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Harris Corporation
2-1
Intraplex Products
Section 2 – Functional Design
2.1 General System Description
The HD Link studio-to-transmitter link (STL) is a transmitter and receiver pair that transports high
quality audio using digital technology over a microwave radio path. Designed for both current analog
and HD Radio
®
, STL applications, the HD Link system can transport multiple channels of high quality
linear (uncompressed) audio or Enhanced apt-X compressed audio, along with HD Radio and other IP
and Ethernet traffic.
In the U.S., FCC Rules Part 74, Subpart E, authorizes a 944-952 MHz band for over-the-air transport
of audio program material. Commonly licensed bandwidths include 200, 300, and 500 kHz. The
theoretical maximum amount of data carried depends on the type of modulation chosen for the link.
In Canada, the 953-960 MHz band is used for the same purpose with bandwidths of 250 and 375 kHz.
The HD Link system supports all these frequencies and bandwidths with a selectable radio frequency
(RF) power output from 1 to 5 watts.
The HD Link digital radio uses digital quadrature amplitude modulation (QAM) with 32, 64, 128, or 256
QAM and low-density parity check (LDPC) coding type forward error correction (FEC). This modulation
and advanced FEC provide very high spectral efficiency (amount of data sent over a given RF
bandwidth), robustness, and high RF gain. This state-of-the-art radio technology provides the highest
performance RF STL available today.
2.2 High Definition (HD) STL Development
2.2.1
Modulation
Analog 950 MHz studio-to-transmitter links transport the baseband FM composite signal. A digital STL
offers significant advantages of flexibility, reliability, and quality. It can multiplex multiple user
channels—such as high quality audio, data, and Ethernet traffic—over the same 950 MHz RF link. The
digital component makes multiplexing seamless; there is no crosstalk or degradation between
adjacent channels. A digital STL also gives errorless end-to-end data transport, unlike analog STLs,
which have some degradation of the RF signal.
The quadrature amplitude modulation (QAM) scheme is widely used in digital STLs and other
communication applications. In digital telecommunications, QAM data is sent as binary symbols in a
grid with the number of points in the grid equaling a multiple of 2 (2, 4, 6, 8, and so on). Here are
common QAM forms:
●
32-QAM
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64-QAM
●
128-QAM
●
256 QAM
By moving to higher-order symbols or constellations, you can transmit more bits in the same RF
bandwidth. In doing so, however, the mean energy between points in the QAM constellation becomes
closer together and more susceptible to noise. So, a higher order QAM can deliver more data than a
lower-order QAM but results in a lower signal-to-noise ratio (SNR).
In a practical QAM application, periodic pilot symbols are inserted in the payload. These pilots carry no
user data and are used for enhanced phase noise immunity and improved adaptation or acquisition by
the receiver. Carefully designing and optimizing the RF receiver can reduce the overhead used for pilot
symbols.
When a digital 950 MHz STL carries Ethernet data to broadcast HD Radio, some problems arise. The
next subsections discuss possible problems and how the HD Link system addresses and corrects them.