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Comment by JoeAltmaier | Hacker News Reader

What kind of havoc does backscatter/reflection wreak on normal tv/radio signals? Do I want this noise in my house?

Short answer: Probably okay if you have only a few of them, but you will probably see degradation if you have lots of them in a cluster.

Longer answer: The Shannon–Hartley theorem tells us how much data can be pumped though a communications channel before we get unrecoverable errors. The data capacity for modulating the backscatter will come out of the same "bucket" as the capacity for the active transmitter, meaning it will slightly reduce the channel capacity available to the active transmitter. In practice, the active transmitter is probably not near to the theoretical limit and has a rate much greater than the backscatter transmitter, so it can easily correct the errors caused by the backscatter modulation. Increase the amount of data on the backscatter channel, by upping the rate or number of transmitters, and the active transmitter probably won't be able to recover from the loss of available capacity, and the user will see an increase in bit error rate.

I'd like to see a MIMO version of the backscatter transmitter!

This communication method creates a type of interference called inter-symbol interference, or ISI, for any intended receivers of the carrier transmission. ISI is basically what happens when echoes cause a signal to interfere with itself. Since this communication method modulates the amount of echo in the carrier transmission, I can only assume that any typical anti-ISI techniques used by most receiver designs (e.g. rake receiver) would be confused by the rapidly-changing ISI caused by these devices, increasing the total noise present in the received signal.

This was my question, too! According to the paper:

>"Finally, we test the interference of ambient backscattering and ﬁnd that, even in less favorable conditions, it does not create any noticeable glitches on an off-the-shelf TV, as long as the device is more than 7.2 inches away from the TV antenna."

It's discussed in more detail in 6.4. So far they've looked mostly at TV interference. I'd be surprised if it interfered with anything else at all, but that's all I could find for now.

My mom would ask: "Does it give me cancer?" (she has the same doubts about microwave ovens)

Does anyone know, with certainty, what to answer to that?

Well, I don't know your Mom, but the answer is 'yes, but 0.0001% less risk than either the sun or time'.

Of course, I'm going by the moto here that 93% of statistics are made up on the spot.

Thanks!

To be slightly pedantic, 0.0001% less risk than the sun is actually a fair bit of risk. 0.0001% of the sun's risk is not much risk.

RF is nonionizing radiation, meaning that it isn't energetic enough to knock electrons off of individual molecules. Obviously it's impossible to say "it can't cause cancer" with 100% certainty, but in all the studies that have been conducted on RF over the last 70 years, there has never been any indication that nonionizing radiation causes an increased incidence of cancer.

Ionizing radiation (X-rays, gamma radation, etc.) is on the other side of the electromagnetic spectrum relative to visible light. These types of radiation are energetic enough to interfere with and ionize molecules, such as your DNA. This can directly lead to cancer.

The only thing dangerous about RF is that if you disperse too much RF energy into something (ie. your body) it usually comes out as an increase in temperature or as an influx of electrical current, which can cause internal/external burns, etc.

Source: mandatory yearly IEEE mandated RF safety training

If I understand this correctly, the backscatter is always there. It's residual radiation from the Big Bang. It's also why analogue tv sets show static when not tuned to a station. TV and Radio stations output a signal that's much stronger than the backscatter, so that's what gets received by the TV or radio set. I could be wrong about any of this though...

I think you may be referring to cosmic background radiation. That's a different thing from backscatter off of an ambient radiation source.

I sense a disturbance in the force!

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