Adrián Agustín de Dios

Cooperative linear precoding for multi-user MISO visible light communications

This paper develops and evaluates precoding techniques for coordinated joint transmission in visible light communications (VLC). A multi-user multiple-input single-output (MISO) setup is adopted. Transmitters are equipped with light emitting diode (LED) arrays, while receivers incorporate a single photodetector (PD). We design zero forcing (ZF) linear precoders considering, as figure of merit, the weighted sumrate for multilevel pulse amplitude modulation (PAM) and take into account the optical power constraints. We present two approaches. First, we formulate the design as a convex optimization problem and find its exact solution. Second, by considering certain approximations, we find an alternative precoder that despite being suboptimal can be computed in an almost closed-form, achieving a good trade-off between performance and complexity. Finally, we demonstrate that the proposed approaches largely outperform the conventional pseudo-inverse precoding and single-user TDMA approaches.

Signal-timing offset compensation in dense TDD OFDM-based networks

Orthogonal Frequency Division Multiplexing (OFDM)-based networks rely on time synchronization to obtain their best performance. Time synchronization with neighboring nodes can be satisfied by increasing the cyclic prefix (CP) length (at the expenses of spectral efficiency reduction) We show that by optimizing the transmit pre-compensation and receive post-compensation we can meet the time synchronization constraints and keep the CP at its minimum value. This concept is applied to paired-bands Frequency Division Duplexing (FDD) systems which tend to show inefficient occupancy of the uplink (UL)-band due to the traffic asymmetry. We consider the possibility of deploying multiple Time Division Duplexing (TDD) small eNBs (SeNBs) in the unused UL spectrum. In this scenario, time synchronization with macro eNB (MeNB) and neighboring SeNBs becomes essential. Two algorithms are proposed in order to ensure orthogonality of OFDM transmissions network-wide.