Teruyuki Miyajima

Second-order statistical approaches to channel shortening in multicarrier systems

In multicarrier systems with a cyclic prefix, interblock interference can be limited by shortening the channel impulse response using time-domain equalizers. In this paper, we present two blind channel-shortening algorithms that exploit the second-order statistics of the channel outputs. The equalizer parameter vector is chosen from a nullspace that forces the length constraint on the effective channel impulse response. The first algorithm is less sensitive to channel-order estimation errors, whereas the second one is simpler to implement. Furthermore, we show that the two algorithms are equivalent for noiseless channels. Simulation examples are provided to demonstrate their superior performance over some existing algorithms.

Subcarrier Nulling Algorithms for Channel Shortening in Uplink OFDMA Systems

This paper considers a blind channel shortening approach based on subcarrier nulling criteria to mitigate the effect of interblock interference in uplink OFDMA systems. We derive a sufficient condition for a method known as carrier nulling algorithm (CNA) to shorten individual uplink user channels. Unlike several known methods, CNA can shorten channels in time domain even if there are many null subcarriers, and is hence suitable for uplink OFDMA in which individual user signals can be modeled as standard OFDM that includes multiple null subcarriers. In addition, we also develop a semi blind implementation of CNA by exploiting potential pilot symbols. The semi blind approach uses the simple idea of treating known pilot subcarriers in the same way as null subcarriers.

Perfect Blind-Channel Shortening for Multicarrier Systems

In multicarrier systems, when the order of a channel impulse response is larger than the length of the cyclic prefix (CP), there is a significant performance degradation due to interblock interference (IBI). This paper proposes a blind-channel shortening method in which the equalizer parameter vector is formed by the noise subspace of the received signal correlation matrix so that the output power is maximized. The proposed method can not only shorten the effective channel impulse response to within the CP length but also maximize the output signal-to-interference-and-noise ratio while eliminating the IBI. We point out that the performance depends on the choice of a decision delay and propose a simple method for determining the appropriate delay. We propose both a batch algorithm and an adaptive algorithm and show by simulation that they are superior to the conventional algorithms.

Multicarrier channel shortening based on second-order output statistics

The paper presents two blind channel shortening algorithms exploiting the second-order statistics of the channel outputs. The shortening equalizer parameter vector is chosen from a null-space that forces a length constraint on the effective channel impulse response. The first algorithm is less sensitive to channel order estimation errors, whereas the second one is simpler to implement. The performance of the proposed algorithms is demonstrated through simulations.

Blind Channel Shortening for MC-CDMA Systems by Restoring the Orthogonality of Spreading Codes

This study considers blind channel shortening to mitigate the effects of inter-block interference and multiple-access interference in both uplink and downlink multicarrier code-division multiple-access systems. We derive the blind channel shortening methods by extending the blind channel equalization method previously proposed by Djebbar, Abed-Meraim, and Djebbari. The basic idea is to restore, at the frequency-domain equalizer output, the orthogonality of spreading codes, which is destroyed when interference is present. We show that the extended methods can perfectly shorten the channels and that the resulting effective channel is identical to the predetermined target channel thus avoiding channel estimation. Computer simulation results illustrate the effectiveness of the proposed methods.

Feasibility of RSSI based access network detection for multi-band WLAN using 2.4/5GHz and 60GHz

This paper describes feasibility of RSSI (Received Signal Strength Indicator) based access network detection for multi-band WLAN (Wireless LAN) using 2.4/5GHz and 60GHz aiming at power-saving of multi-band WLAN device and higher spectrum efficiency. The proposed scheme detects 60GHz coverage by using 2.4/5GHz WLAN signals without using 60GHz WLAN signals for on/off control of 60GHz part of multi-band WLAN device. This makes it possible to turn off 60GHz part whenever a multi-band WLAN device is out of 60GHz coverage. Ray-tracing simulation results indicate that the proposed RSSI based access network detection is feasible.

A continuous-time asynchronous Boltzmann machine

We propose an asynchronous neural network model having the same structure as the binary Hopfield model. Each neuron operates with continuous time and randomly changes its state according only to its membrane potential. The proposed model settles in a steady-state fluctuation, in which the probability distribution of the global state is identical to that of the serial Boltzmann machine with the same synaptic weights. Copyright 1997 Elsevier Science Ltd.

Blind Channel Shortening for Block Transmission of Correlated Signals

In block transmission systems, blind channel shortening methods are known to be effective to reduce the influence of interblock interference which degrades the performance when the length of a channel impulse response is extremely long. Conventional methods assume that the transmitted signal is uncorrelated; however, this assumption is invalid in practical systems such as OFDM with null carriers and MC-CDMA. In this paper, we consider blind channel shortening methods for block transmissions when the transmitted samples within a block are correlated. First, the channel shortening ability of a conventional method is clarified. Next, a new method which exploits the fact that the transmitted samples in different blocks are uncorrelated is introduced. It is shown that the proposed method can shorten the channel properly under certain conditions. Finally, simulation results of OFDM and MC-CDMA systems are shown to verify the effectiveness of the proposed method compared with a conventional one.

Performance Evaluation of SSB Transmission of DFTs-OFDM Using Multi-Level BPSK through Nonlinear HPA

This paper describes performance evaluation results of SSB (Single Side Band) transmission of DFTs-OFDM (Discrete Fourier Transform Spreading Orthogonal Frequency Division Multiplexing) using multi-level BPSK (Binary Phase Shift Keying) through nonlinear HPA (High Power Amplifier). SSB-DFTs-OFDM is generated by eliminating USB (Upper Side Band) or LSB (Lower Side Band) spectrum in frequency domain after DFT spreading of multi-level BPSK signals, thus Hilbert transformer is not necessary to generate SSB signals. The simulation results show that SSB-DFTs-OFDM achieves almost the same PAPR (Peak to Average Power Ratio) and ACLP (adjacent channel power leakage) as conventional DFTs-OFDM and lower PAPR and ACLP than OFDM and required bandwidth can be reduced by half when BPSK modulation is employed. Furthermore, SSB-DFTs-OFDM using multi-level BPSK shows less error floor and/or Eb/N0 degradation compared with conventional DFTs-OFDM using QAM modulation in some cases when HPA OBO (Output Back Off) is small. SSB-DFTs-OFDM is an attractive alternative to DFTs-OFDM for WLAN and cellular applications.

A twin cylinder model for moving human body shadowing in 60GHz WLAN

As link quality in 60GHz WLAN is significantly affected by moving human body shadowing, it is required to introduce shadowing countermeasures such as diversity and fast session transfer. For performance evaluation of these techniques in various environments, we need a deterministic shadowing model for ray-tracing simulations rather than a traditional empirical shadowing model based on experimental data. With this motivation, this paper proposes a twin cylinder model for moving human body shadowing in 60GHz WLAN, where three knife edges for diffraction loss calculation are derived from geometrical positions of a moving human body, a transmitter and a receiver. It is confirmed that calculated results using the proposed model agrees to measured results.