8 research outputs found
Comparative public administration: The dynamics of local administrative reform in Britain and Nigeria.
SIGLEAvailable from British Library Document Supply Centre- DSC:D38907/82 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Quasi-orthogonal space-frequency coding in non-coherent cooperative broadband networks
© 2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.So far, complex valued orthogonal codes have been used differentially in cooperative broadband networks. These codes however achieve less than unitary code rate when utilized in cooperative networks with more than two relays. Therefore, the main challenge is how to construct unitary rate codes for non-coherent cooperative broadband networks with more than two relays while exploiting the achievable spatial and frequency diversity. In this paper, we extend full rate quasi-orthogonal codes to differential cooperative broadband networks where channel information is unavailable. From this, we propose a generalized differential distributed quasi-orthogonal space-frequency coding (DQSFC) protocol for cooperative broadband networks. Our proposed scheme is able to achieve full rate, and full spatial and frequency diversity in cooperative networks with any number of relays. Through pairwise error probability analysis we show that the diversity gain of our scheme can be improved by appropriate code construction and sub-carrier allocation. Based on this, we derive sufficient conditions for the proposed code structure at the source node and relay nodes to achieve full spatial and frequency diversity.Peer reviewe
Efficient space-frequency block coded pilot-aided channel estimation method for multiple-input-multiple-output orthogonal frequency division multiplexing systems over mobile frequency-selective fading channels
© 2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.An iterative pilot-aided channel estimation technique for space-frequency block coded (SFBC) multiple-input multiple-output orthogonal frequency division multiplexing systems is proposed. Traditionally, when channel estimation techniques are utilised, the SFBC information signals are decoded one block at a time. In the proposed algorithm, multiple blocks of SFBC information signals are decoded simultaneously. The proposed channel estimation method can thus significantly reduce the amount of time required to decode information signals compared to similar channel estimation methods proposed in the literature. The proposed method is based on the maximum likelihood approach that offers linearity and simplicity of implementation. An expression for the pairwise error probability (PEP) is derived based on the estimated channel. The derived PEP is then used to determine the optimal power allocation for the pilot sequence. The performance of the proposed algorithm is demonstrated in high frequency selective channels, for different number of pilot symbols, using different modulation schemes. The algorithm is also tested under different levels of Doppler shift and for different number of transmit and receive antennas. The results show that the proposed scheme minimises the error margin between slow and high speed receivers compared to similar channel estimation methods in the literature.Peer reviewe
A Systematic Study of the Behaviour of PMEPR in Relation to OFDM Design Parameters
The design of systems with enhanced quality of service (QoS) and improved power efficiency has evolved into an intensive research area in wired and wireless communications engineering. Orthogonal frequency division multiplexing (OFDM) has been proven to have the potential to achieve high data rates, adapt to severe channel conditions and exhibit spectral efficiency; this has gained its popular support in the design industry, especially for fourth generation (4G) systems. However, the high peak to mean envelope power ratio (PMEPR) exhibited by OFDM signals require linear operation of analog devices, with the associated trade-off of poor power efficiency. Several methods to reduce this PMEPR problem have been effectively researched while revealing the shortcomings. In this study we recognize the need to present the effect of OFDM system parameters on the behaviour of the PMEPR. In order to provide a basis for systematic selection of OFDM design parameters for PMEPR mitigation, we first study the reaction of the PMEPR to OFDM design parameters, we then analyse the effect of OFDM design parameters on the shortcomings of the PMEPR-limiting clipping technique.Peer reviewe
Co-Efficient Vector Based Differential Distributed Quasi-Orthogonal Space Time Frequency Coding
© 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).Distributed space time frequency coding (DSTFC) schemes address problems of performance degradation encountered by cooperative broadband networks operating in highly mobile environments. Channel state information (CSI) acquisition is, however, impractical in such highly mobile environments. Therefore, to address this problem, designers focus on incorporating differential designs with DSTFC for signal recovery in environments where neither the relay nodes nor destination have CSI. Traditionally, unitary matrix-based differential designs have been used to generate the differentially encoded symbols and codeword matrices. Unitary based designs are suitable for cooperative networks that utilize the amplify-and-forward protocol where the relay nodes are typically required to forego differential decoding. In considering other scenarios where relay nodes are compelled to differentially decode and re-transmit information signals, we propose a novel co-efficient vector differential distributed quasi-orthogonal space time frequency coding (DQSTFC) scheme for decode-and-forward cooperative networks. Our proposed space time frequency coding scheme relaxes the need for constant channel gain in the temporal and frequency dimensions over long symbol periods; thus, performance degradation is reduced in frequency-selective and time-selective fading environments. Simulation results illustrate the performance of our proposed co-efficient vector differential DQSTFC scheme under different channel conditions. Through pair-wise error probability analysis, we derive the full diversity design criteria for our code.Peer reviewe
Design and Performance Analysis of Distributed Space Time Coding Schemes for Cooperative Wireless Networks
In this thesis, space-time block codes originally developed for multiple antenna systems are
extended to cooperative multi-hop networks. The designs are applicable to any wireless
network setting especially cellular, adhoc and sensor networks where space limitations
preclude the use of multiple antennas. The thesis first investigates the design of distributed
orthogonal and quasi-orthogonal space time block codes in cooperative networks with single
and multiple antennas at the destination. Numerical and simulation results show that by
employing multiple receive antennas the diversity performance of the network is further
improved at the expense of slight modification of the detection scheme. The thesis then
focuses on designing distributed space time block codes for cooperative networks in which
the source node participates in cooperation. Based on this, a source-assisting strategy is
proposed for distributed orthogonal and quasi-orthogonal space time block codes. Numerical
and simulation results show that the source-assisting strategy exhibits improved diversity
performance compared to the conventional distributed orthogonal and quasi-orthogonal
designs.Motivated by the problem of channel state information acquisition in practical wireless
network environments, the design of differential distributed space time block codes is
investigated. Specifically, a co-efficient vector-based differential encoding and decoding
scheme is proposed for cooperative networks. The thesis then explores the concatenation of
differential strategies with several distributed space time block coding schemes namely; the
Alamouti code, square-real orthogonal codes, complex-orthogonal codes, and quasiorthogonal
codes, using cooperative networks with different number of relay nodes. In order
to cater for high data rate transmission in non-coherent cooperative networks, differential distributed quasi-orthogonal space-time block codes which are capable of achieving full
code-rate and full diversity are proposed. Simulation results demonstrate that the differential
distributed quasi-orthogonal space-time block codes outperform existing distributed space
time block coding schemes in terms of code rate and bit-error-rate performance. A multidifferential
distributed quasi-orthogonal space-time block coding scheme is also proposed to
exploit the additional diversity path provided by the source-destination link.A major challenge is how to construct full rate codes for non-coherent cooperative broadband
networks with more than two relay nodes while exploiting the achievable spatial and
frequency diversity. In this thesis, full rate quasi-orthogonal codes are designed for noncoherent
cooperative broadband networks where channel state information is unavailable.
From this, a generalized differential distributed quasi-orthogonal space-frequency coding
scheme is proposed for cooperative broadband networks. The proposed scheme is able to
achieve full rate and full spatial and frequency diversity in cooperative networks with any
number of relays. Through pairwise error probability analysis we show that the diversity gain
of the proposed scheme can be improved by appropriate code construction and sub-carrier
allocation. Based on this, sufficient conditions are derived for the proposed code structure at
the source node and relay nodes to achieve full spatial and frequency diversity. In order to
exploit the additional diversity paths provided by the source-destination link, a novel multidifferential
distributed quasi-orthogonal space-frequency coding scheme is proposed. The
overall objective of the new scheme is to improve the quality of the detected signal at the
destination with negligible increase in the computational complexity of the detector.Finally, a differential distributed quasi-orthogonal space-time-frequency coding scheme is
proposed to cater for high data rate transmission and improve the performance of noncoherent
cooperative broadband networks operating in highly mobile environments. The approach is to integrate the concept of distributed space-time-frequency coding with
differential modulation, and employ rotated constellation quasi-orthogonal codes. From this,
we design a scheme which is able to address the problem of performance degradation in
highly selective fading environments while guaranteeing non-coherent signal recovery and
full code rate in cooperative broadband networks. The coding scheme employed in this thesis
relaxes the assumption of constant channel variation in the temporal and frequency
dimensions over long symbol periods, thus performance degradation is reduced in frequencyselective
and time-selective fading environments. Simulation results illustrate the
performance of the proposed differential distributed quasi-orthogonal space-time-frequency
coding scheme under different channel conditions
Source-assisting strategy for differential distributed space time block codes
In this paper, a source-assisting differential distributed space time block coding (SA-DDSTBC) scheme is proposed for cooperative networks. Firstly, in most existing works on distributed space time block coding (DSTBC), the destination node is assumed to have perfect channel state information (CSI), thus, signal recovery is straight forward. In practice however, some scenarios exist whereby the destination node is unable to acquire CSI. Consequently, this work incorporates differential concepts with DSTBC to facilitate signal recovery in cooperative networks operating in environments where CSI acquisition is impractical. Secondly, different from most works on DSTBC which assume that the source-destination link is unavailable, the proposed scheme employs a source-assisting (SA) strategy that exploits the additional diversity path provided by the source-destination link. The numerical and simulation results obtained illustrate that compared to the conventional DSTBC schemes, the proposed SA-DDSTBC scheme achieves non-coherent signal recovery and improved BER and diversity performance with negligible increase in decoding complexity
STBC Based Pilot-Aided Channel Estimation Method for SFBC-OFDM Systems
© 2014 PGNetIn this paper, an iterative channel estimation algorithm with joint detection is proposed for Multiple Input Multiple Output (MIMO) systems combined with Orthogonal Frequency Division Multiplexing (OFDM) also known as MIMO-OFDM systems using Space Time Block Codes (STBC) and Space Frequency Block Codes (SFBC). The proposed algorithm is based on an iterative process where pilot subcarriers are used for channel estimation in the first time instant, and then in the second time instant, the estimated channel is used to decode the data symbol in the adjacent data subcarrier. Once data symbols are recovered, the system recursively performs a new channel estimation using the decoded data symbols as pilots. The iterative process is repeated until all MIMO-OFDM symbols are recovered. In addition, the proposed channel estimation technique is based on the maximum likelihood (ML) approach which offers linearity and simplicity of implementation. The method also reduces the processing time via the use of subcarrier grouping for transmitted data recovery. From this we further propose an STBC and SFBC alternating scheme for pilot and data subcarriers. In this scheme STBC and SFBC are used for pilot and data subcarriers respectively, or vice versa. In addition, the efficiency of the method is demonstrated through computer simulation using multiple transmit and receive antennas and different modulation schemes.Peer reviewe