Low Probability of Detection/Low Probability of Interception(LPD/LPI) in layman’s terms refer to microwave signal equipment. As the name suggests, and the image above demonstrates, microwave signals are extremely Line-of -Sight. The use of microwave equipment happens all around you everyday; looking on most communications relay towers, you’ll see microwave relay dishes normally in direct line with another dish in another location. An easy way to understand the concept is that if your standard 2m VHF and 70cm UHF propagation works like lamps in a room, microwave works like a laser pointer.
We know from our early work on covert communications that, if the warden Willie knows the statistics of the noise affecting his receiver, Alice can transmit no more than O(sqrt(n)) covert bits reliably to Bob in n channel uses of an additive white Gaussian noise (AWGN) channel. However, other authors have recently considered the case where Willie has some uncertainty about his noise statistics, in particular the average power of the noise impacting his receiver. These authors have shown that, if Willie does not know his noise statistics exactly but rather only knows that the average noise power falls in some range, Alice can covertly transmit O(n) bits in n channel uses. Here, we take a different approach. Rather than abstracting the error in Willie’s estimation process to parametric uncertainty in his statistical model, we provide Willie with the full collection of channel observations and allow him to use them in whatever manner he chooses (e.g. for first channel estimation and then detection of Alice). In particular, we assume that Willie lacks knowledge of his channel statistics but observes T(n) length-n codeword periods, one of which may be used by Alice to attempt covert transmission. Under most operating conditions, we show that the warden Willie, even without a priori knowledge of his channel statistics, can limit Alice to the same covert performance scaling as when he knows his channel statistics, hence pointing in a different direction from the recent work of other researchers for this important problem. (Paper in preparation for IEEE Wireless Communication Letters ).
Motivated by Willie’s inability to detect transmission by Alice if we can somehow keep him from understanding the background noise power he should be observing when Alice is not transmitting, we consider the case where a single jammer, whom we term “uninformed” because he does not know when Alice might transmit, simply varies the power of the random noise that he puts into the environment. We consider various environments (AWGN, fading) and constructions for the jammer’s strategy under which we are able to demonstrate that Alice can transmit O(n) bits in n channel uses. (Paper submitted to the Allerton Conference on Signals, Systems, and Computers.)
My previous answer to reader questions generated some interesting traffic in linked locations; so in the interest of common readership I’ve posted some links to get the feet wet of the uninitiated. So in case you forgot your electrical engineering degree, I’ll attempt to make things simple (I’m actually one of those knuckle dragging liberal arts fools…) as to why this is important beyond the standard stuff. Admittedly MW is not my forte in the communications world (that’d be HF for me), but contained above is a half decent primer from resources I’ve used to better understand it in the past.
Obscurity is fairly important for certain aspects of your communications plan. As I’ve written before, there’s different tools in the bag for different reasons, and I’ve got at least 15 different types of wrenches at that; radio is no different.
Let’s say you live in an area several miles between homes/BOLs of other folks in your group. In order to keep relatively secure communications, a lot more than just a wire hanging from a tree needs to be done. Heavy use of directional antennas are highly advised, due to the fact that interception requires the listener to be in between the two antennas transmitting point to point.
While the lower VHF bands are great for Rural terrain, they can be eavesdropped on by just about any fool with the ubiquitous Baofeng wandering in between you and the recipients. In addition, your signal is going to travel further than simply the intended target. A means to avoid this is by using parts of the spectrum that aren’t exactly common- and one of the answers to that is Microwave.
While not impossible to intercept, it’s much more difficult than with commonly found or sourced equipment. And yes, it’s construction and implementation takes quite a bit of work and time to get right; but the most worthwhile things always do. It is mostly practical for fixed position transmitting only, but then again, it’s the right tool for the job.
The Alinco DJ-G7, a tri-band HT that utilizes the 23cm band…semi-rugged and a good way to get started. It has many attractive features for what it is, and can be a great multi-use tool on the ground being both a transceiver and a wide-band communications receiver.