IEEE C802.16m-08/116
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Comments on the Super-frame based Framing Structure for 802.16m
Date
Submitted
2008-01-20
Source(s)
Udayan Dasgupta, Muhammad Ikram,
David P. Magee, Murtaza Ali
Texas Instruments Inc.
Re:
IEEE 802.16m-07/047 (“Call for contributions on Project 802.16m SDD”)
udayan,mzi,magee,mali@ti.com
214-480-2779, 214-480-3132
Target topic: “Proposed 802.16m Frame Structure with special attention to legacy support”
Abstract
This document provides some comments and outlines some modifications to the frame structure
as proposed in C80216m-07_354 submitted to the IEEE 802.16 Broadband Wireless Access
Working Group. The changes maintain the overall structure of the super-frame, frame, and the
sub-frame structure proposed in above mentioned document while adding some flexibility,
which may provide performance improvements under certain conditions. The signaling overhead
caused due to the added flexibility is shown to be minimal.
Purpose
For discussion and approval by IEEE 802.16 Working Group
Notice
Release
Patent
Policy
This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It
represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for
discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material
contained herein.
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
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Further information is located at <http://standards.ieee.org/board/pat/pat-material.html> and
<http://standards.ieee.org/board/pat>.
Comments on the Super-frame based Framing Structure for 802.16m
Udayan Dasgupta, Muhammad Ikram, David P. Magee, Murtaza Ali
Texas Instruments Inc., Dallas, TX
Abstract
This document provides some comments and outlines some modifications to the frame structure as proposed in
C80216m-07_354 [3] submitted to the IEEE 802.16 Broadband Wireless Access Working Group. The changes
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IEEE C802.16m-08/116
maintain the overall structure of the super-frame, frame, and the sub-frame structure proposed in [3] while
adding some flexibility, which may provide performance improvements under certain conditions. The signaling
overhead caused due to the added flexibility is shown to be minimal.
Summary of C80216m-07_354
In the above mentioned proposal the authors propose the use of a 20ms super-frame consisting of four 5ms
frames (similar to the 802.16e frames). Each 5ms frame would be further divided into sub-frames, each of
which would be six symbols wide. Frame 0 of the super-frame would contain system configuration, paging and
other broadcast information applicable to the whole super-frame. In the TDD mode of operation, each sub-frame
could be assigned to either uplink or downlink, and each downlink (or uplink) portion of the 5ms frame could be
divided into legacy DL (or UL) zones or the new 802.16m zones. Moreover a DL (or UL) zone would span an
integer number of sub-frames. The duration of the zones may vary from frame to frame. The DL/UL switching
points would be synchronized across the network to reduce interference and would occur on sub-frame
boundaries. In the FDD mode of operation, the UL and DL frames would be transmitted simultaneously (on
different frequencies) and each 5ms frame could contain a mixture of legacy and new DL (and UL) zones.
Analysis of C80216m-07_354
Overall the above proposed structure lends itself to efficient signaling since the system configuration has been
consolidated for every super-frame, thereby reducing overhead. Also the use of 6 symbols as the sub-frame
duration ensures that an integer number of all UL and DL symbols-per-slot (as given in Table 1) can be
accommodated for every 802.16e permutation.
Permutation
Symbols-per-slot
DL FUSC
1
DL PUSC
2
DL AMC/DL Band AMC
3
UL PUSC
3
UL AMC
3
Table 1: Symbols-per-slot for different 802.16e permutations
It also states that “Every frame shall start with a preamble and the MAP followed by legacy DL zone since
legacy terminals/relays expect 802.16e zones here”. However consolidating all the control information at the
start of a 5ms frame would not allow low latency modes. Also, if MIMO permutations are considered, pilot
patterns do not repeat on slot boundaries. For example, 2x2 MIMO requires 4 symbols to transmit the complete
pilot pattern for DL PUSC – meaning that a 6 symbol sub-frame could not accommodate an integer number of
pilot pattern repetitions. Such boundary conditions impact receiver design and degrade channel estimation
performance. Also there is no guarantee that this 6-symbol wide sub-frame would accommodate an integer
number of (yet to be defined) 802.16m pilot pattern repetitions. Finally 5ms frames which do not consist of an
integer number of 6 symbol sub-frames will increase overhead. This occurs for CP lengths other than 1/8 for 5
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IEEE C802.16m-08/116
and 10 MHz systems.
Features of preferred framing structure
To simplify the following discussion, the term “section” is defined for a given zone configuration as either the
minimum number of symbols needed for a slot or the minimum number of symbols over which the pilot pattern
repeats– whichever is larger. Also it should be noted that although some of the following points are not stated in
contribution C80216m-07_354, they are not precluded by it either.
1. Zone dependent sub-frame durations
It is proposed that the sub-frame durations (in terms of symbols) be allowed to change from sub-frame to
sub-frame. It should, however, be fixed to an integer number of sections for each zone configuration, i.e. the
sub-frame duration would be pre-decided based on the zone configuration used for that sub-frame. The subframe durations should also be fixed for the network or allowed to change at a slow rate (thereby minimizing
signaling overhead). This would allow the network to adapt to different latency or bandwidth requirements.
An example set of sub-frame durations corresponding to different zone configurations are given Table 2,
Zone
Configurati
on
Possible sub-frame
duration (symbols)
Section duration
(symbols)
Number of sections per
sub-frame
SISO DL
FUSC
6
2
3
SISO DL
PUSC
6
2
3
SISO/MIMO
DL AMC
and DL Band
AMC
6
3
2
MIMO DL
PUSC
8
4
2
SISO UL
AMC
6
3
2
SISO UL
PUSC
3
3
1
Table 2: Example sub-frame durations for different 802.16e permutations
The zone configuration for the entire super-frame may be part of the broadcast information transmitted at the
beginning of the super-frame or in some other periodic broadcast message. Note that similar to fixing the
sub-frame duration, such a scheme would also not incur any signaling overhead. This is because this
information (i.e. the sub-frame duration and section duration for every zone configuration) is known at both
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IEEE C802.16m-08/116
the transmitter and the receiver.
2. One dimensional user allocations
By limiting the time duration of the sub-frame to a reasonably small value (over which the channel can be
assumed to be invariant), the user allocations can be “one-dimensional” within a sub-frame as shown in
Figure 1. That means that the multiple users may be assigned to different logical frequency bands (or subchannels) spanning the entire sub-frame duration in time. The benefit of such one-dimensional allocations
would be a significant reduction in signaling overhead.
Super-frame (20 ms)
Frame 0 (5 ms)
Frame 1 (5 ms)
SF-0
…
SF-n
Frame 2 (5 ms)
Frame 3 (5 ms)
…
…
1-dimesional user
allocations
Figure 1: Example of sub-frame allocation
3. Control information embedded in sub-frames
The control information (MAP) for the allocations in a sub-frame may be contained at the start of every subframe. This would reduce signaling overhead and allow for per-user dedicated control channel which would
in turn enable efficient signaling and blind coding rate detection at the MS similar to methods mentioned in
[4]. It would also facilitate low latency modes as explained in the next sub-section
4. Configurable low latency modes in TDD
Since the sub-frames may be either UL or DL, it may be possible to interleave UL and DL sub-frames within
a 5ms TDD frame to create low latency modes. To minimize interference the UL/DL switch points may need
to be synchronized across the network. Also using the flexibility of the sub-frame durations, it may be
possible to create short UL PUSC sub-frames (3 symbols – only for feedback) while dedicating most of the
time for downlink – even for low latency modes. A few examples of frame descriptions with low latency are
shown in Figure 2.
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IEEE C802.16m-08/116
Super-frame (20 ms)
Frame 0 (5 ms)
Frame 1 (5 ms)
DL
(36 symbols)
Frame 2 (5 ms)
Frame 3 (5 ms)
UL
(12 symbols)
Frame configuration with 5 ms
latency and (36,12) DL,UL symbols
DL
(12 symbols)
UL
(12 symbols)
DL
(12 symbols)
UL
(12 symbols)
Frame configuration with 2.5 ms
latency and (24,24) DL,UL symbols
DL
(22 symbols)
UL
(3 symbols)
DL
(14 symbols)
UL
(9 symbols)
Frame configuration with 2.5 ms
latency and (36,12) DL,UL symbols
DL
(9)
UL
(3)
DL
(9)
UL
(3)
DL
(9)
UL
(3)
DL
(9)
UL
(3)
Frame configuration with 1.25 ms
latency and (36,12) DL,UL symbols
Figure 2: Low latency modes
Backward compatibility to 802.16e frame structure
In order for a 802.16m BS to support a mixture of IEEE 802.16m and legacy MSs operating on the same RF
carrier, the frame structure needs to be backward compatible to that of 802.16e. In the presence of such a mix,
the first DL sub-frame needs to be long enough (in time) to support all the required legacy traffic. It should
begin with the 802.16e-style FCH and DL MAP combination (and UL MAP, if needed) which can be decoded
by the legacy MSs. The control and data for IEEE 802.16m MSs may reside in the remaining DL sub-frames and
need not be decodable by the legacy MSs. The legacy UL region needs to be present as indicated by the
corresponding UL MAP.
Conclusion and Recommendations
This document outlines a few ways of making the super-frame/sub-frame structure ([3]) more flexible. It has
been explained how such flexibility can be added while minimizing overhead. Additional schemes were also
proposed to facilitate lower overhead and efficient signaling. It is recommended that these and other similar
schemes be evaluated while defining the frame structure for 802.16m.
REFERENCES
[1] IEEE, "IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed
Broadband Wireless Access Systems", IEEE Std 802.16-2004
[2] IEEE, "IEEE Standard for Local and metropolitan area networks Part 16: Air Interface for Fixed and Mobile
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IEEE C802.16m-08/116
Broadband Wireless Access Systems Amendment 2: Physical and Medium Access Control Layers for
Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1",IEEE Std 802.16e-2005 and
IEEE Std 802.16-2004/Cor 1-2005
[3] Views on IEEE 802.16m Frame Structure, Document Number IEEE C802.16m-07/354, Nov. 2007
[4] Downlink Control Channel Design Considerations for IEEE 802.16m, Document Number IEEE C802.16m07/276r1, Nov. 2007
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