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mm-Band MIMO in 5G Mobile

Arogyaswami Paulraj Stanford University

IEEE 5G Summit

Santa Clara University November 16, 2015

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Universal Connectivity Immersive Experience Tele-Control, Tactile, V2X High Speed Low Latency High Reliability Low Power

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Technologies

Virtualization

SDN

WiFi Integration

MTC Support

Multi-Link Integration

U-, C–Plane Splitting

SRAN Multi-Service Platforms

D2D

mm-Band MIMO

Multiple Access, Waveforms Modulation

Dense Cells

Het-Net

CA

CoMP

Multi-RAT

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mm-Band MIMO

Multiple Access, Waveforms and Modulation

Spatial Modes

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Large MIMO

• Antennas

– BS 100 to 10,000 !

– Large number of RF chains, PAs – UE 2 - 8

• BS Beam widths – 10 to 1 deg

• Channel BW 500 - 1500 MHz • Use of LOS – MIMO

• U-plane carrier, Data Only

• Main access mode - MU-MIMO • Back / Front Haul P2P Modes

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mm-Band

• Proposed US Bands

– 28, 37, 39, 57 - 71 GHz • Propagation mode

– Generally LOS, but NLOS also present, strong shadowing, stronger fading

– No significant loss in free space (outside 60 GHz) – but foliage and precipitation induce losses

• Deployment

– Small cells ~ 150m – High BS antennas • Green !

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Some History

• Iospan Wireless (1998) built a Broadband Wireless Internet system. Acq. by Intel in 2003

• Married MIMO & OFDM (DS-SS was then reigning waveform) • Iospan Technology

– Cellular architecture (nomadic access, layer 3 hand over)

– CP-OFDM, MIMO (2 streams) and M-QAM with Rep. Coding, OFDMA, STC, ….

• Iospan technology became precursor for WiMAX (2005) and adopted by LTE (2009) and WiFi 11n (2009)

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Why OFDM + MIMO ?

Time

Pre-MIMO

Waveforms designed to be orthogonal in one dimension and equalized in the other dimension (per user)

Channel Spreading

Waveform Tiling

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Why OFDM + MIMO ?

Time S p a ce

strict orthogonality preferred in Freq. and Time - > CP-OFDM

H(f,t)

Spatial dimension is inherently non-orthogonal, so Post-MIMO

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4G Waveforms

• CP-OFDM

– Sub-Carriers were Freq. Flat and Orthogonal > Great for MIMO, MIMO decoding can be done per sub-carrier

– Pain Points - High PAPR, Guard Time (CP), Guard Band, Out of Band Emission, Strict clock and time Sync. on UL, …

• WiMAX and WiFi stayed with OFDM

• LTE modified UL to DFT - OFDM to reduce PAPR

Ch. BW

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5G Waveform Candidates

• FBMC – Filter Bank Multi Carrier • UFMC - Universally Filtered Multi

Carrier

• f-OFDM - Spectrum Filtered OFDM • GFDM – Generalized FDM

(Windowing Choices)

• Time Orthogonality • Freq. Orthogonality • Out of Band Emission • MIMO friendly

• UL Synchronization • Flexible Raster

Trade Offs • BW < 100 MHz current OFDM LTE is OK

• BW > 100 MHz < 1000 MHz – OFDM needs some changes • BW > 1000 MHz, OFDM not attractive

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5G Waveforms BW > 1000 MHz

• Single Carrier (SC) seems more attractive

• OFDM’s high PAPR complicates high rate ADC / DACs, SC is low PAPR

• OFDM’s PAPR also complicates low power / efficient PA design for large arrays

• Low delay spread of narrow mm-Band beams makes SC equalization manageable. Freq. domain turbo equalization

• SC waveforms – Constant Envelop, Continuous Phase, Linear (QAM) …

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Modulation

• 4G

• M-QAM for high SNRs and Repetition Coding (RC) for low SNRs

• RC is not energy efficient at low SNR (cell edge) and also increases PAPR for

narrowband UL (IoT)

• 5G (< 6 GHz)

• M-QAM at high SNR and FSK at low SNR

• Encode N + 2 bits by choosing one of 2 N sub carriers and

4-QAM modulation on the chosen sub carrier

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Multiple Access

4G

• Time-Frequency - Strict Ortho UE1

UE2 UE3 UE4

• Space

Single User - Quasi Ortho Multi User - Strict Ortho DL

5G

(< 6 GHz)

• Time-Frequency - Quasi Ortho

– SCMA (Sparse Coded

MA) (overloading and

spreading)

– QOMA (Quasi Ortho.

Mul. Access a.k.a NOMA) ~ SP coding with power allocation to exploit path loss differences, SIC Rx • Space

QO-MIMO - Quasi Ortho

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MIMO Modes

• Vertical Dimension (FD – MIMO)

• Single User (streams limited by UE antennas)

• Multi User (streams limited by BS antennas and power) LOS

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Interference Management

• Inter BS interference is localized along beam / sector axis, no

collision - no interference

• Avoid interference collision by inter-BS coordination (topological interference alignment)

• Inter-sector and inter user

interference can be handled by SIC Rx and NAIC (Network Assist IC)

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Large MIMO – Pragmatism

• Grouped analog front end with reduced digital Tx, Rx (hybrid front ends)

• Adaptive sectorizaton to

separate UE clusters followed with per sector MIMO

processing

• Low BPS/Hz waveforms to tolerate MU interference, poor channel estimates. Low PAPR • Also ML and SIC Rx to handle

multi-user interference (QO-MIMO) D AC D AC D AC COD E C

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Channel Estimation - Pilots

• Pilots confined to sector

• UL – Easy, pilot overhead depends on UE antennas

• DL – Hard, pilot overhead depends on BS antennas

• TDD – needs expensive calibration • Model based channel estimation in

sparse scattering environments – Array calibration manageable • Exploit sparsity in all dimensions

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Summary

Large MIMO, mm-Band propagation, large BW and small cell size changes the existing LTE design tradeoffs on

waveforms, multiple access, modulation, RF architecture, interference management and MIMO modes

Hardware integration with 2,3 and 4G (Soft RAN) Many open issues!

References

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