Opinion / Security applications using digital microwave COFDM modulation techniques

By Nathan Lader

As the television broadcast world moved toward digital transmission, it was only natural that with microwave systems, video security has also migrated to digital technologies. The most successful approach to digital microwave transmission for temporary links has been coded orthogonal frequency division multiplexing, or COFDM.

COFDM was developed in Europe in the 1990’s for terrestrial digital television and soon proved its value for digital electronic newsgathering and other portable, non-engineered link applications, such as surveillance and remote monitoring for patrol.

In situations that require field deployment and mobility in uncertain conditions -- such as border patrol -- COFDM microwave links provide an alternative to IP Wireless or SATCOM technologies. These links can deliver range lengths from several hundred meters (low power) to 50 miles or more (for airborne systems). Range is dependent on power, antenna placement, path, and more.

Older, conventional, single-carrier microwave transmission is susceptible to signal fading caused by multipath reflections when a line-of-sight path is not available. Such fading can cause picture distortion in analog receivers, and loss of demodulator lock in digital receivers.

COFDM modulation can overcome these impairments by using multiple carriers rather than single carrier transmission. Even if multiple carriers are lost due to fading, additional error correction techniques enable a receiver to recover the intended video data.

COFDM offers significant advantages when using microwave links to transport video in portable, field-deployable o non-engineered situations.

COFDM systems:

Deliver robust signals, when analog paths don’t work;
Can be used in remote monitoring, mobile command or airborne applications;
Provide received video signals that are equal to the field inputs. After compression, the video signals retain their carrier-to-noise ratio even as the transmitted signal approaches the threshold of the microwave receiver;
Use less bandwidth than analog techniques;
Provide effective adjacent channel and co-channel interference rejection -- reducing interference with the transmission;
Achieve transmission distances greater than analog techniques;
Enable the application of small format, low-profile, covert systems.

Advancements in COFDM technologies have resulted in microwave systems that are ideal for a growing range of homeland security applications, especially in situational awareness monitoring, by minimizing operational failures and maximizing success rates.

There are still extreme circumstances when sufficiently reliable signals cannot be successfully received by a COFDM receiver. This can happen when the fading results in too many lost carriers, or an insufficient signal level.

To make COFDM reception even more robust, many receive sites now incorporate multiple antennas and receivers. Called diversity or “space diversity receive” systems, they typically have two or more antennas aimed at the same transmitter. The physical separation of the antennas makes it less likely that each antenna would encounter the same multipath reception or fading conditions at exactly the same time.

In recent years, diversity solutions have improved by dedicating a receiver for each antenna, and then using a special algorithm, known as “Maximum Ratio Combining,” to combine multiple received signals in a way that strengthens the overall received signal. These receivers are available in rack-mount, portable (suitcase), and hand-held configurations.

Because the maximum ratio combining approach takes advantage of all received data points from all antennas, it offers significant improvements over other older diversity reception techniques.

Receive signal levels improve as much as 10 dB using a 6-way system instead of a single input receiver. This can make the difference in delivering important surveillance information. This technology is effective in moving applications, i.e. body worn, vehicle or airborne.
 
Security applications face challenges that can limit the quality and reliability of video transport via microwave links. Small transmitters, producing low signal level, can be moving or hidden in locations without direct line-of-sight transmission to the receiver. Receive sites can also be mobile or portable and feature very small antennas, due to concealment requirements.

Without maximum ratio combining techniques, surveillance links can suffer from severe multipath interference and signal fading, to the extent that field data cannot be reliably received. With COFDM and maximum ratio combining, debilitating signal impairments can be successfully overcome.

Security applications utilizing digital microwave techniques typically require creative approaches to overcome less than ideal transmission and reception environments. By designing transmission and reception systems that use COFDM, diversity reception and maximum ratio combining, microwave links for monitoring and patrol are more likely to go farther, with higher reliability and simpler operation than alternative approaches.

Nathan Lader is vice president of government programs for RF Extreme. He can be reached at:
Nathan.Lader@vitecgroup.com