We have in our previous posts shown the advantages with digital beamforming over analog beamforming from a performance point of view. Then one may ask; why hasn’t digital beamforming in mobile devices happened yet?
Well, if you ask people in the field, they will say that a digital approach has way too high power consumption due to the need for a complete transceiver chain for each antenna whereas an analog approach only requires a single transceiver chain because beamforming is done in the analog domain at the front-end receiver using phase-shifters. Well, this might have been the truth in the past but let us check the case with the current state of the art technology.
Starting with the analog phase-shifters, which comprise switches and various physical routing of traces on the PCB used in order to make sure of coherent combining of the signal received or transmitted from the respective antenna. In the architecture evaluated in our white paper, the analog phase-shifting and combining network introduces an insertion loss of approximately 8 dB. The actual loss depends on the implementation. That signal power loss needs to be compensated by a high-gain low-noise amplifier (LNA) on the receiver side and a high-gain Power Amplifier (PA) design on the transmitter side. Such high-gain LNAs and PAs consume a significant part of the total power consumption in the radio transceiver.
A digital beamforming architecture performs the combining in the digital domain and therefore the LNA and PA design can be more relaxed for digital architectures saving significant amounts of the power consumption.
Another important factor is that it is commonly believed that the analog-to-digital and digital-to-analog converters (ADC,DAC) need to be designed in the same way for both digital and analog beamforming, and since digital beamforming requires an ADC/DAC pair for each antenna while the analog one only requires a single pair of ADC/DAC the power consumption must be N times larger for these components if N antennas are used. Furthermore, ADC/DAC has also in the past been a very power-hungry component and that (today erroneous) fact is still in many people’s minds in the field.
In fact, the power consumption for ADC/DACs especially when using 4-8 bits resolution, which is what is sufficient from a handheld device point of view, is nowadays, with evolving chip technology, on par with other radio components.
Combining the receiver streams digitally after the ADCs suppresses converter quantization noise. This allows the ADC resolution to be reduced by 1–2 bits compared with analog beamforming. Under the power model used in our white paper, this corresponds to approximately 50–75% lower power consumption per ADC. The reduction applies to each converter, rather than to the total power consumption of all ADCs or the complete radio system.
By understanding and utilizing these facts together with a careful digital beamforming system design, BeammWave can deliver a sustainable and scalable digital beamforming solution with power consumption on par with or better than current analog beamforming solutions for mobile devices! For further information please check out our white paper on The Power Efficient Architecture for 5G on mmWave Frequencies.
