Wireless technology cuts power in high-speed mm-wave communications

Wireless technology cuts power in high-speed mm-wave communications

Technology News |
By Jean-Pierre Joosting

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.

The interleaved-type device allows for more spacing between antenna elements within a subarray (a collection of antenna elements connected to one D/A circuit). When an antenna’s range is spread out, its beam becomes narrower, and as the antenna elements are spread out, a type of undesired emission called a grating lobe can occur. If the elements for different subarrays are placed alternately (see figure 1), then the signals from both subarrays, A and B, are sent as radio waves in both directions the arrays are transmitting.

Figure 1: Interleaved hybrid beam multiplexing.

Signal A, however, is sent in the same phase of radio waves in both directions, while, due to the positional relationship between the antenna elements, the radio wave phase will change for B, depending on the direction.

By coding the signal appropriately between subarrays as shown in figure 2, the interferences can be cancelled out and the beam can be multiplexed.

With this system, an A+B signal is input into one subarray, while an A-B signal is input into the subarray in the other direction, resulting in one direction having an (A+B) + (A-B) = 2A signal, leaving only the radio waves for signal A, while the other direction has (A+B) – (A-B) = 2B, leaving only the radio waves for signal B.

Figure 2: Beam multiplexing with inter-subarray coding.

The newly developed interleaved hybrid beamforming unit leverages this inter-subarray coding scheme in the 60 GHz band to support 10 Gbps links through a narrow multiplexed beam.

Figure 3: Measurement results of inter-subarray coding.

The researchers are now working on further increasing the speed of their wireless millimeter-band equipment while reducing power consumption per bit rate. They hope to make this technology ready for practical implementation by 2020.

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