Robust $\mathcal {L}_{2}$ Stable Networked Control of Wireless Packet Queues in Delayed Internet Connections

This paper presents a networked controller for packet data queue level control between two nodes with an Internet connection. A wireless radio interface empties the packet queue. The control signal is the bitrate from the transmitting node over the Internet connection to the receiving node in which the controlled queue is located. The output signal is the packet dwell time in the packet queue. The networked controller needs to be robust with respect to large and unknown transmission and measurement delays over the Internet. A continuous-time frequency-domain design technique is therefore selected, employing a robust feedback together with a faster feedforward from the measured bitrate emptying the queue. The control signal is limited to positive values since data can only be sent toward the receiving node. Conditions for L2 stability of the networked controller are given by an application of the Popov criterion. Furthermore, the Popov inequality is exploited for robust controller tuning, precomputing the associated stability region. The practical performance in a third-generation cellular radio application is verified by simulation, laboratory measurements, and field trials. Substantial improvements in capacity and robustness were obtained.

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