328 research outputs found
Diversity-Multiplexing Tradeoff of Asynchronous Cooperative Diversity in Wireless Networks
Synchronization of relay nodes is an important and critical issue in
exploiting cooperative diversity in wireless networks. In this paper, two
asynchronous cooperative diversity schemes are proposed, namely, distributed
delay diversity and asynchronous space-time coded cooperative diversity
schemes. In terms of the overall diversity-multiplexing (DM) tradeoff function,
we show that the proposed independent coding based distributed delay diversity
and asynchronous space-time coded cooperative diversity schemes achieve the
same performance as the synchronous space-time coded approach which requires an
accurate symbol-level timing synchronization to ensure signals arriving at the
destination from different relay nodes are perfectly synchronized. This
demonstrates diversity order is maintained even at the presence of asynchronism
between relay node. Moreover, when all relay nodes succeed in decoding the
source information, the asynchronous space-time coded approach is capable of
achieving better DM-tradeoff than synchronous schemes and performs equivalently
to transmitting information through a parallel fading channel as far as the
DM-tradeoff is concerned. Our results suggest the benefits of fully exploiting
the space-time degrees of freedom in multiple antenna systems by employing
asynchronous space-time codes even in a frequency flat fading channel. In
addition, it is shown asynchronous space-time coded systems are able to achieve
higher mutual information than synchronous space-time coded systems for any
finite signal-to-noise-ratio (SNR) when properly selected baseband waveforms
are employed
Distributed space-time coding including the golden code with application in cooperative networks
This thesis presents new methodologies to improve performance of wireless cooperative networks using the Golden Code. As a form of space-time coding, the Golden Code can achieve diversity-multiplexing tradeoff and the data rate can be twice that of the Alamouti code. In practice, however, asynchronism between relay nodes may reduce performance and channel quality can be degraded from certain antennas.
Firstly, a simple offset transmission scheme, which employs full interference cancellation (FIC) and orthogonal frequency division multiplexing (OFDM), is enhanced through the use of four relay nodes and receiver processing to mitigate asynchronism. Then, the potential reduction in diversity gain due to the dependent channel matrix elements in the distributed Golden Code transmission, and the rate penalty of multihop transmission, are mitigated by relay selection based on two-way transmission. The Golden Code is also implemented in an asynchronous one-way relay network over frequency flat and selective channels, and a simple approach to overcome asynchronism is proposed. In one-way communication with computationally efficient sphere decoding, the maximum of the channel parameter means is shown to achieve the best performance for the relay selection through bit error rate simulations.
Secondly, to reduce the cost of hardware when multiple antennas are available in a cooperative network, multi-antenna selection is exploited. In this context, maximum-sum transmit antenna selection is proposed. End-to-end signal-to-noise ratio (SNR) is calculated and outage probability analysis is performed when the links are modelled as Rayleigh fading frequency flat channels. The numerical results support the analysis and for a MIMO system
maximum-sum selection is shown to outperform maximum-minimum selection. Additionally, pairwise error probability (PEP) analysis is performed for maximum-sum transmit antenna selection with the Golden Code and the diversity order is obtained.
Finally, with the assumption of fibre-connected multiple antennas with finite buffers, multiple-antenna selection is implemented on the basis of maximum-sum antenna selection. Frequency flat Rayleigh fading channels are assumed together with a decode and forward transmission scheme. Outage probability analysis is performed by exploiting the steady-state stationarity of a Markov Chain model
Cooperative diversity for the cellular uplink: Sharing strategies, performance analysis, and receiver design
In this thesis, we propose data sharing schemes for the cooperative diversity in a cellular uplink to exploit diversity and enhance throughput performance of the system. Particularly, we consider new two and three-or-more user decode and forward (DF) protocols using space time block codes. We discuss two-user and three-user amplify and forward (AF) protocols and evaluate the performance of the above mentioned data sharing protocols in terms of the bit error rate and the throughput in an asynchronous code division multiple access (CDMA) cellular uplink. We develop a linear receiver for joint space-time decoding and multiuser detection that provides full diversity and near maximum-likelihood performance.;We also focus on a practical situation where inter-user channel is noisy and cooperating users can not successfully estimate other user\u27s data. We further design our system model such that, users decide not to forward anything in case of symbol errors. Channel estimation plays an important role here, since cooperating users make random estimation errors and the base station can not have the knowledge of the errors or the inter-user channels. We consider a training-based approach for channel estimation. We provide an information outage probability analysis for the proposed multi-user sharing schemes. (Abstract shortened by UMI.)
Coherent versus non-coherent decode-and-forward relaying aided cooperative space-time shift keying
Motivated by the recent concept of Space-Time Shift Keying (STSK), we propose a novel cooperative STSK family, which is capable of achieving a flexible rate-diversity tradeoff, in the context of cooperative space-time transmissions. More specifically, we first propose a Coherent cooperative STSK (CSTSK) scheme, where each Relay Node (RN) activates Decode-and-Forward (DF) transmissions, depending on the success or failure of Cyclic Redundancy Checking (CRC). We invoke a bitto- STSK mapping rule, where according to the input bits, one of the Q pre-assigned dispersion vectors is activated to implicitly convey log2(Q) bits, which are transmitted in combination with the classic log2(L)-bit modulated symbol. Additionally, we introduce a beneficial dispersion vector design, which enables us to dispense with symbol-level Inter-Relay Synchronization (IRS). Furthermore, the Destination Node (DN) is capable of jointly detecting the signals received from the source-destination and relay-destination links, using a low-complexity single-stream-based Maximum Likelihood (ML) detector, which is an explicit benefit of our Inter-Element Interference (IEI)-free system model. More importantly, as a benefit of its design flexibility, our cooperative CSTSK arrangement enables us to adapt the number of the RNs, the transmission rate as well as the achievable diversity order. Moreover, we also propose a Differentially-encoded cooperative STSK (DSTSK) arrangement, which dispenses with CSI estimation at any of the nodes, while retaining the fundamental benefits of the cooperative CSTSK scheme
Communication over Asynchronous Networks: Signaling and Rate-Reliability Analysis
Asynchronism inherently exists in many communication systems specially in multi-terminal networks mainly due to the effect of multi-path and propagation delay. While in theoretical analysis
of communication systems perfect synchronization of the terminals is often presumed, in some
cases in which the nodes are randomly distributed over a geometrical area, it might be impossible
to synchronize the nodes even if an ideal infrastructure service provider is used. In this work,
two major categories of multi-user communication systems, i.e., relay networks and interference
channels, are considered and the effect of the asynchronism among the terminals on characteristic
properties of these channels are investigated.
In Chapter 2, the construction of distributed space-time codes for a general two-hop asynchronous cooperative relay network is considered. A novel algebraic structure is proposed and
shown to achieve full diversity for arbitrary number of relays, arbitrary input alphabets, and
arbitrary delay profiles among the relays. Unlike previously proposed delay tolerant schemes,
the new design has minimum length which translates into smaller decoding complexity at the
same transmission rate. Full-rate and full-diversity are achieved by the new designs with or without
the use of guard intervals between successive transmissions. Simulation results confirm the
mathematical analysis of the proposed codes.
In Chapter 3, the underlying asynchronous network is examined for various relaying protocols
such as non-orthogonal selection decode-and-forward, orthogonal selection decode-and-forward,
non-orthogonal amplify-and-forward (NAF), and orthogonal amplify-and-forward (OAF). The
transmitter nodes send pulse amplitude modulation (PAM) signals, in which information symbols
are linearly modulated by a shaping waveform to be sent to the destination, asynchronously. We
consider two different cases with respect to the type of the shaping waveforms used in the structure
of the PAM signals. In the theoretical case where band-limited shaping waveforms are used, it is
shown that the asynchronism does not affect the DMT performance of the system and the same
DMT as that of the corresponding synchronous network is obtained for all the aforementioned
protocols. In the practical case where time-limited shaping waveforms are used, it is shown that better diversity gains can be achieved at the expense of a bandwidth expansion. More precisely, in
the decode-and-forward type protocols, the asynchronous network provides a better diversity gain
than that of the corresponding synchronous network throughout the range of the multiplexing
gain. In the amplify-and-forward type protocols, the asynchronous network provides the same
DMT as that of the corresponding synchronous counterpart under the OAF protocol; however, a
better diversity gain is achieved under the NAF protocol throughout the range of the multiplexing
gain. In particular, in the single relay asynchronous network, the NAF protocol provides the same
DMT as that of the 2 × 1 multiple-input single-output channel.
In Chapter 4, a constant K-user interference channel in which the users are not symbol synchronous
is considered. It is shown that the asynchronism among the users does not affect
the total number of degrees of freedom (DOF) of this channel; however, it facilitates aligning
interfering signals at each receiver node. To achieve the total K/2 DOF of this channel when single
antenna nodes are used, a novel practical interference alignment scheme is proposed wherein the
alignment task is performed with the help of asynchronous delays which inherently exist among
the received signals at each receiver node. The asynchronism causes inter-symbol-interference
(ISI) among transmitted symbols by different transmitters resulting in the underlying quasi-static
links to be converted to ISI and accordingly into time varying channels. It is proved that
this conversion solves the lack of channel variation required for the interference alignment in
quasi-static scenarios. When each node is equipped with M > 1 antennas, it is argued that the
same alignment scheme proposed for the single antenna nodes’ interference channel is sufficient to
achieve the total MK/2 DOF of the medium provided that each pair of the transmitters and the
receivers experience the same asynchronous delay for all the corresponding antennas. In contrast
to previously proposed alignment schemes, the channel state information of the links does not
need to be known at the transmitter nodes. Instead, the relative delays among the received
signals at each receiver node are globally known to the entire network.
While the asynchronism is usually treated as a troublesome factor in communication systems,
in this dissertation, we are interested to introduce it as a useful property of the wireless medium
similar to the fading which can improve the system performance in some communication scenarios
or facilitate signaling over the medium in some other scenarios
Distributed space-time block codes for two-hop wireless relay networks
Recently, the idea of space-time coding has been applied to wireless relay networks wherein a set of geographically separated relay nodes cooperate to process the received signal from the source and forward them to the destination such that the signal received at the destination appears like a Space-Time Block Code (STBC). Such STBCs (referred to as Distributed Space-Time Block Codes (DSTBCs)) when appropriately designed are known to offer spatial diversity. It is known that different classes of DSTBCs can be designed primarily depending on (i) whether the Amplify and Forward (AF) protocol or the Decode and Forward (DF) protocol is employed at the relays and (ii) whether the relay nodes are synchronized or not. In this paper, we present a survey on the problems and results associated with the design of DSTBCs for the following classes of two-hop wireless relay networks: (i) synchronous relay networks with AF protocols, (ii) asynchronous relay networks with AF protocols (iii) synchronous relay networks with DF protocols and (iv) asynchronous relay Fig. 1. Co-located MIMO channel model networks with DF protocols
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