IEEE C802.16m-08/441
`
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
UL Pilot Structure for the IEEE 802.16m
Date
Submitted
2008-05-05
Source(s)
Voice: +82-31-450-7187
E-mail: bcihm@lge.com
Park_SH@lge.com
emptylie@lge.com
wbong@lge.com
Bin-Chul Ihm
Sungho Park
Jinsoo Choi
Wookbong Lee
LG Electronics Inc.
LG R&D Complex, 533 Hogye-1dong,
Dongan-gu, Anyang, 431-749, Korea
Re:
IEEE 802.16m-08/016r1: Call for Contributions on Project 802.16m System Description
Document (SDD)
Target Topic: “Uplink Pilot Structures”
Abstract
This contribution provides the uplink pilot structure for IEEE 802.16m
Purpose
For discussion and approval by TGM
Notice
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UL Pilot structure for IEEE 802.16m
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IEEE C802.16m-08/441
Bin-Chul Ihm, Sungho Park, Jinsoo Choi and Wookbong Lee
LG Electronics
Introduction
In this contribution, we consider the uplink pilot design criteria regarding resource mapping, pilot overhead, channel
estimation performance and some issues related UL MIMO operation. And then, we propose the uplink pilot structure
based on these criteria in the physical resource unit (PRU) and the extended pilot allocation method to avoid pilot
collision when pilot power is boosted in the multi-cell environments.
UL Pilot Design Considerations
1. Pilot Spacing in Time and Frequency Domain
We presented DL pilot design criteria in [1] at previous Orlando meeting. Recommended pilot spacing related to coherent
time and bandwidth are within 3 symbols regarding coherent time and 8~9 subcarriers considering frequency selectivity.
These values are allowed for supporting mobility up to 120km/h and various fading environments according to SRD [2]
and EMD [3] documents.
2. Resource Unit (RU) Structure
Proposed pilot structure should be optimized within a PRU size and applied to all PRU per cell/sector. In [4,] we
proposed DL physical resource unit (PRU) that comprises 18 consecutive subcarriers by 6 consecutive OFDMA symbols
and analyze the advantages of that structure and in [5], many companies reached consensus on that PRU size. For
commonality between DL and UL, we consider the UL PRU [6] which is same to DL as following modes.
A. Legacy Supporting Mode
There are two approaches to support legacy system, which are Frequency Division Multiplexing (FDM) and Time
Division Multiplexing (TDM). In the FDM way, we recommend that the frequency granularity of 16m resource allocation
is same to that of PUSC of the legacy system, because the numerology of resource allocation for 16m can be depending
on that of legacy system. In the TDM way, the legacy UL and new 16m UL subframes are divided by time-division way
and thus the numerology of resource allocation for 16m is not depend on that of legacy system within given subframes.
B. 16m only operating Mode
In this case, there is no problem whether the frequency granularity of 16m resource allocation is same to that of legacy
system or not.
3. Resource Allocation
The uplink pilot design should be optimized based on the both distributed and localized resource allocation reminding
coexisting two permutation modes within a subframe.
A. Distributed Resource Allocation
In this case, subcarriers are distributed across bandwidth to achieve the frequency diversity gain and it could be allocated
by mini-RU spreading due to simple realization for uplink resource allocation. In the distributed resource allocation with
mini-RU spreading, the size of minimum distributed unit becomes a fraction of PRU and these mini-RUs are spread over
time or frequency domains. The pilot design should consider the granularity of PRU/mini-RU and be matched to the
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method of time or frequency domain spreading.
The considerable mini-RU structures are, for example, 6 by 6 or 9 by 6 in the FDM way and 18 by 2 or 18 by 3 in the
TDM way. If same pilot density and pilot boosting are assumed, power fluctuation problem frequently occurs in the FDM
case. Additionally, if multiple antenna transmission is assumed, per antenna power balancing problem is like the
preceding with FDM way. Taking the above points into account, we prefer the TDM approach as a mini-RU spreading
method.
B. Localized Resource Allocation
In the case of localized resource allocation, the pilot design should consider the various pre-coding methods and be
optimized with the proposed UL PRU structure.
4. Pilot Density
Pilot density affects on the data rate as well as channel estimation performance. Generally, in uplink, the channel
estimation performance is limited due to dedicated pilot property. In this contribution, we consider about 11.11% pilot
overhead with single transmit antenna and up to 11.11% ~ 22.22% pilot overhead with two transmit antennas.
5. Pilot Power Boosting
Pilot power boosting is well used to improve channel estimation performance. When pilot power is boosted, data region
should be taken away own power or punctured. If power stealing is used in the data region, it causes power fluctuation
across symbols. However, we can minimize the power fluctuation by placing the same number of pilots per symbol.
6. UL MIMO Support
A. Antenna Power Balancing
When using multiple transmit antennas, it is important to keep balanced powers across the antennas in order to employ a
cheaper amplifier. Imbalanced power per antenna means that each antenna should support more peak power by using an
amplifier of higher cost. To maintain power balance per transmit antenna, each symbol should have same number of pilot
set for all transmit antennas. For example, if there are two antennas, per symbol should have the same number of pilot set
#1 and pilot set #2 for each antenna #1 and #2.
B. STBC/SFBC Support
STBC / SFBC are one of strong candidates of uplink MIMO techniques. To these operations, it is necessary to ensure
stationary channel varying between adjacent subcarriers/symbols in the frequency or time domain. Therefore, the pilot
could be located between N paired subcarriers/symbols in frequency or time domain (N : integer number).
Proposed UL Pilot Structure
 Pilot Patterns within a PRU
In this section, we propose the uplink pilot structure which is based on the UL PRU structure as shown in Figure 1. These
pilot structures are designed for single transmit antenna and two transmit antennas case. The pilot overhead is 11.11% in
both structures. These structures intend to the nested pilot allocation rule for the number of transmit antenna. To
minimize power fluctuation problem all symbols have same number of pilots. The pilot spacing in the frequency domain
is 8~9 subcarriers to support estimation quality against the frequency selectivity. Proposed pilot structure for two transmit
antenna easily supports to SFBC and per antenna power balancing.
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IEEE C802.16m-08/441
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Figure 1: Uplink pilot structure in a physical resource unit; (a) for 1Tx antenna system and (b) for 2Tx antenna system

Pilot Patterns within a mini-RU
Figure 2 shows an exemplary pilot pattern within a mini-RU which comprises 18 subcarriers by 3 OFDM symbols.
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Figure 2: Uplink pilot structure in a mini-RU (18x3); (a) for 1Tx antenna system and (b) for 2Tx antenna system

Extension of pilot allocation by cyclic shifting
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IEEE C802.16m-08/441
Power boosted pilots in the same position have high collision probability in the multi-cell environments so it causes intercell interference. To avoid this problem, we propose cyclic shifted pilot allocation method. In this extended pilot
allocation, single pilot structure is cyclic shifted in the time and/or frequency domain, so the base of pilot structure is
same but differently shifted pilot allocation is used per cell/sector. In this case, pilot collision between the users who have
different number of antenna should be also considered. When TDM way is used for mini-RU spreading distributed
subchannelization, Figure 3 depicts pilot patterns within a mini-RU which comprises 18 subcarriers by 3 OFDM symbols.
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Figure 3: Uplink pilot structure in a mini-RU with 2Tx antenna system ;
(a) shifted pattern #0, (b) shifted pattern #1, and (c) shifted pattern #2
Text Proposal for 802.16m SDD
============================== Start of Proposed Text =================================
Section 11.x: UL Pilot Structure
Section 11.x.1: UL pilot structure within a PRU
For single antenna transmission, there are 96 data subcarriers and 12 pilots within each PRU and for two
antenna transmission, there are 96 data subcarriers and 6 pilots per transmit antenna within each PRU. These
pilot patterns are proposed in Figure xxx.
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IEEE C802.16m-08/441
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Figure xxx: Uplink pilot structure in a physical resource unit; (a) for 1Tx antenna system and (b) for 2Tx antenna system
Section 11.x.2: UL pilot structure within a mini-RU
Figure yyy is proposed for a pilot pattern within a mini-RU which comprises 18 subcarriers by 3 OFDM symbols.
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Figure yyy: Uplink pilot structure in a mini-RU (18x3); (a) for 1Tx antenna system and (b) for 2Tx antenna system
Section 11.x.3: Extension of pilot allocation by cyclic shifting
In case of power boosted pilot allocation, cyclic shifted pilot allocation method can be applied to minimize inter-cell
interference. In this extended pilot allocation, single pilot structure is cyclic shifted in the time and/or frequency domain
so the base of pilot structure is same but differently shifted pilot allocation is used per cell/sector. And pilot collision
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IEEE C802.16m-08/441
between the users who have different number of antenna should be also considered.
=============================== End of Text Proposal ===============================
Reference
[1] IEEE 80216m-08_153r1, “Pilot related to DL MIMO”
[2] IEEE 80216m-07_002r4, “IEEE 802.16m System Requirements”
[3] IEEE 80216m-08_004r1, “Project 802.16m Evaluation Methodology Document (EMD)”
[4] IEEE C80216m-08_152r1, “Physical Resource Allocation Unit in IEEE 802.16m Downlink”
[5] IEEE C80216mDL_PHY-08_009r5, “Proposed 802.16m DL PHY Structure Baseline Content Suitable for Use in the
802.16m SDD”
[6] IEEE 80216m-08_439, “Physical Resource Allocation Unit in IEEE 802.16m Uplink”
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