This paper explores the problem of localizing an emitter of radio frequency energy using a network of mobile receiver nodes, each taking measurements of the emitter's received signal strength (RSS). In this paper, we drop the assumption of receiver calibration, leading to biased measurements for each node. We model these bias effects as additive random variables for each receiver in the log-distance path loss model. We propose two novel estimators to handle these effects as modifications of the nonlinear least squares and the Gaussian particle filter algorithms. The estimators are augmented using the principle of variance least squares, in which the biases' effect on the data covariance is estimated online. These estimates inform subsequent iterations of the nonlinear algorithms. The path loss exponent and emitter power offset are likewise treated as unknowns. Our simulations show the performance improvement evident in various scenarios over the naive approaches, other contemporary algorithms, and with respect to the Cramér-Rao lower bound. We further show the efficacy of our approach through several experiments using real RSS data collected from a mobile network.

This paper considers the problem of tomographic area mapping using radio frequency measurements gathered by a network of mobile nodes. Termed radio tomographic imaging, the technique has shown potential for object tracking, imaging static obstacles, and even through-wall imaging. Our approach addresses substantial issues for the practical implementation of such a system, namely, the mitigation of multipath signal effects and the characterization of a large number of uncalibrated network links. We propose a system that utilizes ultrawideband direct path signal strength measurements as a means of reducing the effects of the multipath fading. Furthermore, we address the estimation of unknown path loss and link bias parameters online through the framework of a linear mixed effects model. This permits the estimation of a static area map without a prohibitive calibration of these parameters prior to deployment, which is crucial in a network that may contain hundreds of links. Our model is posed as a convex optimization problem using the elastic net for regularization. Bayesian performance bounds are derived and our method shows positive results in simulation. We then demonstrate the efficacy of our solution on real tomographic data gathered from our cognitive spectrum operations testbed.

Visible light communications (VLC) rely on white light emitting diodes (LEDs) to provide communication and illumination simultaneously. Orthogonal frequency division multiplexing (OFDM) enables bit loading in VLC to make the best use of available modulation bandwidth of white LEDs. However, OFDM signals in VLC exhibit high peak-to-average power ratio (PAPR) as in radio frequency (RF) communications. VLC-OFDM differs from RF-OFDM in that the baseband signals are real-valued and there are two PAPRs to be reduced, namely upper PAPR and lower PAPR. Moreover, upper PAPR and lower PAPR are not independently distributed, and are often subject to asymmetric constraints. In this paper, we propose a distortion-based PAPR reduction scheme to minimize the weighted upper PAPR and lower PAPR in VLC-OFDM. The proposed method takes bit loading into consideration that ensures allocating less distortions to subcarriers with more bits loaded (higher order constellations).

Recently, visible light communication (VLC) has attracted much attention as a possible candidate technology to meet the ever growing demand in wireless data. However, current low-cost white LED has limited modulation bandwidth, which limits the throughput of the VLC. Optical MIMO can provide spatial diversity and thus achieve high data rate. Traditional multiple-input multiple-output (MIMO) techniques used in wireless communications cannot be directly applied to VLC. This paper studies the precoder and equalizer design of optical wireless MIMO system for VLC. First, we propose a MIMO VLC system, which can effectively support the flickering/dimming control and other VLC-specific requirements. Second, besides the transceiver design with perfect channel state information, we also take into account channel uncertainties for joint optimization in the MIMO VLC system. Numerical results show that the proposed MIMO solution for VLC is robust to combat the influence caused by the channel estimation imperfection. By taking into account the channel estimation errors, the proposed joint optimization method demonstrates the bit error rate (BER) improvements in the scenario of imperfect channel estimation.

This paper addresses the problem of estimating the positions of nodes in a mobile network over time, when pre-surveyed anchor nodes are not available. We do this by employing both inter-node range measurements and odometry data. Both types of measurement data are applied within the multidimensional scaling paradigm, which maps pairwise dissimilarity values into node coordinates. The mathematical treatment is presented, along with several advantages of our proposed approach. We demonstrate its performance through simulation across various parameter values. Further, we show the performance using real range and odometry data gathered from our CSOT mobile robot testbed.

Orthogonal frequency-division multiplexing (OFDM) in visible light communications (VLC) inherits the disadvantage of high peak-to-average power ratio (PAPR) from OFDM in radio frequency (RF) communications. The upper peak power and lower peak power of real-valued VLC-OFDM signals are both limited by the dynamic constraints of light emitting diodes (LEDs). The efficiency and transmitted electrical power are directly related with the upper PAPR (UPAPR) and lower PAPR (LPAPR) of VLC-OFDM. In this paper, we will derive the complementary cumulative distribution function (CCDF) of UPAPR and LPAPR, and investigate the joint distribution of UPAPR and LPAPR.

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 present the privacy rate over Additive White Gaussian Noise (AWGN) and Rayleigh channels when an eavesdropper employs a radiometer detector and has uncertainty about her noise variance. Leveraging recent results on the phenomenon of an Signal to Noise Ratio (SNR) wall when there is eavesdropper noise power measurement uncertainty, we show that a non-zero privacy rate is possible. We also find the SNR wall under Rayleigh fading. We conclude by presenting realizable privacy rates for several practical scenarios.

Visible light communication (VLC) systems can provide illumination and communication simultaneously by way of light emitting diodes (LEDs). The communication function is achieved by employing simple and low-cost intensity modulation and direct detection (IM/DD) schemes. Brightness control and flicker mitigation are two main challenges for the illumination function. Orthogonal frequency division multiplexing (OFDM) has been considered for VLC for its ability to boost data rates. However, OFDM waveforms have high peak-to-average power ratio, and will be clipped if its magnitude is beyond the dynamic range of LEDs. Clipping can cause the performance degradation of communication as well as illumination. In this paper, we will propose an iterative clipping method considering brightness control and flicker mitigation. We will investigate the performance in terms of error vector magnitude (EVM) as well as computational complexity. We will formulate the EVM minimization problem as a convex optimization problem to compare with the iterative clipping method.

In this paper, we study multi-user multi-input single-output (MU-MISO) broadcasting for visible light communication (VLC). VLC differs from radio frequency communication in both baseband signal format and optical power constraints. We propose a precoding and biasing model for MU-MISO transmitter design in indoor VLC. We formulate and solve optimization problems for maximizing max-min fairness or throughput subject to per transmitting unit optical power constraint. We apply linear zero forcing and zero forcing dirty paper coding techniques and compare them in simulations.

Visible light communication (VLC) systems can provide illumination and communication simultaneously via light emitting diodes (LEDs). Orthogonal frequency division multiplexing (OFDM) waveforms transmitted in a VLC system will have high peak-to-average power ratios (PAPRs). Since the transmitting LED is dynamic-range limited, peaks of the OFDM waveform can be clipped causing signal degradation. This same phenomenon also occurs in RF communication system, although in RF systems it is straightforward to quantify the performance in terms of RF power conversion efficiency. Results on quantifying VLC performance are scarce. Specifically, because VLC differs from RF communication in system constraint, baseband signal format, and nonlinearity characteristic of the transmitter, it is not obvious how PAPR is related to illumination-to-communication conversion efficiency in VLC. In this paper, we will attempt to quantify the illumination-to-communication conversion efficiency and clarify how PAPR is related to efficiency in VLC systems. We also present a method to improve the efficiency of VLC OFDM systems.

In this paper we present two results on reducing the peak power of orthogonal frequency division multiplexing (OFDM) symbols via constellation extension (CE). The first result is a derivation of the interior-point method (IPM) algorithm needed to find the optimal distortion set, where the distortion is constrained by convex functions. Next we optimize the parameters of a hybrid CE constraint set to minimize the BER. Numerical examples are provided to illustrate the findings.

Wireless communication systems are increasingly adopting orthogonal frequency division multiplexing (OFDM) to enable efficient high-data rate transmissions. Such systems often employ pilot symbols to estimate wireless channels, and null subcarriers on band edges to reduce adjacent channel interference. Recent work has focused on designing the pilot sequence to improve channel estimation performance. In this paper, we present a new pilot design for OFDM systems based on a arbitrary-order polynomial parameterization of the pilot subcarrier indices. We show our design achieves better performance than existing methods. Theoretical models and simulated performance demonstrate the increased accuracy of our design.

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.

Optimizing the indices and values of free subcarriers (FSs) can significantly reduce the peak-to-average power ratio (PAR) of orthogonal frequency division multiplexing (OFDM) signals. In this paper, a numerical method is proposed to analyze the average PAR reduction performance of the FS optimization method. Different PAR reduction performance is revealed for different FS index assignments and the optimal single FS index can be determined by the proposed approach.

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.

In this paper the effects of receiver nonlinearities are examined for clipping channels. The objective of this work is to determine the optimal receiver functions for additive noise clipping channels. In this case the optimal receiver function will be the one that maximizes the signal-to-noise-plus-distortion ratio (SNDR) between the transmitted and received variables. To solve the problem we utilize functional analysis to find a necessary condition for the SNDR-maximizing receiver function. The results are general and can be applied for any noise and signal distribution. Furthermore, the results show that for the example given, the linear receiver is not SNDR-optimal.

In multiple-access multicarrier systems, it is possible to serve multiple users at high data rates from a single base station. In this paper, we propose the multi-channel partial transmit sequences (MCPTS) method for multiple-access OFDM peak-to-average power ratio (PAR) reduction. By applying the PTS phase rotations to each access channel separately and assuming that the rotations can be detected by each user as a channel fading, MCPTS avoids two major disadvantages of standard PTS: i) the phase rotations can be detected without side information, and ii) the set of possible phase rotation values does not need to be finite. In addition, an iterative method of joint MCPTS and power allocation is proposed to minimize the average BER. The results show a significant BER improvement.

This paper addresses signal classification based on an average power statistic for peak-to-average-power ratio (PAR) reduced signals. Specifically, it is assumed that either a QAM or a Complex Gaussian finite-length symbol is transmitted in a noisy, peak-limited channel with known peak power. The goal is determine the whether an uninformed receiver can distinguish between these signal types using an average power statistic. Several methods for transmitting through peak-power channels are examined including optimal clipping and piecewise linear scaling (PWLS) with selected mapping (SLM) PAR reduction. For the analysis, it is necessary to derive the mean power for each of the transmission methods. Accordingly, we show how the harmonic mean PAR, E[1/PAR], is related to the mean power and derive E[1/PAR] in closed form. We find that average power is a accurate discriminator for low-order QAM and Gaussian symbols. For high-order QAM, accurate discrimination is also possible when the noise level is sufficiently low or when enough signal samples are available.

Pilot symbol assisted modulation (PSAM) orthogonal frequency division multiplexing (OFDM) has proven to be a popular technique for high-speed communication through multipath fading channels. In this paper we examine PSAM pilot design optimization in OFDM systems that employ edge null subcarriers for spectral shaping. Specifically, we show that the commonly used even pilot spacing design is suboptimal in terms of symbol estimate mean squared error (MSE) performance when a sufficient number of null subcarriers are present. We pursue a parametric design of the pilot spacings and use convex optimization techniques in order to find a pilot design that results in near-optimal symbol estimate MSE performance. Finally, we present several example PSAM OFDM pilot designs including one example based on the IEEE 802.16 standard to demonstrate performance improvements over the conventional even-spacing pilot design when null edge subcarriers are present.

In this paper we apply a new joint synchronization-pilot sequence (JSPS) optimization design technique to multiple transmitter OFDM systems. One objective in the design of the JSPS for MISO transmissions is to eliminate the requirement for the different preamble fields used for coarse and fine synchronization and channel training. In this work, independent JSPSs are designed for each transmitter in a multiple antenna system, but are transmitted simultaneously from each antenna. Consequently, JSPSs can potentially reduce the overhead of multiple antenna preamble transmissions (improving bandwidth efficiency). The objective of this paper is to determine the performance advantages of joint carrier frequency offset (CFO) and channel estimation using only the pilot portion of the JSPS. By jointly optimizing the position and power of each pilot across the transmitter antennas in Rayleigh fading channels with CFO, we show ≫ 15 dB SNR improvement in the performance of the channel estimator.

Peak-to-average power ratio (PAR) reduction techniques are often employed to increase the power efficiency of orthogonal frequency division multiplexing (OFDM) systems. A recently proposed PAR optimization method demonstrates how the PAR can be minimized when free subcarriers and a certain distortion allowance on the error vector magnitude (EVM) are available. In this paper, we derive the lower bound on the capacity for such a system and investigate the capacity-maximizing number of free subcarriers that should be used.

In this paper we apply a new pilot design optimization technique to the IEEE 802.16a wireless MAN standard. Using the specifications from 802.16a, we demonstrate that significant SER improvements are possible for 802.16a in a Rayleigh fading channel by judiciously choosing where the pilots are placed and the power contained in each pilot. Specifically, for the OFDM mode, we show that up to 13 dB SNR improvement is possible by simply modifying the pilots. For the OFDMA mode we demonstrate that a more modest but still significant improvement of 1.8 dB is possible with proper pilot design.

Selected mapping (SLM) is a distortionless crest factor reduction (CFR) method for orthogonal frequency division multiplexing (OFDM) transmission. With SLM, it is possible to reduce the peak-to-average power ratio (PAR) of an OFDM symbol by several decibels. In this paper, we propose a method for SLM phase sequence detection that does not require side information transmission. We refer to this method as magnitude-scaled SLM, in the sense that it scales the frequency-domain power profile of the OFDM symbol with an envelope function from a set of pre-determined envelope functions. From the envelope of the received symbol, the receiver can detect which envelope and thus which phase sequence was used in the transmission. Also presented in this paper are the theoretical characterizations of the detection error rate (DER) and symbol error rate (SER) in a magnitude-scaled SLM system. Compared with ordinary OFDM without CFR, magnitude-scaled SLM can achieve an order of magnitude SER improvement in a peak-power-limited channel.

In this paper we build on the orthogonal frequency division multiplexing (OFDM) peak-to-average power ratio (PAR) reduction work by Chen and Zhou. It has been demonstrated that pilot sequences that are constant modulus in the time domain can lead to an ensemble PAR-reduction across all data realizations. However, the problem of creating constant modulus sequences from arbitrary frequency domain power profiles has never been addressed. Often, it is desirable to have a some freedom of choice in how pilot and, possibly synchronization, energy is allocated in the frequency domain. In this paper we present a projection on to convex sets (POCS) method for creating low-PAR synchronization/pilot (S/P) sequences with arbitrary frequency-domain power profiles