We develop a maximum likelihood estimation scheme for correcting the carrier frequency offsets prior to the general intercarrier interference (ICI) self-cancellation in the OFDM systems. Since the same data symbols employed for ICI self-cancellation are also used for frequency offset estimation, the proposed scheme does not consume additional bandwidth. The combined use of the estimation algorithm and ICI self-cancellation scheme provides both frequency offset compensation and ICI reduction hence improves the system performance greatly. The effectiveness of the proposed estimation-cancellation scheme is further verified by calculating the bit error rates of various OFDM receivers, and substantial improvements are found.

In this letter,we design a special preamble composed of two OFDM training blocks with different numbers of identical parts. Based on the designed preamble, we propose a method to estimate frequency offset utilizing initial estimates from the two OFDM training symbols. By elaborately selecting the numbers of identical parts for the two training blocks, the proposed estimator provides a much larger estimate range than conventional estimators using identical parts. Computer simulations show that the proposed estimator exhibits superior estimate performance, while maintaining low computational complexity.

In this letter, we focus on non-pilot-symbol assisted integer frequency offset estimation for multicarrier orthogonal frequency division multiplexing (OFDM) systems. We introduce a frequency offset estimator that is based on the guard interval (GI) present in OFDM signals. We show by simulation that the frequency offset estimator can accurately estimate the frequency misalignment at the sacrifice of limited estimation range.

Accurate and low-cost sensor localization is critical for deployment of wireless sensor networks. Distance between Ultra-wideband (UWB) sensor nodes can be obtained by measuring round trip flying time through two-way ranging (TWR) transaction. Because of difficulties in synchronization and channel estimate, the response delay of UWB node is the order of milliseconds. Comparing with the nanosecond propagation delay, relative clock offset between UWB nodes introduces big error in TWR. This paper presents the management of relative clock offset in TWR transaction. The relative clock offset between sensors is estimated by comparing the claimed and real frame duration. Simulation in the UWB channel model shows the relative clock offset after compensation can be reduced to less than 2 ppm.

In this letter, a blind frequency offset estimation algorithm is proposed for OFDM systems. The proposed method exploits the intrinsic phase shift between neighboring samples in a single OFDM symbol, incurred by a frequency offset. The proposed algorithm minimizes a novel cost function, which is the squared error of the candidate frequency offset compensated signals from two different observation windows. Also, the solution of the proposed algorithm is given by an explicit equation, which does not require any iterative calculation. It is shown that the performance of the proposed method is better than those of the conventional methods, especially in the presence of multipath channels. This is due to the fact that the proposed method is insensitive to inter-symbol interference (ISI).

Orthogonal Frequency Division Multiplexing (OFDM) systems are very sensitive to the frequency offset of the local oscillator at the receiver while the symbol timing offset can be absorbed in the guard interval. For the same reason, estimation of the frequency characteristics, needed for OFDM to be adapted to the frequency selective fading, can only be carried out conventionally after the frequency offset has been compensated. And accurate estimation of large frequency offset certainly requires high precision estimate of the frequency characteristics. In this paper, we propose a new joint estimation method of the frequency offset and the channel frequency response using an Expectation-Maximization (EM) algorithm for OFDM systems. The proposed algorithm overcomes the limitation of the thus far proposed algorithm. By computer simulations, we show the proposed algorithm provides estimation accuracy close to its lower bound in a wide range of the frequency offset.

In the reverse link of orthogonal frequency division multiplexing (OFDM)/ space division multiple access (SDMA) systems, each receive antenna of a base station receives a multiplexed version of signals transmitted from users, where the transmitted signals have individual amounts of frequency offset. Therefore, a frequency offset compensation scheme which is different from those used in general OFDM systems is required. For this requirement, frequency offset compensation schemes using the feedback transmission from the base station to user terminals have been proposed for multiuser OFDM systems. These schemes work with good precision when the feedback information is correct and is transmitted without errors. However, when the offset information is incorrectly received at user terminals, the frequency offset is not accurately compensated for. In OFDM/SDMA systems, one user is enough for causing inter-carrier interference to all users. Therefore, a frequency offset compensation scheme without feedback transmission is sometimes preferable. In this paper, we propose a frequency offset compensation scheme without feedback transmission. To compensate for frequency offset in every transmitted signal, the multiplexed received signals must be separated into each user's component before the offset compensation. Thus, we adopt the principle of the parallel interference cancellation (PIC). By employing PIC, the received signals can be separated before the offset compensation. Thus, the frequency offset of every user's signal can be compensated for. Simulation results show the bit error rate performance of the proposed scheme attains almost the same as that of the conventional scheme using the feedback transmission without errors.

In this letter, two types of blind adaptive frequency offset estimator for multicarrier code division multiple access systems are proposed that do not need any specific training sequence. It can not only accurately estimate the frequency offset, but also improve the revision capability and convergence rate. Several computer simulations confirm the effectiveness of the blind estimate approaches.

In this letter, we present a simple way of estimating the integer frequency offset of orthogonal frequency division multiplexing (OFDM) system over a rapidly time-varying channel. By utilizing the channel responses of neighboring subcarriers within one pilot symbol, the frequency offset estimator is derived. We show by simulation that the proposed estimator can accurately estimate the integer frequency offset with reduced computational burden.

In this letter, a method estimating the intercell carrier frequency-offset (CFO) in orthogonal frequency division multiplexing (OFDM)-based cellular systems is proposed for the user's equipment (UE), especially at the cell boundary, in downlink channels. After describing a new method of deriving the intercell CFO from the signals received by adjacent base stations (BSs), we propose a cell-searching method using the estimated CFOs. It is shown by computer simulation that the proposed methods can uniquely estimate the intercell CFOs and identify the target BS with a high detection probability at the UE.

This paper describes the design of a small-offset 12-bit CMOS charge-redistribution DAC using a weighted-mean flip-around sample-and-hold circuit (S/H). Flip-around S/H topology can realize small-offset characteristics, and it is effective to reduce power dissipation and chip area because independent feedback capacitors are not necessary. In this DAC the small-offset characteristic remains not only in amplification phase but also in sampling phase with the circuit technique. The design of 1.8 V, 50 MS/s fully differential DAC with output swing of 2 Vp-p has very small offset of 100 µV for the reset switch mismatch of 2%. A technique to improve dynamic performance measured by SFDR using damping resistors and switches at the output stage is also presented. The designed 12-bit DAC with 0.25 µm CMOS technology has low-power dissipation of 35 mW at 50 MS/s.

In this letter, we present timing synchronization method using two consecutive orthogonal frequency division multiplexing (OFDM) symbols which are designed to be cyclic-shifted against each other. Our approach can be viewed as an expansion of Minn's correlation methods. Using the proposed OFDM signal, however, we can estimate the timing offset without any training symbol.

In orthogonal frequency-division multiplexing (OFDM)-based wireless local area networks (WLANs), phase noise (PHN) and residual frequency offset (RFO) can cause the common phase error (CPE) and the inter-carrier interferences (ICI), which seriously degrade the performance of systems. In this letter, we propose a combined pilot symbol assisted and decision-directed channel estimation scheme based on the least-squares (LS) and the maximum-likelihood (ML) algorithms. Simulation results present that the proposed scheme significantly improves the performance of OFDM-based WLANs.

This letter evaluates the performance of an uplink multicarrier-code division multiple access (MC-CDMA) system when the frequency offsets of all users are random variables and the frequency offset for the desired user is compensated. The analysis confirms that performance degradation due to frequency offset is negligible if the estimation error of normalized frequency offset for the desired user is less than 10-1.

A novel frequency domain training sequence and the corresponding carrier frequency offset (CFO) estimator are proposed for orthogonal frequency division multiplexing (OFDM) systems over frequency-selective fading channels. The proposed frequency domain training sequence comprises two types of pilot tones, namely distinctively spaced pilot tones with high energies and uniformly spaced ones with low energies. Based on the distinctively spaced pilot tones, integer CFO estimation is accomplished. After the subcarriers occupied by the distinctively spaced pilot tones and their adjacent subcarriers are nulled for the sake of interference cancellation, fractional CFO estimation is executed according to the uniformly spaced pilot tones. By exploiting a predefined lookup table making the best of the structure of the distinctively spaced pilot tones, computational complexity of the proposed CFO estimator can be decreased considerably. With the aid of the uniformly spaced pilot tones generated from Chu sequence with cyclically orthogonal property, the ability of the proposed estimator to combat multipath effect is enhanced to a great extent. Simulation results illustrate the good performance of the proposed CFO estimator.

In this letter, we investigate ESPRIT-based approaches for blind frequency offset estimation on MC-CDMA downlink. By analyzing the signal subspace structures of the MC-CDMA signals, the natures of the spreading codes and the number of users determine the feasibility of the ESPRIT-based approaches for the blind estimation task.

This paper proposes a framework for session-level SLA (Service Level Agreement) and network-level SLA management to provide QoS-oriented application services over DiffServ/MPLS networks. DiffServ and MPLS technologies enhance the capability of QoS guarantee on IP network, and application service provider can provide QoS-oriented application services to their customers based on the transport networks. The example of using our approaches in the VoIP service involving the network service provider, the VoIP service provider, and the VoIP customers are examined. The session-level SLA between VoIP service provider and VoIP customer and the network-level SLA (N-SLA) between network provider and VoIP service provider are constructed in this paper. Besides, the VoIP service provider faces the QoS-mapping issue and the balance between revenue and cost, i.e., how to contract the N-SLA. Therefore, we introduce an off-line SLA evaluation scheme, a heuristic optimization algorithm and an on-line SLA process method to provide VoIP service policy, and then the optimal QoS-mapping can be resolved. The concept of this framework of session-level SLA and network-level SLA management can be extended easily into other real-time multimedia and non-real time data services.

Linearity of a transconductor with a theoretical linear characteristic is deteriorated by mobility degradation, in practice. In this paper, a technique to improve the linearity by combining a source-coupled pair with the transconductor is proposed. The proposed transconductor is the circuit that the deteriorated linearity of the conventional part is compensated by the transconductance characteristic of the source-coupled pair. In order to confirm the validity of the proposed technique, SPICE simulation is carried out. The transconductance change ratio of the proposed technique is about 1% and is 1/10 or less of the conventional circuit.

Admission control is becoming an essential technique for IP networks to provide full-fledged multimedia streaming services. Although signaling-based schemes are utilized to achieve this, these are difficult to deploy and can hardly achieve strict admission control taking the properties of packet arrival into consideration. In this paper, we propose a novel admission control strategy called the Tentative Accommodating and Congestion Confirming Strategy (TACCS). The main idea is to accommodate incoming flows tentatively and confirm congestion after a certain period. Since tentative accommodating enables us to generate the same situation as where incoming flows have been accommodated, TACCS makes it possible to control admission considering the properties of packet arrival after they have been accommodated, without collecting resource information in advance. From the results of mathematical analysis, we confirmed that TACCS enabled a domain to control admission without a centralized management agent and we provided guidelines for configuring parameters of TACCS.

In this letter, a concept of virtual smart antenna (SA) is introduced for OFDM-based cellular systems with a frequency reuse factor equal to 1. A method of estimating intercell symbol timing offsets (STOs) from received OFDM signals impinging on virtual antenna is proposed for users at a cell boundary. Also, adaptive beamforming methods for virtual SA are proposed to reduce intercell interference (ICI) from adjacent base stations (BSs).