Cooperative secrecy transmission in multi‐hop relay networks with interference alignment

In conventional digital communication systems, the quality of the received signal does not improve beyond a certain level as the channel quality increases. Such kind of quality saturation effect is caused by the unrecoverable quantization errors produced by source coding. The Hybrid Digital-Analog (HDA) transmission, where the quantization errors are transmitted in an analog mode along with the quantized data in a digital mode, has been recognized as an effective technique to combat the quality saturation effect. In this paper, we introduce HDA transmission in Wireless Relay Networks (WRNs) over Rayleigh slow-fading channels to eliminate the quality saturation effect and achieve graceful improvement for the better channel quality. Our goal is to minimize the end-to-end distortion by optimal power allocation. We note that digital-analog power allocation involved in HDA transmission is coupled with source-relay power allocation in WRNs. Therefore, the joint power allocation problem should be considered. We investigate this problem for two kinds of relays in WRNs, i.e., Amplify-and-Forward (AF) relays and Decode-and-Forward (DF) relays. In the case of AF relays, we find that the joint power problem is concave and thus derive the explicit expressions of the optimal solution. In the case of DF relays, we formulate the joint power allocation problem as a nonlinear fractional programming problem and then propose an efficient algorithm to search the optimal solution. Simulation results show that the proposed joint power allocation schemes outperform existing schemes in terms of end-to-end distortion in both WRNs with AF relays and that with DF relays under various channel conditions.

Motivated by delay-sensitive information transmission applications, we propose an expected achievable rate maximization scheme with a K-block delay constraint on data transmission using a three node cooperative relay network assuming a block fading channel model. Channel information is fed back to the transmitter only in a causal fashion, so that the optimal power allocation strategy is only based on the current and past channel gains.We consider the two simplest schemes for information transmission using a three node (a source, a relay and a destination) relay network, namely the amplify and forward (AF) and decode and forward (DF) protocols. We use a dynamic programming based methodology to solve the (K-block delay constrained) expected capacity maximization problem with a short term (over K blocks) sum power (total transmission power of the source and the relay) constraint. Furthermore, two simple power allocation schemes for high and low SNR situations are proposed. Extensive numerical results are presented for Rayleigh fading channels, including results demonstrating the accuracy of the high/low SNR approximation based power allocation schemes.

We propose a link adaptation and selection method for the links constituting an orthogonal frequency division multiplexing (OFDM) based wireless relay network. The proposed link adaptation and selection method selects the forwarding, modulation, and channel coding schemes providing the highest end-to- end throughput and decides whether to use the relay or not. The link adaptation and selection is done for each sub-channel based on instantaneous signal to interference plus noise ratio (SINR) conditions in the source-to-destination, source-to-relay and relay- to-destination links. The considered forwarding schemes are amplify and forward (AF) and simple adaptive decode and forward (DF). Efficient adaptive modulation and coding decision rules are provided for various relaying schemes. The proposed end-to-end link adaptation and selection method ensures that the end-to-end throughput is always larger than or equal to that of transmissions without relay and non-adaptive relayed transmissions. Our evaluations show that over the region where relaying improves the end- to-end throughput, the DF scheme provides significant throughput gain over the AF scheme provided that the error propagation is avoided via error detection techniques. We provide a frame structure to enable the proposed link adaptation and selection method for orthogonal frequency division multiple access (OFDMA)-time division duplex relay networks based on the IEEE 802.16e standard.

In this paper, we investigate Raptor-coded distributed unitary space–time modulation (USTM) schemes, where both the amplify-and-forward (AF) and decode-and-forward (DF) relaying scenarios are considered, under a wireless relay network with arbitrary path gains for the channel fading coefficients. First, the AF and DF relaying procedures are investigated based on the use of an inner USTM signal and an outer Raptor code, and the related noncoherent detectors are also derived. The constellation search for the USTM signal, as well as the optimization for the Raptor code, is then investigated in order to enhance the system performance. It is shown that the designed systems effectively approach the predicted limit in throughput performances and outperform the noncoherent distributed MIMO scheme reported in the previous literature.

In this paper, a wireless energy harvesting relay network based on decode-and-forward (DF) mode is considered, where the relay node is energy constrained and it harvests energy through radio-frequency (RF) signals from the source node. Then the relay node uses the harvested energy to decode and forward source information to the destination node. In the hybrid power-time switching based relaying (HPTSR), the block time T is divided into two parts. One is used for energy harvesting and information processing, and the other is used for information transmission from relay to destination nodes. During the part of energy harvesting and information processing, the portion of the received signal is used for energy harvesting (EH) and the other portion of the received signal. The rest of the received signal is sent to transmit source information to the relay node. We derive the analytical expressions of outage probability and ergodic capacity in the two-way DF network. The numerical results provide practical insight into the different system parameters. In particular, the numerical results show that the DF mode significantly outperforms the amplify-and-forward (AF) mode in terms of outage probability and ergodic capacity performance.

In a relay network with a single source-destination pair, we examine the achievable rates with amplify-and-forward (AF) relaying strategy. Motivated by applications in sensor networks, we consider power-constrained networks with large bandwidth resources and a large number of nodes. We show that the AF strategy does not necessarily benefit from the large available bandwidth. We characterize the optimum AF bandwidth and show that transmitting in the optimum bandwidth allows the network to operate in the linear regime where the achieved rate increases linearly with the available network power. We then present the optimum power allocation among the AF relays. The solution, which can be viewed as a form of maximum ratio combining, indicates the favorable relay positions in the network. Motivated by the large bandwidth resources we further consider a network that uses orthogonal transmissions at the nodes. While the above result for the optimum bandwidth still holds, a different set of relays should optimally be employed. In this case, the relay power solution can be viewed as a form of water-filling. The optimum AF bandwidth and the relay powers can be contrasted to the decode-and-forward (DF) solution. In a network with unconstrained bandwidth, the DF strategy will operate in the wideband regime to minimize the energy cost per information bit (S. Verdu, 2002), (O. Oyman et al., 2004). The wideband DF strategy requires again a different choice of relays; in the case of orthogonal signaling, the data should be sent through only one DF relay (I. Maric et al., 2004). Thus, in general, in a large scale network, a choice of a coding strategy goes beyond determining a coding scheme at a node; it also determines the operating bandwidth as well as the best distribution of the relay power.

This paper examines the effect on Outage probability and Detection probability by using Maximal Ratio Combining (MRC), Square Law Combining (SLC) along with the addition of two relay transmission protocols Amplify and Forward (AF) and Decode and Forward (DF) over Rayleigh Fading Channel. In MRC the co-phased signals with highest Signal to Noise Ratio (SNR) are taken where SLC does not rely on phase estimation. The AF protocol works on the principle that the relay receives the signal and amplifies it along with the noise. In DF, the received signal at relay is decoded and again it is re-encoded before sending it to the destination. Single and multiple relays are taken in different scenarios and the diversity techniques (MRC and SLC) along with the relay transmission protocols (AF and DF) are analyzed. The performance of receiver's parameters, i.e. detection probability (P d ) and Outage probability (P out ) are examined. The simulation results clearly show the performance of the efficient combining scheme with the appropriate relay transmission protocol.

In this paper, we study the power consumption in relay networks of the 60 GHz wireless communication based on amplify-and-forward (AF) and decode-and-forward (DF) relaying strategies. We propose a total power consumption model including drive power, decoding power, and power consumption of power amplifier (PA). This model is formulated as a function of drive power, which gives an easy access to the system level optimisation. The optimal drive power that minimises the total power consumption while satisfying the performance requirement can be found by numerical searching method. The impact of relay's locations on the total power consumption is also investigated. We show that, with the same performance requirement, in the small source-relay separation case AF consumes less power than DF, while with larger separation, AF consumes significantly more power than DF. This is different from the common intuition that DF is always more power consuming than AF due to the extra decoding power consumption at relay, which is due to the fact that the large source-relay separation limits the effective destination signal-to-noise ratio (SNR) in AF, leading to more substantial decoding power consumption in the many more decoding iterations than DF.

Multiple-input multiple-output (MIMO) and cooperative communications have been attracted great attention for the improvements of communication capacity, power consumption, and transmission coverage. The conventional fixed relaying protocols, amplify-and-forward (AF) and decode-and-forward (DF), have their own advantages and disadvantages, i.e. AF performs better than DF for low signal-to-noise ratio (SNR) region, while the reverse is true for high SNR region. Therefore, this paper proposes an SNR-adaptive forward (SAF) relaying scheme obtaining the advantages of both AF and DF. Furthermore, the proposed SAF does not need to switch between AF and DF when SNR changes. The main idea is to adaptively derive the soft information at the cooperative relay nodes based on the information of the received signal and the SNR. Besides, based on the theoretical analysis and the simulation results, it is affirmed that the proposed SAF achieves superior performance than both AF and DF for all SNRs. Moreover, the performance gain would be improved with the increasing number of parallel cooperative relay nodes.

The authors propose a physical-datalink cross-layer resource allocation scheme over wireless relay networks for quality-of-service (QoS) guarantees. By integrating information theory with the concept of effective capacity, the proposed scheme aims at maximizing the relay network throughput subject to a given delay QoS constraint. This delay constraint is characterized by the so-called QoS exponent thetas, which is the only requested information exchanged between the physical layer and the datalink layer in our cross-layer design based scheme. Over both amplify-and-forwards (AF) and decode-and-forward (DF) relay networks; the authors develop the associated dynamic resource allocation algorithms for wireless multimedia communications. Over DF relay network, the authors also study a fixed power allocation scheme to provide QoS guarantees. The simulations and numerical results verify that our proposed cross-layer resource allocation can efficiently support diverse QoS requirements over wireless relay networks. Both AF and DF relays show significant superiorities over direct transmissions when the delay QoS constraints are stringent. On the other hand, the results demonstrate the importance of deploying the dynamic resource allocation for stringent delay QoS guarantees.

An iterative game theory based algorithm is proposed to allocate the resources in cooperative schemes for the downlink. Both amplify-and-forward (AF) and decode-and-forward (DF) cooperative schemes are considered with cellular reuse of the relay slot. Multiple antennas can be used at the involved stations. Using the algorithm proposed, the simultaneous relay powers are decided in a decentralized way using mean channel level measures and mean values of noise and interference power. The cell capacity gain for the cooperative schemes using the decentralized algorithm is evaluated by means of simulation for both the AF and DF approaches.

Adaptive decode and forward (DF) relay are studied over independent non-identical (i.n.d.) flat Nakagami-m fading channels. Taking advantage of the moment generating function (MGF), the closed-form expression for the bit error rate (BER) is derived. The behaviors of Space-Time Block Coding (STBC) have also been investigated over the same channel with DF. Taking BER as a metric, cooperation parameter is proposed to conjecture DF cooperative region by comparing with BER of STBC and directive communication. Simulation results show that, under the same channel information, the performances of DF is far superior to directive transmission and better than amplify and forward (AF) strategy, but poorer than STBC strategy. Also, the results indicate that its behavior will be better than STBC strategy when channel performs well, as long as SD and RD channel can reach certain prerequisite.

In this paper, we study the performance of nonregenerative and regenerative cooperation under peak power constraint. We compare the following relay function scenarios: fixed gain amplify and forward (AF) relay, variable gain AF relay, scaled gain AF relay, and decode and forward (DF) relay. We show that under this practical peak-power constraint, the knowledge of the channel state information (CSI) is essential for the AF cooperation in order to improve the power efficiency of the relay and efficiently obtain the diversity gain.

In this paper, a distributed power control algorithm is proposed for wireless relay networks in interference-limited environments. The objective is to minimize the total transmission power while satisfying the signal-to-interference-plus-noise ratio (SINR) requirements. Two forwarding techniques, i.e., decode- and-forward (DF) and amplify-and-forward (AF), are considered. The proposed algorithm only requires locally measured SINR on the relay nodes (RNs) and the destination nodes (DNs), based on which each cooperation unit (defined as one source node (SN) and DN pair with the RN associated to it) iteratively updates the transmission power of the SN and the RN by solving a local optimization problem. We prove that the convergence is guaranteed when the parameters adopted in the algorithm are sufficiently large, and then a parameter adjusting method is also designed. Simulation results indicate that the proposed algorithm converges fast and leads to only 7% more power consumption than the optimal power allocation in the considered scenarios. It is also shown that even in interference-limited environments, relaying can still improve system performance substantially in terms of outage and power consumption.

This paper studies cooperative secure transmission in wireless relay networks, where a source sends a confidential message to a legitimate destination through a set of amplify-and-forward (AF) relays in the presence of multiple eavesdroppers. To prevent the eavesdroppers from retrieving the message, we assume that the relays employ a joint cooperative beamforming (CB) and artificial noise (AN) strategy. Under both the total and individual relay power constraints, we propose an optimal joint CB-AN design based on secrecy rate maximization (SRM). We treat the SRM problem by a combination of two-level optimization and semidefinite relaxation (SDR). For the latter, we prove that SDR always has a rank-one solution for the SRM problem - which identifies that SDR is optimal. Simulation results are presented to show the efficacy of the proposed joint CB-AN design.