Proposal for IEEE 802.16m Downlink Symbol Structure Concept
Document Number: IEEE S802.16m-08/120r1
Date Submitted: 2008-03-12
Source:
Yuval Lomnitz
Huaning Niu
Jong-kae (JK) Fwu
Sassan Ahmadi
Hujun Yin
Intel Corp.
Yuval.Lomnitz@intel.com
Huaning.Niu@intel.com
Jong-kae.Fwu@intel.com
Sassan.Ahmadi@intel.com
Hujun.Yin@intel.com
Venue:
Call for Contributions on Project 802.16m System Description Document (SDD):
Downlink Physical Resource Allocation Unit (Resource blocks and Symbol Structures), and
Pilot Structures as relevant to downlink MIMO
Base Contributions:
This presentation supports two contributions:
IEEE C80216m-08/120: A high level description of a symbol structure concept for 802.16m
IEEE C80216m-08/121: Symbol structure design for 802.16m - resource blocks and pilots
Purpose:
For discussion as introduction to base contributions
Notice:
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.
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contribution may be made public by IEEE 802.16.
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Scope and methodology
•
This contribution proposes an initial symbol structure concept and design
for downlink 802.16m
Symbol structure includes:
•
1. Subchannelization:
– slot (logical “resource block”) definition
– mapping slots to physical subcarriers (“permutation”)
2. Pilots
The two aspects are tightly related
•
Relation between symbol structure and PHY functionality
–
–
–
Symbol structure is a tool to support PHY functionality (such as various
MIMO modes, interference mitigation, etc), so is dependent on this
functionality
However symbol structure affects feasibility of PHY features and a baseline
symbol structure is needed to unify simulation assumptions and align
thoughts
To solve this tie we need to iterate between structure and functionality
Structure
Function
2
Scope and methodology
• Process in 802.16e was
– Symbol structure originally designed for SISO reuse 3
system (PUSC)
– Other features added as patches, creating additional zones,
additional complexity, and performance/functionality
limitations
• We suggest the following methodology:
– Design initial symbol structure based on assumed PHY
feature set which is relatively broad
– Update symbol structure concepts as suitable when features
are determined (added / removed) or additional
considerations are made
3
Key points (1)
• General Concepts
– Design symbol structure to support broadly potential PHY
features (FFR, MIMO modes, precoding, etc) with
considerable flexibility
– Based on SRD explicit and derived requirements and tradeoffs
– Allow flexibility to add future advanced features
• Symbol Structure and Resource Blocks
– Hierarchical symbol structure for dynamic and effective
resource allocation.
– Support 16m subframe or shortened subframe
– Conditionally align with 16e AMC subchannels
4
Key points (2)
• Pilot Design
– Maximize spectral efficiency by considering pilot overhead and
channel estimation loss
– Based on dedicated pilots (for distributed group, dedicated to the
group)
– Support different MIMO modes, # of antennas, and # of streams
– Pilots have different locations in different cells (“randomization”) to
reap the gain of boosting
– Pilot patterns obtained by computer optimization with manual finetuning
• Specific SDD texts are proposed for
– DL Symbol Structure and general concepts
– Various types of DL slot/”resource blocks”
– Various DL Pilot patterns of different overheads and advantages
5
Contents of this contribution
•
•
•
•
•
Requirements and tradeoffs
General concept
Resource block design
Pilot design
Usage scenarios and comparison with baseline
system
• Review of text proposal
6
Requirements
7
Requirements from SRD
• Peak SE
– To support high peak SE numbers in SRD overheads need
to be minimized
– We need low overhead to support the peak SE but can allow
higher overhead in other cases
– To enable satisfying peak SE for 2x2 and 4x4 we limit the
pilot overhead to:
• 11% for 2 streams
• 16% for 4 streams
Peak SE
8
Requirements from SRD
• Mobility support (7.3)
– optimize for low mobility, graceful degradation up to 350Km/h
Mobility
Performance
Stationary, Pedestrian 0 - 10 km/h
Optimized
Vehicular 10 - 120 km/h
Graceful degradation as a function of vehicular speed
High Speed Vehicular 120 - 350 km/h
System should be able to maintain connection
• Support of soft reuse (Annex A.2.1, informative) = FFR
• Support of relay (Annex A.2.2, informative)
• Support of 2,4 TX antennas in DL and 1,2 in UL
(baseline)
– We would like the structure to be extensible to more (e.g. 8 DL)
• VoIP capacity requirements (7.2)
9
PHY functions and implications
• Localized/diversity resources
• Open-loop MIMO transmission
• Precoded MIMO techniques
– Single user / Multi-user MIMO
– Short term / Long term precoding
– Fixed / Adaptive precoding
•
•
•
•
FFR (reuse partitioning)
Relay
MBS
Power control
Localized/distributed resources
 Open-loop MIMO transmission
 Precoded MIMO techniques
 FFR / soft reuse / reuse partitioning
 MBS and Relay
10
Derived Requirements and tradeoffs
Requirements related to frame structure [7,8]
• Fixed-duration 6 symbol subframe
• Subframe shortening (e.g. to 4/5)
– For support of additional switching points in TDD (erase last symbol of
subframe before the switch)
– For support of special transmissions (e.g. sounding, preambles)
– Proposed mechanism is that first/last symbol of the subframe will be erased
– To be considered in pilot and subcarrier mapping design
• Duplexing: similar structure for TDD and FDD
• Slot (RB) size considerations
– Small slot => low pilot efficiency with dedicated pilots
– Large slot => degrades small packet (VoIP, TCP ACK) efficiency
– See [9] and slide: Small packet efficiency
11
Derived requirements and tradeoffs
• Functional unification
– Situation in current 802.16 OFDMA spec:
• Different features such as localized/distributed (AMC/PUSC),
MIMO/BF with different ranks, etc cannot be supported in same zone
– Result: coarse fragmentation of resources and limitation on feature
support
• Each permutation and MIMO mode has different symbol parameters
(useful subcarriers, pilots, etc)
– Result: complexity and inconsistency
– In 802.16m we are likely to have more features than current
spec
We would like to be able to support all PHY features in the
same subframe, with a unified permutation and pilot pattern
12
General concept
13
General concept (1)
• OFDMA parameters:
– No change to sampling factor and CP length
• to allow smooth TDM (/FDM) between legacy and new structures
– Number of used subcarriers as in AMC (maximal Nused in
802.16) = 1+27/32NFFT (865 in FFT=1024)
• All modes supported within the same zone
• One-dimensional resource block indexing over the
subframe
– Motivation is simplicity, scheduling efficiency, sharing
between localized and distributed
14
General concept (2)
• FFR and multi-mode support
– Hierarchy of resources from entire
subframe to single slot
– A top of hierarchy, changes are
infrequent (multi-cell coordination)
– At bottom, changes are per cell and
per user
– Number of resources at each branch
is flexible and controlled
Entire
subframe
FFR
group
FFR
group
Distributed
Localized
Multi cell
Cell/
sector
User
Sc6 Sc5 Sc4
Sc3 Sc2 Sc1
• Pilots
–
–
–
–
Pilots are precoded and organized in clusters (narrow-band)
Channel is estimated independently per cluster or two clusters
Pilots are dedicated to localized subchannels or to a diversity group
Sounding symbols in DL and UL are used to estimate physical
(unprecoded) channel
DL Slot Allocation Process
Pilot design
15
Rationale for precoded pilots
Summary of differences precoded/non precoded –
which is the best case for each type of pilots?
Wideband, not precoded
“open loop”
N streams = N antennas
Flat power
Narrowband, precoded
Precoding/ beamforming
N streams < N antennas
FFR /boosting /
power control
Peak SE
RX
Interference
cancellation
Since we assume use of precoding and FFR in our system,
precoded narrowband pilots are preferred in most factors
Details rationale for precoded pilots
16
Resource block design
Review of the text proposals
Usage scenarios and comparison with baseline system
17
Downlink Resource Block Design
• Flexible Structure
– Support different modes simultaneously
– Different frequency reuse groups (reuse factors, powers,
interference mitigation, etc)
– Localized + distributed allocations
– Different modulation modes (MIMO,BF, etc)
• Hierarchical symbol structure with
– Semi static frequency reuse groups coordinated between
cells
– dynamic resource groups and allocations
– M x N as basic cluster
18
DL Slot/ Resource Block “RB”
Definitions
•
Cluster:
–
–
•
miniBand
–
–
–
•
Logical block of data subcarriers.
Physically contiguous or distributed
Default: 54 (TBD) subcarriers including pilots
Localized RBs
–
–
–
•
Nclusters physically adjacent clusters (default Nclusters =2, TBD)
Minimal unit for FFR group partition, I.e. FFR group is allocated several miniBands
Can be used for more efficient pilot design
Slot / “Resource block (RB)”
–
–
–
•
Physically contiguous group of M by N subcarriers
Default: 9 subcarriers by 6 OFDM symbols (TBD)
A downlink localized RB can be used for frequency-selective scheduling and multi-user diversity.
Localized RBs comprising a subchannel are adjacent/contiguous in frequency
Organized in localized-groups
Distributed RBs
–
–
–
A downlink distributed RB can be used for frequency-non-selective scheduling
distributed RBs comprising a subchannel are spreading across frequency within the frequency reuse
group
Organized in distributed-groups. Several groups in the same FFR group can be used (e.g. for long term
BF)
19
DL Slot Allocation Process
(2)
Distribute miniBands to
localized and diversity
groups
Localized
Resource groups
Inter-cell (semi static)
(Optional) Pseudo- random
distribution
FFR grp 2
(Optional)
Pseudorandom
FFR
grp 2
(Outer)
Permutation
of
miniBands
to FFR
groups
Logical minibands
Physical frequency
FFR grp 1
(1)
Distribute miniBands to
FFR groups
Diversity group
1
(3)
Distribute subcarriers
to subchannels (logical
resources)
00
01
02
03
04
Subcarrier
05
(Inner)
06
07
permutation
08
09
...
Localized
Diversity group
1
Subcarrier
permutation
Diversity group
2
Subcarrier
permutation
Localized
Single resource
Intra-cell (potentially dynamic)
20
Slot allocation process illustrated
E.g. 24 sc.
MiniBand
1 slot
Cluster=subchannel
MiniBand
Localized
1 slot out of 6 in the group
Distributed#1
Subframe
•Divide spectrum to miniBands
•Distribute miniBands to FFR groups
•Collect minibands of each FFR group
•Distribute to localized and distributed groups
•For localized, distribute subchannels, add
pilots
•For distributed, add pilots, distribute
subchannels
6 symbols
21
DL Slot Allocation Process (2)
1.
2.
3.
Form M by N Clusters
Form miniBands by grouping clusters
Mapping miniBands into frequency reuse groups
–
–
–
4.
Mapping is to maximum spreading (Outer Permutation)
Based on number and size of FFR group defined
FFR groups are orthogonal and synchronized across cells
Split each frequency reuse group to localized and distributed resources
–
–
–
Based on number and size of localized & distributed group within each frequency
reuse group
Assign Localized resource based on CQI information
The distributed resources group is analogous to major group in DL-PUSC with
flexible size
Form localized Slot “RB” and distributed Slot “RB”
5.
–
–
Each M by N contiguous localized subcarriers is a localized slot
Interleave (inner Permute) the distributed subcarriers within the group to form
distributed slot
22
Resource Block Comparison –(1)
Cluster &
miniBand Sizes
12x6 cluster /
12x6 miniBand
12x6 cluster /
24x6 miniBand
9x6 cluster /
9x6 miniBand
9x6 cluster /
18x6 miniBand
18x6 cluster /
18x6 miniBand
DL Pilot
Efficiency*
Less efficient,
12x6 pilots
patterns
Efficient, 24x6 and
12x6 pilots patterns
Less efficient, 9x6
pilots patterns
Efficient, 18x6 and
9x6 pilots patterns
Efficient, 18x6
pilots patterns
*24x6 ~3-4% more
efficient than 18x6
UL Pilot
Efficiency
Efficient, 4xM,
6xM tiles
Efficient, 4xM, 6xM
tiles
Less efficient, 3x3 or
3x6 tile
Less efficient, 3x3
or 3x6 tile
Efficient, 3xM,
6xM, 9xM tiles
UL RB design
Can use 16e UL
4x3 tiles
Can use 16e UL 4x3
tiles
Can not reuse 16e
4x3 tiles
Can not reuse 16e
4x3 tiles
Can not reuse 16e
4x3 tiles
DL RB design
Not directly
aligned with
802.16e AMC
subchannel
Not directly aligned
with 802.16e AMC
subchannel
Aligned with 802.16e
AMC subchannel
Aligned with
802.16e AMC
subchannel
Aligned with
802.16e AMC
subchannel
•Efficiency is defined in slide 26
•Alignment with AMC is beneficial for CQI reuse and FDM with 802.16 which is considered for UL.
•We may consider slot size < cluster size. In this case localized users will be limited to K slot
granularity (K=cluster/slot), and distributed users can use a fine granularity (small slot)
23
Resource Block Comparison - (2)
Cluster &
miniBand
Sizes
12x6 cluster /
12x6 miniBand
12x6 cluster /
24x6 miniBand
9x6 cluster /
9x6 miniBand
9x6 cluster /
18x6 miniBand
18x6 cluster /
18x6 miniBand
VoIP / small
packet
Good
Good
Better
Better
Worse
Diversity
Gain (< 1 dB
difference)
Smaller RB has
better DG for
small diversity
group
Slightly less DG
due to larger RB
and miniBand
Smaller RB has
better DG for small
diversity group
Slightly less DG
due to larger RB
and miniBand
Slightly less DG
due to larger RB
and miniBand
Scheduling
Gain (< 1 dB
difference)
Smaller RB has
better SG for
small local
group
Slightly less SG
due to larger RB
and miniBand
Smaller RB has
better SG for small
local group
Slightly less SG
due to larger RB
and miniBand
Slightly less SG
due to larger RB
and miniBand
Complexity
Low
High
Low
High
Low
CQI overhead
High
Lowest
High
Medium
Medium
Overall
Not Favorable
Fair
Not Favorable
Favorable
Fair
Based on this our current recommendation is 9x6 cluster (=subchannel) with 18x6 miniBand
24
Pilot design
25
Pilot targets
Purposes of pilots
1. Channel estimation
2. Measurements: Signal, Noise + interference
• CQI
• interference mitigation/cancellation
3. Frequency offset estimation
4. Time offset estimation (UL)
• Purposes are ordered in decreasing order of estimation difficulty
(estimation error) and effect of estimation error on system performance
• Channel estimation has the most severe impact on system performance
• Our current analysis focuses mainly on optimizing this effect
Non purposes of pilots
• cell identification
• broadcast/control information transmission
26
Pilot Optimization
• Optimization target is maximum spectral efficiency
considering pilot overhead and channel estimation
loss [1,2,3]
– SE abstraction
 Np
 1 Nt
SE  1 
NTX   idealSE SNRdecoder t 
N
t

 N t t 1
Np is the number of pilot, Nt is the number of tones in one cluster
Ideal SE is based on link level simulation with perfect channel knowledge
– Equivalent SNR at the decoder output with MMSE
estimator is

SNRdecoder 
h
1
2
 eff 2

2
h
2
2
e
Eh
 N TX
2

 e2
Eh
1  SNRH ,MMSE
1
SNR1  N TX SNRH ,MMSE
1
2
– Maximize SE by optimizing overhead, pilot location and
boosting
27
Main concepts of pilot design
•
Pilots are dedicated to a localized subchannel or to a distributed group
–
Pilot streams:
•
•
–
–
Pilots are precoded/boosted the same way as data is precoded/boosted
Definitions:
•
•
•
–
•
•
•
“common” = pilots are intended for use by all users in the cell
“dedicated” = pilots are intended for use by a one or more users/bursts
Pilots are boosted
–
•
In precoded case, equals to the number of data streams
Un-precoded case, may equal the number of Tx antennas (or less if CDD/similar used)
Desirable to have equal number of pilot per symbol to reduce power fluctuation (which reduces TX
power and range)
Scattered pilots (in time) are better than concentrated since they allow higher boosting values
For MIMO channel estimation, pilot separation between streams by different locations
combined with nulls (perfect separation)
Pilot separation between BS by randomization (selection from fixed range of options)
For MU-MIMO where interference is small, pilot streams are CDM to reduce overhead
Pilot patterns we propose are a result of computer based optimization with manual fine-tuning
28
DL Pilot Patterns
•
•
Total of 3 pilot patterns to support of 1,2,and 4 pilot streams
Each pattern has several orthogonal versions. In next figures these versions are
shown overlaid
– Pattern A:
•
•
•
•
Designed for 18x6 (mini-band), for support of 1,2 streams
Used for distributed group and localized burst with two clusters based RB
5.5% overhead per pilot stream (6 pilots)
6 versions
– Pattern B:
• Design for 18x6 (mini-band) for support of 4 streams
• 15% overhead
• 1 version
– Pattern C:
•
•
•
•
•
Design for 9x6 (cluster), 1/2 streams
used for localized users with one cluster based RB
7.5% overhead per pilot stream (4 pilots)
6 versions
To support of shortened sub-frame with 5 OFDM symbols, the last OFDM symbol
together with pilots will be punctured
29
Patterns A and C
Patterns A (18 x 6), OH 5.6 % per stream
Patterns C (9 x 6), OH 7.4 % per stream
0
0
-2
-1
-4
-2
-3
subcarrier
subcarrier
-6
-8
-10
-4
-5
-6
-12
-7
-14
-8
-16
-9
-18
-10
0
1
2
3
4
5
6
7
0
1
2
3
symbol
(a) Pattern A: 6 pattern overlay
5
6
7
(b) Pattern C: 6 pattern overlay
Pattern A 1+2 (18 x 6), OH 5.6 % per stream
Pattern C 1+2 (9 x 6), OH 7.4 % per stream
0
0
-2
-1
-4
-2
-6
-3
-8
-4
subcarrier
subcarrier
4
symbol
-10
-12
-5
-6
-7
-14
-8
-16
-9
-18
0
1
2
3
4
5
symbol
(c) Pattern A 1+2 for 2 pilot stream
6
7
-10
0
1
2
3
4
symbol
5
(d) Pattern C 1+2 for 2 pilot stream
6
7
30
Combining Patterns
• The single antenna patterns can be used for
single stream or combined to create 2 antenna
patterns (orthogonal)
• Multiple patterns is to allow randomization, or
planning
– Total of 6 patterns: 2 per sector (MIMO 2x2) X 3
reuse
31
Randomization to enjoy
boosting gain
• The gain from pilot boosting in white noise (when total
TX power is constant) is substantial
• In current spec pilots collide with pilots and therefore
boosting gain is lost in interference limited case
• To gain from boosting in interference scenario pilots
should collide with data subcarriers
• This is always better than having pilots collide with pilots,
since colored interference is better than white noise
Pilots boosted
Data deboosted
Same avg power
Same done for
the interferer
32
Pilots
Data
Noise
Pattern B – 4 Pilot stream
Pattern B (18 x 6), OH 3.7 % per stream
0
-2
-4
-6
subcarrier
• Support 4 TX streams
transmission
• Used also for full
channel sounding for
close loop beamforming
with 4 TX antennas
• Not considered for x3
reuse since boosting
leads to small/zero
power fluctuations
-8
-10
-12
-14
-16
-18
0
1
2
4
3
5
6
7
symbol
33
Boosting values
– Optimal boosting value is
based on white noise
optimization, but is also
good for interference
– The boosting value is the
difference between pilot and
data power
– The average power should
be normalized to 1, and
therefore different power for
data tones may result in each
case – this is not an issue
since the pilots are dedicated
Pattern N
Boost
streams value
A
1
5.6dB
2
7.4dB
B
4
8.2dB
C
1
5dB
2
5dB
34
Analysis for 2x2 MIMO
Effective SE
Effective SE / best SE
12
10
1
Pattern C: 9x6
LTE 2 stream
Pattern A: 18x6
Wideband 18x6
Ideal SE
0.95
0.9
0.85
ratio
SE [B/s/hz]
8
6
4
0.8
0.75
0.7
0.65
Pattern C: 9x6
LTE 2 stream
Pattern A: 18x6
Wideband 18x6
2
0.6
0
-10
0
10
20
SNR[dB]
30
40
0.55
-10
0
10
20
30
40
SNR[dB]
•These figures present spectral efficiency estimated using the aforementioned model, for
V=120Km/h, without precoding.
•The left figure presents SE and the right shows SE normalized by the SE of the best scheme.
•The results are compared against the LTE design for 2 streams with boost=3dB [6]. LTE use
wideband pilots which are optimal for non-FFR non-precoded case.
•The narrow band pilot design we propose performs well even in this scenario. We expect much
35
better performance in FFR and beamforming cases.
Usage scenarios and comparison
with baseline system
36
Support of precoded MIMO options
Mode
Short term precoding (channel
knowledge)
Long term precoding (correlation/DoA
knowledge)
no precoding (OL-MIMO)
•N/A
•distributed group with e.g. 2 pilot streams, pilots
are not precoded.
•For 4 TX, modulation can be STBC/SM (per
user) with CDD (common), or 4 streams for 4x2
configuration (less preferred due to pilot
efficiency)
Fixed precoding:
(“OL-MU-MIMO” or “OL-SUMIMO” with precoder)
•Localized resource with fixed
precoding, MS measures CINR as
basis to select the resource
•Precoder doesn’t have to be
conveyed to MS (and can change,
infrequently)
•Size of group changes slowly
(since requires new
measurements)
•distributed group with fixed precoding and fixed
number of streams.
•Number of pilots streams = number of data
streams.
•Number of different precoders is limited due to
size of distributed groups
•MS can measure CINR on groups to determine
preferred group
•Distributed groups inside FFR group can be
randomized (cell-ID dependent permutation)
Adaptive precoding (“CL-MUMIMO”, “CL-SU-MIMO”)
•Localized resource, adaptive
precoding, MS reports according to
sounding symbols
•distributed group allocated to a single user if
traffic justifies
37
Parameters and comparison with baseline system
Item
WiMAX 1.0
16m proposal
Reasoning
OFDMA parameters
Nused=varying
CP length = 1/8
Nused as in AMC
CP length = 1/8
Subcarrier spacing
remains the same
Symbol alignment with baseline system,
maximum used BW.
Diversity/Localized
support
Open loop /
Precoded MIMO
support
Different zones,
different permutations
(PUSC, AMC)
Unified in same symbol
structure
Reduce frame fragmentation and unify
and simplify solution.
FFR support
Partial & rigid (by PUSC
major groups) in time
domain (zones)
More flexible, multiple
reuse factors in same
symbol
Optimize FFR and make it more usable
Slot size
48 logical subcarriers
54 physical subcarriers,
varying logical
subcarriers
Remove constraint on fixed pilot
overhead. Rate matching used to bridge
the gap.
Size of WiMAX 1.0 slot w.r.t MAC
packets is reasonable choice
Slot duration in time
1-3 symbols
6 symbols (subframe)
Fixed duration in time & frequencydimension scheduling simplify design
Pilot density (DL)
14% PUSC, 11% AMC.
Pilots decimated
between 1TX and 2TX
5.5% 1TX, 11% 2TX,
15% 4TX. More pilots
added with NTX
Optimized overhead per number of
antennas. In WIMAX1 too much
overhead for 1TX and 4TX.
38
Parameters and comparison with baseline system
Item
WiMAX 1.0
16m proposal
Reasoning
Pilot modulation in
precoding
Precoded & dedicated pilots
(mainline)
Precoded pilots
Precoded pilots more efficient
when data is precoded/boosted
Pilot locations per BS
Pilots hit with pilots
Pilots are “randomized”
(choose from set of
locations)
Pilots hitting pilots eliminates
boosting gain in interference
limited case; #1 malady of
WiMAX1 pilot design
Pilot boosting
2.5dB,5.5dB for 1,2 streams
(DL)
5.6dB, 7.4dB, 8.2dB for
1,2,4 streams
Optimized boosting (higher)
Distributed
permutation design
FUSC: wideband pilots,
scattered subchannels
PUSC: narrowband pilots,
major groups, clusters,
subchannels spread over
groups; two layer
permutation.
Use PUSC concept but
extend it to unified feature
support
PUSC concept is good for multimode and FFR support and for
distributed transmission with
precoded pilots
Permutation concept
Fixed and rigid structure,
optimize hit ratio between
subchannels
Flexible structure, less
optimization of hit ratio
possible
Hit ratio less important when
interference is managed (FFR);
tougher to maintain in flexible
and dynamic structure.
Permutation unit
PUSC cluster = 14 sc
AMC subchannel = 18 sc
Cluster = 9 sc
miniBand = 18 sc
See Resource Block
Comparison –(1)
39
Control channels
•
Methodological suggestion:
– First, agree on design optimized for traffic; then attempt to design control channels as a
derivative
•
General considerations for control channels:
– In FFR, DL broadcast maps will use a single FFR group (choose the FFR group that
maximizes normalized SE required to reach a target outage)
– Maps can be per-subframe or per-frame
– UL control channels will use UL diversity resource or pieces of it
•
The straightforward mapping of DL control channels in our proposal is:
– Maps are FDM with data
– Maps use the same formats and structures (pilots, permutations, ..) as data;
– Broadcast control channels are allocated a diversity group within one of the FFR groups
(e.g. can be boosted);
– Formally the pilots of this group are common (all users use them), but they have the
same structure as other pilots.
– Dedicated/sub - maps can have different diversity/FFR group and be beamformed
•
Different/optimized structure for control channels can be considered if the
straightforward approach is proven insufficient
40
Review of text proposal
41
Review of the text proposals
• The text proposals are split as follows:
– Generic/high level part:
• C80216m-08/120: Proposal for IEEE 802.16m
Downlink Symbol Structure Concept
– Detailed part:
• C80216m-08/121: Symbol structure design for 802.16m
- resource blocks and pilots
42
Specific Text Recommendations for
SDD* Section 11 – PHY Layer – high level
11.x: DL Symbol Structure
• The downlink symbol structure will support different modes and resource
allocation schemes simultaneously in the same subframe/zone, for example
(1) Localized and distributed resource allocations,
(2) Different frequency reuse groups (different reuse factors, powers and other properties
related to interference mitigation and management)
(3) Different MIMO modulation modes: Open loop / closed loop, single-user and multiuser.
•
•
The symbol structure is optimized for pre-coded MIMO and fractional frequency
reuse (FFR) operation.
The same OFDMA parameters (i.e., sampling factor and CP length) as the
reference system are used where the number of used sub-carriers is equal to the
Nused of AMC permutation. The resource allocation is performed in frequency
domain only over the 6-symbol subframe, i.e. a slot equals one subchannel by 6
symbols.
* System description document [9]
43
Specific Text Recommendations for
SDD Section 11 – PHY Layer – high level
11.x.1 Subchannelization Concept
• FFR and multiple transmission formats (such as different number of streams, resource
allocation schemes, etc) will be supported by a hierarchical structure as depicted in Figure
11.x-1, where the symbol is first split into FFR groups, then these groups are split into
localized and distributed resource groups, and then these groups are split into individual
subchannels. The number of resources at each branch will be flexible and controlled by
the cell (the level of flexibility and required control channels are TBD); e.g., simply by
naming the number of resources allocated to each branch.
• At the top of this hierarchy, changes are relatively infrequent (involve multi-cell
coordination), and become more frequent (per cell and per user) toward the lower levels.
The motivation for the hierarchical structure is that this structure yields most flexibility
for the cell/user while reducing the amount of coordination/broadcast information required.
Using this structure, the MS is allowed to identify resource groups required for various
measurements.
• 11.x.2 Pilot structure concept
• The pilots are used for channel estimation, measurements (such as Signal, Noise +
interference for CQI and interference mitigation/cancellation), frequency offset estimation
and time offset estimation (UL).
• Pilots will be dedicated to localized subchannels and to each group of diversity resources.
The pilots will be pre-coded, using the same precoding and/or boosting of the information
burst(s) when such is applied (and in general any linear operation on the data subcarriers).
Sounding symbols in DL and UL may be used to estimate physical (non-pre-coded)
channel for the purpose of closed-loop schemes. .
44
Specific Text Recommendations for
SDD Section 11 – PHY Layer (1)
11.x: DL Symbol Structure
• A cluster is physically contiguous group of M subcarriers by N symbols,
with default value of 9 subcarriers by 6 OFDM symbols, i.e. M=9, N=6. A
miniBand consists of Nb number (TBD, default Nb =2) of these adjacent
basic clusters. The miniBand can be used as basic units for CQI reporting,
frequency reuse groups partitions and efficient pilot design. A slot
“Resource block (RB)” is logical block of M subcarriers by N symbols unit,
with default value of 9 subcarriers by 6 OFDM symbols, i.e. M=9, N=6.
The total 54 (M*N) subcarriers in RB include both pilots and data tones.
The number of data subcarriers in slot varies with different pilot densities.
For slots in distributed allocation the 54 subcarriers are randomly
distributed across all available distributed subcarriers with the reuse group.
45
Specific Text Recommendations for
SDD Section 11 – PHY Layer (2)
11.x: DL Subcarrier to Resource Block Mapping
• Add “DL RB Allocation Process” figure here.
• The DL RB allocation process is defined as follows:
(1) A renumbering sequence (TBD), i.e. “Outer Permutation”, is
applied on minibands to maximum spreading and map minibands
into a given size of frequency reuse group
(2) The frequency reuse group is partition into localized and
distributed groups. The size of each group is flexible and the
mapping is uniquely determined by the size of each group.
(3) The localized and distributed groups are further mapped into
localized (by direct mapping) and distributed RBs (by “inner
permutation”).
(4) BS scheduler can semi-dynamically change the Ratio between
localized and distributed RBs for scheduling and diversity gain
based on CQI feedbacks
46
Specific Text Recommendations for
SDD Section 11 – PHY Layer (3)
11.x: DL Localized resource block format
• Localized subchannels contain subcarriers which are
contiguous in frequency. Pilots used for localized subchannels
are dedicated and should be precoded/boosted the same way as
data.
11.x: DL Distributed resource block format
• Distributed subchannels contain subcarriers which are spread
across frequency within the frequency reuse group. The
subcarriers within the distributed groups are permuted (“inner
permutation”) across distributed group to maximize frequency
diversity. Dedicated pilots are used in distributed groups and
are shared within the group. The distributed group may be
precoded in which case the pilots are precoded together with
data.
47
Specific Text Recommendations for
SDD Section 11 – PHY Layer (4)
11.x: Pilot patterns
• Total of 3 pilot patterns are optimized for efficiency and performance. Pattern A and
C should be used for 1 and 2 pilot streams. Pattern B should be used for 4 pilot
streams. Boosting values for different streams with different patterns should be used
as in Table I.
• Pattern A should be used as in distributed resource blocks, and in localized resource
block when the entire miniBand (2 clusters) is used by the same burst. Pattern C
should be used when the localized burst is carried on 1 cluster.
• Add “Pilot Patterns” figure here (as shown below).
11.x: Combining pilot patterns
• Pattern A and C each has 6 single patterns. The single antenna pattern
should be used for single stream or combined to create 2 antenna patterns
(orthogonal). Different base station should coordinate to choose different
combinations of the pilot patterns for randomization.
48
References
[1] Cosovic, I.; Auer, G., "Capacity Achieving Pilot Design for MIMO-OFDM over Time-Varying
Frequency-Selective Channels," Communications, 2007. ICC '07. IEEE International
Conference on , vol., no., pp.779-784, 24-28 June 2007
[2] Hassibi, B.; Hochwald, B.M., "How much training is needed in multiple-antenna wireless
links?," Information Theory, IEEE Transactions on , vol.49, no.4, pp. 951-963, April 2003
[3] Lang Tong; Sadler, B.M.; Min Dong, "Pilot-assisted wireless transmissions: general model,
design criteria, and signal processing," Signal Processing Magazine, IEEE , vol.21, no.6, pp.
12-25, Nov. 2004
[4] IEEE 802.16m-07/002r4, “802.16m System Requirements”
[5] IEEE 802.16m-008/004, “802.16m Evaluation Methodology”
[6] 3GPP TS 36.211 V8.1.0 (2007-11) Technical Specification, 3rd Generation Partnership
Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial
Radio Access (E-UTRA); Physical channels and modulation (Release 8)
[7] IEEE C80216m-08/082, “Proposal for IEEE 802.16m Frame Structure”
[8] IEEE C80216m-08/118r1, “Initial Document output of the IEEE 802.16 TGm Frame Structure
Rapporteur Group”
[9] IEEE 80216m-08/003, “The Draft IEEE 802.16m System Description Document”
49
Backup Slides
50
Peak SE
• Calculation of peak SE
–
–
–
–
–
–
–
Symbol duration incl CP (1/8) = 102.85
Useful subcarriers = 864 @10Mhz (AMC)
MCS: 64QAM 7/8 (5.25 b/s/hz/stream)
Pilot overhead: 11%, 16%
Preamble overhead: not considered yet
Peak thput = (Bits/Subcarrier)#Subcarriers (1-OH) / SymbolTime
Conclusion: with same CP, we barely meet the requirement from below, but the difference doesn’t
seem to justify new CP
Requirement Type
Baseline
Baseline
Target
Target
Link direction
Downlink
Uplink
Downlink
Uplink
2x2
1x2
4x4
2x4
Required Normalized peak rate (bps/Hz)
8
2.8
15
5.6
Assumed theoretical spectral efficiency
10.5
21
Peak throughput @10Mhz
78.5
148.2
SE over entire BW (e.g. 10Mhz)
7.85
14.81
SE over occupied BW (e.g. 9.46Mhz)
8.29
15.66
MIMO Configuration
51
Localized/distributed resources
• Distributed resource (“PUSC”) – spread across
spectrum, yields frequency diversity, mainly used for
high mobility / low feedback
• Localized resource (“AMC”) – localized in
spectrum, enables frequency-selective scheduling,
associated with low mobility / high feedback rate
• Support of both diversity and localized resource is
required in order to guarantee high throughput for
both low and high mobility users
52
Open-loop MIMO transmission
•
•
•
•
•
Open-loop MIMO transmission intended for user with high mobility or low rate of
CSI
Will typically be used in conjunction with distributed resource
We name a few options as reference
For 2TX: STBC (Matrix A), SM (Matrix B)
For 4TX:
– Techniques using 4 channel streams:
•
•
•
•
SM, rate 4
Stacked Alamouti (STBC+SMx2), rate 2
Large delay CDD + STBC, rate 1
May consider STBC design for 4 streams with rate 1
– Techniques using 2 channel streams
•
•
•
•
•
small-delay CDD + STBC, rate 1
small-delay CDD + SM, rate 2
Antenna-Frequency interleaving can be used instead/in addition to CDD
CDD information can be embedded into pilots; the advantage is in lower pilot overhead and
channel estimation loss
For >4TX: combine CDD with 4TX/2TX scheme
53
Precoded MIMO techniques
• Transmitted signal is modulated by a spatial linear precoder
x=Vs
V [NTX  Nstreams] is the precoding matrix
• Precoder may be used to direct energy toward MS / reduce
interference / etc.
• Single user / Multi-user MIMO (SU/MU)
– Single user – all streams directed to same user. Achieves high peak SE.
– Multi user – different streams directed to different users (SDMA).
Achieves high avg SE.
54
Precoded MIMO techniques
• Short term / Long term precoding
– Short term – depends on approximate instantaneous channel knowledge.
Requires frequent feedback, yields best performance.
– Long term – depends on long term channel statistics (e.g. correlation /
geometric direction), requires less feedback, can operate in higher mobility.
• Fixed/Adaptive precoding
– Fixed precoding – BS determines precoder per subchannel, MS/BS selects
subchannel (and/or stream) to best fit conditions. Guarantees predictable
interference, low overhead, multi user diversity. Good solution when number of
users is high.
– Adaptive precoding – precoder is a function of user conditions (by sounding /
report / codebook selection). Good when number of users is small or some
users require higher throughput.
• All combinations are useful in some scenarios
{SU,MU}  {Short,Long}  {Fixed, Adpative}
• We assume most of these combinations will be used in 802.16m
55
FFR / soft reuse / reuse partitioning
• FFR = fractional frequency reuse
– a way to increase system performance by using different reuse factors
for different subchannel/user groups
– An example (supported in 802.16) is combination of reuse 1 and reuse
3
• Use model 1: move cell edge users suffering high interference to reuse 3 to
improve cell edge throughput
• Use model 2: although on average over cell locations reuse 1 provides
better SE than reuse 3, for a specific location/geometry, reuse 3 may be
better. Choosing for each user the best reuse factor improves average and
user throughput
– A softer version applies lower/higher power to some subchannel
groups:
Power
PHigh
PLow
Power
PLow
Power
PHigh
PHigh
PLow
PHigh
w1
Preuse1
Sector
1
Preuse1
Sector
2
Preuse1
Sector
3
PLow
PHigh
PLow
PLow
w2
w3
Reuse 3
PHigh
PLow
PHigh
PLow
PHigh
PHigh
PLow
w12
w23
Reuse 3/2
w
Frequency
w13
w123
Reuse 1
56
FFR / soft reuse / reuse partitioning
• How many FFR groups?
– For 2 power levels and 3 segments (“colors”) we have 7 meaningful
combinations (23-1 since the combination [Plow, Plow,Plow] is not useful)
– Too many groups cause resource fragmentation
– We assume 1-7, typical 4 groups
• It is possible to use other attributes in addition to power.
– For example, group using a single-stream or STBC transmission
improves ability for interference cancellation and may be more suitable
for cell edge
• Dynamics of FFR: FFR can be static / semi-static or dynamic
in two senses:
– The allocation of users to groups – internal in the cell
– The FFR groups (subchannels, attributes) - requires multi cell
coordination; we assume this process is slower than per-cell scheduling
57
FFR / soft reuse / reuse partitioning
• Symbol structure should support FFR in a generic way with
minimum limitation to the scope of the solution
• For the purpose of symbol structure we generalize FFR as
follows:
– We divide the resource (spectrum x time) into orthogonal FFR groups
– Each FFR group may have different attributes such as power, restriction
on type of modulation / number of streams, etc
– We assume the structure changes on a slower basis than per-cell
scheduling and can be shared on slow basis (DCD/UCD)
– FFR groups should be spread in frequency to maximize frequency
diversity and scheduling gains
– As working assumption (for simulation/analysis) FFR groups are equal
(same subcarriers) across cells; in practice they may not be (due to
traffic / deployment asymmetry)
58
MBS and Relay
• Relay support may require
– Concurrent DL transmission from several relays
– Relay DL transmission with BS
– This can be supported in a similar way to FFR, by
groups of clusters with associated pilots
• MBS
– Requires pilot design for large delay spread
59
Small packet efficiency
•
The key constraints for RB size:
– Packing efficiency for small packets generated by certain applications (VOIP packets,
TCP ACKs etc)
– Pilot overhead for the resource block choice as well as the overall capacity of the
system for the given choice of the RB.
•
•
•
•
•
Packing efficiency is defined as the number of data tones wasted in a resource
block when carrying a particular MPDU at a particular MCS scheme, weighted
over the probability distribution of the MCS schemes in the system.
For small packet applications, smaller resource blocks provide better packing
efficiency than the larger resource blocks, especially at higher order MCS
schemes. However it is to be noted that mechanisms such as rate matching can be
used to further improve upon the packing efficiency and the system capacity.
Pilot overhead generally tends to increase with decreasing resource block sizes
(only in localized mode)
Detailed analysis is required for the key metrics of interest (ie packing efficiency,
pilot overhead and system capacity) before the right RB choice is made for
802.16m.
For such an analysis, we recommend a starting RB size of 9x6 which is backward
compatible to 802.16e system
60
Rationale for precoded pilots
• We assume main mode of operation will be
– Precoded (low mobility optimized) with varying rank
– FFR will be used
• Consider two options:
– Precoded & narrowband pilots:
• The channel estimation is narrow-band (as in PUSC, AMC).
• The MS doesn’t have to know the precoder.
• The number of pilot streams is the number of transmitted streams.
– Non-precoded wideband pilots
• Channel estimation can be wide-band (as in FUSC).
• BS has to provide MS with precoder matrix.
• MS estimates physical channel H and uses precoder matrix to calculate
effective channel HW.
• The number of pilot streams is the number of TX antennas.
61
The disadvantages of precoded pilots
• Start with the down side:
– Narrowband pilots/channel estimation is less efficient
locally
• However the difference can be made small (3-5% loss in SE), and
is more than compensated by the other advantages
– Non-precoded pilots allow estimation of the physical
channel for feedback purposes
• With precoded pilots we need to add DL sounding symbols
(“midamble”)
• However, channel estimation quality required for feedback is not
high – therefore small overhead will be required to support the
requirement.
62
The advantages of precoded pilots
• Precoded pilots have the following advantages for a precoded
system:
– Non-precoded pilots limit number of TX antennas, while precoded
pilots determine only number of streams
– Non-precoded pilots require precoding codebook to be predefined +
signaling of the chosen codeword, whereas precoded pilots allow more
flexibility in determining the solution and room for future optimization
– Precoded pilots enjoy the beamforming gain (the pilot energy is
focused toward the MS direction), thus more than compensating for
smaller effective number of pilots (compared to wideband pilots)
– Smaller overhead when number of streams is smaller than num of
antennas.
• This occurs in most scenarios, if typical NTX is
2..4 while typical number of streams is 1-2
• Assuming 5.5% per stream the difference is
significant 5-10%
• A very typical scenario is rank 1, in which we
can enjoy 5.5% overhead (compared to
minimum ~11% with common pilots)
Non
precoded
pilot
63
The advantages of precoded pilots
• Pilot adaptivity: by using local/dedicated pilots we can design
pilots that fit the scenario
• Two examples
– For multi-user MIMO
• Pilots can be optimized for low-cross talk.
• For example, CDM/randomization instead of orthogonal streams can be
used to separate pilots of the users, thus reducing overhead
• This is in addition to the advantages mentioned
– For open-loop MIMO
• At low SNR, considering unknown channel (or channel estimation loss) it
is preferred to transmit on less degrees of freedom (=less antennas or
streams) [Hassibi*]. Dedicated/adaptive pilots enables this matching.
• At high SNR open loop MIMO, narrowband pilots are indeed less
efficient.
* Hassibi, B.; Hochwald, B.M., "How much training is needed in multiple-antenna wireless links?,"
Information Theory, IEEE Transactions on , vol.49, no.4, pp. 951-963, April 2003
64
The advantages of precoded pilots
• For FFR, we compare a case
where pilots are dedicated to
the group (top) to a case the
pilots are common to all
groups (bottom)
Pilots
FFR Group 1
FFR Group 2
Pilots
FFR Group 1
FFR Group 2
• Narrowband pilots belonging to FFR groups have the
following advantages in FFR
– Pilots which boost/deboost with data are more efficient than pilots
which have fixed boosting
– Pilot quality matches data quality; pilots have the same reuse as data
– Allow exact estimation of CINR of each FFR group (compared to
indirect deduction of CINR with pilots common to all groups), thus
enabling robust link adaptation and dynamic choice of FFR group.
65
The advantages of precoded pilots
• RX interference mitigation
– RX interference mitigation such as spatial LMMSE (IRC) yields link
level gains of 1-3dB and system SE gains of 10-15% (2RX)
– Evaluation methodology document requires that usage of LMMSE will
be justified
– Precoded pilots enable instantaneous interference estimation for RX
interference cancellation. Such estimation is not accurate with nonprecoded pilots if interference is precoded / boosted
66
DL RB Allocation Example
Assumptions:
–
–
FFT_Size=1024, Nused=864, M =12, N =6, Nclusters=2
FFR ratio =1/3, Localized Ratio=1/2
Steps:
1.
Form 12 x 6 Clusters  72 clusters (864/12=72)
2.
Form 36 miniBands (assume Nclusters=2) 72/2=36 miniBands
3.
Mapping miniBands into FFR groups (36 x 1/3=12 miniBands)
–
–
4.
Perform Outer permutation to maximum spreading
Assign 12 miniBands to the FFR groups
Assign localized and distributed resources (12 * ½ = 6 miniBands)
–
5.
Assign 6 miniBand to Localized resource and 6 to distributed resource
Form localized RB and distributed RB
–
–
Form 12 localized RBs from the 6 localized miniBands
Form 12 distributed RBs after “Inner Permute” the distributed subcarriers
within the distributed 6 miniBands
67