The Orthogonal Frequency Division Multiplexing (OFDM) system is one of several suitable candidates for use in broadband wireless communications. However, due to multipath propagation, the received signal suffers from frequency-selective fading which causes significant degradation of system performance. Antenna diversity reception is widely used to solve this problem. However, this approach is not suitable for the downlink, because it increases the complexity and power consumption of the mobile station (receiver). In this paper, we consider closed-loop mode transmit diversity for OFDM, instead of diversity reception, to improve the performance in the downlink. The base station (transmitter) has several transmit antennas and each antenna is weighted by a weighting factor calculated based on feedback information (FBI). This system is a closed loop, since the FBI is fed back from the receiver. We propose a new weight generation scheme by making use of the correlation between adjacent OFDM subcarriers. The performance is evaluated under a broadband wireless channel model by computer simulation. The results show that the proposed weight generation scheme exhibits better performance than the conventional weight generation scheme without increasing the number of the FBI bits in the uplink.

Data-Over-Cable Service Interface Specifications v1.1 (DOCSIS v1.1), developed for data transmissions over Hybrid Fiber Coaxial (HFC) networks, defines five upstream services for supporting per-flow Quality of Services (QoS). The cable modem termination system (CMTS) must periodically grant upstream transmission opportunities to the QoS flows based on their QoS parameters. However, packets may violate QoS requirements when several flows demand the same interval for transmission. This study proposes a two-phase, i.e., the scheduling sequence determination phase and the minislot assignment phase, minislot scheduling algorithm to reduce the QoS violation rate. In the scheduling sequence determination phase, the flow whose packets are most unlikely to violate QoS is scheduled first. Then, in the minislot assignment phase, the scheduler allocates to a flow the available interval where the likelihood of packet violation is minimum. Simulation results demonstrate that our scheduling algorithm can reduce the QoS violation rate by 80-35% over that of the first-come-first-serve-random-selection algorithm. It increases the utilization by 25% as well. The two-phase minislot scheduling algorithm can work within the DOCSIS v1.1 framework.

A rapid Pseudo-Noise (PN) acquisition scheme is proposed. The proposed scheme consists of a phase alignment detector and Voltage Controlled Clock (VCC) loop. The VCC loop is used to control the phase update of the local PN signal. It has an auxiliary signal that provides the loop with two stable locking points as well as the direction of each phase update. The performance of the proposed scheme is evaluated by simulation. Results show that the proposed scheme acquires the phase two to three times faster than the conventional coherent serial scheme, and 1.5 times faster than of that in [10], at a small amount of additional hardware.

This paper presents novel flooding schemes for wireless mobile ad hoc networks. Clustering of nodes is assumed as a basic ad hoc network structure. GWF (Gateway Forwarding) and SGF (Selected Gateway Forwarding) are presented based on clustering. A new protocol, termed FGS (Flooding Gateway Selection) protocol, between a cluster head and its gateways to realize SGF is presented. It is shown that SGF significantly improves the packet delivery performance in ad hoc networks by reducing flooding traffic.

This paper proposes a credit-based congestion control scheme for multicast communication which employs application-specific processing at intermediate network nodes. The control scheme was designed not only to take advantage of credit-based flow control for unicast communication, but also to achieve flexibility supported by active network technology. The resultant active multicast congestion control scheme is able to meet the different requirements of various multicast applications in terms of reliability and end-to-end latency. The performance of the proposed control scheme was evaluated using both discrete-event simulations and experiments on a prototype active network implementation. The results show that the proposed scheme performs very well in terms of fairness, responsiveness, and scalability. The implementation experiences also confirmed the feasibility of the scheme in practice.

Progress in the field of broadband access network and information appliances has led to the advent of a new network field called Home Network. In 1999, HomePNA2.0 using phone line was proposed, and we believe that it is one of the most promising solutions because of its cost-effectiveness. However, due to adaptation of the mature IEEE802.3 CSMA/CD technology used for Ethernet, it is not able to guarantee the QoS. We present the design, implementation and empirical evaluation of a new MAC protocol for the Home Network called HomeMAC. In this paper, the software based HomeMAC is implemented by programming the kernel space of FreeBSD. HomeMAC features a hybrid CSMA/CD-Timed Token protocol which combines the CSMA/CD for non-real-time traffic with timed token protocol for real-time traffic. In addition, by providing flexible bandwidth allocation based on QoS Level Table (QLT), HomeMAC can serve high QoS covering the whole offered load. From the results of evaluation of software implementation, we verify that HomeMAC can provide low delay, low loss, and low jitter to the real-time traffic by reservation of the bandwidth.

A robust adaptive beamforming method is proposed to cancel coherent, as well as incoherent, interference using an array of arbitrary geometry. In this method, coherent interferences are suppressed by a transformation of received data with the estimates of their arrival angles and then, to reject incoherent interferences, the array output power is minimized subject to the look direction constraint in the transformed signal-plus-interference (TSI) subspace. This TSI subspace-based beamforming results in robustness to errors in the angle estimations. Its performance is theoretically examined. The theoretic results conform to simulation results. It is straightforward to apply the theoretic results to the performance analysis of subspace-based adaptive beamfomers only for incoherent interference cancellation.

A numerical technique based on Haar wavelets is used for solving transient problems of transmission lines. The approach of our method is to convert the original coupled partial differential equations, the transmission line equations or the telegrapher equations, to a system of ordinary matrix differential equations via Haar wavelets. Then, transient problems of transmission lines can be solved by matrix operations. Numerical examples of homogeneous and dispersive lines, along with both linear and nonlinear loads are verified. In addition, non-sinusoidal signals such as the unit step function and the rectangular pulse for digital applications are included to demonstrate the use of this efficient, easy-to-handle, stable, and versatile method.

We describe the width conversion of an optical signal by using an erbium-doped fiber and an asymmetric optical circuit. The width of an optical signal was measured to be a respective 350 nsec and 200 nsec for a 70 m and 40 m fiber (Lf). The width of the pumping pulse was 5 nsec and the length of erbium-doped fiber was 3 m. We also extended the optical signals to a respective 300 nsec and 150 nsec wide at a pumping pulse 10 nsec by inserting a 60 m and a 30 m fiber (Lf) inside a circuit.

A design method is proposed that yields the optimum remote pre-amplifier (RPRA) parameters considering cable repair, the results of include increased cable loss and insertion position uncertainty. The optimum RPRA location is given by the intersection point of optical SNR (OSNR) vs. RPRA location curves in two cases; the total cable repair loss is assumed to be inserted at the transmitter end and at the receiver end. This RPRA parameter gives the maximum OSNR in the worst loss insertion case by cable repair.

We propose a phase lock detector for 16-QAM systems for high-speed wireless communications. The detector gathers the phase estimates statistically according to the predetermined symbols, filters them through an average filter, and indicates the phase lock state by comparing the filtered resultants to a threshold value. The statistical property of the proposed detector is analyzed using the stochastic process theory. First, we obtain the characteristic function of a random variable describing the filter output. Second, through inverse Laplace transform, we get the probability density function of the random variable. Third, we can obtain the phase lock detection probability using the probability density function. Finally, to investigate its accuracy, we obtain the probability density function of a random variable for the detector output, and compare it to the simulation result.

Transmitter diversity is a powerful technique to improve the transmission quality of downlink in microcellular mobile communications systems. Under cochannel interference (CCI) at the base station (BS), the transmitter diversity is not necessarily effective, because the desired-plus-interference signal power used as a criterion of downlink branch selection is not always relative to the downlink propagation condition. This paper proposes the theoretical derivation of bit error rate (BER) performance in the transmitter diversity under CCI occurring at BS, as parameters of average SIR at BS, normalized Doppler frequency, and so on. It is confirmed from the correspondence of theoretical results with simulation results that the proposed theoretical approach is applicable to the CCI environments at BS.

As a new type of a linear decorrelating receiver, the Pseudo-Decorrelator was presented for asynchronous code division multiple access systems in [6]. In this paper, the concept of the Pseudo-Decorrelator is extended to derive a suboptimal receiver for WCDMA uplink systems in a Rayleigh fading environment. Starting with the analysis of the multiple access components of the decision statistics, a non-square cross-correlation matrix for each bit is obtained. This cross-correlation matrix is then inverted and the inverted matrix is applied to the decision statistics obtained from a conventional receiver. Simulation results are presented for K-user systems over a Rayleigh fading channel. The effects of the synchronization errors, such as time delays and carrier phase errors, are also examined through simulations in this paper.

This letter is concerned with a channel estimation technique with interference cancellation in a wireless personal area network (WPAN) system with complementary code keying (CCK) signaling. The performance of the coherent detector based on the proposed channel estimation scheme is compared with that of a noncoherent detector, and a significant improvement of performance is observed due to the self-interference cancellation.

Existing code division multiple access (CDMA) cellular systems achieve capacity gains over conventional time division multiple access (TDMA) or frequency division multiple access (FDMA) systems, by assuming a large number of low data-rate and hence low power users uniformly distributed over a cell. In current CDMA proposals for providing higher data-rate packet services, however, burst high data-rate users do not satisfy this assumption and thus the capacity advantage is lost. In this letter, we quantify the capacity loss with focusing on location-dependent aspects of the degradation. This work contributes to a better understanding of the capacity problem encountered when introducing high data-rate packet services with the cellular band where existing low data-rate CDMA systems operate.

In this letter, an algorithm that estimates one of the most important channel parameters, maximum Doppler frequency, f_D, is proposed. The algorithm uses phase variations of received pilot signals, which is strongly related with f_D in a fading environment. In addition, a phase variation measurement method for binary phase shift keying (BPSK) modulated signals is also proposed and it makes possible to estimate f_D from BPSK modulated information signals as well as unmodulated pilot signals. The results show that the proposed algorithm is very simple and shows good performance over wide Doppler frequency range.

The major concern at a branch point in asynchronous transfer mode (ATM) networks for point-to-multipoint available bit rate (ABR) services is how to consolidate backward resource management (BRM) cells from each branch for a multicast connection. In this paper, we propose an efficient feedback consolidation algorithm based on an adaptive dynamic threshold (ADT) to eliminate consolidation noise and to reduce consolidation delay. The main idea of the ADT algorithm is that each branch point estimates the ABR traffic condition of the network through virtual queue estimation. Simulation results show that the proposed ADT algorithm can achieve a faster response in congestion status and a higher link utilization compared with the previous works.

A method for fast calibration of digital-beam-forming (DBF) receiving array antennas by means of digital signal processing is described. It uses plane wave arriving from a known direction that contains a known reference sequence. Non-uniformities of the amplitude and phase in the branches are detected and calibrated in real time by the complex correlation of a replica of the known reference sequence with the received signal obtained from the output signals of each element. No special circuit for calibration is required, and the non-uniformities can quickly be compensated for by digital signal processing even for an array antenna with many antenna elements. This method enables highly accurate calibration of large-scale array antennas operating at a high frequency even under a low signal-to-noise power ratio (SNR).

Eigenstructure-based beamformers suffer form performance degradation due to pointing errors when the number of the incident signals is incorrectly detected or when the desired signal is much stronger than the interferences. We present a robust beamformer with the self-correction of look direction errors, based on the Newton method. Even though there are errors in the detection of the incident signal number as well as in the presumed look direction, it can achieve optimum performance with no errors.

Scaled models for an anatomical head model and a simple head model are used to investigate the effects of head size on SAR characteristics for a cellular phone exposure at 835 MHz. From the results, we can see that a larger head produces a higher localized SAR and a lower whole-head averaged SAR.