Typically, research efforts using the millimeter waveband focus on massive multiple input, multiple output (massive MIMO) technology, which controls multiple antenna elements to send radio wave beams to each device. Each antenna element requires D/A circuits to convert digital signals to analog before they are emitted by the antenna. But performing digital beamforming requires the control of multiple high-speed D/A circuits, hence increasing power consumption.
Through what it calls hybrid beamforming, Fujitsu Laboratories aims to reduce the number of circuits in use. By carrying out some of the signal processing in the analog antenna element, multiple antenna elements can be connected to a single D/A circuit, reducing overall power consumption.
Fujitsu Laboratories' first attempts revealed reduced transmission rates, due to signal cross-interferences. In a system with 128 antenna elements, and 8 multiplexed beams, the number of D/A circuits with hybrid beamforming could be reduced to one sixteenth that of digital beamforming, but because the multiplexed beams interfered with each other, the transmission rate fell to one eighth that of a purely digital solution. The researchers managed to cancel out these interferences by crafting an interleaved structure and developing a novel inter-subarray coding format.