174 research outputs found
Cognitive Access Policies under a Primary ARQ process via Forward-Backward Interference Cancellation
This paper introduces a novel technique for access by a cognitive Secondary
User (SU) using best-effort transmission to a spectrum with an incumbent
Primary User (PU), which uses Type-I Hybrid ARQ. The technique leverages the
primary ARQ protocol to perform Interference Cancellation (IC) at the SU
receiver (SUrx). Two IC mechanisms that work in concert are introduced: Forward
IC, where SUrx, after decoding the PU message, cancels its interference in the
(possible) following PU retransmissions of the same message, to improve the SU
throughput; Backward IC, where SUrx performs IC on previous SU transmissions,
whose decoding failed due to severe PU interference. Secondary access policies
are designed that determine the secondary access probability in each state of
the network so as to maximize the average long-term SU throughput by
opportunistically leveraging IC, while causing bounded average long-term PU
throughput degradation and SU power expenditure. It is proved that the optimal
policy prescribes that the SU prioritizes its access in the states where SUrx
knows the PU message, thus enabling IC. An algorithm is provided to optimally
allocate additional secondary access opportunities in the states where the PU
message is unknown. Numerical results are shown to assess the throughput gain
provided by the proposed techniques.Comment: 16 pages, 11 figures, 2 table
Cognitive Interference Management in Retransmission-Based Wireless Networks
Cognitive radio methodologies have the potential to dramatically increase the
throughput of wireless systems. Herein, control strategies which enable the
superposition in time and frequency of primary and secondary user transmissions
are explored in contrast to more traditional sensing approaches which only
allow the secondary user to transmit when the primary user is idle. In this
work, the optimal transmission policy for the secondary user when the primary
user adopts a retransmission based error control scheme is investigated. The
policy aims to maximize the secondary users' throughput, with a constraint on
the throughput loss and failure probability of the primary user. Due to the
constraint, the optimal policy is randomized, and determines how often the
secondary user transmits according to the retransmission state of the packet
being served by the primary user. The resulting optimal strategy of the
secondary user is proven to have a unique structure. In particular, the optimal
throughput is achieved by the secondary user by concentrating its transmission,
and thus its interference to the primary user, in the first transmissions of a
primary user packet. The rather simple framework considered in this paper
highlights two fundamental aspects of cognitive networks that have not been
covered so far: (i) the networking mechanisms implemented by the primary users
(error control by means of retransmissions in the considered model) react to
secondary users' activity; (ii) if networking mechanisms are considered, then
their state must be taken into account when optimizing secondary users'
strategy, i.e., a strategy based on a binary active/idle perception of the
primary users' state is suboptimal.Comment: accepted for publication on Transactions on Information Theor
Cooperative retransmission protocols in fading channels : issues, solutions and applications
Future wireless systems are expected to extensively rely on cooperation between terminals, mimicking MIMO scenarios when terminal dimensions limit implementation of multiple antenna technology. On this line, cooperative retransmission protocols are considered as particularly promising technology due to their opportunistic and flexible exploitation of both spatial and time diversity. In this dissertation, some of the major issues that hinder the practical implementation of this technology are identified and pertaining solutions are proposed and analyzed. Potentials of cooperative and cooperative retransmission protocols for a practical implementation of dynamic spectrum access paradigm are also recognized and investigated. Detailed contributions follow.
While conventionally regarded as energy efficient communications paradigms, both cooperative and retransmission concepts increase circuitry energy and may lead to energy overconsumption as in, e.g., sensor networks. In this context, advantages of cooperative retransmission protocols are reexamined in this dissertation and their limitation for short transmission ranges observed. An optimization effort is provided for extending an energy- efficient applicability of these protocols.
Underlying assumption of altruistic relaying has always been a major stumbling block for implementation of cooperative technologies. In this dissertation, provision is made to alleviate this assumption and opportunistic mechanisms are designed that incentivize relaying via a spectrum leasing approach. Mechanisms are provided for both cooperative and cooperative retransmission protocols, obtaining a meaningful upsurge of spectral efficiency for all involved nodes (source-destination link and the relays).
It is further recognized in this dissertation that the proposed relaying-incentivizing schemes have an additional and certainly not less important application, that is in dynamic spectrum access for property-rights cognitive-radio implementation. Provided solutions avoid commons-model cognitive-radio strict sensing requirements and regulatory and taxonomy issues of a property-rights model
Access Policy Design for Cognitive Secondary Users under a Primary Type-I HARQ Process
In this paper, an underlay cognitive radio network that consists of an
arbitrary number of secondary users (SU) is considered, in which the primary
user (PU) employs Type-I Hybrid Automatic Repeat Request (HARQ). Exploiting the
redundancy in PU retransmissions, each SU receiver applies forward interference
cancelation to remove a successfully decoded PU message in the subsequent PU
retransmissions. The knowledge of the PU message state at the SU receivers and
the ACK/NACK message from the PU receiver are sent back to the transmitters.
With this approach and using a Constrained Markov Decision Process (CMDP) model
and Constrained Multi-agent MDP (CMMDP), centralized and decentralized optimum
access policies for SUs are proposed to maximize their average sum throughput
under a PU throughput constraint. In the decentralized case, the channel access
decision of each SU is unknown to the other SU. Numerical results demonstrate
the benefits of the proposed policies in terms of sum throughput of SUs. The
results also reveal that the centralized access policy design outperforms the
decentralized design especially when the PU can tolerate a low average long
term throughput. Finally, the difficulties in decentralized access policy
design with partial state information are discussed
Opportunistic Secondary Spectrum Sharing Protocols for Primary implementing an IR type Hybrid-ARQ Protocol
In this paper, we propose, analyze and compare three different methods for opportunistic spectrum sharing access when the primary users implements an Incremental Redundancy (IR) type Hybrid Automatic ReQuest (H-ARQ) protocol. The first method consists in allowing the secondary user to communicate only during the first primary transmission round of the IR H-ARQ protocol. In this scenario, if the the secondary receiver fails to decode its message after the first round, it realizes a successive interference cancellation in the subsequent primary HARQ rounds by listening to the primary user. The second method consists in realizing a perfect interference cancellation at the secondary receiver with causal channel state information. In this method, the secondary user communicates only when the secondary receiver succeeds in decoding the primary message.To improve throughput performance at the secondary, the secondary pair is also considering the use of an IR-HARQ protocol. In a third method, the secondary user communicates following the same rule as in the proposed second method, but implementing an Adaptive Modulation and Coding scheme instead of HARQ. In particular, we show that this last protocol with a small number of interfered slots allows to limit the loss in the primary throughput needed for the secondary user to transmit
Cognitive Communications in White Space: Opportunistic Scheduling, Spectrum Shaping and Delay Analysis
abstract: A unique feature, yet a challenge, in cognitive radio (CR) networks is the user hierarchy: secondary users (SU) wishing for data transmission must defer in the presence of active primary users (PUs), whose priority to channel access is strictly higher.Under a common thread of characterizing and improving Quality of Service (QoS) for the SUs, this dissertation is progressively organized under two main thrusts: the first thrust focuses on SU's throughput by exploiting the underlying properties of the PU spectrum to perform effective scheduling algorithms; and the second thrust aims at another important QoS performance of the SUs, namely delay, subject to the impact of PUs' activities, and proposes enhancement and control mechanisms. More specifically, in the first thrust, opportunistic spectrum scheduling for SU is first considered by jointly exploiting the memory in PU's occupancy and channel fading. In particular, the underexplored scenario where PU occupancy presents a {long} temporal memory is taken into consideration. By casting the problem as a partially observable Markov decision process, a set of {multi-tier} tradeoffs are quantified and illustrated. Next, a spectrum shaping framework is proposed by leveraging network coding as a {spectrum shaper} on the PU's traffic. Such shaping effect brings in predictability of the primary spectrum, which is utilized by the SUs to carry out adaptive channel sensing by prioritizing channel access order, and hence significantly improve their throughput. On the other hand, such predictability can make wireless channels more susceptible to jamming attacks. As a result, caution must be taken in designing wireless systems to balance the throughput and the jamming-resistant capability. The second thrust turns attention to an equally important performance metric, i.e., delay performance. Specifically, queueing delay analysis is conducted for SUs employing random access over the PU channels. Fluid approximation is taken and Poisson driven stochastic differential equations are applied to characterize the moments of the SUs' steady-state queueing delay. Then, dynamic packet generation control mechanisms are developed to meet the given delay requirements for SUs.Dissertation/ThesisPh.D. Electrical Engineering 201
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