Technical challenges
The recent emergence of MMW solutions is driven partly by the large swaths of available contiguous bandwidth and recent advances in MMW component technology. MMW spectrum refers to the portion of the radio spectrum corresponding to wavelengths between 1 mm and 10 mm, or the equivalent of a carrier frequency between 30 GHz and 300 GHz, which is between 10-100x higher frequency than what is used for WiFi. However, the advantage of available bandwidth in the MMW spectrum also comes with some well-known challenges:
- RF component technology: For long-range, high-capacity links, high-power RF amplifiers tend to be the largest DC power consuming components of the system. Transmitting high order modulations requires backing off the power amplifiers. However, with increasing power amplifier back-off, the efficiency drops precipitously. RF transceivers and passive components should also be designed carefully to guarantee distortion-free performance of the links over wide bandwidths.
- Antenna design and tracking: Parabolic antennas offer high efficiency and increased gain, but can cause unwanted drag when affixed to a UAV. In addition, as the antenna gain goes up, its beam width narrows and must be pointed at the receiver with high precision. The highly directional antennas needed at both ends presents a challenging pointing and tracking problem; mechanically steered directional antennas introduce additional thermal management issues, while electronic steering affects aperture efficiency and axial ratio.
- Atmospheric attenuation: The atmosphere absorbs the electromagnetic waves in the MMW spectrum. The specific rate of absorption depends on the exact wavelength of the transmission. Rain, clouds, fog, and scintillation result in attenuation of the MMW signal. Specifically, rain and humidity could result in very high attenuation for terrestrial MMW links and could severely compromise their availability.
Building and testing the system
Our first step was to characterize the wireless channel between a mountaintop in Malibu, Calif., and a building rooftop in Woodland Hills, Calif., separated by a line-of-sight distance of 13.2 km. At each location, we installed commercially available E-band equipment that offered data rates from 100 Mbps to 3Gbps based on link conditions, as well as a weather station. This allowed us to develop a long-term characterization of the 13-km link under different weather conditions such as clear skies, clouds, fog, high winds, and rain. We installed a similar setup at one of our data center locations to characterize losses under heavy rain scenarios. The link characterization helped the team pick the right components for the test.
We selected high-linearity RF components that included balanced mixers, frequency multipliers, isolators, ortho-mode transducers, polarizers, low-noise amplifiers, and power amplifiers. The ortho-mode transducers and polarizers used for this test were custom-built to reduce polarization cross coupling. The RF components in our system support more than twice the bandwidth of available off-the-shelf RF communication systems, which, combined with the high linearity, helps us address the high data rate challenge.
We also used custom-built MMW power amplifiers, which use high-efficiency power combining techniques to overcome the transmit power limits of commercially available power amplifier ICs and output 10 times the transmit power compared with the power amplifiers on commercial off-the-shelf radios in the E-band. These improvements addressed both the high data rate and long range challenges.
To address the low DC power consumption constraint, the team developed a technique known as post-amplification spectrum multiplexing. This enables the use of multiple smaller, more efficient power amplifiers instead of one big power amplifier with low efficiency, which avoids the challenges associated with efficient amplification of multi-carrier signals.
