Wireless data flow control
Torbjörn Wigren · Uppsala University Publications (Uppsala University) · 2020
Wireless data flow control Background Data flow control over the internet is one of the most widespread control applications on the planet.TCP is e.g. based on transmit packet window manipulation at the transmitting source, based on ACK/NACK feedback.Hence this control problem is a networked one, with the controller located in another node than the controlled process which resides at the other side of TCP.Wireless data flow control will become increasingly important with the introduction of 5G wireless technology in the coming years, mainly for two reasons.First new use cases appear, like critical machine type communication (C-MTC).C-MTC aims to introduce wireless control of advanced high bandwidth processes in robotics, automotive safety, medicine and power distribution.In these applications quantities like latency and bit error rate will need to be reduced significantly as compared to what can be guaranteed today.Secondly, new wireless spectrum above 10 GHz will be exploited.At these frequencies the rate of variation of the radio channel and the associated wireless data rate both increase due to an increased radio shadowing in more challenging indoor environments.Since the wireless interface is normally the bottle-neck, it needs to be fully utilized.The quickly varying wireless rate, together with the backhaul delays to the node that feeds the wireless transmit node with data, makes it necessary to use transmit data queues.These queues store data for coming wireless transmissions at the wireless interface, thereby avoiding data starvation in case of a sudden increase of the wireless data rate.Since these queues add to the round trip delays, their queuing delays need to be balanced against the likelihood of an empty queue, in order to keep the latencies down.It is the task of flow control to solve this networked control problem.Here the increasing number of users adds to the difficulty.This follows since flow control operates per data stream or bearer.Since there will be thousands of users served by each base station, and since each user may use many bearers, there will be a large amount of flow controller instances operating in parallel.This is why computational complexity needs to be a part of the overall design equation.Three main requirements therefore needs to be met by new wireless flow controllers.They need to• control the data stream so as to guarantee a set delay.• be designed to be globally stable, to guarantee correct operation at all times.• have a low and constant computational complexity.