Digital Signal Processing Reference
In-Depth Information
21.1
Introduction
The strong evolution of inter-vehicle communications in the Intelligent Transportation
System (ITS) sector, along with the widespread adoption of portable devices equipped
with IEEE 802.11 wireless interfaces, fosters the deployment of innovative wireless
communication services based on the real-time streaming of multimedia flows.
Inter-vehicle multimedia streaming has countless applications, ranging from safety
services to collaborative driving and generic value-added services such as advertising
and infotainment.
However, the high variability of intervehicle communication channels based on
the IEEE 802.11 standard makes the transmission of real-time multimedia informa-
tion a very challenging problem [
1
]. Among the main drawbacks of streaming
applications over VANETs is the high percentage of packet losses which can be
experienced over the wireless channel [
2
]. Even if multimedia information is
tolerant to some packet losses, high losses do not actually allow the faithful
reconstruction of the media with respect to its original version, thus not
guaranteeing the quality necessary for object and speech recognition algorithms.
In this chapter, we address the problem of protecting real-time multimedia
communications over intervehicle networks to guarantee the necessary quality for
sophisticated multimedia signal processing techniques.
Let us consider the following scenario, depicted in Fig.
21.1
, where a video-
communication software is installed in two cars that are going over the same path.
The front car is transmitting real-time video information to the back car. As cars
move along the path, the wireless channel experiences noise due to environmental
elements, thus suffering from multipath fading. It causes variable bit error rate,
depending on several parameters such as the distance between the two cars, the
presence of objects between the cars, and the relative speed. Certain combinations
of these parameters can also generate very long bursts of packet losses resulting in
intermittent connectivity. When it happens, the real-time transmission of the con-
sidered video flow is unfeasible, unless appropriate counteractions are taken.
Packet-level Forward Error Correction (FEC) techniques are able to recover
packet losses without resorting to packet retransmission requests (which would
generate too high delays in real-time constrained scenarios). Packets to be trans-
mitted are grouped in blocks, and their loss can be recovered until the packet loss
rate of a given block exceeds the percentage of redundant packets inserted. If made
aware of the channel conditions, the sender can then adapt the percentage of FEC to
match the actual channel conditions.
This mechanism works well with the assumption of uniformly distributed losses.
However, VANET transmissions heavily suffer from burst losses that strongly impact
the possibility to recover data packets belonging to such bursts. To overcome this
problem, in this chapter, we resort to packet-level interleaving to split long consecu-
tive error bursts into smaller lost packets sequences. With adequate constraints, we
show that a joint FEC/interleaving technique is able to consistently improve the
transmission quality while respecting real-time constraints. We then present the
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