OpenAirLink: Reproducible Wireless Channel Emulation using Software Defined Radios

TL;DR

OpenAirLink (OAL) addresses the reproducibility gap in wireless research by delivering an open-source, FPGA-accelerated channel emulator built on SDRs. It implements a finite-impulse-response (FIR) based time-delay line (TDL) channel model on the FPGA, with real-time control of path loss and delay via a host PC. The work demonstrates a low-cost, portable emulator with a 144 dB dynamic range and 5 ns delay resolution, validated against industrial emulators and demonstrated across 5G NR and IEEE 802.15.4 scenarios. This approach lowers hardware barriers and enables open, verifiable wireless experiments, promoting transparency and repeatability in research.

Abstract

This paper presents OpenAirLink(OAL), an open-source channel emulator for reproducible testing of wireless scenarios. OAL is implemented on off-the-shelf software-defined radios (SDR) and presents a smaller-scale alternative to expensive commercially available channel emulators. Path loss and propagation delay are the fundamental aspects of emulating a wireless channel. OAL provides a simple method to change these aspects in real-time. The emulator is implemented using a finite impulse response (FIR) filter. The FIR filter is written in Verilog and flashed on the SDRs Field Programmable Gate Array (FPGA). Most processing transpires on the FPGA, so OAL does not require high-performance computing hardware and SDRs. We validate the performance of OAL and demonstrate the utility of such a channel emulation tool using two examples. We believe that open-source channel emulators such as OAL can make reproducible wireless experiments accessible to many researchers in the scientific community.

Figures (6)

• Figure 1: Emulator Architecture: The software-defined radio (SDR) hardware is used to build the entire channel emulator, integrating the signal conversion hardware with an FPGA-based DSP. The host PC with an ethernet connection is used to control the delay and attenuation of the channel.
• Figure 2: Attenuation Resolution: As the attention increases, the resolution of the emulator degrades. For a $r=15$ bit division of the digital sample, a maximum attenuation of roughly 82 dB is possible. However, the resolution at this attenuation level is almost 6 dB. The bit-shifting operation can greatly improve this resolution. The worst-case resolution when bit shift is employed for the same value of $r$ is 0.000528 dB shown in the dashed blue line.
• Figure 3: Verification Setup: A signal is split into two copies using a passive RF splitter. One copy goes directly to the signal sink and the other to the sink via the channel emulator. The comparison between the two copies allows us to test the latency of the OAL and verify the delay operation.
• Figure 4: OAL Processing Latency: The processing latency of OAL. Each component's variance in processing latency was maximum $\pm 1$ns. This certainty in the latency is helpful for the reproducibility of the wireless channel.
• Figure 5: Attenuation Operation: The attenuation of OAL is compared with a passive variable attenuator and the Spirent Vertex. All the measured values are averages over 1000 measurements. The received power values are lower than the theoretical value due to unaccounted losses in the conducive cables.