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In the case of radio frequency (RF) data links, limited spectrum and the requirement to minimize airborne system size, weight, and power (SWAP) have been strong contributors for limiting data rates. Rates up to 10 Gbps (40 times currently fielded capabilities) are considered possible at current bandwidths by using more bandwidth-efficient modulation methods. At gigahertz frequencies however, RF use becomes increasingly constrained by frequency congestion. This is especially true for the 1-8 GHz range which covers L, S, and C bands. Currently fielded digital data links provide an efficiency varying between 0.92 and 1.5 bps/Hz, where the theoretical maximum is 1.92.
Airborne optical data links, or lasercom, will potentially offer data rates two to five orders of magnitude greater than those of the best future RF systems. However, lasercom data rates have held steady for two decades because their key technical challenge was adequate pointing, acquisition, and tracking (PAT) technology to ensure the laser link was both acquired and maintained. Although mature RF systems are viewed as lower risk, and therefore attract investment dollars more easily, Missile Defense Agency funding in the 1990s allowed a series of increasingly complex demonstrations at Gbps rates. The small apertures (3 to 5 inches) and widespread availability of low power semiconductor lasers explains why lasercom systems typically weigh 30 to 50 percent that of comparable RF systems and consume less power. The smaller apertures also provide for lower signatures, greater security, and provide more jam resistance.
Although lasercom could surpass RF in terms of airborne data transfer rate, RF will continue to dominate at the lower altitudes for some time into the future because of its better all-weather capability. Thus, both RF and optical technology development should continue to progress out to 2025.
4.2.2 Network-Centric Communications
There are several areas of networking technology development that should be identified as critical to the migration path of UAS and their ability to provide network services, whether they be transit networking or stub networking platforms. Highflying UAS, such as the Global Hawk or Predator, have the ability to
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provide coverage that lends itself well to network backbone and transit networking applications. In order to provide these services, the networked communications capabilities need to migrate to provide capacity, stability, reliability and rich connectivity/interoperability options. The following technologies are essential to this development:
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High Capacity Directional Data links
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High capacity routers with large processing capacity - Ruggedized IP enabled Wideband Routers
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Modular and Programmable Router Architecture
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Well-known and Standardized Protocols and Interfaces
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Mobile Ad-hoc quasi-stable mesh - requirement to manage topology
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