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

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System Architecture

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Hardware components

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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

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RRC signaling

Transmission Mode (TM) Configuration in

RRCConnectionReconfiguration

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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

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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

+

+

+

+

+

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

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Cell Specific RS:

Use common BF weights UE Specific RS + data:

Use UE specific BF weights

IFFT and Beam Precoding Parallelization

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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.

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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

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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

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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

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[1] 3GPP TR 38.802 V14.0.0 (2017-03) “Study on New Radio Access Technology Physical Layer Aspects”

APPENDIX

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APPENDIX 1:

STANDARDIZATION FOR

MASSIVE MIMO

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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

+

+

+

+

+

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

+

+

+

+

+

+

+

+

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

APPENDIX 2:

PLL ISSUES FOR LMS6002D

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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

03/05/2016 - p 27

TDD issues on LMS6002D

OAI workshop 2017

Fig.6. LMS6002D layout

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

APPENDIX 2:

TM7 SIMULATION IN OAI

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OAI Downlink simulation on TM7

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Fig.4. TM7 BLER under perfect channel estimation (QPSK, AWGN channel)