PXI vs Alternatives for SDR

Author : Kaue Morcelles, Per Vices

18 June 2023

Software defined radios (SDRs) are becoming increasingly popular in several RF applications.

This is due to their flexibility, cost-effectiveness and ability to adapt to changing requirements without any hardware modifications. Differently from traditional radio systems, SDRs perform most of the radio functions at the software side - thereby enabling a high level of reconfigurability through programming. However, choosing the optimal SDR architecture for a particular application can be a challenging task, with many options available in the market for a variety of performance and size, weight and power (SWaP) requirements. One option is the PCI eXtensions for Instrumentation (PXI) architecture standard - but that’s not the only game in town.

PXI-based systems are rugged PC-based platforms widely implemented in automated test infrastructure, satellite deployments, monitoring instrumentation, etc. They combine the PCI bus architecture and rugged form factor with low cost. However, PXI is not the only option for critical SDR applications. Alternative high-performance off-the-shelf architectures, with internal modules and backhaul connectivity based on 10Gbps, 40Gbps and 100Gbps Ethernet links, are powering state-of-the-art SDRs for mission-critical and performance demanding applications. 

This article will compare PXI-based SDRs with alternative designs based on Ethernet backhauls in terms of performance, cost and ease of use. The advantages and disadvantages of each approach will be discussed and recommendations given on selecting the right architecture for specific application requirements. 
 
Firstly, let us talk about PXI-based SDRs. These devices can be seen as chassis that receive user-selected instrumentation modules and a controller, which fits the chassis in a ‘plug-and-play’ manner. Designers can thus develop completely different test equipment configurations by simply buying and integrating new modules, while keeping the footprint unaltered. Such modularity is one of the most advantageous features in PXI technology, as the SWaP specifications can be easily tailored by simply selecting the right chassis (which can vary from 3U to 6U form factors). Furthermore, the chassis provides strong mechanical ruggedness, which is essential in challenging environments, where temperature extremes, vibrations and shocks may need to be contended with. PXI SDRs are a great choice for automotive and satellite deployments where SWaP considerations are critical. 

One of the other important differences between PXI technology and its alternatives, is the high level of modularity with respect to DSP and other processing scalability. By dividing the system into several different exchangeable modules, a PXI SDR can implement a whole measurement laboratory into a single device, that is scalable in the long-term. However, despite the obvious benefits, PXI systems also present several drawbacks that should be addressed before selecting this technology for SDR applications. The most prominent of these is that PXI systems are not fully integrated solutions, meaning that several components must be obtained separately - including the chassis, controller and modules. Increasing the overall cost and development time of the system. Another major disadvantage is limitations in terms of data transfer rate. Even with PXI Express, only 8GB/s (or 64Gbps) can be achieved in the backhaul, which is not enough to address the current RF trends requiring hundreds of Gbps, such as spectrum monitoring systems. Finally, PXI flexibility is not as great for applications outside the automated test and measurement industry. This is largely because the space inside the chassis is limited, thereby making it difficult to add additional functionalities. Also, any major update will require physical intervention for hardware modification, so they are not as adaptable remotely. Therefore, PXI may not be the best solution for applications that require a high degree of flexibility or a significant amount of customisation, especially if remote updates or automatic adaptation are expected.

An alternative solution to PXI is selecting a commercial off-the-shelf (COTS) modular SDR with Ethernet-based backhaul, such as the Cyan from Per Vices (shown in Figure 1). Different from PXI devices, these SDRs provide integrated solutions (already including the chassis, controllers and RF modules), but still allow for customisation via different numbers of radio boards, types of boards (receive or transmit) and bandwidths (1GHz per radio chain or 3GHz per radio chain) along with additional integration and customisation support. This allows the user to ensure that the purchased device meets their application requirements without needing to dedicate engineering time and costs in the process. One of the best advantages of this architecture is that the SDRs can be easily scaled up or down, allowing for greater flexibility in the number of independent radio chains that can be supported within a single system. This is something that is fundamental for multiple-input multiple-output (MIMO) applications. Additionally, these SDRs are able to provide much higher bandwidth capture per channel and backhaul communication, with up to 3GHz of instantaneous bandwidth per channel and 4x100Gbps qSFP+ Ethernet links, providing a total of 400Gbps of backhaul. This is possible based on the architecture employed - where the device is divided into two main blocks: the radio front-end (RFE) and the digital backend. 

While each channel of the RFE can provide excellent RF capabilities with independent DAC/ADCs, the digital backend implements powerful FPGA DSP operations on-board, capable of parallel computing of multiple channels, and ready-to-use high-speed Ethernet interfaces for high data-throughput. Furthermore, the FPGA of the backend and the digital implementation of most RF functions allows the system to be completely reprogrammed and repurposed to perform different functions, for a wide variety of applications, without any hardware intervention. Unlike PXI solutions, this flexibility is not limited by form-factor, as different functions are added in the digital domain. Thus, the SDR can automatically adapt to different environments and be completely updated remotely and on-the-fly. It also significantly reduces the need for external hardware and simplifies the overall system design. The alternative SDRs can also work out of the box with existing 40/100G networks, making them a cost-effective option for extending and upgrading existing systems, especially for modern applications. 

In terms of software, the digital backend supports different open-source RF toolkits, including GNU Radio and custom C++/Python applications, while offering command line options for higher performance. This modular and flexible architecture makes the SDRs derived from it suitable for a variety of applications beyond automated test and measurement, such as military, aerospace, telecommunications and scientific research. Naturally, there are potential drawbacks that must be addressed before using these devices. For instance, many COTS SDRs may not be rugged enough for use in certain settings, however, some manufacturers offer modifications that enable them to withstand harsh environmental conditions. 

One advantage common to both architectures is access to timing and synchronisation ports. These transceivers can be triggered by an external clock signal or other timing events, allowing synchronisation of multiple instruments, while also providing precise internal clocking for synchronisation between modules. This is particularly useful for data acquisition, where high-speed sampling of multiple signals must be effectively coordinated. 

When it comes to selecting the right SDR for a certain application, there is no ‘one-size-fits-all’ solution, and system integrators must take several factors into account before choosing the correct hardware for their applications. PXI systems are an excellent choice for measurement and automated testing devices, but they are not ideal if high levels of performance, flexibility, and/or high data transfer throughput is required - which is often the case for radar systems, signals intelligence/spectrum monitoring platforms, etc. The COTS Ethernet SDRs can easily be integrated into existing and novel systems, which ultimately reduces the development cost and enables service life extension programs. However, they do not provide the PCI standard operation, which may lead to less acceptance in some industries. 

The choice between PXI and alternative Ethernet SDRs depends on the specific requirements of the system being designed, but the integration between hardware, software and external tools must be carefully considered to ensure a successful deployment. The main advantage of using modular, scalable and flexible solutions, such as COTS SDRs, is that they can be easily modified and upgraded as requirements change. Consequently, unforeseen conditions and future-proof operation for long-term service can both be addressed. As technology continues to evolve, so will the options available to system integrators. Understanding the strengths and limitations of each solution, then balancing performance, cost and compatibility, will remain a critical challenge for engineers working with SDRs.

Conclusion
While PXI is a reliable and widely used standard for automated testing and measurement, it does not address all the requirements of high-throughput SDR applications. COTS SDRs using 40/100G backhaul interfaces, high radio chain count, flexible digital backends and powerful radio front-end can offer several advantages over PXI systems - including higher scalability, bandwidth/tuning range, accuracy and flexibility for integration and extension. Additionally, these SDRs can be modified for use in service life extension programs (SLEPs), working out of the box with existing infrastructure. There can be drawbacks to COTS SDRs, though, unless robustness adaptations are applied. Which option to choose will depend on the specific application needs and the experience of the designer.


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