Continuous Orthogonal Frequency Division Multiplexing Research Articles

Massive Connectivity Over MIMO-OFDM: Joint Activity Detection and Channel Estimation With Frequency Selectivity Compensation

In this paper, we study how to efficiently and reliably detect active devices and estimate their channels in a multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) based grant-free non-orthogonal multiple access (NOMA) system to enable massive machine-type communications (mMTC). First, by exploiting the correlation of the channel frequency responses in narrow-band mMTC, we propose a block-wise linear channel model. Specifically, the continuous OFDM subcarriers in the narrow-band are divided into several sub-blocks and a linear function with only two variables (mean and slope) is used to approximate the frequency-selective channel in each sub-block. This significantly reduces the number of variables to be determined in channel estimation and the sub-block number can be adjusted to reliably compensate the channel frequency-selectivity. Second, we formulate the joint active device detection and channel estimation in the block-wise linear system as a Bayesian inference problem. By exploiting the block-sparsity of the channel matrix, we develop an efficient turbo message passing (Turbo-MP) algorithm to resolve the Bayesian inference problem with near-linear complexity. We further incorporate machine learning approaches into Turbo-MP to learn unknown prior parameters. Numerical results demonstrate the superior performance of the proposed algorithm over state-of-the-art algorithms.

Efficient pilot design scheme for OFDM‐based full‐duplex systems

In orthogonal frequency-division multiplexing (OFDM)-based full-duplex (FD) systems, the terminal is allowed to transmit and receive signals simultaneously in the same frequency band. This leads to new challenges to pilot for channel estimation. The conventional pilot design methods are no longer effective for such systems. In this study, the authors propose a new practical pilot design method for OFDM-based FD systems and its performance is analysed in terms of the self-interference cancellation capability. Unlike the existing OFDM-based FD pilot design schemes, which require the channel to be constant during multiple continuous OFDM symbols, the authors' proposed scheme can be implemented in the block fading channel (i.e. channels that vary from one OFDM symbol to another). Besides, the optimal pilot pattern of the proposed scheme is studied. Their simulation results demonstrate the efficiency of the proposed pilot design.

Quasi-Cyclic Symbol Extensions for Shaping the OFDM Spectrum

$N$ -continuous orthogonal frequency division multiplexing (OFDM) is a promising solution to the out-of-band (OOB) emission issue in OFDM systems where the $N$ th-order continuity between OFDM symbols leads to lower OOB emission than the plain OFDM; however, high self-interference in such systems is an obstacle. In this paper, we propose a new class of quasi-cyclic symbol extensions, which makes the OFDM system $N$ -continuous. The new extension leads to low OOB emission while the additive interference is less than other $N$ -continuous OFDM systems. Moreover, we propose a general analysis of the spectrum and interference that occurs in a linear precoded OFDM system. The framework is applicable to many $N$ -continuous OFDM systems including our proposed method.

Time-Domain <inline-formula> <tex-math notation="LaTeX">$N$</tex-math> </inline-formula>-Continuous OFDM: System Architecture and Performance Analysis

$N$ -continuous orthogonal frequency-division multiplexing (NC-OFDM) is a promising technique to achieve significant sidelobe suppression for baseband OFDM signals. However, NC-OFDM usually has high implementation complexity that limits its practical applications. Based on conventional NC-OFDM, this paper proposes a new technique, termed time-domain $N$ -continuous OFDM (TD-NC-OFDM), which shifts the processing burden from the frequency domain to the time domain. This is achieved by adding a smooth signal, which is linearly combined by rectangularly pulsed OFDM basis signals consisting of a novel basis set. We prove that TD-NC-OFDM with the basis set is essentially equivalent to conventional NC-OFDM. Furthermore, using the time-domain structure, an asymptotic spectrum analysis of $N$ -continuous OFDM signals is obtained by developing a closed-form expression related to sidelobe decaying. This paper also examines the impact of TD-NC-OFDM on the received signal-to-interference-plus-noise ratio (SINR) and derives closed-form expressions.

On the Properties of Cubic Metric for OFDM Signals

As a metric for amplitude fluctuation of orthogonal frequency division multiplexing (OFDM) signal, cubic metric (CM) has received an increasing attention because it is more closely related to the distortion induced by nonlinear devices than the well-known peak-to-average power ratio (PAPR). In this paper, the properties of CM of OFDM signal is investigated. First, asymptotic distribution of CM is derived. Second, it is verified that 1.7 times oversampling rate is good enough to capture the CM of continuous OFDM signals in terms of mean square error, which is also practically meaningful because the fast Fourier transform size is typically 1.7 times larger than the nominal bandwidth in the long-term evolution (LTE) of cellular communication systems.