Wireless communications has been one of the most rapidly expanding areas over the
past decade, and it is expected that this expansion will continue through the next
decades through deployment of 3rd and 4th generation cellular networks, wireless
internet, wireless local area networks, and wireless home networks. In order to fulfill
the promises of wireless, a number of issues have to be addressed. Since the bandwidth
of the wireless medium is limited, in order to accommodate communicating an
increasing amount of information, the communication channel has to be exploited more
efficiently through advanced coding and signal processing technologies. To facilitate
mobility of users and devices and ensure acceptable quality of service (QoS), efficient
networks protocols have to be developed. Finally, to create a trustworthy wireless
network, communications has to be secure, for example to facility mobile commerce (mcommerce). In this project we will work on these aspects of mobile communications in
conjunction through combined expertise in coding, signal processing, networks and
antennas.
Many communication systems in the military and the private sector will consist of a
number of mobile units that must effectively and efficiently communicate over a
wireless medium.
Some mobile wireless networks are formed “on the fly” where a
network is formed because there are mobile devices in proximity of each other and there
is a need to transfer information among them, called ad-hoc networks. Example
applications include people with laptops meeting in a conference room and want to
exchange information, and search and rescue operations. Recently, ad-hoc networks
has started to be employed widely in consumer applications, such as Bluetooth [H00],
HomeRF [N00], and Wireless LAN [PZK97] with a corresponding resurgence of research.
In this research, we address many of the problems that arise in the design of these
types of systems and investigate the capacity of such networks [PK00]. The most salient
feature of wireless networks is user mobility. With mobile users, the connections
between users (network topology) are changing rapidly so that all of the methods for
determining design variables need to be re-examined. This includes routing, flow
control, channel access methods, and so forth. With wireless channels and new access
protocols, new performance evaluation methods need to be developed to characterize
the latency and throughput at the link level [SE00].
To increase the achievable capacity in mobile networks, we will study different signaling
schemes for sending and receiving information streams in mobile environments. This
involves using coding, modulation, equalization, and detection algorithms to transmit
different sources of information. In wireless communication environments performance
of systems can be improved by exploiting diversity that occurs in time, frequency, and
space.
We will consider space time processing and coding [TSC98] in a multiuser
environment where transmitters and receivers may both have multiple antennas to
transmit and receive information. This will be combined with methods for multiuser
communications, such as adaptive multiuser detection for code division multiple access
(CDMA) [HMV95,WP98,HC99,WH99], also in connection with ad-hoc networks [HW99].
We will also consider space time processing in conjunction with unequal error
protection (UEP), a scheme in which a certain percentage of critical information is
always more protected than the remaining part. As a result, a UEP coding scheme
guarantees that the critical information is always retrieved at the receiver, while the
remaining part can be lost in case of unfavorable transmission conditions. We propose
to extend the results derived in [MFLI00,IFMLI00] to space-time coding techniques for
fading channels.
To increase the security, capacity, and flexibility of mobile networks, we will consider a
mobile network based on retrodirective antenna arrays [P64]. A retrodirective antenna,
when illuminated by a source signal, has the ability to retransmit a signal directly back
to the source of the initial transmission without any prior knowledge of its location.
This characteristic provides for a secure link between the sender and receiver. Because
of their self-tracking abilities, retrodirective antennas can be used in space-division
multiple access systems both in cellular networks and ad-hoc networks. We propose to
develop retrodirective arrays based on quasi-optical power combining of solid-state
devices [M86], [AFS00]. Quasi-optical power combining permits hundreds or potentially
thousands of devices to be efficiently combined at millimeter-wave frequencies, which
are now used for local multipoint distribution systems. To provide flexibility, a
reconfigurable aperture for multi-band frequency operation could be achieved by
incorporating MEMS devices into the arrays. These reconfigurable, retrodirective
antenna arrays could be used as the basis for dynamically reconfigurable, secure adhoc communication networks.
[P64]C. Y. Pon, “Retrodirective array using the heterodyne technique,” IEEE
Trans. Ant. and Prop., vol. 12, pp. 176–180, Mar. 1964.
[AFS00] D. M. K. Ah Yo, W. E. Forsyth, and W. A. Shiroma, “A 360°
retrodirective self-oscillating mixer array,” in 2000 IEEE MTT-S Int.
Microwave Symp. Dig., Boston, MA, pp.~813–816, June 2000.
[M86] J. W. Mink, “Quasi-optical power combining of solid-state millimeter-wave
sources,” IEEE Trans. Microwave Theory Tech., vol. MTT-34,
pp. 273–279, Feb. 1986.
[TSC98] V. Tarokh, N. Seshadri and R. A. Calderbank, “Space-Time Codes for High Data
Rate Wireless Communication: Performance Criterion and Code Construction,” IEEE
Transactions on Information Theory, vol. IT-44, pp.744-765, March 1998.
[MFIL00] R. Morelos-Zaragoza, M. Fossorier, S. Lin and H. Imai, “Multilevel Coded
Modulation for Unequal Error Protection and Multistage Decoding; Part-I: Symmetric
Constellations,” IEEE Transactions on Communications, vol. COM-48, pp. 204-213,
February 2000.
[IFMIL00] M. Isaka, M. Fossorier, R. Morelos-Zaragoza, S. Lin and H. Imai, “Multilevel
Coded Modulation for Unequal Error Protection and Multistage Decoding; Part-II:
Asymmetric Constellations,” IEEE Transactions on Communications, vol. COM-48, pp.
774-786, May 2000.
[H00] J.C.Haartsen. The bluetooth radio system. IEEE Personal Communications, 38(2):
28-36, Feb 2000.
[N00] K.J.Negus. HomeRF: Wireless networking or the connected home. IEEE Personal
Communications ,38(2): 20 –27,Feb 2000.
[PZK97] K.Pahlavan, A.Zahedi, and P.Krishnamurthy. Wideband local access: wireless
LAN and wireless ATM. IEEE Communications Magazine, 35(11): 34 –40, Nov.1997.
[HC99] Anders Høst-Madsen and Kyung-Seon Cho: “MMSE/PIC Multi-User Detection
for DS/CDMA Systems with Inter- and Intra-Cell Interference,” IEEE Trans. Comm.,
Vol. 47, No. 2, Feb. 1999, pp. 291-299.
[WH99] Xiaodong Wang and Anders Høst-Madsen: “Group-Blind Multiuser Detection for
Uplink CDMA,” IEEE Jour. Selected Areas in Communications, Vol. 17, No. 11, Nov.
1999, pp. 1971-1984.
[HMV95] M.L. Honig, U. Madhow, and S. Verdú. Blind Adaptive Multiuser Detection.
IEEE Trans. Information Theory, Vol. 41, No. 4, pp. 944-960, July 1995.
[WP98] X. Wang and H.V. Poor. Blind multiuser detection: A subspace approach. IEEE
Trans. Information Theory, Vol. 44, No. 2, pp. 677-690, March 1998.
[SE00] C. Sankaran and A. Ephremides, ``Multicasting with multiuser detection in adhoc wireless networks,'' 2000 International Zurich Seminar on Broadband
Communications. Accessing, Transmission, Networking.
[PK00] P. Gupta and P.R. Kumar, ``The Capacity of Wireless Networks,'' IEEE
Transactions on Information Theory, vol. 46, pp. 388-404, March 2000.
[HW99] N.M.K. Howlader and B.D. Woerner, ``Single-user adaptive and multiuser
receivers for DS-CDMA in peer-to-peer packet radio networks,'' MILCOM 99, pp.10411045.