Projects
Cooperation and Relaying in Wireless Networks
Geolocation in Multipath Environment
Cooperation and Relaying in Wireless Networks
(Possible applications: WiMax, 3GPP-LTE)
This work refers to a group of projects that study the effect of cooperation and relaying in wireless networks. One of the key ideas investigated is that relays provide flexibility in routing signals between sources and destinations. A wide spectrun of scenarios are of interest ranging from the case where the relays are connected to the network infrastructure (base stations) and hence have the ability to cooperate and have access to global network information to the case where relays have only local channel state information and cannot cooperate. Possible applications of this work are the emerging WiMax systems, where a task group is working on a specification to extend base station reach and coverage known as P802.16j (802.16j).
This project is funded by NSF and is a collaboration with Princeton University.
Specific projects and details of the work are provided below.
Multiuser Diversity in Wireless Ad Hoc Networks
PhD student : Shengshan Cui
This work studies the throughput scaling in the limit of large networks for a previously published two-hop opportunistic relaying protocol proposed in previous work (see, “Opportunistic Relaying in Wireless Networks,?submitted to transactions on Information Theory) for more general fading conditions. In the Rayleigh fading environment, the throughput scaling was obtained directly from the closed-form throughput expression of the finite network in previous work. For other fading channels, unfortunately, the exact closed-form throughput analysis turns out to be intractable. Inspired by the multiuser diversity interpretation of the throughput scaling in the Rayleigh case, we introduce the conjecture that for general fading channels, the throughput scaling of the opportunistic scheme is of the same order as the multiuser diversity of the underlying fading model. While the rigorous proof of the conjecture is still ongoing, in this paper, we make two observations that shed additional light on the throughput of wireless networks.
The work is substantiated by simulation results. In Fig. 2 of [1], the average system throughput is plotted for Lognormal fading scenario. Different Lognormal parameters (μ = 1, and different σ alues as 1, sqrt(0.5), and 0.5, respectively), are simulated. The marks in the figure are obtained through simulations. For each simulation parameter, a corresponding theoretical scaling behavior result is shown. A good match is observed between the simulated throughput and the predicted throughput behavior. Similarly in Fig. 3 of [1], the simulation results for Nakagami fading scenario (with various Nakagami parameters) and the corresponding theoretical results are plotted. Again, a good match is observed. These simulations results justify further rigorous proof of this conjecture, which is under investigating.
Publication:
1. Shengshan Cui, Alexander M. Haimovich, “Multiuser Diversity in Wireless Ad Hoc Networks,?submitted to the IEEE Global Communications Conference (IEEE GLOBECOM 2008).
Opportunistic Relaying in Wireless Networks
PhD student : Shengshan Cui
Relay networks having n ad hoc nodes and m half-duplex relays, all operating in the same frequency band in the presence of fading, are analyzed. This setup has attracted significant attention and several relaying protocols have been reported in the literature. However, most of the proposed solutions require either centrally coordinated scheduling or detailed channel state information (CSI) at the source nodes. Here, an opportunistic relaying scheme is proposed, which alleviates these limitations without sacrificing the system throughput scaling in the regime of large n. The scheme entails a two-hop communication protocol, in which sources communicate with destinations only through half-duplex relays. The key idea is to schedule at each hop only a subset of nodes that can benefit from multiuser diversity. To select the source and destination nodes for each hop, it requires only CSI at receivers (relays for the first hop, and destination nodes for the second hop) and an integer-value CSI feedback to the transmitters. Moreover, the relays operate in a completely distributed fashion, with no cooperation. For the case when n is large and m is fixed, it is shown that the proposed scheme achieves a system throughput of m/2 bits/s/Hz. In contrast, the information-theoretic upper bound of (m/2) loglog n bits/s/Hz is achievable only with more demanding CSI assumptions and full cooperation between the relays. Furthermore, it is shown that the system throughput of the proposed scheme scales as the order of log n.
Simulation is used to substantiate the analytical results. In Fig. 4 of [1], the average throughput of the relaying scheme is plotted under the assumption of Rayleigh fading environment. In the figure, different values of number of source/destination nodes, n, are simulated, and for each case, the optimal number of relay nodes, m, is considered. It can be seen from Fig. 4 that the throughput exhibits log n behavior as suggested by the theoretical analysis. It is also seen from Fig. 5 that the optimal number of relay nodes follows the theory as well.
Publications:
1. Shengshan Cui, Alexander M. Haimovich, Oren Somekh, and H. Vincent Poor, Opportunistic Relaying in Wireless Networks, submitted to the IEEE Transactions on Information Theory, Dec. 2007. Also available on arXiv.
2. Shengshan Cui, Alexander M. Haimovich, Oren Somekh, and H. Vincent Poor, Decentralized Two-Hop Opportunistic Relaying With Limited Channel State Information,? in Proceedings of the 2008 IEEE International Symposium on Information Theory, Toronto, ON, Canada, July, 2008.
3. Shengshan Cui and Alexander M. Haimovich, Opportunistic Relaying in Wireless Networks, in Proceedings of Allerton Conference on Communications, Control, and Computing, Monticello, IL, September, 2007.
Interference Management in Wireless Ad-hoc Networks
PhD Student : Bo Niu
A large number of recent works have been looking at the asymptotic capacity of ad-hoc wireless networks with large number of nodes. However, little is known about the capacity region of networks with finite number of nodes. In this work, we look at the multiplexing gain, which is the most basic characterization of network capacity, for both cases of non-relay and relay-helped interference network scenarios. An improved practical scheme based on previous works in the literatures is proposed by using the recent concept of interference alignment. It achieves high multiplexing gain and at the same time requires little overhead of the networks. Then the scheme is considered for an ad-hoc network with relays.
Throughput of Two-hop Wireless Networks with Relay Cooperation
PhD Student : Bo Niu
In this paper, we consider the throughput problem of a wireless fading network with two-hop relaying, where n source-destination pairs communicate through a set of relays using amplify-and-forward strategy. Two different cooperation schemes at the relay nodes are considered, where the relays share either channel state information (CSI) or both CSI and received signals. The high level of cooperation is justified for a case where the relaying role is fulfilled by infrastructure nodes that can communicate through a wired backbone without an overhead on the wireless channel. We show that in the first case, at least n^2 relays are needed to achieve linear scaling of the system throughput versus n. In the second case, exchanging the received signals at the relays can reduce the needed number of relays to n in order to achieve linear scaling. It is also shown that the second cooperation scheme achieves a strictly positive per node throughput, where the total number of nodes accounted for includes the relays.
Publications:
1. B. Niu, O. Simeone, O. Somekh and A. M. Haimovich, Throughput of Two-Hop Wireless Networks with Relay Cooperation, in Proc. of Allerton Conference, Monticello, IL, Sept 2007.
On the Outage Sum-Rate of Broadcast Channels with 1-Bit Feedback
PhD Student : Bo Niu
In this work, the impact of limited channel state information (CSI) feedback on the outage capacity of a network is studied. Specifically, the outage sum-rate of a single-input single-output (SISO) broadcast channel with quasi-static Rayleigh fading and CSI feedback limited to only 1-bit is investigated. The two cases of single-rate and adaptive-rate transmission are analyzed. For single-rate transmission, closed-form expressions for the outage probability and outage sum-rate are derived. The performance analysis is then carried out in the asymptotic regimes of large SNR and large number of users K, respectively. At high SNR, the diversity-multiplexing tradeoff is derived; with increasing number of users K, we show that the outage sum-rate scales as log logK, which is the same scaling law achieved by the optimal full CSI feedback scheme in an ergodic scenario. Extensions are then provided for the adaptive-rate case.
Publications:
1. B. Niu, O. Simeone, O. Somekh and A. M. Haimovich, On the Sum-Rate of Broadcast Channels with Outdated 1-Bit Feedback, in Proc. Asilomar Conference on Signals, Systems and Computers, Pacific Grove, Oct. 2006.
Geolocation in Multipath Environment
(Possible applications: geolocation in sensor networks, geolocation in wireless networks)
In this work, we seek to develop techniques for accurate geolocation of RF sources through the utilization of multiple sensors spatially distributed. Two main approaches are studied. In non-coherent geolocation, the source localization is obtained from estimates of the signal time of arrival (or time difference of arrival if the time reference of the source is not available). Coherent source localization is achieved by exploiting the phase information contained in signals at various points in space. It is shown that coherent localization has the ability to provide highly accurate localization, but has to contend with ambiguity sidelobes. Our investigations are focused on ways to alleviate the sidelobe problem and to calibrate the system against phase errors.
The work is funded by U.S. Army Ft. Monmouth and it is carried out in collaboration with Princeton University.
More details on two topics are given below.
Sidelobe Reduction in Coherent Localization
PhD Student : Vlad Chiriac
A metric derived based on maximum likelihood estimation utilizing phase information is capable of localizing sources with an accuracy of the order of the source carrier wavelength. However, the maximum likelihood metric is subject to high ambiguity sidelobes. Several methods to reduce the sidelobes are investigated. The most basic way of reducing sidelobes is increasing the number of sensors. However, that might be difficult in many practical situations. In this project, we study various other ways to control sidelobes in localization systems utilizing phase information. An alternative to increasing the number of sensors is equipping each sensor with an antenna array. Another method is to place the sensors (or some of them) in a moving platform and utilize multiple locations to observe the signals transmitted by the emitting source. The latter method has the disadvantage of processing non-coherently the signals obtained from different sensor locations.
The capability of multiple sensors to act to reduce sidelobes is degraded when the sensors are widely varying distances from the source. This motivated the development of a localization technique that is robust to distance variations (as long as a required SNR is maintained). The technique is based on the variance of the phase measured at the different sensors.
Calibration Issues in Coherent Localization
PhD Student : Ciprian-Romeo Comsa
The coherent source localization approach requires precise knowledge of the sensor locations. In practice, for a wireless outdoor system the sensor location measurements (usually given by GPS) don’t offer a precision better than 20 meters, while the coherent source localization proved to be very sensitive to errors as low as centimeters in the knowledge of the sensor locations. At the same time, the physical setup inherently involves timing (or equivalent phase) errors which can be limited with an open loop calibration system to about 10 nanoseconds; we found that this is again unacceptable for the requirements of the coherent source localization. Consequently an important part of our work on the coherent source localization is focused on designing a close loop beacon based calibration system, aimed to compensate for both sensor location displacements and timing (or phase) errors inherently induced by the physical equipment.
Publication:
1. C. R. Comsa, J. Luo, A. M. Haimovich, and S. Schwartz, ?span style="color: black">Wireless Localization using Time Difference of Arrival in Narrow-Band Multipath Systems,? in Proceedings of the IEEE International Symposium on Signals, Circuits and Systems, ISSCS 2007, Iasi, Romania, 13-14 July 2007, vol. 2, pp. 469 - 472.
MIMO radar refers to an architecture that employs multiple, spatially distributed transmitters and receivers that operate in close cooperation. The cooperation among radar sensors includes time and, possibly, phase synchronization. The MIMO radar architecture supports highly flexible transmitted waveforms ranging from full correlation to full independence. Additionally and significantly, due to its distributed architecture, this type of MIMO radar has powerful capabilities that are not enjoyed by other architectures MIMO or otherwise. MIMO radar is a new sensing technology that offers many potential advantages. When operated with phase coherency among the receive antennas (and possibly among the transmit antennas), MIMO radar with distributed antennas, supports very high resolution target localization or even target radio frequency (RF) imaging. Multiple transmit antennas can be utilized to either focus a beam (or multiple beams) at desired locations or can emit independent waveforms that illuminate the whole surveillance space. Similarly, post-processing at the receiver can focus the received energy at desired locations. The ability of transmitting independent waveforms makes possible new applications. For example, the MIMO radar could simultaneously transmit waveforms optimized for range estimation and waveforms optimized for Doppler estimation. Multiple transmitting and/or receiving antennas can be used to provide diversity against target radar cross section (RCS) fading. The multistatic nature of MIMO radar with distribute antennas provides a variety of other inherent benefits, such as the ability to overcome low radial velocities in Doppler measurements.
This work is carried out individually at NJIT and in collaboration with Lehigh University. The work is supported by the US Air Force.
Theory of Target Localization with Distributed MIMO Radar
Research Associate : Mohamed Haleem
In this work, we study the the statistical behavior of ambiguity levels arising in location estimation in distributed MIMO radar with phase measurements. We identify the similarities and differences of the problem with that of array antennas of randomly spaced elements used in measuring the angle of arrival of a target. In particular, the problem addressed in this work is concerned with near field in contrast to the far
field radiation pattern of random arrays. The mean pattern of the ambiguity level as a function of location is found to be independent of the number of antennas of the MIMO system. The variance is zero at the location of the scatterer and scales down as 1/(MN) where M and N are respectively the number of transmit and receive antennas. Except in the vicinity of scatterer, the side lobe levels have equal distribution at every location.
Target Localization Accuracy in MIMO Radar
PhD Student : Hana Godrich
In this work, we analyze the target localization accuracy, attainable by the use of MIMO (Multiple-Input Multiple-Output) radar systems, configured with multiple transmit and receive antennas, widely distributed over a given area. The Cramer-Rao lower bound (CRLB) for target localization accuracy is developed for both coherent and non-coherent processing. The localization estimation accuracy is shown to be inversely proportional to the signal effective bandwidth for the non-coherent case while it can be approximated as inversely proportional to the carrier frequency in the coherent case, revealing a significant coherency gain. Further optimization of the CRLB terms, which depend on the radars positioning with respect to the target, is obtained, disclosing additional MIMO related accuracy gain, inversely proportional to the product of the number of transmitting and receiving radars. A set of optimal radar constellation is identified, resulting from the minimization of the target localization error. The best linear unbiased estimator (BLUE) is derived for the MIMO target localization problem. The performance of both the BLUE and the maximum likelihood estimator (MLE) are evaluated with respect to the CRLB. The BLUE is used for further evaluation of the relation between sensors locations, target location, and localization accuracy, by a metric known as geometric dilution of precision (GDOP). GDOP contours map the relative performance accuracy for a given layout of radars over a given geographic area.
Publications:
1. H. Godrich, A. M. Haimovich, and R. S. Blum Target Localization Techniques and Tools for MIMO radar,?presented at the 2008 IEEE Radar Conference, Rome, Italy, May 2008.
2. H. Godrich, A. M. Haimovich, and R. S. Blum Cramer Rao Bound on Target Localization Estimation in MIMO Radar Systems,?presented at CISS 2008, Princeton, March 2008.
1. H. Godrich, A. M. Haimovich and R. S. Blum, Concepts and Applications of a MIMO Radar System with Widely Separated Antennas, book chapter in MIMO Radars, John Wiley 2008.
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