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

CMPT 820Bob McAuliffeJuly 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)

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)

9

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_IE allows support for multiple channels / multiple layers

11

Baseline System

H.264 / AVC

RTP/UDP/IP

transportRTP/UDP/IP

transport

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

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

MBS_MAC_PDU

RTP

22

BS operation

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

contains CID and MCS for MBS_MAC_PDU

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

Possible layer reduction

CID determined

Section Data Units

Ready for OFDMA encapsulation at BS

RTP packets are multi-time and contain only one layer (base or enhanced) for a complete GOP

25

MBS – Client sideClient side “MBS-enhanced Transport-sublayer”

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)

26

Burst scheduling 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

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

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

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

Increased coverage performance

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

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

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