Imperfections in analog circuits are a serious obstacle to the realization of high data rate transmission for OFDM systems. IQ imbalance is one such imperfection. We propose an IQ imbalance estimation scheme with transient local oscillator frequency, which is different from center frequency of transmit signal, to correct the IQ imbalance of the receiver in the time domain. Computer simulations show that the proposed scheme can achieve almost the same performance as that without IQ imbalance.

Channel estimation is a key baseband processing task in wireless systems. Filtering or smoothing algorithms can improve the accuracy of channel estimates and the discrete cosine transformation (DCT) can be used for this purpose. By using the DCT, performance will be improved compared to the straight forward approach of per subcarrier estimation (PSE). However, the complexity of DCT is not negligible. This paper proposes a low complexity channel estimation scheme using the DCT. Simulation results show that the performance is improved by more than 1 dB compared with PSE in MIMO-OFDM system.

Imperfections in analog circuits are a serious obstacle to the realization of high data rate transmission for MIMO-OFDM systems. IQ imbalance is one such imperfection. The paper proposed a compensation scheme using a cascaded connection of MMSE and MLD, which applies MIMO decoding and compensation of IQ imbalance simultaneously. Computer simulations show that the proposed scheme can achieve almost the same performance as the conventional compensation scheme even though the computational complexity is reduced to about 1/3 of that of the conventional compensation scheme.

Orthogonal frequency division multiplex (OFDM) using a high-order modulation suffers from effects of in-phase and quadrature-phase (IQ) imbalance in a front-end analog processing. In this paper, we propose a new IQ imbalance compensation method for OFDM systems. The proposed method estimates the IQ imbalance parameters using pilot signals inserted in frequency domain and then compensates the IQ imbalance effects based on the estimated parameter. The proposed method can be applied regardless of frequency-spacings of pilot signals and can compensate the IQ imbalance effect correctly.

In the IEEE 802.11a/g/n system, frequency offset between a transmitter and a receiver is compensated using the preamble of the frame. Moreover, continuous frequency compensation is possible using pilot subcarriers in cases where the frequency offset varies in following data symbols. This continuous compensation is important since serious phase noise occurs in the frequency synthesizer of 5/2.4 GHz, distorts OFDM symbols, and makes demodulation of 16QAM or 64QAM signals difficult. Recently the problem regarding phase noise has come one of the most prominent issues in development of radio communication systems. Conventionally, a method that measures the phase rotation of the signal using a pilot subcarrier and compensates the rotation by extrapolation is known. However, by the conventional method, a fast phase fluctuation cannot be followed since extrapolation mainly follows the constant phase rotation. Therefore, we propose a phase compensation method which can acquire good receiving characteristics also under phase noise environment. The proposed method applies both measured phase rotation of each OFDM symbol itself and the average of them with constant ratio. Then, tracking for both the steady and the fluctuated frequency offset becomes possible

An antenna-selective transmit diversity (ASTD) technique, applied to a PC card for an orthogonal frequency division multiplexing (OFDM)-based wireless local area network (WLAN) system, is presented. The PC card, which is based on IEEE 802.11 a/g, is equipped with dual-band printed antennas. The ASTD processing is implemented in the developed WLAN system, which has two antennas for transmit and receive modes. The effects of the ASTD technique on the TCP (transmission control protocol) throughput performance are experimentally evaluated. Measurement results with the PC card in an imitated office show that the TCP throughput is improved on both the 2.4 and 5.2 GHz bands under the conditions of an indoor radio propagation channel due to the ASTD technique. In particular, application of the ASTD technique to an access point (AP) achieves four times the average TCP throughput on the 5.2 GHz band.

This paper discusses the problems in teleoperation systems for mobile robots and the use of a virtual world in these systems. In order to achieve smooth operation of the mobile robot via the communication link, we should consider the delays in data transfer. To compensate the incomplete data sets, virtual images can be generated by computer graphics when the information of the working environment can be acquired beforehand. We construct a virtual world teleoperation system. The performance of the system is examined through experiments with an actual mobile robot. It is shown that the virtual robot can be operated by a human operator in almost the same manner as a teleoperated real robot.

An optical code-division multiple-access (CDMA) system with double optical hard-limiters is proposed where the optical hard-limiters are placed before and after an optical correlator. Moreover, the-effect of the optical hard-limiter on the performance of the optical synchronous CDMA systems using modified prime sequence codes as signature codes is analyzed under the assumption of a Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. We evaluate the performance under average power and bit rate constraints. Our results show that using the single optical hard-limiter slightly degrades the performance of the optical CDMA systems under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Moreover, we show that the optical CDMA systems with double optical hard-limiters have better performance than other conventional CDMA systems with and without the optical hard-limiter when the number of simultaneous users is not so large.