Len Cimini

MIMO radar: an idea whose time has come

It has recently been shown that multiple-input multiple-output (MIMO) antenna systems have the potential to improve dramatically the performance of communication systems over single antenna systems. Unlike beamforming, which presumes a high correlation between signals either transmitted or received by an array, the MIMO concept exploits the independence between signals at the array elements. In conventional radar, target scintillations are regarded as a nuisance parameter that degrades radar performance. The novelty of MIMO radar is that it takes the opposite view; namely, it capitalizes on target scintillations to improve the radars performance. We introduce the MIMO concept for radar. The MIMO radar system under consideration consists of a transmit array with widely-spaced elements such that each views a different aspect of the target. The array at the receiver is a conventional array used for direction finding (DF). The system performance analysis is carried out in terms of the Cramer-Rao bound of the mean-square error in estimating the target direction. It is shown that MIMO radar leads to significant performance improvement in DF accuracy.

High Resolution Capabilities of MIMO Radar

Multiple-input multiple-output (MIMO) radar is a multistatic architecture composed of multiple transmitters and receivers, which seeks to exploit the spatial diversity of radar backscatter. In conjunction with centralized processing, MIMO radar has the potential to significantly improve radar functions such as detection and parameter estimation. MIMO radar is distinct from other types of array radars such as phased array or STAP, which process the signals of closely spaced elements and, hence, cannot capitalize on the spatial characteristics of targets. In this work, we explore the ability of MIMO radar and coherent processing to locate a target with high resolution and to resolve targets located in the same range cell. A distributed target model is developed. It is demonstrated that MIMO radar with centralized coherent processing is able to resolve scatterers with a range resolution well beyond that supported by the signal bandwidth. The location estimation capabilities are further illustrated by introducing a new two-dimensional ambiguity function. The analysis is discussed in the context of established results for randomly thinned arrays. The investigation of high resolution MIMO radar also includes comparison with the performance of non-coherent MIMO radar and the effect on performance of the number of sensors and their locations.

New approaches for cooperative use of multiple antennas in ad hoc wireless networks

The paper explores the interaction between cooperative diversity techniques, in the physical layer, and routing, in the network layer. Three approaches are proposed: relay-by-flooding; relay-assisted routing; relay-enhanced routing. In relay-by-flooding, the tradeoff between achieved rate and required power is studied for three selective decode-and-forward schemes: simple relay; space-time-coded relay; best-select relay. In relay-enhanced routing, cooperation is applied to each link of an existing route. Two relay selection approaches are proposed: best-select in the neighbor set; best-select in the decoded set. Best-select in the decoded set is shown to improve the performance significantly.

A simplified opportunistic feedback and scheduling scheme for OFDM

Opportunistic beamforming schedules users when they experience a high instantaneous signal-to-noise ratio. Multiple antennas at the transmitter can he used to induce temporal fading to ensure that all users fade at a rate fast enough to ensure fairness. Because feedback is required, the fading rate must be fast enough to ensure fairness among users, but slow enough so that the feedback information is not out of date. An OFDM system with opportunistic beamforming has the advantage that multiple users can be scheduled at the same time in a frequency-selective channel, thus allowing a slower fading rate. However, the overhead cost of feeding back every sub-carrier for every user is very high. We propose a simplified opportunistic feedback scheme that divides the OFDM symbol into clusters. Each user feeds back a figure-of-merit listing its strongest clusters. This scheme greatly reduces the feedback overhead, without sacrificing performance significantly. In addition, the scheme has an inherent on/off waterfilling property. We compare this simplified feedback scheme in a HIPERLAN/2 scenario to a feeding back of all subcarriers and also to a smart antenna system and show that, when there are many users, it outperforms the smart antenna system.

Networking with cooperative communications: Holistic design and realistic evaluation

Cooperative communications fundamentally changes the abstraction of a wireless link and offers significant potential advantages for wireless networks. However, such technologies are evaluated with idealized scenarios where many control costs are assumed to be negligible. Moreover, even if these costs appear small in simple network scenarios, they increase significantly with the size and traffic level of the network, requiring careful evaluation, or even innovative design, of cooperative protocols to ensure their usefulness in realistic networks. This article describes a realistic evaluation of cooperative communications in a networking context. Insights obtained from this evaluation help guide work on cooperative communications toward practical and potentially beneficial protocols.

On the uplink synchronization of OFDMA systems

In this paper, we study symbol timing synchronization for the uplink of an OFDMA system in a multipath fading channel. The effects of timing offsets on both synchronized and unsynchronized users are investigated and performance degradation is illustrated. Then, a simple timing offset estimator is developed and its performance is evaluated. The symbol-error rate of the compensated system is also presented. Simulation results demonstrate that this estimator performs well even for large offsets and in the presence of significant levels of noise

The impact of the timeliness of information on the performance of multihop best-select

Cooperative relaying enables nodes to actively cooperate to deliver packets to their destination. This cooperation allows nodes to take advantage of the diversity provided by variations in the channel gains between nodes. Best-select, a particular type of cooperation, has been shown to result in significant gains in the performance of source-to-destination communication. However, this increase in performance is achieved by exchanging channel gain measurements, which requires overhead. One way to reduce this overhead is to exchange channel gain measurements less frequently. This paper examines the trade-off between performance and the frequency of exchanging channel gains. This investigation focuses only on the channels that are impaired by multipath fading and shadow fading

Publications: one of the jewels in the crown [The President's Page]

As anticipated in my first message in January, the President,s Pages in February to June will be devoted to the presentation of the five ComSoc Vice Presidents, who will describe their sector activities and programs for the next two years. This months column concerns the Vice President for Publications, Sarah Kate (Katie) Wilson.

A New Policy on Transactions Letters

We want to inform you about some changes that will occur in the IEEE Transactions on Communications (TCOM) and the IEEE Transactions on Wireless Communications (TWireless) in the near future. The primary change is that, starting in early 2012, Letters will no longer be published in TCOM and TWireless. The aim of this editorial is to give some background and further details on this and related changes.

Publications: leading the way in a knowledge-based society [The President's Page]

The IEEE Communications Society (ComSoc) offers a wide range of publications and on-line services, including journals, magazines, and educational products. This portfolio is a shining star of the Society and one of our most important member services. ComSoc publications are a highlyrefined accumulation of new findings in communications and a valuable contribution of ComSoc to the knowledge base of humankind. However, several challenges must be addressed if ComSoc is to maintain its standard of excellence and continue to play an influential role in the rapidly changing landscape of the communications world.