Using concurrent transmissions to improve the reliability and latency of low-power wireless mesh networks
Aachen (2020) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (128 Seiten) : Illustrationen, Diagramme
Concurrent Transmissions (CT) occur when different transmitters simultaneously send the same packet. We analyze how CT distort the received waveform and characterize the Bit Error Rate (BER) of a receiver trying to recover the original bitstream. If collisions are not completely destructive and the information can still be recovered, then simple, robust and latency-optimal wireless mesh protocols can be designed. These protocols are particularly suitable for low-cost and low-power Internet of Things (IoT) applications, especially in high-interference scenarios, where routing efforts can be cumbersome and flooding becomes more effective. Distortion introduced by CT has two main causes. Firstly, simultaneous transmitters are not perfectly synchronized, which introduces intersymbol interference (ISI). The ISI can be reduced by keeping the synchronization mismatch below half the symbol period. Secondly, there is a periodical energy fading (beating) in the received waveform due to alternating patterns of constructive and destructive interference. This beating cannot be avoided, since it is originated by the non-coherent nature of the different local oscillators in the transmitters. Only communication systems able to cope with periodical amplitude and phase distortions are suitable for CT-based protocols. Communication performance is analyzed for typical phase- and frequency-modulation systems, obtaining the first closed-form analytical expression for the BER of two non-coherently received Frequency-Shift Keying (FSK) transmissions. For more complex systems, simulated BER curves are obtained. We conclude that CT are extremely destructive in conjunction with amplitude and phase modulations, but work particularly well with FSK modulations. In non-coherent FSK systems, the efficiency of CT depends on the level of external noise, being very effective in dense and high-noise environments. Demodulators based on non-coherent FSK receivers are typically used in two popular low-power IoT protocols: Bluetooth 5 and IEEE 802.15.4. Both are perfect candidates for CT-based mesh protocols. Finally, two CT-based award-winning protocols are designed: RedFixHop and BigBangBus. RedFixHop is the first protocol using the concept of disseminating information with concurrent packet acknowledgments (ACKs), while BigBangBus proposes the novel usage of longer preambles to decrease the BER introduced by the CT. Both protocols haven been tested in multiple competitions, repeatedly beating state-of-the-art solutions in terms of energy efficiency, reliability and end-to-end latency.