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Examine interoperability issues when selecting airborne downlinks

Posted: 28 Jun 2012     Print Version  Bookmark and Share

Keywords:interoperability  components  downlink system  Digital Video Broadcasting 

Applications of airborne downlink systems are immensely diverse, with users ranging from law enforcement, first-response teams and EMS to fire fighters and border patrol. While its interoperability with other equipment is becoming the key consideration in evaluating a particular downlink system, it's also important to be familiar with the components of a downlink system before investing in one.

A video downlink transmitter's performance depends on the following five main components: operating frequency, modulation form, encryption type, video-encoding standards and auxiliary data or metadata. To maximize the utilization of the downlink, all receivers (fixed, handheld and portable) need to be compatible with the transmitter.

Frequency bands
The transmitting frequency used by law enforcement downlink systems is between 6425 and 6525MHz. There are 12 channels in this band (table). In order for other systems in a facility to interoperate with a downlink system, they must all support these frequencies. This frequency band is subject to licensed use only and is shared with other users, including broadcasters and cable operators, for both electronic news gathering (ENG) and point-to-point relays. Interference with these links can take a TV station off the air. Mobile phone operators also use these frequencies for interconnecting cell sites. Interfering with these links could lead to city-wide outages of cellular services. Under special approval and operating conditions, law enforcement agencies have also been authorized to use the 4.9GHz band.

Table: Law enforcement band.

Modulation
Legacy downlink systems were originally based on frequency modulation, which is often referred to as analog FM. While FM modulation can be useful for line-of-site applications, it suffers severely from multipath, a phenomenon where the transmitted signal will take multiple paths to the receive antenna. This causes signal distortion, limiting the video's usability. To minimize this effect, tracking antennas are used, which further limit the usability to strategic operations. Lastly, analog FM systems lack secure encryption, putting the video at risk of being viewed by unauthorized people.

Common data link (CDL) is a digital form of modulation used by the Department of Defense (DoD). This modulation was developed in 1972 to support high-altitude USAF platforms like U-2 and Global Hawk, which were implemented in response to the Soviet arms buildup. CDL has evolved over the years, but remains a single-carrier modulation format. Because of this, it suffers in a multipath environment in much the same way as FM modulation, and requires similar tracking-antenna systems. Since CDL was originally developed to support DoD applications, it has International Traffic in Arms Regulations (ITAR) restrictions that limit its deployment. Despite the DoD's intent of making CDL a "common" modulation form, in reality, it is largely proprietary to the group of defense contractors who originally developed the protocol.

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