Yasuhiko Tanabe

A Novel IQ Imbalance Compensation Method with Pilot-Signals for OFDM System

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.

Suitable MIMO-OFDM Decoders to Compensate IQ Imbalance

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.

Experimental performance evaluation of IQ imbalance and DC offset estimation and compensation technique for 3GPP LTE base station

In the case of recent wireless communication systems, the need to achieve high performance means that impairments in analog circuits cannot be disregarded. In view of the need for highly accurate compensation, many error compensation techniques using digital signal processing have been studied recently. Digital predistorter (DPD), which can eliminate odd-order distortion caused by PA nonlinearity, is one of the compensation techniques. However, in the case that direct conversion architecture is used in both a forward path and a loop-back path, errors in a quadrature modulator (QMOD) and a quadrature demodulator (QDEMOD) affect the performance of DPD. Previously, we proposed an error estimation technique for a QMOD and a QDEMOD that uses a variable phase shifter and evaluated its performance by an experiment using a narrow-band signal. In this paper, we evaluate the performance of the error estimation technique using the signal based on the 3GPP Long Term Evolution (LTE), whose bandwidth is wider than that in the previous experiment. The results show that image rejection ratio (IRR) after compensation is −59dBc and error vector magnitude (EVM) is −52dBc at the output of the QMOD.

Channel Estimation Scheme with Low Complexity Discrete Cosine Transformation in MIMO-OFDM System

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.

Spatial domain interference canceler using multistage adaptive array with precise timing estimation

In SDMA, a spatial domain interference canceler applying a multistage processing concept to the MMSE multibeam adaptive array has an attractive feature that weak power signals can be detected in the successive stages after removing other strong power signals which are already detected. This idea can be enhanced to the reference timing estimation required in the MMSE algorithm. In this paper the spatial domain interference canceler introducing multistage timing estimation is proposed and its performance is evaluated by computer simulations. The results show that the timing estimation performance is highly improved.

A Novel IQ Imbalance Estimation Scheme using Transient Local Frequency for OFDM Systems

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.

Reduced Complexity Max-Log-MAP Sphere Decoder Using Group Detection in MIMO-OFDM Systems

This paper proposes a Group Detection (GD) algorithm with Max-Log-MAP Sphere Decoder (MLM-SD) in order to reduce the complexity of signal detection in a Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system. The proposed algorithm divides spatial streams into multiple partial spatial streams by using Minimum Mean Square Error (MMSE) detector, followed by multiple MLM-SDs with reduced number of spatial streams. Although the spatial diversity gain in the MLM-SD degrades because of the lack of the degrees of freedom exploited by the MMSE detector, its diversity gain is recovered by combining the metrics obtained by the multiple MLM-SDs. In a MIMO wireless LAN multipath fading environment, the complexity of the proposed algorithm is 10% of that of the original MLM-SD and the performance degradation in terms of SNR is slightly less than that of the original MLM-SD in 4-by-4 MIMO architecture with 64QAM achieving 216 Mbps. It is also found that the proposed algorithm is robust against the limitation of the number of searches in sphere decoder.

Pilot Signals for Multiuser Tomlinson-Harashima Precoding in MIMO-OFDM Systems

This paper describes pilot signals in multiuser multiple-input multiple-output orthogonal frequency division multiplexing systems with nonlinear Tomlinson-Harashima precoding (THP). The transmission processing of THP consists of three parts : feedback processing, modulo operation, and feedforward processing. In particular, the increased transmit power in the feedback processing is reduced by the modulo operation. In the conventional linear precoding systems, the same precoding weight is applied for both pilot and data signals at the transmitter for channel estimation at the receiver. If it is directly applied to THP, however, this will lead to significant increase in transmit power since the modulo operation cannot be applied for pilot signals. In our proposal, only the feedforward processing is applied for pilot signals and the interference due to the lack of feedback processing is removed by using orthogonal pilot signals. Simulation results show that the proposed pilot signals significantly improves the packet error rate performance.

Novel spatiotemporal equalization based on cascaded connection of multibeam adaptive array and multi-input MLSE

This paper proposes a novel spatiotemporal equalizer using a cascaded connection of multibeam adaptive array and multi-input maximum likelihood sequence estimation (MLSE) based on single-user detection. The computational load for the proposed method is much lower than that of the optimal method, and computer simulations revealed that its bit-error-rate (BER) performance is identical to that of the optimal method. Theoretical analysis also supports equivalency between these methods.

Basestation adaptive antennas for a high-speed FDD/TDMA system

Multipath propagation causes serious performance degradation in a high-speed mobile TDMA system. We have proposed the adaptive antenna which copes with the multipath signals. These considerations have been limited to an uplink channel. We propose the basestation adaptive antennas both for the uplink and downlink channels. We assume an FDD/TDMA system where the frequencies are different between the uplink and downlink channels. The weight vectors proposed in this paper are determined using DOA estimation. The problem of the frequency difference in FDD is removed by this scheme. For the uplink channel, we can realize the path-diversity using the adaptive antenna with several weight vectors. For the downlink channel, we propose a single-beam adaptive antenna and two-beam one. It is shown that the combination of the two-beam basestation adaptive antenna and the mobile terminal with two-input (two antenna) MLSE equalizer shows better performance.