Fully GaN-based all-digital transmitter chain for massive MIMO
The digital transmitter (Fig. 1) is a fully transparent replacement for an analog transmitter chain, from the upconverter to the output filter. The upconverter is replaced by a modulator that translates any kind of complex baseband signal into a purely binary data stream. The bit sequence feeds the digital power amplifier (PA), which essentially is a highly efficient voltage-controlled switch. The amplifier output is fed into a band-pass filter, which removes all out-of-band signals, eventually restoring the analog signal at the selected carrier frequency. The transition from a digital (i.e. binary) signal into the analog domain takes place only in the band-pass filter. Right in front of the antenna a T/R switch selects between transmit and receive path of the transceiver. In today’s applications, baseband processors are usually implemented in the digital domain. Extending the boundary of the digital domain even behind the PA yields several benefits, including compactness, less energy consumption and flexibility.
In September 2018, FBH has presented a fully digital GaN-based transmitter module (Fig. 2) for the microwave range at the European Microwave Conference (EuMC) 2018 in Madrid (Spain). This module applies for the first time a novel robust and compact digital GaN PA chip with greatly reduced complexity compared to earlier design. As a result, operation is very stable and insensitive to supply voltages. The chip with a size of 1.8 x 1.8 mm2 requires three voltage sources and an input amplitude of 700 mVpp only. The amplified signal at the chip output pin is then fed into a simple lumped element band-pass filter tuned for 900 MHz signal frequency. The T/R switch is a single-pole double-throw (SPDT) architecture using 8 x 125 µm GaN-HEMTs, which shows an insertion loss of 0.6 dB in the transmit path.
Measurements were carried out using baseband signals of 5 MHz and 20 MHz bandwidth with 6.5 dB peak-to-average power ratio on a 900 MHz carrier. For converting the baseband signal into a binary bit stream, a software modulator (FBH patent) was used. The parameter correction feature of the modulator was enabled for all measurements. Final-stage drain efficiencies of up to 47 % and high linearity at the output have been achieved (see Fig. 3). Full-scale output power at 30 V supply voltage was measured to exceed 3 W at 80 % drain efficiency. The realized module represents an ideal candidate for software-defined radio. Due to its extremely compact size it is predestined for any (massive) MIMO application, utilizing beam forming techniques to multiple receivers. The module can be mounted right underneath each antenna element.
This work has been funded by the German research association (DFG) under ref. WE 6288/1-1 and was supported by the German BMBF within the “Research Fab Microelectronics Germany (FMD)” framework under ref. 16FMD02.
F. Hühn, A. Wentzel, W. Heinrich, "Highly Compact GaN-based All-Digital Transmitter Chain Including SPDT Switch for Massive MIMO", in Proceedings of the 48th European Microwave Conference (EuMC) 2018, pp. 918 - 921, Madrid, Spain.
F. Hühn, A. Wentzel, W. Heinrich, "Highly compact GaN-based all-digital transmitter chain including SPDT T/Rx switch for massive MIMO applications", accepted for publication in International Journal of Microwave and Wireless Technologies, EuMC special issue, 2019.