ALOHA System Research Articles

Capture models for mobile packet radio networks

The probability q/sub i/ of successful reception in a nonfading mobile radio channel with i contending mobiles transmitting to a central base station is studied for a number of different capture and spatial distribution models. It is shown that a generalized capture model can be used to estimate q/sub i/'s for a simplified example system which uses noncoherent frequency shift keying modulation. This model can be applied to other systems as well. An example of the use of the q/sub i/'s in the throughput evaluation of a finite population slotted ALOHA system is given. In most practical systems, the mobiles cannot get arbitrarily close to the base station. The effect of this constraint on q/sub i/ is examined. Finally, the dependence of the capture probability for a test mobile on its distance from the base station is obtained. >

Queueing analysis of a class of star-interconnected networks under Markov modulated output process modeling

A Markov modulated process is developed for the exact characterization of the output process of a class of random access networks (C-RANs) and the mean packet delay induced by the central node of a star interconnecting scheme is calculated. The latter is obtained through an approximate analysis of an appropriate queueing system under Markov modulated arrivals. It is shown that the ALOHA network is a C-RAN. ALOHA networks interconnected according to a star topology are studied and delay results are obtained. These results are compared with those obtained under Bernoulli approximation on the network output processes and simulations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spread slotted ALOHA techniques for mobile and personal satellite communication systems

The objective is to develop a general theoretical framework for a class of spread slotted ALOHA (SSA) systems. The contributions include: modeling of a generalized spread slotted ALOHA (SSA) system; derivation of computationally efficient closed form expressions for the SSA system throughput and delay taking into account receiver complexity; and presentation of numerical results to validate the derivation as well as to substantiate the superior performance of the proposed scheme. The results demonstrate that SSA schemes offer significant improvements in terms of throughput, delay, and network stability against excessive loading at very acceptable levels of receiver implementation complexities. Also, SSA is found to be highly robust to errors in the time of arrivals and eliminates the need for a guard time.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Slotted ALOHA land-mobile radio networks with site diversity

An analytical method for investigating slotted ALOHA land-mobile networks is presented. The probability of successful reception and throughput is assessed for a single base station, taking account of contending transmissions and receiver noise. Receiver capture is assumed to occur if the received signal power exceeds the joint interference power by a certain margin, called the receiver threshold. Rayleigh fading and UHF groundwave propagation are considered. Results are extended for a cellular network considering the interference from packet transmissions in other (co-channel) cells. It is seen that in packetswitched cellular nets, frequency re-use distances may be substantially smaller than in circuitswitched (CW) telephony networks, where each cell has to be safeguarded continuously from cochannel interference. Moreover, a technique to assess the throughput of ALOHA networks with multiple, geographically-separated base stations is presented and numerical results are given for uniform Poisson-distributed packet transmissions in the service area.

Performance approximation of a multi-base station slotted ALOHA for wireless LAN's

It has been reported that the capture effect can improve the performance of slotted ALOHA systems. Further improvement can be expected when the number of base stations is increased. The performance of such slotted ALOHA systems with multiple base stations is analyzed with the aid of the equilibrium point analysis. Not only the capture effect but also the packet dropping due to the finite number of retransmission trials are taken into account. The numerical results indicate that the finite number of retransmission trials mainly contributes to the improvement of the packet dropping probability in the range of the light input traffic, while the number of base stations mainly contributes to the improvement of the overall throughput and the average transmission delay in the range of the heavy input traffic. >

Heterogeneous ALOHA networks: a sufficient condition for all equilibrium states to be stable

This paper models and analyzes the stability of a slotted-ALOHA network hosting heterogeneous users. The scheme is modeled by dividing the total user-population into groups of homogeneous users. The behavior of the system is represented by state equations. The stability analysis is carried out by Liapunov's indirect method, in association with the properties of M-matrix, to obtain the sufficient condition for the structural stability of the networks. The analysis also points out the method of hierarchical grouping of the heterogeneous users.

The effect of capture on performance of multichannel slotted ALOHA systems

The author presents an exact analysis to evaluate the effect of capture on the multichannel slotted ALOHA protocol. The author derives the probabilities of the successful transmission. These probabilities are used to calculate the throughputs, average packet delays for both IFT (immediate-first-transmission) and delayed-first-transmission protocols and numerically compare the performance of the systems with and without capture. Numerical results show that when a quantitative capture restriction u is considered, in a multichannel system having a fixed total bandwidth, depending on parameter u and channel number M, an improved system performance such as the average channel utilization and average packet delay can be obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fast simulation of rare events using reverse-time models

We consider the estimation by simulation of probabilities of rare but potentially damaging events in Markovian systems. The problem is that the rarity of the event mitigates against its frequent occurrence in simulation. Here we show how a reverse-time model may be simulated in place of the forward-time system in order to provide significantly faster estimates of the rare probabilities. An example of an ALOHA system will be used and simulation speedup compared with alternative methods based on Large Deviations theory.

Backlog performance of some controlled slotted ALOHA systems

The backlog performance of the most efficient algorithm used in the controlled slotted Aloha channel is examined by simulation. Some new schemes based on Mikhailov&apos;s algorithm are introduced. The simplest to implement need no traffic estimate and have binary feedback. It is found that such schemes also perform very well, giving an acceptable level of throughput at any load below saturation (1/e).

Path integral evaluation of aloha network transients

The recently proposed Feynman path-integral is applied to the numerical analysis of large transient behavior in ALOHA network models. Path-integral calculations of expected first passage times in the simple diffusion approximation (Wiener action) are shown to be self-consistent and in good agreement with results obtained by other methods.

The ALOHA systems in shadowed mobile radio channels with slow or fast fading

The influence of receiver capture on the performance of the ALOHA protocol in the presence of shadowing is investigated. The combined effect of Rayleigh fading, log-normal shadowing, and spatial distribution of mobile users is also studied. It is shown that shadowing is similar to fading and near/far phenomena, in that it makes the capture effect possible and provides ALOHA systems with substantial improvements in throughput. It is also confirmed that the superimposed Rayleigh fading, log-normal shadowing, and spatial distribution can further enhance the capture effect, resulting in a ALOHA system with higher throughput. The fast fading effect in ALOHA systems is explored. A very fast fading case is examined which may be considered to be equivalent to interleaving in slow fading. It is found that an ALOHA system under fast fading conditions also benefits from the capture results in higher throughput.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stability of mobile slotted ALOHA network with Rayleigh fading, shadowing, and near-far effect

Dynamic network behavior is investigated using the classical ALOHA queueing model extended to mobile channels. The focus is on the assessment of receiver capture probabilities, based on a general propagation model for mobile radio communications. Queueing of packets in a mobile slotted ALOHA network with Rayleigh fading, shadowing, and UHF groundwave propagation is studied, using a finite Markov chain model. It is shown that receiver capture, assisted by all three propagation mechanisms, reduces bistability of the network substantially compared to the retransmission strategy. Results are presented for a uniform spatial distribution of the packet traffic ordered to the ALOHA channel. The near-far effect is highlighted, since the total network performance and the spatial distribution of the attempted traffic interact. >

A slotted ALOHA packet radio system with multiple antennas and receivers

The problem of data transmission in a mobile packet radio system with one central base station and a number of mobile terminals is addressed. A method using multiple directional antennas and receivers at the base station to improve the efficiency of transmission on the inbound channel (from the terminals to the base station) is proposed. A number of channel models are considered, including flat terrestrial propagation loss, Rayleigh fading, and noise. A finite population Markov model is used to obtain the throughput of a multiantenna and multireceiver slotted ALOHA system. Numerical results indicate that substantial gains are possible with the use of several antennas and receivers. The dynamic behavior of the system is also improved. >

Controlled ALOHA with geometric capture probabilities

An infinite user, slotted ALOHA system operating over a capture channel is studied. In a capture channel, differences in the levels of received power and the times of arrival allow the receiver to successfully receive a packet even when two or more have been transmitted simultaneously. The slotted ALOHA system operating over a capture channel is known to be unstable unless a control policy is used. The authors stabilize the system by using a retransmission control algorithm and derive the throughput as a function of the capture parameter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Random access systems: ALOHA's progeny

Random access algorithms allow a large number of bursty data sources to share a common transmission medium. Such algorithms are essentially refinements of the distributed algorithm first introduced in the ALOHA system. Unlike techniques such as time division multiple access (TDMA), frequency division multiple access (FDMA) and polling, the performance of random access algorithms is insensitive to the number of stations in the system. The weakness of early random access systems is extreme sensitivity to traffic, resulting in instability. Later implementations of the technique have improved throughput, eliminating instability. This paper discusses the three forms of random access techniques: stable backoff algorithms, tree search algorithms and carrier sensing techniques. Forms of the last of these have been used in an optical-fibre local area network. The particular application of random access techniques to satellite communication systems is also considered.

ALOHA with capture over slow and fast fading radio channels with coding and diversity

The effects of capture on the average system throughput and delay performance of slotted ALOHA were analyzed for slow and fast Rayleigh fading radio channels. A short-range multipoint-to-base station packet radio network is considered. It is shown that larger capture effects and thus improved network performance can be achieved with proper choice of modulation. It is also shown that the use of simple error-correcting codes improves capture. The use of selection diversity also improves the capture effect both for fast and slow fading. It is concluded that the inverse distance variability of the received signal is the main reason for the capture effect. The Rayleigh fading alone yields a very small contribution in terms of throughput; nonetheless, it helps to stabilize the system. Numerical results are presented for a slotted ALOHA system with 50 users. It is found that the maximum average throughput can be increased from about 36% to almost 60% by using channel coding and space diversity. >

Some remarks on uniformly bounded markov chains: multimodality analysis

Since 1975 when Carleial and Hellman [ IEEE Trans. Commun. COM-23, 401–410 (1975)] published their paper, it has been known that the bistable behavior of the ALOHA system is associated with a bimodal shape of the backlog steady-state distribution. In this paper, we generalize the problem and ask under what conditions a one-dimensional Markov chain possesses a multimodal steady-state distribution. We restrict our analysis to uniformly bounded Markov chains. In this class we distinguish so called near birth and death processes and prove that under some additional assumptions a shape of the distribution is determined by the transition probabilities located on the principal diagonal, subdiagonal and supdiagonal of the transition matrix. This provides a theoretical explanation for the bistable behavior of the ALOHA system. In addition, we establish conditions under which some Markov chains can be approximated by a birth and death process.

Stability of a slotted ALOHA system with capture effect

The stability of a slotted ALOHA system with capture effect is investigated under a general communication environment where terminals are divided into two groups (low-power and high-power) and the capture effect is modeled by capture probabilities. An approximate analysis is developed using catastrophe theory in which the effects of system and user parameters on the stability are characterized by the cusp catastrophe. In particular, the authors focus on the low-power group, since it must bear the strain under the capture effect. The stability conditions of the two groups are given explicitly by bifurcation sets.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the stability of interacting queues in a multiple-access system

The standard discrete-time slotted ALOHA system with a finite number of buffered terminals is considered. The stability (ergodicity) region for this system is known only for the case of two terminals and for the case of any number of symmetric terminals. The stability of the system is studied by means of a simple concept of dominance. It is shown that the stability region for the case of two terminals can be obtained in a simple way. Lower (inner) bounds are obtained for the stability region of the system with an arbitrary finite number of terminals that are tighter than the ones already known. A similarity between these stability results and the achievable region of the no-feedback collision channel is pointed out that suggests a connection between the two problems. >

Performance of a power group division scheme for ALOHA systems in a finite capture environment

A method proposed by Metzner for improving the utilisation in ALOHA systems involves dividing the users into different power groups. This was analysed assuming that a packet can withstand interference from an unlimited number of packets from lower-power groups. The performance of such a scheme when only a limited number of interferers can be tolerated is examined in this paper.