Ka-band

As the C-band and Ku-bands begin to become congested, significant interest has been generated in Ka-band (30/20 GHz)—also commonly called EHF, particularly in the US—for commercial satellite communications applications. Early research was conducted from the 1970s with satellites from Japan, US, Italy and the ESA, followed in 1993 by NASA’s ACTS. While development of Ka-band satellites was slow with only several in orbit by 2003 (Optus C1 was launched in June 2003), interest accelerated rapidly thereafter. By the mid-2000s, several hundred Ka-band satellite network filings had been submitted to the ITU. Ka-band frequencies are also used by systems such as Iridium for inter-satellite links and feeder links to Earth station gateways. The principal commercial driver for Ka-band is the delivery of high-capacity, interactive broadband and multimedia services.

Ka-band systems offer several important advantages. Most significantly, Ka-band provides very large contiguous bandwidths, typically 2–3 GHz, which is approximately twice that available in Ku-band and several times greater than in C-band, and is largely free from sharing with terrestrial services. The shorter wavelength enables smaller RF components and higher-gain antennas for a given physical size, facilitating compact payloads and user terminals. Ka-band spot beams can also be made much smaller—often using the same antenna aperture—enabling aggressive frequency reuse and very high system capacity. For these reasons, Ka-band is now the dominant frequency range for HTS systems and broadband gateway links.

The principal challenge of Ka-band operation is severe propagation impairment, particularly rain attenuation. To maintain acceptable availability, link margins on the order of 10–15 dB may be required, depending on climate and availability objectives. These losses may be mitigated by higher satellite transmit power—often 3–4 dB greater than equivalent C-band or X-band systems—and by larger Earth station antennas. While antenna gain increases with the square of frequency, the corresponding increase in free-space path loss largely offsets this advantage, making higher precision reflector surfaces and tighter pointing tolerances essential. Ka-band systems are therefore generally more expensive, though costs are expected to decrease as component volumes increase. Typical Ka-band LNAs achieve noise figures of 3–4 dB, and traveling-wave tube amplifiers are available with output powers in the range of 45–90 W.

Accurate prediction and mitigation of propagation impairments are critical for Ka-band systems. Adaptive techniques such as uplink power control allow Earth stations to compensate for fading by adjusting transmit power in response to received signal measurements. Beam shaping and adaptive beamforming can be used to increase gain in regions experiencing fading, at the expense of reduced gain elsewhere. Site diversity provides further mitigation by routing traffic through geographically separated Earth stations; separations of more than approximately 10 km typically ensure uncorrelated rain fading. However, site diversity is costly and is generally practical only for large, cooperative gateway networks.