The CommIT hybrid digital-analog (HDA) beamforming testbed is a unique and self developed test system for the hybrid digital-analog approach to massive MIMO. It was developed at the chair within the frame of the A.v.H. Professorship. The testbed can be used to investigate and demonstrate hybrid massive MU-MIMO precoding or other beamforming techniques. It features a scalable structure to support different hybrid architectures and many antennas (up to 64 have been demonstrated, but also more can be connected). It is not constrained to a certain mobile communication protocol or standard and has been demonstrated with e.g. WiFi.
One of the main parts of the testbed is the analog beamforming module. One module is a network of controllable phase shifters and amplitude modulators. The network is passive and hence fully reciprocal. It can be used for TX and RX. The phase and amplitude can be changed with a resolution of 8 bit and a tuning range of 360 degree and 20dB, respectively. The network of a single module has a size of 2 × 16, meaning that two RF chains can be connected to 16 antenna elements. Multiple modules can be connected and thus larger networks realized (e.g. 4 RF chains by 64 antenna elements with 8 modules). One analog module has an insertion loss of 13 dB. This is due to hardware impairments excluding the loss from the power distribution in the beamforming network. The module is a research platform, meant to be reciprocal (hence does not include any active amplification), and is not optimized in terms of loss but rather resolution and tolerances. Nonetheless, it can be used together with external amplifiers to boost the output power.
The WiFi version of the testbed is a unique solution based on the hybrid digital-analog beamforming approach to overcome the limitation of commercial WiFi hardware and effectively increase the MU-MIMO gain and to unlock the potential of MU-MIMO in WiFi. We use commercial and of the shelf WiFi hardware in combination with the analog module explained above and novel algorithms for the hybrid approach, leveraging the MU-MIMO precoding of the WiFi system. The implemented control and signal processing software is fully transparent to the WiFi part, i.e., no protocol changes are needed. We demonstrated the increase in MU-MIMO gain using unmodified commercial end-user terminals. The increase in throughput compared to a four element antenna configuration is show in the picture on the right. The hybrid digital-analog beamforming WiFi testbed has been demonstrated during the MobiCom 2020 ACM SIGMOBILE conference. The corresponding short paper and the presentation video are shown below.
1) Kühne, Thomas and Gawłowicz, Piotr and Zubow, Anatolij and Dressler, Falko and Caire, Giuseppe (2020). Demo: Bringing Hybrid Analog-Digital Beamforming to Commercial MU-MIMO WiFi Networks. 26th ACM International Conference on Mobile Computing and Networking (MobiCom 2020). ACM.
2) Thomas Kühne and Giuseppe Caire (2018). An Analog Module for Hybrid Massive MIMO Testbeds Demonstrating Beam Alignment Algorithms. 22nd International ITG Workshop on Smart Antennas (WSA 2018)