# Spectrum Sensing HW

I am interested to use an existing hardware to perform a spectrum sensing/monitoring in the frequency range of 2.2–2.6 GHz.

Taking into account that the 802.11ac channels are in the range of 2.4–2.5 GHz, and the active/passive scanning feature supported by the standard, seems that the WiFi spectrum scanning can be done using home routers and a modified firmware.

Wonder how the ranges 2.2-2.4 GHz and 2.5-2.6 GHz can be covered as well.

Aware of the fact that SDRs can be used for this purpose, however, the aim of the current study is to find a HW available in rural environment, whether a home router or a base station equipment, which is capable to perform such task.

Taking into account that the 802.11ac channels are in the range of 2.4–2.5 GHz, and the active/passive scanning feature supported by the standard, seems that the WiFi spectrum scanning can be done using home routers and a modified firmware.

Yes. Wifi hardware tunes to a potential channel, senses it for a while, and hops on.

But: that hardware is only designed to detect two kinds of signal:

1. Wifi signals: These are pretty wideband signals, and you won't be able to detect narrowband signals with that
2. specific types of radar signals (I think, mostly weather radar) to avoid interfering with that radar's operation. These are typically detected as chirps by observing the post-DFT data (at least in some wireless chipset's firmware) as appears within the OFDM receiver chain; the signals as such are broadband enough to be relatively similar to Wifi channels.

So, yes, to a very limited amount, you can hypothetically do spectrum sensing with a wifi card. You'd have to find one, however, where you have a very well-documented and most importantly accessible baseband processor firmware. Good luck with that! Also, the analog properties of the receiver chain will very likely not allow you to even detect the presence of narrowband signals, and you won't be able to tune the card to arbitrary frequencies, but only within the channel raster.

Furthermore, don't forget that this is hardware specifically designed as receiver for digital transmissions – there's absolutely no reason to assume you can use what you'd see in digital baseband as being similar to the PSD of the channel. You'll have to deal with an always-on AGC, quite possibly some hardware that might or might not try to correct the sample rates to make sync easier, and very limited processing power for tasks that are not supported by dedicated hardware components within the baseband processor.

whether a home router or a base station equipment, which is capable to perform such task.

I can only guess as to what you need:

• if it is a WiFi device for which a spectrum sensing firmware exists: No. There's no such device that I'd know of, but I'm sure there's some firmwares that can give you something like a "Wifi occupancy" graph. But I'd argue that would only be a very restricted aspect of spectrum sensing – you wouldn't necessarily (or even, likely) detect non-WiFi usage with that, as it works on a much, much higher level.
• if we're talking about finding a device that you can modify yourself: Again, the much more time- and probably cost-effective alternative would be getting proper general-purpose SDR hardware.
Alternatively, there's a lot of different vendors that offer WiFi chipsets. Those are generally SoCs, with a user-programmable CPU core, and a CPU core dedicated to handling WiFi (as that gets quite timing-sensitive). You can usually program the first and must live with the firmware hidden away in some usually inaccessible memory somewhere. You'd have to do a proper market analysis to find a chip where you can do what you need from the user CPU, or you'd need to become a hardware reverse-engineering expert. And that before you could even write a single line of firmware that would allow you to do spectrum sensing.

So, I think it would be very worth writing down a detailed requirement of what you want to sense, and decide whether WiFi hardware has the analog capabilities to do that, at all. If it does, I'd basically go out and buy every single Wifi adapter to which I have access to a firmware blob – that includes the common and cheap atheros9k_htc USB dongles and so on. Then, I'd figure out what part (and if it's the device driver, maybe, even) does the legally required spectrum sensing to avoid interference with primary users (radar). And that's where I'd try to "shim in" my sensing. Again, I do not believe Wifi hardware is what you're aiming for when you're actually after spectrum sensing, since AGCs really don't allow you to compare measurements.

• Hello Marcus, Thank you for such a detailed answer. Several issues: 1. The signals I am interested to detect are wideband as well – mostly OFDM with BW of 10Mhz. 2. Seems that the Weather Radars are using the 5Ghz frequencies (5600-5650 MHz). en.wikipedia.org/wiki/Terminal_Doppler_Weather_Radar At this stage I am interested in the range of 2.2–2.6 GHz. 3. Processing is not required. All is needed is the(swiping)FFT output. The processing will be performed in the centralised host. – SenSen Apr 17 '17 at 8:45
• 4. I am currently working with an SDR(Ettus) for the research purposes. The final product can not be based on SDRs due to relatively high unit cost. The purpose of the study is to figure out a way to use an already available network of HW devices(Home Routers/Base Stations/…) with spectrum sensing capabilities in order to detect specific emitters in the range of 2.2–2.6 GHz. Will be glad to elaborate further in person. Is there an email I can send you more details to? Thanks. – SenSen Apr 17 '17 at 8:47
• @SenSen You're welcome to send me an email to mueller$\text{@}$hostalia.de, but to be honest: I'll answer questions for free in public (i.e. here), but if you need to discuss details of your project in private, we'll have to set up a consultation contract. (this doesn't necessarily apply if you're customer to one of my clients) – Marcus Müller Apr 17 '17 at 9:06
• Thank you Marcus. I am currently studying the topic in depth. Would like to review the datasheets of: Avago bcm43516, Qualcomm Transceiver QCA988 and MediaTek Router SoC - MT7623A/N. The documentation was not found online, as a result, inquiries to the companies were sent. When I will have more info on the issue, will be glad to discuss in private. – SenSen Apr 21 '17 at 11:59
• that makes no sense, these are simply wifi chipsets. You can do this with a purely receiving SDR and any wifi device, as I tried to explain. There's nothing to be done here. – Marcus Müller Apr 21 '17 at 12:42