Importance of Distributed Allocations for Closed-Loop SU- and MU-MIMO Document Number: IEEE C802.16m-08/834 Date Submitted: 2008-07-14 Source: Fred Vook, Bishwarup Mondal, Arvind Krishnamoorthy, E-mail:
[email protected] Kevin Baum, Fan Wang, Mark Cudak Motorola * Venue: TGm – Comments on Project 802.16m System Description Document – Downlink MIMO Schemes Abstract: We propose that the SDD text on DL-MIMO includes the use of distributed allocations for CL-SU-MIMO and CL-MU-MIMO Purpose: Discussion and adoption of recommended text into 802.16m System Description Document for DL-MIMO 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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Problem in C802.16m-08/657r2 • The current SDD text submission for DL-MIMO allows CLSU-MIMO and CL-MU-MIMO to operate only in localized allocations • We propose allowing CL-MIMO and MU-MIMO to operate in both localized and distributed allocations • Also want to ensure dedicated pilots continue to be supported in distributed allocations – Currently included in 16e / WiMAX R1 and 16m SDD draft – Enable vendor-specific precoding to work well in distributed allocations
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Important Deployment Scenario: Correlated Array in Suburban/Rural Environment • Characteristics: – BS above the clutter => Angular spread is low => Correlated array. • Spatial channel characteristics are stable over time/frequency
– Moderate percentage of users experience a time-varying frequency-selective channel • Frequency selectivity not static over time
• Implications for velocity scenarios: – Beamforming / closed-loop transmission works with a wideband precoder • Coherent processing gains can be achieved with wideband precoding • Feedback of wideband PMI is also in the SDD
– Difficulties with Band Selection: • Does not provide gains in velocity scenarios – No gain from frequency selective scheduling
• Has high overhead
• Narrowband allocations cannot exploit frequency diversity! • CL-SU-MIMO & CL-MU-MIMO need to be able to operate with a distributed allocation IEEE C80216m-08/834
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Dedicated Pilots should be an option in Distributed Allocations • Vendor-specific CL-SU-MIMO & CL-MUMIMO require dedicated pilots – E.g., enabled by UL Sounding as in the SDD text – MS will not know the precoder vector/matrix
• CL-SU-MIMO & CL-MU-MIMO should be operable in distributed allocations – See previous slide
• Therefore need dedicated pilots in distributed allocations IEEE C80216m-08/834
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Block-Based Distributed Allocations Needed for Closed-Loop Transmission • Vendor-specific closed-loop transmission requires dedicated pilots (“beamformed” along with the data) – E.g., UL sounding, analog feedback, Codebooks for MU-MIMO – Pilots must be clearly tied to / associated with the user allocation
• Using dedicated pilots in single subcarrier-based distributed allocations (e.g., 16e-PUSC-style) imposes a restriction that all data in the resource block must be precoded / beamformed in the same way – Imposes large scheduling granularity when beamforming to a single user – Need to avoid this in 16m!
• Block-based distributed allocations avoids this restriction – Dedicated pilots within each block allow per-block precoding for tracking frequency selectivity – FEC across blocks exploits frequency diversity
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Response to various concerns • Concern that distributed allocations means more overhead in the precoding feedback. – UL Sounding – enabled precoding • Distributed allocations has no impact on overhead – sounding can be matched to the DL allocation
– Codebook – based precoding • Can have one PMI/PVI applied across the entire band – low overhead on UL & DL
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SU-MIMO Simulations
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Wideband Precoding based on Codebook feedback • Urban Macro, 60kmph
BLER
10
10
10
0
0.5λ,480b,EBF,CE,60kmph,5ms-delay,UrbanMc
Wideband precoding is effective at high velocity in correlated scenarios
-1
CDD subcarrier perm WB PMI blk perm
-2
-8
-6
-4
-2
0
2
4
6
8
10
SNR
* Larger gains observed in suburban channels IEEE C80216m-08/834
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SU-MIMO Simulation Parameters 1 Parameter
Value
NFFT
1024
Carrier frequency
2.6 GHz
# Tx antennas
4
# Rx antennas
2
Antenna spacing
0.5 λ for Tx, 0.5 λ for Rx
MCS
1/2 QPSK
Channel model
SCM Urban Macro 150 (60kmph),
Feedback delay
5ms
Pilots
Dedicated for CL-MIMO and Broadcast for CDD, 2.5dB boost
Codebook Parameters
4-bit LTE codebook, no feedback error
Receiver
MRC
DL channel estimator
2D-MMSE based on 18x6 tile for CL-MIMO, 1-D MMSE for broadcast using WB pilots
Midamble channel estimator
Ideal
Packet size
480 info-bits (5PRU)
DL- Allocation
18x6 block distributed randomly (for CL-MIMO) or subcarrier permutation (for CDD)
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SU-MIMO Simulation Parameters 2 • Pilot – dedicated - 5.56%
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• Pilot – broadcast – 3.7%
10
MU-MIMO Simulations
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Wideband Precoding based on Codebook feedback • Urban Macro, 60kmph, 0.5λ @ BS, 0.5 λ @ MS
Wideband precoding is effective at high velocity in correlated scenarios
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Wideband Precoding based on Codebook feedback • Urban Macro, 60kmph, 4λ @ BS, 0.5 λ @ MS
Wideband precoding performs similar to or slightly better than open-loop at high velocity in uncorrelated scenarios
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MU-MIMO Simulation Parameters Parameter
Value
NFFT
1024
Carrier frequency
2.5 GHz
# Tx antennas
4
# Rx antennas
2
Antenna spacing
4λ or 0.5λ @ BS, 0.5 λ for Rx
Channel model
SCM Urban Macro (Table 10, Sec. 3.2.5.1 of IEEE802.16m-08/004r2)
MS velocity
60 kmph
Feedback delay
5ms
Codebook Parameters
3-bit LTE codebook, no feedback error
Receiver
MMSE for MU-MIMO, MRC for CDD
DL channel estimation
Perfect
Packet size
480 info-bits
DL- Allocation
Spread over entire bandwidth
Number of users
6 for MU-MIMO, 1 for OL-CDD
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