514 research outputs found
Adaptive Modulation and Coding and Cooperative ARQ in a Cognitive Radio System
In this paper, a joint cross-layer design of adaptive modulation and coding
(AMC) and cooperative automatic repeat request (C-ARQ) scheme is proposed for a
secondary user in a shared-spectrum environment. First, based on the
statistical descriptions of the channel, closed-form expressions of the average
spectral efficiency (SE) and the average packet loss rate (PLR) are presented.
Then, the cross-layer scheme is designed, with the aim of maximizing the
average SE while maintaining the average PLR under a prescribed level. An
optimization problem is formed, and a sub-optimal solution is found: the target
packet error rates (PER) for the secondary system channels are obtained and the
corresponding sub-optimal AMC rate adaptation policy is derived based on the
target PERs. Finally, the average SE and the average PLR performance of the
proposed scheme are presented
Green Cellular Networks: A Survey, Some Research Issues and Challenges
Energy efficiency in cellular networks is a growing concern for cellular
operators to not only maintain profitability, but also to reduce the overall
environment effects. This emerging trend of achieving energy efficiency in
cellular networks is motivating the standardization authorities and network
operators to continuously explore future technologies in order to bring
improvements in the entire network infrastructure. In this article, we present
a brief survey of methods to improve the power efficiency of cellular networks,
explore some research issues and challenges and suggest some techniques to
enable an energy efficient or "green" cellular network. Since base stations
consume a maximum portion of the total energy used in a cellular system, we
will first provide a comprehensive survey on techniques to obtain energy
savings in base stations. Next, we discuss how heterogeneous network deployment
based on micro, pico and femto-cells can be used to achieve this goal. Since
cognitive radio and cooperative relaying are undisputed future technologies in
this regard, we propose a research vision to make these technologies more
energy efficient. Lastly, we explore some broader perspectives in realizing a
"green" cellular network technologyComment: 16 pages, 5 figures, 2 table
Cooperative diversity techniques for high-throughput wireless relay networks
Relay communications has attracted a growing interest in wireless communications with application to various enhanced technologies. This thesis considers a number of issues related to data throughput in various wireless relay network models. Particularly, new implementations of network coding (NC) and space-time coding (STC) techniques are investigated to offer various means of achieving high-throughput relay communications.
Firstly, this thesis investigates different practical automatic repeat request (ARQ) retransmission protocols based on NC for two-way wireless relay networks to improve throughput efficiency. Two improved NC-based ARQ schemes are designed based on go-back-N and selective-repeat (SR) protocols. Addressing ARQ issues in multisource multidestination relay networks, a new NC-based ARQ protocol is proposed and two packet-combination algorithms are developed for retransmissions at relay and sources to significantly improve the throughput.
In relation to the concept of channel quality indicator (CQI) reporting in two-way relay networks, two new efficient CQI reporting schemes are designed based on NC to improve the system throughput by allowing two terminals to simultaneously estimate the CQI of the distant terminal-relay link without incurring additional overhead. The transmission time for CQI feedback at the relays is reduced by half while the increase in complexity and the loss of performance are shown to be negligible. Furthermore, a low-complexity relay selection scheme is suggested to reduce the relay searching complexity.
For the acknowledgment (ACK) process, this thesis proposes a new block ACK scheme based on NC to significantly reduce the ACK overheads and therefore produce an enhanced throughput. The proposed scheme is also shown to improve the reliability of block ACK transmission and reduce the number of data retransmissions for a higher system throughput. Additionally, this thesis presents a new cooperative retransmission scheme based on relay cooperation and NC to considerably reduce the number of retransmission packets and im- prove the reliability of retransmissions for a more power efficient and higher throughput system with non-overlapped retransmissions. Moreover, two relay selection schemes are recommended to determine the optimised number of relays for the retransmission.
Finally, with respect to cognitive wireless relay networks (CWRNs), this thesis proposes a new cooperative spectrum sensing (CSS) scheme to improve the spectrum sensing performance and design a new CSS scheme based on NC for three-hop CWRNs to improve system throughput. Furthermore, a new distributed space-time-frequency block code (DSTFBC) is designed for a two- hop nonregenerative CWRN over frequency-selective fading channels. The proposed DSTFBC design achieves higher data rate, spatial diversity gain, and decoupling detection of data blocks at all destination nodes with a low-complexity receiver structure
Permutation Trellis Coded Multi-level FSK Signaling to Mitigate Primary User Interference in Cognitive Radio Networks
We employ Permutation Trellis Code (PTC) based multi-level Frequency Shift
Keying signaling to mitigate the impact of Primary Users (PUs) on the
performance of Secondary Users (SUs) in Cognitive Radio Networks (CRNs). The
PUs are assumed to be dynamic in that they appear intermittently and stay
active for an unknown duration. Our approach is based on the use of PTC
combined with multi-level FSK modulation so that an SU can improve its data
rate by increasing its transmission bandwidth while operating at low power and
not creating destructive interference for PUs. We evaluate system performance
by obtaining an approximation for the actual Bit Error Rate (BER) using
properties of the Viterbi decoder and carry out a thorough performance analysis
in terms of BER and throughput. The results show that the proposed coded system
achieves i) robustness by ensuring that SUs have stable throughput in the
presence of heavy PU interference and ii) improved resiliency of SU links to
interference in the presence of multiple dynamic PUs.Comment: 30 pages, 12 figure
- …