Transcript Slides S802.16m-08/457

IEEE S80216m-08/457
Advanced downlink MIMO design
Document Number: IEEE S80216m-08/457
Date Submitted: 2008-5-11
Source:
Hongwei YANG, Xiaolong ZHU, Keying WU, Song YANG, Liyu CAI
Alcatel Shanghai Bell Co., Ltd
Voice:
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+86-21-58541240 Ext. 7796
[email protected]
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Venue:
IEEE 802.16m-08/016r1 Call for Contributions on Project 802.16m
System Description Document (SDD)
Target topic: Downlink MIMO schemes
Base Contribution:
IEEE C80216m-08/457
Purpose:
To be discussed and adopted by TGm for use in the IEEE802.16m SDD.
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Introduction
• IEEE 802.16m requires higher performances of peak
data rate, sector throughput, cell edge user throughput,
mobility, and coverage
• To achieve these targets, advanced MIMO techniques
shall be an important building block
• To adapt to services and environments, multiple MIMO
modes shall coexist with a uniform architecture
Overview of MIMO modes
• Single-User MIMO (SU-MIMO)
– Each user is served by a single BS
– Mainly target for optimizing the single-user-achievable peak data rate
• Multi-User MIMO (MU-MIMO)
– Multiple users are served by one BS sharing the same radio resource
– Most suitable for heavily loaded systems in cell center, where
maximization of overall system throughput is the primary concern
• Multi-BS MIMO
– Multiple users are served by multiple BSs sharing the same radio
resources
– With BS coordination, the cell-edge user throughput and cell coverage
are improved significantly due to efficient interference reduction
Application scenario of MIMO modes
Cellular system
SU-MIMO
Logical
coordination
entity
Multi-BS MIMO
MU-MIMO
MIMO channel
SU-MIMO
• 2-Tx
– Rate-1
• Space-Time Block Coding: STBC
s
A 1
s 2
– Applied at low mobility
• Space-Frequency Block Coding: SFBC
s*2 

s*1 
– Applied in environments with low delay spread
• Recommend the switched STBC and SFBC
– Both defined as Matrix A, but with different mapping pattern
– Mode adaptation according to mobility and delay spread
s 
B   1
 s2 
– Rate-2
• V-BLAST code as Matrix B
– Sensitive to channel condition
• Full diversity full rate code as Matrix C
– Long decoding delay
– High decoding complexity
C
 s1  jrs4

1  r2  s2  rs3
1
rs2  s3 
1  5
，r 

jrs1  s4 
2
• Recommend a linearly dispersed SM code
– High robustness with low complexity
S
*
*
1 (s1  s2 )cos  (s1  s2 )sin 
1 1 1

，  tan ( )  0.2138
*
*
2
2
2 (s1  s2 )sin  (s1  s2 )cos 
SU-MIMO (Continued)
• 4-Tx
– Rate-1
• Cyclic Delay Diversity: CDD
– Increases the frequency selectivity of channel by artificial multiple paths
– Suitable for control signaling which should be received by all types of terminals
– Performance is highly dependent on the delay setting
• Recommend an antenna permuted CDD to alleviate the impact of delay setting
– Antenna permuted CDD can be combined with STBC and SFBC
SU-MIMO (Continued)
• 4-Tx
– Rate-2
• Scheme 1: Antenna permuted CDD can be combined with SM
• Scheme 2: Recommend layered SM
– Multiplex two Alamouti-coded matrixes. Each Alamouti-coded matrix
is referred to as a layer
– Different layers employ different power levels, which are designed
with the aid of very limited long-term feedback
– Advantages:
• both diversity and multiplexing gains
• Low detection complexity, to the same order as Alamouti
Φ1
d1
FEC
Encoder
c1
π1
π(
1 c1 )
Mod
1
x1
p1
Tx-2
Φ2
d2
FEC
Encoder
c2
π2
Interleaver
π(
2 c2 )
Mod
2
Phase rotation
(e.g., 0 degree)
x2
p2
Power level
Tx-1
Alamouti
Encoder
Alamouti
Encoder
Mapping
Tx-3
Tx-4
SU-MIMO (Continued)
• 4-Tx
– Rate-4
• Recommend layered SM scheme
d1
FEC
Encoder
c1
d2
FEC
Encoder
c2
d3
FEC
Encoder
c3
d4
FEC
Encoder
c4
π1
π2
π(
1 c1 )
π(
2 c2 )
Mod
1
Mod
2
x1
x2
Tx-1
p1
Tx-2
p2
Mapping
π3
π4
π(
3 c3 )
π(
4 c4 )
Mod
Mod
3
4
x3
x4
p3
Tx-3
p4
Tx-4
MU-MIMO
•
MU precoding
– Based on fixed beamforming
– Based on adaptive beamforming
• BS designs the precoding matrixes based on channel information to multiple users
• Recommend the multi-user eigen-mode transmission (MET) algorithm
–
–
–
–
BS performs simultaneous transmissions on the eigen-modes of different users
The eigen-modes to different users are weighted such that they are mutually orthogonal
The number of orthogonal beams is no more than the number of transmit antennas
Co-channel interference (CCI) is effectively controlled, thereby increasing the overall throughput even for
single-antenna users
•
MU scheduling
– To maximize the overall throughput by selecting users for transmission
– With imperfect CSI or precoding information used, increasing the user number may
reduce the throughput when BS fails to control the CCI
– Recommend a feedback-aided scheduling technique
•
•
•
Adjust the user number based on CSIT error covariance or CCI measurement at the user side
Users capable of estimating the covariance of CSIT error feedback the covariance so that BS can refine the
scheduling results
Otherwise, users measure the inter-user CCI power and check if it is too high. They then feed back their
decisions to the BS, which collects feedbacks from all users and adjusts the user number accordingly
MU-MIMO (Continued)
• MU channel measurement
– To provide BS with channel information for precoding and
scheduling
– Recommend a hierarchical sounding/feedback mechanism
to reduce the overhead for MU channel measurement
• Long-term statistical channel information of all users is
used for MU scheduling
– E.g., mean channel matrix, channel quality information
• Instantaneous channel information of selected users is
used for MU precoding
Multi-BS MIMO
•
With BS coordination, some of the cellular mobile network limitations such as
inter-cell interference (ICI) can be reduced significantly by multi-BS MIMO
•
Key features
– Each MS can receive its signals from multiple BSs over the same radio resource
– Each BS can transmit multiple MSs’ signals over the same radio resource
•
Multi-BS transmit solutions
– Different solutions shall give different tradeoffs between performance and
implementation cost
• Optimal performance with high complexity: Network MIMO
• Suboptimal performance with low complexity: Collaborative MIMO
•
Downlink pilot design
– Both flexibility and efficiency should be considered to support different MIMO modes
– Recommend a flexible pilot pattern design with configurable pilot reuse factor
•
Inter-BS exchange of channel information and control signaling
– Backhaul and a certain inter-BS coordination functional entity to be involved in the BS
Text Proposal
[Refer to the base contribution IEEE C80216m-08/457 ]