CWNP CWNA – High Throughput (HT) Part 2

11. Spatial Multiplexing (SM) Part2

So don’t confuse the unique streams of data with the number of transmitters that you have. Again, you don’t have to use all three. It’s just what we really want to know is how many unique streams of data that you’re sending. So let’s say I have an access point and a receiver that can get all three streams, all three unique streams. Notice if I wanted to send you the digits, um, in the numbering system, I can send part of it in one stream, like the one, two, three, and then the 456789. So when you’re done, you effectively in the same amount of time that, if you were in a traditional network, would have only gotten one, two, three, and then four, five, six. So in almost one third of the time, again, tripling, I was able to get all of the information from you.

12. MIMO Diversity Part1

So think of this way. When people get a little bit confused about mimo, just ask the question here. If you were to cover one of your ears with your hand, are you going to hear better or worse with just one ear? And I’d hope you all say worse that you would only hear you’d hear better with two, even though some of the people I’m with think that I have only one good ear. Really? It’s called selective hearing. But that’s another story, isn’t it? Now, do you think you’d be able to hear more clearly if you had three or four ears instead of two? Now, don’t sit there and take this too technical where you’re thinking, wow, I’d be really disoriented. We’re just talking about you have more methods of receiving noise.

I’m sure there’s been plenty of times with just your two good working ears that you hear somebody walking up to you on one side and a car going down the street on the other side, and you’re aware of all of that traffic. You’re getting double the input, like you would if you had two antennas just from the way in which you were able to receive that information. And so having three or four is just going to make it even better. Now, do you think you could hear sounds from greater distances if you had three or four ears instead of two?

And now this one, I’m only going to go by what science says is yes, that if a human being would have more than two ears, they would actually be able to hear more clearly and with greater range. Maybe we just need bigger ears like dogs have. I’m not sure. But anyway, that is in my analogy to try to tell you how having this antenna diversity is helping us in transmitting further distances and in transmitting more information by having the diversity of having two or more antennas.

13. MIMO Diversity Part2

Now, your antenna diversity can sometimes be thought of or mistaken for spatial multiplexing, which are used by mimo. A tenant diversity, meaning receiving and transmitting, is a method of having multiple antennas to survive the effects of multipath. As we talked about already, mimo is taking advantage of multipath with spatial multiplexing to increase the data capacity. But a simple antenna diversity is a way for trying to compensate for the multi path. That was bad to us. Remember when we first talked about the effects of radio frequency? We said that the antenna diversity, in the beginning, where they had the two antennas, which everyone had the better sounding quality, we would take that and drop the other one. So multi path still produces multiple copies of the same signal. That’s still going to happen if I’m transmitting with two or three antennas. And so with each antenna, even though I just got through saying, we’re taking multiple input, multiple output, we are. But each one of those could still have multipath problems. And so the idea of antenna diversity is to recognize that, take the best quality of those signals, and ignore the others.

14. Space -Time Block Coding (STBC)

Now, the spacetime block coding is a method where the same information is transmitted on two or more antennas, and it is a type of transmit diversity. So we call it the stbc. But I like just calling it spacetime because I told you, it sounds cool to me. Anyway, space-time can be used when the number of radio chains exceed the number of spatial streams. Remember, each radio chain was really, in essence, its own antenna. antenna. So by sending copies of the same information on multiple antennas, the actual rate of data transmit is not increasing. But what we’re trying to do is take care of some of the other problems we have in how we transmit information and how it’s being received.

15. Cyclic Shift Diversity (CSD)

The cyclic shift diversity is another transmit diversity that we had in the eight or 211 standard. Unlike the spacetime, a signal from a transmitter that uses csd can be received by the legacy 800 and 211 gs and A devices.

So if you have mixed mode deployments where you have 811 N and G and A devices, then there needs to be a way of being able to transmit the symbols in the legacy OFDM over multiple and transmit antennas. And so that’s where the csd would come in.

16. Transmit Beamforming (TxBF) Part1

Let’s talk about another part of this 811 and amendment, which is the transmit beam forming. It’s just an extra optional physical capability that we can use, and it uses phase adjustments. And the idea here is that if you have two or more transmitting antennas or if you have more antennas than you have spatial data streams that you need to send, then one of the for transmitting antennas can do some things to help us with both the distance and the strength of the signal.

So it still has a mimo transmitter using multiple antennas and it will adjust the phase and amplitude of the outgoing transmissions in a nice coordinated method. And I’ll draw that out a little bit for you. But the results are we want the phase changes to result in a constructive method, right? Not one being 180 degrees out of phase and canceling a signal, but to be constructive in the multipath communications, which hopefully will give you a higher snr and of course a bigger or greater amplitude, letting you then have greater range for the different clients.

17. Transmit Beamforming (TxBF) Part2

Now, this type of communication does require that both sides kind of negotiate that they’re going to be using this type of transmit beam forming, in other words, that either have some implicit feedback or explicit feedback from both sides. And one thing you could do is just send what they call a sounding frame or a null frame to be able to help get an idea as to whether or not not or you’re going to be able to work with this. But when using implicit feedback, the beam former is going to send a sounding frame and receives along training symbols transmitted by the beam form e, which is funny to say, which then the MIMO channel between the beam form e and the beam former can be estimated by the beam former. All right, it sounds like some legal jargon. So what are we doing? We’re sending the same information. We’re sending them all in the same phase. And what happens when you send the same data in the same phase? What do we say happens? That smaller signal, right? That’s now in phase with this other signal is going to make a result of having a stronger signal because they’re going to amplify each other. And we’re hoping that that’s going to be more constructive in our multi path.

18. 20 MHz Non -HT and HT Channels Part1

So we also have to consider what’s changed with the channels in the current four realm. Even in the 5. 0, we’re using 20 mhz channels. But now we’re going to talk about the non high throughput channels and the high throughput channels. So 811 A and G, as I said, use 20 mhz OFDM channels and each one of those channels inside that, remember the channel was kind of a range, had 64 sub carriers. Only 48 of those were to transmit data which are towards the center of that channel. And then four of the sub carriers, the ones on the outside, are used as kind of pilot tones, right, to help try to prevent some of the interference from anybody on either side of them in that channel. And then the remaining of those weren’t being used. Now OFDM by itself has its technology to employ the use of what they called convolutional coating and forward error correction. But in 8211 nht, or in this case the ht channels, they’re going to use the same OFDM technology inside of their 20 mhz channel or even a 40 mhz channel. Now, if they wanted to. But the 20 mhz channels are still there, but they have four extra sub carriers. Instead of the 48, they had four more. Which means they can carry just a little bit more information. So that means that if you’re an ht 20 mhz channel with a single spatial stream, you’re still going to have a little bit better or greater aggregate throughput in that same frequency space. And then if you bond with another 20 mhz channel, you’re going to have even double that.

19. 20 MHz Non -HT and HT Channels Part2

So again, as we talk about each of these channel spaces, right there’s, when we look at the difference here, we call it a 20 mhz channel. Notice I said megahertz because they’re very small channels in this 2. 4 ghz realm and inside of them are all these little subcarriers. And although we’re going to try to use the 48 of them in the middle of that channel, we’re still putting kind of this buffer on the side to prevent interference, which you might call those pilot sub carriers. Well, actually they pilot the signal and then they’re still actually extra at the end of each one. And so here with the ht channel, they just added a few more of the sub carriers. You still have the pilot carriers and you don’t have as much on each end for the guess if you want to call it the protection room. Yeah.

20. 40 MHz Channels

Your eight to eleven N radios can also bond two of the 20 mhz channels. So if I had divided this in half, this was the original 20 mhz. This was the other 20 mhz channel. So now I have 40 so now that I have 40 mhz channels, guess what? I still have what’s close to the center of that channel, which are my subcarriers. I still have my pilot space and I still have some unused at the edge, as I said, kind of protection from any signal trying to overlap. And so that’s the heart of one of the things that they did with 800 and 211 N is to help double the actual amount of data that we can send. So essentially 40 plus 20 or a bonded set of two channels each 120 megahertz and wide you could think of one is primary and one is secondary. But in that case also they have to be adjacent to each other. I just can’t pick. Well, you’re going to find out with some other technologies you could do this, but here you just can’t pick. Say I want to add this channel to this channel over here and bottom that way won’t work. They have to be adjacent to each other.

21. Module 18 Review

So in this module, we talked about the Eight or 211 n 2009 amendment, what the wifi Alliance is looking for. We looked at the benefits now of multiple input, multiple output. And we talked a bit about the ht channels and some of the issues about the Mac layer that was changed with Eight or 211.