576 research outputs found
A universal space-time architecture for multiple-antenna aided systems
In this tutorial, we first review the family of conventional multiple-antenna techniques, and then we provide a general overview of the recent concept of the powerful Multiple-Input Multiple-Output (MIMO) family based on a universal Space-Time Shift Keying (STSK) philosophy. When appropriately configured, the proposed STSK scheme has the potential of outperforming conventional MIMO arrangements
Power Optimisation and Relay Selection in Cooperative Wireless Communication Networks
Cooperative communications have emerged as a significant concept to improve reliability and throughput in wireless systems. In cooperative networks, the idea is to implement a scheme in wireless systems where the nodes can harmonize their resources thereby enhancing the network performance in different aspects such as latency, BER and throughput. As cooperation spans from the basic idea of transmit diversity achieved via MIMO techniques and the relay channel, it aims to reap somewhat multiple benefits of combating fading/burst errors, increasing throughput and reducing energy use. Another major benefit of cooperation in wireless networks is that since the concept only requires neighbouring nodes to act as virtual relay antennas, the concept evades the negative impacts of deployment costs of multiple physical antennas for network operators especially in areas where they are difficult to deploy. In cooperative communications energy efficiency and long network lifetimes are very important design issues, the focus in this work is on ad hoc and sensor network varieties where the nodes integrate sensing, processing and communication such that their cooperation capabilities are subject to power optimisation. As cooperation communications leads to trade-offs in Quality of Services and transmit power, the key design issue is power optimisation to dynamically combat channel fluctuations and achieve a net reduction of transmit power with the goal of saving battery life. Recent researches in cooperative communications focus on power optimisation achieved via power control at the PHY layer, and/or scheduling mechanism at the MAC layer. The approach for this work will be to review the power control strategy at the PHY layer, identify their associated trade-offs, and use this as a basis to propose a power control strategy that offers adaptability to channel conditions, the road to novelty in this work is a channel adaptable power control algorithm that jointly optimise power allocation, modulation strategy and relay selection.
Thus, a novel relay selection method is developed and implemented to improve the performance of cooperative wireless networks in terms of energy consumption. The relay selection method revolves on selection the node with minimum distance to the source and destination. The design is valid to any wireless network setting especially Ad-hoc and sensor networks where space limitations preclude the implementation of bigger capacity battery. The thesis first investigates the design of relay selection schemes in cooperative networks and the associated protocols. Besides, modulation strategy and error correction code impact on energy consumption are investigated and the optimal solution is proposed and jointly implemented with the relay selection method. The proposed algorithm is extended to cooperative networks in which multiple nodes participate in cooperation in fixed and variable rate system. Thus, multi relay selection algorithm is proposed to improve virtual MIMO performance in terms of energy consumption. Furthermore, motivated by the trend of cell size optimisation in wireless networks, the proposed relay selection method is extended to clustered wireless networks, and jointly implemented with virtual clustering technique.
The work will encompass three main stages: First, the cooperative system is designed and two major protocols Decode and Forward (DF) and amplify and forward (AF) are investigated. Second, the proposed algorithm is modelled and tested under different channel conditions with emphasis on its performance using different modulation strategies for different cooperative wireless networks. Finally, the performance of the proposed algorithm is illustrated and verified via computer simulations. Simulation results show that the distance based relay selection algorithm exhibits an improved performance in terms of energy consumption compared to the conventional cooperative schemes under different cooperative communication scenarios
Iterative receiver in multiuser relaying systems with fast frequency-hopping modulation
In this thesis, a novel iterative receiver and its improved version are proposed for
relay-assisted multiuser communications, in which multiple users transmit to a destination
with the help of a relay and using fast frequency-hopping modulation. Each
user employs a channel encoder to protect its information and facilitate interference
cancellation at the receiver. The signal received at the relay is either amplified, or
partially decoded with a simple energy detector, before being forwarded to the destination.
Under flat Rayleigh fading channels, the receiver at the destination can
be implemented non-coherently, i.e., it does not require the instantaneous channel
information to demodulate the users’ transmitted signals. The proposed iterative
algorithm at the destination exploits the soft outputs of the channel decoders to
successively extract the maximum likelihood symbols of the users and perform interference
cancellation. The iterative method is successfully applied for both cases of
amplify-and-forward and partial decode-and-forward relaying. The error performance
of the proposed iterative receiver is investigated by computer simulation. Under the
same spectral efficiency, simulation results demonstrate the excellent performance of
the proposed receiver when compared to the performance of decoding without interference
cancellation as well as the performance of the maximum likelihood multiuser
detection previously developed for uncoded transmission. Simulation results also suggest
that a proper selection of channel coding schemes can help to support significant
more users without consuming extra system resources.
In addition, to further enhance the receiver’s performance in terms of the bit error
rate, an improved version of the iterative receiver is presented. Such an improved receiver
invokes inner-loop iterations between the channel decoders and the demappers
in such a way that the soft outputs of the channel decoders are also used to refine the
outputs of the demappers for every outer-loop iteration. Simulation results indicate
a performance gain of about 2.5dB by using the two-loop receiver when compared to
the performance of the first proposed receiver
Effects of channel estimation on multiuser virtual MIMO-OFDMA relay-based networks
A practical multiuser cooperative transmission scheme denoted as Virtual Maximum Ratio Transmission (VMRT) for multiple-input multiple-output-orthogonal frequency division multiple access (MIMO-OFDMA) relay-based networks is proposed and evaluated in the presence of a realistic channel estimation algorithm and using low-density parity-check (LDPC) codes. It is shown that this scheme is robust against channel estimation errors. It offers diversity and array gain, keeping the complexity low with a multiuser and multiantenna channel estimation algorithm that is simple and efficient. In addition, the combination with LDPC codes provides improved gains; diversity gains larger than 6 dB can be easily obtained with a reduced number of relays. Thus, this scheme can be used to extend coverage or increase system throughput by using simple cooperative OFDMA-based relays.The authors would like to thank Jae-Yun Ko for his valuable
help at the beginning of our work. This work has been partly
funded by the projects MULTIADAPTIVE (TEC2008-06327-
C03-02), COMONSENS (CSD2008-00010) and CODIV
(ICT-2007-215477).Publicad
Effects of channel estimation on multiuser virtual MIMO-OFDMA relay-based networks
In this paper, a practical multi-user cooperative transmission scheme denoted as Virtual Maximum Ratio
Transmission (VMRT) for Multiple-Input Multiple-Output - Orthogonal Frequency Division Multiple Access
(MIMO-OFDMA) Relay-based networks is proposed and evaluated in the presence of a realistic channel estimation
algorithm. It is shown that this scheme is robust against channel estimation errors and offers diversity and array
gain keeping the complexity low, although the multi-user and multi-antenna channel estimation algorithm is simple
and efficient. Diversity gains larger than 4 can be easily obtained with reduced number of relays. Thus, this scheme
can be used to extend coverage or increase system throughput by using simple cooperative OFDMA-based relays
High-Rate Space-Time Coded Large MIMO Systems: Low-Complexity Detection and Channel Estimation
In this paper, we present a low-complexity algorithm for detection in
high-rate, non-orthogonal space-time block coded (STBC) large-MIMO systems that
achieve high spectral efficiencies of the order of tens of bps/Hz. We also
present a training-based iterative detection/channel estimation scheme for such
large STBC MIMO systems. Our simulation results show that excellent bit error
rate and nearness-to-capacity performance are achieved by the proposed
multistage likelihood ascent search (M-LAS) detector in conjunction with the
proposed iterative detection/channel estimation scheme at low complexities. The
fact that we could show such good results for large STBCs like 16x16 and 32x32
STBCs from Cyclic Division Algebras (CDA) operating at spectral efficiencies in
excess of 20 bps/Hz (even after accounting for the overheads meant for pilot
based training for channel estimation and turbo coding) establishes the
effectiveness of the proposed detector and channel estimator. We decode perfect
codes of large dimensions using the proposed detector. With the feasibility of
such a low-complexity detection/channel estimation scheme, large-MIMO systems
with tens of antennas operating at several tens of bps/Hz spectral efficiencies
can become practical, enabling interesting high data rate wireless
applications.Comment: v3: Performance/complexity comparison of the proposed scheme with
other large-MIMO architectures/detectors has been added (Sec. IV-D). The
paper has been accepted for publication in IEEE Journal of Selected Topics in
Signal Processing (JSTSP): Spl. Iss. on Managing Complexity in Multiuser MIMO
Systems. v2: Section V on Channel Estimation is update
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