Recently I had the pleasure to meet Dr. Ted Rappaport and attend to a very interesting talk he gave at NYU Poly. The topic of the talk was his proposed “renaissance of wireless communications“. It was very exciting to meet him in person given the fact that I pretty much started learning all I know from his book “Wireless Communications: Principles and practices“. I actually realized, when sitting there listening to his talk, that I should have bought my copy of the book to get it signed. After all, his book and Proakis’ “Digital Communications” are the two pillars of everything I like. Learning about the Fourier Transform when I was 19 in school was an eye opening and told me that, indeed, I was in the right place (right major). A couple of classes I took over the following years (COM-1, COM-2 and RadioCom at the ETSETB) required me to read those two books and then I knew that I was on the right major and I also knew what I wanted to do.

Anyhow, back to Rappaport’s talk. I find his view very interesting. Essentially he is proposing to design communication systems on the milliliter-wave range, at very high frequencies, and he is actually proving it possible at NYU Wireless.

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These frequency ranges are known for, in some cases, suffer of extreme propagation attenuation due to the interaction of the electromagnetic waves with oxygen molecules, which brings down the signal well over 10dB per kilometer. In this cases with high attenuation, Rappaport is proposing to create “wisper nets” that die off quickly in way less than a meter of propagation. This way, multiple parts of a complex system can be connected, making wires unnecessary. And the fact that these networks have such a short range, one does not have to worry about external attackers sniffing the traffic or injecting stuff in them.

The other frequency ranges suffer from a still reasonable attenuation that, according to some initial results, could host the future 5G wireless systems. These systems would have a huge bandwidth (BW), allowing for great throughput. Although I had the chance to ask a couple of questions, I forgot to ask him whether he thinks that the huge increase in throughput will come purely from increased BW (plenty of available BW at the frequency ranges he is proposing!) or he expects advanced modulation techniques to play a substantial role as well. After all, we are getting close to Shannon’s limits in terms of bits per second per Hertz (bps/Hz).

Based on the observations of Martin Cooper, the capacity of wireless systems has been somehow steadily doubling every 30 months. This increase has been due to (these numbers are extracted from: M.–S Alouini and A.J. Goldsmith, “Area Spectral Efficiency of Cellular Mobile Radio Systems,” IEEE Transactions on Vehicular Technology, vol. 48, no. 4, pp. 1047 – 1066, July 1999) a wider spectrum (25x gain), spectrum splicing (5x), better modulations (5x… these, I believe, was before OFDMA. I wonder if OFDMA increased capacity more than 5 times…) and a huge gain (x1600) by reducing the size of the cells. Although there’s a huge improvement by making cells smaller, it does not make sense to make them much smaller than now (metro-, pico- and femto-cells), so I guess sooner or later we’ll have to look into new directions. And spatial diversity, another topic discussed in Rappaport’s talk, has always been the one I have always seen more promising and suitable. If to that you add a huge BW at the millimeter-wave range, even better!

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