This study presents new results for characterization of optical communication links through seawater-to-air interaction by taking into account the effects of pitching angle, attenuation and underwater turbulence. Pitching angle variation is assumed to present Beckmann distribution while underwater turbulent channel is modeled by Lognormal distribution. The attenuation effect covers both absorption and scattering effects and is modeled by Beer-Lambert law. Probabilistic behavior of seawater-to-air wireless communication channel is obtained with the analytical form expressions of the probability density function (PDF) and cumulative distribution function (CDF). Then, outage performance of optical wireless link is investigated. Results show that moving seawater yields a certain effect on the performance of optical wireless communication systems.

We demonstrate heating, ventilation, and air conditioning (HVAC) ducts as viable and inexpensive high-speed communication links inside of buildings at millimeter (mm-wave) frequencies. HVAC ducts are hollow metal pipes which can be used as waveguides for mm-wave signals. Indoor communication at mm-wave using HVAC ducts outperforms the existing communication methods and the same has been validated through theoretical and extensive experimental results here. Measurement and system calculations show that a received signal strength indicator (RSSI) of -$50\mathrm {d}\mathrm {B}\mathrm {m}$ and data rates in excess of 600 Mbps can be achieved over a distance of up to 8 m using HVAC ducts at 60 GHz.

In this paper, we propose the subcarrier-index modulation with Reed Solomon encoded Optical Orthogonal Frequency Division Multiplexing with index modulation (IM-RS-OFDM) scheme for visible light communication (VLC). In this technique, the data is encoded using an RS codeword and then part of the redundancy introduced by the frame which exceeds the clipping range are punctured and their corresponding subcarriers are set as inactive, where the indices of these inactive subcarriers are used to transmit extra information bits. The puncturing of the redundancy aids in mitigating clipping noise generated due to the dynamic range constraints of the optical transmitter by shortening the number of active subcarriers, while the locations of the punctured subcarriers are exploited to convey more bits in order to compensate for the reduction in spectral efficiency caused by coding redundancy. In the proposed scheme, the bipolar transmitting signal is clipped and biased according to DC bias optical OFDM (DCO-OFDM) system, and a Log-Likelihood ratio (LLR) calculation based detector is used to find the indices of punctured symbols in the codeword. Our simulation results show that the new scheme offers a better bit error rate performance compared to the conventional coded OFDM-based visible light communication.

The problem of optimum signal detection for the channel-aided free-space optics (FSO) communications is addressed. It is assumed that the avalanced-photo-detection (APD) is utilized for the direct-detection (DD) of on-off-keying (OOK) signals impaired by optical turbulence. The average primary photon count due to the background radiation is assumed to be large enough to allow for a Gaussian operation for the APD receiver. In the presence of large signal fluctuations, it is shown that the optimum structure, motivated by the maximum a-posteriori (MAP) rule, is a dual-threshold architecture where the thresholds are functions of the channel state. To remedy the channel effects, the channel state is estimated and the estimates are used to set the receiver thresholds, thereby resulting in a channel-aided scenario. Performance of the proposed optimum receiver is assessed in terms of the overall bit error rate. To assess performance, it is assumed that the turbulent channel obeys a Gamma-Gamma (GG) distribution and that the channel estimator yields an unbiased and efficient estimate of the channel state. This study shows that channel-aided reception reduces the error floor. Further, channel-aided receivers have error-floor-free signal ranges that are larger than those of the standard receivers'.

In many scenarios, the undesirable disturbance at a communication receiver has to be modeled as the sum of Gaussian noise and impulsive interference for which closed form probability density function (pdf) generally does not exist. This makes optimum receiver implementation difficult. In this paper, motivated by the heavy tail nature of the resultant pdf, an alternate parametric solution is proposed. After estimation of the appropriate parameters, the alternate solution may then be written in closed form, leading to a relatively simple architecture. Simulation results show that the proposed receiver performance, in terms of bit error rate (BER), is almost indistinguishable from the optimum receiver while outperforming other sub-optimum receivers. Noise parameter estimation using empirical characteristic functions is also discussed.

In the above titled paper (ibid., vol. 53, no. 9, pp. 1455-1461, Sep. 2005), the probability of error was computed for APD receivers. Due to an unintentional error, the square-root was omitted from the expression for the error rates in Eqs. (13) and (20). The correct expressions are presented here, as well as corrected Figs. 1-4.

In this paper, we present a new partially coherent receiver architecture motivated by optimum detection of quadrature amplitude modulation (QAM) signals in the presence of time-varying Tikhonov-distributed residual phase estimation error due to phase-locked loop (PLL)-aided phase tracking scheme. Performance is established in terms of bit error rate (BER). In this paper, an approximate performance measure motivated by union bound is presented for the proposed receiver architecture for 8- and 16-QAM constellations. The performance measures are assessed via simulation and analytical means for additive white Gaussian noise (AWGN) as well as for Rayleigh and Rician fading channels. The performance measures are shown to follow those of the optimum receiver over a wide range of signal-to-noise ratio (SNR), while outperforming a standard coherent receiver operating in the presence of residual phase error by as much as 2 dB.

Estimators motivated by the minimum mean square error estimation criterion are presented for the estimation of the optical signal intensity in free space optical communication systems impaired by atmospheric-induced scintillation. Joint parameter estimators for the estimation of signal intensities across a photodetector array are investigated. Three scenarios are considered. First, it is assumed that the received signal is detected by using a p-i-n photodiode array and that the receiver operates under a shot-noise-limited condition. In the second scenario, the received signal is detected by using an avalanche photodiode (APD) array and the receiver operates under a shot-noise-limited condition. In the third scenario, the received signal is detected by using an APD array, but the receiver is impaired by thermal noise as well as shot noise. The mean primary electron count received by the APD is assumed to be large enough to justify the Gaussian approximation. The optical channel is modeled using a lognormal distribution with correlated random signal intensities across the array. With the aid of simulation, it is shown that the proposed estimators outperform maximum likelihood estimators for small scintillation indices while providing a performance similar to the maximuma posterioriestimator.

Estimators motivated by the minimum mean-square estimation (MMSE) criterion are presented for the estimation of the optical signal intensity in free-space optical (FSO) communication systems impaired by atmospheric-induced scintillation. Joint parameter estimators for the estimation of signal intensities across a photodetector array are investigated. Three scenarios are considered. First, it is assumed that the received signal is detected using a p-i-n photodiode array and that the receiver operates under a shot-noise-limited condition. In the second scenario, the received signal is detected using an APD array and the receiver operates under a shot-noise-limited condition. In the third scenario, the received signal is detected using an APD array, but the receiver is impaired by thermal noise as well as shot noise. The mean primary electron count received by the APD is assumed to be large enough to justify the Gaussian approximation. The optical channel is modeled using a log-normal distribution with correlated random signal intensities across the array. With the aid of simulation, it is shown that the proposed estimators outperform the maximum-likelihood estimators for small scintillation indices while providing a performance similar to the maximum a posteriori estimator.

In this paper, the performance analysis of a recently proposed multiple-access (MA) scheme for data communication via TDMA wireless channel in terms of Erlang capacity is carried out. The proposed MA scheme takes advantage of time-hopping (TH) within multiple frames of a time-division multiple-access (TDMA) system to provide access to the channel for network subscribers. First, a super-frame for the TDMA signaling is defined. Subsequently, a user is provided with an access code that is used to hop over the indicated TDMA super-frame, hereafter referred to as a TH frame. The assignment codes are those of the {0,1} family codes, recently exploited for fiber-optic code-division multiple-access (FO-CDMA) applications. It is assumed that each assignment code can guarantee no more than 1 hit in a superframe and that code synchronization exists between all users. For differential phase-shift-keying (DPSK) and M-ary frequency-shift-keying (MFSK) modulated THMA systems and when side information is available at the receiver, upper bounds on the overall symbol error rate are derived when the communication channel is impaired by slow frequency non-selective Rayleigh and Rician fading. The capacity of the proposed system is obtained in terms of Erlang/sector for reverse link and is compared to those of TDMA and CDMA systems. It is demonstrated that the power control requirement of the proposed THMA system is identical to that of a conventional TDMA system, and that the proposed system achieves an Erlang capacity identical to that of its CDMA counterpart

In this paper, we address the problem of adaptive resource allocation for the uplink of an orthogonal frequency-division multiple-access (OFDMA) cellular system. In particular, we investigate means to enhance the call carrying capacity of an OFDMA system. To that end, we propose a number of practical resource allocation schemes where either channel usage or total interference level is used in assigning new frequency channels. The performance of the proposed resource allocation schemes are then assessed in terms of Erlang capacity for a call blocking probability of 0.02

Closed-form solutions for the bit-error rate of a freespace, heterodyne optical communication system is derived when the optical beam is subjected to K-distributed optical turbulence. It is assumed that the scintillation index is confined to the range (2,3) or that the number of scatterers in the propagation path is a random variable.

In this paper, we characterize the performance of a direct-detection, avalanche photodiode-based free-space optical (FSO) communication system in terms of the overall bit-error rate. The system of interest uses pulse-position modulation (PPM) and is subjected to scintillation due to optical turbulence. Two scenarios are considered. In one case, a weak turbulence (clear-air) scenario is considered, for which the received signal intensity may be modeled as a log-normal random process. In the other case, we consider a negative exponentially distributed received signal intensity. To arrive at the desired results, it is assumed that the system uses a binary PPM (BPPM) modulation scheme. Furthermore, it is assumed that the receiver thermal noise is nonnegligible, and that the average signal intensity is large enough to justify a Gaussian approximation at the receiver. Union bound is used to assess the performance of M-ary PPM systems using the results of the BPPM scenario. Numerical results are presented for the BPPM case to shed light on the impact of turbulence on the overall performance.

The channel estimation problem is addressed for freespace optical communications impaired by optical turbulence. Estimators which are motivated by the minimum mean square error (MMSE) estimation criterion are proposed for the estimation of optical signal intensity in p-i-n and APD-based freespace optical (FSO) communication systems. A shot noise limited condition is assumed. The received signal intensity is modelled using a log-normal (LN) probability density function (pdf) for low (<0.75) scintillation indices and a negative exponential pdf for scintillation indices in the vicinity of 1. When considering APD-based receivers, only the low scintillation scenario is considered. With the aid of simulation, it is shown that the proposed estimators outperform maximum likelihood (ML) estimators for small scintillation indices.

A new metric is proposed for multiple-symbol differential-detection of phase noise-impaired binary differential phase-shift-keying (DPSK) signals. The performance of the proposed metric is evaluated in terms of the overall bit error rate (BER) using analytical means. It is assumed that the observation interval extends over three consecutive symbol intervals and that the received signal is corrupted by oscillator phase noise.

A multiple-access (MA) scheme for indoor wireless communications is suggested. The proposed MA scheme takes advantage of time-hopping (TH) within multiple frames of a time-division multiple-access (TDMA) system to provide access to the channel for network subscribers. The assignment codes are those of the {0,1} family codes, previously exploited for fiber-optic code-division multiple-access (FO-CDMA) applications. Upper bounds on the overall symbol error rates for differential phase-shift-keying (DPSK) and frequency-shift-keying (FSK) systems are derived when slow, frequency non-selective Rician fading is present.

LAPLUS is a novel switch algorithm for flow control of the available bit rate (ABR) asynchronous transfer mode (ATM) service. It ensures a steady-state rate allocation satisfying the MCR-plus-equal-share criterion. It only requires constant-time processing and two tags to be stored per flow. It is naturally able to take peak cell rates of flows into account. LAPLUS solves in a novel way the problem of selecting a measurement interval. The solution allows it to contain queue growth and keep utilization high on one hand and control low speed flows and operate with stability on the other. We describe results of simulation study of LAPLUS. The results show it to be fair, responsive and stable.

Two novel Viterbi decoders, known as joint Viterbi decoder (JVD) and enhanced Viterbi decoder (EVD), for synchronization and data detection are introduced and analyzed for convolutionally-encoded (CE) optical pulse-position modulation (PPM) and overlapping PPM (OPPM) channels. For the JVD algorithm, a count metric is employed to perform the Viterbi decoding in the absence of synchronization. For the EVD algorithm, a memory array is utilized to reduce the computation time of the JVD algorithm. With the aid of a previously proposed upper-bound and taking advantage of the minimal asymptotic normalized timing error (MANTE) concept, approximate performance measures for the EVD and JVD receivers are obtained.

The performance of a binary differential phase-shift-keying (DPSK) communications system impaired by slow fading, additive noise, timing error, and mixer imbalance is investigated. Mixer imbalance is shown to cause substantial power penalty at error rates less than 10/sup -6/ in the absence of fading. In the presence of Rayleigh fading, the performance is degraded substantially, resulting in a less appreciable deterioration in the error rate due to mixer imbalance.

A packet-switched frequency-hopping multiple-access (FO-FHMA) communication system based on bus topology is proposed and investigated. In this arrangement, access to the optical medium for the network subscribers is provided via frequency-hopping (FH). The proposed FH technique in this analysis takes advantage of the Reed-Solomon (RS) codes to encode information packets and, consequently, to combat possible burst errors at the receiver. At user locations, encoded subpackets are frequency-hopped with the aid of a phase modulator, using a preassigned FH pattern for transmission over the optical medium. At the receiver site, a balanced phase discriminator/detection device followed by a frequency-dehopping receiver is used to recover the desired information. An alternative receiver structure, where the received signal is heterodyned prior to dehopping, is also proposed. The implications of this coherent detection are consequently discussed. Performance measures in terms of throughput and probability of incorrect decoding for chip synchronous model with asynchronous transmission characteristic in the absence of side information are obtained to assess the viability of the proposed packet-switched fiber-optic multiple-access communication system. Numerical results are presented when short RS codes are utilized as FH assignment codes.>

A novel optical phase-locked loop (OPLL) system for the self-homodyne detection of digitally phase modulated optical signals is introduced. A Mach-Zehnder type interferometer is used to self-homodyne binary phase-modulated optical signals with an external phase modulator inserted in the control arm of the interferometer. In the absence of thermal noise, this arrangement is 3 dB less sensitive than a homodyne receiver for a background noise limited operation. For a negligible background noise level and when the receiver thermal noise is ignored, a power penalty of 6 dB is incurred,.>

The power spectral density (PSD) of a direct-detected optical signal with overlapping pulse-position modulation (OPPM) scheme is derived. It is demonstrated that this spectrum contains a strong periodic component at the signal symbol frequency, thereby enabling a simple PLL device to provide joint symbol and slot synchronization. The variance of the timing error for a linearized PLL synchronizer is shown to be mainly dominated by, and is inversely proportional to, the product of the PLL closed-loop bandwidth and symbol duration of the OPPM signal. The performance of a shot-noise limited PLL-based optical OPPM communication system is then assessed in terms of the overall system symbol error rate.>

A clock recovery scheme for direct-detection optical on-off keying (OOK) communication systems with nonreturn-to-zero pulse shaping is proposed and investigated. In the suggested model, the optical field is detected with the aid of an avalanche photodiode (APD) photodetector, which is followed by a clock regeneration subsystem. The proposed clock recovery subsystem consists of a delay-and-multiply nonlinearity followed by a conventional phase-locked loop (PLL), tuned to the slot frequency of the desired optical OOK signal. Performance of the proposed system is obtained in terms of the signal-to-noise ratio (SNR/sub L/) of the linearized PLL device (or, equivalently, the inverse of phase, or timing, error variance) when background noise and receiver thermal noise are present. Numerical results are presented in order to explain the influence of noise processes on the performance of the proposed clock recovery subsystem. The performance of this system is also compared to that of an early-late gate and square-law symbol synchronizers.>

A jointly optimum slot and symbol clock recovery scheme for a direct-detection optical communication system with overlapping pulse position modulation (OPPM) is proposed. To arrive at the suggested optimum synchronizer, the maximum likelihood (ML) estimation technique is used when the observed process is a Poisson point process (i.e., shot noise limited receiver). Two suboptimal synchronizer models computationally superior to the ML model are also proposed.>

The problem of false lock in suppressed-carrier minimum phase-shift keying (MPSK) tracking loops (M>4 in particular), such as Mth power phase-locked loop (PLL) and MPSK Costas loop carrier recovery subsystems, is investigated. It is demonstrated that such tracking loops false lock onto the received signal when the received carrier frequency and the reference signal frequency generated by MPSK tracking loops are mismatched by multiples of 1/M of the MPSK symbol rate. False lock margins (FLMs) for the suppressed carrier MPSK tracking loops are obtained for a noiseless system model as well as for a model corrupted by additive white Gaussian noise (AWGN). Numerical results are presented in order to explain the influence of system parameters, such as the time-bandwidth product of MPSK Costas loop arm filters (or the Mth power PLL prefilter), arm filters' output signal-to-noise ratio and the input signal-to-noise ratio, on the performance of MPSK tracking loops.>

A model for the pseudonoise (PN) code-aided estimation of the transmission path delay and ranging for direct-detection on-off-keying (OOK) optical intersatellite channels is suggested in which the intensity of the optical field is modulated by a PN code and random binary data symbols. A closed-loop code-tracking loop model, motivated by the maximum-likelihood (ML) estimation of the phase difference between the received PN code and a locally generated one, is obtained. The maximum-likelihood model is approximated and, consequently, receiver models for the high and low signal-to-noise ratio (SNR) are suggested. The resulting code tracking loop S-curves of the receivers are that of the S-curves of a noncoherent delay-locked loop (NCDLL). The performance of the two receivers under the influence of background noise, signal count and PN code length is obtained.>

A model for spread-spectrum (SS) communication using a direct-detection optical on-off-keying (OOK) channel is proposed and investigated. In this model, the intensity of the transmitted optical field is modulated by a pseudonoise (PN) code and random binary data symbols. Two PN code acquisition models, namely the maximum-likelihood (ML) and the single dwell-time (SDT) models, are proposed. The performances of the ML and the SDT models are obtained in terms of the probability of missed acquisition, the probability of false alarm, and the probability of missed detection under the Gaussian approximation. The mean acquisition times are obtained and used to provide a comparison between the proposed ML and SDT models.>

The problem of time and frequency transfer between remotely located clocks is investigated for the case when internal and external disturbances are present. A model based on the maximum-likelihood (ML) estimation of the carrier phase and the transmission path delay in the face of white Gaussian noise is suggested. In this model, path delay estimation and, consequently, clock synchronization is achieved by tracking a pseudonoise (PN) code phase-modulated onto the carrier. Phase and frequency estimation are obtained through a closed-loop phase tracking model. The performance of the maximum-likelihood model under high and low signal-to-noise ratio (SNR) is established in terms of probability density functions of the phase increment processes in space and time. Factors which influence performance include external and internal disturbances. These include additive thermal noise, range delay, clock drift, etc. Numerical results are presented to explain the influence of system parameters on performance.>