In this paper, a low-complexity symbol-based equalizer that performs non-linear distortion cancellation is proposed for application at the user terminal in the DVB-S2X satellite forward link. The channel is comprehensively modeled, including the non-linear travelling wave tube amplifier characteristics, the input-multiplexing and output-multiplexing filter responses at the satellite transponder, and the phase noise at the user terminal, according to the very-small aperture terminal reference scenario. Two detectors in the cancellation loop are considered, comparing the packet-error rate (PER) performance of simple maximum likelihood demodulation with hard decision to soft information exchange with low-density parity-check decoder, and showing only marginal improvement with the latter solution. The PER performance is compared against a number of pre-distortion techniques at the transmitter, such as dynamic data pre-distortion, successive data pre-distortion, and static data pre-distortion. The novel receiver demonstrates superior performance even with one iteration of distortion cancellation, while the joint application of successive data pre-distortion and iterative symbol-based equalization shows up to 4.95-dB energy efficiency gain for 32-level amplitude and phase-shift keying (32-APSK). The computational complexity is also evaluated. The improved receiver is particularly suitable for application with higher order modulation, a wide-band carrier, and low roll-off factors.

In this paper, an iterative receiver that performs non-linear distortion noise cancellation is presented. The performance is assessed for time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA), and single-carrier frequency division multiple access (SC-FDMA) waveforms for application over the satellite return link channel. The performance is assessed in terms of energy efficiency and spectral efficiency, where the total degradation (TD) of the received signal-to-noise ratio (SNR) is minimized. The modulation formats of the DVB-RCS2 satellite return link standard and a respective non-linear channel have been used. Two iterations of non-linear noise cancellation across the different modulation orders have shown the following total degradation gains: 1.1-2.5 dB in OFDMA, 0.3-1.1 dB in SC-FDMA, 0.1-0.8 dB in TDMA with 20% roll-off, and 0.2-1.9 dB in TDMA with 5% roll-off.

This video paper describes the practical outcome of the first milestone of a project aiming at setting up a so-called Hybrid Team that can accomplish a wide variety of different tasks. In general, the aim is to realize and examine the collaboration of augmented humans with autonomous robots, virtual characters and SoftBots (purely software based agents) working together in a Hybrid Team to accomplish common tasks. The accompanying video shows a customized packaging scenario and can be downloaded from http://hysociatea.dfki.de/?p=441.

In this paper, an iterative receiver that performs non-linear distortion noise cancellation is presented. The performance is assessed for time division multiple access (TDMA), orthogonal frequency division multiple access (OFDMA) and single-carrier frequency division multiple access (SC-FDMA) waveforms. Even though a return link setup is considered, the receiver is equally applicable in the forward link, taking into account the differences in the data multiplexing and the channel. Analytical modeling of the received electrical signal-to-noise ratio (SNR) is carried out for OFDMA with one iteration of non-linear distortion noise cancellation. The performance is assessed in terms of power efficiency and spectral efficiency, where the total degradation (TD) of the received SNR in a non-linear channel is minimized. The modulation formats of the DVB-RCS2 satellite return link standard and a respective non-linear channel have been used. OFDMA shows the highest power efficiency gain of 1.1-2.5 dB with 2 iterations of non-linear noise cancellation across the different modulation orders. In SC-FDMA, the gain is in the range of 0.3-1.1 dB, while gains of 0.1-0.8 dB and 0.2-1.9 dB are presented in TDMA with 20% roll-off and 5% roll-off, respectively.

In this paper, a framework for optimum signal shaping in multi-carrier modulation is presented for optical wireless communications (OWC). The two fundamental multi-carrier transmission schemes based on orthogonal frequency division multiplexing (OFDM), direct-current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM), are studied. The optimum signal shaping is defined as optimum biasing and optimum scaling of the time domain signal within the optical power constraints of the transmitter front-end. These include the boundaries of the limited linear dynamic range, such as minimum and maximum radiated optical power, and the desired average optical power level. As a result, the minimum required electrical signal-to-noise ratio (SNR) to maintain a target bit-error ratio (BER) is obtained for a desired multi-level quadrature amplitude modulation (M-QAM) scheme and a given combination of optical power constraints. The average optical power is varied over dynamic ranges of 10 dB, 20 dB and 30 dB. With the increase of the dynamic range and for a major portion of the average optical power levels, DCO-OFDM demonstrates a lower minimum electrical SNR requirement for a target BER as compared to ACO-OFDM for modulation orders with similar spectral efficiencies.

This paper constructs a model for Optical Spatial Modulation Orthogonal Frequency Division Multiplexing (O-SM-OFDM). It investigates the effects of different power allocation schemes on bit error ratio (BER) performance. Optical features such as the DCO technique, the LED non-linearity and the indoor optical channel are considered. Under power constraint, when making power difference between transmitters, we discussed trade-off between the accuracy of the spatial domain bits detection and that of signal domain bits detection. We investigated the relationship between transmitter power surplus, spatial BER, signal BER, system total BER and signal to noise ratio (SNR). The optimal point of transmission power difference is found to be around 11.5 dB.

In this paper, the impact of clipping noise on optical wireless communication (OWC) systems employing orthogonal frequency division multiplexing (OFDM) is investigated. The two existing optical OFDM (O-OFDM) transmission schemes, asymmetrically clipped optical OFDM (ACO-OFDM) and direct-current-biased optical OFDM (DCO-OFDM), are studied. Time domain signal clipping generally results from direct current (DC) biasing and/or from physical limitations of the transmitter front-end. These include insufficient forward biasing and the maximum power driving limit of the emitter. The clipping noise can be modeled according to the Bussgang theorem and the central limit theorem (CLT) as attenuation of the data-carrying subcarriers at the receiver and addition of zero-mean complex-valued Gaussian noise. Analytical expressions for the attenuation factor and the clipping noise variance are determined in closed-form and employed in the derivation of the electrical signal-to-noise ratio (SNR). The validity of the model is verified through a Monte Carlo bit-error ratio (BER) simulation. Finally, the BER performance of ACO-OFDM with DCO-OFDM is compared for different clipping levels and multi-level quadrature amplitude modulation (M-QAM) schemes.

In this paper, the impact of double-sided signal clipping on optical wireless communication (OWC) systems employing asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) is investigated. Time domain signal clipping is introduced due to biasing issues or physical limitations of the front-ends. These include insufficient forward biasing of the emitter and the maximum driving power limit of the emitter and/or the detector. The clipping noise can be modeled according to the Bussgang theorem and the central limit theorem as attenuation of the subcarriers at the receiver and addition of zero-mean complex Gaussian noise. The attenuation factor and the clipping noise variance are determined semi-analytically and employed in the derivation of the signal-to-noise ratio (SNR) at the receiver. Finally, the theoretical SNR is verified through a bit-error ratio (BER) simulation.

In this paper, the clipping noise in an asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) wireless communication system is derived semi-analytically. Clipping noise in ACO-OFDM is introduced either because of insufficient forward biasing of the emitter, or a low power sensitivity of the detector. Following the Bussgang theorem, the non-linear distortion caused by the clipping of the time domain signal attenuates the frequency domain subcarriers at the receiver and adds zero-mean Gaussian noise. The attenuation factor and the clipping noise variance are determined and verified through simulation. Finally, bit-error ratio (BER) and goodput simulations compare the performance of ACO-OFDM with a direct-current-biased optical OFDM (DCO-OFDM) system for different clipping levels and multilevel quadrature amplitude modulation (M-QAM) schemes.

In this paper, the infrared optical wireless path loss inside an aircraft cabin is estimated. To this end, a Monte Carlo ray-tracing (MCRT) simulation is performed in a geometric computer-aided design (CAD) cabin model. Position, azimuth (AZ), elevation (EL) and field of view (FOV) properties of transmitters and receivers are defined. Key path loss parameters such as the path loss exponent and the standard deviation of the shadowing component are determined for particular line-of-sight (LOS) and non-line-of-sight (NLOS) cases. A LOS path loss exponent of 1.92 and a shadowing standard deviation of 0.81 dB are obtained. In NLOS conditions, however, the path loss exponent varies depending on the nature of the particular NLOS case considered. The presented scenarios yield NLOS path loss exponent values of 2.26 and 1.28, and shadowing standard deviation values of 1.27 dB and 0.7 dB, respectively.

In this paper, first, path loss models are developed for infrared optical wireless transmission inside an aircraft cabin. Second, a cellular network in the aircraft is considered and signal-to-interference ratio (SIR) maps are determined via simulation. For this purpose, a Monte Carlo ray-tracing (MCRT) simulation is performed in a geometric computer-aided design (CAD) cabin model with defined position, azimuth (AZ), elevation (EL) and field of view (FOV) properties of transmitters and receivers. Mathematical models are developed for line-of-sight (LOS) and non-line-of-sight (NLOS) path losses along particular paths, including estimation of the path loss exponent and the shadowing component. The shadowing is modeled according to a log-normal distribution with zero mean and standard deviation sigma. The validity of this model is confirmed in the paper. It is shown that irradiance distribution under LOS conditions experiences an attenuation with a path loss exponent of 1.92 and a shadowing standard deviation of 0.81 dB. In NLOS conditions, however, the path loss exponent varies, depending on the nature of the NLOS cases considered. The presented NLOS scenarios yield path loss exponent values of 2.26 and 1.28, and shadowing standard deviation values of 1.27 dB and 0.7 dB, respectively. Finally, the cabin is divided into cells and SIR maps are presented for different frequency reuse factors. It is shown that at the edges of the circular cells with diameter of 2.8 m, a SIR of -5.5 dB is achieved in a horizontal cross section of the cabin for frequency reuse of 1, and -2 dB and 3 dB for frequency reuse factors of 2 and 3, respectively. This means that in an aircraft cabin, for reuse factors less than three, viable communication at the cell edges is not feasible without additional interference avoidance or interference mitigation techniques.