The next technological revolution is knocking at the door, with 5G NR and the internet of things driving massive equipment interconnectivity with ultra-low latency, secure communication, cloud and edge computation, and the deployment of distributed low-power devices for industrial and agronomic applications in remote regions. The quick development of new equipment powering 5G NR created a high demand for testing devices, which are crucial to verify the reliability, performance, and cost-effectiveness of novel technology. This is where software-defined radios (SDRs) have a role to play.
In this article, we discuss how SDRs can benefit 5G test and measurement (T&M) in every development aspect, including antenna design, signal-processing algorithms, propagation studies, channel estimation techniques, and latency evaluation. Multiple-input multiple-output (MIMO) high-performance SDRs can also enable T&M in several channels, evaluating system performance in several aspects at the same time, with parallel processing and independent channel configuration. Finally, we discuss the SDR’s compatibility with testing protocols on open radio access networks (O-RAN), an increasingly important network architecture providing interoperability to the immense amount of different radio protocols, frequency tunings, and bandwidth requirements needed for future massive device connectivity.
5G networks
With the exponential increase in the number of wireless user equipment (UE), particularly caused by the advent of wearables, the IoT, and Industry 4.0, it is important to ensure that the network infrastructure can manage the high levels of data throughput and high-speed required for all the UE connected. To address this issue, 5G networks were developed with three services in mind: ultra-reliable low-latency communications (URLLC), massive machine-type communications (mMTC), and enhanced mobile broadband (eMBB), each one with advantages and challenges.
The operating frequency band of the 5G network is not constant, typically varying with the proximity with the base station and the data rates required by the application. Normally, the 5G network is divided into several cell sites, which facilitates the use of higher frequencies by bringing the service close to the supplied region. Coverage area is very dependent on the frequency, so the higher frequencies of 5G require a higher number of small cells forming the radio access network (RAN) (Figure 1), with each cell being connected to the network core through the backhaul link. To adaptively improve the signal strength and communication quality, beam steering and beamforming are commonly applied techniques in 5G that use antenna arrays combined with MIMO radio devices to control the beam direction and distribution within these cells.
Figure 1: 5G network simplified architecture (Source: Per Vices)
The RAN of the 5G infrastructure consists of several base stations, with each station covering a small area of service. The distribution of the base stations is such that the coverage area becomes continuous, offering a stable service over a very large area. The remote base stations, also called remote radio units (RRUs), connect to the baseband units (BBUs) through the fronthaul, while the RAN connects to the backbone (also called 5G core) through the backhaul connection. In this sense, the 5G core is the central part of the network, providing most of the essential architecture functions for a reliable service, including authentication, authorization, mobility and connectivity, and policy management.
The fronthaul, on the other hand, links the BBUs with the RRUs, which in turn connects to the UE via radio waves. The fronthaul can be either wired or wireless, with high-speed optical links (enhanced common public radio interface, or eCPRI) being typically implemented. Every RRU, BBU, and antenna that composes the physical layer of the 5G network handles all wireless communication between the UEs and the 5G core.
Due to the high level of softwarization of the core, 5G functions are independent of the physical infrastructure, which greatly increases the speed of deployment, design flexibility, and interoperability. Service-based architecture (SBA) is a new paradigm in 5G that implements fully cloud-based core functions, using application programming interfaces (APIs) to exchange services between network functions (NFs). It is easy to see that the RAN encompasses a great variety of devices, protocols, and functions, which must be tested exhaustively before deployment to avoid jeopardizing the whole data flow. Therefore, it is crucial to provide flexible and upgradable devices for the T&M industry to keep up with the constant evolution of RRUs, BBUs, and antennas.
SDR use cases for 5G measurements
One of the main issues in 5G networks is latency. To enable the most ambitious networking applications, such as IoT and tactile internet, it is crucial to keep end-to-end latency lower than 1 ms. Therefore, latency T&M is extremely important and challenging to design, as the T&M system itself must be fast. Rico-Martínez et al.1 proposed a new T&M system to measure real-time latency in hybrid optical-wireless links. They used an SDR platform combined with a GNU radio to probe the latency at each step of the signal chain, including before and after encoding/decoding and modulation/demodulation.
They used SDRs as both transmitter and receiver of the test setup, as shown in Figure 2. The transmitter was used to modulate the optical signal, which was then converted to the electrical domain using a photodiode, and both transmission and reception were performed through horn antennas. This proof of concept provided superior performance over PING, with a user-friendly interface in GNU.
Figure 2: Hybrid setup for latency T&M (PD stands for photodiode) (Source: Per Vices)
Another challenge in 5G is signal propagation, as the mmWaves face unfavorable conditions when compared with other bands. Thus, T&M of signal propagation is crucial for antennas. By using an SDR setup with Tx and Rx antennas, one can easily implement a signal-propagation study to evaluate performance indexes, such as path loss, delay, multipath fading, absorption, reflection, angle of arrival, and Doppler shifting.
Furthermore, the MIMO nature of high-performance SDRs enables the development of testbeds for beamforming and beam steering, which improves signal propagation. For instance, Marinho et al.2 proposed a beamforming architecture for 28 GHz using SDRs to coordinate the MIMO signal. It was shown that the use of SDRs provided the flexibility and versatility needed to adjust the radio parameters and achieve the required performance for a variety of applications, including radars, eMBB, and satellite communication.
O-RAN is one of the most prominent 5G RAN architectures in the industry, which is based on the disaggregation of base units and cloud-native infrastructure. Due to its importance in the industry, the development of T&M solutions for O-RAN is highly demanded.
Upadhyaya et al.3 proposed an O-RAN testbed using SDRs. The platform uses an open-source 5G system capable of communicating with a near-real–time RAN intelligent controller (near-RT RIC) using standard interfaces, which is fundamental for O-RAN control and testing. They also implemented srsRAN, an open-source and free software suite for 4G and 5G deployments that is fully compatible with commercial off-the-shelf (COTS) SDRs. By developing the platform, they showed that it is possible to build an O-RAN testbed for research applications using only COTS devices and free software, which can significantly enable future developments in the field.
Network function virtualization (NFV) is a fundamental concept in 5G networking. It decouples the service functions from the physical infrastructure, enabling faster deployments and resource allocation. Particularly in the cloud-RAN (C-RAN) concept, NFVs are fundamental to create virtual base stations over the C-RAN cloud.3 The virtualization of the network functions in the centralized BBUs creates the perfect environment for SDR implementations in the RRUs, so the C-RAN infrastructure is highly dependent on them.
Therefore, SDRs can be used to develop testbeds for C-RAN, enabling T&M of the NFVs. Figure 3 shows the setup proposed by Mufutau et al.,4 which developed a C-RAN platform based on the OpenAirInterface (OAI) to test mobile fronthaul solutions. An OAI-based eNB, evolved packet core (EPC), and UE were implemented to simulate an LTE using general-purpose x86 computing hardware. Two SDRs performed the radio interface for the OAI-UE and the OAI-eNB. This experimental setup allowed the generation of mobile traffic data and characterization of the end-to-end cellular network. The developed C-RAN testbed enabled several T&M verifications, including over-the-air signal capturing and characterization and TCP uplink/downlink traffic.
Figure 3: Experimental setup for C-RAN (Source: Per Vices)
Conclusion
As the world grows more and more interconnected via 5G technology, new T&M solutions are mandatory to keep up with the constant evolution in the field. It became clear that the hardware-based approach from conventional radio technologies does not provide the flexibility and upgradability required to address the new techniques integrating 5G NR, especially due to the high level of softwarization of the network functions.
The new radio paradigm introduced by the SDR, on the other hand, is much more likely to thrive in the 5G world. By decoupling the DSP functions from the hardware to the software, including modulation/demodulation, up-/down-converting, and data packetization, off-the-shelf SDRs can comply with virtually any radio system.
In the context of T&M, SDRs can be applied in the whole testing chain, performing waveform generation, spectrum analysis, power metering, channel emulation, and MIMO protocols for beam steering and beamforming. Because it requires only a few additional components for certain tasks, SDRs can significantly reduce the amount of equipment necessary for T&M. The literature shows applications of SDRs in latency T&M, signal propagation measurements, O-RAN testbeds, and NFV testing.
References
1Rico‐Martínez et al. (2017). “Procedure to measure real time latency using software defined radio in a W‐band fiber‐wireless link.” Microwave and Optical Technology Letters, 59(12), 3147–3151.
2Marinho et al. (2020). “Software-defined radio beamforming system for 5G/radar applications.” Applied Sciences, 10(20), 7187.
3Upadhyaya et al. (2022). “Prototyping Next-Generation O-RAN Research Testbeds with SDRs.” arXiv preprint arXiv:2205.13178.
4Mufutau et al. (2021). “SDR Enabled C-RAN Implementation using OpenAirInterface.”
Brendon McHugh is a field application engineer and technical writer at Per Vices. He has a degree in theoretical physics from the University of Toronto. Per Vices has extensive experience in designing, developing, building, and integrating SDRs for various applications in T&M. Visit pervices.com or contact solutions@pervices.com for more information.
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