1. Video Capacity of WLANs with a Multiuser Perceptual Quality Constraint Authors: Jing Hu, Sayantan Choudhury, Jerry D. Gibson Presented by: Vishwas Sathyaprakash, Aditya Sharma 1

2. Overview Introduction & Motivation Experiment – Simulation Setup Results of Simulation – Packet Loss and Video Quality – Video Data Rate Suggested Improvements Determining Video Capacity Applications Conclusion 2

3. Introduction Wireless LANs gaining popularity Multimedia - Large part of WLAN traffic How many users can be supported ? Video Quality - as perceived by users Fine Balance - Capacity v/s Quality Authors  Define this fine balance in terms of perceived quality of the video being delivered to r% of the users – Example: Streaming a video in this class 3

4. Motivation Video  Compression  Variable Size & Quality Measuring video quality: – Mean Squared Error & Peak Signal-to-Noise Ratio – Poor co-relation to perceived video quality – HVS : Computationally Expensive Capacity, Encoding and transfer rates – Capacity calculation not defined clearly – Transfer rate/Capacity depends on codec used First effort to relate Quality and Capacity: Not been studied yet 4

5. Overview Introduction & Motivation  Experiment – Simulation Setup Results of Simulation – Packet Loss and Video Quality – Video Data Rate Suggested Improvements Determining Video Capacity Applications Conclusion 5

6. Video Over WLAN: Simulation Setup Video Codec: H.264 – Packetized Video; Many coding schemes/options GOPs of 10, 15, 30, 45 3 Videos: 90 Frames each – Silent.cif; Paris.cif; Stefan.cif 6

7. Video Over WLAN: Simulation Setup WLAN: IEEE 802.11a (5GHz; 54 Mbps) Quantization Parameters (QP): 26 (fine) & 30 (coarse) Payload Size (PS) of 100 and 1100 bytes Noise: Additive White Gaussian Noise (AWGN) Packet Loss Compensation: Base Model – I-Frame: Recovery of MB by Spatial Interpolation – P-Frame: Copying MB from reference frames – Lost Frame: Entire frame is copied Measurements: SNR, PER and Data Rates (DR) 7

8. Overview Introduction & Motivation Experiment – Simulation Setup  Results of Simulation  Packet Loss and Video Quality  Video Data Rate Suggested Improvements Determining Video Capacity Applications Conclusion 8

9. Results of Simulation: Packet Loss (1) Number of Realizations 9

10. CDFs of PER > 0 at 0 realizations CDFs of PER < 1 for realizations <1 Average PER over realizations of multipath fading not an appropriate indicator of channe performance Variation of PER of AWGN channel is less, ranges from 1% to 3% Avg. PER of multipath channels = 5.5%  This represents only a small number of total realizations Avg. PER of AWGN channel is much lower than Avg. PER of channels with fading 10 Results of Simulation: Packet Loss (2)

11. Results of Simulation: Packet Loss (3) 70%: No Packet Loss 90%: Packet Loss < 2% Number of Realizations 11

12. Results of Simulation: Packet Loss (3)…contd. Video: Silent.cif QP = 26, 30 GOP = 15 PS = 100 Fading Channels Thick lines represent Average PSNR ‘+’ marks: 70% of the overlapping realizations with no packet loss Frame Index; QP = 26 PSNR of each frame Frame Index; QP = 30 12

13. Video: Silent.cif QP = 26, 30 GOP = 15 PS = 100 Shows behavior of noise channels (AWGN) Prediction in video encoding causes realizations with similar PER: yet, completely different video quality Results of Simulation: Packet Loss (4) PSNR of each frame Frame Index; QP = 26 Frame Index; QP = 30 13

14. Results of Simulation: Data Rate (1) 14

15. Results of Simulation: Data Rate (2) 15

16. Overview Introduction & Motivation Experiment – Simulation Setup Results of Simulation Packet Loss and Video Quality Video Data Rate  Suggested Improvements Determining Video Capacity Applications Conclusion 16

17. PSNR r,f & Perceptual Quality of Multiple Users MOS r Defined as the PSNR achieved by f% of the frames in each one of the r% realizations f%: Captures the majority of the frames r%: Captures the reliability of a channel over many users Claim: Quality Perception doesn’t change for high ‘f’ Observations behind the claim: – Poor quality frames dominate viewers’ experience – Quality drop in a very small number of frames is not perceivable by the human viewer – PSNR > threshold  Increase in PSNR doesn’t translate to increase in perceptual quality 17

18. PSNR f and Perceived Video Quality (1) Experiment to prove the claim: – Same video sequences played side-by-side – Left: Raw video / Perfect Quality – Right: Compressed Video with recovered packet losses and concealment – 3 humans: Rate the videos from 0 – 100% – Scores plotted 18

19. PSNR f and Perceived Video Quality (2) 19

20. Of all f values, f=90% correlates to best opinion score Mean Opinion Score achieved by r% of the transmissions is given by: Dotted Lines = Average PSNR (existing) Problem with regular PSNR: – Quantitative measure of Quality – Underestimates the quality at high quality levels – Overestimates the quality at low quality levels Thick lines = PSNR f (proposed) – Serves as effective quality measure: correctly estimates low quality and high quality as perceived by HVS PSNR f and Perceived Video Quality (3) 20

21. Overview Introduction & Motivation Experiment – Simulation Setup Results of Simulation Packet Loss and Video Quality Video Data Rate Suggested Improvements  Determining Video Capacity Applications Conclusion 21

22. Video Capacity of WLAN with DCF (1) Thumb-rule: Video frames must arrive at the buffers before the playback deadline. DCF: Distributed Coordination Function: based on CSMA/CA Requirement to know the number of users supported: – Network operators get an idea of number of users that can be supported for identical traffic (capacity planning) – Mix of users having different traffic demands  capacity is approximated to an interpolation of capacity values for each traffic category 22

23. Video capacity with no buffer at receiver – I-Frame and P-Frame sizes differ greatly – What happens when: All users are transmitting I-Frame? - Worst Case All users coordinate I-Frame transmission ? - Best Case Video Capacity of WLAN with DCF (2) 23

24. Video capacity with play-out buffer at receiver (C b ) – Play-out buffer b is the length of the buffer (ms) – It is used only for the frames that have more bits than the other frames (I-Frame) – How buffer length compares with S I /S P : Plot – C b fluctuates between C M and a lower bound given by: Video Capacity of WLAN with DCF (2) 24

25. Length of buffer required for video capacity to reach upper bound, for typical S I /S P values: Video Capacity of WLAN with DCF (2) 25

26. Overview Introduction & Motivation Experiment – Simulation Setup Results of Simulation Packet Loss and Video Quality Video Data Rate Suggested Improvements Determining Video Capacity  Applications Conclusion 26

27. Quality Constrained Video Capacity and its Applications (1) 27

28. Quality Constrained Video Capacity and its Applications (2) 28

29. Observations: – Users watching silent.cif  Excellent Quality – Users watching paris.cif  Average Quality – Users watching stefan.cif  Poor Quality Applications: – Link Adaptation based on capacity – System performance evaluation – Accurate System Design Quality Constrained Video Capacity and its Applications (3) 29

30. Overview Introduction & Motivation Experiment – Simulation Setup Results of Simulation Packet Loss and Video Quality Video Data Rate Suggested Improvements Determining Video Capacity Applications  Conclusion 30

31. Conclusion Average PER / Average PSNR  Not a suitable indicator of video quality They should not serve as the basis for video quality assessment Proposed ‘perceptual’ quality indicator matches the quality with the human vision system’s quality perception Video Capacity with/without buffering Quality Indicator + Video Capacity  design better WLAN communication system with importance to both quality and efficient capacity utilization 31

32. Conclusion (2) Some observations: – Video Quality Perception is a highly subjective test – Only 3 human ‘testers’ considered: Results could vary with more humans testing the theory – Ideal test conditions assumed: Packets/Frames received without errors Collisions are not considered Single Hop networks considered. More Packet errors / losses in multi- hop networks: Losses affect video quality directly – 802.11a has been used for testing; Widespread use of 802.11g/n today: Multipath Fading parameters may change due to operating frequency in 802.11g/n (2.4GHz) + Interference – Little description of actual applications of the proposed method – Portability of proposed method assumed: Same results expected in other video coding methods (MPEG-2) but not proved 32

33. Q & A 33 Note: All images are the property of the respective owners. Images used for non-profit / educational purposes only.