An LTE compatible massive MIMO testbed based on OpenAirInterface Xiwen JIANG, Florian Kaltenberger EURECOM
Testbed Overview Open source platform Based on OAI hardware and software 3GPP LTE compatible Incorporate all protocol layers Enable end-to-end experimentation with readily available 4G terminals TDD reciprocity calibration Exploiting TDD reciprocity through relative calibration 03/05/2016 OAI workshop 2017 - p 2
Key Parameters Parameters Value Number of antennas Up to 64 Center Frequency Bandwidth Sampling Rate 2.6GHz 5MHz FFT Size 512 Number of used subcarriers 300 Slot time Maximum simultaneously served users 7.68MS/s 0.5ms Currently 4 (LTE release 10), extendable 03/05/2016 OAI workshop 2017 - p 3
System Architecture 03/05/2016 OAI workshop 2017 - p 4
Hardware components Ettus Research Octo-clock for clock distribution PCIe Chassis 16 EXMIMO2 cards Huawei Antenna array Each EXMIMO2 card contains 4 RF chains 20 patch antennas with 4 antennas each 03/05/2016 OAI workshop 2017 - p 5
Software implementation RRC Signaling Beamforming weights calculation Logical to physical antenna mapping IFFT and Precoding Parallelization UE specific RS 03/05/2016 OAI workshop 2017 - p 6
RRC signaling Transmission Mode (TM) Configuration in RRCConnectionReconfiguration UE RRCConnectionRequest RRCConnectionSetup RRCConnectionComplete EUTRAN Configure the UE to use TM7 RRCConnectionReconfiguration RRCConnectionReconfigurationComplete TM1/2 Data Transmission TM7 03/05/2016 OAI workshop 2017 - p 7
Logical Antenna Ports LTE antenna ports definition Source: 3g4g.blogspot.com 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 03/05/2016 OAI workshop 2017 - p 8
Antenna Port Mapping (TM7) 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. Logical Antenna ports w 0 Physical Antennas Cell Specific 0 w 1 UE Specific 5 w 2 w 3 Cell Specific 1 w 4 w 5 03/05/2016 OAI workshop 2017 - p 9
UE Specific RS Precoding UE specific RS and data with the same weights in order to perform beamforming channel estimation Cell Specific RS: Use common BF weights UE Specific RS data: Use UE specific BF weights 03/05/2016 OAI workshop 2017 - p 10
IFFT and Beam Precoding Parallelization 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 Wakeup all threads only when all threads are in waiting status All threads active Thread pool: each thread is in charge of the IFFT and precoding operation for one physical antenna All threads waiting Some threads finish 03/05/2016 OAI workshop 2017 - p 11
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 OAI workshop 2017 - p 13
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 OAI workshop 2017 - p 16
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 [1] 3GPP TR 38.802 V14.0.0 (2017-03) Study on New Radio Access Technology Physical Layer Aspects 03/05/2016 OAI workshop 2017 - p 17
APPENDIX 03/05/2016 OAI workshop 2017 - p 18
APPENDIX 1: STANDARDIZATION FOR MASSIVE MIMO 03/05/2016 OAI workshop 2017 - p 19
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: TM8: One virtual antenna port p={5} Single codeword for one user Two virtual antenna ports p={7,8} Two codewords for two users Codeword User 2 Codeword User 1 p={8} p={7} Beamforming Filters 03/05/2016 OAI workshop 2017 20
LTE release 10 (transmission mode 9) Superset of all previous transmission modes (supports both cell-specific and UEspecific 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
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 OAI workshop 2017 - p 23
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 OAI workshop 2017 - p 24
APPENDIX 2: PLL ISSUES FOR LMS6002D 03/05/2016 OAI workshop 2017 - p 25
Express MIMO 2 RF RX (4 way) RF TX (4 way) PCI Express (1 or 4 way) Spartan 6 LX150T 4xLMS6002D RF ASICs 12V from ATX power supply 250 MHz 3.8 GHz GPIO for external RF control 03/05/2016 OAI workshop 2017 26
TDD issues on LMS6002D Fig.6. LMS6002D layout 03/05/2016 OAI workshop 2017 - p 27
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 OAI workshop 2017 - p 28
TDD workaround Original Signal Baseband filter = 5MHz Offset RX frequency fc-fs/2 fc fcfs/2 Baseband filter = 10MHz Alias fc-fs/2 fc =fc-fs/4 fcfs/2 Shift baseband signal back by fs/4 0 fc = carrier frequency fs = sampling frequency 03/05/2016 OAI workshop 2017 - p 29
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
APPENDIX 2: TM7 SIMULATION IN OAI 03/05/2016 OAI workshop 2017 - p 31
OAI Downlink simulation on TM7 Fig.4. TM7 BLER under perfect channel estimation (QPSK, AWGN channel) 03/05/2016 OAI workshop 2017 - p 32