1,076 research outputs found
Cooperative Symbol-Based Signaling for Networks with Multiple Relays
Wireless channels suffer from severe inherent impairments and hence
reliable and high data rate wireless transmission is particularly challenging to
achieve. Fortunately, using multiple antennae improves performance in wireless
transmission by providing space diversity, spatial multiplexing, and power gains.
However, in wireless ad-hoc networks multiple antennae may not be acceptable
due to limitations in size, cost, and hardware complexity. As a result, cooperative
relaying strategies have attracted considerable attention because of their abilities
to take advantage of multi-antenna by using multiple single-antenna relays.
This study is to explore cooperative signaling for different relay networks,
such as multi-hop relay networks formed by multiple single-antenna relays and
multi-stage relay networks formed by multiple relaying stages with each stage
holding several single-antenna relays. The main contribution of this study is the
development of a new relaying scheme for networks using symbol-level
modulation, such as binary phase shift keying (BPSK) and quadrature phase shift
keying (QPSK). We also analyze effects of this newly developed scheme when it
is used with space-time coding in a multi-stage relay network. Simulation results
demonstrate that the new scheme outperforms previously proposed schemes:
amplify-and-forward (AF) scheme and decode-and-forward (DF) scheme
Towards the Optimal Amplify-and-Forward Cooperative Diversity Scheme
In a slow fading channel, how to find a cooperative diversity scheme that
achieves the transmit diversity bound is still an open problem. In fact, all
previously proposed amplify-and-forward (AF) and decode-and-forward (DF)
schemes do not improve with the number of relays in terms of the diversity
multiplexing tradeoff (DMT) for multiplexing gains r higher than 0.5. In this
work, we study the class of slotted amplify-and-forward (SAF) schemes. We first
establish an upper bound on the DMT for any SAF scheme with an arbitrary number
of relays N and number of slots M. Then, we propose a sequential SAF scheme
that can exploit the potential diversity gain in the high multiplexing gain
regime. More precisely, in certain conditions, the sequential SAF scheme
achieves the proposed DMT upper bound which tends to the transmit diversity
bound when M goes to infinity. In particular, for the two-relay case, the
three-slot sequential SAF scheme achieves the proposed upper bound and
outperforms the two-relay non-orthorgonal amplify-and-forward (NAF) scheme of
Azarian et al. for multiplexing gains r < 2/3. Numerical results reveal a
significant gain of our scheme over the previously proposed AF schemes,
especially in high spectral efficiency and large network size regime.Comment: 30 pages, 11 figures, submitted to IEEE trans. IT, revised versio
DMT Optimal On-Demand Relaying for Mesh Networks
This paper presents a new cooperative MAC (Medium Access Control) protocol called BRIAF (Best Relay based Incremental Amplify-and-Forward). The proposed protocol presents two features: on-demand relaying and selection of the best relay terminal. âOn-demand relayingâ means that a cooperative transmission is implemented between a source terminal and a destination terminal only when the destination terminal fails in decoding the data transmitted by the source terminal. This feature maximizes the spatial multiplexing gain r of the transmission. âSelection of the best relay terminalâ means that a selection of the best relay among a set of (m-1) relay candidates is implemented when a cooperative transmission is needed. This feature maximizes the diversity order d(r) of the transmission. Hence, an optimal DMT (Diversity Multiplexing Tradeoff) curve is achieved with a diversity order d(r) = m(1-r) for 0 †r †1
DMT Optimal Cooperative Protocols with Destination-Based Selection of the Best Relay
We design a cooperative protocol in the context of wireless mesh networks in order to increase the reliability of wireless links. Destination terminals ask for cooperation when they fail in decoding data frames transmitted by source terminals. In that case, each destination terminal D calls a specific relay terminal B with a signaling frame to help its transmission with source terminal S. To select appropriate relays, destination terminals maintain tables of relay terminals, one for each possible source address. These tables are constituted by passively overhearing ongoing transmissions. Hence, when cooperation is needed between S and D, and when a relay B is found by terminal D in the relay table associated with terminal S, the destination terminal sends a negative acknowledgment frame that contains the address of B. When the best relay B has successfully decoded the source message, it sends a copy of the data frame to D using a selective decode-andforward transmission scheme. The on-demand approach allows maximization of the spatial multiplexing gain and the cooperation of the best relay allows maximization of the spatial diversity order. Hence, the proposed protocol achieves optimal diversitymultiplexing trade-off performance. Moreover, this performance is achieved through a collision-free selection process
Optimal space-time codes for the MIMO amplify-and-forward cooperative channel
In this work, we extend the non-orthogonal amplify-and-forward (NAF)
cooperative diversity scheme to the MIMO channel. A family of space-time block
codes for a half-duplex MIMO NAF fading cooperative channel with N relays is
constructed. The code construction is based on the non-vanishing determinant
criterion (NVD) and is shown to achieve the optimal diversity-multiplexing
tradeoff (DMT) of the channel. We provide a general explicit algebraic
construction, followed by some examples. In particular, in the single relay
case, it is proved that the Golden code and the 4x4 Perfect code are optimal
for the single-antenna and two-antenna case, respectively. Simulation results
reveal that a significant gain (up to 10dB) can be obtained with the proposed
codes, especially in the single-antenna case.Comment: submitted to IEEE Transactions on Information Theory, revised versio
Recovering Multiplexing Loss Through Successive Relaying Using Repetition Coding
In this paper, a transmission protocol is studied for a two relay wireless
network in which simple repetition coding is applied at the relays.
Information-theoretic achievable rates for this transmission scheme are given,
and a space-time V-BLAST signalling and detection method that can approach them
is developed. It is shown through the diversity multiplexing tradeoff analysis
that this transmission scheme can recover the multiplexing loss of the
half-duplex relay network, while retaining some diversity gain. This scheme is
also compared with conventional transmission protocols that exploit only the
diversity of the network at the cost of a multiplexing loss. It is shown that
the new transmission protocol offers significant performance advantages over
conventional protocols, especially when the interference between the two relays
is sufficiently strong.Comment: To appear in the IEEE Transactions on Wireless Communication
Ergodic capacity of the exponentially correlated slotted amplify and forward relay channel
In this paper we analyze the performance degradation of slotted amplify-and-forward protocol in wireless environments with high node density where the number of relays grows asymptotically large. Channel gains between source-destination pairs in such networks can no longer be independent. We analyze the degradation of performance in such wireless environments where channel gains are exponentially correlated by looking at the capacity per channel use. Theoretical results for eigenvalue distribution and the capacity are derived and compared with the simulation results. Both analytical and simulated results show that the capacity given by the asymptotic mutual information decreases with the network density
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
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