Control Channel Design for Many-Antenna MU-MIMO

Clayton Shepard Abeer Javed Lin Zhong

What is the control channel?

Anything that isn’t data

Control Channel Functions

Time-frequency synchronization

Association, paging, and random access

Channel state information (CSI) collection

Gain control

Scheduling, acks, handovers, etc.

4

Traditional control channels aren’t practical for many-antenna MU-MIMO

Single Antenna

MU-MIMO

MU-MIMO Gain Gap

7

Antennas are peak-power constrained!

Gap grows with M2!

Traditional Sync

MU-MIMO

M Base Station AntennasK Users

M2/K

M

9

Single Antenna

MU-MIMO

M-Antenna Gain?

All Antennas?

M2/K

M

10

Multi-antenna techniques don’t work for synchronization or CSI collection!

Existing Solutions Don’t Work!

• Widely adopted/efficient technique is correlation• 802.11 uses Cyclic Delay Diversity for MIMO

systems

• Distortion causes performance to degrade with M!

0 10 20 30 40 50 600

2

4

6

8

10

12

14SIN

R (

dB

)

Number of Antennas (M)

0 10 20 30 40 50 60-15

-10

-5

0

5

12

From-scratch control channel design for many-antenna MU-MIMO

Faros

Key Insight I:

Send as much as possible over MU-MIMO

17

Critical Control Channel Operations

Synchronization

Association

CSI collection

Paging

Random access

18

Key Insight II:

Synchronization and association are not time critical

19

Solving the Gain Gap

Faros Gain Matching: Beamforming

• Sweep open-loop beams!– No time-distortion– Power scales with M2

– Needs many beams (more time)

– Still doesn’t provide full range

Coverage Gap

Traditional Sync

Open-loop Beam

MU-MIMO

21

Gain Gap: Solution

• Use coding gain!– Increase coverage area– Flexible range control

Traditional Sync

Open-loop Beam

MU-MIMO

Faros

– Takes more time

Full Coverage

22

Faros Gain Matching Flexibility

Traditional Sync

Faros

MU-MIMO

23

Faros Control Channel Design

PagingBeacon

MU-MIMO Frame Structure

PagingBeacon

Pilots DownlinkCC UplinkC

C Uplink

25

Synchronization

Paging

Beacon

Wait for beacon to establish synchronization 26

Association

Pilots

Send association request in dedicated slot 27

Association

Pilots

Use MU-MIMO channel to transmit remaining control 28

Random Access

Pilots

Dedicated slots for random access29

Collecting CSI

OFDMA for extra gain

Pilots

Coded for even more gain 30

Paging

Paging

Beacon

Use last known user location to reduce latency! 31

Negligible Overhead

Code Length

Bandwidth

Frame Length

Number of

Beams

Channel Utilizatio

n

Association Delay

Random

Access

128 20 MHz 15 ms 100 0.04% 750 ms 7.5 ms

128 40 MHz 1 ms 100 0.32% 50 ms 0.5 ms

256 20 MHz 10 ms 100 0.13% 500 ms 5 ms

256 20 MHz 5 ms 500 0.26% 1250 ms 2.5 ms

512 40 MHz 2 ms 1000 0.64% 1000 ms 1 ms

1024 80 MHz 1 ms 4000 1.28% 2000 ms 0.5 ms

32

Parameter Overhead

Faros Real World Performance

Method Selection

• Hadamard beamforming weights– Full spatial coverage

• Kasami psuedo-orthogonal coding– Encode base station ID and user ID– Low (bounded) streaming correlation

37

’’’’’’

Base-station LocationsUser Locations

68 Indoor User Locations32 Anechoic User Locations

5 Base-station LocationsOver 1, 400,000 Measurements

38

Faros Drastically Increases Range

39

Method

Beaco

n S

weep

s D

ete

cted (

%)

Faros Decreases Paging Delay 4x

40

Delay (Frames)

CD

F of

Firs

t D

ete

ctio

n (

%)

Broader Implications

• 250 m range line-of-sight with 10 mW power

• Used as realtime framework for Argos

• Faros allows space-time-code resources to be traded off for desired performance and coverage 41

Conclusion

• Faros is a highly efficient from-scratch control channel design for MU-MIMO

• Faros operates in realtime and provides over 40 dB of gain on a 108-antenna array

• Faros solves a critical barrier to the implementation and adoption of massive MIMO 42http://

argos.rice.edu

Acknowledgements

Abeer Javed Lin Zhong

Eugenio Magistretti

Evan EverettHang Yu

43http://argos.rice.edu

Faros Control Channel Design

Highly-Efficient Design

Realtime Implementation

Solves Critical Barrier

44http://argos.rice.edu

Bonus Slides

Single User Beamforming Gain Gap

Single Antenna

MU-MIMO

Single User Beamforming

47

Traditional SyncClient Faros

MU-MIMO

What about the users?User Power ≈ Base Station Power

48

Gain Gap Characterization

M Base Station Antennas

K Users

PBS Power of Single Base Station Antenna

PU Power of User Antenna49

Soft Association

Random Access Request (CSI)

Random Access Request (CSI)

Faros Drastically Increases Range

51

Faros Provides Over 40 dB Gain

52

Faros Reliably Corrects CFO

53

Faros Example

54

Synchronization

• Widely adopted/efficient technique is correlation

• Where R is the received samples and S is the transmitted sequence

55

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