A simple, yet effective geometric method is presented to construct the signature sequences for multicarrier code-division multiple access (MC-CDMA) systems. By minimizing the correlation of the effective signature vectors, the signature sequences are recursively determined via projection onto a properly constructed subspace. Conducted simulations verify the effectiveness of the method.

For an integer d > 0, a d-queue layout of a graph consists of a total order of the vertices, and a partition of the edges into d sets of non-nested edges with respect to the vertex ordering. Recently V. Dujmovi and D. R. Wood showed that for every integer d ≥ 2, every graph G has a d-queue layout of a subdivision of G with 2logd qn(G)+1 division vertices per edge, where qn(G) is the queue number of G. This paper improves the result for the case of a bipartite graph, and shows that for every integer d ≥ 2, every bipartite graph Gm,n has a d-queue layout of a subdivision of Gm,n with logd n-1 division vertices per edge, where m and n are numbers of vertices of the partite sets of Gm,n (m ≥ n).

One approach to accommodating IP traffic on a wavelength division multiplexing (WDM) network is to construct a logical topology, establishing a set of lightpaths between nodes. The lightpaths carry IP traffic but do not require any electronic packet processing at intermediate nodes, thereby reducing the load on those nodes. When the IP and WDM networks have independent routing functions, however, the lightpaths in the WDM network may not be fully utilized by the IP router. It is therefore necessary to integrate the two routing mechanisms in order to utilize resources efficiently and adapt to changes in traffic. In this paper, we propose an integrated routing mechanism for IP over WDM networks. The key idea is to first prepare a set of virtual-links representing the lightpaths that can be established by the WDM network, then calculate the minimum cost route on an IP network including those links. Our simulation results show that when traffic patterns do not change, the throughput of our method is almost the same as that of a logical topology optimally designed for a given traffic demand. When traffic patterns change, the throughput of our method is about 50% higher than that of the logical topology.

This paper proposes the most effective deployment scenario of the passive double-star (PON) system to provide multiple broadband services, such as high speed Internet access and broadcast services. The deployment costs of the two major PON technologies, wavelength division multiplexing (WDM) and 10 Gbps time division multiplexing (TDM), are analyzed using the latest cost trend and the most popular access network architecture. These two technologies are compared using the cost analysis results to identify the cost-effective scenarios of PON system deployment. Based on the comparison, this paper reveals that the WDM network becomes cost effective when the service penetration and the shift ratio becomes high.

Existing vision substitute systems have insufficient spatial resolution to provide environmental information. To present detailed spatial information, we propose two stimulation methods to enhance transfer information using a 2-D tactile stimulator array. First, stimulators are divided into several groups. Since each stimulator group is activated alternately, the interval of stimulations can be shortened to less than the two-point discrimination threshold. In the case that stimulators are divided into two and four groups, the number of stimulators increases to twice and four times, respectively, that in the case of the two-point discrimination threshold. Further, a user selects the measurement range and the system presents targets within the range. The user acquires spatial information of the entire measurement area by changing the measurement range. This method can accurately present a range of targets. We examine and confirm these methods experimentally.

Monolithically integrated four-channel distributed feedback (DFB) laser array has been fabricated by metal organic vapor phase epitaxy (MOVPE) selective area growth for 1.55 µm coarse-wavelength division multiplexing (CWDM) systems. Wide-stripe MOVPE selective area growth and electron-beam lithography are used to obtain wide CWDM channel spacing of 20 nm. Compared to hybrid integration of discrete lasers, monolithic integration of laser array on a single substrate greatly simplifies device alignment and packaging process.

In this letter, the effects of transmit beamforming on downlink performance in DS-CDMA communication systems are examined. We present a simple-to-use expression for the conditional instantaneous SINR after Rake combining. Assuming BPSK modulation, the performance of average bit error rate is evaluated. We compare the average BER performance obtained by different beamforming methods under frequency selective multipath fading channels.

Time-frequency-selective, equivalently time-variant multipath, fading channels in orthogonal frequency division multiplexing (OFDM) systems introduce intercarrier interference (ICI), resulting in severe performance degradation. To suppress the effect of ICI, several symbol detection methods have been proposed, all of which are based on the observation that most of the ICI's power is distributed near the desired subcarrier. However, these methods usually ignore the channel variation in a OFDM symbol block by fixing the number of considered ICI terms. Therefore, we propose a novel frequency-domain symbol detection method with moderate complexity, which adaptively determines the number of ICI terms within each OFDM symbol block.

This paper proposes new resource management schemes for multiple data streams in an orthogonal frequency and space division multiplex access (OFSDMA) system using Radio-on-Fiber (RoF) ubiquitous antennas. The proposed schemes classify the services into some classes in which the number of sub-carriers is dynamically assigned according to the requested data size. The computer simulation results show that the proposed schemes improve the number of users satisfying the required bit error rate (BER) level as well as the average throughput and also show that the RoF ubiquitous antennas can improve system capacity.

This paper presents an admission control technique for multi-carrier systems with an FRF(frequency reuse factor) of 1. The FRF of 1 is very attrative for more improved channel throughput but the forward link capacity is rapidly decreased at the cell boundary region due to the increase in the ICI(InterCell Interference). By measuring a region-based channel capacity and deriving a closed form of blocking probability, a QoS(Quality of Service) maintenance technique and mobility model can be acquired. In the simulation, the proposed scheme demonstrates a blocking probability reduction of up to 40% compared to the cell-based link capacity scheme.

We introduce an adaptive subchannel, bit, and power allocation (ASBPA) algorithm to maximize the bandwidth efficiency of the mobile communication system that use orthogonal frequency division multiplexing (OFDM). We propose a suboptimal rate adaptive ASBPA algorithm that guarantees fairness in resource allocation and overcomes inherent co-channel interference (CCI) in the cellular system. Furthermore, we evaluate the maximum possible bandwidth efficiency of the cellular OFDM system achieved by the ASBPA algorithm which is practical to implement. Our simulation results show that the proposed algorithm outperforms the existing ones and achieves the cellular bandwidth efficiency of up to 5 b/s/Hz/cell. We also investigate some of the conditions that govern the bandwidth efficiency of the cellular OFDM system using the proposed ASBPA algorithm.

In CDMA cellular systems, the frequency reuse factor equals one. Therefore, the soft-handoff technology with combining macroscopic diversity was introduced to enhance the link performance. In this work, a novel macroscopic diversity combining scheme is proposed to enhance the link performance of the forward-link. The basic concept of this scheme is to integrate error correction coding into the soft-handoff technology. According to the number of soft-handoff channels, the source information is encoded by a convolutional code with a lower code rate. The coded symbols are then equally distributed to all channels from different BSs to the MS, and each channel carries a disjointed set of coded symbols. For this proposed scheme, no extra transmission power or bandwidth is required. The only cost is a slight increase of the encoding and decoding complexity of the convolutional codes. Numerical and simulation results show that a performance gain of 1 dB in bit energy-to-total noise power density ratio can be obtained as compared with the conventional scheme in the same conditions.

We derived explicit formulas for evaluating the error vector magnitude (EVM) from the amplitude distortion (AM-AM) and phase distortion (AM-PM) of power amplifiers (PAs) in orthogonal frequency-division multiplexing (OFDM) systems, such as the IEEE 802.11a/g wireless local area networks (WLANs) standards. We demonstrated that the developed formulas allowed EVM simulation of a memoryless PA using only a single-tone response (i.e. without OFDM modulation and demodulation), thus enabling us to easily simulate the EVM using a harmonic-balance (HB) simulator. This HB simulation technique reduced the processing time required to simulate the EVM of a PA for the IEEE 802.11a standard by a factor of ten compared to a system-level (SL) simulation. We also demonstrated that the measured EVM of a PA module for the IEEE 802.11g could accurately be predicted by applying the measured static AM-AM and AM-PM characteristics to the derived formulas.

This paper presents a numerical analysis of reverse link capacity by obtaining a closed form of ICI (InterCell Interference) over OFDM (Orthogonal Frequency Division Multiplexing)-based broadband wireless networks. In the analysis, shadowing factors are taken into account for determining the home BS (Base Station) of each MS (Mobile Station) over multicell environments. Under the consideration, a more accurate analysis of link capacity can be performed compared to Gilhousen's approximation. In the numerical results, it turns out that the actual interference is lower than Gilhousen's approximation with a decrease of around 20% in the interference.

Multiple-input multiple-output (MIMO) systems using eigenbeam space division multiplexing (E-SDM) perform well and have increased capacities compared with those using conventional space division multiplexing (SDM). However, channel state information (CSI) is required at a transmitter, and the performance of E-SDM systems depends much on the accuracy of the CSI at a transmitter and a receiver. In time-varying fading environments, the channel change between the transmit weight determination time and the actual data transmission time causes the system performance to degrade. To compensate for the channel error, a linear extrapolation method has been proposed for a time division duplexing system. Unfortunately, the system performance still deteriorates as the maximum Doppler frequency increases. Here, two new techniques of channel extrapolation are proposed. One is second order extrapolation, and the other is exponential extrapolation. Also, we propose maximum Doppler frequency estimation methods for exponential extrapolation. Simulation results for 4tx 4rx MIMO systems showed that using the proposed techniques, E-SDM system performs better in a higher Doppler frequency region.

We have developed a code-division-multiplexing (CDM) transmission scheme for future road-vehicle communication systems, which uses cyclic shifted-and-extended (CSE) codes generated from a basic code with superior auto-correlation characteristics. This paper proposes to use a Quasi-Orthogonal (QO) sequence as the basic code. Its auto-correlation values are zero except at zero and middle shifts. When the CDM transmission is performed by the CSE codes based on the QO sequence, a desired correlation value is, at a receiver, interfered by the auto-correlation value at middle shift. Therefore, an elimination technique for the interfered correlation value is proposed and realizes zero cross-correlation characteristics within the cyclical shift interval. The new CDM transmission scheme based on the proposed scheme is evaluated by computer simulations in terms of the bit-error-rate performance.

For broadband wireless networks based on OFDM (Orthogonal Frequency Division Multiplexing), an FRF (Frequency Reuse Factor) of 1 has been highly desirable for more improved channel throughput. However, due to the limited power budget of MSs (Mobile Stations) or the increase in ICI (Inter-Cell Interference), a required QoS (Quality of Service) may not be maintained. This paper addresses an optimal LCRTD (Loading Control based on Region-Time Division) over multi-cell environments for an efficient uplink QoS control. In the LCRTD scheme, a cell is divided into several regions by utilizing an optimization approach under QoS constraints, and users in each region are allowed to send their data at the allocated time slots. In the simulation, it is demonstrated that a decrease of 26% in the transmit power can be obtained.

In this paper, we propose a novel spatial filtering technique for orthogonal frequency division multiplexing (OFDM) signals called "VIrtual Subcarrier Assignment (VISA)." Here, virtual subcarrier is a subcarrier which is not used for data transmission. When a wireless terminal is equipped with multiple antennas, VISA can easily achieve a space division multiple access (SDMA) by assigning a different spectral position of virtual subcarrier to a different user. To realize VISA in an already-existing OFDM-based wireless local area network (WLAN), we discuss an antenna weight control method in the preamble of a signal burst format designed for the IEEE802.11a standard and evaluate the bit error rate (BER) performance in typical indoor wireless environments.

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).

In this paper, a multiuser receiver based on a Gaussian Mixture Sigma Point Particle Filter (GMSPPF), which can be used for joint channel coefficient estimation and time delay tracking in CDMA communication systems, is introduced. The proposed algorithm has better improved estimation performance than either Extended Kalman Filter (EKF) or Particle Filter (PF). The Cramer-Rao Lower Bound (CRLB) is derived for the estimator, and the simulation result demonstrates that it is almost completely near-far resistant. For this reason, it is believed that the proposed estimator can replace well-known filters such as the EKF or PF.