15-10-2012, 02:56 PM
H.264/AVC VIDEO FOR WIRELESS TRANSMISSION
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ABSTRACT
H.264/AVC will be an essential component in
emerging wireless video applications thanks to
its excellent compression efficiency and networkfriendly
design. However, a video coding standard
itself is only one component within the
application and transmission environment. Its
effectiveness strongly depends on the selection
of appropriate modes and parameters at the
encoder and decoder, as well as in the network.
In this article we introduce the features of the
H.264/AVC coding standard that make it suitable
for wireless video applications, including
features for error resilience, bit rate adaptation,
integration into packet networks, interoperability,
and buffering considerations. Modern wireless
networks provide many different means to
adapt quality of service, such as forward error
correction methods on different layers and endto-
end or link layer retransmission protocols.
The applicability of all these encoding and network
features depends on application constraints
such as the maximum tolerable delay, possibility
of online encoding, and availability of feedback
and cross-layer information. We discuss the use
of different coding and transport related features
for different applications: video telephony
and conferencing, video streaming, downloadand-
play, and video broadcasting. Guidelines for
the selection of appropriate video coding tools,
video encoder and decoder settings, and transport
and network parameters are provided and
justified. References to relevant research publications
and standards contributions are given.
INTRODUCTION
Most emerging and future mobile client devices
will significantly differ from those used for
speech communications only: handheld devices
will be equipped with a color display and a camera,
and have sufficient processing power to
allows presentation, recording, and encoding/
decoding of video sequences. In addition, emerging
and future wireless systems will provide sufficient
bit rates to support video communication
applications. Nevertheless, bit rates will always
be scarce in wireless transmission environments
due to physical bandwidth and power limitations;
thus, efficient video compression is required.
Nowadays H.263 and MPEG-4 Visual Simple
Profile are commonly used in handheld products,
but it is foreseen that H.264/AVC [1] will
be the video codec of choice for many video
applications in the near future. The compression
efficiency of the new standard outdoes prior
standards roughly by at least a factor of two.
Although compression efficiency is the major
feature for a video codec to be successful in
wireless transmission environments, it is also
necessary that a standard provide means to be
integrated easily into existing and future networks
as well as address the needs of different
applications.
VIDEO OVER WIRELESS
END-TO-END VIDEO TRANSMISSION
Figure 1 attempts to provide a suitable abstraction
level of a video transmission system. In
order to keep this article focused, we have
excluded capturing and display devices, user
interfaces, and security issues; also, most computational
complexity issues are ignored.
The video encoder generates data units containing
the compressed video stream, possibly
stored in an encoder buffer before transmission.
A wireless transmission system might delay, lose,
or corrupt individual data units. The unavailability
of a single data unit usually has significant
impact on perceived quality due to spatio-temporal
error propagation. In modern wireless system
designs, data transmission is usually
supplemented by additional information between
the sender and the receivers, and within the
respective entities. Abstract versions of available
messages are included in Fig.
WIRELESS VIDEO APPLICATIONS
Ideally, high-quality video transmission would
require high transmission bit rates, error-free
delivery, as well as low and constant channel
delays. Obviously, not all of the requests of the
video application can be fulfilled; one has to live
with the features and limitations of wireless systems
as discussed in detail in [2]. Wireless transmission
systems provide different transmission
modes resulting in different quality of service
(QoS) in terms of supported bit rates, bit rate
variations, delay variations, as well as reliable
delivery. The appropriate selection of transmission
modes, adapted to the considered video
application, is discussed. Furthermore, in Table 1
we also categorize video applications with respect
to their maximum tolerable end-to-end delay, the
availability and usefulness of different feedback
messages, the availability and accurateness of
channel state information at the transmitter, and
the possibility of online encoding in contrast to
pre-encoded content. Typical Third Generation
Partnership Project (3GPP) applications within
each category are mentioned. Especially real-time
services streaming and conversational, but also
broadcast, services provide challenges in wireless
environments, as in general reliable delivery cannot
be guaranteed. The suitability of H.264/AVC
for these services is discussed in the following.
For a review of the application standards and
used protocols please refer to [2].
ERROR ROBUSTNESS SUPPORT USING
H.264/AVC
This section discusses the endpoint operation in
a wireless H.264/AVC video system. The provided
H.264/AVC features can be used exclusively
or jointly for error robustness purposes, depending
on the application. It is necessary to understand
that most codec-level error resilience tools
decrease compression efficiency. Therefore, the
main goal when transmitting video goes along
the spirit of Shannon’s famous separation principle
[13]: Combine compression efficiency with
link layer features that completely avoid losses
such that the two aspects, compression and
transport, can be completely separated. Nevertheless,
if errors cannot be avoided, the following
system design principles are essential:
• Loss correction below the codec layer: Minimize
the amount of losses in the wireless channel
without completely sacrificing the video bit rate.
• Error detection: If errors are unavoidable,
detect and localize erroneous video data.
• Prioritization methods: If losses are unavoidable,
at least minimize loss rates for very
important data (e.g., control).
• Error recovery and concealment: In case of losses,
minimize the visual impact of losses on the
actual distorted image.
• Encoder-decoder mismatch avoidance: Limit or
completely avoid encoder and decoder mismatches
resulting in annoying error propagation.
Use cases of the error resilience features for
specific applications are discussed.