Transcript Document

System Architecture and Cross-Layer
Optimization of Video Broadcast over
WiMAX
CMPT 820
Bob McAuliffe
July 24, 2008
Reference
System Architecture and Cross-Layer Optimization of Video
Broadcast over WiMAX
Jianfeng Wang, Muthaiah Venkatachalam, and Yuguang Fang
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Outline
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Introduction
Overview of WiMAX MBS and issues
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MBS -> multicast / broadcast service
Proposed end-to-end solution
Optimization methodology
Results
Conclusions
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Introduction
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Mobile WiMAX (802.16e) operation
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Broadcast TV requires multi-BS operation
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Wireless mobile TV
WiMAX defines only MAC/PHY of wireless link
Synchronization issues
Current
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MBS to BS (base stations)
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transport protocol - RTP / UDP / IP
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Introduction (2)
Areas for improvement…
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Smoothen quality
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during MSS movement
during handoff in Multi-BS environment
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Capacity improvements
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Channel switching time
Synchronization
Spectrum efficiency (number of TV channels)
Increased coverage area
Power efficiency improvement
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Introduction (3)
Viable end-to-end solution proposed
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From MBS Controller
Through BS
To MSS (mobile subscriber stations)
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Overview of WiMAX MBS
and issues
Overview of WiMAX / MBS
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WiMAX / MBS is used as a baseline
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MBS constructs H.264/AVC frame
MBS to BS
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Multiple Base Stations (BS)
Multiple ASN GW (access service network gateways)
H.264/AVC over RTP / UDP / IP transport
OFDMA frame used
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BS to MSS (wireless)
broadcast payload placed in DL (downlink) sub-frame of
OFDMA frame
OFDMA time division duplex used (TDD)
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OFDMA frame structure
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MBS payload
contained in DL
sub-frame
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Multiple MBS
zones supported
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DL MAP contains
multiple
MBS_MAP_IE (info
elements)
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MBS_MAP_IE
allows support for
multiple channels /
multiple layers
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Baseline System
H.264 / AVC
RTP/UDP/IP
transport
RTP/UDP/IP
transport
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MSS operation (baseline)
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MSS reads DL-MAP to determine;
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MBS MAPS
MBS Zones
MBS MAPS point to subsequent MBS MAPS
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Issue #1 – Synchronization
Difficult to achieve
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OFDMA Frame synchronization problems because;
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Each BS makes its own scheduling decision
Each BS independently constructs its own OFDMA
frame
OFDMA frames need to be the same across multiBSs in same geographic zone
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Macro-diversity
Reduced interference
Smooth hand-off
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Issue #2 – Error Protection
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No outer coding in baseline system
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No unequal error protection
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video frame errors not handled
access unit errors not handled
Reduced video quality (during interference or
fading)
More important to preserve video base layer
MAC/PHY error handling only
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Required, but result is low spectral efficiency
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Issue #3 – BS buffer overflow
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BS may have to drop video packets
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Random drop is undesirable
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Buffer overflow
Packet drop is random
Reduced quality
Varied quality
Preferred to drop packets of lower
importance first
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Issue #4 – Energy efficiency
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Burst transmission is not utilized
Burst transmission
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Aggregation possible within single channel
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Used for wireless links to conserve energy
The aggregation of multiple MAC PDUs for simultaneous
transmission
MSS placed in idle state when ever possible
Some / with caution
Ideal for multiple TV channel aggregation
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Simultaneous TV channel broadcast
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Issue #5 – Packet overhead
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Significant packet overhead
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between MBS and BS
RTP, UDP, IP
Approximately 40 bytes per packet
Header compression
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Significant RTP/UDP/IP header reduction is possible
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Proposed end-to-end solution
Key improvements
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Broadcast Synchronization through MBS – BS
cooperation
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RS outer error coding and CTC inner coding used
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Reduce error rate with minimal overhead
Temporal scalability and unequal error protection
Power efficiency improvements
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Same content transmitted from BSs at same time
Burst based multiplexing (channel aggregation)
MSS decodes only needed channel
Header compression reduces burst size
Security
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Encryption to prevent unauthorized viewing
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Proposed end-to-end solution
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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”
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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”
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Receives MBS_MAC_PDUs over wireless link (OFDMA)
De-encapsulates RTP packets (containing H.264/AVC
video)
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Proposed end-to-end Solution
RTP
RTP
MBS_MAC_PDU
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BS operation
contains CID and MCS
for MBS_MAC_PDU
Received from
MBS Server
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BS WiMAX
interface
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MBS_MAC_PDUs
queued and
mapped into
OFDMA frame
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Each
MBS_MAC_PDU is
unique to one
channel
OFDMA Frame
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MBS - Server side
Server Side “MBS-enhanced Transport-sublayer”
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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
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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
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MBS – Client side
Client side “MBS-enhanced Transport-sublayer”
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Receives MBS_MAC_PDUs
over wireless link (OFDMA)
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Decodes only those for the
required channel
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CTC checked
Based on multicast ID (CID)
determined by channel switcher
RS error correction
Decryption
De-encapsulates RTP packets
(containing H.264/AVC video)
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Burst scheduling
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One Burst
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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
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Channel switching (MBS client)
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Improved energy efficiency
MBS client operation
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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
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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
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GOP structure and RTP aggregation
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Improved packet drop handling
GOP structure – I0p1P2p3P4p5P6p7P8p9
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One Base layer - I0P2P4P6P8
One enhancement layer - p1p3p5p7p9
Multi-time aggregation used (RFC3984)
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One RTP packet contains entire base layer of one GOP
(multiple access units)
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More robust error coding applied
Another RTP packet contains entire enhancement layer of
one GOP
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Less error coding
First to be dropped on buffer overflow condition (at BS)
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RS coding / decoding
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Reed-Solomon (RS) error coding
Robust error recovery
RS outer coding applied to each RTP packet
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RTP packet fragmented into M “sections” (SDUs)
N-M Parity “sections” appended (parity SDUs)
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More robust FEC (larger N) applied to base layer RTP
packets
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Where N is the total number of RS sections
Unequal error protection
CRC applied to each SDU and parity SDU
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Efficient for MBS client to detect SDU errors
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MBS_MAC_PDU construction
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MAC header
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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
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Synchronization across multi-BS
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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
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Handoff / Low power mode
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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
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Optimization methodology
Optimization approach
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Goal: to balance the following characteristics:
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Video Quality
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Spectral efficiency
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Represented by effective frame rate (EFR)
Measured as number of channels supported
Coverage
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Distance (size of the cell)
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Video Quality
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Use EFR (effective frame rate) as a measure of quality
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The rate of correct frame decoding at the application
Factors influencing EFR (quality)
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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
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Optimization (quality vs capacity)
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Optimization (quality vs spectral efficiency)
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Determine minimum EFR requirement (i.e. base
layer only) - at cell edge (EFRmin)
Determine maximum K (channels), while
maintaining EFRmin
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Results
Test Environment
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Fixed parameters
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RF environment
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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
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Test environment (2)
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Parameters selected to allow the following;
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targeted cell radius of 2 km
MSS mobility of 30 km/h
Smaller number
indicates more
robust
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Increased coverage performance
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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
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Increased capacity performance
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With same RS error coding rate, and RTP/UDP/IP header compression (left)
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Reduced RS coding on enhancement layer, further reduction on base layer, and
RTP/UDP/IP header compression (right)
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47% increase in channel capacity
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
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Conclusions
Conclusions
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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
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Questions ?
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End
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