This paper proposes a novel frequency-domain channel estimator which mitigates the effects of ICI by jointly finding the frequency offset and channel frequency response (CFR). A binary search algorithm is used to find the present frequency offset and CFR jointly. The variance of the jointly estimated frequency offset is found to be very close to the Cràmer-Rao lower bound.

Spatial correlation among antenna elements both at transmitter and receiver sides in MIMO communications is known to have a crucial impact on system performances. Another factor that can severely degrade receiver performances is the timing offset relative to the channel delay profile. In this paper we derive a novel receiver for turbo MIMO equalization in space-time-trellis-coded (STTrC) system to jointly address the problems described above. The equalizer is based on low complexity MMSE filtering. A joint detection technique of the several transmit antennas is used to reduce the receiver's sensitivity to the spatial correlation at the transmitter and receiver sides. Furthermore, only the significant portion of the channel impulse response (CIR) is taken into account while detecting signals. The remaining portion of CIR is regarded as the unknown interference which is effectively suppressed by estimating its covariance matrix. By doing this the receiver's complexity can be reduced since only a portion of the CIR has to be estimated and used for signal detection. Furthermore, by suppressing the interference from the other paths outside the equalizers coverage the receiver's sensitivity to the timing offset can be reduced. The proposed receiver's performance is evaluated using field measurement data obtained through multidimensional channel sounding. It is verified through computer simulations that the performance sensitivity of the joint detection-based receiver to the spatial correlation is significantly lower than with the receiver that detects only one antenna at a time. Furthermore, the performance sensitivity to the timing offset of the proposed receiver is shown to be significantly lower than that of the receiver that ignores the existence of the remaining multipath CIR components.

Based on traffic prediction, a new reservation method is proposed to reduce delay and void filling ratio at edge node of optical burst switching networks. Simulation studies show that our method achieves an important improvement and has a dynamic optimum weight value in a certain offset time.

This letter considers frame synchronization in non-synchronized sampling discrete multi-tone (DMT) based asymmetric digital subscriber line (ADSL)/very high speed DSL (VDSL) systems in the presence of timing error. We propose a frame synchronization method which is based on the observation that the normalized correlation between two sequences separated by the FFT length is Cauchy random variable. The proposed approach uses less number of correlators, reducing computational complexity as well as demodulation delay than a previous approach. Therefore, ADSL/VDSL modems can be more power efficient and computationally less complex via the proposed frame synchronization method. Simulation results demonstrate the effectiveness of the proposed approach, comparing with the previous approach.

The bit error rate (BER) degradations of fast frequency hopping multiple access (FFH-MA) systems due to the frequency and timing offsets are investigated over a Rician fading channel. It is shown that as the received average SNR increases, the BER is affected much larger by frequency and timing offsets. When the frequency offset or the timing offset exists alone, the BER of the FFH-MA system is degraded much more due to the timing offset than due to the frequency offset. The BER degradation due to both the frequency offset and the timing offset is larger than the sum of the degradations due to each offset.

A new data-aided carrier frequency offset (CFO) estimation technique is presented for correlative coded OFDM systems in the presence of strong multipath. Different from traditional data-aided estimation techniques, the technique estimates CFO by detecting amplitude of pilots rather than their phase shift and removes effects on CFO estimation due to intercarrier interference by an iterative compensation method. A theoretical analysis of its performance has been derived and simulation results comparing the new technique with a traditional data-aided estimation technique are presented.

This paper analyzes the immediate cause of the ICI in OFDM systems due to the Doppler spread and carrier frequency offset. As shown, ICI occurs because of the using DFT (FFT) and IDFT (IFFT) for signal conversion from time domain into frequency domain, and vice versa, when the sampled signal has limited duration. Proposed method refines the spectral density function of subcarriers, when applied in the transmission side, and improves the characteristics of the DFT as a digital filter, when applied in the receiver side. Simulation includes, working under the same conditions, models of conventional OFDM, PCC-OFDM and proposed method. Results of the simulation show that proposed method provides up to the 10 times less BER than PCC-OFDM and up to the 100 times less BER than conventional OFDM, which allows using of the OFDM in the mobile communication for vehicle speed up to the 500 km/hr.

Discrete Wavelet Multi-carrier Transceiver (DWMT) system, which can be viewed as a kind of OFDM, has many advantages because it uses wavelets as its base functions. In this paper we present a new sub-carrier frequency offset correction method for DWMT systems with little assistant information. The essential ideal of this algorithm is: when an orthogonal multi-carrier system is of perfect frequency synchronize, the demodulated signals of different sub-carriers are independent of each other. Whereas when frequency offset exists, intercarrier interference will distort the demodulated signal, i.e. every demodulated signal is the sum of several modulated signals' projects on the demodulating frequency. So the adjacent demodulated signals consist of the element of the same modulated signal, and these demodulated signals are correlated with each other. The degree that they correlated with each other depends on sub-carrier relative frequency offset. Since that little assistant information is used in this algorithm the spectrum efficiency can be largely increased. Simulation results shown that if the number of the sub-carrier of the DWMT system is bigger than 1000, the relative frequency offset can be limited in 2%.

AF service class in Diffserv by realizes minimum bandwidth guarantee with the use of differentiated drop precedence property marked on each packet. In the context of a multiple domains environment, however, QoS of individual flow is not always preserved due to the re-marking behavior forced at the domain boundaries. Focusing on this point, this paper proposes new packet re-marking schemes that can improve the per-flow QoS of AF service traversing multiple domains. The basic concept of the schemes distinguishes packets re-marked to out-of-profile at the domain boundaries from those already marked as out-of-profile at the time of entering the network, and allows the re-marked packet to recover back to in-of-profile, thus regaining its rightful QoS within the networks. The performance of the proposed schemes is evaluated through simulation. The results on UDP flows show the effectiveness of the proposed schemes for reducing packet losses on the flow through multiple domains and preserving fairness between flows. Simulations on TCP flows show that the proposed schemes improve the throughput of the flows through multiple domains. The proposed scheme is especially effective on the transfer time of short TCP flows such as Web traffic, whose throughput is affected more seriously by a single packet loss due to its flow control mechanisms.

In this paper, we term multimedia streaming application traffic "stream flows" and the other usual application traffic "non-stream flows." Many problems occur when both flows are aggregated on a shared link because the different TCP and UDP behaviors cause negative interactions. One way to solve these problems is to isolate stream and non-stream flows to different classes. However, it is difficult to determine the bandwidth allocation for each class and dynamic bandwidth allocation schemes are hard to implement on large scale networks. We therefore propose a dynamic class assignment method that maintains the QoS and that has a higher scalability than dynamic bandwidth allocation schemes. It is workable on Diffserv AF PHB. The outline is as follows. We classify non-stream flows into four classes and dynamically assign stream flows to the classes, taking the conditions and characteristics of the classes into consideration. On assigning classes to stream flows, we map them to a higher drop precedence than non-stream flows not to degrade the QoS of them, based on the assumption that occasional packets being dropped do not create serious problems for them. In this paper, we first discuss our classification of non-stream flows, and present the characteristics of non-stream flows in each class. We then discuss our drop precedence mapping. After this, we propose an algorithm for our method of dynamic class assignment and provide some simulation results where it could provide constant qualities with stream and non-stream flows, adapting to changing traffic.

We analyze the performance of code division multiple access (CDMA) systems with orthogonal frequency division multiplexing (OFDM). We obtain the probability density function (pdf) of the multiple access interference (MAI) of CDMA systems and extend the results to OFDM-CDMA systems to determine the pdf of the MAI and inter-carrier interference (ICI) in terms of the number of users, the spreading length, the crosscorrelation of spreading sequences, the number of sub-carriers and the frequency offset. We consider the synchronous downlink of cellular multi-carrier CDMA systems and derive a Gaussian approximation of the MAI and ICI. The results show that the overall effect of frequency offset varies with system loading for a given crosscorrelation. The performance of OFDM-CDMA in frequency selective fading channels is analyzed in terms of the joint probability of the fading parameter in each sub-carrier.

An analog neurochip for independent component analysis (ICA) is designed with on-line learning capability. Due to the limited dynamic range of analog device, the nonholonomic ICA algorithm is adopted. In order to accommodate the offsets due to device mismatches, a modified algorithm is developed with 2-quadrant multipliers and self-adjusting biases. Performance of the developed system was demonstrated by Monte-Carlo simulation.

In this letter, we propose joint frequency offset and SNR estimation technique. The frequency offset may degrade the system performance greatly by deleterious effect. The proposed frequency offset estimation technique estimates frequency offset by employing the interpolation technique in the frequency domain. Also we propose SNR estimation technique using the estimated frequency offset for FFT-based system. The SNR estimated by the receiver can be used to adapt the demodulation algorithm to enhance its performance, as well as to provide the channel quality information. Simulation results show the performance of frequency offset and SNR estimator.

A fully differential direct conversion receiver IC for W-CDMA is presented. The receiver IC consists of an LNA, a quadrature demodulator, low-pass filters (LPFs), and variable gain amplifiers (VGAs). In order to suppress DC offset, which is the most important issue in a direct conversion system, an active harmonic mixer is applied to the quadrature demodulator. Furthermore, a receiving system, including the LNA and an RF filter, adopts a differential architecture to reduce local signal leakage, which generates DC offset. Performance of the entire receiving system was evaluated and DC offset in steady state was measured at only 40 mV. Moreover, DC offset variation at the LNA gain change, which has the largest affect on the receiving performance, was limited to 70 mV, which is less than -10 dB compared to desired signal strength. It was confirmed by computer simulation that the DC offset variation at the LNA gain change did not degrade bit error rate (BER) performance at all.

A linear-in-dB gain-control amplifier for direct conversion systems employs linearized transconductors in a core amp, a dc offset canceler, and a gain control circuit. The offset compensation circuit achieves a constant corner frequency over a gain range of 14 to 76 dB by simultaneous tuning of the transconductors.

Multi-Carrier Code Division Multiple Access (MC-CDMA) is one of candidates for the next generation wireless communication systems. In an uplink, the MC-CDMA system suffers from the different access timing (asynchronous transmission), the different fading, and the different frequency offsets of each active user. In this paper we analyze the effects of the frequency offset compensation with MMSE-MUD (minimum mean square error based multi-user detection) for MC-CDMA in a quasi-synchronous uplink. We consider the MC-CDMA system with two subcarrier mapping schemes, the continuous mapping scheme and the discrete mapping scheme. From our theoretical analysis and computer simulation, we show that the MMSE-MUD can compensate the different frequency offsets among users. We also show that the MMSE-MUD significantly improves the bit error rate (BER) for the MC-CDMA system with the continuous mapping scheme.

This paper proposes a sensing and a precharge circuit schemes suitable for low-voltage and high-speed DRAM design. The proposed offset-compensated direct sensing scheme improves refresh characteristics as well as speed performance. To minimize the number of control switches for the offset compensation, only the output branches of differential amplifiers are implemented in each bit-line pair with a semi-global bias branch, which also reduces 50-percent of bias current. The addition of the direct sensing feature to the offset-compensated pre-sensing dramatically increases the differential current output. For the fast bit-line equalization, a charge-recycled precharge scheme is proposed to reuse VPP discharging current for the generation of a boosted bias without additional charge pumping. The two circuit schemes were verified by the implementation of a 256 Mb SDRAM with a 0.1 µm dual-doped poly-silicon technology.

In this paper we propose a new blind adaptive compensator associated with the inverse QRD-RLS (IQRD-RLS) algorithm to adaptively estimate the parameters, related to the effects of gain/phase imbalance and DC offsets occur in the Quadrature demodulator, for compensation. In this new approach the power measurement of the received signal is employed to develop the blind adaptation algorithm for compensator, it does not require any reference signal transmitted from the transmitter and possess the fast convergence rate and better numerical stability. To verify the great improvement, in terms of reducing the effects of the imbalance and offset, over existing techniques computer simulation is carried out for the coherent 16 PSK-communication system. We show that the proposed blind scheme has rapidly convergence rate and the smaller mean square error in steady state.

This paper proposes new frame synchronizers that can achieve frame sync in the presence of a frequency offset. In particular, a maximum likelihood (ML) algorithm for joint frame synchronization and frequency estimation is developed for additive white Gaussian noise (AWGN) channels, then the result is extended to frequency selective channels. Computer simulations demonstrate that the proposed schemes can outperform existing methods when a frequency offset exists.

Two simple frequency offset estimators based on projection approaches for multicarrier code-division multiple access systems are proposed, without using specific training sequences. It is not only can estimate and correct frequency offset, but also has less computational load. Several computer simulations are provided for illustrating the effectiveness of the blind estimate approaches.