Genyuan Wang

Imaging Through Unknown Walls Using Different Standoff Distances

In through-the-wall imaging, errors in wall parameters cause targets to be imaged away from their true positions. The displacement in target locations depend on the accuracy of the estimates of the wall parameters as well as the target position relative to the antenna array. A technique using two or more standoff distances of the imaging system from the wall is proposed for application under wall parameter ambiguities. Two different imaging schemes can then be applied to correct for errors in wall characteristics. The first scheme relies on forming target displacement trajectories, each corresponding to a different standoff distance, and assuming different values of wall thickness and dielectric constant. The target position is then determined as the trajectories crossover point. In the second scheme, an image sequence is generated. Each specific image in this sequence is obtained by summing those corresponding to different standoff distances, but with the same assumed wall parameters. An imaging-focusing metric can then be adopted to determine the target position. The paper analyzes the above two schemes and provides extensive simulation examples demonstrating their effectiveness

New approach for target locations in the presence of wall ambiguities

A technique for target location estimation in through-the-wall radar imaging applications is presented. The algorithm corrects for the shifts in target positions due to ambiguities in the wall thickness and dielectric constant. We consider uniform walls and perform imaging using wideband beamforming, with the antennas placed against the wall. Behind-the-wall images are obtained using different structures of transmit and receive arrays. For each array structure, a trajectory of the shifts in the target locations is generated assuming different wall parameters. The target position is estimated as the intersection of the corresponding trajectories. The paper shows that for unknown wall thickness or dielectric constant, the point of intersection is the true target position. In the case when both parameters are unknown, the estimated target location is in close proximity to the target true position. It is demonstrated that the performance of the proposed technique is rather insensitive to the target location behind the wall and to various array structures.

Differential Distributed Space-Time Modulation for Cooperative Networks

In this paper, we develop a protocol for the construction of cooperative networks when the channel state information is not available at the transmitters and the receivers. In the proposed protocol, differential space-time codewords are generated at the source terminal. In the broadcast phase, each row of the differential space-time codeword is transmitted to a different relay, whereas in the relay phase, the relaying terminals retransmit the codeword through simple amplify-and-forward algorithm. The performance of the cooperative diversity system is analyzed for a two-user case for different channel environments in. terms of the diversity gain and the diversity product. The optimization of the power allocation between source and relay terminals is considered for the maximization of the diversity product. When the same modulation scheme is used, the performance of differential detection is degraded by 3 dB noise enhancement compared with coherent detection

Cooperative Spatial Multiplexing in Multi-Hop Wireless Networks

It is well known that a multiple-input-multiple-output (MIMO) system can provide spatial diversity gain as well as spatial multiplexing capability. The MIMO concept has been extended to cooperative wireless networks to form distributed MIMO systems using virtual antennas located at cooperating terminals. The primary interest of cooperative MIMO networks, however, has been focused on the cooperative diversity (C-DIV) approaches to achieve spatial diversity gain. Recent work proposed cooperative spatial multiplexing (C-SM) to simplify the transmit and receive processing requirement on the relay nodes while providing significant energy savings. So far C-SM has been only considered for single-hop relaying. In this paper, we propose the use of multi-hop relaying C-SM systems for transmit energy reduction and performance improvement

Nested cooperative encoding protocol for wireless networks with high energy efficiency

Recently, distributed space-time code designs with high cooperative diversity for wireless communication networks, such as ad hoc and sensor networks, have received much attention. Amplify forward and decoding forward are the widely used protocols for the cooperative diversity in the wireless communication networks. In both protocols, the information received by relay terminals are forwarded to destination or next relay terminals. Since signals sent at relay terminals and ones sent at the source terminal are correlated, there is information redundancy. To improve the energy efficiency of cooperative networks, we propose an encoding protocol, which is referred to as a nested cooperative encoding protocol. In our proposed protocol, the received signal at each relay terminal is divided into several sub-signals with the nest lattice structure of source information. Each of the sub-signals contains only a partial information with a smaller size of constellation compared to the original information sent by the source terminal. We show that our new protocols can achieve both high cooperative diversity and high energy efficiency.

Space-Time Cooperation Diversity Using High-Rate Codes

In a multi-user communication system, cooperative diversity allows single-antenna mobile sets to achieve transmit diversity. Cooperative diversity improves the communication capacity and enhances the robustness of a wireless link when a single channel alone is not reliable. In this paper, a novel cooperative diversity scheme is introduced that enables simultaneous transmission of non-redundant data from all cooperative terminals. By taking advantages of both high-rate full-diversity space-time codes and the cooperative diversity, the proposed method provides high diversity gain beyond the number of physical transmit antennas without compromising the data rates. In essence, the proposed system aims at higher data rates over the non-cooperative counterpart, while maintaining the full diversity gain.

Imperfectly synchronized cooperative network using distributed space-frequency coding

In distributed multiple-input-multiple-output (MIMO) systems, imperfect synchronization causes a unique problem in a coded cooperative diversity system. In the presence of a fractional-symbol delay between the signals transmitted from different relay nodes, the channels become highly dispersive even at a flat-fading environment. Existing methods solve such problem based on time-domain approaches where adaptive equalization is required at the receivers for combining the information transmitted from distributed sources. In this paper, we propose the use of OFDM-based approaches using distributed space-frequency codes. The proposed schemes are insensitive to fractional- symbol delays and lead to higher data rate transmission and simplified implementation. In addition, the proposed schemes permit the use of relatively simple amplify-and- forward algorithm in multi-hop wireless networks without delay accumulations. The proposed methods remove the time delay in each relaying hop by reconstructing the prefix and, as such, improve the spectral efficiency, while keeping a simplified relaying structure.

Space-Time Block Code Designs Based on Quadratic Field Extension for Two-Transmitter Antennas

Space-time block code designs based on algebraic field extension for full rate, large diversity product, and nonvanishing minimum determinant of codewords have received great attention. There are many different types of codes available for two-transmitter antennas, such as cyclotomic space-time block codes, the golden space-time block code, and rotation-based space-time block codes. In this paper, a more general space-time block code design scheme, which is called quadratic space-time block coding, is proposed for the two-transmitter antennas using quadratic field extension. The optimal design of the quadratic space-time block codes in terms of a diversity product criterion is also presented. It is shown that the optimal quadratic space-time block codes designed in this paper do not belong to the existing space-time block code family such as the cyclotomic, golden, and rotation-based space-time block codes. The simulation results demonstrate that the average codeword error rate of the optimal quadratic space-time block code attains about 0.5 dB signal to noise ratio gain over those of the optimal cyclotomic and golden space-time block codes.

A novel technique to overcome wall ambiguities in through-the-wall radar imaging applications

In the through-the-wall radar imaging applications, errors in wall parameters cause targets to be imaged away from their true positions. The shifts of target locations depend on the estimation errors in the wall thickness and dielectric constant as well as the target locations with respect to the antenna array. In this paper, we propose a novel technique that enables proper imaging to be performed with unknown wall characteristics. In this technique, the imaging is performed for different sets of the assumed wall parameters at different standoff distances from the wall. For each set of wall parameters, the combined image for different standoff distances is obtained. A focusing metric is then applied to determine the wall characteristics corresponding to the maximum sharpness of the image. The proposed standoff-based imaging technique has several advantages in practical use, including safety, covertness, and rapid acquisition time.

Distributed MIMO-OFDM in Imperfectly Synchronized Cooperative Network ∗

Coded space-time cooperation is an efficient approach in delivering information over a relay network. Multiple cooperative terminals (nodes) form a distributed multiple-input-multiple-output (MIMO) systems, thus providing high data rates and high diversity gains. However, unlike conventional co-located MIMO systems, it is impractical for distributed MIMO networks to maintain perfect timing synchronization between different transmit terminals. In particular, the presence of a fractional-symbol delay difference between the signals transmitted from different terminals can cause erroneous sampling positions and yield highly dispersive channels even at a memoryless channel environment. Existing methods solve such problem based on time-domain approaches where adaptive equalization is required at the receivers for combining the information transmitted from distributed sources. In this paper, we propose the use of OFDM-based approaches using distributed space-frequency codes. The proposed schemes are insensitive to fractional-symbol delays and lead to higher data rate transmission and simplified implementation. In addition, the proposed schemes permit the use of relatively simple amplify-and-forward algorithm in multi-hop wireless networks without delay accumulations. The time delay in each relaying hop by reconstructing the cyclic prefix and, as such, improve the spectral efficiency, while keeping a simple relaying structure.