Multi-Gigabaud Millimeter-Wave Communication - Challenges and Solutions

Doctoral thesis, 2017

A major challenge in future mobile networks is to overcome the capacity barrier in wireless communication. Utilizing large bandwidth at higher frequencies is key to enabling capacity upgrade for next generation mobile networks (5G). As expected, multi-gigabit wireless communication is needed to support future 5G networks, particularly in the transport capacity of wireless backhaul and fronthaul. To be futureproof, wireless technologies towards 100 Gbps are of great interest. Millimeter-wave (mm-wave) frequency bands (30 to 300 GHz) have sufficient bandwidth to support these high data rates. However, due to hardware limitations, it is challenging to implement multi-GHz modulation bandwidth in actual hardware. For example, limited by the sampling rate of analog-to-digital and digital-to-analog converters, conventional digital modulator and demodulator (modem) designs cannot be applied to the very wide bandwidth required. As proof-of-concept, this thesis presents modems with multi-GHz bandwidth capability [A, B, C, D, E], as required for further capacity enhancement when combined with high-order modulations. The solutions in [C, D, E] do not require any data converters, therefore state-of-the-art energy efficiency is achieved [D, E]. The digital receiver in [B], on the other hand, relaxes requirements on the sampling speed thus being cost and power efficient. Enabled by the proposed modems, multi-gigabit transmission is demonstrated over mm-wave bands [B, C, E] and a short-range optical link [D]. Another aspect that limits the practical use of mm-wave is the degradation of the communication signal quality due to high-frequency hardware impairments. In particular, oscillator phase noise increases with the carrier frequency. An analog phase noise mitigation technique is proposed for arbitrary mm-wave signal waveforms [F]. As a new system application, an analog fronthaul radio link is enabled by implementing phase noise mitigation, where LTE transmission is demonstrated at 70/80 GHz as a step towards future 5G systems [G]. To reach 100 Gbps and beyond, despite the wide bandwidth available at mm-wave bands, the simultaneous use of high-order modulations is also required. However, it is primarily the oscillator noise floor that prevents this combination from being successfully achieved, as confirmed in measurements [H]. A new understanding of performance limitation in wideband communication is provided in a detailed study [I], with guidelines on how to improve hardware designs.