October 2007
doc.: IEEE 802.22-07/0502r0
IEEE P802.22
Wireless RANs
General Superframe Structure and General Frame Structure
Date: 2007-10-26
Author(s):
Name
Wendong Hu
Company
STMicroelectronics
Address
1060 East Brokaw Road, San
Jose, CA 95131
Phone
1-408-467-8410
email
wendong.hu@st.com
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..
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
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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>.
Submission
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Wendong Hu, ST
October 2007
doc.: IEEE 802.22-07/0502r0
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]
Submission
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Wendong Hu, ST
October 2007
...
doc.: IEEE 802.22-07/0502r0
frame n-1
frame n
DS subframe
DSMAP
FCH
USMAP
U
D
C
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.
...
frame n-1
frame n
PS 0
Adaptive
frame n+1
...
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. [Illustrate a time slot]
Submission
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Wendong Hu, ST
October 2007
...
doc.: IEEE 802.22-07/0502r0
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
A frame shall 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 0, the MAC packets are assumed to be
structured in a linear TDM manner (see 0) 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 opportunity window which can use multiple sub-channels in the frequency direction
with CDMA spreading and occupies the complete US sub-channel width, US payload is arranged in a linear time/frequency
structure.
The MAC data elements from 0, starting from the FCH and including the first broadcast burst, are entered into the second
OFDM symbol, as shown in 0, in the increasing order of logical subchannels until all logical subchannels are occupied. Then,
the subsequent data elements, if they have not all been allocated, are placed in the same order on the following OFDM
symbols. The balance of the last OFDM symbol 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 0. They are mapped OFDM symbol by OFDM symbol first, in the same logical subchannel. Once a logical subchannel has
been filled to the maximum capacity, the balance of the MAC data elements are 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 is inserted at the end. Optionally, the horizontal
laying of the MAC data elements may fill one column of 7 symbols at a time. The MAC data elements are then entered in
sequence in an OFDM symbol by symbol order. However, when all logical subchannels have been filled, the next MAC data
elements will be placed starting from the first unfilled OFDM symbol on the first logical subchannel that follows the
Submission
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October 2007
doc.: IEEE 802.22-07/0502r0
opportunity window and has unfilled OFDM symbols available. 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 a US sub-frame, the balance of the last OFDM symbols is then padded with zeros. (Note that the US-MAP indicates
the lengths of the US data bursts, not their absolute position in the US map.)
The downstream’s PHY PDU preamble is followed by a FCH burst. This FCH burst, which is described in Error!
Reference source not found., specifies the burst profile and the length of either a DS-MAP or a US-MAP message. A DSMAP 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 one is 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 the most robust modulation/coding (see
section 8.xx).
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 Error! Reference source not found.) with its basic CID, together with a
bandwidth request subheader (see Error! Reference source not found.) with BR = 0. This would allow the BS to reclaim
this CPE’s allocation and use the resource for some other purpose. Besides these upstream CPE PHY bursts, the BS may
schedule up to four contention windows (see Error! Reference source not found.):[Philips, ETRI, Wendong, Ivan] 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, and the Self-coexistence window is employed by CBP for signaling key information to adjacent WRAN cells for
the purposes of self-coexistence and carrying out geolocation between CPEs of the same WRAN cell.
To deal with coexistence scenarios where multiple 802.22 BSs are deployed in overlapping coverage areas, the Selfcoexistence window together with coexistence beacons (i.e., through CBP) shall be employed. The Self-coexistence window
(depicted in 0) is scheduled at the end of the upstream subframe. These beacons are transmitted by selected CPEs and carry
information, among the other, 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 Error! Reference source not found.).
Whenever a CPE is neither receiving nor sending data to its BS, it is capable of decoding CBP packets transmitted by nearby
stations 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). 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 defines 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.
Submission
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Wendong Hu, ST
October 2007
doc.: IEEE 802.22-07/0502r0
References:
Submission
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Wendong Hu, ST