by: Larry Desjardin
Frequent readers of the Test Cafe blog know that I’ve added a recent focus on 5G characterization and test systems. Why? It’s simple. Test Cafe discusses instrument architectures, particularly modular instrument architectures, and 5G presents a key opportunity for modular instrumentation.
5G (fifth generation) is the next generation of cellular communication standards. For more information about 5G, and why I believe 5G will be a boon for modular instruments, read my recent column, 5G to disrupt the test equipment market. Due to 5G’s greater bandwidth and massive number of channels, I made an unequivocal prediction for vendors: if you don’t have a modular solution, you won’t be playing a significant role in 5G. In a subsequent column I predicted that microwave would be coming to PXI, driven by many of the same dynamics.
Since that time, National Instruments and Keysight Technologies have introduced some exciting solutions in the 5G mmWave space, all based on modular instruments. Several weeks ago I described the internals of an NI system used by Nokia to create a 2x2 MIMO mmWave 5G prototype system, operating at 73 GHz. Now I’d like to take a look at the Keysight 5G channel sounding system.
Before I do, perhaps a little description of what channel sounding is all about. Channel sounding is a technique to measure and characterize the air interface of a RF communication network. That is, to characterize the mathematical transfer function from one or more transmit antennas to one or more receive antennas. 5G channel sounding is both mission critical and difficult. At mmWave frequencies the channel is not only complex (path reflections and attenuation), but dynamic as well. The channel characteristics change as any antenna moves or there is movement of a reflective surface.
The simplest model characterizes the channel between a single transmitter and single receiver:
But in the real world, the channel is dynamic, as exemplified by this image from Nokia Networks:
Here, users 1 and 2 may experience the same channel function due to their proximity, but their movement coupled with the movement of all the surfaces around them mean that the channel is constantly changing its characteristics. It is paramount that researchers characterize exactly what is to be expected in the spectrum of interest before deploying 5G devices (coupled with their complex beam forming algorithms).
So, what has Keysight done? Let's take a look.
Keysight 5G channel sounding system
Keysight has created a reference solution for 5G channel sounding. If you are unfamiliar with the reference solution concept, read my recent article, Keysight mobilizes for modular. Essentially, Keysight creates an example system that can be duplicated or modified by a customer. Typically these are an 80% solution, where customers add the extra 20%, which tailors it to their specific application.
Let’s take a look at the hardware elements of the Keysight reference solution. The diagram below comes from Keysight, but has been modified by me to show the transmit and receive sides (typically in different racks), as well as labeling the PXI and AXIe modular instrument chassis.
The hardware elements of the Keysight 5G channel sounding system are shown above. The transmit system is on the left, the receive system to the right.
The 33511B AWG supplies trigger signals to periodically synchronize the two systems, while the two rubidium clocks keep the systems in lockstep.
On the transmit side, an AXIe AWG creates a specific customized sounding signal that is then up-converted up to 44 GHz with a traditional microwave signal generator. This modulated mmWave signal is routed via a solid state switch to a specific antenna. Though only one antenna may be driven at a time, the solid state switch allows low latency switching between antennas for situations where the channel is changing in real time, retaining a high correlation between successive measurements.
The receive side is multi-channel, and takes advantage of AXIe’s and PXI’s modularity. Either 4 or 8 channels are simultaneously received via the external antennas. These are routed to synchronized down-converters in the PXI chassis. Driven by a common local oscillator (the N5183B), the phase and timing relationships of the received signals are retained. Also in the PXI chassis are variable attenuators for each channel. These allow each signal to be scaled optimally for the M9703A digitizers, in another AXIe chassis. Each of the AXIe digitizers have internal FPGAs that can process the data, and reduce total data bandwidth needed to about 1.6 GB/s. This can be streamed off board for further processing.
That’s a quick look at the hardware elements. Let’s look at the software.
Keysight 5G channel sounding software
The diagram below shows how the Keysight software interacts with the channel sounding hardware.
The user employs Keysight signal generation software to create their own sounding signal. Alternatively, a sample is supplied by Keysight.
On the receive side, the AXIe digitizers can perform a CIR (Channel Impulse Response) via high speed FPGAs. This significantly reduces the data bandwidth required for real-time streaming and storage. Finally, SystemVue is used to post-process the stored data to perform the channel parameter estimation. Alternatively, I/Q data may be analyzed by the end user’s tools of choice.
Now, let’s look at the total solution.
Keysight complete 5G channel sounding system
The image below shows the complete system, racked in two cabinets.
Having the cabinets racked separately allows the systems to be moved independently from each other. The antennas can be seen above the units.
The Keysight system is the first example of a robust 5G channel sounding solution. While limited to 44 GHz, it is straightforward to see how that may be increased. As expected, modular instruments based on AXIe and PXI play a critical role, particularly on the multi-channel receive side.
Today, 5G is embryonic. The total test solutions required will be numerous and complex. Channel sounding is just one of dozens of applications needed before 5G becomes real. But it is a start.