Sher Ali Cheema

On the performance of block transmission schemes in optical channels with a Gaussian profile

OFDM and its real valued version discrete multitone transmission (DMT) are popular schemes to compensate dispersion in direct detection optical systems. They employ an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver, whereas the data symbols are processed block-wise. Block transmission combined with frequency domain equalization (FDE) has been recognized as a possible alternative to the DMT schemes, where the IFFT is moved from the transmitter to the receiver. This paper compares various bit loading enhanced DMT schemes such as asymmetrically clipped DMT and DC-biased DMT to PAM block transmission with FDE and to various single subcarrier FDE schemes. Moreover, a new approach termed asymmetrically clipped orthogonal PAM, is proposed in this work. Simulation results are presented for theoretical channels with a Gaussian low-pass profile. It will be shown that PAM-FDE performs best for this kind of channel, and neither bit loading enhanced DMT nor single subcarrier block transmission with FDE can offer a higher data rate at a given average optical power.

Transmission schemes for visible light communications in multipath environments

Discrete multitone transmission (DMT) is a useful approach to cope with the dispersive nature of VLC indoor channels. In such environments, the dispersion occurs either due to the LED in combination with its driver and or the multipath propagation channel. In this paper, we consider the impact of both types of dispersions on the bit error rate performance of a DMT based Li-Fi system. However, the main goal of this work is to compare the DMT scheme results with the transmission strategies employing frequency domain equalization. Here, we distinguish between pulse-amplitude modulation (PAM), partial-response coding, single-subcarrier (SSC) modulation, and “carrier-less amplitude and phase” (CAP) modulation.

Efficient techniques for multi-gigabits/s transmission over plastic optical fiber

Due to their dispersive nature, large core step index plastic optical fibers (SI-POF) suffer from a bandwidth limitation. At high data rates or transmission distances, this leads to severe inter symbol interference (ISI). Efficient transmission techniques at the transmitter and robust equalization techniques at the receiver are needed to mitigate the effect of ISI. Discrete multitone transmission (DMT) is one of the most popular schemes to compensate dispersion in direct detection optical systems. Recently, non-linear equalization technique such as decision feedback equalization (DFE) and Tomlinson- Harashima precoding (THP) are being proposed for such dispersive channels. A possible low complex alternative to the such schemes can be a PAM block transmission with frequency domain equalization (PAM-FDE) at the receiver. This paper compares the bit loading enhanced DMT (namely asymmetrically clipped (AC) DMT) with systems employing nonlinear equalization techniques such as DFE and THP, and PAM-FDE for theoretical POF channels with a Gaussian profile. Although it is widely believed that non-linear equalization schemes will outperform the linear schemes such as PAM-FDE, we show that almost similar or better performance can be achieved by using PAM-FDE when system complexity is also taken into account.

Linear detection schemes for MIMO UW-OFDM

A new signaling concept, known as unique word — orthogonal frequency division multiplexing (UW-OFDM), has been introduced recently. It has been shown that UW-OFDM has better spectral properties and also a superior bit error ratio (BER) performance over conventional cyclic prefix (CP) based OFDM. In UW-OFDM, the CPs are replaced by deterministic sequences, the so-called unique words (UWs). This is achieved by adding redundancy in the frequency domain with the help of a code generator matrix at the transmitter which introduces correlation between the data symbols. A superior BER performance is achieved if the code generator matrix is utilized efficiently in the detection procedure. In wireless communication systems multiple-input multiple-output (MIMO) techniques are widely used to improve the overall spectral and energy efficiency by exploiting the multiple antennas. So far, the performance of UW-OFDM has been well investigated for single-input single-output (SISO) systems. In this work, we expand our investigation of UW-OFDM to MIMO systems and propose two detection approaches. We show that similar to CP-OFDM, a subcarrier wise detection is also possible for UW-OFDM. But in addition an efficient code generator demodulator is required to take advantage of the correlation introduced in the data symbols. The results show that UW-OFDM outperforms CP-OFDM significantly but requires a higher computational complexity.

Block Transmission with Frequency Domain Equalization for VLC

OFDM and its real-valued version discrete multitone transmission (DMT) are popular schemes to compensate dispersion in direct detection optical systems. They employ an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver, whereas the data symbols are processed block-wise. Pulse-amplitude modulation (PAM) or single subcarrier (SSC) modulation combined with block transmission and frequency domain equalization (FDE) has been recognized as a possible alternative to the DMT schemes. Here, the IFFT is moved from the transmitter to the receiver, since the modulation takes place directly in the time domain. In this chapter, we investigate the suitability of FDE for Li-Fi (light fidelity) systems based on visible light communications. Such systems require nonnegative and real-valued signals, which additionally offer a DC-balance. We discuss the theoretical background of FDE under the intensity modulation constraint and compare its performance with the performance of bit-loading enhanced DC-biased DMT in multipath indoor channels.

PAM-transmission with optimal detection for dispersive optical channels with intensity modulation and direct detection

Digital transmission over dispersive optical channels suffers from intersymbol interference (ISI).We study themutual information of PAM transmission under the intensity modulation and direct detection (IM/DD) constraint, where we model the optical channel as a Gaussian low-pass filter, which is a valid attempt for polymer optical fibers (POF) and allows closed form expressions. In particular, we study the properties of the sample whitened matched filter, which enables the derivation of the mutual information for PAM with IM/DD. The information rate (lower bound) of PAM with equally probable input symbols is compared to an information rate approximation of DC biased multiple sub-carrier modulation (MSM), where we approximate the effect due to threshold clipping as Gaussian distributed noise, and to the information rate of asymmetrically clipped MSM. The required optical power for uncoded block transmission at a bit error rate of 10−3 is also presented. In the case of PAM, a unique word approach is used, and decision-feedback equalization is performed directly at the output of a sample whitened matched filter realized in the frequency domain. The results show that PAM with a rectangular pulse shape outperforms the other schemes.

Perturbation analysis of Joint Eigenvalue Decomposition Algorithms

Joint EigenValue Decomposition (JEVD) algorithms are widely used in many application scenarios. These algorithms can be divided into different categories based on the cost function that needs to be minimized. Most of the frequently used algorithms in the literature use indirect least square (LS) criteria as a cost function. In this work, we perform a first order perturbation analysis for the JEVD algorithms based on the indirect LS criterion. We also present closed-form expressions for the eigenvector and eigenvalue matrices. The obtained expressions are asymptotic in the signal-to-noise ratio (SNR). Additionally, we use these results to obtain a statistical analysis, where we only assume that the noise has finite second order moments. The simulation results show that the proposed analytical expressions match well to the empirical results of JEVD algorithms which are based on the LS cost function.

First-order perturbation analysis of low-rank tensor approximations based on the truncated HOSVD

The truncated version of the higher-order singular value decomposition (HOSVD) has a great significance in multi-dimensional tensor-based signal processing. It allows to extract the principal components from noisy observations in order to find a low-rank approximation of the multi-dimensional data. In this paper, we address the question of how good the approximation is by analytically quantifying the tensor reconstruction error introduced by the truncated HOSVD. We present a first-order perturbation analysis of the truncated HOSVD to obtain analytical expressions for the signal subspace error in each dimension as well as the tensor reconstruction error induced by the low-rank approximation of the noise corrupted tensor. The results are asymptotic in the signal-to-noise ratio (SNR) and expressed in terms of the second-order moments of the noise, such that apart from a zero mean, no assumptions on the noise statistics are required. Empirical simulation results verify the obtained analytical expressions.

A Maximum Likelihood “identification-correction” scheme of sub-optimal “SeDJoCo” solutions for semi-Blind Source Separation

The “Sequentially Drilled” Joint Congruence (SeDJoCo) transformation is a set of matrix transformation equations, which coincide with the Likelihood Equations for semi-blind source separation, when each source is modeled as a zero-mean Gaussian process with a known (and distinct) temporal covariance matrix. Therefore, with such a model a solution of SeDJoCo can lead to the Maximum Likelihood (ML) estimate of the separating matrix, which is asymptotically optimal. However, as we have shown in previous work, multiple solutions of SeDJoCo may exist, and the selection of the optimal solution among these (corresponding to the global maximum of the likelihood function) is therefore of considerable interest. In this paper we further extend our results by proposing a new ML approach for the identification and correction of a sub-optimal solution, assuming sources of unrestricted, general temporal covariance structures. We demonstrate the resulting improvement in simulation with non-stationary sources.

Design of space-time block coded unique word OFDM systems

In this paper we develop space-time block codes for unique word - orthogonal frequency division multiplexing (UW-OFDM) systems to fully exploit the diversity gain when the channel state information is not available at the transmitter. To this end, we propose two novel space-time block codes (STBCs) for UW-OFDM systems, namely a frequency domain space-time block code and a time-reversal space-time code (TR-STC). The former one is an extension of the traditional space-frequency block codes (SFBCs) for CP-OFDM systems to UW-OFDM systems while the latter one makes use of the frame structure of the UW-OFDM symbols and has a low complexity decoder. Simulation results show that both of the proposed space-time block codes achieve a significant gain compared to the SFBC based CP-OFDM. Moreover, the frequency domain STBC yields a slightly better performance as compared to the TR-STC but has a higher computational complexity.