G. Tong Zhou

A Hammerstein predistortion linearization design based on the indirect learning architecture

Power amplifiers (PAs) are inherently nonlinear devices and are used in virtually all communications systems. Digital baseband predistortion is a highly cost effective way to linearize the PAs, but most existing architectures assume that the PA has a memoryless nonlinearity. For wider bandwidth applications such as WCDMA, PA memory effects can no longer be ignored, and memoryless predistortion has limited effectiveness. In this paper, we model the PA as a Wiener system and construct a Hammerstein predistorter, obtained using an indirect learning architecture. Linearization performance is demonstrated on a 3-carrier UMTS signal.

Multi-user MISO broadcasting for indoor visible light communication

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.

Spectral analysis of polynomial nonlinearity with applications to RF power amplifiers

The majority of the nonlinearity in a communication system is attributed to the power amplifier (PA) present at the final stage of the transmitter chain. In this paper, we consider Gaussian distributed input signals (such as OFDM), and PAs that can be modeled by memoryless or memory polynomials. We derive closed-form expressions of the PA output power spectral density, for an arbitrary nonlinear order, based on the so-called Leonov-Shiryaev formula. We then apply these results to answer practical questions such as the contribution of AM/PM conversion to spectral regrowth and the relationship between memory effects and spectral asymmetry.

Nonlinear distortion mitigation in visible light communications

Many physical devices used in VLC systems exhibit nonlinear effects, which can significantly degrade the overall system performance. For example, the LED is the major source of nonlinearity. The forward current is zero unless the forward voltage exceeds a turn-on value. The forward current is also limited with a maximum permissible value. In addition, the electrical-tooptical conversion is also nonlinear. In this article, we provide an overview of techniques related to modeling and distortion mitigation techniques of the nonlinearity in VLC systems. Appropriate models and robust nonlinearity mitigation techniques are crucial to support high-speed transmissions in practical VLC systems.

Superimposed periodic pilots for blind channel estimation

Multipath is a major impairment in a wireless communications environment. Blind channel estimation methods are of interest because they avoid training and thus make efficient use of the available bandwidth. In the past few years, we have seen an evolution from higher-order statistics-based, to cyclic second-order statistics based approaches for blind channel estimation. We propose here a simple, alternative scheme which employs only a first-order statistic. Pilot symbols are periodically added to the intended symbol stream prior to transmission. There is no loss of information rate in our approach, and neither is there any restriction on the FIR channel to be identified. We derive the variance expression of our linear channel estimate and compare with the Cramer-Rao Bound (CRB). Numerical examples indicate that the variance approaches the CRB at high SNR.

Peak-to-average power ratio and illumination-to-communication efficiency considerations in visible light OFDM systems

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.

Error vector magnitude optimization for OFDM systems with a deterministic peak-to-average power ratio constraint

Orthogonal frequency division multiplexing (OFDM) has been adopted by several wireless transmission standards. A major disadvantage of OFDM is the large dynamic range of its time-domain waveforms, making OFDM vulnerable to nonlinearities (including clipping effects) of the power amplifier (PA) and causing the PA to yield low efficiency on the RF to dc power conversion. A commonly used metric to characterize a signals dynamic range is the peak-to-average power ratio (PAR). To suppress the nonlinear effects, one may want to reduce the signal PAR. However, this results in the increase of error vector magnitude (EVM), and may violate the spectral mask. In this paper, we formulate the problem as an EVM optimization task subject to a deterministic PAR constraint and a spectral mask constraint. A low-complexity customized interior-point algorithm is developed to solve the optimization problem. We also discuss extensions of the optimization framework, whereby we optimize the parameters with respect to two metrics on signal-to-noise-and-distortion ratio (SNDR) and mutual information, respectively.

Designing Peak Power Constrained Amplify-and-Forward Relay Networks with Cooperative Diversity

Relay networks with amplify-and-forward (AF) protocol have attracted attention due to its high performance and low implementation complexity at the relay. To collect full cooperative diversity, the AF relay networks are facing two main issues. One is that the channel state information (CSI) of the source to relay link (i.e., two-hop information) is needed at the destination. The other concern is that the power scaling factor (PSF) and output signals at the relay are unbounded. These two issues make AF less practical in resource constrained networks, e.g., blind and peak power constrained relays. In this paper, we reveal the sufficient and necessary conditions on the PSF for the maximum ratio combining (MRC) receiver with two-hop CSI to achieve full cooperative diversity. Furthermore, we also provide necessary conditions on the PSF design so that MRC with one-hop CSI still collects full cooperative diversity. Based on these design criteria, we show a practical peak power constrained AF strategy so that neither the relay nor the destination node needs the source-relay CSI, yet full cooperative diversity can still be achieved.

Achievable data rate analysis of clipped FLIP-OFDM in optical wireless communication

Optical wireless communication (OWC) systems employ simple low-cost intensity modulation and direct detection (IM/DD) techniques, which require that the electric signal must be real-valued and unipolar. Flip orthogonal frequency division multiplexing (Flip-OFDM) is one of the unipolar OFDM techniques developed for OWC. In this paper, we will analyze the clipping effects on Flip-OFDM and derive the achievable data rates under both average optical power and dynamic optical power constraints. Finally, the Flip-OFDM will be compared with the DC biased optical OFDM (DCO-OFDM).

Iterative Clipping for PAPR Reduction in Visible Light OFDM Communications

Visible light communications (VLC) rely on white light emitting diodes (LEDs) to transmit information and provide illumination. White LEDs act as low pass filters and thus place a limitation on achievable data rates in VLC. Orthogonal frequency division multiplexing (OFDM) has been applied to VLC to best make use of the available modulation bandwidth through bit loading. However, VLC-OFDM waveforms exhibit high peak-to-average power ratio (PAPR). This drawback makes VLC-OFDM signals very sensitive to nonlinerarity of LEDs and requires large biasing. We propse an iterative clipping method to reduce upper PAPR and lower PAPR of over sampled VLC-OFDM signals without introuducing in-band distortions. Theoretical analysis of PAPRs distributions, and simulations of PAPR reduction scheme are presented in this paper.