Virtual Carriers Research Articles

Blind maximum likelihood CFO estimation for OFDM systems via polynomial rooting

The blind carrier frequency offset estimation problem has been well studied by exploiting the virtual carriers existing in practical orthogonal frequency division multiplexing transmissions. A highly efficient approach by rooting a polynomial has been proposed in the literature. However, this rooting method is suboptimal when noise is present. In this letter, we propose an improved polynomial rooting method that is shown to be the maximum likelihood estimator for both the noisy and the noise-free case.

Blind Carrier Frequency Offset Estimation in MIMO-OFDM

We propose a DOA-Matrix based blind CFO estimation method for a MIMO-OFDM system with Virtual Carriers. The method estimates CFO in closed-form by jointly exploiting the shift-invariant structure of time-domain signal received on multiple receive antennas. In contrast to previous training-based methods, the proposed method is bandwidth efficiency. We also present numerical simulation results for different transmit and receive antenna configurations and for different record lengths.

Blind Estimation of OFDM Carrier Frequency Offset via Oversampling

Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed.

Subspace-based blind channel estimation for OFDM by exploiting virtual carriers

Reliable channel estimation is indispensable for orthogonal frequency-division multiplexing (OFDM) systems employing coherent detection and adaptive loading in order to achieve high data rate communications. Several options exist in practical OFDM systems-including training symbols, cyclic prefix, virtual carriers, pilot tones, and receiver diversity-to facilitate channel estimation. In this paper, a subspace blind channel estimation method based on exploiting the presence of virtual carriers is proposed for OFDM systems over a time-dispersive channel. The method can be applied to conventional OFDM systems with cyclic prefix as well as OFDM systems with no cyclic prefix. The reduction/elimination of cyclic prefix thereby provides the OFDM systems the potential to achieve higher channel utilization than most previously reported cyclic prefix based estimators. Sufficient channel identifiability condition is developed as well. Comparison with two other recently reported subspace methods is presented via computer simulations to support the effectiveness of the proposed method.

A semi-analytic method for the design of OFDM systems

This work presents a semi-analytic method for the design of orthogonal frequency division multiplexing (OFDM) systems, thereby avoiding the use of intensive simulation runs. In particular, the power of the distortion on each sub-channel is evaluated analytically for any possible choice of system parameters (namely the number of virtual carriers, the length of the cyclic prefix, the delay spread of the channel, the impulse response of the transmit and receive filters). By this means, we may calculate the values of the above parameters which guarantee a desired system performance. Copyright © 2003 AEI.

Equalization methods in OFDM and FMT systems for broadband wireless communications

Multicarrier systems are adopted in several standards for their ability to achieve optimal performance in very dispersive channels. In particular, orthogonal-frequency division multiplexing (OFDM) and filtered multitone (FMT) systems are two examples where the modulation filter has an ideal rectangular amplitude characteristic in time and frequency domains, respectively. In this letter, we propose new equalization schemes for FMT and compare their performances with OFDM. In general, FMT has a greater spectral efficiency than OFDM, due to the absence of the cyclic prefix and a reduced number of virtual carriers. However, it exhibits a higher distortion per subchannel, due to the imperfect equalization of the transmit filters. As a performance comparison, we considered both the achievable bit rate (ABR) and the bit error rate (BER) in a multipath Rayleigh fading channel. We note that while ABR gives a theoretical bound on the system bit rate, assuming the knowledge of the channel at the transmit side, the BER refers to an uncoiled system with a fixed modulation. Although FMT requires a fixed structure with a higher computational complexity than OFDM, it turns out that FMT, even with the simplest one tap per subchannel adaptive equalizer, yields a better performance than OFDM, both in terms of ABR and BER. Hence, FMT can be a valid alternative to OFDM for broadband wireless applications, also.

Maximum likelihood estimation of OFDM carrier frequency offset

We develop a maximum likelihood estimate for orthogonal frequency division multiplexing (OFDM) carrier frequency offset in the presence of virtual carriers. It is found that the resulting estimate has an identical form to that of a previously proposed blind OFDM carrier frequency offset estimate by Liu and Tureli (see IEEE Commun. Lett., vol.2, p.104-106, 1998). Insights are provided as to why these two algorithms are equivalent.

Ultrafast optical generation of carriers in a dc electric field: Transient localization and photocurrent.

Recent experimental observations of terahertz radiation produced by the ultrafast optical excitation of surface depletion layers of GaAs and InP have generated a lot of interest but its physical origins are still not fully understood. The source of the radiation is believed to be either the time-dependent transport current produced by the optically generated carriers in the depletion field, or the displacement current due to creation of polarized electron-hole pairs. We show that, in general, both mechanisms must be included. The ultrafast optical generation in a dc field is shown to result in the creation of carriers in transiently localized states that evolve into delocalized states, causing transport current in the process. By calculating the dipole moments of these localized states, we are able to determine the time-dependent polarization and photocurrent including both transport and displacement contributions. We find that the displacement contribution comes mainly from the virtual carriers while the real carriers are responsible for the transport current. The competition between transport and displacement current leads to a nontrivial dependence of the overall signal on the dc field strength, the excitation duration, and the detuning. In particular, we predict a sign reversal of the signal at sufficiently high detunings in agreement with recent experimental findings.

Coulomb enhancement of ultrafast nonlinearities in quantum-well structures

The Coulomb interaction among photoexcited virtual carriers or virtual excitons in two-dimensional heterostructures is theoretically considered by using bare-states and the dressed-states approaches. It is shown that, if the wave functions of the ground and the excited states are spatially separated, then the Coulomb interaction causes coherent pulsations of the transition energy. As a result, significant (an order of magnitude) enhancement of the dynamic Stark shift and of other ultrafast nonlinearities can be achieved. The enhancement of ultrafast nonlinearities is evaluated as a function of the geometry, the doping level, broadening, and other parameters. The effects of the Coulomb interaction on coherent transient phenomena is considered. The problem of practical realization of the predicted effects is addressed.

Interaction of a Localized Spin Moment with Phonons Due to the Exchange Scattering of Virtual Carriers in Semiconductors

AbstractA new theory of the spin‐lattice interaction mechanism is developed for localized spins in semiconductors. In this mechanism, the coupling with phonons results from exchange scattering on a localized spin of the electron‐hole pairs accompanying the phonons. The process leads to a relaxation flip of the localized spin, provided the intermediate‐state carrier spins are flipped by spinorbit interaction. A detailed analysis of the mechanism is carried out in the approximation of the isotropic band structure of the semiconductor and the deformation electron‐phonon interactions. The experimental manifestations of the mechanism are discussed.