Lee Center for Advanced Networking

Lee Center member Babak Hassibi is working on wireless networks. Wireless networks differ from wireline networks in four fundamental ways. First, since wireless is a shared medium, transmission is in the form of broadcast meaning that each node transmits the same message to all nodes in the network. Second, there is interference in reception so that if more than one node is transmitting, other nodes in the network receive a linear superposition of the transmitted signals. Third, there is path-loss which means that further away nodes receive weaker signals, so that the performance of the network depends not only on its topology, but also on its “geometry”. Fourth, there is fading, i.e., the random fluctuations that appear in wireless channels due to the mobility of the transmitter/receiver and the scattering of electromagnetic waves off different objects.

With the exception of cellular communications systems that explicitly use path-loss to enable frequency reuse (and thereby increase the capacity of the system), traditional wireless communications systems treat broadcast, interference and fading as nuisances that have to be dealt with. Fading is mitigated by the use of channel coding and interference is avoided by not allowing close-by nodes to simultaneously transmit. As a result, wireless networks are “forced” to behave as wireline networks.

Hassibi and his coworkers, however, take the view that fading and interference present opportunities that should be exploited, rather than problems that should be avoided. They have shown that traditional approaches for the design of wireless networks do not come near to harnessing the potentials and capabilities of wireless networks. To do so requires new techniques and approaches.
Hassibi and his coworkers have pioneered the use of multiple antenna systems (at both the transmitter and receiver) that increase the capacity and reliability of wireless links.

The idea comes from sorting out the multiple fading signals going back and forth, especially when these signals are ricocheting off various objects—so-called “rich scatter.” In a rich-scattering environment, the right kind of signal processing at the transmitter or receiver can open up parallel communications channels. Hassibi studies both the information and coding aspects of such multiple antenna systems and is developing transmission algorithms that will rapidly code and decode the data. In multiple antenna systems, signaling occurs over both space (because of the multiple antennas) and time, so such coding schemes are called “space-time codes.”

Hassibi and his colleagues have further shown that interference can be exploited to boost the power efficiency of wireless networks: in a network with 100 nodes, to maintain a fixed communications rate, the total transmit power can be reduced 10-fold. This cannot be done without interference. They have also shown how to harness fading to improve the performance of a wireless network above that of a non-fading network. The idea is to exploit the “diversity”, essentially the randomness, in the network. Compared to multiple antenna systems, says Hassibi, the challenge in wireless networks is that the communication, modulation, coding, etc., must be done in a distributed fashion.