IEEE C80216m-08/423
Project
IEEE 802.16 Broadband Wireless Access Working Group <http://ieee802.org/16>
Title
Multiple Base Station MIMO
Date
Submitted
2008-05-05
Source(s)
Keying WU, Yang SONG, Liyu CAI,
Hongwei YANG, Xiaolong ZHU
Alcatel Shanghai Bell Co., Ltd.
Voice: + 86-21-50554550;
50554554
Fax:+ 86-21-
Mailto:Hongwei.Yang@alcatel-sbell.com.cn
Krishna Balachandran, Howard Huang,
Srinivas Kadaba, Kemal Karakayali,
Pantelis Monogioudis, Ashok Rudrapatna,
Sivarama Venkatesan
Alcatel-Lucent
Re:
IEEE 802.16m-08/016: Call for Contributions on Project 802.16m System Description
Document (SDD)
Target topic: Downlink MIMO schemes
Abstract
This contribution discusses the multi-BS MIMO technique, which can greatly improve the sector
throughput of 16m systems, especially at cell edges. The information included in this
contribution includes the concept of multi-BS MIMO, the basic operations involved, its
advantages and the impact on systems. We recommend supporting multi-BS MIMO as a
supplement to single-BS MIMO techniques such as single-user MIMO and multiuser MIMO in
IEEE 802.16m.
Purpose
Adopt concept and proposed text into SDD.
Notice
Release
Patent
Policy
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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>.
Multiple Base Station MIMO
Keying WU, Yang SONG, Liyu CAI, Hongwei YANG, Xiaolong ZHU
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IEEE C80216m-08/423
Alcatel Shanghai Bell Co., Ltd.
Krishna Balachandran, Howard Huang, Srinivas Kadaba, Kemal Karakayali, Pantelis Monogioudis, Ashok
Rudrapatna, Sivarama Venkatesan
Alcalte-Lucent
Background
IEEE802.16m system targets for a great improvement over 16e system in terms of the peak data rate, average
sector throughput, and cell-edge user throughput. MIMO is an important building block to meet the requirement.
However, performances of MIMO are always limited by inter-cell interference (ICI), especially with universal
frequency reuse. To solve this problem, various techniques have been proposed and widely discussed in many
standardization organizations like 3GPP LTE, IEEE802.16e, etc. Typical examples include ICI-aware power
control, flexible frequency reuse, macro diversity, ICI randomization with interference cancellation at receiver,
and so on. These techniques can be used on the top of MIMO to improve the MIMO performance in interference
limited scenarios. Most of these techniques can effectively improve cell edge user throughput, but at the cost of
either loss of average sector throughput or increased receiver complexity. The concept of multi-BS MIMO
provides a new solution to for ICI control, which can improve the cell-edge performance and the sector
throughput at the same time, with limited complexity increase at the terminal.
The idea of multi-BS MIMO is to perform joint MIMO transmission and reception between multiple
coordinated BSs and multiple MSs over the same radio resources. Similar concepts have been proposed by
many companies, such as the BS cooperation from Mitsubishi [1], collaborative MIMO from Alcatel Shanghai
Bell [2][3], network MIMO from Alcatel Lucent [4], and Co-MIMO from ETRI [5]. The advantage of multi-BS
MIMO has been demonstrated by preliminary simulation results from different companies, especially for cell
edge users. For example, in [1] and [2] a potential throughput gain of up to 100% has been demonstrated for cell
edge users.
Recently, other IMT-advanced standardization organizations like 3GPP LTE+ have also started to pay a close
attention to the multi-BS MIMO concept. Similar ideas have been proposed by many companies to LTE+
[6]~[10], including Ericsson, Motorola, DoCoMo, Alcatel Lucent, Samsung, LGE, Nortel, Hitachi, and etc.
Multi-BS MIMO has been generally regarded as a promising technique to achieve the requirement of IMTadvanced. To enhance the competitiveness of 16m against other standards, multi-BS MIMO should be
supported. Therefore, we propose to support multi-BS MIMO as an enhanced MIMO option in IEEE 802.16m.
Concept of Multi-BS MIMO and Basic Approach
The basic concept of multi-BS MIMO is joint MIMO transmission and reception between multiple coordinated
BSs and a single MS or multiple MSs over the same radio resources. It has two basic features:
1) Each MS can be jointly served by multiple BSs through BS coordination over the same radio resource. By
doing so, the ICI can be mitigated or even changed into useful signal power.
2) Each BS can serve multiple MSs over the same radio resource. By doing so, the overall sector throughput
can be improved.
An example of multi-BS MIMO on the downlink is shown in Fig. 1, in which two BSs jointly serve two MSs
through coordination over the backhaul. Similar concepts can be applied on the uplink for coordinated reception
at the base stations.
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Figure 1. An illustration of multi-BS MIMO on the downlink
Multi-BS MIMO involves three basic operations:
1) Each BS acquires the channel information of all MSs that it serves. The channel information acquisition can
be realized in various ways:
i.
For the uplink, base stations estimate the channel directly from the MS pilots or other reference
signals. This applies to both TDD and FDD.
ii.
For the TDD downlink, uplink estimates could be used for downlink transmission. For the FDD
downlink, channel information is acquired through feedback from the MSs.
Also, different types of channel information can be used in this operation:
i.
Short-term channel information, i.e., the channel matrix of each user, can be used.
ii.
Long-term channel information such as the channel covariance matrix can be used.
2) BSs exchange information needed for coordination. Various ways can be used to implement information
exchange:
i.
Full channel information exchange: Each BS may need to share the complete channel information,
e.g. short-term channel matrices, between itself and MSs that it may serve through multi-BS
MIMO.
ii.
Partial channel information exchange: Each BS may only need to share a part of the channel
information, e.g. the long-term CQI, RSSI in conjunction with other DL preamble measurements or
the covariance channel matrices between MSs and itself.
iii.
Scheduling information (e.g., time, frequency, transport format, etc.) may also be shared between
BSs.
3) Based on the information obtained in 1) and 2), multi-BS transmitter/receiver processing is applied on the
downlink/uplink for the MSs being jointly served by the coordinating BSs. Different multiuser transmitter
and receiver processing techniques such as block diagonalization, SINR maximization, interference
cancellation, beamforming, etc. may be used. Various technical alternatives can be considered to provide
different tradeoffs between performance and implementation cost.
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Potential System Impact
Multi-BS MIMO can be regarded as an evolution of macro-diversity handover (MDHO) and spatial division
multiple access (SDMA) that allows the benefits of both MDHO and SDMA to be realized across a set of
coordinating BSs. Both MDHO and SDMA (single-BS) have been supported in 16e, so multi-BS MIMO can
reuse and extend these mechanisms to realize most of its functions [1][3]. In particular:
 In multi-BS MIMO, the BSs should be grouped into coordinating sets, containing the BSs that are likely to
coordinate with each other. This function is similar to the concept of a diversity set in the MDHO, and can
be implemented in the similar way.
 Data transfer between multiple BSs and the same MS bears significant resemblance to MDHO where
multiple BSs communicate with one MS, e.g. radio resource synchronization and synchronous data
distribution to multiple BSs.
 Data transfer between a BS and multiple MSs is similar to SDMA, where one BS communicates with
multiple MSs.
The similarity between multi-BS MIMO and MDHO/SDMA minimizes its impact on the standard, so that we
can reuse the existing standard with limited modifications and enhancements to realize multi-BS MIMO. The
key modifications/enhancements needed are discussed below.
Overhead on the backhaul:
Coordination among BSs needs information exchange through backhaul, which will cause additional overhead
on the backhaul.
Channel measurement
In 16e, the channel measurement mechanism has already been defined for single-BS MIMO in both TDD and
FDD mode. Multi-BS MIMO can reuse this mechanism by introducing some enhancements. In particular, in
single-BS MIMO, each BS only needs the channel information of users associated to it, while in multi-BS
MIMO, each BS needs the channel information of users associated to other BSs as well. The existing channel
measurement mechanism cannot support this function and needs to be improved.
Pilot design
16e defines a preamble on the downlink, uplink and downlink pilots, and a sounding channel on the uplink that
can be used for channel estimation. Multi-BS MIMO can leverage these mechanisms with some minor
enhancements. In particular, in single-BS MIMO, the channel only needs to be estimated between each MS and
its associated BS, while in multi-BS MIMO channel estimation may need to be performed between each MS and
multiple coordinating BSs as well. For this purpose, it is preferred to use orthogonal pilots among coordinating
BSs so that the channels can be separated correctly. Existing pilot mechanism in 16e cannot support this
function and enhancement is required.
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Conclusions
We propose to support the multi-BS MIMO technique in IEEE 802.16m as an enhanced MIMO option to
mitigate ICI as well as improve the sector throughput. Based on the existing simulation and analysis results from
various companies, multi-BS MIMO serves as an important supplement to the existing single-BS MIMO
techniques such as SU-MIMO and MU-MIMO. In particular, single-BS MIMO can be used at the cell center to
improve the peak data rate (by SU-MIMO) and sector throughput (by MU-MIMO), while multi-BS MIMO can
be used at the cell edge for better ICI control and to improve the cell-edge throughput. Therefore, we
recommend supporting three MIMO options, namely single-BS SU-MIMO, single-BS MU-MIMO and multiBS MIMO in IEEE 802.16m.
Text Proposal to SDD
Include the following text in Section 11.5 of IEEE 802.16m-08_003r1.
------------------------------------Start text proposal-----------------------------------MIMO mode
In 16m, three MIMO modes should be supported:
-
Single-BS single-user MIMO (SU-MIMO)
-
Single-BS multi-user MIMO (MU-MIMO)
-
Multi-BS MIMO:
Multi-BS MIMO performs a joint MIMO transmission/reception between multiple coordinated BSs and
multiple MSs. It can improve the cell-edge performance as well as increase sector throughput. Multi-BS
MIMO has two basic features: Each MS can be jointly served by multiple BSs over the same radio
resource and each BS can serve multiple MSs over the same radio resource, for inter-cell interference
mitigation and for improving sector throughput. Multi-BS MIMO involves three major operations:
1) Each BS acquires the channel information of all MSs that it may serve jointly with other BSs.
2) BSs exchange information needed for coordination.
3) Based on the information obtained in 1) and 2), multi-BS transmitter/receiver processing is applied on
the downlink/uplink for the MSs being jointly served by the coordinating BSs.
------------------------------------End text proposal------------------------------------
References
[1] IEEE C802.16m-08/183, “Base Station Cooperation and Channel Sounding”
[2] IEEE C802.16m-07/244r1, “Collaborative MIMO”
[3] IEEE C802.16m-07/243, “Closed-loop MIMO Macro Diversity”
[4] IEEE C802.16m-08/044r1, “Network MIMO for Inter-cell Interference Mitigation”
[5] IEEE 802.16m-08/017, “Frame Structure to Support Inter-cell Interference Mitigation for Downlink Traffic
Channel using Co-MIMO and FFR”
[6] 3GPP REV-080009, “Technical Proposal for IMT-Advanced”
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[7] 3GPP REV-080019, “Consideration on Technical Candidates for IMT-Advanced”
[8] 3GPP REV-080030, “LTE-Advanced – LTE evolution towards IMT-Advanced: Technology components”
[9] 3GPP REV-080033, “Technology considerations for LTE_Advanced”
[10] 3GPP REV-080011, “LTE Advanced Technical Proposals”
[11] 3GPP REV-080026, “Proposals for LTE-Advanced Technologies”
[12] 3GPP REV-080037, “Future 3GPP Radio Technologies for IMT-Advanced”
[13] 3GPP REV-080044, “Requirements and Concepts for LTE Advanced”
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