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To enable user diversity and multiplexing gains, a fully digital precodingmultiple input multiple output (MIMO) architecture is typically applied.However, a large number of radio frequency (RF) chains make the systemunrealistic to low-cost communications. Therefore, a practical three-stagehybrid analogue-digital precoding architecture, occupying fewer RF chains, isproposed aiming for a non-orthogonal IoT signal in low-cost multiuser MIMOsystems. The non-orthogonal waveform can flexibly save spectral resources formassive devices connections or improve data rate without consuming extraspectral resources. The hybrid precoding is divided into three stages includinganalogue-domain, digital-domain and waveform-domain. A codebook based beamselection simplifies the analogue-domain beamforming via phase-only tuning.Digital-domain precoding can fine-tune the codebook shaped beam and resolvemultiuser interference in terms of both signal amplitude and phase. In the end,the waveform-domain precoding manages the self-created inter carrierinterference (ICI) of the non-orthogonal signal. This work designs over-the-airsignal transmission experiments for fully digital and hybrid precoding systemson software defined radio (SDR) devices. Results reveal that waveform precodingaccuracy can be enhanced by hybrid precoding. Compared to a transmitter withthe same RF chain resources, hybrid precoding significantly outperforms fullydigital precoding by up to 15.6 dB error vector magnitude (EVM) gain. A fullydigital system with the same number of antennas clearly requires more RF chainsand therefore is low power-, space- and cost- efficient. Therefore, theproposed three-stage hybrid precoding is a quite suitable solution tonon-orthogonal IoT applications.

Design and Prototyping of Hybrid Analogue Digital Multiuser MIMO Beamforming for Non-Orthogonal Signals