Analysis of TCP Performance in 5G mm-wave Mobile Networks

Millimeter-wave (mm-wave) bands will play an essential role in SG mobile networks in supporting the increasing demand for higher data rates. Communications at mm-wave frequencies pose unique challenges. The high propagation loss and unfavorable atmospheric absorption make the channel quality highly variable - short communication ranges and blockage through obstacles may prevent communication altogether. The use of directional antennas helps to achieve higher communication ranges and provides better spatial reuse and lower interference compared to omni-directional communications. At the same time, this introduces the problem of beam misalignment. Mm-wave research has primarily focused on the PHY and MAC layers, whereas the transport layer aspects of mm-wave systems require further attention. In this article, we analyze the behavior of TCP in mm-wave networks and study its impact on system-level performance. Through extensive simulations, we show the effect of different types of blockages on the behavior of the congestion control in the presence of handovers, and when small, medium and long flows coexist. Protocols like CUBIC that target high throughput benefit significantly when jointly optimizing link layers buffers and timeouts. While the optimization fosters prompt reaction to short-term blockages, the performance of such protocols significantly decreases when obstacles degrade the channel quality for longer time periods. Hybrid-designs like TCP YeAH are more robust to blockage, but fail to recover quickly and to ramp up to the link capacity after timeouts.