an increasing subchannel order. This process continues until all of the subchannels and symbols
alloted to the burst are filled. If the quantity of MAC data elements is insufficient to fill in the burst, padding shall be inserted
at the end.
Optionally, the horizontal laying of the MAC data elements may fill one column of 7 OFDM symbols at a time. The MAC
data elements are then entered in sequence, on an OFDM symbol-by-symbol order. However, when all logical subchannels
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have been filled, the next MAC data elements will be placed in the first available logical subchannel in the following column.
The width of the last vertical column will be between 7 and 13 symbols depending on the total number of symbols in the
upstream sub-frame. When all MAC data elements have been placed in the US subframe, the balance of the last OFDM
symbols shall then bepadded with zeros. (Note that the US-MAP indicates the length of the US MAC element, not their
absolute position in the US map.)
The downstream’s PHY PDU preamble is followed by a FCH burst. This FCH MAC burst, which is described in 6.5.2,
specifies the burst profile and the length of the DS-MAP, if transmitted or the US-MAPs. The DS-MAP message, if
transmitted, shall be the first MAC PDU in the burst following the FCH. An US-MAP message shall immediately follow
either the DS-MAP message, if transmitted, or the FCH. If UCD and DCD messages are transmitted in the frame, they shall
immediately follow the DS-MAP and US-MAP messages. The symbols containing these broadcast MAC control messages
shall be modulated using QPSK, rate: ½ (see section 8.2) with the mandatory convolutional code mode (see section 8.6.2.1).
In the upstream direction, if a CPE does not have any data to be transmitted in its US allocation, it shall transmit an US PHY
burst containing a general MAC header (see 6.6.1.1) with its basic CID, together with a bandwidth request subheader (see
6.6.1.3.1) with BR = 0. This would allow the BS to reclaim this CPE’s allocation and use the resource for some other
purpose.
The BS may schedule up to four types of contention windows (see 6.14): the Ranging window is used for initializing the
association, the BW request window is for CPEs to request upstream bandwidth allocation from the BS, the UCS Notification
window is used by CPEs to report an urgent coexistence situation with incumbents, while the Self-coexistence window is
employed by CBP burst for signaling key information to adjacent and overlapping WRAN cells for the purpose of selfcoexistence and for carrying out geolocation between CPEs of the same WRAN cell.
The Self-coexistence window is scheduled at the end of the upstream subframe as depicted in Figure 11. The CBP bursts are
transmitted by selected CPEs and carry information, among other things, about the transmitter’s ongoing service flows with
the BS and also about the 802.22 cell as a whole. With this information, CPEs associated with other nearby BSs are capable
of implementing mechanisms that allow better inter-cell coexistence such as “interference-free” scheduling (see 6.20.2).
Whenever a CPE is neither receiving nor sending data to its BS (idle state), it shall be capable of decoding CBP packets
transmitted by nearby CPEs belonging to other BSs operating on the same TV channel (N), on adjacent channels (N+/-1),
and/or on alternate channels (N+/-2 and beyond). This mechanism shall also be available at CPE initialization. In addition, a
frame synchronization mechanism is defined so that multiple collocated 802.22 cells can efficiently communicate with each
other and align their quiet periods for sensing incumbents.
In addition, the BS has the possibility of using the Self-coexistence window and define what should be done during this time
window. The Self-coexistence window may have two purposes as indicated by the BS: listening for beacons (from nearby
CPEs associated with the same BS or associated with other BSs) or transmitting beacons.
References:
Submission
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