Brett T. Walkenhorst

Adaptive polarization transmission of OFDM signals in channels with polarization mode dispersion and polarization-dependent loss

A 2times2 multiple-input multiple-output (MIMO) architecture using dual-polarized antennas (DPAs) is considered with orthogonal frequency division multiplexing (OFDM). The performance of DPAs is evaluated for adaptive polarization (AP) transmission techniques in time-varying multipath channels impaired by polarization mode dispersion (PMD) and polarization-dependent loss (PDL). AP transmission techniques considered include power gain maximization, polarization dispersion minimization for interference avoidance, polarization multiplexing with water-filling, and a suboptimal multiplexing strategy that enables direct recovery of the polarization multiplexed streams, thereby simplifying the design of the receiver. Measured time-varying dual-polarized channel realizations from mobile-to-mobile experiments are used to estimate the capacity, diversity, and interference avoidance performance of the adaptive approaches.

A high-speed four-transmitter four-receiver MIMO OFDM testbed: experimental results and analyses

By adopting multiple-input multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) technologies, indoor wireless systems could reach data rates up to several hundreds of Mbits/s and achieve spectral efficiencies of several tens of bits/Hz/s, which are unattainable for conventional single-input single-output systems. The enhancements of data rate and spectral efficiency come from the fact that MIMO and OFDM schemes are indeed parallel transmission technologies in the space and frequency domains, respectively. To validate the functionality and feasibility of MIMO and OFDM technologies, we set up a four-transmitter four-receiver OFDM testbed in a typical indoor environment, which achieves a peak data rate of 525 Mbits/s and a spectral efficiency of 19.2 bits/Hz/s. The performances including MIMO channel characteristics, bit-error rate against signal-to-noise ratio curves, the impairments of carrier frequency offset and channel estimation inaccuracy, and an asymmetric MIMO scheme are reported and analyzed in this paper.

Dual-polarized architectures for sensing with wireless communications signals

Methods for detecting object translation and/or motion using differential polarization-based sensing are described. Both time and frequency-based polarimetric sensing strategies are considered. The time-based detection statistic is formed using the differential time-averaged received signal polarization state. This approach suffers from the impact of polarization-sensitive channel impairments, especially polarization mode dispersion (PMD). As PMD effects become more prominent, the variability of the time-domain signal polarization increases and the average polarization over a detection interval may be substantially different from the instantaneous signal polarization states, detracting from the overall sensitivity of the detection statistic. Frequency-domain approaches, on the other hand, can leverage the relatively stable polarization-frequency signature of the received signal, and can be designed to easily exploit PMD using differential detection in the subbands. Integration of the detections over the full complement of subbands is then used to arrive at a final detection statistic. Using indoor measurements, the resulting subbanded architecture is found to provide improved detection sensitivity in comparison to the time-based version as well as to more conventional power-based techniques, including a subbanded power-based architecture.

Achieving Undetectable Communication

In this paper we consider the problem of achieving a positive error-free communications rate without being detected by an eavesdropper-we coin this the privacy rate. Specifically, we analyze the privacy rate over additive white Gaussian Noise (AWGN) channels with finite and infinite number of samples and Rayleigh single input-single (SISO) and multiple input-multiple output (MIMO) channels with infinite samples when an eavesdropper employs a radiometer detector and has uncertainty about his noise variance. Leveraging recent results on the phenomenon of a signal-to-noise ratio (SNR) wall when there is eavesdropper noise power measurement uncertainty, we show that a nonzero privacy rate is possible. We also show that in this scenario, the detector should not necessarily take as many samples as possible.

Complex Gaussian Ratio Distribution with Applications for Error Rate Calculation in Fading Channels with Imperfect CSI

Communications systems rarely have perfect channel state information (PCSI) when demodulating received symbols. This paper shows that the symbol error rate (SER) of a flat fading communications system can be expressed in closed form by expressing the demodulator outputs as random variable (RVs) that have a complex ratio distribution, which is the ratio of two correlated complex Gaussian RVs. To complete the analysis, the complex ratio probability density function (PDF) and cumulative distribution function (CDF) are both derived. Finally, using several scenarios based on M-QAM signaling, the SER performance of imperfect channel state information (ICSI) systems is analyzed.

Polarization-based interference mitigation for OFDM signals in channels with polarization mode dispersion

A dual-polarized antenna architecture is used in channels exhibiting polarization mode dispersion to investigate polarization-based interference suppression. In a wireless experiment, orthogonal frequency division multiplexing (OFDM) signals are transmitted from a slant-45deg polarized antenna and received with a dual-polarized antenna. The vertical (V) and horizontal (H) received complex baseband samples are corrupted by synthesized broadband interference with arbitrary polarization. Channel estimates are formed for each subcarrier of the received OFDM signal and minimum mean-squared error (MMSE) weights are computed to maximize the signal to interference plus noise ratio (SINR) with a single interferer. We find that this sub-band processing approach improves the performance relative to full-band processing because of the polarization mode dispersion found in a typical wireless channel.

Improving MIMO Capacity in a Line-of-Sight Environment

In this paper, the optimal form of a channel matrix is derived to maximize the MIMO capacity in a LOS environment. Antenna response functions are proposed to achieve the optimal form and suboptimal results are considered. In designing an antenna to maximize the capacity, the objective is to increase the slope of the far-field phase response without sacrificing too much antenna gain. Such a constraint has not typically been considered in antenna design and should be further explored.

Repeater-Assisted Capacity Enhancement (RACE) for MIMO Links in a Line-of-Sight Environment

A single on-frequency, full duplex wireless repeater is considered for enhancing the capacity of a long-distance 2 times 2 multiple-input multiple-output (MIMO) wireless link with a dominant line-of-sight (LOS) component. Such links might be used for high-speed communication between buildings or towers. For practical reasons, the aperture sizes of the transmit (TX) and receive (RX) arrays may be limited, which tends to make the rank of the long-distance MIMO channel matrix close to unity. The paper shows that the addition of just one repeater can bring the channel to full rank, approximately doubling the capacity of the link. Typical values of repeater isolation are assumed. For small TX and RX array element spacings, there is considerable robustness in optimal repeater location. Even with some multipath, most of the capacity improvement is retained.

Multiple Repeater Placement for Assisting Long-Range LOS MIMO Links

A wireless MIMO link is considered in a LOS configuration with multiple SISO wireless repeaters assisting. A theoretical analysis is presented in which repeater positions are considered and sufficient conditions proposed and evaluated for maximizing the MIMO multiplexing gain. The assisting repeaters would be useful when space constraints, for example, on a building top, would prohibit the wide element spacings needed by a MIMO terminal to reach full capacity over a long range LOS link. Sufficient conditions include, among other things, mutual orthogonality of steering vectors from one MIMO terminal pointing toward the opposite terminal and toward each of the repeaters. Once n - 1 such repeaters have been properly placed, where n represents the smaller dimension of the channel matrix, the matrix becomes full rank and the multiplexing gain is maximized.

An Empirical Doubly-Selective Dual-Polarization Vehicular MIMO Channel Model

To optimally design vehicle-to-vehicle (VTV) communication systems, it is necessary to first fully characterize the response of the communications channel. Significant research attention has been focused on describing channels for various scenarios with both analytical and statistical models based on empirical data (empirical models). This paper extends this field by proffering a doubly selective (i.e., time- and frequency-selective) channel for VTV 2x2 multiple input multiple output (MIMO) systems where the pair of transmit and the pair of receive antennas each have orthogonal polarizations. The proposed empirical model is based on collected channel data in an urban setting and is described by a four-basis function Doppler spectrum for each delay tap. For verification, it is demonstrated that the modeled channel closely matches the collected channel data.