On the efficient use of multiple channels by single‐receiver clients in wireless data broadcasting

This letter proposes an adaptive wireless push system for wireless data broadcasting environments, where multiple channels are available for broadcasting data from a broadcast server to a large number of mobile clients. We address the general case where the client demands are not dependent on client locations. The efficiency of the proposed system lies in the fact that it offers significant performance improvements to the system clients with the need of only one receiver at each client device. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimal resource allocation for pilot symbol aided multiuser receivers in Rayleigh faded CDMA channels

We consider a synchronous code-division multiple-access system where each user undergoes independent frequency-flat Rayleigh fading, and where pilot symbols are periodically inserted into the data stream of each user in order to assist in the coherent demodulation of the data symbols. The motivating question for this work is: for any given set of system parameters, how often should we insert pilot symbols? Along the way to answering this question, we: (1) derive and analyze the performance of the linear minimum mean-squared-error channel estimator and (2) study the performance of a linear minimum mean-squared-error data estimator which is coupled to the channel estimator. We are able to obtain a very compact expression for the average signal-to-interference ratio in terms of the key system parameters: pilot insertion period, channel fading rate, signal-to-noise ratio, and the ratio of the number of users to the spreading gain. The average signal-to-interference ratio is numerically optimized and results are presented to illustrate the optimal rate of inserting pilot symbols for a range of system parameters.     

Optimization of Switched MIMO Systems Over Rayleigh Fading Channels

This paper proposes a new switched MIMO system where the MIMO includes both diversity-based MIMO and spatial multiplexing MIMO (SM-MIMO). The proposed scheme is a closed-loop system where the feedback bits are sent back to the transmitter to indicate the channel quality. The switching operation is employed among groups of a certain number of transmit antennas. For diversity-based MIMO systems, the switching operation aims to improve the link performance, while the goal of the switching for the SM-MIMO systems is to increase the data rate. The authors analyze and optimize the proposed systems and show that there is considerable benefit in terms of performance and practical use     

A methodology for predicting service life and design of reliability experiments

This paper summarizes a methodology for reliability prediction of new products where field data are sparse, and the allowed number & length of experiments are limited. The methodology relies on estimating a set where the unknown parameters are most likely to be found, calculation of an upper bound for the reliability metric of interest conditioned that the parameters reside in the estimated set, and tightening the bounds via design of experiments. Models of failure propagation, failure acceleration, system operations, and time/cycle to failure at various levels of fidelity & expert elicited information may be incorporated to enhance the accuracy of the predictions. The application of the model is illustrated through numerical studies.     

Optimum alternatives of tandem G/G/K queues with disaster customers and retrial phenomenon: interactive voice response systems

In this paper, we study a tandem queue with retrials where the queue experiences disasters. The probability of system failure depends on the strength of equipment, which makes servers idle and causes the removal of all customers in queues and service areas at once. The customers in the queue are forced to orbit in a retrial queue during the system failure where they decide whether or not to come back to the system. Reducing the disaster arrival rate (the probability of system failure) by employing more servers and reducing the number of lost customers is very costly. Moreover, it is important to service the customers with no interruption and reduce the time in system. The developed scenarios are compared in five dimensions including time in system, cost of lost customer, operator cost, the number of uninterrupted service customers and cost of reducing disaster arrival rate (or empowering system cost). The scenarios are modeled by computer simulation. Then, the optimal scenario is chosen using data envelopment analysis. The optimal scenario maximizes system efficiency in terms of disaster arrival rate, cost of lost customers and the number of satisfied customers. In the main problem, the disasters arrive at the system according to Poisson process; the effect of changing the distribution function of disaster arrival has been investigated finally. We are among the first ones to study and optimize G/G/K tandem queuing systems with system failures and retrial phenomena in interactive voice response systems.     

Multiuser Detection in a Dynamic Environment– Part I: User Identification and Data Detection

In random-access communication systems, the number of active users varies with time, and has considerable bearing on receiver's performance. Thus, techniques aimed at identifying not only the information transmitted, but also that number, play a central role in those systems. An example of application of these techniques can be found in multiuser detection (MUD). In typical MUD analyses, receivers are based on the assumption that the number of active users is constant and known at the receiver, and coincides with the maximum number of users entitled to access the system. This assumption is often overly pessimistic, since many users might be inactive at any given time, and detection under the assumption of a number of users larger than the real one may impair performance. The main goal of this paper is to introduce a general approach to the problem of identifying active users and estimating their parameters and data in a random-access system where users are continuously entering and leaving the system. The tool whose use we advocate is random-set theory (RST): applying this, we derive optimum receivers in an environment where the set of transmitters comprises an unknown number of elements. In addition, we can derive Bayesian-filter equations which describe the evolution with time of the a posteriori probability (APP) density of the unknown user parameters, and use this density to derive optimum detectors. In this paper, we restrict ourselves to interferer identification and data detection, while in a companion paper we shall examine the more complex problem of estimating users' parameters.     

Asymptotic analysis of LMMSE multiuser receivers for multi-signature multicarrier CDMA in Rayleigh fading

This paper considers a multicarrier (MC) code-division multiple-access system where each user employs multiple signatures. The receiver is linear and minimizes the mean square error of the data estimate. Both multiple-user and single-user systems are considered, as well as single and multiple signatures per user. In each case, an asymptotic analysis is used to derive the output signal-to-interference-plus-noise ratio (SINR) as a function of the system loading, the noise power, and the fading properties of the channel. Asymptotic in this case means that the number of independent subcarriers and number of signatures per user each tends to infinity with fixed ratio. The associated bit-error rate (BER) is evaluated for binary phase-shift keying symbols. Simulations show that the asymptotic SINRs and BERs derived in each case are accurate for realistic finite systems.     

Analysis of new methods for broadcasting digital data to mobileterminals over an FM-channel

Two new methods using an FM-radio channel for transmission of digital data to mobile terminals are examined: 1. A modification of the radio data system (RDS). In RDS, additional digital information is multiplexed with a stereo sound signal. A new system is suggested where the data signal can be multiplexed with a mono audio signal. This causes extension to the bandwidth available for the data signal, and therefore the RDS bitrate can be increased. Error calculations are performed both for the original RDS system and for the new system. 2. Orthogonal frequency division multiplexing (OFDM). OFDM is used in the digital audio broadcasting system (DAB), which is designed to transmit digital audio in the FM band. In OFDM a signal is divided over a large number of 2- or 4-PSK modulated orthogonal subcarriers. The subcarriers of 6 different programmes are multiplexed in one beam to reduce the effects of frequency selectivity of the transmission channel. A new system based on OFDM is proposed, in which the carriers of each programme are transmitted in one FM-channel with a bandwidth of 200 kHz instead of multiplexed with the carriers of other programmes. Error calculations are performed for the subcarriers used in the OFDM modulation method     

Channel reassignment in TDMA systems: single-carrier transmitting,multiple-carrier hop receiving

Channel reassignment schemes are described for a time-division multiple-access (TDMA) system where a single carrier is available on the transmitting side (Tx-SC), while multiple-carriers are hopped on the receiving side (Rx-MCH). The Tx-SC/Rs-MCH TDMA system configuration is presented as being logically equivalent to a three-stage switching network. This equivalence means that rearrangement algorithms for three-stage switching networks proposed to date can be applied to channel reassignment in the Tx-SC/Rx-MCH system. The maximum number of channels that must be reassigned is shown to be M-1, where M is the number of simultaneous carriers     

A multicarrier allocation (MCA) scheme for variable-rate 3Gwirelesssystems

High spectral efficiency and flexible data rate access are the main focus of future wireless networks. Multiple channel allocation schemes have the potential of achieving this goal. By assigning multiple slots and/or multiple carriers to one user, it is possible to provide a flexible data rate with quite low complexity. In this article we propose a simple allocation scheme where each user is assigned a fixed group of carriers. These carriers are adaptively used depending on the interference situation within the system. The system performance in terms of average throughput is investigated for two different types of allocation schemes: a fully centralized scheme and a distributed one that uses frequency diversity as a mean of improving the user link quality. The obtained results show that both schemes improve the system throughput over single carrier allocation without affecting the capacity of the system in terms of number of users per cell     

Performance analysis of a slotted CDMA ALOHA network

ALOHA is a simple and efficient way of allowing many machines with bursty data streams to communicate with a central computer. For cases where machines are equally likely to transmit to one another, CDMA ALOHA, which allows for full connectivity, may be a better multiple access protocol than slotted ALOHA through a central machine. This paper first describes a model for a fully connected, full duplex, and slotted CDMA ALOHA network where the receiver-based code access protocol is used. The stations can send data to, and receive data from, different stations simultaneously. The model is analyzed using discrete-time Markov chain, and some numerical results are presented. For a system with a large number of users where Markov analysis is impractical, equilibrium point analysis is used to predict the stability of the system and determine the throughput as well as the delay performance of the system when it is stable. It is shown that a CDMA slotted ALOHA network has a much better performance compared to simple slotted ALOHA networks.     

Performance evaluation of underwater wireless optical CDMA system for different water types

In this paper, an analytical approach is presented to investigate the bit error rate (BER) performance of underwater wireless optical code division multiple access system for different water types. Optical orthogonal codes (OOC) are used as the user-specific address sequence to encode and decode the user’s data. LED with an operating wavelength of 532 nm is used as the optical source, and silicon avalanche photodiode is used as the optical detector. The system BER performance is analyzed on account of transmission distance, transmitted power, number of simultaneous users, and other system parameters where different water types (i.e., pure sea water, clear ocean water, and coastal ocean water) are considered as the communication channel qualities. It is found that the BER performance of this system significantly depends on the water types. However, the best overall system performance is achieved in pure sea water where clear ocean water provided better results than coastal ocean water.

     

Synchronous data flow

Data flow is a natural paradigm for describing DSP applications for concurrent implementation on parallel hardware. Data flow programs for signal processing are directed graphs where each node represents a function and each arc represents a signal path. Synchronous data flow (SDF) is a special case of data flow (either atomic or large grain) in which the number of data samples produced or consumed by each node on each invocation is specified a priori. Nodes can be scheduled statically (at compile time) onto single or parallel programmable processors so the run-time overhead usually associated with data flow evaporates. Multiple sample rates within the same system are easily and naturally handled. Conditions for correctness of SDF graph are explained and scheduling algorithms are described for homogeneous parallel processors sharing memory. A preliminary SDF software system for automatically generating assembly language code for DSP microcomputers is described. Two new efficiency techniques are introduced, static buffering and an extension to SDF to efficiently implement conditionals.     

Broadcast scheduling for information distribution

Broadcast data delivery is encountered in many applications where there is a need to disseminate information to a large user community in a wireless asymmetric communication environment. In this paper, we consider the problem of scheduling the data broadcast such that average response time experienced by the users is low. In a push-based system, where the users cannot place requests directly to the server and the broadcast schedule should be determined based solely on the access probabilities, we formulate a deterministic dynamic optimization problem, the solution of which provides the optimal broadcast schedule. Properties of the optimal solution are obtained and then we propose a suboptimal dynamic policy which achieves average response time close to the lower bound. The policy has low complexity, it is adaptive to changing access statistics, and is easily generalizable to multiple broadcast channels. In a pull-based system where the users may place requests about information items directly to the server, the scheduling can be based on the number of pending requests for each item. Suboptimal policies with good performance are obtained in this case as well. Finally, it is demonstrated by a numerical study that as the request generation rate increases, the achievable performance of the pull- and push-based systems becomes almost identical.     

Design and Use of an Integrated Cambridge Ring

The Cambridge ring is a local area network used both in universities and industry. It is based on the empty slot principle and data are transmitted using minipackets containing two bytes of data. This paper describes the design process, decisions, and tradeoffs in implementing an integrated system which incorporates both analog and digital components. The technology chosen is a bipolar gate array. A number of options are provided for the implementor who can optimize network parameters such as minipacket size and transmission speed to his needs. He can also choose the style of interface between the communicating device and the network. An important option provided by the integrated Cambridge ring is the ability to simultaneously transmit short control minipackets and long data packets. A system exploiting this feature has been built and is described. Its proposed uses are to interconnect telephones and other real-time systems as well as computers where the partitioning of bandwidth and precise performance specification are important.     

Throughput of strongly interfering slow frequency-hopping networks

With the increasing use of short-range wireless devices for high-data-rate communication in the shared frequency bands, the level of interference can be expected to increase. The ability to estimate the performance degradation of these devices due to increasing interference is, therefore, important. In this paper, the throughput of devices that perform frequency hops after each transmitted packet in order to achieve diversity is investigated. The system model allows for an analysis of systems where packets of variable durations are used, and the throughput derivation is based on the assumption that collisions result in a total loss of the data in the colliding packets. The resulting expression for the throughput is given as a function of the number of frequency channels used for frequency hopping, the number of interfering networks, the durations of the packet types available, and the probability of networks selecting a certain packet type for transmission. An approximation of the exact expression for the throughput is also derived, and the results are applied to an example system consisting of Bluetooth piconets.     

Capacity analysis of reservation-based random access for broadband wireless access networks

In this paper we propose a novel model for the capacity analysis on the reservation-based random multiple access system, which can be applied to the medium access control protocol of the emerging WiMAX technology. In such a wireless broadband access system, in order to support QoS, the channel time is divided into consecutive frames, where each frame consists of some consequent mini-slots for the transmission of requests, used for the bandwidth reservation, and consequent slots for the actual data packet transmission. Three main outcomes are obtained: first, the upper and lower bounds of the capacity are derived for the considered system. Second, we found through the mathematical analysis that the transmission rate of reservationbased multiple access protocol is maximized, when the ratio between the number of mini-slots and that of the slots per frame is equal to the reciprocal of the random multiple access algorithm?s transmission rate. Third, in the case of WiMAX networks with a large number of subscribers, our analysis takes into account both the capacity and the mean packet delay criteria and suggests to keep such a ratio constant and independent of application-level data traffic arrival rate.     

Memory customisations for image processing applications targeting MPSoCs

Multiprocessor System on Chips (MPSoCs) are quickly becoming the mainstay in embedded processing platforms due to their hardware and software design flexibility. This flexibility increases the design space for developers, introducing trade-offs between performance and resource/power consumption. This paper presents a comprehensive evaluation of memory customisations for MPSoCs. Custom arrangements of instruction and data cache are presented to optimise off-chip memory consumption and improve system performance. Off-chip memory management and threading are presented to balance the computational load on available processors and improve system performance. The proposed methods are applied to an object detection case study, where performance increases of up to 2.93x are achieved when compared to standard memory designs. Furthermore, the proposed techniques can increase the number of possible processors in an MPSoC by reducing the number of bus interconnects.     

Laser cross-link systems and technology

Laser communications offer a viable alternative to RF communications for intersatellite links and other applications where high-performance links are a necessity. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communications that offer a number of potential advantages for system design. This article presents an overview and tutorial of laser cross-link systems and technologies     

Partially Loaded Superimposed Training Scheme for Large MIMO Uplink Systems

This paper proposes a new superimposed training (ST) scheme for uplink multi-user multi-cell system, where each base station, equipped with a large number of antennas (M), communicates to single antenna users. In uplink training phase, large number of users within limited coherence time introduces the pilot contamination, which causes two types of interferences in data estimation. The first type, which is referred as self interference, arises due to the dependence between channel estimate and estimation error of the same user, while the second type, known as cross interference, occurs because of the correlation between ST vectors of different users. In this paper, an ST scheme with variable data length is proposed for Rician fading channels. For simplicity of analysis, a single cell model is considered first to derive mean squared error and signal to interference plus noise ratio. The analysis is further extended to multi-cell system. Various limiting cases are investigated, and the design parameters viz., power allocation factor and length of data vector, are optimized. Simulation results verify that the proposed ST scheme reduces self interference, and yields sum rate improvement over conventional ST scheme.