OFDM under Oscillator Phase Noise : Contributions to Analysis and Estimation Methods

Most modern transmitters and receivers involve an analog front-end unit and a digital back-end unit. The digital back-end is responsible for information processing which involves thefollowing: redundancy removal from information; information representation; improvinginformation resilience; and information correction. The analog front-end is responsible forinformation transmission and reception. The information processing algorithms developedand implemented in the digital back-end assume a linear and noiseless analog front-end which,in reality, is not the case. This renders some of the information processing algorithms to be lesseffective in practice. The focus of this thesis is on orthogonal frequency-division multiplexing(OFDM) systems under the influence of oscillator phase noise. OFDM is an efficientinformation representation technique used in many communication systems. On the otherhand, phase noise is one type of undesired noise that occurs in the oscillator device used in theanalog front-end. It arises due to the imperfect task of frequency conversion, performed by theoscillator device, between baseband and radio frequency.  This thesis contributes to the areas of analysis and estimation in OFDM systems under theinfluence of oscillator phase noise. With regard to analysis, this thesis contributes by derivingthe channel capacity assuming a Gaussian input alphabet. The aim here is to show bothquantitatively and qualitatively the degradation in performance of the OFDM system in thepresence of phase noise. The analysis is conducted for phase noise processes that occur in bothfree-running and phase-locked loop based oscillators and also extended to include the effect ofcarrier frequency offset. With regard to estimation, two new phase noise estimation algorithmsare proposed in this thesis. In particular, these algorithms restrict the search space to a specific subset, where the desired phase noise parameter of interest is shown to lie. For example, in the first estimation method, possible subspaces in which the desired phase noise spectral vector may lie are used in the estimation step. In the second method, the geometry of the desired phase noise spectral vector is used in the estimation step. Specifically, this geometry corresponds to a non-convex set described by a set of quadratic forms that involve permutation matrices. By restricting the search space to this set, the accuracy of phase noise estimation can be improved

Constrained Phase Noise Estimation in OFDM Using Scattered Pilots Without Decision Feedback

In this paper, we consider an OFDM radio link corrupted by oscillator phase noise in the receiver, namely the problem of estimating and compensating for the impairment. To lessen the computational burden and delay incurred onto the receiver, we estimate phase noise using only scattered pilot subcarriers, i.e., no tentative symbol decisions are used in obtaining and improving the phase noise estimate. In particular, the phase noise estimation problem is posed as an unconstrained optimization problem whose minimizer suffers from the so-called amplitude and phase estimation error. These errors arise due to receiver noise, estimation from limited scattered pilot subcarriers and estimation using a dimensionality reduction model. It is empirically shown that, at high signal-to-noise-ratios, the phase estimation error is small. To reduce the amplitude estimation error, we restrict the minimizer to be drawn from the so-called phase noise geometry set when minimizing the cost function. The resulting optimization problem is a non-convex program. However, using the S-procedure for quadratic equalities, we show that the optimal solution can be obtained by solving the convex dual problem. We also consider a less complex heuristic scheme that achieves the same objective of restricting the minimizer to the phase noise geometry set. Through simulations, we demonstrate improved coded bit-error-rate and phase noise estimation error performance when enforcing the phase noise geometry. For example, at high signal-to-noise-ratios, the probability density function of the phase noise estimation error exhibits thinner tails which results in lower bit-error-rate

Preamble-Based Channel Estimation for CP-OFDM and OFDM/OQAM Systems: A Comparative Study

In this paper, preamble-based least squares (LS) channel estimation in OFDM systems of the QAM and offset QAM (OQAM) types is considered, in both the frequency and the time domains. The construction of optimal (in the mean squared error (MSE) sense) preambles is investigated, for both the cases of full (all tones carrying pilot symbols) and sparse (a subset of pilot tones, surrounded by nulls or data) preambles. The two OFDM systems are compared for the same transmit power, which, for cyclic prefix (CP) based OFDM/QAM, also includes the power spent for CP transmission. OFDM/OQAM, with a sparse preamble consisting of equipowered and equispaced pilots embedded in zeros, turns out to perform at least as well as CP-OFDM. Simulations results are presented that verify the analysis

Performance and Compensation of I/Q Imbalance in Differential STBC-OFDM

Differential space time block coding (STBC) achieves full spatial diversity and avoids channel estimation overhead. Over highly frequency-selective channels, STBC is integrated with orthogonal frequency division multiplexing (OFDM) to achieve high performance. However, low-cost implementation of differential STBC-OFDM using direct-conversion transceivers is sensitive to In-phase/Quadrature-phase imbalance (IQI). In this paper, we quantify the performance impact of IQI at the receiver front-end on differential STBC-OFDM systems and propose a compensation algorithm to mitigate its effect. The proposed receiver IQI compensation works in an adaptive decision-directed manner without using known pilots or training sequences, which reduces the rate loss due to training overhead. Our numerical results show that our proposed compensation algorithm can effectively mitigate receive IQI in differential STBC-OFDM.Comment: 7 pages, 2 figures, IEEE GLOBECOM 201