In my last two blog posts, I presented the MO1000 FPGA mezzanine card (FMC), our new multi-channel digital-analog converter (DAC) FMC. The FMC is still under development but in this post, I show how the MO1000 FMC and what is being developed around it will be useful in the creation of massive multiple-input/multiple-output (MIMO) channel emulators for testing the next generation of communication protocols.

First, I’d like to introduce a great new feature that will be brought to the market by Nutaq with the upcoming release of the MO1000 FMC in a few months: hybrid stacks. This innovation is what will help the development of massive MIMO channel emulators.

In previous blog posts, I explained Nutaq’s double-stack FMC (for details, see this presentation). Now, the double-stack design was developed with the goal of increasing our FMC’s channel density. The MO1000 is the fourth FMC from Nutaq implementing this in-house standard. The following table summarizes the FMC combinations available thanks to the double-stack.

 

Radio420

LVDS-xIn-xOut*

MI125

MO1000

Single stack

1 RX channel /

1 TX channel

16 input channels / 16 output channels

16 ADC channels

8 DAC channels

Double stack

2 RX channels /

2 TX channels

32 input channels / 32 output channels

32 ADC channels

16 DAC channels

* Standard LVDS-16In-16Out configuration

If we want to design a massive MIMO channel emulator, we need a few ingredients:

–          A powerful processing unit – the Virtex-6 FPGA of a Perseus601x or Perseus611x

–          An interface board with multiple ADC channels – the MI125 FMC

–          An interface board with multiple DAC channels – the MO1000 FMC

We also need to make sure that the design of a system with all these pieces put together will introduce as little phase difference or distortion as possible on every channel of the system.

In designing the MO1000, we took special care to make it as generic as possible as far as the FMC connections involved in the double-stack design were concerned. The idea was to enable the MO1000 FMC to be part of a new kind of FMC double-stack made up of two different FMCs – a hybrid stack. As soon as the MO1000 FMC is released, so will the MI125/MO1000 hybrid stack, which will implement 16 ADC input channels and 8 DAC output channels on the same FMC site.

The following diagram shows the channel routing in the hybrid double-stack.

MO1000/MI125 Hybrid FMC Stack

Not only will this hybrid stack implement 16 ADC and 8 DAC channels on a single FMC site, it will also bring a great new capability to the MI125, which will be sitting on top of a MO1000 in such a stack. In a double-stack configuration, we refer to the FMC closest to the carrier as the “master” because when the stack is used as a single FMC system (MI125-32 or MO1000-16), it is responsible for the clocking of the whole stack. On the MI125, there is no PLL available to generate a custom onboard clock. When used as a slave on an MO1000, however, it will be possible to use the MO1000’s CDCE62005 clock generator to generate a custom clock for the MI125 on the OUT3 output clock signal, as shown below in red.

MI125 clocking using a MO1000 master FMC

Because of the way the MI125 and MO1000 are designed, this will create a system with a phase from input-to-processing-to-output across all channels that is as constant as it can get. This makes a single Perseus processing board with an MO1000/MI125 hybrid stack an ideal system to implement an 8 by 8 MIMO channel emulator. The number of channels in the emulator can then be scaled by carefully synchronizing multiple instances of this system together.

The concept of double-stack FMC adds a lot of room for increasing the density of an FMC-based system. By adding the ability to mix different FMC in a single stack on the same site, it opens up a lot of possibilities for the development of more complex FMC systems that could not otherwise fit on the form factor of a single FMC. We are thrilled about this new hybrid FMC stack concept and see a lot of potential in it going forward­.