In this paper, we present a novel geometry-based statistical model for small-scale non-wide-sense stationary uncorrelated scattering (non-WSSUS) mobile-to-mobile (M2M) Rayleigh fading channels. The proposed model builds on the principles of plane wave propagation to capture the temporal evolution of the propagation delay and Doppler shift of the received multipath signal. This is different from existing non-WSSUS geometry-based statistical channel models, which are based on a spherical wave propagation approach, that in spite of being more realistic is more mathematically intricate. By considering an arbitrary geometrical configuration of the propagation area, we derive general expressions for the most important statistical quantities of nonstationary channels, such as the first-order probability density functions of the envelope and phase, the four-dimensional (4-D) time-frequency correlation function (TF-CF), local scattering function (LSF), and time-frequency-dependent delay and Doppler profiles. We also present an approximate closed-form expression of the channel's 4-D TF-CF for the particular case of the geometrical one-ring scattering model. The obtained results provide new theoretical insights into the correlation and spectral properties of non-WSSUS M2M Rayleigh fading channels.

Power control is an important problem in today's wireless systems, which is related to battery utilisation in the mobile units. In the present study, this problem is addressed using a distributed approach. The uplink of a direct-sequence code-division multiple-access communication (DS-CDMA) system is studied, and through a proper selection of the tracking error function, the non-linear coupling among active users is transformed to individual linear loops, where equivalent power references incorporate the information of the remaining active users in the cell. It is concluded that the uplink channel variations do not destroy the stability of these feedback structures. However, the delays in the closed-loop paths can severely affect the stability and performance of the resulting feedback schemes. A linear quadratic (LQ)-optimal power control strategy is derived and compared to a dead-beat approach. In this sense, the dead-beat algorithm shows sensitivity to the roundtrip delays estimation, but the LQ-optimal control can be adjusted to be robust to this estimation error. In fact, there is a severe compromise between robustness and performance. But through an appropriate selection of the LQ parameters, a stable closed-loop system can be always guaranteed independently on the uncertainty in the estimation of the roundtrip delay. Simulation results are presented to compare the proposed control algorithms to a standard single-step power correction approach.

Color Shift Keying (CSK) is a new modulation scheme for visible light communication systems using RGB LEDs which has been standardized in the PHY III level of the IEEE 802.15.7. This paper proposes some modifications so as to include multiuser capabilities provided by a time-based multiplexing, with the modulation constellation symbols being adapted to encode data with the luminux powers of the red, green and blue color bands respectively. This is achieved by employing a simple and low-complexity time-based pulse signals structure to separate the users’ data symbols, while a three-dimensional signal constellation design is merged to improve data throughput. Numerical simulations are carried out to assess the performance of this novel architecture. The statistical properties of the transmitted RGB signals ensure dimming capabilities and that the illumination function is unaffected by flickering.

Distance estimation plays an important role in location-based services, which has become very popular in recent years. In this paper, a new short range cricket sensor-based approach is proposed for indoor location applications. This solution uses Time Difference of Arrival (TDoA) between an optical and an ultrasound signal which are transmitted simultaneously, to estimate the distance from the base station to the mobile receiver. The measurement of the TDoA at the mobile receiver endpoint is proportional to the distance. The use of optical and ultrasound signals instead of the conventional radio wave signal makes the proposed approach suitable for environments with high levels of electromagnetic interference or where the propagation of radio frequencies is entirely restricted. Furthermore, unlike classical cricket systems, a double-way measurement procedure is introduced, allowing both the base station and mobile node to perform distance estimation simultaneously.

Colour‐shift keying (CSK) has emerged as a new modulation scheme for indoor visible light communication (VLC) systems, standardised in IEEE 802.15.7. This scheme transmits data by modulating the intensity of red‐green‐blue light‐emitting diodes using a colour coding format. To support VLC, CSK requires the application of multiple light sources and detectors without changing the white light target condition. In this study, a low‐complexity VLC system is proposed for multiple light sources that offers important reductions to the overall system complexity while maintaining the compatibility with the premises of the IEEE 802.15.7 standard. The contributions of this work are two‐fold. First, a new RGB‐based modulation scheme is proposed. This modulation scheme suggests to replace the CSK‐based colour coding format by a more flexible, versatile and simplified mapping structure without the need of sending target colour information to the receiver. The second contribution is a novel single‐photodiode based receiver that reduces the number of inputs to demodulate RGB signals. Closed‐form error performance expressions are verified to analyse the proposed VLC system performance which are verified through Monte Carlo simulations. Finally, the performance of the proposed system is also presented and compared in terms of complexity and throughput efficiency.