This letter deals with adaptive array beamforming based on a minimum variance distortionless response (MVDR) technique with robust capabilities for code-division multiple access signals. It has been shown that the MVDR beamformer suffers from the drawback of being very sensitive to pointing error over the eigenspace-based beamformers. For the purpose of efficient estimation and calibration, a highly efficient approach has been proposed that is implemented on polynomial rooting rather than spectral searching. However, this rooting method is suboptimal in the presence of the noise and multiple access interference (MAI). In this letter, we propose an improved polynomial rooting calibration method that is robust in both of the low signal-to-noise ratio and large MAI scenarios. Several computer simulations are provided for illustrating the effectiveness of the proposed method.

We design M(≥3)-phase spreading sequences of Markov chains optimal in terms of bit error probabilities in asynchronous SSMA (spread spectrum multiple access) communication systems. To this end, we obtain the distributions of the normalized MAI (multiple access interference) for such systems and find a necessary and sufficient condition that the distributions become independent of the phase shifts.

To meet users' multi-service requests under dynamic and heterogenous environment with high-assurance, the Autonomous Network-Based Integration System based on Faded Information Field (FIF) has been proposed, which permits to actively integrate the correlated information services according to the current situation of the system. However, the increase in the total number of users' requests and changes in users' preferences cause the unbalancing load in the system and the overload in the locality. In this paper, based on the autonomous access distribution in the locality, a new approach of autonomous correlated services access is proposed to reduce the load of the system and achieve the adaptability and timeliness of correlated services utilization. We proved the effectiveness of the proposed technology through the simulation and the results show that the system can improve the average response time not only for joint requests of correlated services, but also for separate requests of each service under changing environments.

This paper presents interference suppression using a subband adaptive array (SBAA) for uplink space-time block coding (STBC) code division multiple access (CDMA) under a frequency selective fading (FSF) channel. The proposed scheme utilizes CDMA with STBC and a receive array antenna with SBAA processing at the receiver. The received signal is converted into the frequency domain before despreading and adaptive processing is performed for each subband. A novel SBAA construction is introduced to process CDMA signals based on STBC. To improve the performance of the proposed scheme, we evaluate STBC-SBAA using spreading codes cyclic prefix (CP). Simulation results demonstrate an improved performance of the proposed system for single and multiuser environments compared to competing related techniques.

This paper generalizes parallel error correcting codes proposed by Ahlswede et al. over a new type of multiple access channel called parallel error channel. The generalized parallel error correcting codes can handle with more errors compared with the original ones. We show construction methods of independent and non-independent parallel error correcting codes and decoding methods. We derive some bounds about the size of respective parallel error correcting codes. The obtained results imply a single parallel error correcting code can be constructed by two or more kinds of error correcting codes with distinct error correcting capabilities.

Multiple input multiple output (MIMO) systems represent a very attractive candidate for future high data rate wireless access systems to increase the channel capacity. To obtain a further increase in the channel capacity, a distributed wireless communication system that has multiple access points (APs) and an access controller (AC), was proposed. This system increases not only the peak transmission rate but also the overall transmission rate in the service area. In this paper, we consider a cooperative multiple AP system where APs are synchronized in terms of time, but not in terms of phase. We propose a transmission method for the cooperative multiple AP system that enables the mobile station to obtain a high achievable bit rate regardless of decoding algorithms. The performance of the multiple AP systems is evaluated in comparison to the conventional single AP system. To evaluate the effectiveness of the proposed method in the cooperative multiple AP system, computer simulations are performed using the i.i.d. channel in Rayleigh and Ricean fading environments. Furthermore, we evaluate the performance of the cooperative multiple AP system using the channel measured in an actual indoor environment.

In orthogonal frequency-division multiplex access (OFDMA) downlink systems, the carrier-frequency offset (CFO) between the multiple transceivers introduces inter-carrier interference (ICI). In this letter, we propose an iterative precoding scheme to suppress the ICI due to CFO. This scheme is applied at the transmitter, and can jointly cancel the ICI for all the receivers. Moreover, by the studies of the convergence behavior of the iterations, a sufficient condition for the convergence is presented. The theoretical analysis and simulation results both show that this iterative scheme is equivalent to the zero-forcing (ZF) scheme in function, but with much lower complexity.

Multiple access points (APs) are much more likely to be available for stations (STAs) due to the popularity of wireless LANs. The serious issue of how an appropriate AP is selected from those that are available in a wireless LAN therefore arises. We discuss the development of a decentralized architecture for selecting APs, and examine its fundamental characteristics. The proposed architecture should be introduced without adversely affecting the performance of the existing common architecture that is currently being deployed. Therefore, the deployability of our architecture is examined in this respect. Furthermore, the dynamic behavior of the proposed architecture is studied in addition to static characteristics to evaluate its robustness against various dynamic changes in situation due to AP breakdowns and bursty arrivals of STAs. Simulations revealed that the proposed architecture can attain excellent performance in all the cases treated here.

China has experienced fast growth in mobile communications. Now, China is the world largest mobile communication country with about 500 million users. Wide applications of mobile communications are giving strong pull to the research and development on the broadband wireless communication technology to meet the fast growing demand for high speed access into the information infrastructure. This makes the R&D on wireless technology play great role in the Chinese High-Tech program. This paper will review the key project--FuTURE (Future Technology for Universal Radio Environment)--development of the 863 program, which represents the Chinese efforts towards IMT-advance. Taking some works done in the Tsinghua National Laboratory for Information Science and Technology as examples, the paper will show what has been made in China on the broadband wireless technology, including the trial network in Shanghai.

We point out the unstable operation of reverse traffic management in the cdma2000 1xEV-DO system, and propose a new rate control scheme that controls the reverse traffic load more precisely. The proposed scheme is modeled as a multidimensional Markov process and compared with the conventional scheme. The analysis results show that the proposed rate control scheme has a lower overload probability and higher reverse link throughput than the conventional one.

An integration of the access/backbone network is expected to become indispensable in the future. We analyze the current and future optical networks and we describe the promising technologies. GMPLS architecture in backbone networks and WDM PON architecture in access networks will play the most important roles. We overview recent studies on the access/backbone integrated network to achieve guaranteed QoS. We also describe the developed system architecture as a milestone toward the access/backbone integrated network.

Ethical and legal requirements have made accessibility a crucial feature in any information systems. This paper presents a content adaptation framework, based on the MPEG-21 standard, to help low-vision users have better accessibility to visual contents. We first present an overview of MPEG-21 Digital Item Adaptation (DIA) and the low-vision description tool which enables interoperable content adaptation. This description tool lists seven low-vision symptoms, namely loss of fine detail, lack of contrast, central vision loss, peripheral vision loss, hemianopia, light sensitivity, and need of light. Then we propose a systematic contrast-enhancement method to improve the content visibility for low-vision users, focusing on the first two symptoms. The effectiveness of the low-vision description tool and our adaptation framework is verified by some experiments with an adaptation test-bed. The major advantages of the proposed approach include 1) support of a wide range of low-vision conditions, and 2) customized content adaptation to specific characteristics of each user.

We propose an effective subcarrier allocation scheme for multiuser orthogonal frequency division multiple access (OFDMA) system in the downlink transmission with low computational complexity. In the proposed scheme, by taking multiple attributes of a user's channel, such as carrier gain decrease rate and variation from the mean channel gain of the system, to determine a rank for the user, subcarriers are then allocated depending on the individual user's rank. Different channel characteristics are used to better understand a user's need for subcarriers and hence determine a priority for the user. We also adopt an attribute weighing scheme to enhance the performance of the proposed scheme. The scheme is computationally efficient, since it avoids using iterations for the algorithm convergence and also common water-filling calculations that become more complex with increasing system parameters. Low complexity is achieved by allocating subcarriers to users depending on their determined rank. Our proposed scheme is simulated in comparison with other mathematically efficient subcarrier allocation schemes as well as with a conventional greedy allocation scheme. It is shown that the proposed method demonstrates competitive results with the simulated schemes.

This study employs secret codes and secret keys based on the elliptic curve to construct an elliptic curve cryptosystem with a dynamic access control system. Consequently, the storage space needed for the secret key generated by an elliptic curve dynamic access control system is smaller than that needed for the secret key generated by exponential operation built on the secure filter (SF) dynamic access control system. Using the elliptic curve to encrypt/decrypt on the secure filter improves the efficiency and security of using exponential operation on the secure filter in the dynamic access control system. With the proposed dynamic elliptic curve access control system, the trusted central authority (CA) can add/delete classes and relationships and change the secret keys at any time to achieve an efficient control and management. Furthermore, different possible attacks are used to analyze the security risks. Since attackers can only obtain the general equations for the elliptic curve dynamic access control system, they are unable to effectively perform an elliptic curve polynomial (ECP) conversion, or to solve the elliptic curve discrete logarithm problem (ECDLP). Thus, the proposed elliptic curve dynamic access control system is secure.

A method for flexibly allocating and reallocating optical access network (OAN) resources, including fibers and equipment, using the constraint satisfaction problem (CSP) is described. OAN resource allocation during service delivery provisioning involves various input conditions and allocation sequences, so an OAN resource allocation method has to support various workflow patterns. Furthermore, exception processing, such as reallocating OAN resources once they are allocated, is inevitable, especially during the spread of service using optical fiber and during the deployment of an optical access network. However, it is almost impossible to describe all workflow patterns including exception processes. Improving the efficiency of these exception processes, as well as that of the typical processes, is important for reducing the service delivery time. Describing all these patterns and process flows increases development cost. The CSP can be used to search for solutions without having to fix the process sequence and input conditions beforehand. We have formulated the conditions for OAN resource allocation and reallocation as a CSP. Use of this method makes it possible to handle various allocation workflow patterns including exception processes. Evaluation of the solution search time demonstrated its feasibility.

In orthogonal frequency-division multiplex access (OFDMA) uplink, the carrier-frequency offsets (CFOs) between the multiple transmitters and the receiver introduce inter-carrier interference (ICI) and severely degrade the performance. In this paper, based on the perfect estimation of each user's CFO, we propose two low-complexity iterative algorithms to cancel ICI due to CFOs, which are denoted as the basic algorithm and the improved algorithm with decision-feedback equalization (DFE), respectively. For the basic one, two theorems are proposed that yield a sufficient condition for the convergence of iterations. Moreover, the interference-power-evolution (IPE) charts are proposed to evaluate the convergence behavior of this interference cancellation algorithm. Motivated by the IPE chart, the procedure of DFE is introduced into the iterations, which is the basic idea of the improved algorithm. For this improved algorithm, the error-propagation effect are analyzed and suppressed by an efficient stopping criterion. From IPE charts and simulation results, it can be easily observed that the basic algorithm has the same capability of ICI cancellation as the linear optimal minimum mean square error (MMSE) method, but offers lower complexity, while the improved algorithm with DFE outperforms the MMSE method in terms of the bit-error rate (BER) performance.

Internet access traffic follows hourly patterns that depend on various factors, such as the periods users stay on-line at the access point (e.g. at home or in the office) or their preferences for applications. The clustering of Internet users may provide important information for traffic engineering and billing. For example, it can be used to set up service differentiation according to hourly behavior, resource optimization based on multi-hour routing and definition of tariffs that promote Internet access in low busy hours. In this work, we propose a methodology for clustering Internet users with similar patterns of Internet utilization, according to their hourly traffic utilization. The methodology resorts to three statistical multivariate analysis techniques: cluster analysis, principal component analysis and discriminant analysis. The methodology is illustrated through measured data from two distinct ISPs, one using a CATV access network and the other an ADSL one, offering distinct traffic contracts. Principal component analysis is used as an exploratory tool. Cluster analysis is used to identify the relevant Internet usage profiles, with the partitioning around medoids and Ward's method being the preferred clustering methods. For the two data sets, these methods lead to the choice of 3 clusters with different hourly traffic utilization profiles. The cluster structure is validated through discriminant analysis. It is also evaluated in terms of several characteristics of the user traffic not used in the cluster analysis, such as the type of applications, the amount of downloaded traffic, the activity duration and the transfer rate, resulting in coherent outcomes.

The performance of the blind multiuser detector for a DS-CDMA system with linearly constrained constant modulus (LCCM) criterion is known to highly depend on the exact knowledge of the desired user amplitude; it is usually not available at receiver end. In this paper, we propose a novel LC adaptive CM RLS (LC-ACM-RLS) algorithm to adaptively implement the optimal solution of the LCCM receiver, and to track the desired user's amplitude, simultaneously. From computer simulations, we verify the superiority of the new proposed algorithm over the conventional LCCM-RLS algorithm for multiple access interference (MAI) suppression. Also, for time-varying channel during the adaptation processes, if the amplitude of desired user is not available and varies with time, such as hand-off and Rayleigh fading environments, we show that the proposed LC-ACM-RLS algorithm has better tracking capability compared with the conventional approaches.

This letter yields a security evaluation of certain broadcast encryption (BE) schemes regarding the generic vulnerability of the textbook BE schemes. The considered vulnerability can be effectively explored assuming known plaintext attacks which in a realistic scenario, corresponding to a legitimate user being the attacker, appears as a ciphertext only attack. Employing the birthday paradox, a dedicated time-data trade-off based algorithm for cryptanalysis is proposed. The developed algorithm is applied to cryptanalysis of particular recently reported class of BE schemes, implying additional insights regarding motivations for their security improvements.

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.