This letter proposes a novel nonlinear distortion for the unique identification of receiving room impulses in stereo acoustic echo cancellation when applying the frequency-domain adaptive filtering technique. This nonlinear distortion is effective in reducing the coherence between the two incoming audio channels and its influence on audio quality is inaudible.

In a CDMA non-linear interference canceller, a generated replica of an interference signal is multiplied by a positive number smaller than unity, which is called cancellation moderating factor (CMF), to prevent interference enhancement due to inaccurate replica subtraction. In this paper, two CMF controlling schemes applicable to a multistage parallel interference canceller with multi-antenna (spatial diversity) reception are proposed. They control CMF by using the mean square error of the complex channel gain or by using the ratio of the estimated power of each interference signal to remaining interference signals' power, in order to mitigate the replica subtraction error due to inaccurate channel estimation. The performance of the proposed schemes are evaluated by computer simulations assuming an asynchronous uplink single chip-rate variable spreading factor DS-CDMA system. The simulation results show that the proposed schemes with higher order diversity reception improve the bit error rate (BER) performance compared with a conventional scheme considering the tentative decision error or fixed CMF settings. Their performance improvement is by 0.1-0.9 dB in terms of the required Eb/N0 at an average BER of 10-5 over exponentially decaying 5-path Rayleigh distributed channels when the number of receiving antennas is 6.

A feedforward-based active shielding technique for digital noise suppression is more preferred for its capability of reducing the noise on the entire area inside the guard ring. In order to compensate for the variation of substrate parameters, an automatic control scheme to tune the gain of the active shield circuit is proposed. Simulation results show the effectiveness of the proposed system in reducing the digital noise regardless of circuit layout. Simulation results also show that noise suppression improvement from passive guard ring to active shield with tuning is 20 dB or one tenth while that from active shield without tuning to active shield with tuning is 12 dB.

High-Speed High-Resolution Comparators are integral parts of very high-speed high-resolution Analog-to-Digital Converters (ADC). Parallel successive-approximation and flash ADCS can boost conversion rates while providing high resolution, provided that accurate and fast offset-cancelled comparators could be implemented. Moreover, accurate offset cancellation is needed in accurate gain stages of other types of high speed ADCs as well. This has never been easy and creates a bottle neck for high-speed high-resolution ADCs. The reason is that conventional offset cancellation methods, suffer either from inaccurate cancellation or from slow operation. Hence, either speed or accuracy is compromised. This is due to the trade off of gain (accuracy) for bandwidth (speed) in conventional methods. Here, we introduce a new offset cancellation method which satisfies the need for both high-speed and accurate offset cancellation simultaneously.

Packet-recognition/code-acquisition (PR/CA) is one of the most important issues in packet communication systems. In a CDMA Unslotted ALOHA system, Multiple Access Interference (MAI) may bring about errors in PR/CA. The MAI mainly stems from already recognized packets and newly arriving packets under the execution of PR/CA. This characteristic of asynchronous transmission in CDMA U-ALOHA systems implies that only one or a few packets arrive at the receiver within a short interval of a execution. Furthermore, newly arriving packets are recognized and code-acquired by using a short preamble part. Consequently, the MAI from the packets under the execution of the PR/CA will be small. Focusing on that point, this paper proposes applying the IC scheme in order to suppress the MAI from the already recognized and code-acquired packets. A performance evaluation demonstrates that such an application is valid due to the small amount of MAI from the packets under the execution of PR/CA. In addition, we demonstrates that the scheme reduces false recognition rather than mis-recognition. Such a scheme improves the performance of not only PR/CA, but also the throughput.

This paper aims to improve the performance of the soft canceller followed by simplified minimum mean-square error (SC/S-MMSE) turbo receiver for multiple-input and multiple-output space-division multiplexing/orthogonal frequency division multiplexing (MIMO-SDM/OFDM) transmission; it performs iterative parallel soft interference cancellation and MMSE filtering, and stream-wise soft-input and soft-output decoding. For this aim, we newly introduce two detection techniques: 1) serial interference cancellation, and 2) cyclic redundancy check (CRC)-assisted interference cancellation and MMSE filter tap computation. Various computer simulations are conducted to evaluate the performance enhancement obtained via the use of the two detection techniques. The computer simulation results show that this paper's proposed serial SC/S-MMSE turbo receiver with CRC achieves frame error rate (FER) performance gain over existing MIMO receivers (MMSE receiver, V-BLAST receiver, parallel SC/MMSE-matched filter (MF) turbo receiver, and parallel SC/S-MMSE turbo receiver) for QPSK, 16QAM and 64QAM modulation while keeping the comparable complexity order.

An active shield circuit which effectively reduces the substrate noise on the entire area inside the guard ring regardless of the noise source position is proposed. Simulation result shows that the proposed circuit can reduce the noise level to -85 dB while a conventional guard ring gives -52 dB.

A design optimization of active shield circuit using noise averaging method is proposed. The relation between the averaged noise and the design parameters of the active shield circuit such as circuit gain and on-chip layout is examined. A simple design guideline is also provided. Simulation results show that the active shield circuit designed by the proposed optimization method gives a better noise suppression performance of about 28% than the conventional one.

Polynomial cancellation coding (PCC) was proposed to mitigate the sever inter-carrier-interference (ICI) in an orthogonal frequency division multiplexing (OFDM) system caused by frequency offset. In this paper, we consider the effectiveness of PCC under time-variant multi-path Rayleigh fading analytically and by simulations. We first consider an analytical expression of the signal-to-interference plus noise power ratio (SINR) and then derive an approximation of the bit-error-rate (BER) of the OFDM-PCC system under the assumption that ICI is well approximated by a white Gaussian noise. Since the bandwidth efficiency of OFDM-PCC is half of that of normal OFDM, we compare the BER performance of the scheme with the normal OFDM system of the same bit-rate when low, medium, and high level modulations are used. Our results show that OFDM-PCC performs well even for high modulation level under time-varying multi-path fading.

In this letter, parallel interference cancellation (PIC) in code division multiple access (CDMA) was performed with two different structures by using a neural network (NN). In the first structure (receiver-1) the NN was used as a front-end stage of a one stage PIC circuit. In the second structure (receiver-2), the NN was used instead of the one stage PIC circuit and it was trained as a multiple access interference (MAI) detector to perform the PIC process by subtracting the MAI from the outputs of the matched filter. The PIC is a classical technique in multi user detection process and its bit error rate (BER) performance is not good in one stage for most of the applications. For improving its BER performance, generally a multi stage PIC which has the high computational complexity is used. In this study, we have gotten a better BER performance than a three stages PIC receiver with both proposed receivers that have the lower computational complexity.

We propose a new diversity scheme for orthogonal frequency division multiplexing/multi-input multi-output (OFDM/MIMO) systems. The proposed scheme, named turbo layered space-frequency coded OFDM (TLSFC-OFDM), exploits the turbo principle with space hopping (SH). The TLSFC-OFDM system with SH provides a spatial coding so that we can obtain the transmit diversity. We also introduce a successive interference cancellation (SIC) algorithm that requires no ordering and fewer iterations to converge. As a result, this scheme reduces computational complexity. Computer simulation results show that the unordered SIC-based TLSFC-OFDM system outperforms the OFDM/H-BLAST system. It is also shown that the proposed system can operate even with fewer receive antennas than transmit antennas.

In this paper, we propose an adaptive multistage fuzzy-based partial parallel interference cancellation (FB-PPIC) multiuser detector for multi-carrier direct-sequence code-division multiple-access (MC-CDMA) communication systems over frequency selective fading channels. The partial cancellation tries to reduce the cancellation error in parallel interference cancellation (PIC) schemes due to the wrong interference estimations in the early stages and thus outperforms the conventional PIC (CPIC) under the heavy load for MC-CDMA systems. Therefore, in this paper, the adaptive cancellation weights are inferred from a proposed multistage fuzzy inference system (FIS) to perform effective PPIC multiuser detection under time-varying frequency selective fading channels in MC-CDMA systems. Simulation results show that the proposed adaptive four-stage FB-PPIC scheme outperforms both CPIC and constant weight PPIC (CW-PPIC) schemes, especially in near-far environments.

This paper proposes a scattered-pilot-OFDM reception scheme employing turbo inter-carrier interference (ICI) cancellation in the fast varying fading environments of mobile communications. In the OFDM transmission, the orthogonality among the subcarriers cannot hold due to large Doppler shift, and the OFDM signal suffers from severe degradation due to ICI. The proposed receiver carries out two modes: (i) a coherent detection (CD) mode, and (ii) a turbo ICI cancellation (TC) mode. Initially, the receiver performs the CD mode. When any decision errors are detected, it shifts from the CD mode to the TC one that carries out both the ICI cancellation and the channel estimation by using the decoder output (the log likelihood ratio). In addition, the iteration of the TC mode can improve the accuracy of the channel estimation and ICI cancellation ability. Computer simulations following specifications for the mobile reception mode in the digital terrestrial television broadcasting demonstrate that the receiver can effectively cancel ICI due to the fast varying fading, and that its average BER performance is much better than that of CD.

An adaptive interpolated FIR (IFIR) echo canceller was recently proposed for xDSL applications. This canceller consists of an FIR filter, an IFIR filter, and a tap-weight overlapping and nulling scheme. The FIR filter is used to cancel the short and rapidly changing head echo while the IFIR filter is used to cancel the long and slowly decaying tail echo. This echo canceller, which inherits the stable characteristics of the conventional FIR filter, requires low computational complexity. It is well known that the interpolation filter for an IFIR filter has great influence on the interpolated result. In this paper, a least-squares method is proposed to obtain optimal interpolation filters such that the performance of the IFIR echo canceller can be further improved. Simulations with a wide variety of loop topologies show that the optimal IFIR echo canceller can effectively cancel the echo up to 73.0 dB (for an SHDSL system). About 57% complexity reduction can be achieved compared to a conventional FIR filter.

In this paper, a sub-optimal Rake receiver combined with a Wiener Filter is investigated for use in an indoor environment. Inner-Chip-interference is dominant when the application is indoors, so the inner-chip-interference rejection function becomes critical for the receiver. Pilot symbols in each slot are used for channel estimation and weight calculation of Rake combining through Wiener Filter. Compared to conventional combining which uses maximum ratio combining, Wiener combining using IRC (Interference rejection combining) achieves better ICI (Inner-chip-Interference) rejection. This paper clarified that the sub optimal Rake receiver using Wiener Filter is 4 dB better than the conventional Rake receiver under the indoor application.

Some conventional beamformers require the direction of the desired signal. The performance of such beamformers can substantially be degraded even in the presence of small error on the directional information. In this letter, we propose a prefilter-type beamforming scheme robust to directional error by employing a simple compensator. The performance of the proposed scheme is verified by computer simulation.

This letter studies the impact of chip waveform shaping on the multiple access interference (MAI) in band-limited direct sequence code-division multiple access (DS-CDMA) systems. The family of band-limited waveforms with zero interchip interference (ICI) and with an excess bandwidth β in the range 0β 1 is considered. A criterion for the performance comparison of various band-limited chip waveforms based on the elementary density function is established. The effect of varying the roll-off factor of a band-limited chip waveform on the MAI level is also investigated.

We have developed a code-division-multiplexing (CDM) transmission scheme for future cellular communication systems, which uses cyclic shifted-and-extended (CSE) codes generated from an M-sequence to enable seamless communication in highly mobile environments. Because the correlation characteristics of CSE codes are determined by the M-sequence, the cross-correlation values are accumulated as a result of combining transmitted signals with opposite polarities in parallel channels. The accumulated cross-correlation values significantly degrade transmission performance, especially with multi-level modulation schemes such as quadrature amplitude modulation (QAM). We thus propose a cancellation technique to eliminate the accumulated cross-correlation values. We have evaluated the transmission performance of the CDM transmission scheme with the proposed technique by computer simulation. The new scheme enables high-quality data transmission in fast-fading channels.

This letter considers multiple access systems without bandwidth expansion. To improve the spectral efficiency, each user employs a QPSK modulation. The orientation of QPSK constellations is designed to maximize the minimum distance of the superimposed symbol constellation. The upper and lower bounds for the error performance of the proposed design demonstrate its advantage.

The non-individualized head related transfer function (HRTF) is known to have a few problems, which are referred to as the 'hole in the middle' phenomenon and 'front-back reversals.' To overcome these problems, an HRTF refinement technique was introduced, but unfortunately, this refinement technique causes sudden degradation in sound quality and difficulty in cross-talk cancellation because of notch frequency exaggeration. In this paper, an HRTF refinement using directional weighting function is proposed. This newly proposed technique weights ordinary HRTF according to its direction to amplify frontal sound intensity. Since the proposed technique does not exaggerate the notch frequency, spectral differences in the 'cone-of-confusion' region become more pronounced within overall audible frequencies, resulting in mitigating the sound degradation. In addition, the cross-talk cancellation can be done more easily. We verified the superiority of the proposed technique over the existing one by means of the sound localization and sound quality tests in headphone and loudspeakers.