Massive MIMO – a overview

Chandrasekaran

CEWiT

Outline

• Introduction

• Ways to Achieve higher spectral efficiency

• Massive MIMO basics

• Challenges and expectations from Massive MIMO

• Network MIMO features

• Summary

Wireless traffic growth

• Growth in Wireless Traffic

– One Million times in last 45 years

Martin Cooper’s law

The number of simultaneous voice/data connections has

doubled every 2.5 years (+32% per year) since the

beginning of wireless

Src: Martin Cooper

Spectrum and Limitations

• Radio spectrum is a very scarce resource – Limited availability for cellular communications

• Spectral efficiency needs to be improved

• Spectral efficiency of point-to-point transmission – Shannon’s capacity limit

– 𝑙𝑜𝑔2 1 + 𝑆𝐼𝑁𝑅

– Cannot do much: 4 bps/Hz -> 8 bps/Hz costs 17 times more power

Spectral Efficiency

• Frequency reuse – Dense deployments using smaller cells

• Inter-cell interference • Diminishing returns with increase in number of smaller cells

• MIMO – Capacity increasing linearly with factor of min{Nt, Nr} – SM for SU-MIMO

• Limited by number of antennas at the user in cellular frequency

– SDMA for MU-MIMO • Larger number of antennas from distributed user terminals

Conventional MU-MIMO

• Performance depends on scheduler and link adaptation • CSI feedback from UE report (FDD mode) • Precoder design using uplink channel (Reciprocity principle for TDD)

MU-MIMO in LTE

MU-MIMO

MU-MIMO (TM5)

Dual layer DMRS based SM (TM8)

8 layer DMRS based SM (TM9)

• Code book based scheme • Maximum 2 user pairing • Single layer transmission

to each UE

• Non Code book based scheme • Adaptive SU/MU MIMO • More user pairing • Higher rank transmission to each UE

FD-MIMO in LTE Release 13

• Vertical sectorization – Creation of vertical sectors – Just like having multiple sectors in the horizontal direction

• Beam formed CSIRS based scheme – Virtual sectorization using beam selection

• Kronecker based precoding – Vertical and horizontal precoder reporting from UE – Forming the final precoder using KP

• SRS based scheme (TDD) – Precoder selection using uplink channel – Reciprocity property

Massive MIMO

Hundreds of BS antennas

Tens of active users

Higher order improved MU-MIMO

• MU-MIMO Scheduler challenges • User pairing algorithm • MU-CQI prediction assuming co-user interference

Active Antenna Array (AAS)

Antenna element

Sub array TXRU model Full connection TXRU model

Antenna element

AAS Structure

Src : TR 36.897

Massive MIMO deployment

• Linear • Rectangular array • Cylindrical

Virtual sectorization

Conventional beamforming in horizontal direction

3D beamforming for single UE

3D beamforming for multiple UEs

Beamforming using user location

Active Antenna System (AAS)

Base station

Virtual beamforming

Horizontal beamforming

Elevation beamforming

Challenges of Massive MIMO

UE specific beamforming

Cell wide coverage for broadcast/control channels

• To achieve cell wide coverage for broadcast /control channels • Narrow beam for data channels • CQI estimation for the UE specific beam

Further challenges

• Feedback and codebook design in FDD – How to reduce the feedback overhead? – How FDD reciprocity can be used?

• Uplink sounding in TDD – How to accurately estimate a large number of channels? – Channel estimation complexity

• Precoding, Scheduling & Link adaptation – CQI prediction and scheduling – Beam identification

• With and without UE location information

– Antenna grouping

• High mobility scenarios – Channel ageing effects – Need for better diversity scheme

• Pilot contamination – It limits MU-MIMO performance – Coordination between BSs is needed

Network MIMO

Backhaul network

• Coordinated transmission from multiple base stations • a.k.a. CoMP • Fast backhaul is a challenge • Improves area spectral efficiency, system capacity

Summary

• Massive/ FD MIMO is a promising technology to significantly improve cellular capacity

• Pilot design, Channel estimation, Precoder estimation are the main challenges

• Wider coverage for control and broadcast channels using larger MIMO

• Design of better Diversity schemes for high mobility and low SINR users

Thank you