The multiple-input-single-output (MISO) Gaussian broadcast channel (BC) under per-emitter peak-amplitude constraint arises in naturally indoor optical wireless communication (OWC) applications due to the availability of many luminaires and simple photo receivers. In this paper, we study the achievable rate region of the two-user peak-amplitude constrained MISO BC with the goal of increasing the rate region beyond the one achieved using zero-forcing (ZF) precoding. To this end, a novel precoding scheme is proposed that combines a type of discrete-state dirty paper channel (DPC) studied recently and ZF precoding for the MISO BC, termed the DPC-ZF precoding. We derive the achievable rate region of the proposed precoding scheme, and compare it with the one achieved by ZF precoding numerically by simulating a two-user indoor OWC broadcasting system. Numerical results show a significant improvement in the achievable rate region through DPC-ZF precoding compared to ZF precoding, implying the existence of more powerful interference management schemes for OWC broadcasting.

Enabling high-speed internet access in rural areas is vital for advancing education, healthcare, environmental protection, and economic growth. However, the cost of backhaul poses a significant barrier to achieving broadband connectivity in these regions. Optical satellite-ground feeder links offer an affordable solution for rural backhauling, delivering high-speed capabilities. Nevertheless, cloud-induced attenuation poses a major challenge, often leading to substantial degradation or complete disruption of such optical satellite feeder links. To address this issue, we propose an optical feeder link utilizing fiber tethered aerostats. Our study focuses on evaluating the performance of this proposed approach in Canada's expansive northern geography, where we analyze the statistical distributions of cloud-induced attenuation using ERA5 data. We compare the availability of the proposed approach with traditional ground diversity approaches. The numerical results demonstrate that even during the warm months when liquid clouds cause significant attenuation, the proposed approach achieves a link availability of over 95%.

Visible light communications (VLC) is a promising technology that combines the ubiquity of illumination devices with broadband communication to provide connectivity. The most recent VLC communication standards specify orthogonal frequency division multiplexing (OFDM) as the underlying modulation due to its spectral efficiency and compatibility with existing broadband modems. However, a key challenge remains in realizing VLC modulators capable of generating OFDM signal compatible with the primary illumination function, while ensuring the efficacy of the luminaires. In this paper, a novel method is proposed to integrate OFDM modulation into a switching power supply. Commutating diodes in the DC/DC converter are replaced by light-emitting diodes (LEDs) which are used for communication while the larger illumination string of LEDs is left unmodulated preserving the quality of illumination and luminaire lifetime. Biasing the switching MOSFET to its saturation region during its off states allows for the MOSFET to act as an amplifier, amplifying the OFDM communication signal and injecting it to the light-emitting commutating diode (LECD). As a result, energy efficient LED drivers capable of OFDM signal transmission are realized. Both simulation and experimental results are provided to demonstrate the feasibility of this approach, showcasing superior performance compared to existing literature in terms of component count, efficacy, illumination quality, and reliability. Notably, based on simulation results, the efficacy of our approach surpasses the bias-T approach by approximately 6.25%.

Though visible light communication (VLC) systems are contained to a given room, improving their security is an important criterion in any practical deployment. This paper studies the design of artificial noise (AN) to enhance physical layer security in VLC systems in the context of input signals with no explicit amplitude constraint (e.g., multicarrier systems). In such systems, clipping is needed to constrain the input signals within the limited linear ranges of the LEDs. However, this clipping process gives rise to non-linear clipping distortion, which must be incorporated into the AN design. To solve the design problem, a sub-optimal approach is presented using the Charnes-Cooper transformation and the convex-concave procedure (CCP). Then, a novel AN transmission scheme is proposed to reduce the impact of clipping distortion, thus improving the secrecy performance. The proposed scheme exploits the typical structure of LED luminaries that are composed of multiple light-emitting chips. Specifically, LED chips in each luminaire are divided into two groups driven by separate driver circuits. One group is used to transmit the information-bearing signal, while the other group transmits the AN. Numerical results show that while clipping distortion can significantly reduce the secrecy level, with proper design AN significantly improves the secrecy performance using the proposed design methodology.

Though visible light communication (VLC) systems are contained in a given room, ensuring their security amongst users in a room is essential. In this paper, the design of artificial noise (AN) to enhance physical layer security in VLC systems is studied in the context of input signals with no explicit amplitude constraint (such as multicarrier systems). In such systems, clipping is needed to constrain the input signals within the limited linear ranges of the LEDs. However, this clipping process gives rise to non-linear clipping distortion, which must be incorporated into the AN design. To facilitate the solution of this problem, a sub-optimal design approach is presented using the Charnes-Cooper transformation and the convex-concave procedure (CCP). Numerical results show that the clipping distortion significantly reduces the secrecy level, and using AN is advantageous over the no-AN scheme in improving the secrecy performance.

Conventional spatial diversity techniques do not reduce the severe impact of the main impairments in underwater optical wireless communication (UOWC) such as absorption and scattering. This paper addresses a complete comparison of Multiple-Input/Single-Output (MISO) UOWC systems by using spatial repetition coding (SRC) and transmit laser selection (TLS) strategies over salinity-induced oceanic turbulence in clear ocean and coastal waters. We consider an SRC system with sufficient space between sources to efficiently increase the total transmitted power while satisfying eye-safety requirements by assuming a maximum optical power per source in order to obtain the same ergodic capacity performance of a TLS system with the same number of sources. Novel closed-form expressions and asymptotic results are derived to compute such a performance and optimize the UOWC system design. The presented results demonstrate that MISO UOWC systems achieve a greater ergodic capacity as the number of laser sources increases in all kinds of waters. This improvement is greater when SRC scheme is adopted for all the presented UOWC scenarios except when extreme oceanic turbulence conditions are considered. Monte Carlo simulations verify analytical and asymptotic results.

In visible light communication (VLC) systems, the uplink and downlink channel gains depend on environmental geometry. This geometric nature induces a relationship between the uplink and downlink VLC channel gains. In this paper, the reciprocity relation between the uplink and downlink VLC channel gains are analyzed. It is shown that the reciprocity relationship can be accurately modeled as affine or polynomial. This reciprocity analysis can be a useful tool that enables several VLC system analysis and design. As an interesting application, using this channel reciprocity and model of the solar cell, the relationship between the downlink rate, harvested power and uplink rate are obtained for a solar cell based VLC-SLIPT system. This enables the formulation of the uplink-downlink rate region for a solar cell-based SLIPT system. The rate region gives the set of all simultaneously achievable rates in the uplink and downlink. The rate analysis, enabled by the reciprocity result, gives the trade-off between downlink and uplink rates that is essential in the system design of solar cell based SLIPT receivers.

Indoor optical wireless communication (IOWC) is a key technology complementing radio communication in 6G. Typically, the downlink of IOWC is implemented using visible light and uplink is implemented using infrared. Though the downlink of IOWC has been studied well, the uplink of multiple-access IOWC requires further research. In this paper, power allocation strategies in the uplink of IOWC with multiple non-cooperative users is studied. Optimizing power allocation in IOWC that maximize sum-rate with power constraints becomes a non-convex problem. First, this paper proposes an alternating maximization algorithm to compute near optimal power allocation policies for small number of users. Next, a phenomenon of invariance is observed when the number of users in the system is large. Under invariance, the optimal power allocation policy of a user becomes independent of other users, thereby facilitating fast and decentralized computation of power allocation strategies. From simulations, it was observed that the invariance can occur even with five to ten users. For the first time, a rigorous analysis of the invariance phenomenon is provided. An algorithm to compute the optimal power allocation at invariance is proposed; this algorithm is proven to achieve the global maxima in sum-rate without the channel knowledge of other users.

A passive indoor visible light positioning system is proposed that does not require active participation from the user and is suitable for IoT sensor networks. This approach does not require a line-of-sight path and measures the impulse response (IR) between sources and receivers installed in the room. The presence of an object of interest (OI), i.e., a person to be localized, disrupts the IRs among the source–receiver pairs, which can be related to its position. A deep learning framework is developed that learns the relationship between changes in sets of IRs and the OI position through a set of training data obtained by placing the OI at random locations in the room. This approach shows that the OI can be localized using a very limited set of training data under a wide range of illumination levels. In order to represent a realistic scenario, a room with furniture is modeled in the optical system design software. The ray trace information of the modeled room is used to construct IR measurements among different source–receiver pairs that include multiorder reflections. The results show that localization performance is crucially related to the signal-to-noise ratio and the number of training data points used in the learning process. A root-mean-square error (RMSE) near 30 cm is possible in the case of high SNR and a large training set. However, even with a very limited training set and over a range of dimming levels, RMSEs of near 80 cm were obtained without the need for explicit user involvement.

Enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) and layered asymmetrically clipped optical OFDM (LACO-OFDM) are spectrally efficient modulation techniques for intensity modulated systems which layer multiple non-negative signals. In this paper, we propose absolute value layered asymmetrically clipped optical OFDM (ALACO-OFDM), which further improves the spectral efficiency while using a smaller number of layers and no explicit direct current (DC) bias. In ALACO-OFDM, asymmetrically clipped optical OFDM (ACO-OFDM) signals are sent at the first L layers and absolute value optical OFDM (AVO-OFDM) is used for the remaining subcarriers and transmitted simultaneously. Analysis indicates that ALACO- achieves higher spectral efficiency than eU- or LACO-OFDM while using a smaller number of layers. Bounds on achievable information rates of ALACO-OFDM and related layered ACO-OFDM techniques are also developed. Two optical power allocation schemes over the layers of ALACO-OFDM are developed with the objective of optimizing uncoded transmission performance and the achievable information rate respectively. Additionally, a theoretical bound on the uncoded BER of ALACO-OFDM is derived. Monte Carlo simulation results indicate ALACO-OFDM with the optimum power allocation achieves significant uncoded BER performance gains compared to its counterparts at the same spectral efficiency while having a smaller peak-to-average power ratio.

In this paper, antisymmetry-constructed clipped optical OFDM (AC- OFDM) is proposed for intensity modulation with direct detection (IM/DD) systems, in which an antisymmetry property is imposed directly in time domain. AC-OFDM has the same spectral efficiency as traditional asymmetrically clipped optical OFDM (ACO-OFDM) but is less complex to implement. Layered AC-OFDM (LAC-OFDM) is then proposed as an extension to further improve spectral efficiency, where different layers of AC-OFDM signals are added in the time domain and transmitted simultaneously. Computational complexity analysis and numerical results show that LAC-OFDM has the same spectral efficiency as layered asymmetrically clipped optical OFDM (LACO-OFDM) and enhanced unipolar OFDM (eU-OFDM) but is less complex in terms of both the number of real multiplications and real additions required. Specifically, LAC-OFDM requires less than half the multiplication and addition operations compared to the comparable LACO-OFDM scheme. Monte Carlo simulation results show that LAC- OFDM has the same bit error rate (BER) performance as LACO- and eU-OFDM.

Both enhanced unipolar orthogonal frequency division multiplexing (eU-OFDM) and layered asymmetrically clipped optical OFDM (LACO-OFDM) are among the most spectrally efficient modulation techniques for intensity modulated links that layer multiple non-negative signals. In this paper, we propose absolute value layered asymmetrically clipped optical OFDM (ALACO-OFDM) which further improves the spectral efficiency while using a smaller number of layers and no direct current (DC) bias. In ALACO-OFDM, asymmetrically clipped optical OFDM (ACO-OFDM) signals are sent at the first L layers and absolute value optical OFDM (AVO-OFDM) is used for the remaining subcarriers and transmitted simultaneously. Analysis indicates that ALACO-OFDM achieves higher spectral efficiency and smaller peak-to-average power ratio (PAPR) compared to eU-OFDM, LACO-OFDM and asymmetrically clipped absolute value optical OFDM (AAO-OFDM). Monte Carlo simulation results also indicate that ALACO-OFDM achieves significant bit error rate (BER) performance gains compared to its counterparts at the same spectral efficiency.

A hybrid two-level multipulse pulse-position modulation-M-ary differential phase-shift keying (2L-MPPM- MDPSK) technique is proposed which achieves high power and spectral efficiencies. The proposed 2L-MPPM-MDPSK scheme is shown to have the highest spectral efficiency in comparison with other hybrid modulation schemes. Additionally, the proposed technique has the advantage of removing the complex variable delay-line required for the direct- detection of other hybrid MPPM-MDPSK systems. The performance of 2L-MPPM-MDPSK is quantified in free-space optical (FSO) channels modeled with an exponentiated Weibull (EW) fading distribution. Closed-from expressions for both bit-error rate (BER) and outage probability upper-bound are derived and verified using Monte Carlo (MC) simulation. At the same data rate and bandwidth, 2L-MPPM-MDPSK techniques outperform traditional MPPM-MDPSK scheme by approximately 2dB at a BER of 10-4.

In this paper we present a new methodology for indoor localization using visible light emissions which does not require users to actively participate in the process. Our passive positioning approach utilizes a network of luminaires and receivers on the ceiling and measures the impulse responses (IRs) between each source-receiver pair. This channel sounding approach leverages changes in the measured IRs to localize an object in the room. Simulation results show that with a database of sampled IRs, the root mean squared (RMS) positioning error can be made on the order of 4 cm. In order to lower storage requirements, a simplified database based on fitting to an integrating sphere model of the IR yields localization performance with RMS error around 6 cm.

A pixelated MIMO wireless optical communication system is introduced, which transmits a series of time-varying coded images that can be received and decoded by commercial digital cameras. The system exploits the bokeh effect to obtain fixed-scale images at all link distances by placing a convex lens in front of the transmitter array at its focal length and focusing the receiver at infinity. This spatial-angular mapping simplifies the receiver structure requiring no re-focusing as the receiver moves. As an additional benefit, this mapping can also be exploited to provide location information to the receiver. The channel model is measured and modeled and rateless codes are applied to track the truncation of receive images for various link ranges and angular offsets. A proof-of-concept optical communication system is implemented with an LCD display and a high-speed CMOS camera.

Visible light communication (VLC) is an emerging short-range optical wireless communication technology that is driven by the widespread deployment of light-emitting diodes (LEDs). Indoor VLC users may experience noticeable interference when independent transmitters utilizing wide-beam luminaires are closely located. In this paper, a coordinated beamforming (CB) scheme is proposed for downlink interference mitigation among the coexisting VLC attocells utilizing multi-luminaire transmitters. Compared to joint transmission (JT) schemes, wherein the data and channel state information are shared among transmitters of different attocells, the proposed CB scheme places lower requirements on the network in terms of backbone traffic, and is easier to implement in a practical deployment, though at the cost of compromised performance. In this paper, we investigate the downlink transmission of coordinated VLC attocells and focus on its transmitter design. The weighted sum mean square error (WSMSE) is adopted as the performance metric to take into consideration interference, noise, and fairness among users in system optimization. The WSMSE minimization problem is considered with linear beamforming restricted by amplitude constraints. Such constraints arise from the dynamic range limitations in typical LEDs. Moreover, we extend our design method to take into account possible mismatches in channel information available to the transmitters. Numerical examples are provided to illustrate the performance of the proposed CB scheme in typical VLC scenarios. We also quantify the performance gap among several coordination schemes including JT and CB.

This paper studies an indoor communication system that integrates visible light communication (VLC) and power line communication (PLC) technologies. We exploit the application of the same orthogonal multicarrier modulation format for both PLC and LC link and present the framework for the capacity analysis of the hybrid VLC-PLC (HVP) system. In addition, several computationally efficient subcarrier allocation schemes are proposed to maximize the system capacity. The results demonstrate that the proposed subcarrier allocation in the HVP system can provide an improvement in the system capacity over random subcarrier allocation.

Bandlimited optical intensity channels, arising in applications such as visible light communications, require that all signals satisfy a bandwidth constraint as well as average, peak, and non-negativity amplitude constraints. In this paper, a 2-D signal space for bandlimited optical intensity channels is presented. A novel feature of this model is that the non-negativity and peak constraints are relaxed, and the signal space parameterizes the probability of the negative or peak amplitude excursions in the output. Although the intensity channel only supports non-negative amplitudes, the impact of clipping on system performance is shown to be negligible if the likelihood of the negative excursion is small. A tractable approximation using finite series is used to accurately compute the probability of clipping under average and peak optical power constraints. The optical power and spectral efficiencies using hexagonal lattice constellations are computed. Schemes designed in this paper have higher average and peak optical power gains than $M$ -ary pulse amplitude modulation (PAM) using previously established techniques for spectral efficiencies greater than 2.5 and 3.5 bits/s/Hz, respectively. For high spectral efficiency, e.g., greater than 6 bits/s/Hz, the proposed scheme attains a more than 2-dB average and peak optical power gain over 16-PAM using the previous approaches.

Location-based services have experienced substantial growth in the last decade. However, despite extensive research efforts, sub-meter location accuracy with low-cost infrastructure continues to be elusive. In this paper, we propose IDyLL -- an indoor localization system using inertial measurement units (IMU) and photodiode sensors on smartphones. Using a novel illumination peak detection algorithm, IDyLL augments IMU-based pedestrian dead reckoning with location fixes. We devise a robust particle filter framework to mitigate identity ambiguity due to the lack of communication capability of conventional luminaries and sensing errors. Experimental study using data collected from smartphones shows that IDyLL is able to achieve high localization accuracy at low costs. Mean location errors of 0.38 m, 0.42 m, and 0.74 m are reported from multiple walks in three buildings with different luminary arrangements, respectively.

Visible light communication (VLC) reuses illumination devices, particularly light-emitting diodes (LEDs), for communication purposes. It has great potential to alleviate the strain on radio-frequency spectrum in indoor environments. VLC-enabled LED luminaries form VLC attocells that carry downlink data traffic to indoor mobile or stationary terminals. While one of the advantages of indoor VLC is low interference due to natural cell boundaries such as walls, multiple VLC attocells within a room would interfere. This is because illumination requirements often mandate a rich overlap of emissions of luminaries in a room. In this paper, we suggest the coordination of multiple VLC attocells (i.e., VLC-enabled LED luminaries) to turn the problem of overlap and thus interference into an advantage. We stipulate that this coordination can be accomplished through power line communication, which has been considered before as a means to transport data to VLC transmitters. Borrowing from concepts developed for radio-frequency wireless communications, we develop several precoding schemes for the new coordinated VLC broadcasting architecture. These include designs for the case of imperfect channel knowledge at the VLC transmitter, since channel information is usually provided through a low-rate feedback channel. The performance advantages for VLC transmission due to the proposed coordination and precoding designs are demonstrated based on a set of numerical results.

Visible-light communication (VLC) systems leverage solid-state illumination devices to create high-speed communication links. Orthogonal frequency-division multiplexing (OFDM) has been considered for these intensity-modulated/direct-detection (IM/DD) optical channels, however, it suffers from high peak-to-average power ratio (PAPR). Moreover, the implementation of linear, power-efficient, high-current, wideband drivers is challenging. In this paper, the concept of spatial summing is developed where wideband, high PAPR OFDM signals are partitioned into many low-PAPR narrowband signals that are transmitted from multiple LEDs. The signals from different LEDs are allowed to sum in space before being detected by a conventional OFDM receiver. Spatial optical-OFDM (SO-OFDM) is proposed in which filtered subsets of carriers are emitted by each LED. In addition, low-PAPR optical single-carrier FDMA (OSC-FDMA) is developed where different collections of LEDs act as virtual users in a multiple-access scheme. The different variations of SO-OFDM and OSC-FDMA are compared to conventional DC-biased optical (DCO) OFDM and are shown to achieve lower PAPR and more robustness to LED nonlinearities leading to error rate performance gains at high signal-to-noise ratios.

Despite being an accepted model for a wide variety of optical channels, few general results on optimal signalling for discrete-time Poisson (DTP) channels are known. Among the most significant is that under simultaneous peak and average constraints, the capacity-achieving distributions are discrete with a finite number of mass points. In this paper, several fundamental properties of capacity-achieving distributions for DTP channels are established. In particular, we demonstrate that all capacity-achieving distributions of the DTP channel have zero as a mass point. In the case of only a peak constraint, it is further shown that the optimal distribution always has a mass point at the maximum amplitude. Finally, under solely an average power constraint, it is shown that a finite number of mass points are insufficient to achieve the capacity. In addition to these analytical results, a numerical algorithm based on deterministic annealing is presented which can efficiently compute both the channel capacity and the associated optimal input distribution under peak and average power constraints. Numerical lower bounds based on the envelope of information rates induced by the maxentropic distributions are also shown to be extremely close to the capacity, especially in the low power regime.

Discrete-time Poisson (DTP) channels exist in many scenarios including space laser communication systems which operate over long distances and which can be corrupted by reflected and scattered light. Through simulation, binary-input distributions have been observed to be optimal in many cases, however, little analytical work exists on conditions for optimality or the form of optimal signalling. In this second part, the general properties of Part I are extended to the case of DTP channels where binary-inputs are optimal. Necessary and sufficient conditions on the optimality of binary (i.e. two mass point) distributions are presented by leveraging the general properties of DTP capacity-achieving distributions. Closed-form expressions of the capacity-achieving distributions are derived in several important special cases including zero dark current and for high dark current. Numerical results are presented to elucidate the developed analytical work.

In this paper, rate-adaptive free-space optical (FSO) communication is studied using experimental data measured over a 1.87 km terrestrial FSO link in different weather conditions. To accommodate to the channel gain fluctuations induced by atmospheric turbulence and/or weather variations, a rate-adaptive communication system is implemented by puncturing low-density parity-check (LDPC) codes in a rate-compatible fashion. Beside the conventional random puncturing method, an optimized intentional puncturing technique is employed. Using experimental data and on-off-keying (OOK) modulation, the performance of the rate-adaptive FSO system operating at different signaling rates is evaluated. Unlike uncoded OOK, the rate-adaptive technique provides reliable and efficient FSO communication under different weather conditions and scintillation indices. Applying intentional puncturing can further improve the efficiency of the FSO system in terms of throughput.

In this paper, the performance of multi-beam free-space optical (FSO) communication systems are studied through an accurate analytical approach which does not rely on far-field assumptions commonly used in the literature. A framework is presented for analytical and numerical performance analyses of multi-beam FSO systems employed in a diversity or multiplexing scheme. The performance analyses show that the far-field assumptions may not correctly estimate the system behavior in many geometrical scenarios within practical interest. The results demonstrate the degrading effects of diffraction and spatial correlation of atmospheric turbulence in the form of diversity gain reduction in diversity systems and crosstalk in multiplexing systems. These effects have been mostly neglected in the literature by applying far-field assumptions. Our results can thus be useful in the design of practical diversity or multiplexing FSO systems especially when a compact design is desired.

There is recent interest to establish non-line-of-sight (NLOS) links in the solar-blind ultraviolet (UV) region for outdoor optical wireless communications. A new channel model is proposed based on the propagation and detection statistics of the channel. Unlike previous work, in this paper, the performance of NLOS UV communication systems is investigated by numerically computing the information rates for a uniform binary input distribution. Simulation results indicate that conventional uncoded transmission is far from capacity and that large gains in data rate can be had by applying error control coding.

In this paper, the performance of a multiple-input/single-output (MISO) free-space optical (FSO) systems is studied through an analytical approach which more accurately models the atmospheric channel. The diversity gain is evaluated based on measures determined by up to second-order statistics of the received power. The numerical results demonstrate the effects of diffraction and spatial correlation of turbulence-induced fading on the performance of multi-beam diversity systems. These effects have been mostly neglected in the literature by applying far-field assumptions. Our results can thus be useful in the design of practical multi-beam FSO systems especially when a compact design is desired.

Peer-to-peer (P2P) traffic and other latency-tolerant data sources form a large portion of present-day backbone traffic. Traffic shaping has been introduced to reduce the amount of P2P traffic at the expense of the end-user experience. In this work, free-space optical (FSO) links are proposed as a means to connect internet service providers to the backbone to carry latency-tolerant traffic. Although FSO links are less reliable than fibre, they are inexpensive and able to provide high throughput for traffic, which is able to tolerate delay. Single and adaptive modulation schemes are designed with the goal of maximising the throughput of the FSO link over a period of time rather than maximising the instantaneous reliability. Real world weather and visibility data from a major Canadian city are used to evaluate various techniques in terms of throughput, availability and outage. For a fixed modulation FSO system, four-pulse amplitude modulation (PAM) supports a yearly average rate of near 1.5 Gbps for a 1 km range, whereas conventional on–off-keying (OOK) achieves 880 Mbps. With additional channel feedback data, adaptive modulation has over double the average throughput of OOK using four different PAM modulation schemes.

A novel statistical channel model for multiple-input multiple-output (MIMO) free-space optical (FSO) communication systems impaired by atmospheric and misalignment fading is developed. A slow-fading channel model is considered and the outage probability is derived as a performance measure. The diversity gain defined as the signal-to-noise ratio (SNR) exponent at high SNR is analyzed. Interestingly in the presence of misalignment fading the diversity gain depends only on the misalignment variance and is independent of the number of transmitters M and receivers N. Increasing the number of transmitters and receivers only results in a lower probability of outage for a given SNR, however, the rate of change is unaffected. Contrary to this case, the diversity gain of MIMO FSO systems in the presence of atmospheric fading and no misalignment is shown to be proportional to the number of transmitters and receivers, in particular the product MN.

Lower and upper bounds on the capacity of wireless optical intensity pulse amplitude modulation channels under nonnegativity and average optical power constraints are derived. A lower bound is derived based on source entropy maximization over a family of discrete nonuniform distributions with equally spaced mass points. A closed form for the maxentropic discrete input distribution is provided. Compared to previously reported bounds, the derived lower bound is tight at both low and high signal-to-noise ratios (SNRs). In addition, a closed-form upper bound is derived based on signal space geometry via a sphere packing argument. The proposed bound is tight at low SNRs and incurs a small gap to the channel capacity at high SNRs. The derived bounds asymptotically describe the optical intensity channel capacity at low SNRs, where a majority of such links operate.

The design and analysis of capacity-approaching input signalling for optical intensity channels are presented. Both peak and average optical power constraints are considered in the analysis. The capacity-achieving distribution for this channel is discrete with a finite number of mass points. In practice, finding this distribution requires solving a complex non-linear optimization at every SNR. In this work, we present a closed form discrete capacity-approaching distribution derived via source entropy maximization. The computation of this distribution is substantially less complex than previous optimization approaches and can be easily computed for different SNRs. The information rates using the derived maxentropic distribution are shown to be negligibly far away from the channel capacity found by non-linear optimization in the SNR range -6 to 6 dB.

Free-space optical (FSO) links are competitive wireless links offering high data rate, security and low system complexity. For mobile applications, e.g., from a ground base station to an unmanned aerial vehicle (UAV), the FSO channel can suffer from severe instantaneous misalignment. This time varying misalignment is unknown to the transmitter and causes data packet corruption and erasure. As a result, the application of traditional fixed-rate erasure coding techniques is difficult. In this paper, we consider the application of rateless Raptor codes for such mobile FSO channels. Due to the high data rates required, short-length (16-1024) Raptor codes are designed and simulated on a severe jitter FSO channel. A key advantage of Raptor codes is their independence on channel state, no matter how large the misalignment. With a 1 Gbps transmitter, the designed Raptor code with k = 64 message packets offers 560 Mbps data rate and decoding cost of 4.14 operations per packet when transmitting power is 20 dBm. In contrast, a traditional automatic repeat- request (ARQ) algorithm technique on the same FSO jitter channel achieves a rate of 60 Mbps.

Optical Wireless link provides high bandwidth solution to the last mile access bottleneck. However, an appreciable availability of the link is always a concern. Free Space Optics (FSO) links are highly weather dependent and fog is the major attenuating factor reducing the link availability. However the traffic requirement of a LAN may not require 99.999% availability of FSO at all instants. A hybrid network with intelligent switch over can fulfil the availability and bandwidth requirement. In this paper statistical data of fog and FSO availability in Graz (Austria) has been compared to the traffic data of Technical University Graz (Austria).

This work considers the design of capacity-approaching, non-uniform optical intensity signalling in the presence of average and peak amplitude constraints. Although it is known that the capacity-achieving input distribution is discrete with a finite number of mass points, finding it requires complex non-linear optimization at every SNR. In this work, a simple expression for a capacity-approaching distribution is derived via source entropy maximization. The resulting mutual information using the derived discrete non-uniform input distribution is negligibly far away from the channel capacity. The computation of this distribution is substantially less complex than previous optimization approaches and can be easily computed at different SNRs. A practical algorithm for non-uniform optical intensity signalling is presented using multi-level coding followed by a mapper and multi-stage decoding at the receiver. The proposed signalling is simulated on free-space optical channels and outage capacity is analyzed. A significant gain in both rate and probability of outage is achieved compared to uniform signalling, especially in the case of channels corrupted by fog.

Free-space optical (FSO) links offer gigabit per second data rates and low system complexity, but suffer from atmospheric loss due to fog and scintillation. Radio-frequency (RF) links have lower data rates, but are relatively insensitive to weather. Hybrid FSO/RF links combine the advantages of both links. Currently, selection or "hard-switching" is performed between FSO or RF links depending on feedback from the receiver. This technique is inefficient since only one medium is used at a time. In this paper, we develop a "soft-switching" scheme for hybrid FSO/RF links using short-length Raptor codes. Raptor encoded packets are sent simultaneously on both links and the code adapts to the conditions on either link with very limited feedback. A set of short-length Raptor codes (κ = 16 to 1024) are presented which are amenable to highspeed implementation. A practical Raptor encoder and decoder are implemented in an FPGA and shown to support a 714 Mbps data rate with a 97 mW power consumption and 26360 gate circuit scale. The performance of the switching algorithms is simulated in a realistic channel model based on climate data. For a 1 Gbps FSO link combined with a 96 Mbps WiMAX RF link, an average rate of over 472 Mbps is achieved using the implemented Raptor code while hard-switching techniques achieved 112 Mbps on average.

This paper introduces the dynamic spot diffusing (DSD) configuration for high-speed indoor wireless optical communications. In this configuration, data are modulated onto a moving spot which is translated over the ceiling. A multi-element imaging receiver is pointed upward and acquires data whenever the transmitter spot is in its field-of-view (FOV). We develop expressions for the channel capacity of such DSD links and discuss design techniques to maximize these information theoretic bounds. Rather than tracking the transmitter spot, we apply rateless erasure correcting codes to approach the capacity of the simulated DSD links. This technique is demonstrated to have better flexibility, greater multipath immunity, higher data rates and simpler transmitters than previously defined multi-spot and diffuse architectures. In a 6 x 6 x 3 m room, simulated data rates vary between 7 Mbps to 25 Mbps at different positions using a single 100 Mbps transmitter and between 35 Mbps to 84 Mbps using 6 spots and the designed erasure correction code. Using power efficient modulation and power gain due to the spot motion of the DSD system, proportionally higher rates are estimated when faster 1 Gbps and 10 Gbps transmitters are employed.

Current lasers and LEDs have far higher pulse rates than can be supported by the lowpass indoor diffuse optical wireless channel. Although high-frequency emissions are attenuated by the channel and are not detected by the receiver, a key insight of this paper is that these bands can be used to satisfy the channel non-negativity constraint. We define optical impulse modulation (OIM) in which data are confined to the lowpass region while the highpass region, which is attenuated by the channel, is used to satisfy the channel amplitude constraints. A mathematical framework for OIM is presented, and a simple suboptimal receiver filter is designed which is channel independent. Using a well-known exponential model for indoor diffuse optical channels, at a normalized delay spread of 0.2, the gain in optical average power of OIM with a simple lowpass receiver is shown to be 4.9 dBo which exceeds the gain of rectangular on-off keying (Rect-OOK) with a complex decision feedback equalizer. From an information theory point of view, at the same normalized delay spread of 0.2, the information rate of OIM with a lowpass receiver is shown to be 11.5% higher than that of Rect-OOK with a more complex whitened matched filter receiver.

This paper presents a novel application of compressive sensing (CS) to provide spatial diversity for free-space optical communication links. A CS receiver estimates the focal-plane signal distribution due to atmospheric turbulence. This estimate is then employed by a digital micromirror device to select the portions of the focal plane which contain significant energy for detection. The CS receiver is less complex, requires fewer highspeed optoelectronic components, has lower preamplifier noise and can operate at higher rates than a comparable multiple-detector array receiver. Bit-error rates are simulated for a photon counting channel and performance improvements near 2.4 dB can be obtained over a single detector receiver.

Two-dimensional (2D) optical intensity channels exist in a variety of applications including holographic storage, page-oriented memories, optical interconnects, 2D barcodes, as well as MIMO wireless optical links. This paper considers the capacity of such channels when the transmitted signal is binary-level. Strict spatial alignment between transmitter and receiver is not required nor is independence among the spatial channels. Spatial discrete multitone modulation is combined with digital image halftoning to produce a binary-level transmit image. Unlike earlier work, this paper considers imagers with pixels of fixed size and quantifies the tradeoff between frame rate, array size and capacity per frame. An experimental prototype pixelated wireless optical channel is constructed, and the channel parameters are measured. With a measured channel model, rates on the order of 450 Mbps are predicted for aim link using 0.5 megapixel arrays at a frame rate of 7 kfps.

Wireless mesh networks (WMN) must often be upgraded as usage demands evolve. This is usually done by adding gateways which serve to increase the backhaul capacity of the network. In this paper we study the problem of this type of capacity augmentation using free-space optical (FSO) backhaul links. A joint clustering and gateway placement problem is formulated which includes the strong rate-distance dependence of practical FSO links. The formulation incorporates the positions of existing wireline gateways and minimizes the number of additional hybrid-FSO/RF gateways which are needed to satisfy the new target capacity requirements. A genetic algorithm solution is proposed, and the performance of our algorithm is compared to an optimal solution generated using an integer linear programming (ILP) formulation. Various scenarios are considered which demonstrate the value of using FSO backhaul links to obtain post-deployment capacity upgrades in response to evolving user traffic.

Analysis and design of near capacity approaching non-uniform signaling for optical intensity pulse amplitude modulation (PAM) channels is presented. A discrete nonuniform source distribution with quantized-level probabilities is presented. The proposed source distribution approaches the channel capacity while remaining practical for implementation. A significant gap in the mutual information between the non-uniform quantized-level distributions and uniform M-ary PAM is obtained. For example, at low SNR, non-uniform binary signalling has over four times the rate of uniform constellations. Multilevel coding and multistage decoding are proposed to approach the rate gains due to non-uniform signaling.

The capacity of two-dimensional (2D) optical intensity channels in which transmit images are constrained to be binary-level has been considered. Examples of such links exist in holographic storage, page-oriented memories, optical interconnects, 2D barcodes as well as multiple-input/multiple-output wireless optical links. Data are transmitted by sending a series of time-varying binary-level optical intensity images from transmitter to receiver. Neither strict spatial alignment between transmitter and receiver nor independence among the spatial channels is required. The approach combines spatial discrete multitone modulation developed for spatially frequency selective channels with halftoning to produce a binary-level output image. Data are modulated in spatial frequency domain as dictated by a water pouring spectrum over the optical transfer function as well as channel and quantisation noise. A binary-level output image is produced by exploiting the excess spatial bandwidth available at the transmitter to shape quantisation noise out of band. A general mathematical framework has been presented, in which such systems can be analysed and designed. In a pixelated wireless optical channel application, halftoning achieves 99.8% of the capacity of an equivalent unconstrained continuous amplitude channel using 1megapixel arrays.

In this paper, two novel techniques are proposed to reduce the average optical power in wireless optical multiple- subcarrier modulated (MSM) systems, namely in-band trellis coding and out-of-band carrier design. Data transmission is confined to a bandwidth located near DC. By expanding the signal set and coding over the increased degrees of freedom, an in-band trellis coding technique achieved an average optical power reduction up to 0.95 dBo over conventional MSM systems while leaving the peak optical power nearly unaffected. With a symbol-by-symbol bias, the received DC level can be detected to provide a degree of diversity at the receiver. In this manner, an additional average optical power reduction up to 0.50 dBo together with a peak power reduction of 0.46 dBo is achieved. Moreover, the unregulated bandwidth available in wireless optical channels is exploited and out-of-band carrier signals are designed outside the data bandwidth to reduce the average optical power. Average optical power reduction as high as 2.56 dBo is realized at the expense of 4 out-of-band carriers and an increase in the peak optical power. Finally, combining the three techniques achieves the best average optical power reduction of 2.63 dB optical.

Power efficient signaling over indoor diffuse wireless optical channels is considered. Present-day laser diodes have pulse rates many times higher than the bandwidth of multipath distorted diffuse channels. Despite the fact that the transmitter extra degrees of freedom are not supported by the low-pass channel, they can be used to satisfy the channel non-negativity constraint. In this paper, we define optical impulse modulation (OIM) in which data are confined to the low-pass region while the high-pass region, which is distorted by the channel, is used to satisfy the channel amplitude constraints. A mathematical framework for OIM is presented, and an optimal receiver filter is designed. At a normalized delay spread of 0.2, the gain in average optical power of using decision feedback equalization (DFE) with rectangular pulse-amplitude modulation (PAM) is 4.8 dBo, while that of using the less complex unequalized OIM receiver is shown to be 4.9 dBo.

Improved upper and lower bounds on the capacity of wireless optical intensity channels under non-negativity and average optical power constraints are derived. We consider intensity modulated/direct detection (IM/DD) channels with pulse amplitude modulation (PAM). Utilizing the signal space geometry and a sphere packing argument, an upper bound is derived. Compared to previous work, the derived upper bound is tighter at low signal-to-noise ratios. In addition, a lower bound is derived based on source entropy maximization over discrete distributions. The proposed distribution provides a tighter lower bound compared to previous continuous distributions. The derived bounds asymptotically describe the capacity of PAM optical intensity channels at both low and high SNR.

The indoor diffuse wireless optical intensity channel is bandwidth-limited due to multipath distortion, and all transmitted signal amplitudes are constrained to be nonnegative. In order to control the impact of intersymbol interference (ISI) on this channel, pulse shaping is required. This paper derives the minimum bandwidth, ISI-free Nyquist pulse which satisfies the amplitude nonnegativity constraint. The minimum bandwidth required is twice that of conventional electrical channels. With the addition of excess bandwidth, the optimal bandlimited optical intensity pulse, in the sense of minimizing average optical power, is shown to be a squared double-jump pulse. Thus, a bandwidth versus optical power efficiency tradeoff in pulse design is quantified. The impact of timing jitter on the probability of symbol error for various excess bandwidths is quantified via simulation. Further, it is shown that there are no bandlimited root-Nyquist pulses satisfying the amplitude nonnegativity constraint. In fact, all practical optical intensity root-Nyquist pulses are shown to be necessarily time-limited to a single symbol interval

In this paper, the unregulated bandwidth available in wireless optical channels is exploited to reduce the average optical power in multiple-subcarrier modulated (MSM) systems. Data transmission is confined to a bandwidth located near DC, while out-of-band subcarrier signals are designed outside the data bandwidth to reduce the average optical power. Although the out-of-band signals at higher frequency are subject to severe corruption induced by multipath distortion, they are removed at the receiver and not used for detection. Optimizing the out-of-band carrier amplitudes over the set of real numbers yields gains as high as 2.6 dB over conventional MSM systems at the same bandwidth efficiency. We additionally apply in-band trellis coding and out-of-band signals, whose amplitudes are optimized over a discrete constellation. When no out-of-band signals are used, the system achieves an average optical power reduction of 0.9 dB over uncoded systems with a simultaneous peak power reduction of 0.4 dB. An additional average optical power reduction of 0.9 dB is realized at the expense of 4 out-of-band carriers and a moderate increase in peak power.

In this work we introduce a novel high-speed architecture for indoor wireless optical communications termed the dynamic spot diffusing (DSD) architecture. In this configuration the transmitter modulates data onto a spot on the ceiling which moves in a closed path. An imaging receiver acquires data whenever the transmitter spot is in its field-of-view (FOV). We show that this technique is more flexible, has greater multipath immunity, permits higher data rates and has a simpler transmitter than previously defined multi-spot and diffuse architectures. We compare the available channel capacities for a family of spot paths and receiver FOV. Instead of synchronizing to the spot location, rateless erasure correcting codes are applied to approach the capacity of the simulated DSD links. In a 6×6×3 m room, simulated data rates vary between 12.1 Mbps to 40 Mbps at different positions using a single 100 Mbps transmitter and the designed erasure correction code.

Indoor diffuse wireless optical intensity channels are bandwidth constrained channels in which all transmitted signals must be non-negative. In order to control the impact of inter-symbol interference (ISI) on this channel, pulse shaping is required. This paper derives the minimum bandwidth, ISI-free Nyquist pulse that satisfies the amplitude non-negativity constraint. The minimum bandwidth required is twice that of conventional electrical channels. Further, it is shown that there are no bandlimited root-Nyquist pulses satisfying the amplitude non-negativity constraint. In fact, all optical intensity root-Nyquist pulses are time-limited. Examples of optical intensity root-Nyquist pulses which have minimum fractional energy bandwidth are presented.

A conditionally Gaussian channel is a vector channel in which the channel output, given the channel input, has a Gaussian distribution with (well-behaved) input-dependent mean and covariance. We study the capacity-achieving probability measure for conditionally Gaussian channels subject to bounded-input constraints and average cost constraints. Many practical communication systems, including additive Gaussian noise channels, certain optical channels, fading channels, and interference channels fall within this framework. Subject to bounded-input constraint (and average cost constraints), we show that the channel capacity is achievable and we derive a necessary and sufficient condition for a probability measure to be capacity achieving. Under certain conditions, the capacity-achieving measure is proved to be discrete.

Short-range wireless optical channels provide high-data rate indoor links free of spectral licensing issues. In this work, we present a point-to-point, multiple-input/multiple output (MIMO) optical channel, termed the pixelated wireless optical channel, which exploits the inherent spatial diversity of the channel to achieve gains in spectral efficiency. Information is conveyed through the transmission of a series of pixelated images to a receiver array. An experimental prototype point-to-point link is constructed using a 512 /spl times/ 512 pixel LCD panel and 154 /spl times/ 154 pixels of a CCD camera. Based on channel measurements, a channel model amenable to computer simulation is developed. Spatial discrete multitone modulation is proposed for this MIMO wireless optical channel to combat the low pass spatial response of the channel. The capacity of a given channel realization is estimated by way of the water-pouring spectrum. Multi-level coding and multi-stage decoding over the spatial frequency bins is shown to yield spectral efficiencies of approximately 1.7 kbit/s/Hz over a range of 2 m.

We determine upper and lower bounds on the channel capacity of power- and bandwidth-constrained optical intensity channels corrupted by white Gaussian noise. These bounds are shown to converge asymptotically at high optical signal-to-noise ratios (SNRs). Unlike previous investigations on low-intensity Poisson photon counting channels, such as some fiber optic links, this channel model is realistic for indoor free space optical channels corrupted by intense ambient light. An upper bound on the capacity is found through a sphere-packing argument while a lower bound is computed through the maxentropic source distribution. The role of bandwidth is expressed by way of the effective dimension of the set of signals and, together with an average optical power constraint, is used to determine bounds on the spectral efficiency of time-disjoint optical intensity signaling schemes. The bounds show that, at high optical SNRs, pulse sets based on raised-quadrature amplitude modulation (QAM) and prolate spheroidal wave functions have larger achievable maximum spectral efficiencies than traditional rectangular pulse basis sets. This result can be considered as an extension of previous work on photon counting channels which closely model low optical intensity channels with rectangular pulse shapes.

Traditional approaches to constructing constellations for electrical channels cannot be applied directly to the optical intensity channel. This work presents a structured signal space model for optical intensity channels where the nonnegativity and average amplitude constraints are represented geometrically. Lattice codes satisfying channel constraints are defined and coding and shaping gain relative to a baseline are computed. An effective signal space dimension is defined to represent the precise impact of coding and shaping on bandwidth. Average optical power minimizing shaping regions are derived in some special cases. Example lattice codes are constructed and their performance on an idealized point-to-point wireless optical link is computed. Bandwidth-efficient schemes are shown to have promise for high data-rate applications, but require greater average optical power.