Multiple Antenna Technology for Faster Downloads
Dave - As the demand for mobile communication devices increases, we're demanding progressively faster data transmission rates. One option to improve this is to install multiple antennae into the device so there's lots of streams of data can be handled in parallel, but this comes at a cost in size, or does it? Scientists at Bristol University have come up with a way to share the antennae of nearby devices to get around this problem. To explain how this works, one of the inventors, Professor Mark Beach, is with us. Now Mark, should we start off really simple? What limits the amount of data which two straight aerials can transmit between one another?
Mark - Well, we are very limited by the channel bandwidth that we can provide to mobile communication systems. So the bandwidth is the main driver, followed by the amount of power that we allocate to the transmitted signal which we can increase to improve the data communications. But that causes interference with other users, and we then reduce the capacity of the system.
Dave - So the bandwidth, this is the effect that if you send data on a radio frequency, the more data you send, the more frequencies around the original one it uses.
Mark - Yes, that's right. As we expand the data rates, we get a corresponding increase in the bandwidth of the signal needed to transmit information between A and B.
Dave - I guess there's a limited number of frequencies out there that I can use, so there's a limit to the number of people who can talk at the same time or surf the internet at the same time.
Mark - Yes, it's true. The frequencies around 1 to 2 Gigahertz are the sweet spot for mobile communications where the signal propagates well with less attenuation, and the physical size of the antennas used allows us to put them into mobile devices.
Dave - I guess the whole of this problem is a bit like having lots of people talking in a room. One person on one side of the room is attempting to listen to someone on the other side of the room but filter out all the other people talking at the same time. Like all these different mobile phones. So, one technique is to talk at different pitches. So you might be able to hear a high pitch at the other side of the room if everyone else is speaking at a low pitch. But you're using a slightly different approach. What are you actually doing?
Mark - What we've been looking at is the use of multiple antennas to increase the transmission bandwidth of information by exploiting the multipath in the channels that we heard about a few minutes ago. So the multiple antennas excite the multipath and at the receiving end we can separate out the multipath's signals. We resolve these individual spatial channels that we can push information down in parallel. We can do this on the same frequency and at the same time.
Dave - I guess this multipath is like standing next to a big reflective building. The mobile phone message can get between your phone and the mast in two ways. It can either go straight there or reflect off the building. As you get a very complicated system there's lots and lots of different reflections and so you somehow take advantage of this.
Mark - Yes, we are taking advantage of the multipath's signature in the channel and when we have multiple antennas, we will actually see slightly different signatures, and those signatures can be quite unique, and we can use those special paths independently to support communications.
Dave - Does this mean that if you've got lots of different aerials in one place, you can actually build a directional aerial - you can put them together in one way so you can just listen to the reflection from the building, and you can add the reception from your aerials together in a different way to listen to the direct signal from the mobile mask, so doubling your data rate?
Mark - It's along those lines. We can actually beam from the information from these spatial paths and extract them at the receiver. The theory tells us that if we doubled our number of antennas, we can double the data rates, and if we got up to four times the number of antennas, we'll have four times the rate, providing there is sufficient multipath richness in the channel. There is eventually a trade-off with the number of antennas and the actual benefits that you will see.
Dave - Can you just carry on adding more and more antenna into something the size of a mobile phone with no problems at all?
Mark - Well there is a problem. Although I mentioned that the frequencies around 2 Gigahertz make quite small antennas, there's a limited amount of real estate in a mobile phone where you can actually place these devices because of the other electronics inside the box. In particular, the display on Smartphones. So we are somewhat limited to between probably two and four antennas. It is probably about the limit that you could actually put in a portable device.
Dave - So what are you doing to get around this problem?
Mark - Right. What we've been looking at is sharing antennas. If there was somebody else's mobile phone which was enabled with multiple antennas, and if they were not using it in conversation, we could share those antennas and use them to form our communications link. This would make the two phones slightly simpler in terms of the number of active antennas that they would need.
Dave - So you'd be essentially joining the two mobile phones together with, instead of the wire, another wireless link which is very short range.
Mark - That's right, so we would form a virtual MIMO system between the two devices and then have a short range low power link between the actual users.
Dave - I hear you've been using this in practice. How has it been working?
Mark - We've been looking at this using quite a sophisticated channel sounder that measures the multipath signatures of the channel. We could see if we separate the antennas and have two with one user and another two antennas with another user, does that actually form a good 4 X 4 MIMO system?
Dave - Were you getting an increase in speed on average?
Mark - On average, yes. One of the things about MIMO communications is that you require this independent fading so we can get these separate spatial channels. So moving the antennas further apart normally gives you much better characteristics. We found from real measurements done in the city of Bristol that for at least half the locations we chose, that was certainly the case.
Dave - Brilliant! Thank you very much, Mark.