# Performance Evaluation of MPTCP on Simultaneous Use of 5G and 4G Networks
## 1 Introduction  
The worldwide implementation of the 5G cellular network has been opening a new era of communication, facilitating high data transfer speed and low latency compared to previous mobile generations such as 4G [1]. The 5G network has been evolving, focusing on three major supports: enhanced broadband mobile, low latency and ultra-reliable communications, and massive integration for machine communications [2, 3]. The upcoming 5G cellular network deployment will bring an estimated USD 13.1 trillion-worth of business globally [4]. The still deploying 5G network leaves many places out of the network coverage, leading to frequent handovers to 4G network and vice versa [5]. Moreover, 5G mmWave signals are comparatively more susceptible to obstacles. Sometimes, even penetrating through a house can substantially reduce its signal strength [6]. This problem yields significant packet losses and more frequent handoff between 5G and 4G networks [7, 8, 9]. From the transport layer's point of view, these handoffs can potentially cause significant delay and low throughput in data transfer in the event of reliable data delivery [10, 11, 12, 13]. According to [PERSON] et al. [14], transmission control protocol (TCP), which is the key transport layer protocol responsible for reliable end-to-end communication, frequently comes across spurious retransmission timeouts (RTOs) for such handovers causing drastically degraded performance in terms of delay and throughput. Nowadays, most modern devices come with multiple communication interfaces (e.g., 4G, 5G, WiFi, etc.). Generally, they use any one from these available communication interfaces for data communication. Instead of using a single interface at a time, simultaneous use of multiple communication interfaces is expected to provide better performance in terms of throughput and delay, especially in the case of large file transfers [15]. Moreover, keeping the communication up through all the communication interfaces can be a better solution for resolving the spurious RTO problem experienced during the 4G/5G handoffs. On the other hand, most mobile operators are deploying their 5G infrastructure along with their existing 4G infrastructure to reduce costs. Even in a large number of instances, the 4G and 5G antennas are installed in the same tower in a non-standalone (NSA) setup [16], making both the networks available to the user equipment (UE), as can be seen in Figure 1. A simultaneous communication through both the 4G and 5G network interfaces can be a promising solution to produce a better throughput [17]. While the 5G access traffic steering, switching, and splitting (5G ATSSS) focuses on extending support for non-3 GPP (third generation partnership project) access networks (e.g., WiFi) to access the 5G core [18; 19], following [17], it can be highly effective for the UE to concurrently use both 4G and 5G radios for obtaining high throughput, especially while worldwide full 5G implementation still needs significant time [20]. Multipath TCP (MPTCP) is the modified version of TCP that enables a device to use multiple communication interfaces at the same time [21]. The scheduler and congestion control algorithm (CCA) are the two major parts of MPTCP that mainly controls which path a data packet travels and how much data can be sent on a path, respectively [22]. The key responsibility of the scheduler is to remove head-of-line (HoL) blocking at the receiver, while the CCA is responsible for the maximum utilization of an available path. Together, they try to fulfill the aim of providing high throughput with low latency, which is the ultimate goal of end-to-end communication. MPTCP can be a promising solution for utilizing both 4G and 5G paths at the same time while resolving spurious RTO problems [14]. Similarly, as previously indicated, combining both 5G and 4G networks concurrently should deliver a higher throughput than using a single path, according to Qualcomm [17].