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JPL's Wireless Communication Reference WebsiteChapter: Network Concepts and Standards
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Figure 2: Different system configurations
The operators of vehicle-roadside communication systems require
so-called multi-lane systems, which allow to
pass a beacon site without any restriction
(no reduction of vehicle speeds, lane changing and overtaking
possible etc.) and which enable fast installation (minimal road work,
no physical barriers). In order to realize efficient and reliable
multiple access in a multi-lane vehicle-beacon system,
the interference between vehicles, which need to communicate at the
same time, has to be reduced to a minimum.
System configurations using different
access techniques have been proposed (see figure 2) in order
to separate up-link transmissions from each other.
The Space Division Multiple Access SDMA approach (see figure 2/I) makes use of
antennas with very narrow beams (usually phased arrays),
which are able to provide a lane-specific and
very reliable up-link communication zone.
The size of each communication zone guarantees, that not more than one
vehicle uses a communication zone for up-link transmissions (no data
collisions). Therefore parallel transmissions by vehicles in
different communication zones can be received by the beacon.
In order to avoid interference effects from
adjacent communication zones, the total available bandwidth often
has to be divided into different frequency bands on the up-link,
which reduces the available up-link data rate of each zone.
Using a Random Time Division Multiple
Access (RTDMA) approach, one communication zone is provided for all
lanes (see figure 2/II) and
multi-access interference has to be reduced to a minimum by using
suitable medium access control protocols, which are able to avoid
or resolve data collisions (see next section).
Since there is only one up-link channel,
the full bandwidth can be used, which usually leads to a higher
available up-link data rate than in a SDMA approach.
Shadowing effects, which are caused by vehicles
(especially vans), as well as effects caused by multi-path fading,
can be reduced by optimized antenna configurations using
diversity techniques (e.g. space diversity,
see figure 2/III).
