Bo-k Seo

Maximum ratio combining for OFDM systems with cochannel interference

In this paper, we investigate a receive diversity combining scheme for OFDM wireless LAN systems with cochannel interference. The scheme utilizes signal to interference plus noise power ratio (SINR) to obtain the weighting factors in maximal ratio combining. To estimate SINR, we use the coherency between the subchannels, which is caused by the limited delay spread of the multipath channel.

OFDM receivers using oversampling with rational sampling ratios

In this paper, we propose orthogonal frequency division multiplexing (OFDM) receivers using oversampling, where the oversampling ratio is extended from integers to rational numbers. We propose several receiver structures, which reveal the same performance, with the same oversampling ratio. Since the low-pass filters in general OFDM receivers are not ideal, additional noise is introduced into the side bands of the demodulated signal. If we increase the sampling frequency above the symbol rate, subcarriers with higher frequencies than we want are obtained and composed of noise only. Removing the unwanted higher frequency components we can reduce the noise and obtain signal-to-noise power ratio gain compared to the conventional symbol rate receivers. Through computer simulations, we verify the proposed method.

Automatic Classification of Digitally Modulated Signals Based on K-Nearest Neighbor

In this paper, we propose an automatic classification method for eight digitally modulated signals, such as 2FSK, 4FSK, MSK, BPSK, QPSK, 8PSK, 16QAM, and 64QAM. The method uses spectral correlation density and high-order cumulants as features. For feature classification, K-nearest neighbor algorithm is used. Simulation results are demonstrated to evaluate the proposed scheme.

Compensation of multiple carrier frequency offsets in amplify-and-forward cooperative networks

In this paper, we propose a method to find an optimal correction value for multiple carrier frequency offsets (CFO) compensation, where two received signals have different path gains, in orthogonal frequency division multiplexing (OFDM) systems. Multiple CFOs are occurred when two spatially-separated transmitters are used to transmit the same signal simultaneously. In this case, the sidelobes of the subcarrier spectrums of the undesired signals have similar magnitude with opposite signs. Based on this fact, we propose a self-cancellation method of intercarrier interference caused by multiple CFOs. In the proposed scheme, we compensate the multiple CFOs for the signal to intercarrier interference power ratio of the received signal to be maximized, instead only the signal power is maximized in the ordinary methods.

Self-cancellation of interchannel interference for OFDM systems in amplify-and-forward relay communication networks

We propose a method to compensate the effects of multiple carrier frequency offsets (CFOs) for orthogonal frequency division multiplexing (OFDM) systems. The multiple CFOs, we considered, are caused by several spatially separated frequency-unsynchronized transmitters as in amplify-and-forward relay communication networks or wireless ad hoc sensor networks. Despite its appealing features, OFDM is extremely sensitive to CFO, especially to multiple CFOs. At the receiver, the residual CFO which appears due to incomplete compensation of multiple CFOs destroys orthogonality among subcarriers and brings interchannel interference (ICI). In the proposed scheme, we compensate the CFOs of the received signal by the predesigned value, which results in cancellation of the opposite ICI by each other. This is based on the property that the spectrum of ICIs shown in a subcarrier has similar magnitudes and opposite signs from the fact that spectrum shapes of the ICI are of sinc function. Simulation results are shown to demonstrate the effectiveness of the proposed method.

Comparison of MUSIC and ESPRIT for direction of arrival estimation of jamming signal

For antijamming design of global positioning system (GPS) in military applications, the direction of arrival (DOA) estimation of jamming signal is positively necessary. The DOA estimation is also used in mobile communication systems to expand the coverage area or to increase the channel capacity, where the base station forms a beam to the direction of the user terminal. In this paper, we compare the performance of MUSIC and ESPRIT algorithms with two-dimensional antenna arrays. MUSIC and ESPRIT algorithms are two typical DOA estimation methods in nonparametric and parametric methods, respectively. With ESPRIT, we derive the estimates of two-dimensional azimuth and elevation from two one-dimensional DOAs. Computer simulations are shown to demonstrate the performance.

Frequency offsets compensation for multiuser interference cancellation in OFDMA uplink systems

In this paper, we propose a multiuser interference cancellation scheme for orthogonal frequency division multiple access (OFDMA) uplink systems. In the scheme, two steps of compensation and cancellation in time and frequency domains, respectively, are applied. In the frequency domain, we only consider the interference from both adjacent sides of the band to reduce the computation complexity and parallel interference cancellation is employed. Simulation results demonstrate that the proposed method reveals improved performance.

Noise Whitening Decision Feedback Equalizer for SC-FDMA Receivers

In this paper, we propose a noise whitening decision feedback equalizer for single carrier frequency division multiple access (SC-FDMA) receivers. SC-FDMA has the same advantage as that of orthogonal frequency division multiple access (OFDMA) in which the multipath effect can be removed easily, and also solves the problem of high peak to average power ratio (PAPR) which is the main drawback of OFDMA. Although SC-FDMA is a single carrier transmission scheme, a simple frequency domain linear equalizer (FD-LE) can be implemented as in OFDMA, which can dramatically reduce the equalizer complexity. Moreover, some residual intersymbol interference in the output of the FD-LE can be further removed by an additional nonlinear decision feedback equalizer (DFE) in time domain, because the time domain signal is a digitally modulated symbol. In the conventional DFE, however, the noise is not white at the input of the decision device and correspondingly the decision is not optimum. In this paper, we propose an improved DFE scheme for SC-FDMA systems where a linear noise whitening filter is inserted before the decision device of the conventional DFE scheme. Through computer simulations, we compare the bit error rate performance of the proposed DFE scheme with the conventional equalizers.

Fast reference frame selection algorithm for H.264/AVC based on reference frame map

The variable block size motion estimation (ME) and compensation (MC) using multi-reference frames is adopted in H.264/AVC to improve coding efficiency. However, the computational complexity of the ME/MC increases proportional to the number of reference frames. In this paper, we propose a new efficient reference frame selection algorithm to reduce the complexity. The proposed algorithm selects suitable reference frames by employing the spatial and temporal correlation of video sequence. The experimental results show that the proposed algorithm decreases video encoding time while keeping the similar visual quality and bitrates.

ICI cancellation for relay communication systems with multiple frequency offsets

In this paper, an interchannel interference (ICI) cancellation method is proposed for multiple frequency offsets in orthogonal frequency division multiplexing (OFDM) systems. When signals are received from different transmitters simultaneously like in a relay communication, multiple frequency offsets may appear at the receiver. This causes interchannel interference (ICI) and severe performance degradation because OFDM systems are very sensitive to the frequency offsets. In this paper, we propose a method to eliminate the effect of the multiple frequency offsets for OFDM systems. The method is accomplished in two steps: compensation of the frequency offset in time domain and subsequent cancellation of the ICI in frequency domain. Through computer simulations, we verify the effectiveness of the proposed ICI cancellation method.