MIMO: Challenges and Opportunities

Lili QiuUT Austin

New Directions for Mobile System Design Mini-Workshop

Motivation

• Benefits of MIMO – Large capacity increase– High reliability

• Challenges in achieving MIMO gain– Power efficiency– Distributed MIMO in WLANs– Distributed MIMO in multihop networks

Model-Driven Energy-Aware MIMO Rate Adaptation [MobiHoc’13]

• Why simple rule doesn’t work?– Highest throughput ≠ lowest energy– One antenna ≠ lowest energy– The min energy rate depends on channel

condition and energy profile of WiFi device

Why Model-Driven?

• Probing may take a long time and may not find the optimal rate by the time channel changes– Probing space is large especially w/ MIMO

• Model-driven– Estimate power consumption for each

rate– Directly select the one w/ lowest power

Measurement-Driven Energy Model

• Etx = a ETT + b

• Erv = c ETT + d

where a, b, c, d depend on the WiFi card Intel Atheros

a 0.24 ntx + 0.425 MIMO + 1.02

0.38 ntx + 0.108

b 0.045 ntx + 0.108 0.040 ntx + 0.062

c 0.30 nrv + 0.61 0.142 nrv + 0.3

d 0.064 nrv + 0.167 0.048 nrv + 0.106

Model Validation

Intel WiFi transmitter Intel WiFi receiver

Error is within 5%.

Model Validation (Cont.)

Atheros WiFi transmitter Atheros WiFi receiver

Error is within 5%.

Energy Aware Rate Adaptation

Measure CSI

Compute post-processed CSI

Compute ETT

Compute energy using model

Select rate with min energy

It reduces energy by 15-40%.

Multi-point to Multi-point MIMO in WLANs [INFOCOM’13]

AP 1 AP 2 AP n…

ClientClient Client … Client

n concurrent uplink or downlink streams

Downlink: Zero-Forcing Precoding

• APs precode the signal so that the receiver can decode it with one antenna

• Each client separately gets its intended signal

[𝑥1𝑥2]=𝐻− 1[𝑝1𝑝2]

[𝑦1𝑦2]=𝐻 [𝑥1𝑥2]=𝐻𝐻−1[𝑝1𝑝2]=[𝑝1𝑝2]Client Client

AP 1 AP 2

𝒙𝟏 𝒙𝟐

h11 h21 h12h22

𝒚𝟏=𝒑𝟏𝒚𝟐=𝒑𝟐

Uplink: Joint Decoding

• APs share their received signals and jointly decode

Client Client

AP 1 AP 2

𝑝1❑ 𝑝2

❑

Share the received signals over the Ethernet

h11 h21h12h22

[𝑦1𝑦2]=[h11 h12h21 h22 ][𝑝1𝑝2]

[𝑝1𝑝2]=𝐻−1[𝑦 1𝑦 2]=𝐻−1𝐻 [𝑝1𝑝2]

Our Contributions

• Demonstrate the feasibility and effectiveness of multi-point to multi-point MIMO on USRP and SORA– Downlink: phase and time

synchronization– Uplink: time synchronization

• Design multi-point to multi-point MIMO-aware MAC

MAC Design

• Medium Access• Support ACKs• Rate adaptation• Dealing with losses and collisions• Scheduling transmissions• Limiting Ethernet overhead• Obtaining channel estimation

MAC Design

• Medium Access• Support ACKs• Rate adaptation• Dealing with losses and collisions• Scheduling transmissions• Limiting Ethernet overhead• Obtaining channel estimation

Medium Access

• 802.11 compatible MAC design– CSMA/CA– A winning AP/client triggers the selected

APs/clients to join its transmission– Trigger frame has NAV set till the end of

data transmission

Supporting ACKs

• ACKs enjoy the same spatial multiplex in the reverse direction

• Downlink – Clients multiplex ACK to APs and APs

jointly decode

• Uplink– APs multiplex ACK to clients via

precoding

Rate Adaptation (downlink)

• Challenges– Receiver receives a combination of

signals from all of the transmitting APs– Per link SNR based rate adaptation does

not work

Rate Adaptation (downlink)

• Error vector magnitude (EVM) based SNR– Distance between the received symbol

and the closest constellation point

Evaluation

• Implementation using USRP and SORA

• Performance evaluation– Phase alignment– Downlink throughput– Uplink throughput– Rate adaptation (downlink)

Downlink Phase Misalignment

0.000.02

0.040.06

0.080.10

0.120.14

0.160.18

00.20.40.60.8

1

Phase misalignment (radian angle)

CDF

Median phase misalignment is 0.078 radianand reduces SNR by 0.4 dB.

Downlink Throughput

Downlink throughput almost linearly increases with # antennas across different APs or clients.

116QAM

216QAM

3QPSK

4QPSK

5BPSK

0

1

2

3individual2x2 downlink3x3 downlink

Location ID

Thro

ughp

ut (M

bps)

Uplink Throughput

116QAM

2QPSK

3QPSK

4 5BPSK

0

10

20

30 individual2x2 uplink3x3 uplink

Location ID

Thro

ughp

ut (M

bps)

Uplink throughput almost linearly increases with # antennas across different APs or clients.

Rate adaptation (downlink)

0 24 48 72 96 1201441681922162402642880

1

2Best fixed ESNR

Packet Trace Index (x 20)

Thro

ughp

ut (M

bps)

Achieves close to 96% throughput of best fixed rate.

Distributed MIMO in Multihop Wireless Networks

• How to relay signals while achieving spatial multiplexing?

Distributed MIMO in Single-hop Wireless Networks

APs share received signals over Ethernet to jointly decodeClients

Ethernet

Distributed MIMO in Multihop Wireless Networks

• Receivers can’t share received signals for free!• How can they relay signals without decoding them while still allowing the destination to decode?

Distributed MIMO in Multihop Wireless Networks

• How to relay while achieving spatial multiplexing?

• How to select distributed MIMO routes?

• How to design a practical routing protocol?

Thank you!

Challenge of downlink

• Each AP generate signal based on its own clock

• Signals from two APs have different phase rotation

Client Client

AP 1 AP 2

𝒆 𝒋∆𝟏 𝒆 𝒋∆𝟐 [𝑥1𝑥2]=[𝒆 𝒋 ∆𝟏   00 𝒆 𝒋 ∆𝟐  ]𝐻 −1[𝑝1𝑝2]

29 / 40

Handling phase difference

• The reason of different phase rotation: different center frequency offset (CFO) by using separate clock

• How to synchronize it? 1. Measurement of CFO at the receiver side2. Feedback to the transmitter3. Compensation at the transmitter

𝑒 𝑗 ∆1(𝑡 )=𝑒 𝑗2𝜋 𝑓 𝑐❑1 𝑐𝑡

30 / 40

Handling phase differenceCFO measurement and feedback

• AP 1 sends LTS (long training sequence) to clients

• Client measures CFO (carrier frequency offset) based on it

Client Client

AP 1 AP 2

LTS 1

31 / 40

Handling phase differenceCFO measurement and feedback

• AP 1 sends LTS (long training sequence) to clients

• Client measures CFO (carrier frequency offset) based on it

• AP 2 sends LTS to clients• Client measures CFO based on it

Client Client

AP 1 AP 2

LTS 2

32 / 40

Handling phase differenceCFO measurement and feedback

• AP 1 sends LTS (long training sequence) to clients

• Client measures CFO (carrier frequency offset) based on it

• AP 2 sends LTS to clients• Client measures CFO based on it• Client feedbacks them to APs

Client Client

AP 1 AP 2

FEEDBACK

33 / 40

Handling phase differenceCFO measurement and feedback

• AP1 sends precoded signal with phase rotation

Client Client

AP 1 AP 2

𝑥1′ (𝑡 )

34 / 40

Handling phase differencePhase rotation compensation

• AP1 sends precoded signal with phase rotation

• AP2 sends phase rotation compensated precoded signal

=Client Client

AP 1 AP 2

𝑥1′ (𝑡 ) 𝑥2

′ (𝑡 )

35 / 40

Handling phase differencePhase rotation compensation

• Clients receive the signals with unified phase rotation

• Each client separately compensates during its CFO compensation process

Client Client

AP 1 AP 2

𝑝1𝒆𝒋∆𝟏

𝑝2𝒆𝒋∆𝟏

36 / 40

Multi-point to Multi-point MIMO in WLANs [INFOCOM’13]

• Motivation–MIMO promises a capacity increase• 802.11n, 802.11ac, …

– But usually limited by # antennas at a client

–Multi-point to multi-point MIMO achieves a higher capacity and overcomes the limitations