188 research outputs found
Performance analysis for a stabilized multi-channel slotted ALOHA algorithm
We study slotted ALOHA with multiple random access channels, the so called multi-channel ALOHA (MC-ALOHA). It is well known that single-channel ALOHA (SC-ALOHA) is unstable. Not surprisingly, MC-ALOHA is also unstable. A stabilization algorithm for MC-ALOHA has been proposed in [10], in which the pseudo-Bayesian algorithm in SC-ALOHA was extended to achieve stabilized MC-ALOHA. The idea is to estimate the number of attempting users so that user transmission probability can be adjusted accordingly. In this paper, we give a theoretical analysis on the algorithm performance for cases with limited and unlimited number of users by assuming perfect estimate. The theoretical results are validated by simulation, which shows the stabilization algorithm performs close to a system with perfect estimate. The simulation results also show that the performance of the stabilized algorithm is much better than the non-stabilized algorithm. With the stabilized algorithm, the system is always stable when the new packet arrival rate is less than system capacity. Even when the arrival rate is higher than capacity, system throughput can still be guaranteed. © 2003 IEEE.published_or_final_versio
Stabilized multi-channel ALOHA for wireless OFDM networks
Multiple access based on orthogonal frequency division multiplexing (OFDM), or OFDMA, enables multiple users to simultaneously access the media by using different subcarriers. This leads to the convenient realization of multi-channel ALOHA, in which each user transmits with a group of subcarriers. In this paper, we first introduce the multi-channel slotted ALOHA algorithm to OFDM, which is called OFDMA-based multi-channel ALOHA (OMC-ALOHA). Since ALOHA is an unstable algorithm, we show OMC-ALOHA is also unstable. To solve this stability problem, we extend the pseudo-Bayesian algorithm to achieve stabilized OMC-ALOHA.published_or_final_versio
Stability Region of a Slotted Aloha Network with K-Exponential Backoff
Stability region of random access wireless networks is known for only simple
network scenarios. The main problem in this respect is due to interaction among
queues. When transmission probabilities during successive transmissions change,
e.g., when exponential backoff mechanism is exploited, the interactions in the
network are stimulated. In this paper, we derive the stability region of a
buffered slotted Aloha network with K-exponential backoff mechanism,
approximately, when a finite number of nodes exist. To this end, we propose a
new approach in modeling the interaction among wireless nodes. In this
approach, we model the network with inter-related quasi-birth-death (QBD)
processes such that at each QBD corresponding to each node, a finite number of
phases consider the status of the other nodes. Then, by exploiting the
available theorems on stability of QBDs, we find the stability region. We show
that exponential backoff mechanism is able to increase the area of the
stability region of a simple slotted Aloha network with two nodes, more than
40\%. We also show that a slotted Aloha network with exponential backoff may
perform very near to ideal scheduling. The accuracy of our modeling approach is
verified by simulation in different conditions.Comment: 30 pages, 6 figure
On the Stability of Contention Resolution Diversity Slotted ALOHA
In this paper a Time Division Multiple Access (TDMA) based Random Access (RA)
channel with Successive Interference Cancellation (SIC) is considered for a
finite user population and reliable retransmission mechanism on the basis of
Contention Resolution Diversity Slotted ALOHA (CRDSA). A general mathematical
model based on Markov Chains is derived which makes it possible to predict the
stability regions of SIC-RA channels, the expected delays in equilibrium and
the selection of parameters for a stable channel configuration. Furthermore the
model enables the estimation of the average time before reaching instability.
The presented model is verified against simulations and numerical results are
provided for comparison of the stability of CRDSA versus the stability of
traditional Slotted ALOHA (SA). The presented results show that CRDSA has not
only a high gain over SA in terms of throughput but also in its stability.Comment: 10 pages, 12 figures This paper is submitted to the IEEE Transactions
on Communications for possible publication. The IEEE copyright notice applie
Age of Information in Random Access Channels
In applications of remote sensing, estimation, and control, timely
communication is not always ensured by high-rate communication. This work
proposes distributed age-efficient transmission policies for random access
channels with transmitters. In the first part of this work, we analyze the
age performance of stationary randomized policies by relating the problem of
finding age to the absorption time of a related Markov chain. In the second
part of this work, we propose the notion of \emph{age-gain} of a packet to
quantify how much the packet will reduce the instantaneous age of information
at the receiver side upon successful delivery. We then utilize this notion to
propose a transmission policy in which transmitters act in a distributed manner
based on the age-gain of their available packets. In particular, each
transmitter sends its latest packet only if its corresponding age-gain is
beyond a certain threshold which could be computed adaptively using the
collision feedback or found as a fixed value analytically in advance. Both
methods improve age of information significantly compared to the state of the
art. In the limit of large , we prove that when the arrival rate is small
(below ), slotted ALOHA-type algorithms are asymptotically
optimal. As the arrival rate increases beyond , while age
increases under slotted ALOHA, it decreases significantly under the proposed
age-based policies. For arrival rates , , the
proposed algorithms provide a multiplicative factor of at least two compared to
the minimum age under slotted ALOHA (minimum over all arrival rates). We
conclude that, as opposed to the common practice, it is beneficial to increase
the sampling rate (and hence the arrival rate) and transmit packets selectively
based on their age-gain
Analysis of Slotted ALOHA with an Age Threshold
We present a comprehensive steady-state analysis of threshold-ALOHA, a
distributed age-aware modification of slotted ALOHA proposed in recent
literature. In threshold-ALOHA, each terminal suspends its transmissions until
the Age of Information (AoI) of the status update flow it is sending reaches a
certain threshold . Once the age exceeds , the terminal
attempts transmission with constant probability in each slot, as in
standard slotted ALOHA. We analyze the time-average expected AoI attained by
this policy, and explore its scaling with network size, . We derive the
probability distribution of the number of active users at steady state, and
show that as network size increases the policy converges to one that runs
slotted ALOHA with fewer sources: on average about one fifth of the users is
active at any time. We obtain an expression for steady-state expected AoI and
use this to optimize the parameters and , resolving the
conjectures in \cite{doga} by confirming that the optimal age threshold and
transmission probability are and , respectively. We find that
the optimal AoI scales with the network size as , which is almost half
the minimum AoI achievable with slotted ALOHA, while the loss from the maximum
throughput of remains below . We compare the performance of this
rudimentary algorithm to that of the SAT policy that dynamically adapts its
transmission probabilities
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