Parallel transmission based on OFDM is undoubtedly an excellent solution for frequency-selective and bandlimited channels. For general linear channels with additive Gaussian noise, the channel capacity can be achieved in combination with water pouring and ideal coding. In conjunction with optical transmission based on intensity modulation and direct detection (IM/DD), however, non-negative signals are required, so that the matter is no longer so simple. Here, the actual OFDM signal is usually provided with a DC-offset that corresponds to the mean optical power to be used. Since signals always tend to be Gaussian distributed in parallel transmission, the information bearing component is preferably clipped before biasing, or an adaptive bias is used. However, what about alternative solutions? What are the benefits of Kramers-Kronig optical OFDM or of “single side-band OFDM” under the IM/DD constraint? What if we clip only a single modulated subcarrier (that has lower peak values) symmetrically, apply a bias and equalize the received signal in the frequency domain? In this context we consider carrierless amplitude-phase (CAP) modulation, where the bandpass pulses are generated directly by means of FIR-filtering. Of course, “unfiltered” pulse-amplitude modulation could also be an alternative.

Lighting requirements and their impact on visible light communication (VLC) systems based on multi-level pulse-amplitude modulation are discussed. We will show that the moving average of the signal must not fluctuate too much. Two solutions are presented. One is based on a simple extension of the well-known 5B6B line coding to codes with M-ary symbols. The other is based on the so-called Hadamard-coded modulation. Closely related to the topic of DC balance is the resistance of the received signal to high-pass filtering. The impact of such filtering on multi-level PAM systems with frequency domain equalization is investigated. We will show that the demand for flicker-free lighting can easily be met with line codes. Together with an equalization, which has to be used for multi-level PAM anyway, the AC coupling at the receiver is no problem either. The feasibility of an adaptive modulation is also discussed.

Discrete multitone transmission (DMT) schemes employing real-valued constellations such as PAM (PAM-DMT) are popular multi-carrier transmission schemes for optical systems using intensity modulation and direct detection. PAM-DMT employs a data dependent cyclic prefix (CP) to maintain cyclicity. In this letter, we propose a novel DMT scheme using real-valued modulation formats where the usual CPs are replaced by a unique word (UW) that is a known deterministic sequence. This is achieved by designing a code generator matrix in the frequency domain that introduces correlation among the data symbols. The proposed PAM UW-DMT scheme shows a superior bit error ratio performance as compared with PAM CP-DMT. Moreover, the UW can be used for other purposes, such as synchronization and channel estimation.

Digital transmission over dispersive optical channels suffers from intersymbol interference (ISI). We study the mutual information of PAM transmission under the intensity modulation and direct detection (IM/DD) constraint, where we model the optical channel as a Gaussian low-pass filter, which is a valid attempt for polymer optical fibers (POF) and allows closed form expressions. In particular, we study the properties of the sample whitened matched filter, which enables the derivation of the mutual information for PAM with IM/DD. The information rate (lower bound) of PAM with equally probable input symbols is compared to an information rate approximation of DC biased multiple sub-carrier modulation (MSM), where we approximate the effect due to threshold clipping as Gaussian distributed noise, and to the information rate of asymmetrically clipped MSM. The required optical power for uncoded block transmission at a bit error rate of 10(-3) is also presented. In the case of PAM, a unique word approach is used, and decision-feedback equalization is performed directly at the output of a sample whitened matched filter realized in the frequency domain. The results show that PAM with a rectangular pulse shape outperforms the other schemes.

The timing synchronization of OFDM/DMT systems is often based on auto-correlation algorithms. However, these algorithms do not provide a sharp correlation peak. In this work, we present a novel timing synchronization and channel estimation concept for optical block transmission systems with intensity modulation and direct detection. This concept is based on Golay complementary sequences. As these sequences are binary, multiplier-less matched filters can be implemented in order to use the cross-correlation function for the timing synchronization. Since Golay complementary sequences have ideal correlation properties, they enable an estimation of the channel impulse response directly in the time domain. We present an appropriate preamble structure and receiver concept for PAM-block transmission systems with frequency domain equalization and demonstrate the performance exemplary for 7B8B-coded binary PAM.

A next generation optical wireless communications standard is currently established, denoted as IEEE 802.15.7r1, targeting data rates between 1 Mbit/s and 10 Gbit/s. Selecting an appropriate transmission scheme is one of the critical tasks. Following this issue, this paper gives a review on PAM-FDE in VLC. Based on PAM-FDE laboratory measurements, we demonstrate the need to include highpass filtering into the propagation model. In this paper, numerical analyses investigate the impact of the resulting baseline wander on the performance. We include a multi-level 5S6S coding technique in order to prepare PAM-FDE for practice. Finally, research topics are summarized.

Channel characterisation for indoor visible-light communication systems is revisited. The purpose of this study is to evaluate the channel frequency selectivity, or in other words, the significance of inter-symbol interference (ISI) at the receiver and the necessity of channel equalisation to recover the transmitted data. The authors focus on the effect of the indoor channel by assuming no bandwidth constraint on the light-emitting-diodes and by considering a simple intensity modulation technique, excluding discrete multi-tone modulation. The channel impulse response (CIR) is first simulated using the iterative site-based method. Then, conventional metrics for evaluating channel frequency selectivity, that is, the root-mean-square delay spread and channel frequency response are investigated and their practical interest is discussed. The authors also consider the signal-to-ISI ratio (SIR), which they define based on the sampled (i.e. discrete-time) signal at the receiver, and demonstrate its usefulness in determining the necessity of channel equalisation at the receiver. They consider several link scenarios in a medium-size and a large room, and show the significance of the LOS components of CIR in determining the channel frequency selectivity. They also discuss the choice of the receiver filter and explain how it affects the SIR.

Due to their dispersive nature, large core step index plastic optical fibers (SI-POF) suffer from a bandwidth limitation. At high data rates or transmission distances, this leads to severe inter symbol interference (ISI). Efficient transmission techniques at the transmitter and robust equalization techniques at the receiver are needed to mitigate the effect of ISI. Discrete multitone transmission (DMT) is one of the most popular schemes to compensate dispersion in direct detection optical systems. Recently, non-linear equalization technique such as decision feedback equalization (DFE) and Tomlinson-Harashima precoding (THP) are being proposed for such dispersive channels. A possible low complex alternative to the such schemes can be a PAM block transmission with frequency domain equalization (PAM-FDE) at the receiver. This paper compares the bit loading enhanced DMT (namely asymmetrically clipped (AC) DMT) with systems employing non-linear equalization techniques such as DFE and THP, and PAM-FDE for theoretical POF channels with a Gaussian profile. Although it is widely believed that non-linear equalization schemes will outperform the linear schemes such as PAM-FDE, we show that almost similar or better performance can be achieved by using PAM-FDE when system complexity is also taken into account.

Optical wireless technology uses light for mobile communications. The idea is to simultaneously combine the illumination provided by modern high-power light-emitting diodes (LEDs) with high-speed wireless communications. There have been numerous practical demonstrations of this concept, and the technology is now well matured to be deployed in practice. Independent market analysts forecast a high-volume market for mobile communication devices connected to the ubiquitous lighting infrastructure. This paper aims to make optical and wireless industries aware of the requirement for standardization in this area. The authors present the view of the European COST 1101 research network OPTICWISE towards a next-generation optical wireless standard aiming at data rates from 1 Mbit/s to 10 Gbit/s. Besides key technical insights, relevant use cases and main features are described that were recently adopted by the IEEE 802.15.7r1 working group. Moreover, a channel model is introduced to enable assessment of technical proposals.

Carrier-less amplitude and phase (CAP) modulation has recently been considered as an alternative to optical orthogonal-frequency-division-multiplexing (O-OFDM) in visible light communication (VLC) systems. The main advantages of CAP over O-OFDM are its lower implementation complexity at the transmitter side and its lower peak-to-average power ratio. Here, we consider CAP modulation where matched filtering and equalization at the receiver are performed in the frequency domain, resulting in reduced system complexity. Through numerical results we investigate the performance of this transmission scheme and, in particular, study the impact of the roll-off factor and the length of the pulse shaping filters.

Two high-speed angle diversity optical wireless systems have recently been implemented, as part of a European Community funded project. One operates at 1.25 Gb/s offering a limited coverage area, and the other at 280 Mb/s, with room scale coverage. In this paper, we summarize the design approach for these systems and their performance. Implications of these results for the design and implementation of future systems are also discussed.

The design, simulation and practical implementation for a bidirectional 1.25 Gbit/s indoor optical wireless communications system has been presented. It is a part of the European community funded hOME Gigabit Access project (OMEGA). The line-of-sight (LOS) system uses angle-diversity transceivers enabling discrete beam steering. Each transceiver uses three transmitting and receiving elements giving an overall field of view of similar to 25 x 8 degrees and a transmission range of 3 m. Measurement shows that the system can operate at a bit error rate below 1029 without channel coding and the handover between cells is <400 ns. Detail of a demonstrator of high-definition (HD) video embedded in gigabit ethernet stream using this system is also reported.

Block transmission with frequency-domain equalization (FDE) has been proven as an attractive alternative to orthogonal frequency division multiplex (OFDM) for radio frequency (RF) communication, especially at 60 GHz. In this paper, we show that FDE can be advantageously applied for optical transmission over dispersive channels as well, and in particular, in direct-detection systems. We review the optimal coefficients for zero forcing (ZF) and minimum mean square error (MMSE) equalization - both in the case of symbol-spaced and fractionally-spaced sampling, and present promising results for an illustrative visible-light communications (VLC) link based on non-return-to-zero (NRZ) on-off keying (OOK).

In this paper we describe an angle diversity optical wireless system that operates at 280Mbits/s and provides bidirectional data transmission over a wide coverage area. The system uses commercially available components and operates at a wavelength of 860nm. Three terminals, each using seven transmitter and receiver channels were implemented, and the system was successfully tested in a wide range of different conditions. Implementation challenges, design and performance are also discussed, together with future directions for this work.

We propose a Time-Hopping (TH) M-ary Walsh transmission scheme using a one-bit non-coherent receiver for power efficient and low complexity low data rate Ultra Wideband (UWB) communications. The analytical Bit Error Rate (BER) bounds of the M-Walsh modulation and the widely used M-ary Pulse Position Modulation (M-PPM) are derived and validated in the multipath scenario. We compare both schemes in terms of the receiver implementation and complexity, the quantization loss due to the one-bit receiver, and the multiple access performance. Taking into account the perfect/imperfect power control, the Multi-User Interference (MUI) resistance is evaluated via Monte Carlo simulations. According to the performance comparison and analysis, we show the suitability of the proposed TH M-Walsh scheme using a one-bit non-coherent UWB receiver to achieve a high power efficiency.

High-speed optical wireless systems are challenging to implement, due to limitations in available components, and implementation of the necessary high speed electronics. In this paper we report on the development of a gigabit/s class infrared indoor optical wireless system that uses commercially available components. System challenges and design choices are discussed together with details of demonstrator construction. Results from the implementation of a demonstration system are also detailed, together with a discussion of how this might scale in the future.

Achieving high data-rates in optical wireless involves theoretical limits and practical constraints. In this paper we discuss these, with reference to two system examples. A 1.25 Gigabit/s demonstrator that has been fabricated as part of the European Community Framework 7 project OMEGA is described, and a 280Mbit/s demonstrator that is currently under development as part of the same project. In each case the compromises required, and implementation issues are discussed.

In this paper, the link budget of Gbps wireless infrared indoor communication is analysed. We particularly focus on the receiver sensitivity and identify the most suitable wavelengths range. We show that an optical receiver operating at 1 Gbps will hardly achieve the shot noise limit, which is determined by the received amount of background light. Regarding the link budget, we present two case studies. One deals with (very) short range communication, the other one with a wireless personal area network. We reveal that a network demands for avalanche photodiodes as well as beam steering. This clearly causes major challenges regarding compact and inexpensive components.

The hOME Gigabit Access (OMEGA) home-area-network project aims at bridging the gap between home and access network and providing Gb/s connectivity to users. The project considers a combination of various technologies such as radio-frequency and wireless optical links operating at infrared and visible wavelengths. When combined with power-line communications (PLC), this enables a home backbone that meets the project's "without new wires" vision. A technology-independent MAC layer will control this network and provide services as well as connectivity to any number of devices the user wishes to connect to in any room of a house/apartment. In order to make this vision come true, substantial progress had to be achieved in the fields of optical wireless physical layer development and data-link-layer protocol design. This paper reports an experimental demonstration of an indoor visible-light wireless link including a MAC layer protocol adapted to optical wireless communications systems. The system operates at 84 Mb/s broadcast and was successfully used to transmit three highdefinition video streams.

This letter reports an experimental demonstration of an indoor angle-diversity optical wireless communications system. This operates at 1.25 Gb/s and provides bidirectional communications between two terminals. Each terminal uses three transmitting "cells" giving a field of view of approximately 25° × 8° over a range of approximately 3 m. Data is transmitted to a terminal that uses three receivers to obtain a similar reception field of view. Link operation at a bit-error rate <;; 10e-9 is reported, together with an overview of the system configuration.