CWNP CWNA – Spread Spectrum Technologies Part 3

13. Hopping Sequence

So the hopping sequence that I was just talking about is very well published. And so that means they had a predefined hopping sequence. I mean, really, if you think about it, if I had some antenna that was transmitting on different channels, hopping from one to the other to a receiver. And remember back in the day when I was working with this, when it first at least arrived in my life, it was on the 900 MHz range, and it was about putting what they called mobile data transmitters inside of the police cars so they could have computer access to records and everything else from the police cars.

Believe it or not, that was in the 1990s, so how long ago was that? But anyway, they would use the FHSS because that was really the encoding traffic at the time. And unlike the 1990s, today they get so much more information. I mean, those police officers that have the computers can pull up your driver’s license, they can look at your picture, make sure you’re not lying about who you are, scan it. I mean, they can do a lot more than in those days when they could just run a registration, which, by the way, if you care, it’s called a 1028 in police code. Or they could run warrants, see if you had any, which is called a 1029. Or they could run your driver’s license, which was called a 1027 just in case any of you listen to a police scanner. So you know, that kind of cool stuff.

Anyway, that was mostly just text information. So the low speed and the frequency worked out very well, but they still use the frequency hopping. Now, the problem that I saw when I first was learning about it, I thought to myself, well, if the antenna decides it’s time to change the frequency, how does the other side know what’s the next frequency that they’re going to go to? I mean, is it randomly picked? No, it’s not. They have what’s called a hopping sequence or a hopping pattern.

And so both sides, when they talked to each other, would know what that pattern was and they would know how often in time cycles or time slices that they would have to go from one channel to the next. So again, it was very small carrier frequencies. We didn’t carry a lot of data and we didn’t stay on one channel for a finite amount of time. And so the FHSS radio would have to know what that hopping pattern was to be able to get that non stop flow of information.

14. Dwell Time

Now, the dwell time with FHSS is defined as the amount of time that a system transmits on a specific frequency before it switches to the next frequency in the hop set or that hop pattern. Now, the FCC specified what the maximum dwell time is going to be, usually about 400 milliseconds per carrier frequency. During any 32nd period period of time, your typical dwell times are between 100 milliseconds and 200 milliseconds.

And I know that sounds like incredibly fast time. Go from one to the other, to the other, to the other. But remember, in the world of computers, you can send a lot of data between 100 milliseconds and 200 milliseconds. So it wasn’t that interruptive. And the standard from the eight or 211, the old or the original standard, specified that the hopping sequence must have at least 75 different frequencies, each one of them only 1 MHz wide.

15. Hop Time

Another part of FHSS is what we call the hop time. Now, the hop time is not a specified period of time, but really it’s a measurement of the amount of time it would take for the transmitter to change from one frequency to the other. In other words, if I’m going to keep hopping between channels, it does take a little bit of time to get from one channel and switch to a different channel. Remember, panels mean frequency, so the radio has to change the frequency that it’s transmitting on. So hop time, though, typically, is a very small number, often about 200 to 300 microseconds that we would use.

16. Direct Sequence Spread Spectrum Part1

Another type of encoding is what we call the DSS. Again, to me it’s another tongue twister, the direct sequence spread spectrum type of technology. Originally it was specified as the primary or the root method of being able to give you from one to two megabits per second of radio frequency communication in the two 4 GHz band. In other words, words, the technology did not take you up to the eleven megabits per second or the 54 that we got with some of the others, but we kept improving the technology about how we encoded ones and zeroes through the radio frequency. Now, the 800 and 211 B addendum provided a 5. 5 and an eleven megabits RF communication using that same band. That just meant they had to have a little bit wider channels to be able to send that to you, which is why we saw only three channels in that spectrum rather than twelve like we saw in the 5. 0

GHz range with 800 and 211 B. When we wanted to get the 5. 5 or the eleven megabits per second, we used what was called the high rate DSS, which was just a little bit of a twist on how we did the direct sequence spread spectrum, but it was a way of being able to encode more ones and zeroes in that same frequency. And that’s what you’re going to tell as we go through the different options that we have of how we continue to find better ways to send more data over the same frequency. Now, in that case, the DSS, unlike frequency hopping, chose a channel, stayed on that channel, and used just that channel for the communications. The idea was no more hopping from one very narrow frequency range to the next one. Now, the data that is being transmitted is basically going to be spread across the range of frequencies that make up that channel.

17. Direct Sequence Spread Spectrum Part2

So when we continue to look at DSS, one of the things was finding a task or a way as a task of adding additional redundant information to the data. And if we did that, it was called processing gain. Redundancy was to try to take care of interference or something else that might cause a problem with the transmission. Now, in this day and age of what we call data compression, it seems strange that we would actually use a technology that adds data to our transmission, but by doing so, the communication is more resistant to data corruption.

Some of you might call it parity. In other words, we might send X number of bits, that is the actual data, and then add a few extra bits so the receiver could tell if the data they received had been corrupted by some sort of interference or not. So the idea is that the system would convert the one bit of data into a series of bits that are referred to as chips. Now, this is important because we’re going to talk about the chipping codes later on. So, to create the chips, we basically did a Boolean exclusive, or what they call the XOR on the data and the fixed length bit sequence to create what we called a pseudorandom number. In other words, it was kind of like a checksum, if you would, the PNSE random number.

So PN code is known as the Barker code, and the binary data of one and one can be represented by the following types of chip sequences. So when we look at the table for exclusive or, it’s what we would call in Boolean mathematics, a logic table, if you would. And the idea was we had some things that were true, which would be a one. We would have some things that are false, a zero, and then we would compare that statement to get the ultimate answer if it was true or false.

Now, with an and statement, I would have to say that if I had two arguments, they would both have to be true for the answer to be true. If either one of them was false, then the answer was false. Exclusive or is a little bit different, an exclusive or. For the statement to be true, one of the two arguments has to be true, but not both of them. And if they’re both false, then it’s false. So, you know, if I said and I created a statement that says it’s either Monday or Tuesday, right? So if on the false statement, I know I put a zero there, but let’s just say this is the true and that’s the false, if I said I had a statement that said it’s either it’s either Monday or Tuesday today, well, if you look at that, they can’t both be true. It either is Monday or it is Tuesday, but we can’t be both.

And so the answer when both sides of the argument are true would actually be false. That same question. It’s Monday or Tuesday. Well, now we can look at that and say, well, if it’s Monday, it’s true. And so when one of the two sides is true, but not both of them true, then we get a true statement. If I said it’s Monday or Tuesday, and let’s say it was actually Wednesday, that means both were false, the answer would be false. So the only time exclusive or gives you a true answer is if one of the two sides of the argument are actually true, and where one is considered a true and zero is considered a false. We could compare data, I could take the data being sent with an exclusive or and calculate for each row what the answer was going to be. And when I look at this, the answer would be all ones, because it’s either true or it’s false. It’s either false or it’s true. And so we can use some of that especially important when we’re doing some of the error correction.

18. Direct Sequence Spread Spectrum Part3

Now, with the DSS, the sequence of chips is going to be spread across a wider frequency space. Although one bit of the data might need only 2 frequency space, the eleven chips that we use some for error correction will require 22 frequency carrier space. Now, did you get that for one bit of data? Next we’re going to have, as a part of the actual chips, an extra eleven bits, just to make sure that the other side understood that I was sending a one instead of a zero.

So it really looks like a waste of bandwidth to be able to send one bit. But then again, what would we say about spread spectrum? We have more bandwidth than we need for the data that we’re sending. Now. The process of converting a single data bit into a sequence is a part of what we call the spreading or the chipping. When the Barker code is used, as many as nine of the eleven chips can be corrupted, still giving you the opportunity to know what the real data was. So if I’m sending one chip and then one bit at I’m sorry, and eleven bits for the chipping sequence, if I lost nine of those, you would still know if I was sending you a one or a zero. So it became very resilient, especially with any other type of radio frequency interference.