Increase System Capacity Research Articles

Low Complexity Antenna Selection Algorithms for downlink Distributed MIMO Systems

Distributed Multiple Input Multiple Output system (D-MIMO) becomes an attractive topic for increasing system capacity more efficiently. In this paper, we propose a novel two-step antenna selection algorithm f or downlink D- MIMO systems. First, the distributed antenna cluster is selected based on maximum path fading, and then the antennas in the selected antenna cluster are chosen utilizing the permutation and QR decomposition (IC-PQRD) method for V-BLAST systems with Ordered Successive Interference Cancellation detection (OSIC) . The analysis and simulation results show that the proposed algorithm could greatly enhance the average channel capacity of the downlink distributed antenna systems. Furthermore, in the edge region of a cell, both the system capacity and symbol error probability (SER) are significantly improved due to the antenna cluster selection process. For in-cluster antenna selection in the second step, the IC-PQRD algorithm could largely improve the system outage capacity with low computational complexity and low SER.

Adaptive Equalizers in Smart Antenna Systems

The massive expansion of mobile communications over the recent years has meant that telecommunications companies continually need to increase the capacity and coverage of their networks to keep up with demand. This has required more base station antennas to be erected, resulting in increased costs and visual pollution. Smart antenna systems have gained increased interest as they promise to provide significant increases in system capacity and performance, and greater coverage, meaning less base stations are needed to cover the same area compared to conventional antennas. The increase in demand for fast data transmission rates in today’s society leads to greater inter-symbol interference (ISI) in the received signal, due to multipath propagation. Adaptive equalizers can be used to periodically estimate the communication channel and then perform equalization to reduce the effects of ISI.Smart antennas have promised to provide significant increases in system capacity and performance in wireless communication systems[1]. In turn, this leads to increased revenue for the telecommunications companies and also a reduction in dropped and blocked calls. Other benefits include greater coverage, meaning less base stations are needed to cover the same area compared to conventional antennas. For these reasons, smart antennas have gained greater interest over the recent years

An Improved Power Estimation for Mobile Satellite Communication Systems

In this paper, in order to increase system capacity and reduce the transmitting power of the user's equipment, we propose a efficient power estimation algorithm consisting of a modified open-loop power control (OLPC) and closed-loop power control (CLPC) for mobile satellite communications systems. The improved CLPC scheme, combining delay compensation algorithms and pilot diversity, is mainly applied to the ancillary ter-restrial component (ATC). ATC link in urban areas, because it is more suitable to the short round-trip delay (RTD). In the case of rural areas, where ATCs are not deployed or where a signal is not received from ATCs, transmit power monitoring equipment and OLPC schemes using efficient pilot diversity are combined and ap-plied to the link between the user's equipment and the satellite. Two modified power control schemes are ap-plied equally to the boundary areas where two kinds of signals are received in order to ensure coverage conti-nuity. Simulation results show that the improved power control scheme has good performance compared to conventional power control schemes in a geostationary earth orbit (GEO) satellite system utilizing ATCs.

Code Rate-Diversity-Multiplexing Tradeoff

Multiple antenna systems can be used to increase system reliability or to increase system capacity. Initially, space-time codes were designed to achieve one of these two types of gain. Recently, though, a tradeoff between these two system resources has been characterized by the diversity-multiplexing tradeoff. Achieving this optimal performance frontier requires proper coding. For diversity optimality, the signal transmitted from each antenna must redundantly describe the message bits. This redundancy has been quantified by the rate of a space-time code, which relates space-time codebook size to constituent single-input-single-output (SISO) constellation size. Achievable diversity has also been shown to decrease with increasing rates, which establishes the diversity-rate tradeoff. In this work, we consider a generalized notion of the rate of the space-time code, which we refer to as the code rate Rc, and the associated diversity-code rate tradeoff. We then generalize the diversity-multiplexing and diversity-code rate tradeoffs and find that a new diversity-multiplexing tradeoff exists as a function of the code rate.

On the sum-rate scaling law of downlink MU-MIMO decomposition transmission with fixed quality imperfect CSIT

In recent years, downlink multi-user multiple-input multiple-output (MU-MIMO) transmission techniques have attracted much attention because of their potential to significantly increase system capacity compared to single-user (SU) transmission. Unfortunately, accurate channel state information at the transmitter (CSIT) is crucial for these techniques to efficiently separate multiple users in the space domain so as to realize their capacity gain. In practice however accurate CSIT is difficult to obtain. In this letter we investigate the impact of imperfect CSIT on the system performance in terms of sum-rate scaling law for systems employing MU-MIMO decomposition scheme for downlink transmission. In particular, we derive the sum-rate scaling factors for various system configurations and find that imperfect CSIT has different effects on different categories of system configurations. Simulation results demonstrate the accuracy of our analysis.

An Innovative Receiver for Incoherent SAC-OCDMA Enabling SOA-Based Noise Cleaning: Experimental Validation

We propose a new low complexity receiver for spectral amplitude optical coded division multiple access (SAC OCDMA) that enables intensity noise reduction using semiconductor optical amplifiers (SOAs). Compared to the standard receiver requiring two optical filters at the receiver side, our receiver requires only one optical filter. While a 1.4-dB power penalty in incurred, network capacity is unchanged, i.e., BER floors due to intensity noise have the same level. The primary motivation for the low complexity receiver is not reduced component count, but rather modifying the receiver so that promising SOA noise mitigation techniques might be employed to increase system capacity. SOA noise cleaning suffers from a major limitation: filtering after the SOA can negate most of the signal enhancement, the so-called post SOA filtering issue. The only solution to date for the post-SOA filtering effect in SAC-OCDMA is prohibitively complex McCoy , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J.</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Lightw.</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Technol.</i> , vol. 25, no. 1, pp. 394-401, Jan. 2007, i.e., requiring multiple SOAs per client. We demonstrate that our proposed receiver drastically limits the client side filtering, thus maintaining noise suppression and overcoming the post-SOA filtering effect. We compare BER at up to 10 Gb/s with and without noise cleaning. When a noise cleaning module is used, BER improvement of several orders-of-magnitude is observed when only a few users are active in the network. Examination of the noise properties, however, leads us to conclude that highly populated networks will have diminished improvement.

Modified Power Control Algorithms for Mobile Satellite Communication Systems with ATCs

In this paper, in order to increase system capacity and reduce the transmitting power of the user's equipment, we propose a modified power control scheme consisting of a modified closed-loop power control (CLPC) and open-loop power control (OLPC). The modified CLPC algorithm, combining delay compensation algorithms and pilot diversity, is mainly applied to the ancillary terrestrial component (ATC) link in urban areas, because it is more suitable to the short round-trip delay (RTD). In the case of rural areas, where ATCs are not deployed or where a signal is not received from ATCs, transmit power monitoring equipment and OLPC algorithms using efficient pilot diversity are combined and applied to the link between the user's equipment and the satellite. Two power control algorithms are applied equally to the boundary areas where two kinds of signals are received in order to ensure coverage continuity. Simulation results show that the modified power control scheme has good performance compared to conventional power control schemes in a geostationary earth orbit (GEO) satellite system utilizing ATCs.

Coherent detection associated with digital signal processing for fiber optics communication

Abstract Optical transmission systems have seen their performance (system capacity and system reach) drastically improved in the last 15 years by using new technologies such as wide band optical amplification, wavelength division multiplexing (WDM), dispersion management, introduction of 10 Gbit/s channel rate and now 40 Gbit/s, Differential Phase Shift Keying (DPSK) modulation… . In order to further increase system capacity while relaxing the constraints associated with higher channel rate (i.e. 100 Gbit/s), the association of multilevel modulation format with coherent detection and digital signal processing appears as a key enabler. This article describes the actual modulation and detection methods in order to explain how a coherent receiver including digital signal processing works. The efficiency of coherent receiver to compensate for linear distortions induced by fiber optic propagation will also be detailed. The promising association of coherent receiver with multilevel modulation format, especially for 100 Gbit/s channel rate will be described. Eventually, the limitations of optical communication systems relying on coherent detection will be explained. To cite this article: G. Charlet, C. R. Physique 9 (2008).

User Cooperation in TDMA Wireless System

Reliability of radio link is limited, owing to path loss, shadowing and multi-path fading. This necessitates the use of a certain type of diversity. In recent years, cooperative diversity has gained considerable attention. Here, wireless nodes cooperate in such a way that they share their antennas and other resources, to create a virtual array through distributed transmission and signal processing. This increases coverage and reduces transmitted power, thereby bringing down co-channel interference, which results in increased system capacity. This paper gives an overview of the state of art of various cooperation schemes and issues related to their implementation.

Model Predictive Control of Parallel-Connected Inverters for Uninterruptible Power Supplies

Uninterruptible power supplies (UPSs) have been used in many installations for critical loads that cannot afford power failure or surge during operation. It is often difficult to upgrade the UPS system as the load grows over time. Due to lower cost and maintenance, as well as ease of increasing system capacity, the parallel operation of modularized small-power UPS has attracted much attention in recent years. In this paper, a new scheme for parallel operation of inverters is introduced. A multiple-input-multiple-output state-space model is developed to describe the parallel-connected inverters system, and a model-predictive-control scheme suitable for paralleled inverters control is proposed. In this algorithm, the control objectives of voltage tracking and current sharing are formulated using a weighted cost function. The effectiveness and the hot-swap capability of the proposed parallel-connected inverters system have been verified with experimental results.

On multicell cooperative transmission in backhaul-constrained cellular systems

Recent work has shown that multicell cooperative signal processing in cellular networks can significantly increase system capacity and fairness. For example, multicell joint transmission and joint detection can be performed to combat intercell interference, often mentioned in the context of distributed antenna systems. Most publications in this field assume that an infinite amount of information can be exchanged between the cooperating base stations, neglecting the main downside of such systems, namely, the need for an additional network backhaul. In recent publications, we have thus proposed an optimization framework and algorithm that applies multicell signal processing to only a carefully selected subset of users for cellular systems with a strongly constrained backhaul. In this paper, we consider the cellular downlink and provide a comprehensive summary and extension of our previous and current work. We compare the performance obtained through centralized or decentralized optimization approaches, or through optimal or suboptimal calculation of precoding matrices, and identify reasonable performance–complexity trade-offs. It is shown that even low-complexity optimization approaches for cellular systems with a strongly constrained backhaul can yield major performance improvements over conventional systems.

WiMAX throughput evaluation of conventional relaying

The paper analyses performance of IEEE 802.16 standard, also known as WiMAX, when relay stations are introduced. Thanks to this feature, coverage area and throughput of the system may be significantly enhanced. The article defines the increase of system capacity of the relay based cell deployment compared to a conventional single hop deployment of the same area size. Further, the parameters that have main impact on the system with relays and its capacity are determined.

The impact of space division multiplexing on resource allocation: a unified treatment of TDMA, OFDMA and CDMA

Space division multiple access (SDMA) with an antenna array at the transmitter is a promising means for increasing system capacity and supporting rate-demanding services. However, the presence of an antenna array at the physical layer raises significant issues at higher layers. In this paper, we attempt to capture the impact of SDMA on access layer channel allocation, reflected on channel reuse. This impact obtains different twists in TDMA, CDMA and OFDMA due to the different nature of co-channel and cross-channel interference and the different interaction of user spatial channel characteristics with system channels, namely time slots, codes and subcarriers. We consider these access schemes in a generalized unified framework and propose heuristic algorithms for channel allocation, downlink beamforming and transmit power control so as to increase total provisioned system rate and provide QoS to users in the form of minimum rate guarantees. We study the class of greedy algorithms that rely on criteria such as induced or received interference and signal-to-interference ratio (SIR), and a class of SIR balancing algorithms. Results show superior performance for SIR balancing resource allocation and expose the performance benefits of cross-layer design.

A topology control-based self-organisation in wireless mesh networks

An algorithm for self-organisation that assigns the channels intelligently in multi-radio wireless mesh networks (MR-WMN) is important for the proper operation of MR-WMN. The aim of the self-organisation algorithm is to reduce the overall interference and increase the aggregate capacity of the network. In this paper, we have first proposed a generic self-organisation algorithm that addresses these two challenges. The basic approach is that of a distributed, light-weight, cooperative multiagent system that guarantees scalability. Second, we have evaluated the performance of the proposed self-organisation algorithm for two sets of initialisation schemes. The initialisation process results in a topology control of MR-WMN by way of spatial distribution of connectivity between the mesh nodes. The results have been obtained for realistic scenarios of MR-WMN node densities and topologies. We have shown in addition the need to develop non-transmit power control based algorithms to achieve a further increase in system capacity.

Hybrid Reduced-Complexity Multiuser Detector for CDMA Communication Systems

In a code-division multiple-access (CDMA) system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase system capacity. However, the optimum MUD for CDMA systems requires the solution of an NP-hard combinatorial optimization problem. It is well known that the computational complexity of the optimum MUD is exponential with the number of active users in the system. In this paper, we apply a hybrid algorithm to develop a suboptimal MUD strategy. The result of the symmetric-successive-overrelaxation-preconditioned conjugate-gradient (SSOR-CG) method is first used to initialize the reduced-complexity recursive (RCR) MUD. Then, the RCR algorithm is applied to detect the received data bit by optimizing the likelihood function. Simulation results for the synchronous and asynchronous case show that the performance of our proposed SSOR-RCR MUD is promising and outperforms the decorrelator, linear minimum mean-squared error, and original RCR MUD.

MU-MISO Transmission with Limited Feedback

Downlink multi-user transmission techniques that allow several users to be served simultaneously in frequency and time have been recognized as a promising means to increase system capacity. However they require accurate channel state information (CSI) at the base station (BS) such that appropriate signal processing can be performed to separate multiple users in the space domain. In practice however, CSI cannot be perfectly known to the BS due to the capacity-limited feedback channel in FDD systems for instance. In this letter we investigate multiuser multiple-input single-output (MU-MISO) transmission with limited feedback. In particular we identify the useful information that the BS needs to deal with inter-user interference and a codebook design is derived accordingly. In addition, an adaptive threshold-based feedback approach is proposed, where only users with good feedback quality are allowed to do feedback. It is shown that the adaptive scheme can effectively improve the system performance given a fixed number of feedback bits.

Reduced-Rank Adaptive Filtering Using Localized Processing for CDMA Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> An integrated beamforming (spatial processing) and multiuser-detection (temporal processing) scheme is an effective approach to increase system capacity but is also impractical due to the high associated computational costs. To overcome this problem, researchers have developed reduced-rank approaches. Adding to this class of algorithms, this paper introduces a joint-domain adaptive algorithm, which processes spatial and temporal data within a localized region after transformation to a “beamspace.” This new joint-domain-localized (JDL) adaptive algorithm is developed for single-cell uplink code-division multiple-access (CDMA) systems with a receive-antenna array. Given a rank constraint on the JDL transformation, this paper develops the optimal choice of transformation matrix and justifies a simpler suboptimal choice. The JDL algorithm is shown to have relatively low computation load. We also introduce the JDL-Z algorithm that combines JDL processing with zero forcing for multicell uplink CDMA systems. At the cost of higher computational complexity, this new scheme provides better performance and a faster convergence rate than the JDL algorithm. However, the key contribution is a framework wherein adaptive processing can be followed by yet another stage of adaptive processing. Simulations are used to illustrate the efficacy of the two algorithms. </para>

Particle-Swarm-Optimization-Based Multiuser Detector for CDMA Communications

In a code-division multiple-access system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase system capacity. However, the optimum MUD can be characterized as an NP-hard combinatorial optimization problem such that the computational complexity increases exponentially with the number of users. In this paper, we apply a new evolutionary algorithm, called particle swarm optimization (PSO), to develop a suboptimal MUD strategy. The decorrelating detector (DD) or linear minimum mean square error (LMMSE) detector is used as the first stage to initialize the PSO-based MUD. Then, the PSO algorithm is applied to detect the received data bit by optimizing an objective function incorporating the linear system of the DD or LMMSE detector. Simulation results show that the performance of our proposed decorrelating PSO and LMMSE-PSO MUD are promising and outperform the decorrelating and LMMSE MUD with a slight increase in computation.

Source-Matched Spreading Codes for Optical CDMA

Puncturing is studied as a physical layer mechanism for efficiently transmitting datastreams containing bits of unequal priority via wavelength-time optical code-division multiple access. Puncturing increases system capacity, while ensuring that important bits are received with low bit-error rate

Downlink performance and capacity of distributed antenna systems in a multicell environment

Distributed antenna systems (DAS) have been widely implemented in state-of-the art cellular communication systems to cover dead spots. Recent academic studies have shown that in addition to coverage improvements, DAS can also have potential advantages such as reduced power and increased system capacity in a single cell environment. This paper analytically quantifies downlink capacity of multicell DAS for two different transmission strategies: selection diversity (where just one or two of the distributed antennas are used) and blanket transmission (where all antennas in the cell broadcast data). Simple repeaters are a special case of our analysis. A generalized information theoretic analysis is provided to illuminate the fundamental limits of such systems in the cellular context. The results show that DAS reduces other-cell interference in a multicell environment and hence significantly improves capacity (by about 2x), with particularly large improvements for users near cell boundaries. Less obviously, from a communication theory standpoint, it is shown that selection diversity is preferable to blanket transmission in terms of achievable ergodic capacity. For blanket transmission, we show that the optimal transmission strategy is just phase steering due to the per antenna module power constraints in DAS