123 research outputs found
Optimal Spectrum Access for Cognitive Radios
In this paper, we investigate a time-slotted cognitive setting with buffered
primary and secondary users. In order to alleviate the negative effects of
misdetection and false alarm probabilities, a novel design of spectrum access
mechanism is proposed. We propose two schemes. First, the SU senses primary
channel to exploit the periods of silence, if the PU is declared to be idle,
the SU randomly accesses the channel with some access probability .
Second, in addition to accessing the channel if the PU is idle, the SU possibly
accesses the channel if it is declared to be busy with some access probability
. The access probabilities as function of the misdetection, false alarm
and average primary arrival rate are obtained via solving an optimization
problem designed to maximize the secondary service rate given a constraint on
primary queue stability. In addition, we propose a variable sensing duration
schemes where the SU optimizes over the optimal sensing time to achieve the
maximum stable throughput of the network. The results reveal the performance
gains of the proposed schemes over the conventional sensing scheme. We propose
a method to estimate the mean arrival rate and the outage probability of the PU
based on the primary feedback channel, i.e., acknowledgments (ACKs) and
negative-acknowledgments (NACKs) messages.Comment: arXiv admin note: substantial text overlap with arXiv:1206.615
To Sense or Not To Sense
A longer sensing time improves the sensing performance; however, with a fixed
frame size, the longer sensing time will reduce the allowable data transmission
time of the secondary user (SU). In this paper, we try to address the tradeoff
between sensing the primary channel for seconds of the time slot
proceeded by randomly accessing it and randomly accessing primary channel
without sensing to avoid wasting seconds in sensing. The SU senses
primary channel to exploit the periods of silence, if the primary user (PU) is
declared to be idle the SU randomly accesses the channel with some access
probability . In addition to randomly accesses the channel if the PU is
sensed to be idle, it possibly accesses it if the channel is declared to be
busy with some access probability . This is because the probability of
false alarm and misdetection cause significant secondary throughput degradation
and affect the PU QoS. We propose variable sensing duration schemes where the
SU optimizes over the optimal sensing time to achieve the maximum stable
throughput for both primary and secondary queues. The results reveal the
performance gains of the proposed schemes over the conventional sensing scheme,
i.e., the SU senses the primary channel for seconds and accesses with
probability 1 if the PU is declared to be idle. Also, the proposed schemes
overcome random access without sensing scheme.
The theoretical and numerical results show that pairs of misdetection and
false alarm probabilities may exist such that sensing the primary channel for
very small duration overcomes sensing it for large portion of the time slot. In
addition, for certain average arrival rate to the primary queue pairs of
misdetection and false alarm probabilities may exist such that the random
access without sensing overcomes the random access with long sensing duration
Spectral and Energy Efficiency in Cognitive Radio Systems with Unslotted Primary Users and Sensing Uncertainty
This paper studies energy efficiency (EE) and average throughput maximization
for cognitive radio systems in the presence of unslotted primary users. It is
assumed that primary user activity follows an ON-OFF alternating renewal
process. Secondary users first sense the channel possibly with errors in the
form of miss detections and false alarms, and then start the data transmission
only if no primary user activity is detected. The secondary user transmission
is subject to constraints on collision duration ratio, which is defined as the
ratio of average collision duration to transmission duration. In this setting,
the optimal power control policy which maximizes the EE of the secondary users
or maximizes the average throughput while satisfying a minimum required EE
under average/peak transmit power and average interference power constraints
are derived. Subsequently, low-complexity algorithms for jointly determining
the optimal power level and frame duration are proposed. The impact of
probabilities of detection and false alarm, transmit and interference power
constraints on the EE, average throughput of the secondary users, optimal
transmission power, and the collisions with primary user transmissions are
evaluated. In addition, some important properties of the collision duration
ratio are investigated. The tradeoff between the EE and average throughput
under imperfect sensing decisions and different primary user traffic are
further analyzed.Comment: This paper is accepted for publication in IEEE Transactions on
Communication
Cooperative Access in Cognitive Radio Networks: Stable Throughput and Delay Tradeoffs
In this paper, we study and analyze fundamental throughput-delay tradeoffs in
cooperative multiple access for cognitive radio systems. We focus on the class
of randomized cooperative policies, whereby the secondary user (SU) serves
either the queue of its own data or the queue of the primary user (PU) relayed
data with certain service probabilities. The proposed policy opens room for
trading the PU delay for enhanced SU delay. Towards this objective, stability
conditions for the queues involved in the system are derived. Furthermore, a
moment generating function approach is employed to derive closed-form
expressions for the average delay encountered by the packets of both users.
Results reveal that cooperation expands the stable throughput region of the
system and significantly reduces the delay at both users. Moreover, we quantify
the gain obtained in terms of the SU delay under the proposed policy, over
conventional relaying that gives strict priority to the relay queue.Comment: accepted for publication in IEEE 12th Intl. Symposium on Modeling and
Optimization in Mobile, Ad Hoc, and Wireless Networks (WiOpt), 201
Throughput and Collision Analysis of Multi-Channel Multi-Stage Spectrum Sensing Algorithms
Multi-stage sensing is a novel concept that refers to a general class of
spectrum sensing algorithms that divide the sensing process into a number of
sequential stages. The number of sensing stages and the sensing technique per
stage can be used to optimize performance with respect to secondary user
throughput and the collision probability between primary and secondary users.
So far, the impact of multi-stage sensing on network throughput and collision
probability for a realistic network model is relatively unexplored. Therefore,
we present the first analytical framework which enables performance evaluation
of different multi-channel multi-stage spectrum sensing algorithms for
Opportunistic Spectrum Access networks. The contribution of our work lies in
studying the effect of the following parameters on performance: number of
sensing stages, physical layer sensing techniques and durations per each stage,
single and parallel channel sensing and access, number of available channels,
primary and secondary user traffic, buffering of incoming secondary user
traffic, as well as MAC layer sensing algorithms. Analyzed performance metrics
include the average secondary user throughput and the average collision
probability between primary and secondary users. Our results show that when the
probability of primary user mis-detection is constrained, the performance of
multi-stage sensing is, in most cases, superior to the single stage sensing
counterpart. Besides, prolonged channel observation at the first stage of
sensing decreases the collision probability considerably, while keeping the
throughput at an acceptable level. Finally, in realistic primary user traffic
scenarios, using two stages of sensing provides a good balance between
secondary users throughput and collision probability while meeting successful
detection constraints subjected by Opportunistic Spectrum Access communication
On Spectrum Sharing Between Energy Harvesting Cognitive Radio Users and Primary Users
This paper investigates the maximum secondary throughput for a rechargeable
secondary user (SU) sharing the spectrum with a primary user (PU) plugged to a
reliable power supply. The SU maintains a finite energy queue and harvests
energy from natural resources and primary radio frequency (RF) transmissions.
We propose a power allocation policy at the PU and analyze its effect on the
throughput of both the PU and SU. Furthermore, we study the impact of the
bursty arrivals at the PU on the energy harvested by the SU from RF
transmissions. Moreover, we investigate the impact of the rate of energy
harvesting from natural resources on the SU throughput. We assume fading
channels and compute exact closed-form expressions for the energy harvested by
the SU under fading. Results reveal that the proposed power allocation policy
along with the implemented RF energy harvesting at the SU enhance the
throughput of both primary and secondary links
Sensing-Throughput Tradeoff for Interweave Cognitive Radio System: A Deployment-Centric Viewpoint
Secondary access to the licensed spectrum is viable only if interference is
avoided at the primary system. In this regard, different paradigms have been
conceptualized in the existing literature. Of these, Interweave Systems (ISs)
that employ spectrum sensing have been widely investigated. Baseline models
investigated in the literature characterize the performance of IS in terms of a
sensing-throughput tradeoff, however, this characterization assumes the
knowledge of the involved channels at the secondary transmitter, which is
unavailable in practice. Motivated by this fact, we establish a novel approach
that incorporates channel estimation in the system model, and consequently
investigate the impact of imperfect channel estimation on the performance of
the IS. More particularly, the variation induced in the detection probability
affects the detector's performance at the secondary transmitter, which may
result in severe interference at the primary users. In this view, we propose to
employ average and outage constraints on the detection probability, in order to
capture the performance of the IS. Our analysis reveals that with an
appropriate choice of the estimation time determined by the proposed model, the
degradation in performance of the IS can be effectively controlled, and
subsequently the achievable secondary throughput can be significantly enhanced.Comment: 13 pages, 10 figures, Accepted to be published in IEEE Transactions
on Wireless Communication
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