Electrically reconfigurable antennas are capable of dynamically reshaping themselves and their radiation characteristics in response to the needs of the overlying communication link and network. Most past work on reconfigurable antennas focused on antenna geometries that provide agility in frequency. DWSL has pioneered the application of pattern reconfigurable antennas to multiple input multiple output (MIMO) communication systems and has, more importantly, demonstrated how reconfigurable antennas can be treated as a valuable additional degree of freedom for software defined radio, cognitive radio, and systems "beyond cognitive radio"1. Sample DWSL reconfigurable antennas are shown in Figure 1.
DWSL's research in the area of reconfigurable antennas for wireless systems, supported directly through several NSF grants spans the layers of the protocol stack, includes the following research topics:
Reconfigurable antenna geometries: Reconfigurable antennas can be realized by integrating PIN/varactor diodes and/or MEMS devices into the structure of the antenna. By electrically controlling these components, the radiation patterns of the antennas can be modified. The vast literature on directional or beam-steerable antennas as well as the literature on pattern diversity antenna systems often make unrealistic assumptions about antenna capabilities, resulting in systems that can only be realized using antenna arrays that are prohibitively large for mobile wireless devices. DWSL reconfigurable antenna arrays are all designed to operate in a compact form factor; for several prototypes, this involves using multiple feed points on a single compact physical antenna to realize an antenna array. Through international collaboration with Politechnico di Milano in Italy, DWSL has designed and demonstrated pattern diversity-based reconfigurable length dipole antennas, pattern diversity-based reconfigurable radius/polarization circular patch antennas, as well as leveraged new design techniques in metamaterial transmission line theory to realize reconfigurable "leaky wave" antennas that are capable of transmitting focused directional beams in a very compact form factor.
Link adaptive modulation: Traditional literature in the area of link adaptive modulation considers how signal constellations and coding can be intelligently adapted to the current state of the wireless propagation channel. This previous work assumes that the communications channel is a "black box" that cannot be controlled. Reconfigurable antennas act as a sort of changeable "lens" through which the communication channel can be viewed by the nodes in the network. Thus, by intelligently setting the configuration of the reconfigurable antenna, a desirable channel can be "selected." DWSL research has experimentally shown how link adaptive modulation techniques can be extended by the degrees of freedom provided by a reconfigurable antenna.
Co-channel interference mitigation: DWSL's research with link adaptive modulation has shown how the throughput of individual links can be optimized by the use of reconfigurable antennas. In multi-link co-channel networks, the reconfigurable antenna state selection problem can be recast to consider how antenna state impacts (and is impacted by) interference levels in the network. DWSL's research has looked at developing models for these channels and interference, as well as considered how practical reconfigurable antenna architectures perform in measured networks.
Machine learning for antenna state selection: The incorporation of reconfigurable antennas in practical wireless systems requires the use of channel training in order to gain information about the state of the channel. However, when the number of reconfigurable antenna configurations is high, this training overhead can negate the benefits of using reconfigurable antennas in the first place. Machine learning techniques, applied by DWSL to the reconfigurable antenna state selection problem, allow for the development of a formal framework to tradeoff antenna state exploration and exploitation in practical networks.
Physical layer security: The changeable perspective of the communication channel provided by reconfigurable antennas can be used as a sort of "fingerprint" to differentiate the signals of multiple users. Thus, we have applied reconfigurable antennas to physical layer security techniques for user authentication in networks as well as the generation of encryption keys. These techniques can be used to augment conventional security mechanisms implemented at other layers of the protocol stack.
Interference alignment: Interference alignment is an emerging technique in the wireless communications community in which signal and interference subspaces can be manipulated to enable simultaneous, high throughput communications to multiple communication links. The techniques work by pre-coding signals sent from multiple disconnected transmitters so that the desired signal and interference subspaces are easily separable at multiple disconnected receivers. DWSL's research has shown that the additional degrees of freedom provided by reconfigurable antennas, in both signal and interference subspaces, has the potential to greatly enhance interference alignment techniques.
Dynamic spectrum access: Cognitive radio is most commonly defined as the capability of wireless network nodes to sense and adapt their carrier frequency in response to network conditions. The interference mitigation capabilities of reconfigurable antennas provide a natural way to introduce enhanced frequency re-use in cognitive dynamic spectrum access algorithms. This research is the cornerstone of DWSL's contribution to the NSF WiFiUS consortium2 encouraging collaborative wireless research by teams of researchers in the U.S. and Finland.
Tactical networking: In addition to the commercial interest in DWSL reconfigurable antenna system technology, the Department of Defense has also taken note of DWSL reconfigurable antennas. DWSL research efforts have shown how reconfigurable antennas can be used to enable tactical, multi-hop wireless networks in rural areas (i.e., wireless test-range at Ft. Dix for the U.S. Army), as well as onboard naval vessels (i.e., on-ship characterization of naval vessels docked in Philadelphia for the U.S. Navy).