Random Access System Research Articles

Performance analysis of backoff algorithms under unsaturated conditions

AbstractThis paper is concerned with the backoff algorithm for a random access system on the basis of slotted ALOHA. The performance of the algorithm in which the backoff counter is reset after the packet transmission success (conventional scheme), and the performance of the algorithm in which the backoff counter is successively decremented (modified scheme), are analyzed for the unsaturated condition. Based on the concept of equilibrium point analysis, the packet transmission failure probability, the throughput, the average transmission delay, and the uniformity of the transmission delay variation are evaluated. It is shown by a numerical example that the modified scheme exhibits better performance than the conventional scheme, even in the unsaturated condition, as well as in the saturated condition, with appropriate setting of the backoff window width. In addition, the modified scheme is shown to be better from the viewpoint of uniformity, in the sense that the packet transmission delay variation is smaller, even if the parameters are set to values that degrade the throughput. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 88(4): 32–43, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20176

Optimal Power and Retransmission Control Policies for Random Access Systems

We consider in this study dynamic control policies for slotted Aloha random access systems. New performance bounds are derived when random access is combined with power control for system optimization, and we establish the existence of optimal control approaches for such systems. We analyze throughput and delay when the number of backlogged users is known, where we can explicitly obtain optimal policies and analyze their corresponding performance using Markov Decision Process (MDP) theory with average cost criterion. For the realistic unknown-backlog case, we establish the existence of optimal backlog-minimizing policies for the same range of arrival rates as the ideal known-backlog case by using the theory of MDPs with Borel state space and unbounded costs. We also propose suboptimal control policies with performance close to the optimal without sacrificing stability. These policies perform substantially better than existing Certainty Equivalence controllers.

Reference Intervals for Serum Calcitonin in Men, Women, and Children

The strategy currently used to investigate thyroid nodules in most cases involves measurement of calcitonin to exclude the possibility of medullary thyroid cancer (MTC). Techniques used to measure calcitonin have become more reliable, sensitive, and easy to perform. Automated nonisotopic techniques now exist, such as the Advantage® system. We found that some children have particularly high calcitonin. Previous reports also have shown that calcitonin is higher in young children (1)(2)(3)(4)(5)(6)(7)(8)(9), but most of these studies are now outdated and are based on immunoradiometric or even radioimmunologic (RIA) methods, which sometimes involve an extraction step. These techniques are also not very sensitive or specific (interference by procalcitonin). Because most studies addressing this subject investigated specific populations (premature infants, newborns, or hypotrophic or hypocalcemic children), we concluded that it would be useful to determine reference intervals for blood calcitonin in children with the Advantage system. Because the manufacturer (Nichols Institute Diagnostics) found a significant difference between men and women, we also evaluated this finding. The Advantage is an automated multiparametric random-access system that uses an acridinium ester chemiluminescence technique and a magnetic separation step (10)(11). The monoclonal antibodies used recognize the 11–23 (capture monoclonal antibody) and 21–32 (labeled monoclonal antibody) fragments of human calcitonin. …

Precoded Random Spreading Multiple Access System in AWGN Channels

In this paper, we compare the asymptotic performance of the decorrelator approaches to multiuser detection in code-division multiple-access (CDMA). Precoding is the decorrelation which is performed at a common transmitter such as the base station or satellite that allows single-user detection at the handset receiver. Receiver-based decorrelator decorrelates multiple user signals at the receiver side. We analyze the performance of the aforementioned decorrelation approaches with time-varying random spreading sequences in additive white Gaussian noise channels. The results demonstrate that the performance of precoding and that of receiver-based decorrelator are asymptotically equivalent.

Robustness of Q-ary collision resolution algorithms in random access systems

The throughput characteristics of a random access system (RAS) which uses Q-ary tree algorithms (where Q is the number of groups into which colliding users are split) of the Capetanakis–Tsybakov–Mikhailov–Vvedenskaya type are analyzed for an infinite population of identical users generating packets. In the standard model packets are assumed to be generated according to a Poisson process. In this paper we greatly relax this assumption and consider a rich class of Markovian arrival processes, which, in general, are non-renewal. This class of arrival processes is known to lend itself very well to modeling bursty and correlated arrival processes commonly arising in computer and communication applications. Blocked and grouped channel access protocols are considered in combination with Q-ary collision resolution algorithms that exploit either binary (“collision or not”) or ternary (“collision, success or idle”) feedback. For the resulting RASs the corresponding maximum stable throughput is determined. It is concluded that the resulting RASs maintain their good stability characteristics under the wide range of arrival processes considered, thereby further extending the theoretical foundations of tree algorithms.

Performance of CDMA random access systems with packet combining in fading channels

We analyze the system performance of code-division multiple-access (CDMA) random access systems with linear receivers and packet combing in multipath fading channels. Both slotted and unslotted CDMA systems with random spreading codes are considered. The analysis is based on large systems in which both the offered load and the processing gain tend to infinity but their ratio is fixed. It is relatively easy to characterize the traffic in such large systems, which enables us to derive the system throughput and average delay. From the analysis results, it is observed that multiuser detection and packet combining substantially improve the system performance.

Analysis of the priority stack random-access protocol in W-CDMA systems

The use of the stack protocol (also called tree protocol) to introduce a priority mechanism on the random access stage in wideband code-division multiple access (W-CDMA) seems more robust when compared with slotted ALOHA. In fact, the protocol stability in infinite population and the guaranteed low-contention delay appear very attractive for a random-access system proposal. Also, for third-generation networks, defining and offering priorities in wireless communication systems appears to be more important for operators and system users. Indeed, after voice only 2G networks, 3G systems (UMTS) should offer more services with quality and priority. However, all priorities in the UMTS system are based on the dedicated channel, which is offered after the random-access mechanism that uses the weak access protocol, slotted ALOHA. We analyze theoretically the possibility of applying the tree random-access protocol for the W-CDMA part of the UTRA radio interface proposition. We also study a priority system applied for random access directly. This seems to be very useful to propose priority for some applications, especially emergency applications. The analytical model uses generating functions and an algebraic method in order to show the stack protocol performance. Also, numerical and simulation results are presented and show the predominance of this protocol compared with the slotted-ALOHA mechanism.

A one-shot LDD with pre-SDC/post-MRC for random CDMA over asynchronous Rayleigh channels

It is well known that conventional detectors of direct sequence-code division multiple access systems are interference-limited. Multiuser detection has been considered as a promising approach to combat this problem. Since an optimal multiuser detector is too complex to implement, several suboptimal multiuser detector techniques have been developed. The one-shot linear decorrelating detector (LDD) was proposed to reduce the computational complexity and delay required in the conventional LDD, but it still require high complexity. Another one-shot LDD with low complexity was proposed. To improve its performance, this detector uses a maximal ratio combining (MRC) at the two outputs of the left piece of the present bit and the right piece of the next bit in an one-shot window; this detector is called as a one-shot LDD with post-MRC. In spite of the aid of MRC, the performance of the one-shot LDD with post-MRC is worse than the one-shot LDD. To overcome this problem, we propose the one-shot LDD with pre-SDC/post-MRC that employs selection diversity combining (SDC) with multiple antenna elements in front of the one-shot LDD with post-MRC. In asynchronous random code division multiple access systems over fading channels, the largest signal-to-noise ratio (SNR) of SDC, near-far resistance (NFR) of the one-shot LDD, and the output SNR of MRC are random variables. We derive the probability density functions (PDFs) for these random variables and calculate the bit error rate) of the proposed detector using the obtained PDFs. In a comparison of mathematical analysis and computer simulation results, we observe good agreement.

Power levels and packet lengths in random multiple access

Multiple-power-level ALOHA has been proposed to take advantage of the capture phenomenon in order to improve the throughput of a multiple random access system. We study the effect of the use of multiple transmission power levels and of the corresponding packet lengths on the system throughput and energy efficiency. We prove that the single-power-level system in which all transmit at the maximum allowable power level achieves both optimal throughput and energy usage efficiency under a condition on the decodability threshold value.

Per-queue stability analysis of a random access system

We have extended our previous studies (2000) of the system stability of buffered ALOHA systems to study an individual queue's stability, i.e., per-queue stability. The main result obtained in this work is a necessary and sufficient per-queue stability condition, which can be computed analytically only for several cases. For other incomputable cases, we evaluated several inner and outer bounds. They are generally quite tight for not-so-asymmetric systems.

Stability of N interacting queues in random-access systems

We revisit the stability problem of systems consisting of N buffered terminals accessing a common receiver over the collision channel by means of the standard ALOHA protocol. We find that in the slotted ALOHA system queues have based on their individual average arrival rates and transmission probabilities. If a queue is stable, then the queue with lower instability rank is stable as well. The instability rank is used to intelligently set up the dominant systems. And the stability inner and outer bounds can be found by bounding the idle probability of some queues in the dominant system. Through analyzing those dominant systems one by one, we are able to obtain inner and outer bounds for stability. These bounds are tighter than the known ones although they still fail to identify the exact stability region for cases of N>2. The methodology used is new and holds promise for successfully addressing other similar stability problems.

Performance analysis of inhibit sense multiple access with mixed priority classes

A random access system is considered promising in wireless communication where multiple terminals send packets to a single base station, using a single channel. Among these systems, the inhibit sense multiple access (ISMA) system is known as a system that can realize a high throughput. It is also desirable in wireless communication to provide a priority function to handle various kinds of traffic with different quality requirements, in order to cope with future multimedia systems. A problem within the conventional ISMA system, however, is that all packets are handled on an equal basis; traffic with priority is not considered. This paper proposes an ISMA system, where the base station sends multiple idle signals with a time difference, so that traffic with priority can be handled. The throughput and the delay performances are evaluated. It is shown that priority can be adjusted by adjusting the time difference of idle signals. © 1998 Scripta Technica. Electron Comm Jpn Pt 1, 81(7): 47–54, 1998

Capture probabilities in random-access mobile communications in the presence of Rician fading

The capture probabilities, C/sub n/, defined as the probability that any of the n, contending packets are correctly received, are found with reference to a random-access mobile radio communications system in the presence or fading, shadowing, and near-far effect. A Rician model for fading that is more general than the well-studied Rayleigh fading is considered. The capture probabilities are computed, and their dependence on the system parameters and, in particular, on the Rice factor, K, is investigated. As an example of application, a slotted ALOHA system is considered, and the performance of a simple control strategy is studied. Also, some analytical results are given as to the value of C/sub /spl alpha//=lim/sub n-/spl alpha//C/sub n/, which is relevant to the study of the system stability.

Optimal blocked random access algorithm for CDMA–random access systems

Code-division multiple-access-random access systems are considered. Specifically, the authors consider a generalised common receiver model with transmitter-based code assignment and study a class of random access algorithms known as blocked random access algorithms. It is assumed that the number of active users at the beginning of a collision resolution epoch is known. An appropriate state of the system and control variables that constitute a Markovian decision process (MDP) are identified aid it is shown that the problem of optimal collision resolution algorithm is equivalent to the first passage problem of this MDP. The optimal control actions specify the number of users to be enabled separately for each code. The algorithm is illustrated with numerical examples.

Stability conditions for some distributed systems: buffered random access systems

We consider the standard slotted ALOHA system with a finite number of buffered users. Stability analysis of such a system was initiated by Tsybakov and Mikhailov (1979). Since then several bounds on the stability region have been established; however, the exact stability region is known only for the symmetric system and two-user ALOHA. This paper proves necessary and sufficient conditions for stability of the ALOHA system. We accomplish this by means of a novel technique based on three simple observations: applying mathematical induction to a smaller copy of the system, isolating a single queue for which Loynes' stability criteria is adopted, and finally using stochastic dominance to verify the required stationarity assumptions in the Loynes criterion. We also point out that our technique can be used to assess stability regions for other multidimensional systems. We illustrate it by deriving stability regions for buffered systems with conflict resolution algorithms (see also Georgiadis and Szpankowski (1992) for similar approach applied to stability of token-passing rings).

Stability conditions for some distributed systems: buffered random access systems

We consider the standard slotted ALOHA system with a finite number of buffered users. Stability analysis of such a system was initiated by Tsybakov and Mikhailov (1979). Since then several bounds on the stability region have been established; however, the exact stability region is known only for the symmetric system and two-user ALOHA. This paper proves necessary and sufficient conditions for stability of the ALOHA system. We accomplish this by means of a novel technique based on three simple observations: applying mathematical induction to a smaller copy of the system, isolating a single queue for which Loynes' stability criteria is adopted, and finally using stochastic dominance to verify the required stationarity assumptions in the Loynes criterion. We also point out that our technique can be used to assess stability regions for other multidimensional systems. We illustrate it by deriving stability regions for buffered systems with conflict resolution algorithms (see also Georgiadis and Szpankowski (1992) for similar approach applied to stability of token-passing rings).

Throughput analyses of packet radio communications systems

AbstractCollision of packets is inevitable and is a problem in contention‐type random access systems. In radio systems, however, the throughput of the systems is improved as a result of the capture effect on power differences between colliding packets caused by propagation characteristics of channels such as fading, shadowing, and distance problems.In this paper, analyses are made of the capture effect by considering channel characteristics of radio channels and modulating and demodulating schemes. First, a theoretical equation for the packet reception success probability of the ALOHA system under frequency nonselective slow Ray‐leigh fading has been derived, and the validity of this equation has been proven by computer simulation. Thus, throughput characteristics of slotted ALOHA and pure ALOHA system have been analyzed by means of this theoretical formula. In addition, it has been proven theoretically that the channel capacity of the ALOHA system is improved two times by the capture effect for the case of a large number of mobile stations and high mean power versus noise power ratios.

Delay upper bounds for a finite user random-access system with bursty arrivals

The authors study the effect of burstiness of packet arrivals on the performance of a buffered finite-user random-access system. Primarily, they formulate and analyze a protocol that exploits the burstiness of arrivals and the presence of buffers. In addition, it is shown that the boundary of the region in which infinite user models best approximate a finite-user system is dependent on the burstiness of the arrival process. They also show that an accurate comparison between time-division multiaccess (TDMA) and random-access schemes depends crucially on the number of users in the system. >

A multiuser random-access communication system for users with different priorities

A binary feedback collision resolution algorithm is developed for a multiuser random access communication system with nonhomogeneous user population. The user population is split into two classes with different priorities. The throughput and delay analysis of the proposed algorithm are performed, and numerical results are obtained. >

Clinical evaluation of five therapeutic drugs using dry film multilayer technology on the opus™ immunoassay system

Five therapeutic drug assays, carbamazepine, phenobarbital, phenytoin, theophylline, and valproic acid, were evaluated using an automated random access system for performing thin dry film multilayer competitive immunoassays, the OPUS analyzer. All reagents for the therapeutic drug assays are contained in a coated multilayer film chip encased within a plastic bar-coded test module and require no external or supplementary reagents. A serum or plasma sample is applied to the test module by the instrument and the fluorescence intensity from the module is measured after 6 minutes. We found the OPUS assays acceptable for clinical use. Within-run coefficient of variations were 2.3-6.7%, between-run, 2.9-7.6%. These methods correlated well with the Abbott TDx, having correlation coefficients of 0.92-0.97. Because of the instrument design and the stability of the reagents, weekly calibration is not needed and samples can be run immediately upon receipt in a random access fashion or can be batched together.