Basic features of fretting and factors affecting its deleterious effects on the performance of electrical/electronic connection were reviewed. It was shown that although the fretting cannot be eliminated completely, its deleterious effects can be substantially reduced by lubrication and also connection design.

In this letter, we present the impact of carrier frequency offset (CFO) in dual-hop orthogonal frequency division multiplexing (OFDM) systems with a fixed relay for frequency-selective fading channels. Approximate expressions of the average signal-to-noise ratios (SNRs) for both downlink and uplink are obtained and validated by simulations. It is shown that dual-hop systems have slightly worse average SNR degradation than single-hop systems. We also show that the average SNR degradation due to the CFO varies according to the gap between average received SNRs for the first and the second hop.

Ultrawide-band impulse radio (UWB-IR) technology and multiple-input multiple-output (MIMO) systems have attracted interest regarding their use in next-generation high-speed radio communication. We have studied the use of MIMO ultrawide-band (MIMO-UWB) systems to enable higher-speed radio communication. We used frequency-domain equalization based on the minimum mean square error criterion (MMSE-FDE) to reduce intersymbol interference (ISI) and co-channel interference (CCI) in MIMO-UWB systems. Because UWB systems are expected to be used for short-range wireless communication, MIMO-UWB systems will usually operate in line-of-sight (LOS) environments and direct waves will be received at the receiver side. Direct waves have high power and cause high correlations between antennas in such environments. Thus, it is thought that direct waves will adversely affect the performance of spatial filtering and equalization techniques used to enhance signal detection. To examine the feasibility of MIMO-UWB systems, we conducted MIMO-UWB system propagation measurements in LOS environments. From the measurements, we found that the arrival time of direct waves from different transmitting antennas depends on the MIMO configuration. Because we can obtain high power from the direct waves, direct wave reception is critical for maximizing transmission performance. In this paper, we present our measurement results, and propose a way to improve performance using a method of transmit (Tx) and receive (Rx) timing control. We evaluate the bit error rate (BER) performance for this form of timing control using measured channel data.

In this letter, we propose a novel frequency-domain equalizer (FDE) for single-carrier systems characterized by severe inter-symbol interference (ISI) channels; it consists of a linear FDE and an iterative block noise-predictor (IBNP). Unlike the FDE with time-domain noise predictor (FDE-NP), the proposed scheme allows the feedback equalizer being an uncausal filter, and performs the noise prediction in an iterative manner. For this reason, FDE-IBNP can remove both precursor and postcursor ISI, and alleviate the impact of error-propagation. Besides, our scheme has lower computational complexity than the present iterative block equalizers.

In OFDM systems, the estimation/correction of carrier frequency offset (CFO) is crucial to maintain orthogonality among subcarriers. However, the CFO estimation suffers from DC offset (DCO) generated in low-cost direct-conversion receivers (DCRs). More seriously, in practice, DCO is time-varying due to the automatic gain control. In this paper, a novel CFO estimator in the presence of time-varying DCO is proposed. It is shown the residual DCO after high-pass filtering varies in a linear fashion. Based on this observation and the periodicity of the training sequence, we derive a CFO estimator independent of DCO. Also, the residual DCO can be estimated, using the obtained CFO. The validity of the proposed estimation method is demonstrated by simulations.

Let G(s)=C(sI - A)-1B+D be a given system where entries of A,B,C,D are polynomials in a parameter k. Then H∞ norm || G(s) ||∞ of G(s) is a function of k, and [9] presents an algorithm to express 1/(||G(s) ||∞)2 as a root of a bivariate polynomial, assuming feedthrough term D to be zero. This paper extends the algorithm in two ways: The first extension is the form of the function to be expressed. The extended algorithm can treat, not only H∞ norm, but also functions that appear in the celebrated KYP Lemma. The other extension is the range of the frequency. While H∞ norm considers the supremum of the maximum singular value of G(i ω) for the infinite range 0 ≤ω ≤ ∞ of ω, the extended algorithm treats the norm for the finite frequency range ω ≤ ω ≤ ω- (ω, ω- ∈ R ∪ ∞). Those two extensions allow the algorithm to be applied to wider area of control problems. We give illustrative numerical examples where we apply the extended algorithm to the computation of the frequency-restricted norm, i.e., the supremum of the maximum singular value of G(i ω) (ω- ≤ ω ≤ ω-).

This letter presents recursive frequency offset estimation for MIMO (Multiple Input Multiple Output) system in flat-fading channels. With the recursive estimation, the frequency offset range in MIMO system can be extended compared with normal estimation. Simulation results show that the recursive frequency offset estimation is valid for large frequency offset.

One major drawback of orthogonal frequency-division multiplexing is the high peak-to-average power ratio (PAPR) of the output signal. The selected mapping (SLM) and partial transmit sequences (PTS) methods are two promising techniques for PAPR reduction. However, to generate a set of candidate signals, these techniques need a bank of inverse fast Fourier transforms (IFFT's) and thus require high computational complexity. In this paper, we propose two low-complexity multiplication-free conversion processes to replace the IFFT's in the SLM method, where each conversion process for an N-point IFFT involves only 3N complex additions. Using these proposed conversions, we develop several new SLM schemes and a combined SLM & PTS method, in which at least half of the IFFT blocks are reduced. Computer simulation results show that, compared to the conventional methods, these new schemes have approximately the same PAPR reduction performance under the same number of candidate signals for transmission selection.

We present a novel high-speed transmitter consisting of a frequency modulated DBR laser and optical filters. The refractive index modulation in the phase control region of the DBR laser allows high-speed frequency modulation. The generated frequency modulated signal is converted to an intensity modulated signal using the edge of the optical filter pass band. We present theoretical simulations of high-speed modulation characteristics and extension of transmission reach. With the proposed transmitter, we review the experimental demonstration of 180-km transmission of a 10-Gb/s signal with a tuning range of 27 nm and 60-km transmission of a 20-Gb/s signal.

In this letter, a relay-assisted transmission scenario over frequency-selective fading channels perturbed by different random carrier frequency offsets is considered. OFDM and block-double differential (BDD) design are implemented to overcome the problem of intersymbol interference (ISI) and carrier frequency offsets (CFOs). We analyze the symbol error rate (SER) performance of decode-and-forward relaying with BDD design in wireless cooperative communications over frequency-selective fading channels and derive a theoretical upper bound for average SER when the relay (R) is error free. It can be seen from our analysis that the system performance is influenced by the ability of R to decode, and when R decodes without error, both spatial and multipath diversity can be obtained without requiring any knowledge of channel state information and CFO information at the receivers. Numerical examples are provided to corroborate our theoretical analysis.

In direct-conversion orthogonal frequency division multiplexing (OFDM) receivers, the impact of frequency-dependent I/Q mismatch (IQ-M) with carrier frequency offset (CFO) must be considered. A preamble-assisted estimation is developed to circumvent the frequency-dependent IQ-M with CFO. The results of a simulation and an experiment show that the proposed method could provide good estimation efficiency and enhance the system performance. Moreover, the proposed scheme is compatible with current wireless local area network standards.

Kalman filters are effective channel estimators but they have the drawback of having heavy calculations when filtering needs to be done in each sample for a large number of subcarriers. In our paper we obtain the steady-state Kalman gain to estimate the channel state by utilizing the characteristics of pilot subcarriers in OFDM, and thus a larger portion of the calculation burden can be eliminated. Steady-state value is calculated by transforming the vector Kalman filtering in to scalar domain by exploiting the filter charactertics when pilot subcarriers are used for channel estimation. Kalman filters operate optimally in the steady-state condition. Therefore by avoiding the convergence period of the Kalman gain, the proposed scheme is able to perform better than the conventional method. Also, driving noise variance of the channel is difficult to obtain practical situations and accurate knowledge is important for the proper operation of the Kalman filter. Therefore, we extend our scheme to operate in the absence of the knowledge of driving noise variance by utilizing received Signal-to-Noise Ratio (SNR). Simulation results show considerable estimator performance gain can be obtained compared to the conventional Kalman filter.

Threshold-based ordered successive interference cancellation (OSIC) detection algorithm is proposed for per-antenna-coded (PAC) two-input multiple-output (TIMO) orthogonal frequency division multiplexing (OFDM) systems. Successive interference cancellation (SIC) is performed selectively according to channel conditions. Compared with the conventional OSIC algorithm, the proposed algorithm reduces the complexity significantly with only a slight performance degradation.

Some wireless OFDMA communication systems support the frequency reuse factor of 1. In order to reduce co-channel interference (CCI) caused by neighbor cells, the fractional frequency reuse (FFR) can be employed. A promising frequency partitioning policy and subcarrier allocation for FFR are essential. In this letter, we employ an efficient frequency partitioning mechanism with less interference and propose an efficient subcarrier allocation algorithm to maximize the sum of users capacity under FFR. We show that the proposed algorithm has higher spectral efficiency than the conventional method as well as significantly high system fairness.

This letter presents a noise-robust sampling frequency offset (SFO) estimation scheme for OFDM-based WLAN systems. Mean square error of the proposed estimation scheme is derived and simulation results are provided to verify our analysis. The proposed SFO estimator has an improved performance over the existing schemes with a reduction of the estimation range.

This paper proposes a frequency domain nulling filter and Turbo equalizer to suppress interference in the uplink of one-cell reuse single-carrier time division multiple access (TDMA) systems. In the proposed system, the desired signal in a reference cell is interfered by interference signals including adjacent-channel interference (ACI), co-channel interference (CCI), and intersymbol interference (ISI). At the transmitter, after a certain amount of spectrum is nulled considering the expected CCI, the suppressed power due to nulling is reallocated to the remaining spectrum components so as to keep the total transmit power constant. In this process, when mitigation of ACI is necessary, after a certain amount of spectrum at both edges is nulled using an edge-removal filter, the aforementioned process is conducted. At the receiver, frequency domain SC/MMSE Turbo equalizer (FDTE) is employed to suppress ISI due to spectrum nulling process in the transmitter as well as the multipath fading. Computer simulations confirm that the proposed scheme is effective in suppression of CCI, ACI and ISI in one-cell reuse single-carrier TDMA systems.

This paper proposes a one-way ranging method using reference-based broadcasting messages. The method is based on impulse radio UWB (Ultra-wideband) for wireless sensor networks. The proposed method reduces traffic overheads and increases the ranging accuracy using frequency offsets and counter information based on virtually synchronized counters between RNs (Reference Nodes) and MNs (Mobile Nodes). Simulation results show that the proposed method can alleviate the ranging errors comparing to SDS-TWR (Symmetric Double-Sided Two-Way Ranging) method in terms of the frequency offset.

Frequency dependent properties of accumulation-mode MOS varactors, which are key elements in many RF circuits, are dominated by Non-Quasi-Static (NQS) effects in the carrier transport. The circuit performances containing MOS varactors can hardly be reproduced without considering the NQS effect in MOS-varactor models. For the LC-VCO circuit as an example it is verified that frequency-tuning range and oscillation amplitude can be overestimated by over 20% and more than a factor 2, respectively, without inclusion of the NQS effect.

Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can provide better downlink bit error rate (BER) performance of direct sequence code division multiple access (DS-CDMA) than the conventional rake combining in a frequency-selective fading channel. FDE requires accurate channel estimation. In this paper, we propose a new 2-step maximum likelihood channel estimation (MLCE) for DS-CDMA with FDE in a very slow frequency-selective fading environment. The 1st step uses the conventional pilot-assisted MMSE-CE and the 2nd step carries out the MLCE using decision feedback from the 1st step. The BER performance improvement achieved by 2-step MLCE over pilot assisted MMSE-CE is confirmed by computer simulation.

Inductive effect becomes important for on-chip global interconnects, like the power/ground (P/G) grid. Because of the locality property of partial reluctance, the inverse of partial inductance, the window-based partial reluctance extraction has been applied for large-scale interconnect structures. In this paper, an efficient method of partial reluctance extraction is proposed for large-scale regular P/G grid structures. With a block reuse technique, the proposed method makes full use of the structural regularity of the P/G grid. Numerical results demonstrate the proposed method is able to efficiently handle a P/G grid with up to one hundred thousands wire segments. It is several tens times faster than the window-based method, while generating accurate frequency-dependent partial reluctance and resistance.