Heterogeneous networks represent a practical way to face high traffic demands in densely populated areas. Nevertheless, to achieve these advertised data rates, advanced interference management techniques are required. Trying to tackle this problem, this paper presents a stochastic scheduling approach for two-layer LTE-Advanced networks. The proposed stochastic frequency-domain scheduling (SFDS) algorithm seeks the avoidance of cross-layer and colayer interference, and is based on allocating different portions of the transmission bandwidth probabilistically. The SFDS algorithm is first presented as an interference management solution for an orthogonal frequency-division multiple access system (i.e. LTE-Advanced downlink). Later on, the SFDS algorithm is extended to be compatible with single-carrier frequency-division multiple access transmissions, and its performance is evaluated when combined with open-loop power control techniques (i.e. LTE-Advanced uplink). Extensive system level simulations are carried out to evaluate the performance of SFDS algorithm in a predefined LTE-Advanced scenario that combines both macrolayer and femtolayer. Results show that the proposed algorithm allows to balance the cross-layer and colayer interferences, extending the performance trade-offs that well-known full-range spectrum allocation and orthogonal spectrum allocation approaches provide.

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Machine-to-Machine (M2M) communication represents a new paradigm for mobile cellular networks, where a massive number of low-cost devices requests the transfer of small amounts of data without human intervention. One option to tackle this problem is obtained by combining Random Beamforming (RBF) with opportunistic scheduling. RBF can be used to induce larger channel fluctuations and opportunistic scheduling can be used to select M2M devices when their overall channel quality is good. Traditional RBF does not fulfill M2M requirements because overall channel quality needs to be tracked continuously. In order tackle this limitation, a novel codebookbased RBF architecture that identifies in advance the time instants in which overall channel quality should be reported, within a coherence time window, is proposed. This opportunistic feedback mechanism reduces signaling overhead and enables energy saving at M2M devices. A simplified methodology is presented to evaluate the system mean data rate, using for this purpose closed form formulas derived from SNR distribution approximations. Results reveal that the performance loss that is experienced for introducing the proposed modifications to traditional RBF scheme is negligible. The concepts analyzed in this paper provide useful insights, and show that codebook-based RBF with simplified opportunistic scheduling algorithms is an excellent combination to provide wide-area M2M services with low-cost devices and limited signaling overhead.

Joint transmission (JT)-based Coordinated Multipoint (CoMP) systems achieve high performance gains by allowing full coordination among multiple cells, transforming unwanted intercell interference into useful signal power. In this paper, we present an analytical model to perform adaptive modulation for a typical JT CoMP system, consisting of three transmission points, under a target bit error rate (BER) constraint. Probability density functions of the signal-to-interference-plus-noise ratio (SINR) are derived for different JT CoMP schemes, and based on them, closed-form expressions for the average spectral efficiency (ASE) are obtained when adopting continuous-rate adaptive modulation. The study of ASE is also extended for the case of discrete-rate modulations, where the performance comparison of different practical quantized modulation schemes is carried out. Index Terms-Average spectral efficiency (ASE), continuous-rate adaptive modulation, Coordinated Multipoint (CoMP), discrete-rate adaptive modulation, joint transmission (JT).

Abstract-Visible Light Communication (VLC) is a promising technology to achieve high data rates in heterogeneous scenarios. However, VLC strongly depends on the existence of a Line-of-Sight (LoS) link between transmitter and receiver to guarantee a good data rate performance, which is often a condition that is difficult to satisfy in practice. In this paper, a novel cooperative multicarrier transmission scheme is proposed, where neighboring attocells smartly cooperate to decrease the probability of blockage in the LoS link. This approach is compared to single-cell transmission schemes, obtaining notable gains in both received Signal-to-Interference-plus-Noise Ratio and cell data rate when blockage of the LoS link occurs towards the nearest Base Station.

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Transmit diversity techniques have received a lot of attention recently, and open-loop and closed-loop downlink transmit diversity modes for two transmit antennae have been included into universal terrestrial radio access (UTRA) frequency division duplex (FDD) specification. Closed-loop modes provide larger system capacity than openloop modes, but they need additional side information of the downlink channel in the transmitter. In FDD systems this requires a separate feedback channel. Quantization of channel state information (CSI) in closed-loop transmit diversity schemes decreases the performance when compared to a closed-loop system where the transmitter has access to complete CSI. In this paper, we analyze the effect of quantization of CSI and deduce approximate capacity formulae for closed-loop transmit diversity schemes that are generalizations of the closed-loop schemes included in UTRA FDD specification. Moreover, we calculate approximation error and show by simulations that our approximation is tight for flat Rayleigh fading environments with and without fast transmit power control.

This paper studies the sum data rate that a Visible Light Communication (VLC) system with multiple users can achieve when phosphor-converted white LED panels are used to provide illumination and communication simultaneously. Three different transmission schemes based on Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing (ACO-OFDM) are considered to allocate the communication resources in each transmission point: Frequency Reuse (FR), Joint Transmission Coordinated Multi-Point (JT-CoMP), and a Hybrid combination of them. Since phosphor-converted white LEDs have a larger modulation bandwidth for blue optical wavelengths rather than green-yellow-red ones, the performance of each transmission method is also evaluated using a visible and/or blue light optical filter in front of the Photodetector (PD). When the transmission points apply FR, strong inter-cell interference results in high variability of the achievable data rate at different locations. On the other hand, when JT-CoMP is applied, a more homogeneous data rate coverage is achieved, reducing the peak data rate in the inner parts of the cells but improving notably the data rate in cell-edge areas.