Vislink Strata RX, Technical Reference Manual

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Link Quality D-5
Strata RX
Technical Reference Manual
D.4 Technical Background: LQ
Factors
Each of the factors used to derive LQ is important, but none of
them alone is sufficient to monitor and optimize the quality of a
digital microwave link.
The following sections explain these factors in more detail.
This is technical background information to help you understand
LQ and make best use of its capabilities.
D.4.1 RCL and SNR
In analog systems, efforts to optimize the link quality are usually
directed at improving the received signal strength (RCL), with the
Table D-3: Setting Changes to Improve Link Quality
Link Capability
Least
Robust
Most
Robust
Modulation
64 QAM 16QAM QPSK
FEC
7/8 5/6 3/4 2/3 1/2
GI
1/32 1/16 1/8 1/4
BW
8 MHz 7 MHz 6 MHz
Note - receiver BW must be manually set to match transmitter.
Highest Lowest
Data Transfer Rate
Factory Default Settings are highlighted in greeen.
goal of having a strong signal relative to the RF noise present
(SNR).
However, in digital systems, having a solid RCL with good SNR
is not enough to ensure a good link. If the signal has picked up
errors in the digital data, the link quality can still be poor. Worse,
it can be unstable, with the video appearing and disappearing as
the error sources change over time.
Optimizing a digital link, then, needs to also consider errors in
the data, as measured by the Bit Error Ratio (BER). The ability to
correct these errors will depend in part on the settings chosen for
Forward Error Correction (FEC) and Guard Interval (GI).
D.4.2 FEC
One of the advantages of digital transmission is the ability of the
receiver to automatically correct errors in the incoming data. This
is possible because the transmitter adds extra “error correction”
bits to the data. These extra bits describe attributes of the
original data so it can be reconstructed in the receiver if needed.
The transmitter segments the digital data (the “payload”) into
small groups of bits called packets. The error correction bits are
likewise grouped into packets, which are added into the data
stream. The number of packets allocated to error correction is
controlled by the FEC setting in the transmitter.
• An FEC setting of 1/2 means that 1 packet of payload is
accompanied by 1 packet of error correction. In effect, 1/2
of the packets are payload.
This is the most robust FEC setting, capable of correcting
the most errors. However, it has the lowest data transfer
rate.
• By contrast, an FEC setting of 7/8 would mean that every
7 packets of payload are accompanied by 1 packet of