Pilot-assisted OFDM Research Articles

Pilot-Assisted OFDM for Underwater Acoustic Communication

Multicarrier techniques have made it possible to wirelessly transmit data at higher rates for underwater acoustic (UWA) communication. Several multicarrier techniques have been explored in the past for wireless data transmission. OFDM is known to fight off inter-symbol interference due to the orthogonality of its subcarriers. However, due to time variations, OFDM suffers from intercarrier interference. As the UWA channel is both a time and frequency variant, channel estimation becomes complex. We propose a pilot-based channel estimation technique and explore two equalizers for improving the error performance of an OFDM-based UWA system. Both the equalizers employ pilot subcarriers to estimate the UWA channel. One equalizer is a least squares (LS) equalizer and the other is a zero forcing (ZF) equalizer. Using computer simulations, it is observed that, for an acceptable error performance, the number of pilots should be one-fourth the number of subcarriers. Moreover, if the energy of the pilots is increased without changing the overall symbol energy, the error performance degrades. It is also noted that both the LS and ZF equalizers give an acceptable error performance with the ZF performing marginally better than the LS. Furthermore, the error performance of the proposed system is evaluated as a function of the transmitter-receiver distance and an acceptable error performance is observed even at 1250 m.

Deterministic Pilot Pattern Placement Optimization for OFDM Sparse Channel Estimation

Compressed sensing (CS) technologies have been widely adopted to pilot-assisted orthogonal frequency division multiplexing (OFDM) sparse channel estimation. However, only few works have focused on the location optimization of the pilot pattern. This paper investigates the pilot location optimization for the measurement matrix construction based on the minimum mutual coherence (MC) rule. We consider the design of the deterministic OFDM pilot pattern for solving a combinatorial optimization problem. The proposed approach utilizes the advantages of the Q-bit to update the location of the OFDM pilot pattern. The obtained results show that the proposed approach can form measurement matrix with a smaller MC and the estimated performance can be essentially improved compared with the standard genetic algorithms or random search method.

Modified PTS for PAPR Reduction in Pilot-Assisted Ofdm: Generating New Phase Weighting Factors from the Paley-I Type Hadamard Matrices

High Peak to Average Power Ratio (PAPR) is still a most important challenge in Orthogonal Frequency Division Multiplexing (OFDM) system. This paper proposes a modified Partial Transmit Sequence (PTS) technique which acquires its phase weighting factors from the Paley-I type Hadamard matrices with an objective to reduce the high PAPR in OFDM systems. The proposed technique achieves better PAPR reduction than traditional PTS-OFDM and it also obviate the need for exhaustive search for the viable phase weighting factors in PTS-OFDM, thereby reducing the computational overhead and also avoids the unwanted data rate loss.

None pilot-tones and training sequence assisted OFDM technology based on multiple-differential amplitude phase shift keying

This paper proposes a novel none pilot-assisted orthogonal frequency division multiplexing (OFDM) technology based on multi-differential amplitude phase shift keying (mDAPSK) for optical OFDM system. It doesn't require any bandwidth-consuming pilot tones or training sequence for channel estimation due to the differential detection during demodulation. In the experiment, a 41.31 Gb/s 64DAPSK-OFDM signal without pilot tones is successfully transmitted over 160-km single mode fiber (SMF). The performance comparison between multi-quadrature amplitude modulation (mQAM) and mDAPSK is also given in the experiment, and the results indicate a prospect of this technology in optical OFDM system.

A Study of Pilot-Assisted OFDM Channel Estimation Methods With Improvements for DVB-T2

We provide a comparative study of channel estimation methods for pilot-assisted orthogonal frequency-division multiplexing (OFDM) communication systems in time-varying frequency-selective fading channels. We also propose an improved low-complexity least-squares (LS) estimation method based on the domain-transform process, which estimates time-domain channel impulse response (CIR) using the LS estimate of pilot subchannels. Our method is particularly suited to commercial OFDM systems such as the second generation of digital terrestrial television broadcasting systems (DVB-T2) because it has the following properties: 1) It uses only one OFDM symbol; 2) it does not require knowledge of channel statistics; 3) it works for any pilot pattern; and 4) the size of the estimation matrix for obtaining CIR only depends on the number of pilot subcarriers, but not on the size of OFDM symbol. Because of the first three properties, our method is robust to high Doppler frequency shifts, timing synchronization errors, and pilot patterns in commercial systems. Moreover, through computer simulations, we verify that our method provides competitive bit error rate and channel estimation error performance, compared with some well-known methods, which suffer from either a much higher computational load or less robustness to Doppler shifts or timing errors. For irregular pilot patterns, efficient yet simple regularization methods are suggested to solve the ill-conditioned problem of the estimation matrix. An upper bound on the mean squared error of the estimated CIR is also provided.

Estimation of time-varying channels for pilot-assisted OFDM systems

Orthogonal frequency division multiplexing (OFDM) systems encounter performance degradations because of the time-varying (TV) channels common in wireless environments. The channel variations within one OFDM symbol introduce intercarrier interference. In this case, the frequency domain channel matrix is no longer diagonal, hence the corresponding channel estimation is challenging. In this article, two novel TV channel estimation approaches are proposed for the pilot-assisted OFDM systems, where the channel is approximated by the high-order linear model or the piece-wise linear model in time domain. The least square estimation is derived for the two kinds of channel approximations. The simulation is performed based on realistic TV channels with a fairly high Doppler spread. The results show the significant decreasing of the estimation mean square error using the proposed approaches.