MIMO: Challenges and Opportunities

MIMO: Challenges and Opportunities LiliQiu UT 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

Model Validation Intel WiFi receiver Intel WiFi transmitter Error is within 5%.

Model Validation (Cont.) Atheros WiFi receiver Atheros WiFi transmitter 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] … • n concurrent uplink or downlink streams … Client Client Client Client AP 1 AP 2 AP n

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 Client Client AP 1 AP 2

Uplink: Joint Decoding Share the received signals over the Ethernet • APs share their received signals and jointly decode Client Client AP 1 AP 2

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

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 Median phase misalignment is 0.078 radian and reduces SNR by 0.4 dB.

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

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

Rate adaptation (downlink) 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 Ethernet APs share received signals over Ethernet to jointly decode Clients

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

Handling phase difference • The reason of different phase rotation: different center frequency offset (CFO) by using separate clock • How to synchronize it? • Measurement of CFO at the receiver side • Feedback to the transmitter • Compensation at the transmitter

Handling phase differenceCFO measurement and feedback • AP 1 sends LTS (long training sequence) to clients • Client measures CFO (carrier frequency offset) based on it LTS 1 Client Client AP 1 AP 2

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 LTS 2 Client Client AP 1 AP 2

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 FEEDBACK Client Client AP 1 AP 2

Handling phase differenceCFO measurement and feedback • AP1 sends precoded signal with phase rotation Client Client AP 1 AP 2

Handling phase differencePhase rotation compensation • AP1 sends precoded signal with phase rotation • AP2 sends phase rotation compensatedprecoded signal = Client Client AP 1 AP 2

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

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

MIMO: Challenges and Opportunities