By integrating the design of antenna and electronics, researchers have boosted the energy and spectrum efficiency for a new class of millimeter wave transmitters, allowing improved modulation and reduced generation of waste heat. The result could be longer talk time and higher data rates in millimeter wave wireless communication devices for future 5G applications.
The new co-design technique allows simultaneous optimization of millimeter wave antennas and electronics. The hybrid devices use conventional materials and integrated circuit (IC) technology, meaning no changes would be required to manufacture and package them. The co-design scheme allows fabrication of multiple transmitters and receivers on the same IC chip or the same package. This potentially enables multiple-input-multiple-output (MIMO) systems as well as boosting data rates and link diversity.
Key to the new design is maintaining high energy-efficiency regardless of whether the device is operating at its peak or at its average output power. The efficiency of most conventional transmitters is high only at the peak power but drops substantially at low power levels, resulting in low efficiency when amplifying complex spectrally efficient modulations. Moreover, conventional transmitters often add the outputs from multiple electronics using lossy power combiner circuits, exacerbating the efficiency degradation.
The innovation in this design is combining the output power through a dual-feed loop antenna to merge the antenna and electronics for so-called outphasing operation that dynamically modulates and optimizes the output voltages and currents of the power transistors. The new designs have been implemented in 45-nanometer CMOS SOI IC devices and flip-chips packaged on high-frequency laminate boards, where testing has confirmed a minimum two-fold increase in energy efficiency.
Beyond energy efficiency, the co-design also facilitates spectrum efficiency by allowing more complex modulation protocols. This will enable transmission of a higher data rate within the fixed spectrum allocation that poses a significant challenge for 5G systems. Within the same channel bandwidth, the proposed transmitter is able to transmit at a six to ten times higher data rate.
An antenna structure with multiple feeds makes it possible to use multiple electronics to drive the antenna concurrently. Different from conventional single-feed antennas, multi-feed antennas can serve not only as radiating elements, but can also function as signal processing units that interface among multiple electronic circuits. This opens the new design paradigm of having different electronic circuits driving the antenna collectively with different but optimized signal conditions.
The cross-disciplinary co-design could also facilitate fabrication and operation of multiple transmitters and receivers on the same chip, allowing hundreds or even thousands of elements to work together as a whole system. Having large numbers of elements working together becomes more practical at millimeter wave frequencies because the wavelength reduction means elements can be placed closer together to achieve compact systems. These factors could pave the way for the new types of beamforming that are essential for future millimeter wave 5G systems.
The technology will be ideal for battery-powered devices but could also be useful for grid-powered systems such as base stations or wireless connections to replace cables in large data centers. In those applications, expanding data rates and reducing cooling needs could make the new devices attractive. Higher energy efficiency also means less energy will be converted to heat that must be removed to satisfy thermal management requirements. In large data centers, even a small reduction in thermal load per device can add up.