2 – Functional Design
HD Link Installation & Operation Manual
Version 2, April 2010
Harris Corporation
2-3
Intraplex Products
There are several ways to increase a given RF link’s performance:
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Using a more powerful RF transmitter and/or higher gain antennas
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Optimizing the RF hardware circuits to
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Minimize circuit noise
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Reject unwanted signals
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Maximize receive signal sensitivity
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Adding forward error correction (FEC)
The first two ways are common to any RF system and present no special challenge in the design of a
combined FM/HD transport system. FEC essentially sends additional information on the link along with
the payload (audio and data) information and uses this additional information to detect data errors
and rebuild any missing information at the receive end. There are several types of error correction
available. However, FEC adds overhead to the total data transport requirement. When trying to
compensate for poor link quality caused by using high QAM orders, and using those high QAM orders
to carry more information, adding FEC can hinder the information rate gains. Therefore, the key is to
use the most efficient form of FEC possible, with efficiency defined as the ability to correct the
maximum amount of errors while sending the minimal amount of FEC overhead. One of the most
efficient FEC schemes is low-density parity-check (LDPC) codes or turbo codes.
In information theory, the noisy-channel coding theorem (called Shannon’s theorem) establishes that,
however contaminated a communicate channel is with noise interference, it is possible to
communicate digital data information nearly error free up to a given maximum rate through the
channel. The Shannon limit of a communications channel is the theoretical maximum information
transfer rate of the channel for a particular noise level.
While LDPC and other error correcting codes cannot guarantee perfect transmission, the probability of
lost information can be greatly lessened. LDPC was the first code to allow data transmission rates
close to the theoretical maximum, the Shannon limit. In fact, it has been shown that LDPC codes can
reach within 0.0045 dB of the Shannon limit.
In contrast to less efficient error correction schemes, such as Reed-Solomon, LDPC offers a clear
advantage in terms of performance for a given signal-to-noise ratio. Specifically, for equal amounts of
FEC overhead, LDPC requires 3 dB to 5 dB less signal for the same BER performance than does Reed-
Solomon. This difference can translate to using the next-higher QAM order while maintaining the same
BER and RF power level.
2.2.5
Data Quality
2.2.5.1 Network Layer
One of the core components of the HD Link system is its network layer capability, which works with
data bandwidth management to determine the quality of the data information transfer. The network
layer set of functionalities meets challenges of a typical STL transport application and integrates
advanced data networking features, thus providing a reduction in capital and operational expense. The
HD Link system contains an IP gateway architecture that support bandwidth management to efficiently
prioritize and transport critical and non-critical traffic across the STL system and be able to operate in
one-way as well as full-duplex link configurations.
Figure 2-1 depicts a high-level view of the functionalities in the HD Link system’s IP gateway. In
addition to core features, the platform manageability of this gateway contributes directly to minimizing
the device’s operational cost. The system’s support of standard network management protocol
functions (such as HTTP, SNMP, FTP, Telnet, and ICP), allowing for more effective troubleshooting and
management.