115 research outputs found
Novel Subcarrier-pair based Opportunistic DF Protocol for Cooperative Downlink OFDMA
A novel subcarrier-pair based opportunistic DF protocol is proposed for
cooperative downlink OFDMA transmission aided by a decode-and-forward (DF)
relay. Specifically, user message bits are transmitted in two consecutive
equal-duration time slots. A subcarrier in the first slot can be paired with a
subcarrier in the second slot for the DF relay-aided transmission to a user. In
particular, the source and the relay can transmit simultaneously to implement
beamforming at the subcarrier in the second slot for the relay-aided
transmission. Each unpaired subcarrier in either the first or second slot is
used by the source for direct transmission to a user without the relay's
assistance. The sum rate maximized resource allocation (RA) problem is
addressed for this protocol under a total power constraint. It is shown that
the novel protocol leads to a maximum sum rate greater than or equal to that
for a benchmark one, which does not allow the source to implement beamforming
at the subcarrier in the second slot for the relay-aided transmission. Then, a
polynomial-complexity RA algorithm is developed to find an (at least
approximately) optimum resource allocation (i.e., source/relay power,
subcarrier pairing and assignment to users) for either the proposed or
benchmark protocol. Numerical experiments illustrate that the novel protocol
can lead to a much greater sum rate than the benchmark one.Comment: 6 pages, accepted by 2013 IEEE Wireless Communications and Networking
Conferenc
Weighted Sum Rate Maximization for Downlink OFDMA with Subcarrier-pair based Opportunistic DF Relaying
This paper addresses a weighted sum rate (WSR) maximization problem for
downlink OFDMA aided by a decode-and-forward (DF) relay under a total power
constraint. A novel subcarrier-pair based opportunistic DF relaying protocol is
proposed. Specifically, user message bits are transmitted in two time slots. A
subcarrier in the first slot can be paired with a subcarrier in the second slot
for the DF relay-aided transmission to a user. In particular, the source and
the relay can transmit simultaneously to implement beamforming at the
subcarrier in the second slot. Each unpaired subcarrier in either the first or
second slot is used for the source's direct transmission to a user. A benchmark
protocol, same as the proposed one except that the transmit beamforming is not
used for the relay-aided transmission, is also considered. For each protocol, a
polynomial-complexity algorithm is developed to find at least an approximately
optimum resource allocation (RA), by using continuous relaxation, the dual
method, and Hungarian algorithm. Instrumental to the algorithm design is an
elegant definition of optimization variables, motivated by the idea of
regarding the unpaired subcarriers as virtual subcarrier pairs in the direct
transmission mode. The effectiveness of the RA algorithm and the impact of
relay position and total power on the protocols' performance are illustrated by
numerical experiments. The proposed protocol always leads to a maximum WSR
equal to or greater than that for the benchmark one, and the performance gain
of using the proposed one is significant especially when the relay is in close
proximity to the source and the total power is low. Theoretical analysis is
presented to interpret these observations.Comment: 8 figures, accepted and to be published in IEEE Transactions on
Signal Processing. arXiv admin note: text overlap with arXiv:1301.293
Optimization Framework and Graph-Based Approach for Relay-Assisted Bidirectional OFDMA Cellular Networks
This paper considers a relay-assisted bidirectional cellular network where
the base station (BS) communicates with each mobile station (MS) using OFDMA
for both uplink and downlink. The goal is to improve the overall system
performance by exploring the full potential of the network in various
dimensions including user, subcarrier, relay, and bidirectional traffic. In
this work, we first introduce a novel three-time-slot time-division duplexing
(TDD) transmission protocol. This protocol unifies direct transmission, one-way
relaying and network-coded two-way relaying between the BS and each MS. Using
the proposed three-time-slot TDD protocol, we then propose an optimization
framework for resource allocation to achieve the following gains: cooperative
diversity (via relay selection), network coding gain (via bidirectional
transmission mode selection), and multiuser diversity (via subcarrier
assignment). We formulate the problem as a combinatorial optimization problem,
which is NP-complete. To make it more tractable, we adopt a graph-based
approach. We first establish the equivalence between the original problem and a
maximum weighted clique problem in graph theory. A metaheuristic algorithm
based on any colony optimization (ACO) is then employed to find the solution in
polynomial time. Simulation results demonstrate that the proposed protocol
together with the ACO algorithm significantly enhances the system total
throughput.Comment: 27 pages, 8 figures, 2 table
Enhancing Physical Layer Security in AF Relay Assisted Multi-Carrier Wireless Transmission
In this paper, we study the physical layer security (PLS) problem in the dual
hop orthogonal frequency division multiplexing (OFDM) based wireless
communication system. First, we consider a single user single relay system and
study a joint power optimization problem at the source and relay subject to
individual power constraint at the two nodes. The aim is to maximize the end to
end secrecy rate with optimal power allocation over different sub-carriers.
Later, we consider a more general multi-user multi-relay scenario. Under high
SNR approximation for end to end secrecy rate, an optimization problem is
formulated to jointly optimize power allocation at the BS, the relay selection,
sub-carrier assignment to users and the power loading at each of the relaying
node. The target is to maximize the overall security of the system subject to
independent power budget limits at each transmitting node and the OFDMA based
exclusive sub-carrier allocation constraints. A joint optimization solution is
obtained through duality theory. Dual decomposition allows to exploit convex
optimization techniques to find the power loading at the source and relay
nodes. Further, an optimization for power loading at relaying nodes along with
relay selection and sub carrier assignment for the fixed power allocation at
the BS is also studied. Lastly, a sub-optimal scheme that explores joint power
allocation at all transmitting nodes for the fixed subcarrier allocation and
relay assignment is investigated. Finally, simulation results are presented to
validate the performance of the proposed schemes.Comment: 10 pages, 7 figures, accepted in Transactions on Emerging
Telecommunications Technologies (ETT), formerly known as European
Transactions on Telecommunications (ETT
Resource Management in Multicarrier Based Cognitive Radio Systems
The ever-increasing growth of the wireless application and services affirms the importance of the effective usage of the limited radio spectrum. Existing spectrum management policies have led to significant spectrum under-utilization. Recent measurements showed that large range of the spectrum is sparsely used in both temporal and spatial manner. This conflict
between the inefficient usage of the spectrum and the continuous evolution in the wireless communication calls upon the development of more flexible management policies. Cognitive radio (CR) with the dynamic spectrum access (DSA) is considered to be a key technology in making the best solution of this conflict by allowing a group of secondary users (SUs) to share the radio spectrum originally allocated to the primary user (PUs). The operation of CR should not negatively alter the performance of the PUs. Therefore, the interference control along with the highly dynamic nature of PUs activities open up new resource allocation problems in CR systems. The resource allocation algorithms should ensure an effective share of the temporarily available frequency bands and deliver the solutions in timely fashion to cope with quick changes in the network.
In this dissertation, the resource management problem in multicarrier based CR systems is considered. The dissertation focuses on three main issues: 1) design of efficient resource allocation algorithms to allocate subcarriers and powers between SUs such that no harmful interference is introduced to PUs, 2) compare the spectral efficiency of using different multicarrier schemes in the CR physical layer, specifically, orthogonal frequency division multiplexing (OFDM) and filter bank multicarrier (FBMC) schemes, 3) investigate the impact of the different constraints values on the overall performance of the CR system.
Three different scenarios are considered in this dissertation, namely downlink transmission, uplink transmission, and relayed transmission. For every scenario, the optimal solution is examined and efficient sub-optimal algorithms are proposed to reduce the computational burden of obtaining the optimal solution. The suboptimal algorithms are developed by separate the subcarrier and power allocation into two steps in downlink and uplink scenarios. In the relayed scenario, dual decomposition technique is used to obtain an asymptotically optimal solution, and a joint heuristic algorithm is proposed to find the suboptimal solution. Numerical simulations show that the proposed suboptimal algorithms achieve a near optimal performance and perform better than the existing algorithms designed for cognitive and non-cognitive systems. Eventually, the ability of FBMC to overcome the OFDM drawbacks and achieve more spectral efficiency is verified which recommends the consideration of FBMC in the future CR systems.El crecimiento continuo de las aplicaciones y servicios en sistemas inal´ambricos, indica la
importancia y necesidad de una utilizaci´on eficaz del espectro radio. Las pol´ıticas actuales de
gesti´on del espectro han conducido a una infrautilizaci´on del propio espectro radioel´ectrico.
Recientes mediciones en diferentes entornos han mostrado que gran parte del espectro queda
poco utilizado en sus ambas vertientes, la temporal, y la espacial. El permanente conflicto
entre el uso ineficiente del espectro y la evoluci´on continua de los sistemas de comunicaci´on
inal´ambrica, hace que sea urgente y necesario el desarrollo de esquemas de gesti´on del espectro
m´as flexibles.
Se considera el acceso din´amico (DSA) al espectro en los sistemas cognitivos como una
tecnolog´ıa clave para resolver este conflicto al permitir que un grupo de usuarios secundarios
(SUs) puedan compartir y acceder al espectro asignado inicialmente a uno o varios usuarios
primarios (PUs). Las operaciones de comunicaci´on llevadas a cabo por los sistemas radio
cognitivos no deben en ning´un caso alterar (interferir) los sistemas primarios. Por tanto, el
control de la interferencia junto al gran dinamismo de los sistemas primarios implica nuevos
retos en el control y asignaci´on de los recursos radio en los sistemas de comunicaci´on CR. Los
algoritmos de gesti´on y asignaci´on de recursos (Radio Resource Management-RRM) deben
garantizar una participaci´on efectiva de las bandas con frecuencias disponibles temporalmente,
y ofrecer en cada momento oportunas soluciones para hacer frente a los distintos cambios
r´apidos que influyen en la misma red.
En esta tesis doctoral, se analiza el problema de la gesti´on de los recursos radio en sistemas
multiportadoras CR, proponiendo varias soluciones para su uso eficaz y coexistencia con los
PUs. La tesis en s´ı, se centra en tres l´ıneas principales: 1) el dise˜no de algoritmos eficientes de gesti´on de recursos para la asignaci´on de sub-portadoras y distribuci´on de la potencia en
sistemas segundarios, evitando asi cualquier interferencia que pueda ser perjudicial para el
funcionamiento normal de los usuarios de la red primaria, 2) analizar y comparar la eficiencia
espectral alcanzada a la hora de utilizar diferentes esquema de transmisi´on multiportadora en
la capa f´ısica del sistema CR, espec´ıficamente en sistemas basados en OFDM y los basados en
banco de filtros multiportadoras (Filter bank Multicarrier-FBMC), 3) investigar el impacto de
las diferentes limitaciones en el rendimiento total del sistema de CR.
Los escenarios considerados en esta tesis son tres, es decir; modo de transmisi´on
descendente (downlink), modo de transmisi´on ascendente (uplink), y el modo de transmisi´on
”Relay”. En cada escenario, la soluci´on ´optima es examinada y comparada con algoritmos sub-
´optimos que tienen como objetivo principal reducir la carga computacional. Los algoritmos
sub-´optimos son llevados a cabo en dos fases mediante la separaci´on del propio proceso de
distribuci´on de subportadoras y la asignaci´on de la potencia en los modos de comunicaci´on
descendente (downlink), y ascendente (uplink). Para los entornos de tipo ”Relay”, se ha
utilizado la t´ecnica de doble descomposici´on (dual decomposition) para obtener una soluci´on
asint´oticamente ´optima. Adem´as, se ha desarrollado un algoritmo heur´ıstico para poder obtener
la soluci´on ´optima con un reducido coste computacional.
Los resultados obtenidos mediante simulaciones num´ericas muestran que los algoritmos
sub-´optimos desarrollados logran acercarse a la soluci´on ´optima en cada uno de los entornos
analizados, logrando as´ı un mayor rendimiento que los ya existentes y utilizados tanto en
entornos cognitivos como no-cognitivos. Se puede comprobar en varios resultados obtenidos
en la tesis la superioridad del esquema multiportadora FBMC sobre los sistemas basados en
OFDM para los entornos cognitivos, causando una menor interferencia que el OFDM en
los sistemas primarios, y logrando una mayor eficiencia espectral. Finalmente, en base a lo
analizado en esta tesis, podemos recomendar al esquema multiportadora FBMC como una
id´onea y potente forma de comunicaci´on para las futuras redes cognitivas
Radio resource allocation in relay based OFDMA cellular networks
PhDAdding relay stations (RS) between the base station (BS) and the mobile stations (MS) in a cellular system can extend network coverage, overcome multi-path fading and increase the capacity of the system.
This thesis considers the radio resource allocation scheme in relay based cellular networks to ensure high-speed and reliable communication. The goal of this research is to investigate user fairness, system throughput and power consumption in wireless relay networks through considering how best to manage the radio resource.
This thesis proposes a two-hop proportional fairness (THPF) scheduling scheme fair allocation, which is considered both in the first time subslot between direct link users and relay stations, and the second time subslot among relay link users.
A load based relay selection algorithm is also proposed for a fair resource allocation. The transmission mode (direct transmission mode or relay transmission mode) of each user will be adjusted based on the load of the transmission node.
Power allocation is very important for resource efficiency and system performance improvement and this thesis proposes a two-hop power allocation algorithm for energy efficiency, which adjusts the transmission power of the BS and RSs to make the data rate on the two hop links of one RS match each other.
The power allocation problem of multiple cells with inter-cell interference is studied. A new multi-cell power allocation scheme is proposed from non-cooperative game theory; this coordinates the inter-cell interference and operates in a distributed manner. The utility function can be designed for throughput improvement and user fairness respectively.
Finally, the proposed algorithms in this thesis are combined, and the system performance is evaluated. The joint radio resource allocation algorithm can achieve a very good tradeoff between throughput and user fairness, and also can significantly improve energy efficiency
OFDMA-based resource allocation for wireless communication systems
Ph.DDOCTOR OF PHILOSOPH
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