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In this paper, we make an investigation on the sum-mean-square-error (Sum-MSE) performance gain in full-duplex orthogonal frequency-division multiplexing (OFDM) systems in the presence of colored interference-plus-noise (IPN). This gain is defined as the ratio of Sum-MSE of frequency-domain least-square (LS) channel estimator to that of DFT-based LS one. The closed-form formula of the gain is derived. And, its simple upper and lower bounds are given using inequalities of matrix eigenvalues. The exact value of Sum-MSE gain depends heavily on the correlation factor of the IPN covariance matrix. More importantly, we also find that the Sum-MSE performance gain grows from 1 to $N/L$ as the correlation factor gradually decreases from 1 to 0, where $N$ and $L$ denote the number of total subcarrier and the length of cyclic prefix, respectively. Also, via theoretical analysis, the exact Sum-MSE gain degenerates into 1 and $N/L$ in two extreme scenarios: fully-correlated and white, respectively. The former 1 means there is no performance gain, while the latter $N/L$ corresponds to the maximum Sum-MSE performance gain achievable. Numerical simulation further validates the above results. Additionally, the derived lower bound is shown to be closer to the exact value of Sum-MSE gain compared to the upper bound.

In this paper, the problem of content caching in 5G cellular networks relying on social-aware device-to-device communications (DTD) is investigated. Our focus is on how to efficiently select important users (IUs) and how to allocate content files to the storage of these selected IUs to form a distributed caching system. We aim at proposing a novel approach for minimizing the downloading latency and maximizing the social welfare simultaneously. In particular, we first model the problem of maximizing the social welfare as a many-to-one matching game based on the social property of mobile users. We study this game by exploiting users’ social properties to generate the utility functions of the two-side players, i.e., content providers (CPs) and IUs. Then we model the problem of minimizing the downloading latency as a many-to-many matching problem. For solving these games, we design a many-to-one IU selection (MOIS) matching algorithm and a many-to-many file allocation (MMFA) matching algorithm, respectively. Simulation and analytical results show that the proposed mechanisms are stable, and are capable of offering a better performance than other benchmarks in terms of social welfare and network downloading latency.

Medium-scale or large-scale receive antenna array with digital beamforming can be employed at receiver to make a significant interference reduction but leads to expensive cost and high complexity of the RF-chain circuit. To deal with this issue, classic analog-and-digital beamforming (ADB) structure was proposed in the literature for greatly reducing the number of RF-chains. Based on the ADB structure, in this paper, we propose a robust hybrid ADB scheme to resist directions of arrival (DOAs) estimation errors. The key idea of our scheme is to employ null space projection (NSP) in the analog beamforming domain and diagonal loading (DL) method in digital beamforming domain. The simulation results show that the proposed scheme performs more robustly, and moreover, it has a significant improvement on the receive signal-to-interference-plus-noise ratio compared to NSP ADB scheme and DL method.

In a passive multisatellite location system with Earth constraint (EC), the localization problem of time differences of arrival (TDOAs) is formulated as a quadratic optimization (QO) problem with two equality constraints: EC and variable constraint (VC). Which constraint is more important? To evaluate the importance of the two constraints, the original QO problem with both EC and VC is relaxed into two individual QO problems with only one equality constraint EC or VC being kept. A convex combination operation is performed on the two optimal solutions associated with the two relaxed QO problems to form a new weighted solution. Numerical simulation results show that the new weighted coefficient can achieve the Cramer–Rao lower bound with EC as the weighted factor of the solution exploiting only VC tends to zero, and increasing the value of this weighted coefficient will gradually degrade the overall localization performance. Thus, we conclude that the EC plays an extremely important role in improving the performance of TDOA-based localization.

In this paper, we make an investigation of robust beamforming for secure directional modulation in the multiuser multiple-input and multiple-output systems in the presence of direction angle measurement errors. When statistical knowledge of direction angle measurement errors is unavailable, a novel robust beamforming scheme of combining main-lobe-integration and leakage is proposed to simultaneously transmit multiple different independent parallel confidential message streams to the associated multiple distinct desired users. The proposed scheme includes two steps: designing the beamforming vectors of the useful confidential messages and constructing artificial noise (AN) projection matrix. Here, in its first step, the beamforming vectors for the useful confidential messages of desired user $k$ are given by minimizing the useful signal power leakage from the main-lobe of desired user $k$ to the sum of main-lobes of the remaining desired directions plus main-lobes of all eavesdropper directions. In its second step, the AN projection matrix is constructed by simultaneously maximizing the AN power leakage to all eavesdropper directions such that all eavesdroppers are disrupted seriously, where AN is viewed by the transmitter as a useful signal for eavesdroppers. Due to independent beamforming vectors for different desired users, a more secure transmission is achieved. Compared with conventional nonrobust methods, the proposed method can provide a significant improvement in bit error rate along the desired directions and secrecy-sum-rate toward multiple desired users without needing the statistical property or distribution of angle measurement errors.

In this letter, we propose an auction-based resource trading scheme between a network service provider (NSP) and multiple content providers (CPs). The NSP is in charge of a number of small-cell base stations (SBSs), while each CP provides content downloading services to its affiliated mobile users (MUs). By treating the SBSs as a kind of resources, the NSP leases SBSs to the CPs for caching their popular contents, thereby reducing the downloading latency of the MUs. We first establish the utility functions of the NSP and CPs. Then, we maximize the utilities of both sides by formulating an iterative auction problem, in which the NSP (auctioneer) communicates with the CPs (bidders) in an iterative manner for achieving optimal price and resource allocations. Furthermore, to avoid being cheated from the CPs, we adopt an ascending-bid auction mechanism such that the CPs do not have motivations to cheat the NSP for a lower price. Numerical results show that our proposed scheme can effectively allocate SBSs to the CPs and prevent each CP from cheating the NSP for gaining a higher profit.

For original paper see N. Tayem et al., ibid., vol.53, no.5, p.1622-30, (2005). We indicate that the two-dimensional (2-D) direction-of-arrival (DOA) estimation method proposed by N. Tayem has an error in the pair matching between the azimuth and elevation angle estimates. A feasible solution for pair matching is proposed.