580 research outputs found
Study of Gaussian Relay Channels with Correlated Noises
In this paper, we consider full-duplex and half-duplex Gaussian relay
channels where the noises at the relay and destination are arbitrarily
correlated. We first derive the capacity upper bound and the achievable rates
with three existing schemes: Decode-and-Forward (DF), Compress-and-Forward
(CF), and Amplify-and-Forward (AF). We present two capacity results under
specific noise correlation coefficients, one being achieved by DF and the other
being achieved by direct link transmission (or a special case of CF). The
channel for the former capacity result is equivalent to the traditional
Gaussian degraded relay channel and the latter corresponds to the Gaussian
reversely-degraded relay channel. For CF and AF schemes, we show that their
achievable rates are strictly decreasing functions over the negative
correlation coefficient. Through numerical comparisons under different channel
settings, we observe that although DF completely disregards the noise
correlation while the other two can potentially exploit such extra information,
none of the three relay schemes always outperforms the others over different
correlation coefficients. Moreover, the exploitation of noise correlation by CF
and AF accrues more benefit when the source-relay link is weak. This paper also
considers the optimal power allocation problem under the correlated-noise
channel setting. With individual power constraints at the relay and the source,
it is shown that the relay should use all its available power to maximize the
achievable rates under any correlation coefficient. With a total power
constraint across the source and the relay, the achievable rates are proved to
be concave functions over the power allocation factor for AF and CF under
full-duplex mode, where the closed-form power allocation strategy is derived.Comment: 24 pages, 7 figures, submitted to IEEE Transactions on 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
On the optimization of distributed compression in multirelay cooperative networks
In this paper, we consider multirelay cooperative networks for the Rayleigh fading channel, where each relay, upon receiving its own channel observation, independently compresses it and forwards the compressed information to the destination. Although the compression at each relay is distributed using Wyner-Ziv coding, there exists an opportunity for jointly optimizing compression at multiple relays to maximize the achievable rate. Considering Gaussian signaling, a primal optimization problem is formulated accordingly. We prove that the primal problem can be solved by resorting to its Lagrangian dual problem, and an iterative optimization algorithm is proposed. The analysis is further extended to a hybrid scheme, where the employed forwarding scheme depends on the decoding status of each relay. The relays that are capable of successful decoding perform a decode-and-forward (DF) scheme, and the rest conduct distributed compression. The hybrid scheme allows the cooperative network to adapt to the changes of the channel conditions and benefit from an enhanced level of flexibility. Numerical results from both spectrum and energy efficiency perspectives show that the joint optimization improves efficiency of compression and identify the scenarios where the proposed schemes outperform the conventional forwarding schemes. The findings provide important insights into the optimal deployment of relays in a realistic cellular network
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
PRACTICAL QUANTIZE-AND-FORWARD SCHEMES FOR THE FREQUENCY RELAY CHANNEL
International audienceWe consider static and quasi-static relay channels in which the source-destination and relay-destination signals are assumed to be orthogonal and thus have to be recombined at the destination. We propose cheap relaying schemes that are optimized from the knowledge of the signal-to-noise ratios (SNRs) of the source-relay and relay-destination channels at the relay. For this purpose the scheme under investigation is assumed to be scalar and have to minimize the mean square error between the source signal and its reconstructed version at the destination. We propose a quantize-and-forward (QF) scheme, which is a generalization of techniques based on joint source-channel coding. To further improve the receiver performance when the source-relay SNR is relatively poor we propose a Maximum Likelihood detector (MLD) designed for the QF protocol
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