System Architecture and Cross-Layer Optimization of Video Broadcast over WiMAX

1. System Architecture and Cross-Layer Optimization of Video Broadcast over WiMAX CMPT 820 Bob McAuliffe July 24, 2008

2. Reference System Architecture and Cross-Layer Optimization of Video Broadcast over WiMAX Jianfeng Wang, Muthaiah Venkatachalam, and Yuguang Fang

3. Outline • Introduction • Overview of WiMAX MBS and issues • MBS -> multicast / broadcast service • Proposed end-to-end solution • Optimization methodology • Results • Conclusions

4. Introduction • Mobile WiMAX (802.16e) operation • Wireless mobile TV • WiMAX defines only MAC/PHY of wireless link • Broadcast TV requires multi-BS operation • Synchronization issues • Current • MBS to BS (base stations) • transport protocol - RTP / UDP / IP

5. Introduction (2)Areas for improvement… • Smoothen quality • during MSS movement • during handoff in Multi-BS environment • Channel switching time • Synchronization • Capacity improvements • Spectrum efficiency (number of TV channels) • Increased coverage area • Power efficiency improvement

6. Introduction (3)Viable end-to-end solution proposed • From MBS Controller • Through BS • To MSS (mobile subscriber stations)

7. Overview of WiMAX MBS and issues

8. Overview of WiMAX / MBS • WiMAX / MBS is used as a baseline • Multiple Base Stations (BS) • Multiple ASN GW (access service network gateways) • MBS constructs H.264/AVC frame • MBS to BS • H.264/AVC over RTP / UDP / IP transport • OFDMA frame used • BS to MSS (wireless) • broadcast payload placed in DL (downlink) sub-frame of OFDMA frame • OFDMA time division duplex used (TDD)

10. OFDMA frame structure • MBS payload contained in DL sub-frame • Multiple MBS zones supported • DL MAP contains multiple MBS_MAP_IE (info elements) • MBS_MAP_IEallows support for multiple channels / multiple layers

11. Baseline System H.264 / AVC RTP/UDP/IPtransport RTP/UDP/IPtransport

12. MSS operation (baseline) • MSS reads DL-MAP to determine; • MBS MAPS • MBS Zones • MBS MAPS point to subsequent MBS MAPS

13. Issue #1 – SynchronizationDifficult to achieve • OFDMA Frame synchronization problems because; • Each BS makes its own scheduling decision • Each BS independently constructs its own OFDMA frame • OFDMA frames need to be the same across multi-BSs in same geographic zone • Macro-diversity • Reduced interference • Smooth hand-off

14. Issue #2 – Error Protection • No outer coding in baseline system • video frame errors not handled • access unit errors not handled • No unequal error protection • Reduced video quality (during interference or fading) • More important to preserve video base layer • MAC/PHY error handling only • Required, but result is low spectral efficiency

15. Issue #3 – BS buffer overflow • BS may have to drop video packets • Buffer overflow • Packet drop is random • Random drop is undesirable • Reduced quality • Varied quality • Preferred to drop packets of lower importance first

16. Issue #4 – Energy efficiency • Burst transmission is not utilized • Burst transmission • Used for wireless links to conserve energy • The aggregation of multiple MAC PDUs for simultaneous transmission • MSS placed in idle state when ever possible • Aggregation possible within single channel • Some / with caution • Ideal for multiple TV channel aggregation • Simultaneous TV channel broadcast

17. Issue #5 – Packet overhead • Significant packet overhead • between MBS and BS • RTP, UDP, IP • Approximately 40 bytes per packet • Header compression • Significant RTP/UDP/IP header reduction is possible

18. Proposed end-to-end solution

19. Key improvements • Broadcast Synchronization through MBS – BS cooperation • Same content transmitted from BSs at same time • RS outer error coding and CTC inner coding used • Reduce error rate with minimal overhead • Temporal scalability and unequal error protection • Power efficiency improvements • Burst based multiplexing (channel aggregation) • MSS decodes only needed channel • Header compression reduces burst size • Security • Encryption to prevent unauthorized viewing

20. Proposed end-to-end solution • Additional transport sub-layer implemented on MBS and on MSS (end-to-end) • Layered between RTP and UDP in protocol stack • Server side “MBS-enhanced Transport-sublayer” • H.264/AVC video packets provided (RTP encapsulated) • MBS_MAC_PDUs are prepared for UDP / IP transport to BS • Client side “MBS-enhanced Transport-sublayer” • Receives MBS_MAC_PDUs over wireless link (OFDMA) • De-encapsulates RTP packets (containing H.264/AVC video)

21. Proposed end-to-end Solution RTP RTP MBS_MAC_PDU

22. BS operation contains CID and MCS for MBS_MAC_PDU • BS WiMAX interface • MBS_MAC_PDUs queued and mapped into OFDMA frame • Each MBS_MAC_PDU is unique to one channel Received from MBS Server OFDMA Frame

23. MBS - Server sideServer Side “MBS-enhanced Transport-sublayer” • Map video channel to CID • Shaping to reduce layers (if necessary) • Encryption done on “sections” of AU (access units) • Reed-Solomon (RS) outer error coding applied • Construct MBS_MAC_PDU • Apply CTC inner error encoding (convolutional turbo code) • Burst scheduling (aggregate of multiple TV channels) • Map into OFDMA frame (region allocation) • Buffer for transmission Refer to diagram – next slide

24. RTP packets are multi-time and contain only one layer (base or enhanced) for a complete GOP CID determined Possible layer reduction Section Data Units Ready for OFDMA encapsulation at BS

25. Receives MBS_MAC_PDUs over wireless link (OFDMA) CTC checked Decodes only those for the required channel Based on multicast ID (CID) determined by channel switcher RS error correction Decryption De-encapsulates RTP packets (containing H.264/AVC video) MBS – Client sideClient side “MBS-enhanced Transport-sublayer”

26. Energy efficiency improvement Round-robin channel to channel Determined at MBS server A burst contains many/all channels and multiple MBS_MAC_PDUs per channel Burst size chosen to ensure max efficiency and reasonable switch delay between channels MBS client set to idle mode between bursts Burst scheduling One Burst

27. Channel switching (MBS client) • Improved energy efficiency • MBS client operation • determines desired CID • Looks in MBS MAP of received OFDMA frame (via WiMAX) • Locates MBS_DATA_IEs for new CID • Begins decoding corresponding MBS_MAC_PDUs for new CID • Stops decoding previous MBS_MAC_PDUs • Power not wasted decoding MBS_MAC_PDUs not associated with channel being viewed

28. Channel switching (MBS client) Channel switching time Ti– transmission time for one GOP for channel i Tcs – average channel switching time K – total number of video channels

29. GOP structure and RTP aggregation • Improved packet drop handling • GOP structure – I0p1P2p3P4p5P6p7P8p9 • One Base layer - I0P2P4P6P8 • One enhancement layer - p1p3p5p7p9 • Multi-time aggregation used (RFC3984) • One RTP packet contains entire base layer of one GOP (multiple access units) • More robust error coding applied • Another RTP packet contains entire enhancement layer of one GOP • Less error coding • First to be dropped on buffer overflow condition (at BS)

30. RS coding / decoding • Reed-Solomon (RS) error coding • Robust error recovery • RS outer coding applied to each RTP packet • RTP packet fragmented into M “sections” (SDUs) • N-M Parity “sections” appended (parity SDUs) • Where N is the total number of RS sections • More robust FEC (larger N) applied to base layer RTP packets • Unequal error protection • CRC applied to each SDU and parity SDU • Efficient for MBS client to detect SDU errors

31. MBS_MAC_PDU construction • MAC header • compressed RTP header including sequence number and timestamp • Type of RS section (data or parity) • RS section sequence number, size and code book index • Modulation coding scheme (MCS) used • RS section data • CRC

32. Synchronization across multi-BS • OFDMA frames are the same for all BSs • ODFMA frame numbers allocated at server side • Region of OFDMA for MBS_MAC_PDUs allocated at server side • “schedule-to-transmit” OFDMA frame set by server side • All BSs follow same procedure • Same schedule-to-transmit (determined by server) • Same OFDMA coding and PHY coding • Server sets suitable delay guard • allows time for most/all PDUs to arrive at BS. Those arriving after delay guard are dropped • Synchronization is achieved • All BSs transmit same OFDMA frame at same time • Macro-diversity, smooth hand-off

33. Handoff / Low power mode • Lower power operation / efficient hand-off • MSS registers at BS to join an MBS geographic zone • Security parameters consistent throughout zone • synchronized for effective hand-off • Available channels determined by higher level protocol • Broadcast / multicast service flows maintained even if no active MSS • MSS goes into lower power operation (sleep / idle) • When no video channel being viewed • Between bursts • MSS can migrate to alternate MBS geographic zone • Re-registers at new BS for changed parameters • Less synchronization • Continue receiving same multicast / broadcast content

34. Optimization methodology

35. Optimization approach • Goal: to balance the following characteristics: • Video Quality • Represented by effective frame rate (EFR) • Spectral efficiency • Measured as number of channels supported • Coverage • Distance (size of the cell)

36. Video Quality • Use EFR (effective frame rate) as a measure of quality • The rate of correct frame decoding at the application • Factors influencing EFR (quality) • Distance (d) • Speed (s) • RS section size (L) - base and enhancement layers • RS coding rate (p) - base and enhancement layers • MCS (modulation coding scheme) for base and enhancement layers • CTC inner coding scheme • Base layer frame rate – fb • Enhancement layer frame rate – fe

37. Optimization (quality vs capacity) • Optimization (quality vs spectral efficiency) • Determine minimum EFR requirement (i.e. base layer only) - at cell edge (EFRmin) • Determine maximum K (channels), while maintaining EFRmin

38. Results

39. Test Environment • Fixed parameters • RF environment • carrier 2.5GHz, BW 10 MHz, etc (Ref: table III) • OFDMA slot rate set at 144 kps • H.264/AVC – QVGA 240*320, 30 fps • GOP structure – IpPpPpPpPp • Robust error encoding for MAP_DATA_IE so that error probability is negligible

40. Test environment (2) • Parameters selected to allow the following; • targeted cell radius of 2 km • MSS mobility of 30 km/h Smaller number indicates more robust

41. 178% higher coverage at EFR of 14.5 fps (Ref-1 to Pro-1) 67% higher coverage at EFR of 28.5 fps (Ref-1 to Pro-1) 195% higher coverage at both EFR (Ref-2 to Pro-2) Increases largely due to increased macro-diversity and frequency-time diversity (synchronization) Note: There could be some inconsistencies with this as the Baseline (Ref) parameters are stated here as including RS coding Increased coverage performance

42. Increased capacity performance • With same RS error coding rate, and RTP/UDP/IP header compression (left) • 47% increase in channel capacity • Reduced RS coding on enhancement layer, further reduction on base layer, and RTP/UDP/IP header compression (right) • 38% increase in channel capacity • Note: There could be some inconsistencies with this as the Baseline (Ref) parameters are stated here as including RS coding

43. Conclusions

44. Conclusions • End-to-end solution provides increased macro-diversity • improved synchronization, therefore improved coverage and capacity • Improved hand-off • Improved error coding (2 levels) to reduce error rate while minimizing frame overhead • Temporal scalability and unequal error protection • Provides smoother quality degradation • Therefore greater effective range /capacity • Energy efficiency improvement • Burst based • Increased MSS idle mode

45. Questions ?

46. End