# IEEE Transactions on Communications

Updated: hace 13 horas 19 mins

Vie, 09/01/2017 - 00:00
Categorías: research

### IEEE Communications Society

Vie, 09/01/2017 - 00:00
Categorías: research

### Braided Convolutional Codes With Sliding Window Decoding

Vie, 09/01/2017 - 00:00
In this paper, we present a novel sliding window decoding scheme based on iterative Bahl–Cocke–Jelinek–Raviv decoding for braided convolutional codes, a class of turbo-like codes with short constraint length component convolutional codes. The tradeoff between performance and decoding latency is examined and, to reduce decoding complexity, both uniform and nonuniform message passing schedules within the decoding window, along with early stopping rules, are proposed. We also perform a density evolution analysis of sliding window decoding to guide the selection of the window size and message passing schedule. Periodic puncturing is employed to obtain rate-compatible code rates of 1/2 and 2/3 starting from a rate 1/3 mother code and a code rate of 3/4 starting from a rate 1/2 mother code. Simulation results show that, with nonuniform message passing and periodic puncturing, near capacity performance can be maintained throughout a wide range of rates with reasonable decoding complexity and no visible error floors.
Categorías: research

### Ambient Backscatter: A New Approach to Improve Network Performance for RF-Powered Cognitive Radio Networks

Vie, 09/01/2017 - 00:00
This paper introduces a new solution to improve the performance for secondary systems in radio frequency (RF) powered cognitive radio networks (CRNs). In a conventional RF-powered CRN, the secondary system works based on the harvest-then-transmit protocol. That is, the secondary transmitter (ST) harvests energy from primary signals and then uses the harvested energy to transmit data to its secondary receiver (SR). However, with this protocol, the performance of the secondary system is much dependent on the amount of harvested energy as well as the primary channel activity, e.g., idle and busy periods. Recently, ambient backscatter communication has been introduced, which enables the ST to transmit data to the SR by backscattering ambient signals. Therefore, it is potential to be adopted in the RF-powered CRN. We investigate the performance of RF-powered CRNs with ambient backscatter communication over two scenarios, i.e., overlay and underlay CRNs. For each scenario, we formulate and solve the optimization problem to maximize the overall transmission rate of the secondary system. Numerical results show that by incorporating such two techniques, the performance of the secondary system can be improved significantly compared with the case when the ST performs either harvest-then-transmit or ambient backscatter technique.
Categorías: research

### Optimal Training Design for MIMO-OFDM Two-Way Relay Networks

Vie, 09/01/2017 - 00:00
In this paper, we study a training design problem for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) amplify-and-forward (AF) two-way relay networks. Unlike the existing studies, we assume the spatially correlated fading and consider the nonreciprocal channel condition, which is a more practical assumption but makes the training problem more challenging. The equivalent channels of bidirectional relaying links, which consist of self-interfering channels and information-bearing channels, are estimated at each source node based on a linear minimum mean square error (LMMSE) approach. The total mean square error (MSE) of the channel estimation is minimized under the transmit power constraints at the source nodes and at the relay. To solve this problem, we first derive an optimal structure of the training signals, and then, convert the optimization problem into a tractable convex form, from which the optimal training scheme is designed efficiently. Furthermore, for a practical special case, the optimal training design is derived in semi-closed form, which provides useful insights. To reduce the required complexity, a low-complexity training scheme is also derived in closed-form. This scheme is shown to be asymptotically optimal in the high signal-to-noise ratio (SNR) regime and gives further insights into the optimal training. The performance of the proposed schemes is demonstrated through numerical simulations.
Categorías: research

### Joint Transceiver Design for Secure Downlink Communications Over an Amplify-and-Forward MIMO Relay

Vie, 09/01/2017 - 00:00
This paper addresses joint transceiver design for secure downlink communications over a multiple-input multiple-output relay system in the presence of multiple legitimate users and malicious eavesdroppers. Specifically, we jointly optimize the base station (BS) beamforming matrix, the relay station (RS) amplify-and-forward transformation matrix, and the covariance matrix of artificial noise, so as to maximize the system worst-case secrecy rate in the presence of the colluding eavesdroppers under power constraints at the BS and the RS, as well as quality of service constraints for the legitimate users. This problem is very challenging due to the highly coupled design variables in the objective function and constraints. By adopting a series of transformation, we first derive an equivalent problem that is more tractable than the original one. Then, we propose and fully develop a novel algorithm based on the penalty concave-convex procedure (penalty-CCCP) to solve the equivalent problem, where the difficult coupled constraint is penalized into the objective and the resulting nonconvex problem is solved at each iteration by resorting to the CCCP method. It is shown that the proposed joint transceiver design algorithm converges to a stationary solution of the original problem. Finally, our simulation results reveal that the proposed algorithm achieves better performance than other recently proposed transceiver designs.
Categorías: research

### A Utility-Based Joint Subcarrier and Power Allocation for Green Communications in Multi-User Two-Way Regenerative Relay Networks

Vie, 09/01/2017 - 00:00
In this paper, we investigate utility-based joint subcarrier and power allocation algorithms for improving the energy efficiency (EE) in multi-user two-way regenerative relay networks. With the objective of determining the best subcarrier allocation for each user pair, subcarrier pairing permutation, and power allocation to all the nodes, a network price is introduced to the power consumption as a penalty for the achievable sum rate, followed by the examination of its impact on the tradeoff between the EE and spectral efficiency. The formulated optimization problem is a non-convex mixed-integer nonlinear programming problem, and thus a concave lower bound on the objective function and a series of convex transformations are applied to transform the problem into a convex one. Through dual decomposition, we propose a utility-based resource allocation algorithm for iteratively tightening the lower bound and finding the optimal solution of the primal problem. By exploring the structure of the obtained optimal solution, an optimal price that enables green resource allocation is found from the perspective of maximizing EE. Additionally, a suboptimal algorithm is investigated to strike a balance between computational complexity and optimality. Simulation results evince the effectiveness of the proposed algorithms.
Categorías: research

### Perfect Gaussian Integer Sequences of Period $p^{k}$ With Degrees Equal to or Less Than $k+1$

Vie, 09/01/2017 - 00:00
This paper presents the construction of perfect Gaussian integer sequence (PGIS) of period $N=p^{k}$ with degrees equal to or less than $k+1$ , where $p$ is a prime number and $k\geq 2$ . The study begins with the partitioning of $\mathbb {Z}_{N}$ into $k+1$ subsets, from which $k+1$ base sequences are defined to construct degree- $(k+1)$ PGISs. The $k$ constraint equations derived from matching the ideal periodic autocorrelation function criterion in the time domain are nonlinear. We propose a decomposition method that can transform these $k$ nonlinear equations into $2k-2$ linear equations with $2k-2$ variables. The number of nonlinear constraint equations derived from the frequency domain is $k+1$ , where the $k+1$ sequence coefficients can be derived sequentially by an iteration algorithm. These two proposed schemes simplify the construction of the degree- $(k+1)$ PGIS. PGISs having less th- n $k+1$ degrees can be constructed in two steps. First, some sequence coefficients are assigned a zero value or one or more pairs of two consecutive base sequences are combined into one. The second step involves the adjustment of new sequence coefficients for the associated sequence to be perfect. We show that there exist at least $2k+(({k(k-1)(k-2)})/{6})$ distinct PGIS patterns of period $N=p^{k}$ with degrees equal to or less than $k+1$ .
Categorías: research

### Massive MIMO Performance With Imperfect Channel Reciprocity and Channel Estimation Error

Vie, 09/01/2017 - 00:00
Channel reciprocity in time-division duplexing (TDD) massive multiple-input multiple-output (MIMO) systems can be exploited to reduce the overhead required for the acquisition of channel state information (CSI). However, perfect reciprocity is unrealistic in practical systems due to random radio-frequency (RF) circuit mismatches in uplink and downlink channels. This can result in a significant degradation in the performance of linear precoding schemes, which are sensitive to the accuracy of the CSI. In this paper, we model and analyse the impact of RF mismatches on the performance of linear precoding in a TDD multi-user massive MIMO system, by taking the channel estimation error into considerations. We use the truncated Gaussian distribution to model the RF mismatch, and derive closed-form expressions of the output signal-to-interference-plus-noise ratio for maximum ratio transmission and zero forcing precoders. We further investigate the asymptotic performance of the derived expressions, to provide valuable insights into the practical system designs, including useful guidelines for the selection of the effective precoding schemes. Simulation results are presented to demonstrate the validity and accuracy of the proposed analytical results.
Categorías: research

### Beamforming Optimization for Full-Duplex Wireless-Powered MIMO Systems

Vie, 09/01/2017 - 00:00
We propose techniques for optimizing transmit beamforming in a full-duplex multiple-input-multiple-output wireless-powered communication system, which consists of two phases. In the first phase, the wireless-powered mobile station (MS) harvests energy using signals from the base station (BS), whereas in the second phase, both MS and BS communicate to each other in a full-duplex mode. When complete instantaneous channel state information (CSI) is available, the BS beamformer and the time-splitting (TS) parameter of energy harvesting are jointly optimized in order to obtain the BS–MS rate region. The joint optimization problem is non-convex, however, a computationally efficient optimum technique, based upon semidefinite relaxation and line-search, is proposed to solve the problem. A sub-optimum zero-forcing approach is also proposed, in which a closed-form solution of TS parameter is obtained. When only the second-order statistics of transmit CSI is available, we propose to maximize the ergodic information rate at the MS while maintaining the outage probability at the BS below a certain threshold. An upper bound for the outage probability is also derived and an approximate convex optimization framework is proposed for efficiently solving the underlying non-convex problem. Simulations demonstrate the advantages of the proposed methods over the sub-optimum and half-duplex ones.
Categorías: research

Vie, 09/01/2017 - 00:00
The extension of long term evolution (LTE) networks in unlicensed spectrum areas under the licensed assisted access concept aims at achieving higher transmission rates via the aggregation of the aforementioned bands along with the licensed ones within the 3G Partnership Project framework. A prospect carrier aggregation (CA) scheme should handle efficiently the coexistence of the LTE systems that compete for the same unlicensed spectrum areas along with their incumbent users (i.e., Wi-Fi). In this paper, a novel CA scheme is proposed for licensed/unlicensed MIMO LTE systems that allocates optimally the resources (power and resource blocks) of an evolved Node B to user equipments. Furthermore, the proposed approach handles the coexistence matters within the unlicensed bands with an efficient decentralized way. The new scheme involves the solution to a mixed integer nonlinear programming problem and thus, an optimal low complexity method is proposed based on the Lagrange dual decomposition. Furthermore, the proposed technique is extended to the imperfect channel state information (CSI) case. To that end, a novel listen-before-talk scheme is developed via which the required unlicensed bands CSI are estimated in a blind manner. The performance of all of the proposed techniques is verified via indicative simulations.
Categorías: research

### Spectral and Energy Efficiency of Uplink D2D Underlaid Massive MIMO Cellular Networks

Vie, 09/01/2017 - 00:00
One of the key 5G scenarios is that device-to-device (D2D) and massive multiple-input multiple-output (MIMO) will be co-existed. However, interference in the uplink D2D underlaid massive MIMO cellular networks needs to be coordinated, due to the vast cellular and D2D transmissions. To this end, this paper introduces a spatially dynamic power control solution for mitigating the cellular-to-D2D and D2D-to-cellular interference. In particular, the proposed D2D power control policy is rather flexible, including the special cases of no D2D links or using maximum transmit power. Under the considered power control, an analytical approach is developed to evaluate the spectral efficiency (SE) and energy efficiency (EE) in such networks. Thus, the exact expressions of SE for a cellular user or D2D transmitter are derived, which quantify the impacts of key system parameters, such as massive MIMO antennas and D2D density. Moreover, the D2D scale properties are obtained, which provide the sufficient conditions for achieving the anticipated SE. Numerical results corroborate our analysis and show that the proposed power control solution can efficiently mitigate interference between the cellular and the D2D tier. The results demonstrate that there exists the optimal D2D density for maximizing the area SE of D2D tier. In addition, the achievable EE of a cellular user can be comparable with that of a D2D user.
Categorías: research

### Spectral and Energy Efficiency of Multi-Pair Massive MIMO Relay Network With Hybrid Processing

Vie, 09/01/2017 - 00:00
We consider a multi-pair massive multiple-input multiple-output relay network, where the relay is equipped with a large number, $N$ , of antennas, but driven by a far smaller number, $L$ , of radio-frequency (RF) chains. We assume that $K$ pairs of users are scheduled for simultaneous transmission, where $K$ satisfies $2K=L$ . A hybrid signal processing scheme is presented for both uplink and downlink transmissions of the network. Analytical expressions of both spectral efficiency (SE) and energy efficiency (EE) are derived with respect to the RF chain number under imperfect channel estimation. It is revealed that, under the condition $N>\left \lfloor{ 4L^{2}/\pi }\right \rfloor$ , the transmit power of each user and the relay can be, respectively, scaled down by $1/\sqrt {N}$ and $2K/\sqrt {N}$ if pilot power scales with signal power, or they can be, respectively, scaled down by $1/N$ and $2K/N$ if the pilot power is kept fixed, while maintaining an asymptotically unchanged SE. While regarding EE of the network, the optimal EE is shown to be achieved when $P_{r} = 2K P_{s}$ , where $P_{r}$ and $P_{s}$ , respectively, refer to the transmit power of the relay and each source terminal. We show that the network EE is a quasi-concave function with respect to the number of RF-chains which, therefore, admits a unique globally optimal choice of the RF-chain number. Numerical simulations are conducted to verify our observations.
Categorías: research

Vie, 09/01/2017 - 00:00
Quadrature spatial modulation (QSM) is a recent multiple input multiple output transmission scheme that attracted significant research interest. QSM expands the spatial constellation diagram of spatial modulation (SM) to enhance the overall spectral efficiency while retaining all SM inherent advantages. In this paper, differential QSM (DQSM) is proposed to alleviate the requirement of channel knowledge at the receiver side. Receiver channel knowledge is crucial in QSM as part of the data are encoded in the Euclidean difference among different channel paths. Time dimension and orthogonal in-phase and quadrature spatial dimensions of QSM are exploited to facilitate differential modulation and demodulation while maintaining single RF-chain transmitters. In addition, a systematic design of the transmission blocks is provided for arbitrary number of transmit and receive antennas. Besides, a novel analytical framework for analyzing the performance of DQSM is derived and shown to predict accurate performance for differential SM and differential space shift keying systems as well. Analytical and simulation results are shown to match closely over a wide range of signal to noise ratios and for different system parameters.
Categorías: research

### Performance Analysis of Multiple Association in Ultra-Dense Networks

Vie, 09/01/2017 - 00:00
In this paper, we propose a general mathematical framework to compute the average downlink rate in a multiple connectivity context considering ultra-dense network (UDN) environment. UDN is a dense small cells network featured by the high density of small cells that may exceed the density of active users. In multiple association, a user connects to $M$ base stations (BSs) that provide the maximum average received power forming a multicell. This provides the user with a “data-shower,” where the user’s traffic is split into multiple paths, which helps overcoming the capacity limitations imposed by the backhaul links. The developed framework significantly simplifies the computation of the average downlink rate of the individual connections to the cells of a multicell. Moreover, the accuracy of the mathematical framework is confirmed by extensive simulations. The simulation results show a perfect match with the numerical results computed from the mathematical framework in different combinations of the system parameters including multicell size, small cells density, active users density, pathloss exponent, and fading channel distribution of the signal link.
Categorías: research

### Comparison of Collision-Free and Contention-Based Radio Access Protocols for the Internet of Things

Vie, 09/01/2017 - 00:00
The fifth-generation (5G) cellular networks will face the challenge of integrating the traditional broadband services with the Internet of Things (IoT), which is characterized by sporadic uplink transmissions of small data packets. Indeed, the access procedure of the previous generation cellular network (4G) is not able to support IoT traffic efficiently, because it requires a large amount of signaling for the connection setup before the actual data transmission. In this context, we introduce two innovative radio access protocols for sporadic transmissions of small data packets, which are suitable for 5G networks, because they provide a resource-efficient packet delivery exploiting a connectionless approach. The core of this paper resides in the derivation of an analytical framework to evaluate the performance of all the aforementioned protocols. The final goal is the comparison between 4G and 5G radio access solutions employing both our analytical framework and computer simulations. The performance evaluation results show the benefits of the protocols envisioned for 5G in terms of signaling overhead and access latency.
Categorías: research

### Designing Optimal Interlink Patterns to Maximize Robustness of Interdependent Networks Against Cascading Failures

Vie, 09/01/2017 - 00:00
In this paper, we consider the optimal design of interlinks for an interdependent system of networks. In contrast to existing literature, we explicitly exploit the information of intra-layer node degrees to design interdependent structures such that their robustness against cascading failures, triggered by randomized attacks, is maximized. Utilizing percolation theory-based system equations relating the robustness of the network to its degree sequence, we characterize the optimal design for the one-to-one structure, with complete interdependence and partial interdependence, under randomized attack. We also extend our study to the one-to-many interdependence structure and the targeted attack model. The theoretically derived optimal interdependence structures have been verified using simulations on scale-free networks.
Categorías: research

Vie, 09/01/2017 - 00:00
Orthogonal frequency division multiplexing with index modulation (OFDM-IM) performs transmission by considering two modes over OFDM subcarriers, which are the null and the conventional $M$ -ary signal constellation. The spectral efficiency (SE) of the system, however, is limited, since the null mode itself does not carry any information and the number of subcarrier activation patterns increases combinatorially. In this paper, a novel IM scheme, called multiple-mode OFDM-IM (MM-OFDM-IM), is proposed for OFDM systems to improve the SE by conveying information through multiple distinguishable modes and their full permutations. A practical and efficient mode selection strategy, which is constrained on the phase shift keying/quadrature amplitude modulation constellations, is designed. Two efficient detectors that provide different tradeoffs between the error performance and detection complexity are also proposed. The principle of MM-OFDM-IM is further extended to the in-phase and quadrature components of OFDM signals, and the method of generating multiple modes from the $M$ -ary pulse amplitude modulation constellation for this modified scheme is also introduced. Bit error rate (BER) analyses are provided for the proposed schemes. Monte Carlo simulations on BER corroborate the analyses and show that the proposed schemes appear as promising multi-carrier transmission alternatives by outperforming the existing OFDM-IM counterparts.