The 240GHz band is a frequency range over 100 times wider than that used by typical mobile devices today (0.8-2GHz), which should enable a 100-fold increase in communications capacity. To achieve such an increase, however, requires amplifiers with high amplification ratios that can receive signals that have become very faint when transmitted through the air.
A portion of these research results were obtained through "R&D Program on Multi-tens Gigabit Wireless Communication Technology at Subterahertz Frequencies," a research program commissioned by
Details of this technology are being presented at CSICS 2013, the Compound Semiconductor IC Symposium, opening
The explosive growth of smartphones and other wireless devices has brought about an increase in the use of mobile data communications for browsing the web or downloading music, alongside conventional voice communication. With an expected shift toward high-capacity data communications, including videos and movies, there is an expectation that demand will grow for devices that can instantly download such data. In order for that to happen, high-capacity wireless devices will need to use wider frequency ranges than they do now. Wireless devices that could use millimeter-wave frequencies would be able to take advantage of a frequency range 100 times wider than that used by today's wireless devices, so it is expected that they would also be able to handle communications speeds 100 times greater.
Millimeter-wave transmissions, particularly at such high frequencies as 240GHz, however, become severely attenuated as the radio waves travel through the air. Receiving such a faint signal requires a highly sensitive receiver (comprised of an antenna, amplifier, and wave detector). The introduction of an amplifier with a high amplification ratio shows has been sought after as an effective way of improving reception sensitivity.
A common way of increasing the amplification ratio of the amplifier is to connect multiple amplifiers as part of a staged construction, but more stages result in bigger chip sizes. When applying this technique in the 240GHz band, the wavelength of the signal is very short - less than 1 mm - so the length can be shorter than the chip itself. This creates technical problems that do not exist at the frequencies used by today's cellular phones (that use the 2GHz band). Output signals from the amplifier can leak to ground(2) on the chip's surface, and these leaked signals return to the amplifier's input pin, resulting in double amplification. When the leaked signal re-enters the amplifier, it is amplified again and produces even more signal leakage, which returns to the input pin yet again, creating what is known as the oscillator effect, making it difficult to receive these signals correctly. For this reason, creating a high amplification ratio with millimeter waves requires a technology that can suppress these oscillations without losing amplification levels.
About the New Technology
Building on indium phosphide high electron-mobility transistor (InP HEMT)(3) technology developed by
1. Multistage amplifier suppresses oscillator effect, increases amplification ratio
The leaked signal from an amplifier will always have "antinodes" at specific sites where that signal is at its greatest amplitude, and "nodes" where it has no amplitude at all. If the amplifier's input pin is located in a leaked signal's antinode, then a stronger leaked signal will feed back into the amplifier, creating the oscillation effect. Conversely, if the input pin is located in a node, the leaked signal has no amplitude, and the amplifier will not re-amplify the leaked signal.
2. Impedance-matching technology efficiently transmits amplifier output signal to next stage
Efficiently transmitting the output signal from an amplifier to the next stage requires impedance matching on the lines that connect the amplifiers, which, in turn, requires that the lines be of a uniform length. But aligning the input and output pins of the amplifiers with the nodes limits the dimensions of the amplifier, while the length requirements on the lines create another set of constraints, complicating the task of impedance matching. To resolve this problem, a U-shaped line was deployed and the length and width of the U were adjusted to enable impedance matching regardless of the limitations on the dimensions of the amplifier.
These technologies have produced a roughly tenfold improvement in the sensitivity of a receiver chip compared with previous designs. These receivers could be used in smartphones and other wireless devices equipped with compact antennas. And because this would also allow for antennas with broader relative directionality(4) than existing devices, there would be no need to precisely align the transmitter to the handset, resulting in greater convenience for users.
(1) Millimeter-band:Radio waves with frequencies from 30GHz to 300GHz.
(2) Ground:In any electrical circuit, not only amplifiers, an electrical baseline surface that applies a baseline voltage of 0 V.
(3) Indium phosphide high electron-mobility transistor (InP HEMT):Invented in 1979 by
(4) Directionality:A measure of the extent to which the radio waves emitted from an antenna tend to spread through space. An antenna with high directionality tends not to spread through space but to be carried in a straight line.
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