Lecture Series in Mobile Telecommunications and Networks (1583KB)

Questions & Answers
Mike Walker: Thank you, Professor Kumar, for that fascinating talk which took us from ad hoc networks, radio networks,
through cars changing their engine while still moving, to traffic control and collision avoidance systems.
Professor Kumar has agreed to take some questions. Let me start because, in our business, which is based on cellular
radio systems, they are all very old-fashioned. It is that very old spatial diversity planning scenario that you painted right
at the beginning. You showed that in fact Maxwellian networks is the way we ought to look at things, and this is the
best possible. However, we have seen many attempts to build ad hoc networks with direct communication from one
mobile device to another. These have all more or less failed commercially. Do you have a feeling about why that is so,
and whether there will be a turning point so that we will see some of these networks being deployed commercially for
real?
PR Kumar: There are two answers to that. If we look at the pace of evolution of communication technology, it is just
amazing. Telephones are about 100 years old, cellular phones are about 35 years old and the internet is about 25 years
old. Who knows what it will be like in 20 years from now? There could be an information fabric connecting individuals
and so on. One cannot rule out such things, and we may see them in the future.
One of the issues has been what is the application of these ad hoc networks. Of course, the military is always hungry for
any application or any technology but what about in the civilian world? There have been some applications and, for
example, when Hurricane Katrina struck New Orleans, there was actually a team from Champaign-Urbana Wireless who
went down there to set up the emergency network without any infrastructure, so you have seen those kinds of things.
One big way it could potentially take off would be in vehicular networks. In the US, there has already been spectrum set
aside for vehicular networks. Those are systems with some structure, when cars go along the road and so on, so that it
is not completely unstructured. This actually facilitates people who are trying to design systems, because at least they
have a target in mind. People are thinking of safety applications, infotainment applications, highspeed tolling and so on.
I think vehicular networks could be a domain that you will see being realised sooner.
Professor Lajos Hanzo (University of Southampton): I very much enjoyed your lecture and you set out a number of
interesting challenges for us. One of these is in the field of cross-layer optimisation. We set up the seven-layer OSI
architecture and it is very convenient, as you said, to have our little systems optimised on a layer by layer basis, but of
course mobile communications does not quite fit into that mould. For example, we have power control and all the
related functions which are stuck to the side of the seven-layer architecture.
You then went beyond that and spoke about these complex systems where we now integrate control, communications
and even mechanical systems. You also provided a very interesting architecture for that. However, in a way, you are still
also suggesting that perhaps even these complex structures will have to use some form of cross-layer optimisation as
well as some form of logical ordering of the different functions into a greater entity. Do you have any comments on
this?
PR Kumar: Some of the theory that I talked about earlier clearly ignores multiplicative cost – so that factors of 200 or
300 are irrelevant in these theories. You are searching for an architecture for a Maxwellian network in this infinite
dimensional space, whereas there are all kinds of possibilities. The theory gives you some guidance on what the
architecture is but of course in the real world we want to improve performance by factors of two, three and so on.
A great deal of design work is being done and so, in that sense, there is a little bit of a disconnect between the
architecture that this theory says, and the real world.
A very good example, as you mentioned, is power control. Power control is, how loudly should you talk? When I send a
packet, I can decide at what power level to transmit it, and that has all kinds of implications. For example, if you think
that the power level is affecting signal quality, then that is a physical layer issue that classical communication engineers
are worried about. On the other hand, when I talk really loudly, it causes interference to somebody else, and
interference is treated at the condition control layer of the transport layer, which is somewhat higher up. On the other
hand, power control also affects connectivity and so, when I talk loudly, I can get to my final destination in three hops
rather than in 30 hops if I were to talk softly. That affects the network layer.
18 The Royal Academy of Engineering