In channels with a wide available bandwidth, M-ary pulse amplitude modulation (M-PAM) with a small number of signal levels is a popular modulation method. The capacity of such optical channels has been investigated before as a function of the energy per bit and a limitation in average or peak optical power. However, in practice, the transmit energy consumption can be a complicated function of the signal waveform. In this work, we study a channel in which, firstly, the power associated with radiating a signal level is not a quadratic function of the signal amplitude, because the emitter may not act as a linear ohmic resistor. Secondly, level transitions consume additional power, for instance, because an intrinsic capacitance in the emitter diode junction needs to be charged up quickly. Light-emitting diodes (LED)s or lasers used in optical wireless communications are prime examples. We optimize their source entropy for a limited power budget. This not only leads to asymmetric probabilities for signal levels, as known from previous theory, but also introduces a dependency on previous signal probabilities. For example, our results show that in an energy-constrained system that employs on-off keying (OOK), the conditional probability of transmitting a logical 1 following a logical 0 should be lower than that of transmitting a second 1 after an initial 1.

To allow a good signal-to-noise ratio (SNR) over a wide bandwidth for signals arriving from an unknown angle, many optical wireless communication (OWC) systems need a wide-aperture high-bandwidth detector. However, the physical properties of a photodiode (PD) challenge the design for high bandwidth and a large detection area, simultaneously. This problem is also known as the bandwidth versus area trade-off of an amplified PD receiver. In this paper, we study the limitations of a single-PD OWC receiver on throughput, particularly on bandwidth, photon capturing area, and electrical noise, to formulate a model that evaluates and optimizes the throughput of a communication link for different signal modulation schemes. We optimize the signal power spectral density (PSD) of pulse-amplitude modulation (PAM) and of DC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) modulation for throughput, considering that large modulation bandwidths suffer disproportionally from noise enhancement in typical receiver amplifiers. This worsens if large PDs are used, but we show that to a large extent, the corresponding stronger signal can compensate for the increased noise. We study the optimum detector size, in combination with adaptations of the spectrum and, in particular, the maximum frequency of the modulation. While the limitations of photonic emitters have been extensively studied before, this is one of the first papers that models how the receiver limits the throughput of PAM and OFDM.

The bit rate, power consumption, security and user density achieved by Optical Wireless Communication (OWC) systems can be improved by ensuring that the modulated light is only sent in the direction of the target client device. This paper compares the potential benefits from sectorization at the transmitter versus at the receiver, which turn oput to be very different. A system can perferm well with many transmit sectors but only one or few receive sectors. A mechanism to activate right sector for the outgoing beam without relying on a detector with high angular resolution has not yet been published. We developed a non-mechanical beam control mechanism. It is designed for a bi-directional system. Each sector carries a unique identifier which is used to dynamically control the choice of the emitter sector(s). We optimized the design by minimizing the overhead and signal processing associated with it.

Light mainly propagates along a line-of-sight without the rich multipath scattering that can be exploited in radio frequency (RF) multiple-input multiple-output (MIMO) systems. To ensure outage-free communication MIMO can nonetheless be used, provided that multiple ceiling nodes are spatially distributed. Angular-selective client nodes can to some extent, but not perfectly, separate spatial streams coming from different ceiling points. In addition, many optical wireless communication (OWC) links have a low-pass frequency response. Because of these properties, indoor distributed (D)-MIMO OWC differs in many aspects from RF links. This paper explores how OWC D-MIMO systems can best be operated. The optimization of how to spread power over spatial streams and frequencies comes to different settings than typically seen for radio. We evaluate algorithms by an analytical foundation and by simulation based on a commonly accepted channel model. We derive a simple criterion to select the number of spatial streams which can be used. This also can serve as a selector for switching between spatial diversity and spatial multiplexing transmission modes. Moreover, we derive a simple formula for how to best spread power among spatial streams. Besides theoretically optimum solutions, we propose a very-near-optimum algorithm that is highly efficient for practical systems in rapidly varying channels.

Automatic sleep staging from single-channel electroencephalography (EEG) using artificial intelligence (AI) is emerging as an alternative to costly and time-consuming manual scoring using multi-channel polysomnography. However, current AI methods, mainly deep learning models such as convolutional neural network (CNN) and long short-term memory (LSTM), struggle to detect the N1 sleep stage, which is challenging due to its rarity and ambiguous nature compared to other stages. Here we propose SSC-SleepNet, an automatic sleep staging algorithm aimed at improving the learning of N1 sleep. SSC-SleepNet employs a pseudo-Siamese neural network architecture owing to its capability in one- or few-shot learning with contrastive loss. SSC-SleepNet consists of two branches of neural networks: a squeeze-and-excitation residual network branch and a CNN-LSTM branch. These two branches are used to generate latent features of the EEG epoch. The adaptive loss function of SSC-SleepNet uses a weighing factor to combine weighted cross-entropy loss and focal loss to specifically address the class imbalance issue inherent in sleep staging. The proposed new loss function dynamically assigns a higher penalty to misclassified N1 sleep stages, which can improve the model's learning capability for this minority class. Four datasets were used for sleep staging experiments. In the Sleep-EDF-SC, Sleep-EDF-X, Sleep Heart Health Study, and Haaglanden Medisch Centrum datasets, SSC-SleepNet achieved macro F1-scores of 84.5%, 89.6%, 89.5%, and 85.4% for all sleep stages, and N1 sleep stage F1-scores of 60.2%, 58.3%, 57.8%, and 55.2%, respectively. Our proposed deep learning model outperformed most existing models in automatic sleep staging using single-channel EEG signals. In particular, N1 detection performance has been markedly improved compared to the state-of-the-art models.

In an optical wireless communication (OWC) system, the choice of the photo-diode (PD) receiver faces a trade-off between detecting area and bandwidth. If the PD size is increased to receive a strong signal, the capacitance also increases, hence the receiver suffers not only from bandwidth limitations but also from noise amplification. This paper studies adaptive bit and power loading to cope with gain and noise issues. The paper also improves the design of a TransImpedance Amplifier to improve throughput. We propose to use one cascode switch per receiver segment, and we also propose a second cascode amplifier stage. Through circuit simulations, we show that this extends the receiver bandwidth and reduces the noise. To analyze the communication performance of such a system, we analytically model the electro-opto-electro conversion of the transmit signal power. The throughput achieved by various receiver alternatives is estimated for DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM). Our new segmentation approach significantly improves throughput compared to known receiver designs.

1In Optical Wireless Communication (OWC), the use of narrow beams can enhance the achievable bit rates and reduces interference. However, this requires a mechanism to find the direction in which the transmitter must send its beam towards the receiver. It turns out that it may be counterproductive to widen the beam during the direction search phase. This paper analyzes a system with a feedback loop that allows the receiver to report back, once it sees an incoming signal. However, this feedback channel may not have a high time resolution and may have a relatively long and random delay. This paper studies the appropriate beam width adaptation during direction search and acquisition, taking into account the received (electrical) energy per bit, and noise limitations of the transimpedance amplifier. The paper also introduces a new address coding scheme. It uses continuous addressing, thereby eliminating overhead from packet headers, error correction coding or synchronisation words, by adopting maximum-length Linear Feedback Shift Register sequences. This accelerates the beam direction search.

Optical wireless communications systems are preferably designed to receive a strong signal over a wide field of view and with a large bandwidth. These are conflicting requirements. Recently it has been proposed and experimentally tested to split the surface area of the photodiode (PD) into Nsby Npsubareas, and to place Nprows in parallel with Nsof these in series. This paper generates a deeper insight in this solution by analyzing the performance, in particular in combination with the low-noise transimpedance amplifier (TIA). In fact, by creating a virtual-ground (very low impedance) input, a TIA flattens the frequency response thereby mitigates parasitic PD capacitances. With decreased capacitance due to serial connection of PDs, noise is also less boosted at high frequency. Our analysis is compared with SPICE simulations.

This paper addresses the question of whether an optical wireless communication (OWC) LiFi system preferably uses LEDs near their most efficient operating point, which is preferred for environmentally-friendly highly efficient illumination, or that one should preferably drive the LED into its droop regime to speed up its response. In fact, additional non-radiative recombination speeds up the LED response to modulation but spoils efficiency. We introduce the differential (Internal and External) Quantum Efficiency (dIQE; dEQE), as a particular derivative of quantum efficiencies. It quantifies the efficiency at which a current modulation translates into an optical signal. We optimize the current density, thus how deeply the LED is to be driven into droop, using DC-offset (DCO) OFDM throughput expressions for the first–order low–pass LED response. Short– range small–coverage systems prefer high current densities to achieve very high throughputs. LiFi with wide opening angles, e.g. covering tens of square meters are better off with a high dIQE, thus lower current densities.

In this paper we report, for the first time, measured performance results of the concatenation between two different plastic optical fibre (POF) fronthaul approaches and distributed multiple-input multiple-output (D-MIMO) LiFi using commercial G.hn chipsets. These integrated circuits (IC)s are capable of performing, among other features, MIMO signal processing, orthogonal frequency-division multiplexing (OFDM) modulation, and adaptive bit-loading, which are important features for LiFi systems. The use of commercially available ICs with the aforementioned features, accelerates the mass market adoption of LiFi technology. We consider the POF fronthaul approaches: spatial division multiplexing (SDM) and wavelength division multiplexing (WDM). POFs are interesting for industrial scenarios due to its electromagnetic interference immunity, their low weight and cost. The measured throughput results show that a WDM POF fronthaul, using a red light-emitting diode (LED) and a green laser diode (LD), has ample throughput to support the DMIMO LiFi link, which proves the viability of the use of POFs as fronthaul for D-MIMO LiFi systems. Our results give input for possible enhancements to the ITU G.vlc standard G.9991.

This paper studies the response of a Light Emitting Diode (LED) to a stepwise change of the power delivered by a current or voltage supply. This study contributes to the design of efficient electronic circuits to generate On-Off Keying modulation for optical wireless communication using LEDs as an emitter. We combine the Shockley diode equation with the ABC model for electron-hole recombination to propose a new model for rise and fall curves. The fall curve differs for an open or shunted LED, as in the latter case, the carriers (electrons and holes) are pulled out of the LED junction. We quantify these effects and exploit this difference to propose a tristate optimized modulator circuit, which is highly power efficient. In this design, we allow modulating currents to be switched a bit earlier or later than then the bit transition itself.

Optical Wireless Communication (OWC) is gaining popularity as it potentially offers interference–free communication in dense environments. By using LEDs, cells of very small size can be created with sharp boundaries thus bandwidth can be reused very densely. Building an effective communication systems starts from understanding the channel. In scientific literature, various models for the LED response are in use. This overview paper starts from the non-linear recombination of electrons and holes in the LED semiconductor junction, which then appears to lead to a very tractable and simple signal processing model. In fact, the LED channel substantially differs from a radio channel. The paper summarizes recent progress in developing an LED channel model, based on the physics of photon generation in the active region. The LED Quantum Well (QW) creates a first–order low–pass effect and the hole-electron re-combinations are inherently non-linear. Thereby the LED exhibits dynamic distortion, that is, non-linearities that are are intertwined with memory effects. The model can be used to reduce power consumption and to increase throughput of LED OWC systems.

Intensity Modulated (IM) optical communication over an LED channel requires the use of a non-negative signal that can also cope with the low-pass nature of LEDs. For this purpose, dedicated schemes such as Flip-OFDM and Assymetrically Clipped Optical (ACO)-OFDM have been proposed. We derive a common mathematical description on which both schemes rely. Exploiting this insight, we propose Continuous Phase Flip-OFDM (CP-Flip-OFDM) as an enhancement to Flip-OFDM. It ensures phase continuity at the transition between the two successive copies of the OFDM blocks, thereby it obviates the Cyclic Midfix between the first OFDM block and its flipped copy. Simultaneously, we derive a less compute-intensive way to generate and detect ACO-OFDM. Instead of creating an Hermitian-symmetry at the transmit Inverse FFT input, which is common in optical IM OFDM, we use zero padding. After the transmit IFFT, a phase ramp-up, i.e., a multiplication with a complex-valued exponential, is applied before truncating to a real and non-negative signal.

In modern society, humans spend over 90% or their time indoors. However, despite the growing scientific understanding of the impact of light on biological mechanisms, benefits of this understanding are not harvested in practical systems. Existing light in the built environment, offices in particular, is designed predominantly to meet visual performance requirements. Increasing attention is being given to the biological effects of light, especially how it could be used to promote occupants' health and well-being through the circadian functions that regulate sleep, mood, and alertness. While Human Centric Lighting is being offered based on generic insights on population average experiences, in this paper, we suggest a personalized bio-adaptive office lighting system, controlled to emit a lighting recipe tailored to the individual employee. We introduce a novel optimization algorithm that finds the best office lighting profile to achieve circadian alignment to the 24-hour cycle. The system aims to support employee's circadian rhythm and ensure that they receive the right light at the right time of the day. In particular, we use existing, commonly accepted chronobiologic models to find the most effective light exposure pattern.

LEDs, particularly those used for Visible Light Communications (VLC), have a limited bandwidth, while above their 3 dB bandwidth, the roll-off is relatively gentle. If the modulation bandwidth would be limited to the 3 dB LED bandwidth, the achievable rate would be unacceptably constrained. Hence, effective communication systems need to optimize the use of bandwidth significantly above this 3 dB point. Orthogonal Frequency Division Multiplexing (OFDM) is a popular method to fine-tune the amount of power and constellation as a function of the channel response over different frequencies. Various power and bit loading strategies have been proposed and simulated in literature, but their performance was not captured in expressions. This manuscript derives these for optimal waterfilling, uniform and pre-emphasized power loading for the LED channel, that severely attenuates high frequencies. We also investigate the influence of practical discrete constellations and verify our new results experimentally. Interestingly, simple uniform loading only falls less than 1~2% short of the throughput achieved by waterfilling, but when we restrict OFDM to discrete QAM constellation sizes, the penalty for uniform loading is 1.5 dB. Inspired by the good performance of uniform power loading, we propose an algorithm to find the best discrete bit loading for uniform power within an optimized band. As pre-emphasis is nonetheless attractive because a flattened channel does not need adaptive sub-carrier loading, we quantify its penalty. This can be modest provided that the system can adapt its transmit bandwidth, thereby adaptively switching upper sub-carriers to zero power.

This work investigates handover in hybrid light fidelity (LiFi) and wireless fidelity (WiFi) networks (HLWNets). In such a network, the handover process becomes challenging due to two main factors: i) the relatively short coverage range of a single access point (AP), and ii) the largely overlapping coverage areas of different networks. As a result, HLWNets are susceptible to frequent handovers. To reduce the handover rate, the concept of handover skipping (HS) was introduced, which enables handovers between non-adjacent APs. However, conventional HS methods rely on knowledge about the user's trajectory, which is not readily available at the AP. In this paper, a novel HS scheme is proposed on the basis of reference signal received power (RSRP) and its rate of change, with an adaptive network preference adopted. Since RSRP is commonly used in the existing handover schemes, the proposed method requires no additional signalling between the user and the AP. Simulation results show that the new method can effectively reduce unnecessary handovers, especially those between LiFi and WiFi. Compared to the standard and trajectory-based handover schemes, the proposed method improves network throughput by up to about 120% and 30%, respectively.

This work addresses the problem of presence detection as Distributed Sensing of a Hidden Markov Model (DS-HMM). We propose an efficient communication strategy to limit sensor communication. For optimum detection under constrained communication, it does not suffice to just send only the latest sensor observations. It requires tracing back in the HMM algorithm to the latest moment that all data from all sensors was available. Alternatively, we investigate a more pragmatic approach, that is, to send a few recent observations in every message (packet) instead of the full history. The effect of unreliable communication, subject to channel erasure, is also investigated.

A ZigBee (802.15.4) lighting network is potentially vulnerable to interference by other wireless technologies that operate in the same Industrial, Scientific and Medical (ISM) frequency band, such as Wi-Fi (802.11). Therefore the study of coexistence between IEEE 802.15.4 and IEEE 802.11/Wi-Fi is becoming more relevant than ever. In this area, literature approaches to the co-existence issue currently fall into two main categories: experimental or simulation results and analytical models which are either asymptotic (e.g., using Bianchi's assumption) and/or stationary. In contrast, in this paper we present an analytical framework in which a comprehensive study of the dynamic influence of Wi-Fi interference on ZigBee networks of arbitrary size becomes possible. We first present an application of mean field approximation to an intractable Markov model of ZigBee networks. We use this modeling approach to investigate the effect of the Wi-Fi traffic pattern on the ZigBee network performance. By taking this dynamic interaction into account, we notice major performance differences depending on the type of traffic. It also reveals that even light traffic patterns, that offer long gaps of inactivity, can hamper the ZigBee performance significantly more than previously reported Wi-Fi coexistence studies suggest.

The visible light communication (VLC) can provide data transmission via the illumination light emitting diodes (LED). This paper introduces a new model to analyze the bit error rate (BER) of binary phase modulation in VLC for an arbitrary modulation depth and data duty cycle while taking into account both the Gaussian and signal-dependent shot noise. The impact of the driver design on the BER and its impact on ripple have not been considered in detail before. We compare two different LED driver schemes, namely directly adapting the driver control loop and binary shunting. We address data rate, BER, and power efficiency, for which we propose to use the extra energy per symbol above unmodulated light. We further introduce an analysis of the effect that (truncated) ripple interference has in the (matched or other) filter of the receiver. Ripple causes a harmful interference in VLC, and thus a BER expression is derived to include its effect. Two approximations are proposed to model the ripple interference, and their accuracies are compared by simulations. A low-pass filtering is proposed to alleviate the impact of ripple interference in VLC system.

This paper addresses the nonlinear memory effects in the response of typical illumination light emitting diodes (LEDs), in order to enhance the performance of visible light communication (VLC) systems. These LEDs have a limited bandwidth of only several MHz. To reflect the physical mechanisms in the quantum well, we describe the LED transient response by a nonlinear dynamic differential equation. Three different mechanisms of the nonlinearity are relevant in the double hetero-structure LEDs, which result in dynamic nonlinearities, that is, a mixture of nonlinearities and memory effects. Hitherto, generic pre-distorter and non-linear equalizers have been studied for the LEDs. Yet this paper shows that recombination rates of photon generation can be translated into an equivalent discrete-time circuit that can be inverted. This allows us to develop a new pre-distorter with a simpler and more efficient structure than previously studied and overly generic approaches. The novel pre-distorter along with a parameter estimation can effectively overcome LED nonlinearity for high-speed VLC with amplitude-based single carrier modulations, including ON-OFF keying and pulse amplitude modulation-4 systems, and with the multi-carrier orthogonal frequency-division multiplexing. We report experimentally obtained eye-diagrams, first to justify our choice for the LED model on which our nonlinear pre-distorter have been based, and second to verify the effectiveness in enhancing the VLC link performance to the extent predicted by our model.

Wireless sensors with radio frequency (RF) energy scavenging capabilities can operate batteryless and thus are promising for a wide range of applications. For sensor nodes with a single antenna, radiation from certain incoming directions cannot be received due to zeros in the antenna radiation pattern. This can result in sensor nodes that will not power up and thus are not functional. To overcome these spatial zeros and to improve the RF-DC rectification efficiency, multi-antenna rectification schemes are considered. Multi-antenna schemes known from communications, e.g. beam steering architectures, are not suitable for ultra-low power sensor nodes, as they require an on-chip power supply as well as a large area. With these practical constraints on power and area, we address an alternative approach to multi-antenna rectification schemes dedicated to RF energy scavenging. Using circuit simulations, we show that the proposed scheme achieves a good balance between directional uniformity and RF-DC rectification efficiency.

In this paper the architecture and performance of a co-integrated 60 GHz on-chip wireless energy harvester and ultra-low power (ULP) radio in 65-nm CMOS are discussed. Integration of an on-chip antenna with wireless power receiver and wireless data transfer module is the crucial next step to achieve compact and high efficiency fully-integrated monolithic wireless sensor nodes. A single-pole-single-throw 60 GHz RF switch is proposed and simulated to decouple the power harvesting and data transfer module. The designed on-chip RF switch has -2 dB insertion loss in EM simulations, and achieves -18 dB isolation between the energy harvesting module and the data transfer module. A single on-chip monopole antenna for power reception and data transfer is proposed with an adapted layout to reduce power coupling to undesired substrate modes. The simulated antenna shows a gain of -1.68 dBi. The power harvesting performance of the co-integrated antenna, switch and Dickson type multistage rectifier is simulated and leads to a DC output voltage of 1.2 V for -5 dBm input power at 60 GHz. With 15 dBm power transmitted to the tag at 30 GHz, the output voltage is 1.14V in the measurement. This paper is the first to demonstrate the 30 GHz mm-wave wireless energy harvesting with fully on-chip wireless energy receiver.

Monolithic wireless sensors with integrated antenna, on-chip transceiving, sensing and energy scavenging are low-cost and robust, thus very suitable for mass production and deployment. The design of such a sensor node requires a proper architecture with careful trade-offs and joint considerations over different building blocks. In this paper, we focus on the energy scavenging and receiver part of such a sensor node. A radio-triggered receiver architecture is proposed to achieve the extreme low energy budget. Energy/power models for different building blocks are developed that show the tradeoffs between available energy and sensor performance. A system-level analysis identifies the 60GHz mm-wave band is suitable for such applications. Moreover, a design example of receiver front-end in 65nm CMOS technology is presented to demonstrate the potential performance of the proposed architecture.

RF energy scavenging enables batteryless operation of wireless sensors. In particular, a system with a central controller that transfers wireless energy to and exchanges information with RF energy scavenging sensors is very suitable for a wide range of applications. State-of-the-art analysis of RF energy scavenging is mostly based on RF-DC rectifier models operating with relatively high input power to achieve high rectification efficiency. However, to enable larger distance between the central controller and sensors and/or to increase the operating frequencies, which can lead to small and low-cost smart dust like sensors, a good model describing the RF-DC rectification with low input power is needed to aid system design and optimization. In this paper, we develop such a model. Using the model, we derive closed-form solutions for the equilibrium voltage and the input resistance of the rectifier. We further propose a quasi-static model to describe the dynamic charging of the capacitor in the rectifier. A comparison with circuit simulations using Cadence Virtuoso Spectre circuit simulator shows good match between our model and the circuit simulation.

Radar sensors are widely used in intelligent transportation systems. The performance of radar sensors in practical setups depends on many factors, such as sensor specifications, choice of locations, height and orientations of sensor installation. Currently, most of these choices are made by a "rule of thumb". In many cases, trial and error changes of settings are required and several rounds of on-site measurements and adjustments are incurred, which are both inefficient and costly. To overcome these problems, this paper reports a 3D virtual radar system that can accurately predict and evaluate the performance of actual radar sensors using a standard computer. The proposed system builds upon a 3D virtual model of real world. It allows users to freely enter specifications for radar sensors and the underlying algorithm can efficient calculate and represent graphically in the 3D model by the coverage area of the chose radar sensors at the specified location in the virtual world. Comparison with experimental data obtained using hardware testbeds shows the result obtained by the virtual radar system is accurate.

Biometrics gain increasing interest as a solution for many security issues, but privacy risks exist in case we do not protect the stored templates well. This paper presents a new verification scheme, which protects the secrets of the enrolled users. We will show that zero leakage is achieved if certain criteria are met and we benchmark the performance of this scheme. We quantify performance loss in terms of detection and false acceptance rate and capacity of the biometric channel, which are slightly worse than those of the current leaky methods.

Maximizing the battery life time of mobile devices and sensor nodes increasingly becomes a challenge, and receiver power consumption tends to become more problematic than delivering adequate transmit power. We address the challenge of achieving the highest possible throughput per Watt of receiver circuit power. Our results show that optimum and adaptive tuning of the front-end parameters of the receiver can result in substantial power savings, compared to the common practice of a design for worst case conditions. We obtain a closed form solution for maximum throughput and the corresponding optimal overall system specifications. We confirm that handling the interference from nearby channels has a large influence, and our analysis concludes that adaptive control of the IP3 performance has an overarching impact. We further describe how the adaptive overall system settings can be translated into optimum gain and IP3 specifications of each of the individual stages that form the receiver cascade, considering both the accumulation of circuit noise and distortion products. The example of a WLAN system is elaborated to illustrate our method.

In many wireless systems, the energy consumed by the receiver is significantly larger than the energy consumed by transmitter, possibly even by orders of magnitudes. This paper derives an analytical solution to maximize the throughput per unit of available receiver circuit power. Adaptive control of the IP3, to handle the time-varying adjacent channel interference level, can substantially improve the power efficiency, compared to the common practice of design for worst case. There appears to be an optimum throughput at a specific (non zero) power consumption level. For large SNR, the optimal throughput per Joule of receiver circuit power occurs for a modulation rate of 2.3 bits/s/Hz, irrespective of interference power level. If the receiver has less power available than the optimum setting requires, a duty cycling scheme can still achieve optimum operation.

IEEE 802.15.4 Wireless Sensor Networks (WSNs) share the 2.4 GHz Industrial, Scientific, and Medical (ISM) license-free band with many other wireless technologies such as IEEE 802.11b/g Wireless Local Area Networks (WLANs). Because of the low-power, however, IEEE 802.15.4 WSNs are potentially vulnerable to the interference introduced by the other wireless technologies such as IEEE 802.11b/g WLANs, which have much higher power. Particularly, in the presence of heavy interference, IEEE 802.15.4 nodes can hardly get a chance to access the channel, which could result in discarding a large amount of packets. In this paper, we propose a decentralized approach to help IEEE 802.15.4 nodes mitigate interference. By adaptively and distributively adjusting Clear Channel Assessment (CCA) thresholds of IEEE 802.15.4 nodes in the presence of heavy interference, the approach can substantially reduce the amount of discarded packets due to channel access failures, and therefore significantly enhance the performance of IEEE 802.15.4 WSNs. The approach is robust, responsive and easy to be implemented at a low cost. The effectiveness of the approach is validated by OPNET simulation.

We investigate packet-by-packet rate adaptation so as to maximize the throughput. We consider a finite-state Markov channel (FSMC) with collisions, which models channel fading as well as collisions due to multi-user interference. To limit the amount of feedback data, we only use past packet acknowledgements (ACKs) and past rates as channel state information. The maximum achievable throughput is computationally prohibitive to determine, thus we employ a two-pronged approach. Firstly, we derive new upper bounds on the maximum achievable throughput, which are tighter than previously known ones. Secondly, we propose the particle-filter-based rate adaptation (PRA), which employs a particle filter to estimate the a posteriori channel distribution. The PRA can easily be implemented even when the number of available rates is large. Numerical studies show that the PRA performs within one dB of SNR to the proposed upper bounds for a slowly time-varying channel, even in the presence of multi-user interference.

We consider packet-by-packet rate adaptation to maximize the throughput over a finite-state Markov channel. To limit the amount of feedback data, we use past packet acknowledgements (ACKs) and past rates as channel state information. It is known that the maximum achievable throughput is computationally prohibitive to determine. Thus, in this paper we derive two upper bounds on the maximum achievable throughput, which are tighter than previously known ones. We compare the upper bounds with a known myopic rate-adaptation policy. Numerical studies over a wide range of SNR suggest that the myopic rate-adaptation policy is close to the upper bounds and may be adequate in slowly time-varying channels.

In recent years, LED technology emerged as a prime candidate for the future illumination light source, due to high energy efficiency and long life time. In addition, LEDs offer a superior flexibility in terms of colors and shapes, which leads to a potentially infinite variety of available light patterns. In order to create these patterns via easy user interaction, we need to sense the local light contribution of each LED. This measurement could be enabled through tagging of the light of each LED with unique embedded IDs. To this end, we propose a new modulation and multiple access scheme, named as code- time division multiple access - pulse position modulation (CTDMA- PPM): a form of PPM which is keyed according to a spreading sequence, and in which the duty cycle is subject to pulse width modulation (PWM) according to the required lighting setting. Our scheme considers illumination constraints in addition to the communication requirements and, to our best knowledge, it has not been addressed by other optical modulation methods. Based on the proposed modulation method and multiple access schemes, we develop a system structure, which includes illumination sources, a sensor receiver and a control system. Illumination sources illuminate the environment and transmit information, simultaneously. According to our theoretical analysis, this system structure could support a number of luminaries equal to the size of the CDMA codebook times the dimming range.

We consider a wireless ad-hoc network with random ALOHA transmissions between nodes. However nodes can decide to use the Request-to-Send (RTS) / Clear-to-Send (CTS) protocol for occasional long packets. Via the RTS/CTS protocol, nodes proactively make a temporal and spatial reservation of the channel before sending the actual data payload (DATA). The aim of this paper is to explore the effectiveness of the RTS/CTS protocol in such a wireless network, in particular by alleviating the commonly made assumption that an entire RTS/CTS/DATA cycle is always successful if only the RTS packet is recovered. Our results show that this assumption loses its accuracy if the data rate is optimized for the link throughput or if one aims for allowing dense frequency reuse. We quantify the spatial and temporal impact of the RTS/CTS reservation on the network traffic, as well as the link throughput achieved. Numerical results demonstrate that for sufficiently long packets, a significant throughput gain can be achieved by employing the RTS/CTS reservation.

OFDM signals suffer from a large Peak to Average Power Ratio, which requires large power back-offs in the transmit and receive chains. This paper studies two digital postprocessing methods of different complexity that mitigate clipping by the analog-to-digital converter in the receiver. Clipped peaks cause spurious signals on empty subcarriers, which can be used to eliminate clipping artifacts and to recover the original signal. Simulations show that a significant reduction of 3 dB in the headroom of the A/D converter (ADC) is possible, when an elaborate MMSE clip correction algorithm is used. A simple algorithm still allows for 1 dB reduction of the headroom. As the ADC is consuming an ever increasing fraction of the total receiver power, the results are believed to be relevant for low-power design of OFDM receivers, for instance to prolong battery life during digital television reception on mobile phones.

We consider the problem of detecting a primary user signal over a wireless channel by a cognitive radio with multiple antennas. We present a multiple antenna processing based energy detector. In this detection method, the multiple copies of the primary user signal at the antennas of the cognitive radio receiver are combined and input to an energy detector. We consider two multiple antenna processing methods and analyze their detection performance - the achieved probability of detection for a pre-specified probability of false alarm and given signal-to-noise ratio. We derive closed-form expressions for the probabilities for detection and false alarm for each of the processing schemes. Finally, we show through numerical results the potential improvement in primary user detection with multiple antenna processing based energy detection at the cognitive radio receiver.

The Request-to-Send (RTS) / Clear-to-Send (CTS) scheme uses a four-way handshake to ensure proper spatial and temporal reservation of the multiple-access wireless LAN channel. By considering the (interdependence of) successive capture probabilities for a cycle of an RTS, CTS, data packet and acknowledgement, we analyze the throughput of the RTS/CTS scheme in a new mathematical framework. We also propose closed-form bounds which are useful for rate optimization.

In mobile reception, the reliability of orthogonal frequency division multiplexing (OFDM) is limited because of the time-varying nature of the channel. This causes intercarrier interference (ICI) and increases inaccuracies in channel tracking. We model the ICI using derivatives of the channel amplitude. This allows us to design a relatively simple receiver scheme that iteratively cancels the ICI. The design of the canceler aims at maximizing the signal-to-noise-plus-ICI ratio at the detector input. We also propose a new channel estimator, and we show that it achieves reliable mobile reception in practical situations that are relevant to terrestrial Digital Video Broadcasting (DVB-T). Extensive simulations for a receiver with one or two antennas show that a small number of iterations between ICI cancellation and channel estimation allow a reliable reception at vehicle speeds above 100 km/h.

In orthogonal frequency-division multiplexing, time variations of a multipath channel lead to a loss of orthogonality between the subcarriers, and thereby limit the achievable throughput. This paper proposes a general framework for a controlled removal of intercarrier interference (ICI) and channel acquisition. The core idea behind our method is to use a finite power series expansion for the time-varying frequency response, along with the known statistical properties of mobile radio channels. Channel acquisition and ICI removal are accomplished in the frequency domain and allow for any desired tradeoff between the residual ICI level, the required training for channel acquisition, and processing complexity. The proposed approach enables a high spectral efficiency (64-quadrature amplitude modulation mode) of digital video broadcasting-terrestrial in highly mobile environments.

In orthogonal frequency division multiplexing (OFDM), time variations of a fading multipath environment lead to a loss of orthogonality between the subcarriers and thereby limit the achievable throughput. This paper suggests a general framework for a controlled removal of intercarrier interference (ICI) and channel acquisition. The core idea behind our method is to use a finite Taylor expansion for the time-varying frequency response along with the known statistical properties of mobile radio channels. Channel acquisition and ICI removal are accomplished in the frequency domain and allow for any desired tradeoff between the residual ICI level, the required training for channel acquisition and processing complexity.

The reception of the DVB-T signal on a mobile equipment is strongly affected by Doppler spread. Most of the current proposals require the use of two or more antennas at the receiver and a computationally expensive signal processing. In this paper for a DVB-T receiver with a single antenna we propose new schemes based on the interference cancellation principle. With respect to previously studied techniques, our solution is able to deliver DVB-T services at higher speeds, while a more flexible architecture allows a wider range of trade-off between complexity and performance. In order to improve the performance in terms of maximum achievable speed, an iterative scheme is proposed which performs both the interference cancellation and the channel estimation. A decoding and re-encoding of the inner convolutional code is inserted into the iteration process in order to boost the performance. Moreover, by applying the interference cancellation principle also for the estimation of the channel parameters, we obtain a scheme with a reasonable complexity for the DVB system. Simulation results for various DVB-T modes, constellations and code rates show that the proposed schemes allow the correct reception of DVB-T for moderate and high speeds.

The paper proposes and analyses a new receiver structure to mitigate the effect of Doppler on the reception of OFDM signals. A discrete-frequency channel representation is developed for the link between the input of the transmit I-FFT and the output of the receive FFT. It is based on a Taylor expansion of the time variations of the received subcarrier amplitudes. The model realistically addresses the correlation of fading at neighboring subcarriers. We study a new type of receiver which estimates not only amplitudes but also derivatives of subcarriers amplitudes. An adaptive MMSE filter is proposed to cancel the intercarrier interference (ICI) resulting from Doppler. This results in a substantial improvement of the link performance.

Electronic watermarking is an active area of research with many applications being foreseen. Watermarks may become an essential tool for copy management in future consumer electronic or PC devices. With simple circuits, detection of watermarks after noise addition, MPEG compression, D/A conversion, pixel shifts appears feasible, but detection after transformations, such as cropping and stretching, remains a challenge. We propose a model to evaluate the effect of scaling on the detector reliability and verify it with experiments.

The prospect of consumer digital versatile disk (DVD) recorders highlights the challenge of protecting copyrighted video content from piracy. We describe the copy-protection system currently under consideration for DVD. The copy-protection system broadly tries to prevent illicit copies from being made from either the analog or digital I/O channels of DVD recorders. An analog copy-protection system is utilized to protect the NTSC/PAL output channel by preventing copies to VHS. The digital transmission of content is protected by a robust encryption protocol between two communicating devices. Watermarking is used to encode copy-control information retrievable from both digital and analog signals. Hence, such embedded signals avoid the need for metadata to be carried in either the digital or analog domains. Finally, the copy-protection system provides the capability for one-generation copying. We discuss some proposed solutions and some of the implementation issues that are being addressed.

A digital watermark is a perceptually unobtrusive signal embedded in some multimedia asset carrying additional information: e.g. copyright information of a video clip. For nearly all watermarking schemes published so far, detection is based on some form of correlation. We show that the detection reliability can be significantly improved by applying filtering prior to correlation. This improvement is analysed using a theoretical model based on statistical communication and detection theory. Finally, the improvements predicted by the theory are verified in a number of experiments.

A watermark is a perceptually unobtrusive signal embedded in an image, an audio or video clip, or any other other multimedia asset. Its purpose is to be a label which is holographically attached to the content. Moreover, it can only be removed by malicious and deliberate attacks (without a great loss of content quality) if some secret parameter K is known. In contrast, a watermark should be readily detectable by electronic means. This implies that electronic watermark detection is only feasible if the watermark detector is aware of the secret K. In many watermarking business scenarios the watermark detector will be available to the public as a black box D. The following question is therefore justified: can the secret K be deduced from the operation of the black box D? And if yes, what is the complexity of this process? We address these questions for a large class of watermarking schemes.

Watermarks allow embedded signals to be extracted from audio and video content for a variety of purposes. One application is for copyright control, where it is envisaged that digital video recorders will not permit the recording of content that is watermarked as "never copy". In such a scenario, it is important that the watermark survive both normal signal transformations and attempts to remove the watermark so that an illegal copy can be made. We discuss to what extent a watermark can be resistant to tampering and describe a variety of possible attacks.

In a wireless network with path loss and fading channels, receiver capture is known to substantially enhance the performance of the slotted ALOHA random access protocol. The efficiency of narrowband slotted ALOHA radio networks can be enhanced further by using sector antennas, each receiving signals from a particular segment of the network area. This paper investigates the effect of realistic, i.e., partially overlapping antenna patterns and the resulting correlation of received power levels at different receiver branches. A method is derived for computing the joint throughput from two base station receivers with overlapping antennas patterns. The a posteriori information provided by the event of one message capturing one antenna is used to find conditional probabilities of capturing the other antennas as well. The paper shows that any overlap in the antenna patterns decreases the throughput, but transmissions from the overlapping area may face a larger probability of capture than signals from directions in which one antenna has maximum gain.

The paper introduces a new protocol for wireless cellular networks by combining a time reservation scheme with a dynamic space reservation scheme. The proposed new protocol, known as the space and time reservation multiple access (STRMA) protocol, does not use a fixed reuse pattern but allows a dynamic reservation of bandwidth in both time and space. The performance of this protocol found by simulation is compared with the performance of a cellular network that uses PRMA with a fixed frequency reuse pattern. It is found that for homogenous traffic STRMA offers improved performance over a wide range of traffic loads. For inhomogeneous traffic the improvement is even more pronounced.

We address the transmission of data packets from base stations to mobile terminals in a one-dimensional cellular network. The probability that a data packet successfully arrives at the destined mobile terminal highly depends on the activity of nearby co-channel base stations. Motivated by favorable results from earlier studies we consider a network in which all base stations use the same radio channel. Packets lost because of mutual interference are retransmitted with a random backoff mechanism. Recursive arguments are applied for the calculation of the mean time necessary for a base station to deliver the packet, provided that in an initial state all stations have packets to transmit. The average delivery time, maximized over all base station locations, is also analyzed. The relations obtained give some further insight into the behavior of a (highway) cellular data network.

Multi-carrier (MC) multi-user CDMA is a new transmission method in which every user bit is sent simultaneously over multiple subcarriers (or multiple tones), each of which has a time-constant 0 or /spl pi/ phase offset. The offsets corresponding to each user form a unique code signature sequence, orthogonal to those of any other user, or pseudo orthogonal as with Gold codes or Kessami code sequences. We address a moderately dispersive channel in which each user-subcarrier channel has a different random amplitude and phase. We estimate the performance of a receiver matched to such a multicarrier, multi-user scheme. Then we implement a decorrelating interference canceler at the output and examine its effect on performance.

The paper developed a refined model for DS spread spectrum transmission over a Rician fading channel. A prevalent model in previous papers has been to group all signals received other than the dominant LOS path as interference. A new approach is taken, whereby some components of the multipath are included in the desired signal. An expression for the local-mean BER is derived from this model, and an analysis of BER as a function of the number of users is performed.

This paper reviews progress in developing a mathematical framework to evaluate the performance of mobile radio links. It takes into account multi-ray multipath fading with Rician or Nakagami probability density, lognormal shadowing and path loss of wanted and interfering transmitters, additive white Gaussian noise, man-made burst noise and discontinuous transmission. The method uses Laplace transforms of the PDF of the power of each interfering signal. This method is computationally efficient and allows inclusion of a wide range of link aspects. We also show that, in contrast to common belief, it is not realistic to approximate the Rician fading of the wanted signal by a Nakagami model. New results are given for microcells along highways.

The spectrum efficiency of cellular radio data networks is maximized by increasing the cell throughput and decreasing the cluster sizes. However, smaller cluster sizes lead to higher cochannel interference and hence lower throughput. A multiple access and collision resolution scheme that accounts for the spatial aspects of cochannel interference is introduced and analyzed for roadside base station to vehicle packet communications in intelligent vehicle highway systems (IVHS). In a wireless network with base stations having full knowledge of neighboring station activity, cochannel interference and destructive collisions are diminished by using discontinuous transmissions and coordination: a base station is silenced (or power-controlled) by a neighboring station in order to improve the latter's chance of successful transmission and decrease retransmission attempts. Interaction among base stations makes efficient use of channel resources for both low- and high-intensity traffic.

This paper proposes a radio architecture that offers two-way transmission services essential to intelligent vehicle highway systems (IVHS) using only a single (30 kHz) radio channel. Within this bandwidth multiple communication services can be supported, including datacasting, packet switched transmission to and from vehicles, collection of traffic data from probe vehicles and transmission of emergency messages. The design is based on theoretical investigations reported at PIMRC '93 and VTC '94 on packet-switched downlink transmissions and ALOHA random access for traffic reports from probe vehicles, respectively. These analyses revealed that contiguous frequency reuse gives optimum packet delay and highest spectrum efficiency. New results are included for datacasting messages to groups of vehicles.

This paper evaluates the performance of a packet radio network employing code division multiple access and a stack algorithm to resolve message collisions. We investigate the effects of bandspreading assuming a fixed total system bandwidth. Although bandspreading with perfectly orthogonal signals enhances system capacity, it does not resolve the instability of ALOHA random access unless special measures are taken to control retransmission traffic. Using a stack algorithm, at small traffic loads the packet delay is minimized if no spreading is applied. At large traffic loads, perfect CDMA enhances performance, but for imperfect signal separation at the receiver, advantages of CDMA are lost. The capacity of the stack algorithm addressed here can be enhanced from 0.32 to at least 0.40 if a large spreading factor with perfect signal separation is employed.

Analyzes the application of Wiener filtering to the detection of multi-carrier code division multiple access (MC-CDMA) signals. Simulation results are included to test the validity and accuracy of the analytical results.

Optimum frequency reuse patterns for roadside base station to vehicle communications in intelligent vehicle/highway systems (IVHS) are determined based on optimal delay/throughput and packet erasure rates for BPSK transmission over narrowband dispersive Rician channel with ISI and (Rayleigh-faded) co-channel interference. For fixed cluster size and fixed total system bandwidth, a reuse pattern of 2 for continuously transmitting (hence interfering) base stations gives minimum average delay and maximum spectrum efficiency: this is a denser pattern than suggested for highway cellular telephony networks. Spectrum efficiency is enhanced by using a dynamic cluster size which depends on the distance from the base station, taking advantage of the IVHS environment where vehicle position may be known.>

A new cellular communication architecture, called a 'virtual cellular network' (VCN) is proposed and its performance is investigated in terms of probability of capture, throughput and delay. A VCN does not use a conventional cellular frequency reuse concept. In a VCN, each terminal sends packets using the entire system bandwidth while any nearby port can pick up the signal. Then, the ports relay the packets to the port server (PS) over a fixed (wired) network. This simplifies the functions of a port, and the placement of ports will be easier than that in a conventional cellular network. Both the present analysis and simulation show that despite its simplicity this network has a larger user capacity than conventional cellular reuse patterns.>

The lack of proficient real time traffic monitoring systems is one of the major bottlenecks of Advanced Traveller Information Systems (ATIS) and Advanced Traffic Management Systems (ATMS). The authors describe and analyze the performance of a method for collecting real-time traffic data from probe vehicles automatically sending traffic reports to one or more base stations, connected to a traffic center by a wired communications network. The results reveal that random access (ALOHA) transmission of traffic reports is a (spectrum) efficient, inexpensive and flexible method for collecting road traffic data and that this approach can provide reliable traffic monitoring. Not only highly accurate real time link travel times can be estimated, but also Automatic Incident Detection (AID) can be performed.>

Vehicle-to-vehicle radio links suffer from power variations, multipath fading and associated Doppler spreading in frequency, as well as interference from other vehicles. These effects are investigated in the context of an automated vehicle control systems (AVCS) employing vehicle platoons. A statistical model for the channel is considered and the performance of the network involving many links is evaluated. We compare the performance of time division multiple access (TDMA), direct sequence code division multiple access (DS-CDMA), and multi-carrier code division multiple access (MC-CDMA). Reliability of the radio link is investigated by specifying the radio spectrum occupation for a given required reliability of the radio link.>

In this paper, a novel digital modulation technique called Multi-Carrier Code Division Multiple Access (MC-CDMA) is analyzed. With MC-CDMA, each data symbol is transmitted at multiple subcarriers with each subcarrier modulated by "1" or "-1" based on a spreading code. Analytical results are presented on the performance of this modulation/multiple access scheme in the downlink of an indoor wireless Rician fading channel. The performance of a controlled equalization technique that attempts to restore the orthogonality between users is evaluated.>

A large number of terminals transmitting data packets over a common radio channel to a central base station is studied. In inhibit sense multiple access (ISMA), the base station broadcasts a busy signal when an incoming packet is being received, to inhibit other terminals from colliding transmissions. This busy signal arrives at each terminal with a propagation delay, proportional to the distance between the base station and the terminal. This leads to unfairness in the probability of successfully transmitting a data packet, since nearby terminals have more up-to-date information on the actual channel status than remote terminals. This unfairness is additional to the advantage that nearby terminals have because of the capture effect, which is also considered. The paper applies nonstationary Poisson processes to describe the random arrivals of data packets at the central receiver. It is shown that the probability of a successful attempt to transmit a packet decreases, approximately linearly with the distance between the transmitter and the central receiver. The total throughput is also assessed, and it is found that the assumption of a fixed propagation delay adopted in CSMA studies by others gives too optimistic results for ISMA. Moreover, the analysis suggests a subtle change of the fixed delay approximation that enhances its accuracy, without adding complexity. The effect of propagation delays in random access radio networks are of increasing importance, since newly developed systems are designed to transmit with increasingly high bit rates.>

An analytical model is developed for the performance of a microcellular radio network in the presence of cochannel interference and additive white Gaussian noise. The modulation schemes considered are binary phase-shift keyed (BPSK), binary frequency-shift keyed (BFSK), and quadrature phase-shift keyed (QPSK). The multiple-access channel is statistically modeled by one Rician-distributed desired signal and several uncorrelated Rayleigh plus log-normally shadowed interfering signals, propagating according to dual path loss law with a turning point. The performance is determined in terms of bit error rate (BER), outage probability, block error probability, crosstalk probability, and spectrum efficiency, considering both fast and slow multipath fading. The effect of error correction codes, consisting of blocks with equal number of bits, on the performance parameters is also studied. The computational results show that the propagation loss exponents, Rician factor, turning point, and cell size all plays a major role in the design of an efficient microcellular system.>

The performance of a slotted ALOHA-network in a slow Rayleigh-fading channel for mobile data communications is investigated considering error correction and detection coding with binary phase shift keying (BPSK) modulation. The performance is measured in terms of receiver error probability and throughput. An enhanced performance is obtained by using error correction coding. In spite of the use of error correction coding, a packet can be in error, i.e. the receiver erroneously detects a codeword that none of the terminals transmitted. Bounded distance decoding can be used effectively to reduce the packet error probability. A comparison of the performance is presented for a specific code with equal Hamming distance d/sub min/, exchanging between the error correcting and error detecting capability of that code. Also, the effects of signal carrier detection on receiver error probability and throughput are investigated.>

A novel mathematical method is presented to express the outage probability for a desired radio signal received from a mobile transmitter in the presence of multiple interfering signals with combined log-normal and Rayleigh fading. In contrast to previously reported analyses, this exact method avoids approximation of the PDF of the received powers of the various signals. This is useful in determining the spectrum efficiency and performance of (interference-limited) radio networks for high-capacity cellular telephony, two-way paging, packet radio and other mobile data networks.>