Just about every mobile manufacturer is now concerned about releasing their 5G products. Samsung, Huawei, OnePlus and even Oppo have already released their phones, whereas Apple and Google are on the way. Let’s leave all the branding and advertising aside for a second. Real questions to raise are, “What is 5G?” and “Do we really need it?”
The key requirement for the enhancement of mobile communication is to satisfy the requirement for high data rates. Starting from 2G (GPRS), 3G and 4G (LTE) have increased the data rate; so that users could switch from downloading 480p movies to streaming high quality - 4K videos. Though it may sound cool, engineers and scientists behind the stage have been working their butts off to make high data rates a possibility for the commercial use.
If you could recall those boring physics lessons from the high school (or middle school?), you might remember electromagnetic signals at known frequencies are being used as carrier waves in wireless communication. The bandwidth is the fluctuation of the frequency required for communication. Simply, all the information you need to send or receive would lie in this region of the spectrum as shown in the figure.
That explains why we would need a higher bandwidth to achieve high data rates. Higher the bandwidth, more data we could pack in the spectrum. As it may sound pretty obvious now, the bandwidth is much smaller than the carrier frequency. In the case of LTE, the carrier frequency lies between 700 MHz to 2.7 GHz, whereas the bandwidth is less than 20 MHz.
As you may have already heard, FCC is an agency responsible for managing the frequency spectrum and they have allocated frequency bands from 9 kHz to 275 GHz for use. For a better understanding, below is a graphical illustration of the range.
The obvious solution for the limited bandwidth problem is to increase the carrier frequency. Imagine what if we had increased it all the way up to the mm-wave range in the spectrum? That is beyond 30 GHz carrier frequency. This is an easy way to increase the bandwidth by a huge factor.
As interesting as it may sound, mm-wave communication has some obstacles to win.
Back to high school physics!
Remember the wave equation c = fλ which states that the wavelength of an electromagnetic wave reduces with the frequency? In other words, the wavelengths of mm-waves would literally lie in the range of millimeters!! High school physics also states that signals with such small wavelengths could get diffracted by small objects. Believe it or not, even dust particles and water droplets could attenuate mm-wave propagation.
Let’s say if you buy a 5G mobile phone, which operates at mm-wave spectrum. In one hand, receiver antenna of your phone would not receive strong signals as in 3G or 4G. On the other hand, as the aperture of an antenna is equal to λ/2, mm-wave antennas have smaller apertures. Small antenna apertures cannot capture enough energy. As a result, the range of mm-wave communication drastically reduces to several meters.
What are we going to do about it?
Well… How about using multiple antennas instead of one?
We know the antenna aperture has to be small for mm-wave communication. Yet no one said anything about the number of antennas we could use. Therefore, the solution is to use multiple antenna elements and create an antenna array instead of one antenna.
Imagine it is raining and you need to collect water. But all you have is a pack of small cups. What can you do? I would place them in a row and collect water separately. You can simply pour all the water into one big container later, but these are electromagnetic signals we are talking about. How do you combine them? Well, that’s a story I better save for another article!!
Now, this method is going to increase the hardware complexity of the RF system. Instead of using one RF chain, these systems have to use multiple chains, such that every element has an RF chain for itself. (An RF chain is the set of components connected together to retrieve the baseband signal from the received signal which includes mixers, low noise amplifiers and filters.) Last year, my colleagues at FIU and I have built a 4-element array operating at 28 GHz as a part of our course project.
Cool stuff. Right?
Back to the story!
At FIU, we research on methods we could use to reduce the hardware complexity in antenna array systems. As most of the signal processing systems, these RF systems prefer processing in the digital domain rather than analog to reduce the complexity. Therefore, we need an analog-to-digital converter per every antenna element. Since the frequency of operation is high in these systems (some GHz range) these ADCs have to sample the analog signal at higher rates. Of course, these ADCs going to be very expensive. Well, my research is to find alternative methods to reduce this requirement.
In the next articles, I will share more about the methods we have explored so far with results, and I will try to explain some multi-dimensional signal processing theories in a more comprehensive manner. Until then,