Using FPGAs to add IEEE 802.11n channel emulation capabilities to wireless test equipment

If you are already a hero with regard to designing and testing wireless systems, then today’s announcement from octoScope will make perfect sense. For myself, it made me realize just how little I know…

Fortunately, I got to chat with Fanny Mlinarsky from octoScope, and she was kind enough to explain things using simple words with few syllables so that (eventually) even I could understand what was going on.

Of course the only way we will know if this “stuck” is if I can explain it to you without totally confusing the issue.

Multipath and Doppler
Let’s start with the fact that when a radio signal is travelling from a transmitter to a receiver it may be subject to a variety of different phenomena that cause interference and fading. Two terms that we might hear in this context are “multipath” and “Doppler”.

Multipath refers to a radio signal that reaches the receiving antenna by two or more paths. This may be caused by reflection from bodies of water or terrestrial objects such as mountains and buildings (also things like atmospheric ducting and ionospheric reflection and refraction). The effects of multipath include constructive and destructive interference, and phase shifting of the signal.

Now consider what happens if you are standing still while a fire engine races past. The siren will sound different when the fire engine is approaching you to the way it sounds when the engine is receding from you. (You would perceive the same effect is the fire engine was stationary and you were the one that was moving.) The change is caused by the sound waves being compressed when the engine is approaching you and stretched out after it has passed you.

Known as the Doppler effect (or Doppler shift), this phenomenon was named after Austrian physicist Christian Doppler, who proposed it in 1842. The Doppler effect describes the change in frequency of a wave for an observer moving relative to the source of the wave.

The point is that the same thing happens with radio transmissions. If you are in a car talking on your cell phone, then the radio signals passing between you and the nearest base station radio tower are going to be undergoing some amount of Doppler shift.

In reality, the sort of radio signals we use with our computers (e.g. WiFi) and cell phones (e.g. LTE) are subject to both multipath and Doppler effects.

Multiple-input and multiple-output, or MIMO
In the case of radio systems, MIMO (commonly pronounced "my-moh" or "me-moh") refers to the use of multiple antennas at both the transmitter and receiver to improve communication performance. This is one of several forms of smart antenna technology. (Note that the terms "input" and "output" refer to the radio channel carrying the signal, not to the devices having antennas.)

Traditional test environments
If you are designing a wireless system like a cell phone, you obviously want to test it before you send it out to the end users. Furthermore, you want to test in in a variety of different environments – in a small room, in a larger building, in a street surrounded by skyscrapers.

One way would be to walk around with the cell phone continually calling home base (“Can you hear me now?”), but this would be time-consuming, expensive, subjective, and hard to repeat exactly. The alternative is to model all of the channels being used. Not surprisingly this is known as “channel modeling”.

Consider the scenario shown below; this is based on the use of a traditional channel emulator, which typically costs around $200K


So we have a signal generator (or some other piece of test equipment) which is generating our MIMO signal. The output from this signal generator is at the radio frequency (RF) level; however, instead of feeding this signal to an antenna and broadcasting it, the RF signal is fed directly into the traditional channel emulator. Thus, the first thing that the traditional channel emulator unit does is to perform a down-conversion. Then it models the effects of multipath and Doppler on the signals. Then it performs an up-conversion to re-generate the RF signal that is fed to the device under test (DUT).

In addition to being horribly expensive, the additional down-conversion and up-conversion employed by the traditional channel emulator can introduce unwanted noise and artifacts. If only there was some other way…

octoScope’s octoFade approach
Now, this is where things start to get very interesting – especially for those of us who bask in the delights of programmable logic space (where no one can hear you scream). The clever guys and gals at octoScope, who have a tremendous amount of expertise in channel modeling, have created some software called octoFade, which accurately models multipath and Doppler effects on MIMO wireless systems.

Actually, when I say that octoFade is software, it might be better to think of this in terms of a library of algorithms (we will return to this point shortly). The thing is that the signal generator (or whatever piece of test equipment is being used) almost certainly already contains FPGAs. These will be located prior to the RF signal being generated as illustrated in the following diagram:


The obvious difference between this diagram and the previous one is that we’ve removed the $200K channel emulator from the picture. As a by-product we’ve also removed any errors associated with its down- and up-conversions.

Now, the octoFade channel modeling algorithms are being performed in the signal generator (test equipment) before these signals are converted into RF. In some cases, the existing FPGAs have enough unused resources to execute the octoFade algorithms as-is. In other cases they need to be swapped out for larger parts. In other cases it may be necessary to use an FPGA daughter card. But in all cases, the folks at octoScope say they can help wireless test equipment vendors and integrators to quickly and easily implement this functionality at a fraction of the cost associated with traditional channel emulation techniques.

So how did I do? Did I make this understandable? Did it make sense?

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