With users ranging from law enforcement, first-response teams and EMS to fire fighters and border patrol, airborne downlink systems are tremendously diverse in their applications. 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 6525 MHz. There are 12 channels in this band (Table 1: Law Enforcement Band). 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 4.9-GHz 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.
CDL (common data link) 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.
COFDM (coded orthogonal frequency division multiplexing) was designed to meet the demands of a highly multipath-rich environment. Developed to support commercial markets, it has no ITAR restrictions and is a widely deployed, truly open standard published by Digital Video Broadcasting (DVB, also known as DVB-T). DVB-T has been adopted universally in the commercial markets because of its unmatched ability to move video reliably. Deploying DVB-T on airborne downlink platforms removes much of the restrictions on how video can be shared; making real-time video available even on small handheld receivers equipped with omni-directional antennas. COFDM digital transmission can be secured with encryption, an important feature for command and control, which makes it ideal for tactical, covert and surveillance operations.
Figure 1: Example of the effects of multipath on a FM signal
Figure 2: Example of the effects of multipath in a CDL signal
Figure 3: Example of the effects of multipath on a COFDM signal
In mainland USA XM / Sirius uses COFDM towers in urban areas to overcome loss of satellite links due to high rise buildings.
But the quality of COFDM goes a little beyond good video quality. It also affords better penetration into buildings. Though Sirius / XM is basically an audio service, the digital stream allows video streaming to back seat passengers or hotels / motels can use it to distribute video within their facility.