Scattered Pilots and Virtual Carriers Based Frequency Offset Tracking for OFDM Systems: Algorithms, Identifiability, and Performance Analysis

In this paper, we propose a novel carrier frequency offset (CFO) tracking algorithm for orthogonal frequency division multiplexing (OFDM) systems by exploiting scattered pilot carriers and virtual carriers embedded in the existing OFDM standards. Assuming that the channel remains constant during two consecutive OFDM blocks and perfect timing, a CFO tracking algorithm is proposed using the limited number of pilot carriers in each OFDM block. Identifiability of this pilot based algorithm is fully discussed under the noise free environment, and a constellation rotation strategy is proposed to eliminate the c-ambiguity for arbitrary constellations. A weighted algorithm is then proposed by considering both scattered pilots and virtual carriers. We find that, the pilots increase the performance accuracy of the algorithm, while the virtual carriers reduce the chance of CFO outlier. Therefore, the proposed tracking algorithm is able to achieve full range CFO estimation, can be used before channel estimation, and could provide improved performance compared to existing algorithms. The asymptotic mean square error (MSE) of the proposed algorithm is derived and simulation results agree with the theoretical analysis

Effect of Synchronization error in Multi Carrier systems

Communications as an important aspect life plays a major role in our daily routine. With the progress in age and growth in its demand, there has been rapid development in the field of communications. Analog signals which were used to send information previously are now sent in digital domain in a much wider range. For better performance in terms of transmissions, single carriers are replaced by multi-carriers Some of the methods that use multi-carriers for transmission are Orthogonal Division Frequency Multiplexing (OFDM), Code Division Multiple Access (CDMA). In OFDM system, orthogonally placed subcarriers are used to carry the data from the transmitter to the receiver. The guard band present in this system avoids the system from being effected by Inter Symbol Interference (ISI). However, Doppler Shifts and delays induced in the channel due to various reasons impose frequency offset to the carrier. This results in an error in the Synchronization between the transmitter and receiver. This leads to the loss of orthogonally between the subcarriers and thus degrades the performance of the OFDM system.In this project, the Synchronization error in OFDM is discussed in particular and a technique of estimating the Carrier Frequency Offset (CFO) using Null Subcarriers is studied

Waveform Advancements and Synchronization Techniques for Generalized Frequency Division Multiplexing

To enable a new level of connectivity among machines as well as between people and machines, future wireless applications will demand higher requirements on data rates, response time, and reliability from the communication system. This will lead to a different system design, comprising a wide range of deployment scenarios. One important aspect is the evolution of physical layer (PHY), specifically the waveform modulation. The novel generalized frequency division multiplexing (GFDM) technique is a prominent proposal for a flexible block filtered multicarrier modulation. This thesis introduces an advanced GFDM concept that enables the emulation of other prominent waveform candidates in scenarios where they perform best. Hence, a unique modulation framework is presented that is capable of addressing a wide range of scenarios and to upgrade the PHY for 5G networks. In particular, for a subset of system parameters of the modulation framework, the problem of symbol time offset (STO) and carrier frequency offset (CFO) estimation is investigated and synchronization approaches, which can operate in burst and continuous transmissions, are designed. The first part of this work presents the modulation principles of prominent 5G candidate waveforms and then focuses on the GFDM basic and advanced attributes. The GFDM concept is extended towards the use of OQAM, introducing the novel frequency-shift OQAM-GFDM, and a new low complexity model based on signal processing carried out in the time domain. A new prototype filter proposal highlights the benefits obtained in terms of a reduced out-of-band (OOB) radiation and more attractive hardware implementation cost. With proper parameterization of the advanced GFDM, the achieved gains are applicable to other filtered OFDM waveforms. In the second part, a search approach for estimating STO and CFO in GFDM is evaluated. A self-interference metric is proposed to quantify the effective SNR penalty caused by the residual time and frequency misalignment or intrinsic inter-symbol interference (ISI) and inter-carrier interference (ICI) for arbitrary pulse shape design in GFDM. In particular, the ICI can be used as a non-data aided approach for frequency estimation. Then, GFDM training sequences, defined either as an isolated preamble or embedded as a midamble or pseudo-circular pre/post-amble, are designed. Simulations show better OOB emission and good estimation results, either comparable or superior, to state-of-the-art OFDM system in wireless channels

Dual-Polarization OFDM-OQAM Wireless Communication System

In this paper we describe the overall idea and results of a recently proposed radio access technique based on filter bank multicarrier (FBMC) communication system using two orthogonal polarizations: dual-polarization FBMC (DP-FBMC). Using this system we can alleviate the intrinsic interference problem in FBMC systems. This enables use of all the multicarrier techniques used in cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM) systems for channel equalization, multiple-input/multiple-output (MIMO) processing, etc., without using the extra processing required for conventional FBMC. DP-FBMC also provides other interesting advantages over CP-OFDM and FBMC such as more robustness in multipath fading channels, and more robustness to receiver carrier frequency offset (CFO) and timing offset (TO). For DP-FBMC we propose three different structures based on different multiplexing techniques in time, frequency, and polarization. We will show that one of these structures has exactly the same system complexity and equipment as conventional FBMC. In our simulation results DP-FBMC has better bit error ratio (BER) performance in dispersive channels. Based on these results, DP-FBMC has potential as a promising candidate for future wireless communication systems.Comment: 1.This paper is accepted to be published in IEEE Vehicular Technology Conference (VTC) FALL 2018. 2.In this new submitted version authors have revised the paper based on the VTC FALL reviewers comments. Therefore some typos have fixed and some results have change

Performance Evaluation of a Hybrid Fractional Carrier Frequency Offset Estimator in OFDM

The major drawback of the orthogonal frequency division multiplexing (OFDM) system is high sensitivity to synchronization errors caused by carrier frequency offsets (CFOs), which result in degradation in the bit error rate (BER) performance. This paper investigates the performance of a hybrid fractional carrier frequency offset estimator (FCFOE) for frequency synchronization in the OFDM system. The hybrid FCFOE exploits the pilots inserted within the OFDM symbol for channel estimation together with the information inherent in the cyclic prefix (CP), with a view to improving the estimation of the CP-based FCFOE. The performance of the developed hybrid FCFOE was evaluated in terms of the mean squared error (MSE) and bit error rate (BER) using OFDM-QPSK and OFDM-16QAM schemes it turn. The simulation results show that the hybrid FCFOE only gives slightly better performance over the CP-based FCFOE; but the performance enhancement of the hybrid FCFOE is noticeable in OFDM-16QAM. Keywords: Carrier frequency offset estimation, Hybrid, Orthogonal frequency division multiplexing, Synchronization, Maximum likelihood, Cross-correlation

Experimental demonstration of data-dependent pilot-aided phase noise estimation for CO-OFDM

We demonstrate a novel phase noise estimation scheme for CO-OFDM, in which pilot subcarriers are deliberately correlated to the data subcarriers. This technique reduces the overhead by a factor of 2