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This letter investigates an amplify-and-forward two-way relay network (TWRN) for short-packet communications. We consider a classical three-node TWRN consisting of two sources and one relay. Both two time slots (2TS) scheme and three time slots (3TS) scheme are studied under the finite blocklength regime. We derive approximate closed-form expressions of sum-block error rate (BLER) for both schemes. Simple asymptotic expressions for sum-BLER at high signal-to-noise ratio (SNR) are also derived. Based on the asymptotic expressions, we analytically compare the sum-BLER performance of 2TS and 3TS schemes, and attain an expression of critical blocklength, which can determine the performance superiority of 2TS and 3TS in terms of sum-BLER. Extensive simulations are provided to validate our theoretical analysis. Our results discover that 3TS scheme is more suitable for a system with higher differences between the average SNR of both links, and relatively lower requirements on data rate and latency. [ABSTRACT FROM AUTHOR]

We introduce the superimposed training strategy into the multiple-input multiple-output (MIMO) amplify-and-forward (AF) one-way relay network (OWRN) to perform the individual channel estimation at the destination. Through the superposition of a group of additional training vectors at the relay subject to power allocation, the separated estimates of the source-relay and relay-destination channels can be obtained directly at the destination, and the accordance with the two-hop AF strategy can be guaranteed at the same time. The closed-form Bayesian Cramér-Rao lower bound (CRLB) is derived for the estimation of two sets of flat-fading MIMO channel under random channel parameters and further exploited to design the optimal training vectors. A specific suboptimal channel estimation algorithm is applied in the MIMO AF OWRN using the optimal training sequences, and the normalized mean square error performance for the estimation is provided to verify the Bayesian CRLB results. [ABSTRACT FROM AUTHOR]

In this paper, we propose a relay selection amplify-and-forward (RS-AF) protocol in general bidirectional relay networks with two sources and N relays. In the proposed scheme, the two sources first simultaneously transmit to all the relays, and then, a single relay with a minimum sum symbol error rate (SER) will be selected to broadcast the received signals back to both sources. To facilitate the selection process, we propose a simple suboptimal min–max criterion for relay selection, where a single relay that minimizes the maximum SER of two source nodes will be selected. Simulation results show that the proposed min–max selection has almost the same performance as the optimal selection with lower complexity. We also present a simple asymptotic SER expression and make a comparison with the conventional all-participate AF relaying scheme. The analytical results are verified through simulations. To improve the system performance, optimal power allocation (OPA) between the sources and the relay is determined based on the asymptotic SER. Simulation results indicate that the proposed RS-AF scheme with OPA yields considerable performance improvement over an equal-power-allocation scheme, particularly with a large number of relay nodes. [ABSTRACT FROM PUBLISHER]

In the paper, we propose a novel satellite-based spectrum sensing method for cognitive radio networks (CRNs) using multibeam antennas in large-scale coverage ground areas. Our objective is to achieve the dynamic spectrum sharing easily in both temporal and geographical dimensions, such that the primary networks and the CRNs can better coexist. By simple linear transformation and energy detection at the satellite multibeam receiver, spectrum sensing can be simultaneously performed for multiple cells. This approach overcomes the limitations of the traditional sensing method, which therefore considerably improves the spectrum utilization. To verify the efficiency of the proposed approach, theoretical expressions of the average probabilities of miss detection and false alarm are derived. Simulation results show that the satellite-based spectrum sensing method is very efficient for the CRNs in large-scale scenarios under different conditions (equivalent signal-to-noise ratio, threshold, antenna beam pattern, etc.). In addition, the proposed sensing method provides a new vision on the large-scale dynamic spectrum sharing system design to effectively integrate the terrestrial and the satellite communication systems for the future CRN development. [ABSTRACT FROM AUTHOR]

Context-aware computing is an important aspect of the pervasive computing environment and its various dynamic context information brings new challenges to access control systems. In this paper a new access control model, relation based access control ( RelBAC), is provided for context-aware environment with a domain specific Description Logic to formalize the model. The novelty of RelBAC is that permissions are formalized as binary relations between subjects and objects which could evolve with the dynamic contexts. The expressive power of RelBAC is illustrated in a case study of a project meeting event. [ABSTRACT FROM AUTHOR]

In this correspondence, we propose an analog network coding scheme with differential modulation (ANC-DM) using amplify-and-forward protocol for bidirectional relay networks when neither the source nodes nor the relay knows the channel state information (CSI). The performance of the proposed ANC-DM scheme is analyzed and a simple asymptotic bit error rate (BER) expression is derived. The analytical results are verified through simulations. It is shown that the BER performance of the proposed differential scheme is about 3 dB away from that of the coherent detection scheme. To improve the system performance, the optimum power allocation between the sources and the relay is determined based on the simplified BER. Simulation results indicate that the proposed differential scheme with optimum power allocation yields 1-2 dB performance improvement over an equal power allocation scheme. [ABSTRACT FROM AUTHOR]

In this paper, we consider a general bidirectional relay network with two sources and N relays when neither the source nodes nor the relays know the channel-state information (CSI). A joint relay-selection and analog-network coding using differential modulation (RS-ANC-DM) is proposed. In the proposed scheme, the two sources employ differential modulations and transmit the differential modulated symbols to all relays at the same time. The signal received at the relay is a superposition of two transmitted symbols, which we call analog network-coded symbols. Then, a single relay that has minimum-sum symbol error rate (SER) is selected out of N relays to forward the ANC signals to both sources. To facilitate the selection process, in this paper, we also propose a simple suboptimal Min--Max criterion for relay selection, where a single relay that minimizes the maximum SER of two source nodes is selected. Simulation results show that the proposed Min--Max selection has almost the same performance as the optimal selection but is much simpler. The performance of the proposed RS-ANC-DM scheme is analyzed, and a simple asymptotic SER expression is derived. The analytical results are verified through simulations. [ABSTRACT FROM AUTHOR]