MIMO: Challenges and Opportunities
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Transcript of MIMO: Challenges and Opportunities
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
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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 )
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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
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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