Offset Quadrature Amplitude Modulation-based Filter Bank Multicarrier Research Articles

Symbol Encryption and Placement Design for OQAM/FBMC Systems

In this paper, we propose a symbol encryption method based on the complex-valued symbol multiplication and design the encryption symbol placement for offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly analyze the encryption and decryption effects of complex-valued symbol multiplication on the original OQAM symbols. Then, by utilizing the intrinsic interference characteristics of prototype filters, a symbol encryption method and the placement design are proposed to simultaneously achieve the encryption of all OQAM symbols and low complexity decryption. Finally, the symbol error rate (SER), average signal transmit power, computational overhead and power spectral density (PSD) are analyzed. Simulation results are consistent with the theoretical analysis and demonstrate that the proposed method outperforms the traditional method based on imaginary-valued encryption symbol insertion in terms of the SERs of both legitimate and illegitimate receivers.

Distribution Line Fitting-Based Channel Estimation Without Guard Symbols for OQAM/FBMC Systems

In this paper, we propose a distribution line fitting (DLF) based channel estimation method for offset quadrature amplitude modulation based filter-bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly analyze the distribution characteristics of demodulated OQAM symbols at the receiver, and derive an intrinsic interference free relation between the real and imaginary parts of demodulated OQAM symbols. Then, based on the above derivation, we design a pilot structure of arbitrary columns of pilot symbols without guard symbols between pilot symbols and data symbols. Finally, the distribution lines can be fitted by utilizing the distribution characteristics of demodulated OQAM symbols and the channel frequency response can be estimated. Simulation results demonstrate that the proposed method with one column of pilot symbols outperforms the interference approximation method (IAM) in high signal-to-noise (SNR) regions and outperforms the pair of pilots (POP) method in all SNR regions in terms of bit error ratio (BER), and the proposed method with two columns of pilot symbols outperforms both IAM and POP in all SNR regions in terms of both BER and complexity.

Channel Estimation and Pilot Symbol Optimization Based on Intrinsic Interference Utilization for OQAM/FBMC Systems

In this paper, we propose a novel channel estimation method and optimize the pilot symbols based on intrinsic interference utilization for offset quadrature amplitude modulation based filter-bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly propose a line fitting based channel estimation method by analyzing and utilizing the distribution characteristics of the demodulated OQAM symbols, where no guard symbols are needed between pilot symbols and data symbols. Then, we formulate a pilot symbol optimization problem by utilizing the intrinsic interferences to improve the accuracy of the line fitting. Finally, we obtain the optimal solution of the optimization problem and present the corresponding pilot structure. Simulation results demonstrate that the proposed method with the optimized pilot structure performs the best among candidate pilot symbols and outperforms the traditional interference approximation method (IAM) with the optimal pilot structure.

Block-Wise Preamble Design in OQAM/FBMC Systems With Interference Cancellation

In this letter, we propose a novel complex-valued offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) system with interference cancellation. Specifically, we construct the frame structure of OQAM/FBMC in a frequency-reversal form, where complex-valued symbols are transmitted repeatedly at paired symmetric subcarriers, with appropriate sign changes being inserted to achieve a complete self-cancellation of the intrinsic interference. Besides, we further present a block-wise preamble design method with frequency reversal to reduce the pilot overhead. The whole preamble is divided into multiple subblocks of equal size, each of which is optimally designed to provide a virtual boost of the pseudo-pilot power. Simulation results validate the effectiveness of the proposed complex-valued OQAM/FBMC and significant performance gains are demonstrated against conventional methods with the block-wise preamble design method.

Pairs of Pilots Design for Preamble-Based Channel Estimation in OQAM/FBMC Systems

In this letter, we propose an improved pairs of pilots (POP) design method for channel estimation in offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) systems. Based on an error analysis of the conventional POP channel estimation method, we first develop a heuristic pilot design in which each pilot pair is inserted at two consecutive subcarriers. Besides, we further propose to build a preamble by quartet of subcarriers and conduct a pilot optimization subject to a pilot power constraint, aiming to suppress the noise effect as much as possible. To obtain its optimal solution, we convert the original non-convex pilot optimization problem into a convex semidefinite programming problem, which is shown to be computationally efficient and readily solvable. Numerical simulations validate the effectiveness and superiority of the proposed pilot design method compared to the conventional POP and interference approximation methods.

Preamble Power Optimization Based on Intrinsic Interference Utilization for OQAM/FBMC Channel Estimation

Benefiting from high spectral efficiency, agility of carrier aggregation and support for asynchronous transmissions, offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) is being developed as an attractive alternative to cyclic-prefix based orthogonal frequency division multiplexing (CP-OFDM) as the air interface technology for future communications. However, its non-orthogonal nature results in intrinsic interference among symbols and subcarriers, which makes channel estimation in OQAM/FBMC significantly more difficult than that in CP-OFDM. In this paper, we propose a novel preamble design method based on intrinsic interference utilization for OQAM/FBMC channel estimation. Specifically, we build a preamble by triplets of subcarriers, each of which is determined by a preamble power optimization problem that minimizes the channel estimation mean square error (MSE) at the central subcarrier. We derive the optimal closed-form solution by exploiting the linearly dependent relationship between preamble symbols and intrinsic interference weights. Moreover, extensions to multiple-input multiple-output (MIMO) setup capable of improving both spectral efficiency and channel estimation performance are investigated. Theoretical analysis and simulation results show the superiority of the proposed preamble design method over the conventional methods in terms of channel estimation MSE, bit error ratio (BER) and peak-to-average power ratio (PAPR).

Short Prototype Filter Design for OQAM-FBMC Modulation

Offset Quadrature Amplitude Modulation based Filter-Bank Multicarrier (OQAM-FBMC) has gained research interest for future wireless communication. Conventional OQAM-FBMC systems are mainly designed with long filters that offer better spectral shape. However, latency is also another key parameter when considering low response systems. Therefore, in this context, short prototype filters should be applied for OQAM-FBMC. In this letter, we propose to design short filters with low undesirable spectral emission by using an optimization method. In this approach, we propose a novel objective function to be minimized. Simulation results show that the proposed filters exhibit better behavior in comparison with the existing short filters.

Preamble-Based Channel Estimation for OQAM/FBMC Systems With Delay Diversity

Delay diversity (DD) is a low-complexity and flexible transmit diversity technology for coded offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) transmission systems. However, the introduction of delay parameters in DD-OQAM/FBMC systems leads to increased channel frequency selectivity, which makes the channel estimation problem more complicated. In this paper, we analyze and develop preamble-based channel estimation methods in DD-OQAM/FBMC systems from the perspective of frequency-domain, transform-domain, and time-domain aspects. Besides, channel estimators that can obtain the minimum channel estimation mean square error under least squares (LS) and linear minimum mean square error (LMMSE) criteria are derived according to different levels of channel knowledge. Specifically, the proposed frequency-domain method estimates each channel frequency response individually based on a simplified frequency-domain channel model. The transform-domain method first estimates the equivalent single-input-single-output channel observed at the receiver assuming subchannel flatness, and then uses a window function in the time domain to separate the channels of different transmit antennas. The time-domain method is constructed based on an accurate time-domain channel model to directly estimate the channel impulse response without making any assumption about the subchannel flatness. A series of simulation experiments is conducted to verify the effectivity of these channel estimation methods for DD-OQAM/FBMC systems.

Short Prototype Filter With Constrained Frequency Spreading for OQAM/FBMC Systems

In this letter, we propose a short filter (i.e., the overlapping factor is one) with constrained frequency spreading (CFS) for offset quadrature amplitude modulation-based filter bank multicarrier (OQAM/FBMC) systems. In detail, a system of equations in the filter's frequency coefficients is formulated and solved in the sense of least squares, then the filter is obtained through an inverse discrete Fourier transform. Simulation results reveal that the proposed CFS filter is able to achieve comparable performance to existing short filters in the presence of different channel impairments with less frequency spreading, which leads to lower implementation complexity of OQAM/FBMC using frequency spreading structure, thus it is a valuable choice when considering both the CFS and a near perfect reconstruction of the OQAM/FBMC signal.

Prototype Filter Design to Minimize Stopband Energy With Constraint on Channel Estimation Performance for OQAM/FBMC Systems

In this paper, we propose a prototype filter design method to minimize the stopband energy of the filter with guaranteed time-domain channel estimation performance for offset quadrature amplitude modulation based filter bank multicarrier (OQAM/FBMC) systems. First, we analytically derive the total mean squared error (MSE) of estimated channel impulse response with two channel estimation schemes, i.e., the linear minimum mean square error and weighted least square, where the impact of intrinsic interference from data symbols is taken into account. Then, we formulate an optimization problem of filter coefficients to minimize the stopband energy with constraint on the total MSE. By exploiting the Heisenberg–Gabor uncertainty principle, the original optimization problem is transformed into an equality-constrained one. Finally, we employ the Lagrange multiplier method to solve the transformed problem and obtain the filter coefficients with the Newton’s method. Simulation results demonstrate that the proposed filter outperforms the isotropic orthogonal transform algorithm and extended Gaussian function filters considering both the MSE and stopband energy performances.

New multiple‐input multiple‐output‐based filter bank multicarrier structure for cognitive radio networks

Cognitive radio (CR) has been proposed for spectral efficiency improvement while avoiding interference with licensed users. The offset quadrature amplitude modulation-based filter bank multicarrier (OQAM-FBMC) modulation is a promising candidate for CR-based systems. Although it behaves better in many aspects than orthogonal frequency division multiplexing (OFDM), the application of some techniques with FBMC becomes more challenging due to the imaginary interference, as in the case of multiple-input multiple-output (MIMO) technique. Here, the authors first propose a new MIMO-based FBMC transceiver structure for CR systems. The proposed structure is based on QAM transmission to handle the interference presented in conventional OQAM-FBMC systems. The authors also introduce two different methods to satisfy the orthogonality conditions and cancel the residual interference in QAM-FBMC systems. Both methods are based on discrete cosine transform (DCT) to achieve superior spectrum confinement and hence better spectral efficiency. Moreover, the authors present a resource allocation algorithm for the proposed structure. Simulation results demonstrate that the proposed structure outperforms the conventional FBMC-based MIMO techniques in terms of bit error rate (BER) and computational complexity. The results also show that the spectral efficiency of the proposed structure could be further improved by applying the proposed resource allocation algorithm.