Enabling High-Quality Untethered Virtual Reality (MoVR)

Abari et al., USENIX NSDI ‘17

Motivation/Introuction

• Streaming Virtual Reality (VR) systems
  • Multi-Gbit/s data rates from game console to wearable headset
  • 100 frames per second frame rate to make it realistic, avoid nausea
• Replace cable from console to headset with RF link
  • mmWave: only practical RF technology that supports those rates
    • > 24 GHz, 24 GHz in this paper
    • Highly directional, like light
    • Problem: Blocked by hand, head, obstacles, like light
  • Challenge: User is mobile, creates dynamic blockages
• Interest from Google, many other smaller companies

Design (§3)

• Overview shown in Figure 2: wall-mounted mmWave relay module (referred to as the “mirror” throughout)
  • AP chooses to send direct (above), or thru relay if direct path blocked
• Quantifying blockage (Figure 4)
  • Hand: -14 dB, head: -20 dB, body: -25 dB
  • Status quo: AP finds a “bounce” path off a wall or object (best case of that is -15 dB, since longer distance and bounce scattering)

MoVR Mirror Design (§4.2)

• Overall design (Figure 6): receive antenna to amplifier to transmit antenna
  • Phased array antennas for both transmit & receive
    • Create narrow transmit, receive beams
  • No radio, just a relay
  • Problem: Some signal transmitted from mirror is received by mirror, resulting in feedback loop that saturates the amplifier, generates garbage (Fig. 6b, right)
  • So set the amplifier so that forward gain minus leakage is negative, so the feedback loop dies away
    • So need forward gain less than leakage
    • But leakage varies with angles of antennas (Figure 7)
    • So need adaptive algorithm to set the forward amplifier gain
  • Challenge: No radio to measure signal levels on the mirror
  • MoVR approach: Monitor current draw of the amp, increase gain continuously until high current draw indicates saturation, then back off the gain. Repeat continuously.

Alignment and Tracking (§§5.1, 5.2)

• Very sensitive to alignment (-20 dB for 10 degrees misalignment as shown in Figure 9), so need precise alignment

  • For Direct Mode: Need AP to headset alignment
  • For Relay Mode: Need AP to mirror, and mirror to headset alignment

• b) AP to headset alignment:

  • Just leverage VR laser tracker: gives location and orientation

• a) AP to mirror alignment:

  • Challenge: The mirror can’t receive, just relay
  • AP transmits on some frequency, mirror reflects back to the AP on a neighboring frequency, AP measures power of the reflection
    • Frequency shift overcomes problems with listening and transmitting on the same frequency
    • Need two angles: Angle at AP \(\theta_1\), angle at mirror \(\theta_2\)
      • Exhaustive two-dimensional search in those two angles to maximize power

• c) Mirror to headset alignment:

  • Leverage AP to headset alignment from VR laser tracker, from step (a) above
  • Then, translate coordinates from AP to mirror: have angle from step (b) above, need distance as well to translate coordinates
  • Approach: Triangulate mirror location as shown in Figure 10
    • \(\phi_{AP}\) is \(\theta_1\) from previous step
    • \(\phi_H\) is found by a similar two-dimensional exhaustive search with AP sending to mirror, searching over mirror and headset angles

Evaluation (§7)

• HTC VIVE VR system hardware with custom mirror hardware in a 5 meter by 5 meter room
• LoS blocked 20 times over 5 minutes while playing a game (Fig. 12 shows blockage durations, 245 milliseconds median, i.e. 25 frames, so would cause a noticeable glitch)
• Mirror Performance (no blockage = 0 dB reference level in Fig. 13)
  • Compare no blockage versus blockage without MoVR
    • Fig. 13: -27 dB loss from bouncing off a wall
  • Compare no blockage versus blockage with MoVR
    • Fig. 13: Improvement in SNR from amplification, shorter paths
  • Why sometimes 3 dB worse? No explanation, possibly amplifier saturation, beam alignment problems
• Beam Alignment and Tracking Performance (§7.3)
  • Two degree accuracy for AP to mirror beam alignment (Fig. 14), ground truth coming from laser distance measurement tool
  • Whole system (Fig. 15) versus exhaustive search in all three angles: MoVR 4 dB worse sometimes about the same number of times to possibly explain Fig. 13
• Beam Alignment Latency
  • Important: needs to be fast to track – claimed 0.9 \(\mu\)s to align, 1.7 \(\mu\)s to reconfigure phase shifters, compared to 1 ms delay in the VR tracker so MoVR is much faster
• System Performance (§7.4)
  • Compare no mirror (bounce off walls), fixed gain mirror, MoVR with LoS path blocked by hand
  • Where’s the AP in Fig. 16?
  • Better with MoVR’s adaptive gain control