Millimeter-wave frequency synthesis systems for 5G and 6G using compound semiconductors

The demand for higher data rates in wireless communications seems unstoppable. The frequency spectrum below 6 GHz that has been used for the predecessors of 5G, is being extended with frequency bands above 20 GHz, extending into the millimeter-wave region. For 6G communications even higher data rates are foreseen than for 5G, reaching 100 Gbit/s.

Increasing the data rate can be obtained by occupying more bandwidth and by using more complex modulation with more bits per second and per Hertz. More bandwidth is available at higher frequencies, and therefore 6G will use frequency bands above 100 GHz for its applications with the highest data rates. The use of complex modulation requires a high signal-to-noise ratio (SNR) that should be preserved by the electronics in the radio transceivers. Experience has shown that in mm-wave transceivers a major source of SNR degradation is the phase noise that originates from the local oscillator (LO). This LO is made by a phase-locked loop, which is a feedback loop that regulates the frequency of a voltage-controlled oscillator (VCO). The phase noise of the VCO is a major contributor to the total PLL phase noise. It can be lowered by circuit design techniques, by the use of high-quality passive components in the LC-tank of the VCO and by increasing the oscillation amplitude in the VCO.

High-end mm-wave transceivers will be using a combination of CMOS, for the complex electronics, and compound semiconductors. The latter ones are typically used to generate wireless power at a higher efficiency than CMOS. In this PhD we want to explore if and how compound semiconductors can lower the phase noise of a PLL. In communication systems operating below 100 GHz, GaN is being used more and more for efficient power generation. For communication above 100 GHz, InP is superior to silicon technologies for wireless power generation. These compound semiconductors typically use a higher supply voltage than CMOS, giving an outlook to higher oscillation amplitudes and hence less phase noise. Further, InP HBTs do not exhibit flicker noise, in contrast with CMOS devices, such that 1/f noise upconversion into phase noise is not present.

The frequency range of the VCO does not need to correspond with the carrier frequency of the RF circuits in the signal path. You will select the frequency range that gives the lowest phase noise and then use frequency multipliers to attain the required LO frequency.