an lte compatible massive mimo testbed based on ... · huawei antenna array pcie chassis + 16...
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An LTE compatible massive
MIMO testbed
based on OpenAirInterface
Xiwen JIANG, Florian Kaltenberger
EURECOM
Testbed Overview
03/05/2016 OAI workshop 2017 - p 2
Open source
platform
3GPP LTE compatible
TDD reciprocity calibration
• Based on OAI hardware
and software
• Exploiting TDD
reciprocity through
relative calibration
• Incorporate all
protocol layers
• Enable end-to-end
experimentation with
readily available 4G
terminals
Key Parameters
Parameters Value
Number of antennas Up to 64
Center Frequency 2.6GHz
Bandwidth 5MHz
Sampling Rate 7.68MS/s
FFT Size 512
Number of used subcarriers 300
Slot time 0.5ms
Maximum simultaneously
served users
Currently 4 (LTE release 10),
extendable
03/05/2016 OAI workshop 2017 - p 3
Hardware components
03/05/2016 OAI workshop 2017 - p 5
Ettus Research Octo-clock for clock
distribution
Huawei Antenna array
PCIe Chassis + 16 EXMIMO2 cards
Each EXMIMO2 card contains 4 RF chains
20 patch antennas with 4 antennas each
Software implementation
RRC Signaling
Logical to physical antenna mapping
UE specific RS
IFFT and Precoding
Parallelization
Beamforming weights
calculation
03/05/2016 OAI workshop 2017 - p 6
RRC signaling
Transmission Mode (TM) Configuration in
RRCConnectionReconfiguration
03/05/2016 OAI workshop 2017 - p 7
UE EUTRAN
RRCConnectionRequest
RRCConnectionSetup
RRCConnectionComplete
RRCConnectionReconfiguration
RRCConnectionReconfigurationComplete
TM1/2
TM7
Configure the
UE to use TM7
Data Transmission
Logical Antenna Ports
LTE antenna ports definition
03/05/2016 OAI workshop 2017 - p 8
Antenna Ports DL RS 3GPP Release
Port 0-3 Cell Specific RS Release 8
Port 4 MBSFN-RS Release 8
Port 5 UE Specific RS for single-layer Beamforming (TM7) Release 8
Port 6 Positioning RS Release 9
Port 7-8 UE specific RS for Dual-layer Beamforming (TM8) Release 9
Port 9-14 UE specific RS for up to 8 layers Beamforming (TM9) Release 10
Port 15-22 Channel State Information (CSI) RS Release 10
Port 15-30 CSI-RS (precoded or standard) Release 13
Port 15-46 CSI-RS (precoded or standard) Release 14
Source: 3g4g.blogspot.com
Antenna Port Mapping (TM7)
03/05/2016 OAI workshop 2017 - p 9
+
1
5
0
Logical Antenna ports Physical Antennas
w0
w1
w2
w3
w4
w5
Cell Specific
UE Specific
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Cell Specific
Cell specific and UE specific antenna port mapping
Logical antenna ports are mapped to physical antennas;
Precoding the control channel with common beam weights;
Precoding data with the UE specific weights.
UE Specific RS
Precoding UE specific RS and data with the same
weights in order to perform beamforming channel
estimation
03/05/2016 OAI workshop 2017 - p 10
Cell Specific RS:
Use common BF weights UE Specific RS + data:
Use UE specific BF weights
IFFT and Beam Precoding Parallelization
03/05/2016 OAI workshop 2017 - p 11
All threads active
Some threads finish
All threads waiting
…
… …
Thread pool: each thread is in
charge of the IFFT and precoding
operation for one physical antenna
Wakeup all threads
only when all threads
are in waiting status
Real time IFFT and beam precoding
Challenge: Impossible to perform IFFT and beam precoding
sequentially when the number of physical antennas go large
Solution: using a thread pool to parallelize the IFFT and beam
precoding for all physical antennas
Beamforming Weights Calculation
CSIT acquisition
Challenge: LTE CSIT Feedback mechanism not feasible in massive
MIMO.
Solution: TDD channel reciprocity, estimate UL channel to assess
DL channel.
Implementation:
– Use SRS (UL RS) to estimate UL channel,
– Calibrate the UL channel to have DL CSIT (reciprocity calibration).
Beamforming weights calculation
Same weights for the whole frame (since based on TDD
reciprocity).
MRT is implemented, ZF, MMSE are to be accomplished.
03/05/2016 OAI workshop 2017 - p 12
TDD Reciprocity Calibration
TDD Reciprocity and hardware non-symmetry
TDD DL/UL physical channels enjoy reciprocity, implying that we
can obtain DL CSI from UL channel estimation
TX/RX RF chains are not symmetric, broking the channel reciprocity
TDD Reciprocity Calibration
The RF chain non-symmetry are stable during time, and can be
estimated
We perform offline reciprocity calibration to obtain the hardware
non-symmetry
BS internal calibration within the 64 antenna array so that the UE is
not evolved in the calibration
03/05/2016 - p 13 OAI workshop 2017
Demo at WSA 2017 @TU Berlin
Reduced scale demo with 4
antennas
TDD band 38
Motorola phone
Reciprocity calibration
Beamforming based on reciprocity
03/05/2016 OAI workshop 2017 - p 14
Massive MIMO and C-RAN
Massive MIMO is currently implemented centralized as an enhanced 3GPP eNodeB function
New functional splits (ongoing development) allow flexible (co-located or distributed) C-RAN deployments RRC (IF1’) (RAU + RRU): L1/L2 processing in the frontend
RRC (IF1’) RAU (IF4’5) RRU: one RAU for multiple sites, high speed fronthaul
RRC (IF4’5) RRU: several virtual cells, high speed fronthaul
Use synchronized low-cost RRUs to create (distributed) massive MIMO array
03/05/2016 OAI workshop 2017 - p 15
Conclusions
FDD vs TDD Massive MIMO FDD: UE beam-selection among a set of fixed beams
TDD: Can use TDD reciprocity calibration -> better performance due to better quality CSIT
Eurecom massive MIMO testbed based on ExpressMIMO2 is being phased out
Alternative scalable hardware solutions Synchronized, low-cost RRUs based on USRP B2x0 (mini)
– like in C-RAN testbed
USRPs X3x0 can be scaled & synchronized using
– PXIe (NI based solution)
Very expensive
Not supported by UHD and thus OAI
– Gbit Ethernet switch
Skylarke, Other?
03/05/2016 - p 16 OAI workshop 2017
Outlook for 5G New Radio (3GPP Rel 15)
Designed for massive MIMO from the start:
At least, the 8 orthogonal DL DMRS ports are supported for SU-
MIMO and maximum 12 orthogonal DL DMRS ports are supported
for MU-MIMO [1]
FDD and dynamic TDD
Reciprocity based beamforming still possible
Hybrid analogue digital antenna systems supported
Challenges for reciprocity calibration
OAI-NR project starting now
Will lay the groundwork for massive MIMO
03/05/2016 OAI workshop 2017 - p 17
[1] 3GPP TR 38.802 V14.0.0 (2017-03) “Study on New Radio Access Technology Physical Layer Aspects”
LTE release 8/9 (transmission modes 7/8)
Unspecified number of TX antennas
UE-specific reference signals to which the same beamforming is
applied as for PDSCH
Means to derive beamforming is unspecified
TM7: One virtual antenna port p={5}
Single codeword for one user
TM8: Two virtual antenna ports p={7,8}
Two codewords for two users
03/05/2016 OAI workshop 2017 20
p={7} Codeword
User 1
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p={8} Codeword
User 2
Beamforming
Filters
LTE release 10 (transmission mode 9)
Superset of all previous transmission modes (supports both cell-specific and UE-specific pilots) UE specific reference signals (p={7,8,…,6+})
CSI reference signals (p={15,16,…,22})
If UE-specific pilots are used Arbitrary number of antennas
Up to 8-layer SU/MU-MIMO (max 2 codewords per UE)
No. concurrent users limited by PDCCH
Feedback (UE-selected) of multiple precoding matrix indicators (quantized as in Rel-8)
Measurements made using CSI reference signals
03/05/2016 OAI workshop 2017 21
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LTE release 11/12 (transmission mode 10)
LTE release 11 (transmission mode 10)
Scrambling identities for DMRS can be assigned for better
orthogonality in CoMP scenarios
ePDCCH: same beamforming applied to control and data
More than 8 UEs possible (per subframe) by using virtual cells (with
same eNB id)
LTE release 12
Mainly small cell enhancements
Not many change regarding MIMO
03/05/2016 OAI workshop 2017 22
LTE release 13/14
LTE release 13
New CSI reference signals for up to 16 antennas
No new feedback scheme or transmission modes
Unfinished work?
LTE release 14
Work Item on Enhancements on Full-Dimension (FD) MIMO for LTE
CSI reference signals for up to 32 antennas
Enhancement on CSI reports
Support for providing higher robustness against CSI impairments
(such as inter-cell interference or higher-speed UEs) and higher CSI
accuracy
New transmission mode?
03/05/2016 - p 23 OAI workshop 2017
Summary
TDD Massive MIMO feasible even with current Rel 10/11
Using transmission mode 9 or 10
Massive MIMO could even be applied to earlier releases
Beamforming of all signals in transmission mode 1
Similar to Artemis private cell concept [Forenza, 2015]?
No explicit UE support for relative calibration
Not absolutely needed (can be done internally or by proprietary
calibration Ues)
Maybe work in Rel 13/14 could also be exploited for that
FDD Massive MIMO partially feasible
Release 14 should support up to 32 antennas + feedback modes
Can be also used for fixed beam-switching
03/05/2016 - p 24 OAI workshop 2017
Express MIMO 2
03/05/2016
RF RX (4 way)
RF TX (4 way)
PCI Express (1 or 4 way)
4xLMS6002D RF ASICs 250 MHz – 3.8 GHz GPIO for external RF control
Spartan 6 LX150T
12V from ATX power supply
26 OAI workshop 2017
PLL issues
LMS6002D limitations
ExpressMIMO2 uses LMS6002D as RF front-end chips
Tx and Rx RF chains use different PLLs (initially designed for FDD
mode)
– If we set both PLLs to the same frequency as in the FDD mode,
they interfere each other;
– LMS6002D turns on/off alternatively the PLLs for Tx and Rx in
TDD mode, resulting in a random modulation phase and making
it impossible to perform MIMO precoding.
Solutions
Set a ¼*fs frequency shift in Tx and Rx RF chains in the analogue
domain, where fs is the sampling frequency
Draw back the frequency shift in the digital domain
03/05/2016 - p 28 OAI workshop 2017
Offset RX frequency
TDD workaround
03/05/2016 - p 29
fc
Baseband filter = 5MHz
Baseband filter = 10MHz
fc+fs/2 fc-fs/2
fc’=fc-fs/4 fc+fs/2 fc-fs/2
Alias
Original Signal
fc = carrier frequency
fs = sampling frequency
Shift baseband signal back by fs/4
0
OAI workshop 2017
TDD workaround Drawbacks
Drawbacks
LO leakage (issue mostly for UE)
Will only work if (left-) adjacent channel is free
03/05/2016 OAI workshop 2017 - p 30
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