Targeting 5G with powerful building blocks
Image courtesy of National Instruments
Figure 1 shows the basic block diagram of a 5G transceiver. To the left are the mmWave air interfaces, while the rightmost edge shows the intense FPGA processing. In between are the up and down converters and the baseband elements, the latter typically being high-speed analog-to-digital and digital-to-analog converters.
Frequent readers of the Test Cafe blog will recognize this diagram from when I first reported on it in April of last year. At that time, NI had announced the industry’s first 5G prototyping system, and you can read my architectural analysis here. This first system was deployed by Nokia to create a 10Gb/s 5G prototype in the 71-76 GHz band.
NI’s announcements at NIWeek effectively built on this same system architecture. Since a year ago, the 28GHz band has become the prime candidate for many first deployments, particularly in the US, Japan, and Korea, according to James Kimery, director of RF research and SDR marketing at NI. It also aligns with the so-called “Verizon specification” and the latest 3GPP specifications. To meet this requirement, NI did what they said they would do a year ago—offer modular heads for various bands that are interchangeable with others. This is what NI did with the MTS-28, short for 28GHz mmWave transceiver system. The new head actually covers 27.5 to 29.5GHz. Less you think this can only be used for prototyping, the same head can be used in many test applications. Senior Editor Martin Rowe reported on an AT&T-developed mobile channel sounding application last week that uses the 28GHz heads, and many of the same system components.
Once the signal has been downconverted and digitized, the real heavy lifting begins. This brings us to the “multi-FPGA processing” shown to the extreme right of Figure 1. 5G is fundamentally challenging to process, as it involves processing multiple channels, each with bandwidths much greater than a gigahertz of bandwidth. This is where NI’s FlexRIO architecture comes in. FlexRIO pertains to FPGAs, either in an instrument or as standalone modules, which can be programmed by a user using LabView FPGA.
The PXIe-7915 is the latest, and most powerful, of NI’s standalone FlexRIO coprocessor modules. It is based on Xilinx’s Kintex UltraScale technology, their fastest and densest yet. Each module is capable of performing 7 million 1024-point FFTs per second. They can be ganged together for more processing power, and a 24 million FFT/second application was shown at NIWeek. The module itself supports 6.7GByte/second peer-to-peer streaming over the PXIe backplane. This is getting close to the PCI Express Gen 3 ×8 theoretical limit.
For even higher speeds of streaming, the UltraScale’s 16Gb/sec serial lines may be used. Each PXIe-7915 brings eight bi-directional serial lanes out. Multiply eight by 16Gb/s, and you are close to 16GB/s of total throughput, though tempered a bit by the 64/66 encoding. These lines may be used for communicating with other FlexRIOs, or to a data converter module (ADC or DAC) that also has a FlexRIO serial interface.
Figure 2 On this NI PXIe-7915 FlexRIO module, the Xilinx UltraScale FPGAs can be seen in the rear, while a customizable mezzanine module is shown up front.
Figure 2 shows a photo of the new module. The shielded area is a user-customizable mezzanine. Essentially, a user or NI partner can design their own FlexRIO-based instruments using this approach. Grab the latest front end and data converter technology, lay it down on a mezzanine board, and interface it with the FlexRIO serial lines. Indeed, NI has developed a white paper detailing how to do exactly that. Using JESD204B data converters (serial based data converters) allows a user to stream data to and from the data converters of their choice. Most importantly, the mezzanine is on the inside. Previous generations of FlexRIO only offered a pluggable external faceplate module for custom circuitry. This limited the custom electronics to 5W, a mismatch to the highest speed FPGAs. Now users can take advantage of the PXI chassis’ forced air cooling and high power density to use the data converters of their choice. Whether or not this is widely adopted by users, this approach will most certainly be adopted by NI themselves as they deliver new FlexRIO-based data converters in the future. As I’ve pointed out in the past, modularity is not just a benefit for the users; it can also be a powerful tool for vendors to achieve time to market.
5G challenges conventional instrument architectures. Multi-channel, high streaming rates, unprecedented processing, and mmWave spectrum collude to make 5G a difficult architecture to prototype, emulate, and test. NI is using their unique combination of modular architecture, FlexRIO, and LabView FPGA to address these challenges.
Larry Desjardin is a regular contributor to EDN's Test Cafe. He served in several R&D and executive management positions with Hewlett-Packard and Agilent Technologies, and current manages a consulting company, Modular Methods.
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