Underlay signaling is a robust physical layer technique, allowing for transmitting a very low power signal in conjunction with the primary signals across the entire frequency band of the primary signals. The secondary users of the secondary network (i.e., a wireless network consisting of primary and secondary networks) primarily utilize the underlay, which increases spectral efficiency and improves the network capacity. This thesis focuses on underlay signaling in the context of the cellular (primary) network, where the underlay is an auxiliary channel made available to the primary users and the network, that is, the base stations and users of the cellular network. The current fifth-generation (5G) cellular networks are constructed using Orthogonal Frequency Division Multiplexing (OFDM) modulation. Hence, this thesis delves into the study of underlay coexistence with OFDM, specifically 5G, by performing extensive simulations and analytical analysis and investigating the impact of underlay signaling on the throughput performance of 5G networks. We develop the underlay signaling based on the frequency-domain spread spectrum and add the underlay signal prior to the Inverse Fast Fourier Transform (IFFT) operation of OFDM. Furthermore, we present a real 5G setup built on the srsRAN project, where we showcase a proof-of-concept demonstration of underlay coexistence with the 5G over the air, where the 5G base station transmits both 5G NR and underlay signal simultaneously. Through our research, we conclusively demonstrate that a low-data rate underlay signal can be successfully transmitted alongside the existing 5G signal. Our study concludes by carefully selecting the appropriate design parameters, such as the signal-to-interference power level (5G power in relation to underlay), spreading factor, and coding gain at which we can reliably detect and decode underlay signals having no impact on the 5G performance. The integration of underlay in 5G brings forth a multitude of benefits using underlay for military and tactical applications, massive Machine Type Communications (mMTC) alongside Ultra-Reliable Low Latency Communications (URLLC), and the offloading of crucial control information of 5G to the underlay channel. Thus, this underlay operates as a low-data rate error-free conduit, with the potential to provide Low Probability of Interception (LPI) and Low Probability of Detection (LPD) attributes and heightened reliability while concurrently transmitting with the 5G NR, bolstering the overall effectiveness of the communication. / Master of Science / Underlay signaling is a technique that allows for transmitting a low-power signal alongside the primary signals, occupying the entire frequency band of the primary signals. The secondary users of the secondary network (i.e., a wireless network consisting of primary and secondary networks) primarily utilize the underlay, which increases spectral efficiency and improves the network capacity. This thesis focuses on underlay signaling in the context of cellular (primary) networks where the underlay is an auxiliary channel made available to the primary users and network, that is, the base stations and users of the cellular network. The current fifth-generation (5G) cellular networks are constructed using Orthogonal Frequency Division Multiplexing (OFDM) modulation. OFDM is a multicarrier modulation scheme that divides the available frequency band into multiple narrow subcarriers, each carrying a portion of the data. The key advantage of OFDM is its ability to combat frequency-selective fading, where different frequencies experience different levels of fading and interference. By using many closely spaced sub-carriers, OFDM can effectively mitigate the impact of fading, allowing for robust and reliable communication even in challenging channel conditions. Thus, this thesis investigates the co-existence of underlay signaling and OFDM in 5G. We develop the underlay signaling based on the frequency-domain spread spectrum. Extensive simulations and analytical analysis are performed to understand the impact of underlay signaling on OFDM performance in terms of bit error rates and throughput. Additionally, a real 5G setup is presented, demonstrating a proof-of-concept of underlay co-existence with 5G NR, where the 5G base station transmits both 5G NR and underlay signal simultaneously. Through the research, it is conclusively demonstrated that a low-data rate error-free underlay signal can be successfully transmitted alongside the existing 5G signal. The integration of underlay in 5G brings forth a multitude of benefits using underlay for military and tactical applications, massive Machine Type Communications (mMTC) alongside Ultra-Reliable Low Latency Communications (URLLC), and the offloading of crucial control information of 5G to the underlay channel. Thus, this underlay operates as a low-data rate error-free conduit, characterized by its low interception and low detection attributes, enhancing reliability while concurrently transmitting with 5G NR, bolstering the overall effectiveness of the communication.