Katsumi Yamashita

Blind Estimation of Carrier Frequency Offset and DC Offset for OFDM Systems

In this letter, a blind joint carrier frequency offset and DC offset (DCO) estimator for OFDM systems is proposed. By exploiting the underlying subspace of the received OFDM signals after the time-domain-average based coarse DCO cancellation, the proposed estimator can achieve excellent performance, which is demonstrated by simulations.

Carrier Frequency Offset and I/Q Imbalances Compensation in OFDM Systems

Carrier frequency offset (CFO) and I/Q imbalances severely degrade the performance of direct-conversion OFDM receiver. In this paper, based on the subcarrier allocation (SCA) of OFDM signal, a novel approach for compensating CFO and I/Q imbalances is proposed. First, we derive a useful matrix representation of OFDM signal with CFO and I/Q imbalances. Then, maximum likelihood CFO estimation, as well as the associated SCA, are given. With the CFO estimate, a SCA-based algorithm which has a close-form solution, is proposed for the compensation of I/Q imbalances. Furthermore, due to the correspondence between the CFO estimate and the compensation of I/Q imbalances, the proposed approach can be extended to a blind compensation scheme, suitable for OFDM signals with asymmetric SCA. Finally, computer simulations demonstrate the validity of the proposed approach.

Subspace-based OFDM Carrier Frequency Offset Estimation in the Presence of DC Offset

In an OFDM receiver with direct-conversion architecture, DC offset (DCO) and carrier frequency offset (CFO) cause a severe performance degradation. Recently, by investigating the subspace of OFDM signal after removing the coarse DCO estimate obtained via a time-domain average, we have proposed a null space based estimator (NSE), for blind CFO and DCO estimation. In this paper, we analyze the cost function of the NSE, and show that the full performance NSE is complexity reducible, which can indeed lead to a common null space based estimator (CNSE). Also, we develop a maximum-likelihood estimator (MLE), and prove that the CNSE is equivalent to the MLE. Furthermore, the Cramer-Rao bound of CFO estimation in the presence of DCO is given. Finally the analysis results are verified by simulations.

A Novel Blind Integer CFO Estimator for OFDM Systems

Carrier frequency offset (CFO) in orthogonal frequency divisional multiplexing (OFDM) system severely degrades the transmission performance. This frequency offset can be divided into an integer part and a fractional part. The fractional part mainly introduces inter-carrier interference (ICI), while the integer part causes frequency ambiguity. In this paper, based on the underlying structure of the received OFDM symbols, a novel blind algorithm is proposed to estimate the integer part of CFO. Compared with the conventional M&Ms blind integer CFO estimator, the proposed one has considerably reduced computational load. Furthermore, simulation results show that under the multipath fading channel, the proposed estimator has lower probability of failure than the M&Ms estimator

Iterative Equalization Technique for Double-Selective Channel Estimation in OFDM Systems

In OFDM based mobile communication systems, channel variation during one symbol period introduces intercarrier interference (ICI). Conventional pilot-aided equalization mitigates the ICI at the price of band inefficiency. On the other hand, the blind or semi-blind equalization method, which utilizes the known statistic properties of the transmitted data, will raise system complexity. In this letter, without bandwidth-consuming pilots, a novel channel estimation and tracking method based on an iterative equalization technique (IET) is proposed. The proposed approach successfully achieves a good compromise between bandwidth efficiency and system complexity, and its validity is demonstrated by numerical simulations, especially for fast fading channel.

Multiuser Detection for MIMO-OFDM Systems Based on Known Partial CSI

In this paper,a novel method for signal recovery in multiuser multiple-input multiple-output(MIMO) orthogonal frequency division multiplex(OFDM) systems with known partial channel state information(CSI) is proposed. The effect of the dispersive channel in a multiuser MIMO-OFDM system can be treated as a set of instantaneous mixtures from the viewpoint of frequency bin(FB). Upon the arrival of a new user,the new interferences are introduced which results an expanding mixture. If the CSI before the arrival of new user is known, it can be employed to recover the transmitted signals, rather than the bandwidth waste pilot symbols. In the proposed method, from the second order statistics of the received signals, the parameters of the mixture caused by the new user are extracted. Then the signals belong to different users are separated at each FB, where the new joined users signals still retain a phase distortion. This distortion can be removed by the existing non-coherent detection method.

PAPR Reduction for PCC-OFDM Systems Using Neural Phase Rotator

This paper proposes a novel Orthogonal frequency-division multiplexing (OFDM) system based on polynomial cancellation coded OFDM (PCC-OFDM). This proposed system can reduce peak-to-average power ratio (PAPR) by our neural phase rotator and it does not need any side information to transmit phase rotation factors. Moreover, this system can compensate the common phase error (CPE) by a proposed technique which allows estimating frequency offset at receiver. From numerical experiments, it is shown that our system can reduce PAPR and ICI at the same time and improve BER performance effectively.