# mmS-TCP: Scalable TCP for Improving Throughput and Fairness in 5G mmWave Networks
## 1 Introduction  
Transport layer protocols affect the performance of end-to-end connections over the Internet. Transmission control protocol (TCP), a transport layer protocol used by most applications on the Internet, controls transmission speed by properly adjusting the congestion window (cwnd) indicating the maximum amount of data the sender can transmit based on the congestion control algorithms (CCA) applied to manage network congestion [1]. In addition, when packet loss occurs, reliable packet delivery is ensured by retransmitting and recovering the lost packets based on the acknowledgement (ACK) message delivered from the receiver. With the rapid development of network hardware and the expansion of wireless Internet, various TCP congestion control studies have been conducted to achieve better throughput and low latency [2, 3, 4, 5]. According to the performance requirements of 5G networks, the 5G networks should support gigabits per second (Gbit/s) at their best, and the user bitrate should provide at least 50 megabits per second (Mbit/s) [6]. High-bandwidth millimeter wave (mmWave) communications could play a key role in achieving the throughput goals of these 5G networks. The mmWave band that corresponds to a frequency of 24 GHz or higher provides a wider available bandwidth than do existing mobile communication networks, so higher end-to-end throughput can be expected. Furthermore, it is expected that the demand for high-capacity, low-latency networks caused by the rapid increase in mobile Internet use will be met and resolved through the mmWave band. However, the high-bandwidth wirelesschannel has a highly variable characteristic whereby extensive signal attenuation occurs even with small obstacles, and the mmWave signal can be greatly attenuated by the human body as well as ordinary buildings [7]. This highly flexible radio channel presents a variety of challenges to existing transport layer protocols and end-to-end applications [8; 9; 10; 11; 12; 13]. Although the mmWave band is considered a promising technology for achieving the goals of 5G cellular networks with multi-gigabit per second data rates and ultra-low latency, the end-to-end performance experienced by users is ultimately determined by the interaction with the transport layer protocol. In some recent studies, it was noted that the available bandwidth could not be fully utilized due to the large and frequent fluctuations of signals in the mmWave channel, which reduce the end-to-end throughput [14; 15]. In order to fully utilize the available bandwidth of the mmWave link, it is necessary to maintain the TCP cwnd value corresponding to the best channel condition, but frequent degradation of the quality of the wireless channel changes the usable capacity of the link. Even if the capacity of the link decreases due to poor radio channel quality, the TCP sender cannot quickly reduce the cwnd and transmits a large number of packets; as a result, the number of packets exceeding the capacity of the wireless link accumulates in the buffer of the base station and generates a long queue. The long queuing delay caused by this bufferbloat [16] problem also increases the round-trip time (RTT), and thus mobile device users experience low quality service as a result. In previous studies, we analyzed the problems that could arise when TCP operates in a 5G mmWave cellular network, and we conducted related experiments [17; 18]. CUBIC [19], the basic TCP congestion control of a representative operating system, could guarantee sufficient throughput when a first-in-first-out (FIFO) queue management scheme was applied to the buffer of the base station. However, the application of FIFO resulted in the creation of a long queue in the radio link control (RLC) buffer, and the RTT greatly increased; this does not achieve the design goal of 5G.