5G networks are becoming more widespread around the world. Many consumer devices that support 5G are already enjoying increased speeds and lower latency. However, some frequency bands allocated for 5G are not used efficiently due to technological limitations. These frequency bands include the New Radio (NR) 39 GHz band, but actually range from 37 GHz to 43.5 GHz, depending on the country. The NR band offers notable performance advantages over other lower frequency bands that 5G networks use today. For example, it enables ultra-low latency in communication along with data rates of over 10Gb/s and massive capacity to accommodate multiple users.
However, these exploits come at a cost. High frequency signals are quickly attenuated as they travel through space. It is therefore crucial that the transmitted power is concentrated in a narrow beam aimed directly at the receiver. This can, in principle, be achieved by using phased array beamformers, transmission devices consisting of an array of carefully phase-controlled antennas. However, working in high frequency regions of the NR band decreases the efficiency of power amplifiers because they tend to suffer from non-linearity problems, which distort the transmitted signal.
To address these issues, a team of researchers led by Professor Kenichi Okada of the Tokyo Institute of Technology (Tokyo Tech), Japan, recently developed, in a new study, a novel phased array beamformer for 5G base stations. Their design adapts two well-known techniques, namely the Doherty amplifier and digital predistortion (DPD), into an mmWave phased array transceiver, but with a few twists. The researchers will present their findings at the upcoming IEEE 2022 Symposium on VLSI Technology and Circuits.
The Doherty amplifier, developed in 1936, has enjoyed a resurgence in modern telecommunications devices due to its good power efficiency and suitability for signals with a high peak-to-average ratio (such as 5G signals). The Tokyo Tech team modified the design of the conventional Doherty amplifier and produced a bi-directional amplifier. This means that the same circuit can both amplify a signal to be transmitted and a signal received with low noise. This fulfilled the crucial role of amplification for both transmission and reception. “Our proposed bi-directional implementation for the amplifier is very area efficient. Additionally, by co-designing with chip-scale packaging technology, it enables low insertion loss. This means less power is lost when the signal passes through the amplifier,” says Professor Okada.
Despite its many advantages, however, the Doherty amplifier can exacerbate nonlinearity problems that result from offsets in the phased array antenna elements. The team approached this problem in two ways. First, they used the DPD technique, which involves distorting the signal before transmission to effectively cancel out the distortion introduced by the amplifier. Their implementation, unlike conventional DPD approaches, used a shared look-up table (LUT) for all antennas, minimizing circuit complexity. Second, they introduced inter-element mismatch compensation capabilities into the phased array, thereby improving its overall linearity. “We compared the proposed device with other state-of-the-art 5G phased array transceivers and found that by compensating for inter-element mismatches in the shared DPD LUT module, ours demonstrates leakage of lower adjacent channel and transmission error,” remarks Professor Okada. “Hopefully, the device and techniques described in this study will allow us all to experience the benefits of 5G NR sooner!”