Increase System Capacity Research Articles

A Reduced-Complexity Maximum-Likelihood Method for Multiuser Detection

Multiuser or joint detection has recently been receiving significant research interest because of its potential for the significant increase in system capacity and performance. Among the conventional multiuser detection schemes such as the minimum mean-squared error and successive interference cancellation-based ones, maximum-likelihood multiuser detection has the best system performance. However, the complexity of maximum-likelihood detection (MLD) increases exponentially in the number of users and constellation size. In this paper, a low-complexity MLD scheme based on the use of a sensitive-bits algorithm is proposed. It is demonstrated that the proposed method can greatly reduce the computational complexity with a minimal penalty in performance compared with the exhaustive optimal MLD scheme.

Optimisation of radio coverage in urban microcells using a UTD based ray-tracing model

In cellular communications, requirements for increased system capacity, improved coverage, better quality and reduced transmitter power, along with the need to rationalise cellular system planning, have prompted a considerable interest in the development of valid and efficient radiowave propagation models. This work utilises recent advances that improve the accuracy and efficiency of ray-tracing models, using a three-dimensional representation of the terrain and urban topographies, in conjunction with adaptive and selective ray-tracing. These improvements allow utilisation of the field computation algorithms in the network design and radio coverage planning processes. They include optimum automated selection of base station locations and phased array excitation. A reliable ray-tracing scheme can be utilised directly in the optimum and, potentially, adaptive beamforming of base station antenna systems by interfacing field computations with design aims. The paper addresses a cellular planning process, which involves determination of the antenna array position and beam shape or, equivalently, the array excitation coefficients, for a desired power distribution over the defined cell area.

Downlink radio resource allocation for multibeam satellite communications

To increase system capacity and allow downlink beams to share orthogonal radio resources, a synchronous transmission for downlink signals in a multibeam satellite communication system is proposed. Also presented is an optimum radio resource allocation for packet transmission in the synchronous multibeam system.

Identification modale sous excitation ambiante : Application à la surveillance des ponts

Vibration measurements on bridges are not recent and many studies were carried out in the past by the LCPC and many French or foreign agencies or companies. What is new today, it is the increase of the number of dynamic tests carried out on bridges for a better assessment of their structural behaviour. The prime objective in a dynamic assessment is the determination – or identification – of the modal characteristics. That includes natural frequencies, damping ratios and mode shapes. The use of tests under ambient vibration is increasingly popular today because they make it possible to measure the structural response in service. The increase in the data acquisition system capacity and data storage also largely supported the development of tests under ambient vibration. The excitation can be induced by wind, pedestrians or vehicles traffic or any other live loads. In this case, identification methods which do not require measurement of the excitation source must be used. After pointing out the objectives and the stakes of bridge dynamic testing, this paper highlights some of these methods and provides some practical applications.

Layered space-time multiuser detection over wireless uplink systems

In this paper, we investigate the use of layered space-time (also known as the vertical Bell Laboratories layered space-time (V-BLAST) scheme) for multiuser detection in fading channels. The multiple transmit antennas in V-BLAST are treated as individual mobile station transmitters, while the base station consists of multiple receive antennas. In the proposed system, users are organized in groups and allocated a unique spreading code within the same group. Using these codes, we are able to separate the different groups, and layered space-time algorithm is then invoked to further remove the remaining interference between users. A decorrelator-type receiver-based layered space-time detection is proposed for both complex and real constellations. For the latter case, we derive our receiver after evaluating and comparing the performance of two decorrelators based on the V-BLAST scheme. It is demonstrated that a significant performance improvement and increase in system capacity is obtained with very low spreading factors. Further results are also introduced by considering reduced complexity receivers based on serial layered space-time group multiuser detection, and parallel layered space-time group multiuser detection.

A method for increasing downlink capacity by coded multiuser transmission with a base station antenna array

In this paper, a simple structure is proposed for accommodating reuse-within-cell in the downlink of a time-division multiple-access system using a transmit antenna array at the base station and joint detection of the cochannel signals at the single-antenna mobile receivers. The transmit array is used solely for system capacity increase; diversity is obtained in the traditional way through standard coding and interleaving. Due to the spatial formatting of the coded symbols prior to transmission, a single multiantenna channel usage spans several trellis transitions - an unconventional situation which is handled through a trellis. Using the merged trellis, a combined joint detector/soft-decision decoder is constructed, and an analytical expression is derived for the coded bit-error rate based on the exact computation of pairwise error probabilities. It is shown that while the system suffers a reduction in diversity order as the number of intracell cochannel users increases, the reduction may be countered by increasing the constraint length of the code. Although this increases complexity, it is shown that multiple users may be supported, all enjoying good performance at low signal-to-noise ratios, while keeping the complexity in check.

Optimal bandwidth allocation for bandwidth adaptation in wireless multimedia networks

The concept of bandwidth adaptation which additionally allocates the terminated bandwidth to ongoing calls is one of the promising methods to reduce the new call blocking and handoff call dropping probabilities. In this paper, we formulate the bandwidth allocation problem for the bandwidth adaptation as a binary linear integer program which maximizes the satisfaction degree of customers. Since this optimization should be performed in real time, we presented a computationally efficient heuristic algorithm for the problem based on the Lagrangean relaxation procedure. Our simulation test shows that the algorithm finds a sub-optimal solution within 0.5% in average from the true optimal solution. The solutions of our scheme show better performance than other scheme in both handoff call dropping and new call blocking probabilities. Scope and purpose With the emergence of various multimedia applications, it becomes more important that wireless networks satisfy quality of service (QoS) requirements. Each base station has a specific amount of bandwidth to be used within a small geographical area called cell. As the number of users increases, the amount of bandwidth assigned to a cell becomes insufficient to support the required number of users. Therefore, wireless networks will employ smaller cells to perform channel reuse in the cells at specific distance and to accommodate the overall increase in system capacity. Since employing smaller cells implies higher rate of handoff, the continuous support of QoS guarantees of multimedia calls poses a major challenge in the wireless networks. Such a support requires the development of an adaptable network resource management framework. In this paper, we deal with the multimedia adaptive service where the bandwidth of an ongoing call can be varied during its lifetime. When an ongoing call is completed or is handed off from a current cell to another, its bandwidth can be reallocated to other ongoing calls. The concept of this bandwidth adaptation is one of the promising methods to reduce the new call blocking and handoff call dropping probabilities. If the bandwidth is optimally allocated among all ongoing calls which need more bandwidth, then the satisfaction grade of the customers can be increased. We formulate the bandwidth allocation problem for this bandwidth adaptation as a binary linear integer program which maximizes the satisfaction degree of customers.

Modeling and performance analysis of resource allocation strategies for real-time services in UMTS using TIPPtool

The third generation UMTS system will be able to offer a wide variety of multimedia services (speech, teleconferencing, browsing on the net, etc.). In order to allocate resources to these heterogeneous traffic sources in an efficient way, shared channels have been introduced in addition to the traditional transfer on dedicated channels. Although initially conceived for not real-time packet users, shared channels can satisfy the performance requirements of real-time traffic, thanks to the short and efficient channel reservation signaling phase. This paper compares the performance of the radio resource allocation strategies based on dedicated and shared channels, and quantifies the increase in system capacity of an hybrid solution, where shared channels are used to convey the bursty low priority component of a real-time video service. Since the low priority packets that cannot be delivered are dropped, a specific minimum amount of shared resources is required to match the global quality of service requirements of the video service. Modeling and numerical analysis are carried out with the TIPPtool package. With its formal and effective notation and the provision of stochastic features, a compact system specification can be obtained and validated for proper behavior, and performance measures can be derived. In order to numerically evaluate the huge state-space of the complex system under study, a compositional approach is proposed: the system is decomposed into simpler components, which are analyzed separately and whose numerical results are associated for global performance evaluation.

Forward-link performance of satellite CDMA with linear interference suppression and one-step power control

Wideband direct-sequence (DS)-code-division multiple-access (CDMA) is a strong candidate for both terrestrial and satellite components of UMTS. The forward-link capacity of a satellite DS-CDMA system with a conventional matched filter (MF) receiver is limited by interference from adjacent beams and possibly overlapping beams from multiple satellites. In this paper, we study the performance of the linear minimum mean squared error (MMSE) receiver for the satellite forward link. System constraints are long propagation delay, which prevents accurate closed-loop power control, and low on-board power consumption, which implies a low received bit energy to noise density ratio at the mobile receiver. We consider a one-step power adjustment algorithm which attempts to compensate for random shadowing and path loss, and compare the associated performance of the MMSE and MF receivers. Dual-satellite diversity is also considered. The effect of code rate on performance is studied through the use of punctured convolutional codes and the evaluation of random coding bounds. Our results indicate that linear MMSE interference suppression can improve the quality of service and increase system capacity significantly.

Wireless simulation and self-organizing spectrum management

Wireless service providers are continually looking for new features and products to improve quality of service, increase system capacity, and reduce administrative overhead. The simulation tool W provides a flexible platform for the exploration of a broad range of system-level design and performance issues in wireless networks. Investigation of network issues using W gives Lucent Technologies the ability to design new features and products more effectively by reducing development intervals and implementing a one-pass design on field-quality products. We describe the scope of the simulation platform and illustrate the specific use of the W tool for examination of a next-generation channel assignment algorithm currently under development. The algorithm's adaptive nature provides automatic configuration at system initialization, as well as adaptation to system expansion and traffic patterns with spatial or temporal variations, thus ensuring ease of operation for service providers. Initial simulation experiments confirm its self-organizing capability and suggest a significant improvement in quality of service in terms of call blocking and dropping.

A low-complexity combined antenna array and interference cancellation DS-CDMA receiver in multipath fading channels

A simple direct sequence-code division multiple access receiver that combines adaptive beamforming with parallel interference cancellation in a multipath fading channel is proposed and analyzed. A fast adaptation, conjugate gradient algorithm is used to find the optimum beamformer weights. By beamforming, the desired user's signal is enhanced and the cochannel interference from other directions is reduced. For in-beam multiple access interference reduction, a parallel interference canceller is used in each RAKE finger. In the demodulation process, we propose a new demodulation method in which the incoming signal is correlated with the effecting spreading code rather than the physical spreading code called the effective matched filter. A new combining method called equivalent maximal ratio combining is also proposed and analyzed. The average uncoded bit error rate as a function of the average antenna signal-to-noise ratio and the number of receiving antennas is examined in a frequency selective Rayleigh fading channel for all proposed receiver structures. Both simulation and analysis show an increase in system capacity as a function of the number of antennas and the number of interferers canceled per finger.

Optimizing time and space MIMO antenna system for frequency selective fading channels

Smart or adaptive antennas promise to provide significant increases in system capacity and performance in wireless communication systems. In this paper, we investigate the use of adaptive antennas at the base and mobile stations, operating jointly, to maximize the average signal-to-interference and noise ratio (SINR) of each packet in the system for frequency selective channels with prior knowledge of the channel at the transmitter. Our approach is based on deriving an analytic formula for the average packet SINR and using the Lagrange multiplier method to determine an optimum. We derive necessary conditions for an optimum solution and propose an analytical expression for the optimum. Our analytical expression is not guaranteed to be the global optimum but it does satisfy the derived necessary conditions and, in addition for frequency flat channels, our results reduce to expressions for optimal weights previously published. To demonstrate the potential of the proposed system, we provide Monte Carlo simulation results of the system bit-error rates and make comparisons with other adaptive antenna systems. These show that significant improvements in performance are possible in a wireless communications context.

Four-sector cross-shaped urban microcellular systems with intelligent switched-beam antennas

A four-sector cross-shaped urban microcellular system with intelligent switched-beam antennas is proposed. Each sector covers a street block with a base station located at an intersection, and an intelligent beam-switching scheme is used to locate mobile users in the most suitable beam coverage. Due to directional narrow-beam patterns and waveguide effects of tall buildings, radio signals along vertical and horizontal streets do not interfere with each other. Therefore, a channel can be reused simultaneously in multiple neighboring cells as long as cochannels do not encounter each other along the line of sight. The proposed scheme has a channel reuse efficiency of 0.95 for a traffic load of 0.02 [new-call arrivals/s/cell]. The system also increases system capacity more than three times with a blocking probability of 1% and considerably reduces handoff traffic when compared with a conventional cross-shaped microcellular system with an omnidirectional beam pattern.

Dynamic slot allocation (DSA) in indoor SDMA/TDMA using a smart antenna basestation

We introduce and study the use of dynamic slot allocation (DSA) in packet-switched space-division-multiple-access (SDMA) systems. In conventional SDMA, a smart antenna is used at the basestation to simultaneously communicate with multiple stations on the same frequency channel. When dynamic slot allocation is added, the basestation uses uplink channel measurements to intelligently construct future SDMA/TDMA frames. It is shown that under a simple minimum signal-to-interference-plus-noise ratio (SINR) constraint, the problem of performing optimal dynamic slot allocation is NP-complete. Heuristic slot allocation algorithms are introduced which are capable of greatly increasing SDMA/TDMA frame capacity compared with a random allocation of stations. The paper uses both theoretical results and measured data from an experimental testbed to characterize the performance of dynamic slot allocation. The experimental system operates at a carrier frequency of 1.86 GHz and uses an eight-element circular antenna array. It is demonstrated that significant increases in system capacity are possible using DSA in the indoor situations that were tested. Dynamic slot allocation requires the channel to be essentially constant from the time that channel measurements are made until the SDMA/TDMA frame is transmitted. We also present channel measurements which show the effects of channel time coherence in the presence of indoor pedestrian movement. This and other results we have taken suggest that dynamic slot allocation is possible at the frequency considered, provided turnaround times are in the low-to-mid tens of milliseconds.

Maximizing signal strength for OFDM inside buildings

Propagation inside buildings suffer from large shadowing and high multipath effects. This is a serious problem for wireless local area network (WLAN) systems. This paper shows that shadowing and path loss can be minimized by exploiting the multipath tolerance of orthogonal frequency-division multiplexing (OFDM). This can be achieved by using multiple transmission antennas spread over the area of a WLAN cell. These antennas act as repeaters, transmitting and receiving the same signal at the same time. This decreases the average path loss, but increases the multipath delay spread. Using OFDM allows the advantage of reduced path loss to be utilized without detrimental effects of inter-symbol interference caused by the increased delay spread. The reduced path loss allows an increased system capacity, quality of service, or a decrease in intercellular interference in a cellular WLAN.

Joint power control and intracell scheduling of DS-CDMA nonreal time data

The performance of DS-CDMA systems depends on the success in managing interference arising from both intercell and intracell transmissions. Interference management in terms of power control for real time data services like voice has been widely studied and shown to be a crucial component for the functionality of such systems. In this work we consider the problem of supporting downlink nonreal time data services, where in addition to power control, there is also the possibility of controlling the interference by means of transmission scheduling. One such decentralized schedule is to use time division so that users transmit in a one-by-one fashion within each cell. We show that this has merits in terms of saving energy and increasing system capacity. We combine this form of intracell scheduling with a suggested distributed power control algorithm for the intercell interference management. We address its rate of convergence and show that the algorithm converges to a power allocation that supports the nonreal time data users, using the minimum required power while meeting requirements on average data rate. Numerical results indicate a big potential of increased capacity and that a significant amount of energy can be saved with the proposed transmission scheme.

Theoretical model to determine the blocking probability for SDMA systems

Antenna array technology has attracted the attention of the research community as a means to increase system capacity and improve the signal reception. Space division multiple access (SDMA) is a multi-access scheme based on the use of antenna arrays to separate users by exploiting their positions in space. Several works have been carried out to examine the improvement in the system capacity provided by SDMA. A theoretical model to determine the blocking probability for SDMA is derived. A closed-form linear system of equations is obtained whose numerical solution gives the blocking probability. The formulation is employed to assess the capacity gain improvement of a single-cell system under specific conditions. It is found from the results that SDMA is not efficient for low traffic loads, whereas it is so for high traffic.

Benefits and feasibility of the partial loading approach in cellular mobile radio systems

This paper analyzes the feasibility and potential benefits of following a novel design strategy for cellular systems known as partial loading. This strategy seeks low channel occupancy per cell at the capacity limit, as opposed to the high channel occupancy per cell of more conventional design strategies. By doing so, a considerable flexibility for the radio resource management is obtained, which can be exploited both to increase system capacity and to ease the accommodation of highly fluctuating traffic. A comprehensive system capacity analysis has been performed, based on an analytical model, and it is shown that the introduction of flexibility through partial loading has a capacity cost associated with it. However, it is shown that when that flexibility is properly exploited by adaptive radio link control techniques, such as link adaptation, frequency hopping, or call admission control policies, that cost is more than compensated for, making the partial loading approach a very attractive strategy for the evolution of current TDMA systems and the design of future ones.

High reuse efficiency of radio resources in urban microcellular systems

A new microcell configuration scheme in urban areas is proposed in order to increase the reuse efficiency of radio resources. Multiple directional-beam antennas lined up along a street form a cigar-shaped cell. Antennas are located at intersection and are directed along the intersecting streets. Two antennas facing each other form a microzone and, therefore, a cigar-shaped cell consists of multiple microzones in a line. Each microzone is protected against cochannel interference from neighboring microzones by confinement of transmitted signals to a microzone using down-tilting of antenna beams. Therefore, a radio channel which is occupied in one microzone can be used in adjacent microzones. An increase in system capacity is achieved by the high reuse efficiency of radio resources. The proposed system increases system capacity up to 560% with a call blocking probability of 1% when compared with a conventional system. In addition, a macrodiversity scheme using two antennas in a microzone improves the call quality on the reverse link.

Performance of a Multiple-Antenna Multiple-Equalizer System for a CDMA Indoor Wireless System

A multiple-antenna multiple-equalizer (MAME) system for interference suppression in some code-division multiple-access (CDMA) indoor wireless systems is investigated. The MAME system offers enhanced interference suppression capabilities over existing approaches under certain conditions and can thus increase system capacity. The details of the MAME system are discussed and the unique features of this work as compared with many other studies on space-time processing are explained using spectral correlation and diversity domain concepts. The effects of the number of antennas, tap spacing of the equalizers, spectral correlation, and diversity domains are discussed using simulation results. Adaptation performance results are also presented and a recently proposed quasi-Newton algorithm is recommended for use in the MAME system. Decision-directed equalization results are also studied under various conditions and bit-error rate results are presented. Simulation results illustrate that the MAME system can offer enhanced interference suppression capabilities in CDMA indoor wireless systems.