In Broadcast - July 2013 - 54

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WWW.INBROADCAST.COM | ISSUE 21 - JULY 2013

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InTECH

ABonAir Part II – Wireless Video Links
Continuing our review of microwave video links, we take a closer look at the issues surrounding
uni-directional / bi-directional technologies and transmission range...

By Eran Igler
CEO, ABonAir

I

54

n our previous ar ticle we
reviewed different wireless video
transmission technologies that
allow camera teams to transmit
video content from cameras back
to outside broadcasting vans
(OB-vans) or central stations.
Broadcasters increasingly
appreciate the value of wireless
links to create attractive, compelling
content along with easy video
shooting. For this reason the use of
wireless link solutions is on the rise.
In our last issue’s overview,
we discussed the way different
technologies suit different broadcast
uses, and indicated a set of buyingdecision parameters: transmission
stability, picture quality, compression,
licensed / un-licensed frequencies,
delay, uni-directional / bi-directional
technologies and transmission range.
The last two parameters will be
elaborated in this article.

Uni / Bi-Directional
Technologies
Most microwave broadcast
systems today are based on unidirectional (or half-duplex) RF links,
where the transmitter sends video
content to the receiver and does
not receive any feedback about the
transmission quality. Uni-directional
systems have no acknowledgement
conﬁrmation or error notiﬁcation
from the receiver.
With no content conﬁrmation or

acknowledgment mechanism, unidirectional links cannot identify
errors while they occur, and so they
need to allocate large amount of the
bandwidth for redundant information
transmission. The redundancy
enables the receiver to correct some
of the errors in the received data and
improve the overall error rate to an
acceptable level.
In bi-directional links, however,
both transmitter and receiver can
send and receive data and control
information. The technology enables
the receiver to acknowledge each
pixel transmitted over the air
and thus guarantees zero errors
in transmission. The packet
acknowledgment mechanism
identiﬁes errors and quickly ﬁxes
them (by re-transmitting the
corrupted data) before the frame
is presented.
By acknowledging the correct
acceptance of each packet of
pixels, full-duplex systems enable
continuous and reliable content
transmission and allow broadcast
professionals to rely on stable and
accurate transmission.
In addition, bi-directional links
double the system efﬁciency and
provide higher net throughput
since there is no need to constantly
transmit redundant data, with only
error information being re-sent.
For example: let’s assume we
have a bandwidth of 20Mbps and

Outdoor applications require large transmission range. ABonAir exclusively
enabled a live coverage of the Israeli Parliament ceremony

a typical 1% PER (Packet Error
Rate). A uni-directional transmitter
would need approximately half of
the bandwidth for redundancy, or:
FEC = ½ (FEC is the technical term
used for redundancy – Forward Error
Correction) as the system cannot
identify when in time errors occur
and is forced to send the redundant
information of all the data.
So with 20Mbps available
bandwidth, the actual bit rate
decreases to only 10Mbps
because the system wastes 50%
on redundancy (FEC = ½), even
though there is only 1% error rate!
In bi-directional solutions the
numbers are totally different.
Since we know that errors occur
only 1% of the time, and we
know exactly when they occur
(due to the acknowledgment

mechanism), we then need to save
only 1% of the original bandwidth
for re-transmission of the wrong
information. It means that from
20Mbps, as much as 19.8Mbps
can be utilised to transfer the data
and only 0.2Mbps is dedicated to
error correction.
This high efficiency provides
two valuable advantages: higher
picture quality – since the available
bandwidth for compressed video is
almost double – and superior link
robustness, as simpler and more
robust modulation can be used due
to the relatively smaller bandwidth
requirement.
Bi-directional technology also
enables fully automated system
conﬁguration based on changing
conditions to ensure robust link
performance - for example, since
there is full communication between
the transmitter and receiver, both
can identify new interference in the
frequency they use and decide to
move to a clearer frequency.
In conclusion, bi-directional
solutions assure that all the data
is well received without errors, and
increase efﬁciency, picture quality,
stability and robustness.

Transmission Range
The range between receiver and
transmitter may vary from 10 meters
to 2.5 Km. A few meters range may
satisfy in-studio applications like
video-assist or monitoring, or indoor
sport coverage (basketball, billiard
or ping-pong games) while outdoor
applications like live news broadcast,
open-air sports coverage or
Bi-directional technology enables robust and reliable video links

CONTINUED ON PAGE 56

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