Intraplex® HD Link™

The ability to do more
Our RF and audio engineering teams designed HD Link for maximum reliability and to carry higher-quality audio and more data over the STL path than was possible with earlier 950 MHz STLs.

• More power
  Sometimes you just need a little more RF power to get the job done. Out of the box, HD Link can operate at 1 W, 2 W, or 5 W, selectable in the field. It’s the most powerful 950 MHz digital STL you can buy.
  
• Advanced error correction
  HD Link is the only radio STL using LDPC coding for forward error correction (FEC). This highly efficient scheme contributes to increased data throughput.
  
• 21st-century design
  By applying Harris know-how to the technologies not available when legacy STLs were designed, we optimized HD Link for state-of-the-art performance with minimal circuit noise.

The advantages to you
The combined benefits of more transmitter power, LDPC error correction and enhanced circuit design add up to as much as 10 dB signal improvement over older digital STLs. Depending on your requirements, this can enable any or all of the following:

• Use of smaller antennas, for less tower load and lower costs
• Increased path distance
• Improved fade margin
• Operation at higher quadrature amplitude modulation (QAM) orders for increased carrying capacity

Sounds great
As with any Intraplex product, audio quality is a top priority. Uncompressed audio trans-port is fully transparent. In the event bandwidth limitations require audio compression, HD Link offers Enhanced apt-X® coding to prevent the occurrence of audio artifacts that can occur with multiple generations of MPEG compression in the air chain.

Audio and data. And more data.
The architecture of HD Link starts with an integrated IP gateway, the first in a radio STL. This provides you with options for nearly every practical combination of audio, data, control and status used by radio stations — as well as future media applications. With the HD Link system’s 1536 kb/s of allocatable IP data, you’ll no longer consider your STL to be merely an audio transport system that happens to provide some data options. In addition, an asynchronous RS-232 serial data channel accompanies each main program audio channel for use with legacy control systems.

IP done right — the Intraplex way
If you like what Intraplex has done for you with T1 and IP audio and data codecs, you’ll love our implementation in HD Link. The integrated IP gateway includes numerous features:

• IP prioritization allows it to carry other IP data, such as control signals and LAN traffic, separately from the high-priority HD Radio stream.

• Dual domain access control provides the tightest, most practical security. One access list is for control of the HD Link units. Another manages the firewall that keeps unwanted traffic off the transport link.

• Layer 3 switching supports an asymmetric TCP return path, allowing integrated “plug and play” support for TCP wherever a return path exists. Using TCP allows quality HD Radio transport under 30 times higher bit error rate (BER) conditions than UDP (3x 10-4 for TCP versus 10-5 for UDP).

• IP tunneling allows TCP return packets with private addresses to traverse third-party networks like ISPs.

HD Link IP Gateway Architecture

How HD Link Technology Improves Link Quality
Like most digital microwave systems, HD Link uses quadrature amplitude modulation to maximize data throughput. All other factors being equal, higher-order QAM can deliver more data using the same RF bandwidth than lower-order QAM (Figure 1), but each step up in QAM order requires about 3 dB to 4 dB better signal-to-noise ratio on the RF link to maintain the same level of quality. RF link quality is thus a critical factor in determining the amount of data that any given 950 MHz STL can carry.

Forward error correction is a useful tool for improving link quality in digital transmission. Essentially, it involves adding redundant data to the transmission stream to allow the detection and reconstruction of missing information. All digital RF STLs use some form of FEC, typically using either the Reed-Solomon or the Viterbi coding method.

However, adding FEC increases the bit rate of the STL. Adding enough extra bits may require the use of a higher QAM order, which places greater demands on the link qual-ity, thereby creating a Catch-22 situation — adding FEC to improve link quality may cause an increase in QAM order, which requires yet higher link quality.

Figure 1. Higher-order QAM levels enable greater data carrying capacity

With HD Link, Harris has taken significant steps to improve this situation.

First, HD Link uses LDPC to do its FEC coding. LDPC is more efficient than either Reed-Solomon or Viterbi, requiring less overhead data to provide the same level of er-ror correction; given the same overhead, it typically provides about 3 dB improvement in link performance over traditional FEC methods (Figure 2). While the mathematical principles underlying LDPC have been understood for some time, only recently have DSP chips become available with the speed and processing power to run LDPC on digital STL signals in real time.

Figure 2. The HD Link system’s advanced error correction handles difficult links
better than STLs using older technology

In addition, whereas the performance of Reed-Solomon FEC falls off directly if the link quality is degraded due to environmental or other interference, LDPC maintains supe-rior performance in the face of link deterioration.

Further, where other STLs add a fixed level of FEC regardless of the system configura-tion, HD Link uses adjustable amounts of FEC and employs a sophisticated algorithm to calculate the optimum balance between QAM order and FEC overhead to achieve the maximum data throughput for any given configuration.

The result? Combined with the increased performance afforded by its 5 W amplifier, HD Link can provide up to 10 dB of system enhancement, right out of the box, compared with older digital STL systems (Figure 3).

 
Figure 3. The combination of HD Link technology advantages can offer you
a dramatically improved STL system.

Elegantly simple installation, configuration and operation
Your time is valuable, so HD Link is designed to make setup quick and painless. The front-panel display is bright and clear, with easy-to-understand labels. Menu hierar-chies are shallow, so accessing most functions takes only a few steps.

Modem settings automatically synchronize between the transmitter and receiver units. With both units set for the same frequency and RF bandwidth, changes to QAM, IP bandwidth, etc., on the transmitter automatically sync to the receiver.

HD Link has tools to help you in planning your overall STL system. Select the ac-tive audio channels and IP bandwidth you want it to carry, and HD Link automatically calculates the optimum settings for QAM and forward error correction, along with the receive signal strength necessary to achieve the desired level of throughput. These auto-calculated values enable you to prepare a reliable path calculation and determine the requisite transmitter power, antenna size and other variables.

HD Link provides numerous interfaces for setup, diagnostics and updates, with Ether-net and USB ports on each unit, plus remote access via HTTP, FTP and SNMP.

Additional convenience features include a headphone jack and AES/EBU sync port on the receiver, and variable-speed fans to minimize noise. And HD Link is physically com-pact at only two rack units per end.

Built-in backup
When your HD Link is connected to a bidirectional IP network with sufficient bandwidth, the program channels will automatically switch to IP should the 950 MHz link fail. In ef-fect, HD Link is both a 950 MHz RF STL and an IP audio STL in one box, with automatic fallback from one to the other.