Visible Light Communication (VLC) technology using light emitting diodes (LEDs) has been gaining increasing attention in recent years as it is appealing for a wide range of applications such as indoor positioning. Orthogonal frequency division multiplexing (OFDM) has been applied to indoor wireless optical communications in order to mitigate the effect of multipath distortion of the optical channel as well as increasing data rate. In this paper, we investigate the indoor positioning accuracy of optical based OFDM techniques used in VLC systems. A positioning algorithm based on power attenuation is used to estimate the receiver coordinates. We further calculate the positioning errors in all the locations of a room and compare them with those using single carrier modulation scheme, i.e., on-off keying (OOK) modulation. We demonstrate that OFDM positioning system outperforms its conventional counterpart.

A major performance degrading factor in free space optical communication (FSO) systems is atmospheric turbulence. Spatial diversity techniques provide a promising approach to mitigate turbulence-induced fading. In this paper, we study the error rate performance of FSO links with spatial diversity over atmospheric turbulence channels described by the Double Generalized Gamma distribution which is a new generic statistical model covering all turbulence conditions. We assume intensity modulation/direct detection with on-off keying and present the BER performance of single-input multiple-output (SIMO), multiple-input single-output (MISO) and multiple-input multiple-output (MIMO) FSO systems over this new channel model.

In this paper, we investigate and compare the performance of single- and multi-carrier modulation schemes for indoor visible light communication (VLC). Particularly, the performances of single carrier frequency domain equalization (SCFDE), orthogonal frequency division multiplexing (OFDM) and on-off keying (OOK) with minimum mean square error equalization (MMSE) are analyzed in order to mitigate the effect of multipath distortion of the indoor optical channel where nonlinearity distortion of light emitting diode (LED) transfer function is taken into account. Our results indicate that SCFDE system, in contrast to OFDM system, does not suffer from high peak to average power ratio (PAPR) and can outperform OFDM and OOK systems. We further investigate the impact of LED bias point on the performance of OFDM systems and show that biasing LED with the optimum value can significantly enhance the performance of the system. Bit-interleaved coded modulation (BICM) is also considered for OFDM and SCFDE systems to further compensate signal degradation due to inter-symbol interference (ISI) and LED nonlinearity.

The unit-norm constraint optimization for joint shortening of channel and echo impulse response is presented in this paper. The optimization is performed in the mean square sense (MSE) and compared to unit-tap constraint joint shortening optimization. The proposed method could serve in performing joint echo cancellation and equalization for data transmission over category cables for the next IEEE standard on shortrange ultra high-speed copper interconnections used in data servers, high-performance computing centers, local area networks, etc. The main objective of the presented algorithm is to extensively reduce the power and implementation complexities of a 40GBASE-T system.

Free Space Optical (FSO) communications is a practical solution for creating a three dimensional global broadband communications grid, offering bandwidths far beyond possible in Radio Frequency (RF) range. However, attributes of atmospheric turbulence (scintillation) and obscurants such as clouds impose perennial limitations on availability and reliability of optical links. To design and evaluate optimum transmission techniques that operate under realistic atmospheric conditions, a good understanding of the channel behavior is necessary. In most prior works, Monte-Carlo Ray Tracing (MCRT) algorithm has been used to analyze the channel behavior. This task is quite numerically intensive. The focus of this paper is on investigating the possibility of simplifying this task by a direct extraction of state transition matrices associated with standard Markov modeling from the MCRT computer simulations programs. We show that by tracing a photon's trajectory in space via a Markov chain model, the angular distribution can be calculated by simple matrix multiplications. We also demonstrate that the new approach produces results that are close to those obtained by MCRT and other known methods. Furthermore, considering the fact that angular, spatial, and temporal distributions of energy are inter-related, mixing time of Monte-Carlo Markov Chain (MCMC) for different types of aerosols is calculated based on eigen-analysis of the state transition matrix and possibility of communications in scattering media is investigated.

Free-space optical communication (FSOC) is a method by which one transmits a modulated beam of visible or infrared light through the atmosphere for broadband applications. FSOC makes connectivity possible between high-rise buildings and metropolitan and intercity communication infrastructures. While radio frequency (RF) communication still dominates the commercial communications networks, it has not been able to meet the increasing demand for larger data rates. Reliable communications require a good understanding of the surrounding environment that affects the communications channel. The fundamental limitation of FSOC arises from the environment through which light propagates. The presence of particles, such as fog and clouds in the propagation channel introduces spatial and angular dispersion to the transmitted light signals. Attenuation of the light signals through the propagation medium is a combined effect of dispersion and absorption by particles. A mode of ray tracing is developed to simulate light going through the medium. A 2-D histogram is presented to investigate correlation of spatial and angular dispersion of the received light.

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel.

Home users are in need for broadband communications access globally. Broadband powerline communications has been advanced through last decade and it is going to be a mature broadband access in near future. Meanwhile, indoor optical wireless communications through lighting LEDs has been investigated, recently. Suitable channel models are proposed for each of these systems and the corresponding transmission capacity values are calculated. It is shown that the combination of these two technologies makes an efficient method for fulfilling the premise of broadband access for home networking, while providing efficient and low-cost lighting.

Broadband communications for indoor power-line networks with impulsive noise using Orthogonal Frequency Division Multiplexing (OFDM) is considered in this paper. From earlier investigations, it is known that this channel suffers from multipath fading and frequency selectivity along with man-made impulsive bursty noise. Nevertheless, the calculated channel capacity limit promises very high data rates over this channel. Enhancement techniques, such as concatenated coding can help an OFDM system to tackle impulsive noise burst. In this paper we employ Digital Fountain codes, which are a new class of erasure detecting codes. These codes are considered the state-of-the-art discovery in coding theory due to their simplicity and performance.

Broadband communications for indoor powerline networks with impulsive noise using Orthogonal Frequency Division Multiplexing (OFDM) is considered. From earlier investigations, it is known that this channel suffers from multipath fading and frequency selectivity along with manmade impulsive bursty noise. Nevertheless, the calculated channel capacity limit promises very high data rates over this channel. In this paper the bit error rate (BER) performance of the OFDM system under impulsive noise and frequency fading is theoretically analyzed and closed form formulas for this performance is derived. Furthermore, a theoretical upper bound on the performance of coded OFDM system is obtained, given perfect interleaving and the effect of the interleaver length on coding performance is also studied.

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. In this paper, we derive error performance bounds for coded FSO communication systems operating over temporally correlated lognormal turbulence fading channels. Our analytical derivations build upon an approximation to the sum of correlated lognormal random variables. The derived bounds are tighter than the previously published results and are valid for a wider range of turbulence conditions. Simulation results are further demonstrated to confirm the analytical results

This paper presents a channel model suitable for multiwire overhead medium voltage lines. This model, incorporating ground admittance, is more appropriate at higher frequencies than predicted by Carson's model of 1926. The proposed model is further used to evaluate the multipath channel impulse response and associated capacity limit in sample power distribution grids for applications in broadband over power lines communications. For a sample grid model, comparison is made to the capacity value predicted based on the Carson's model, and it is demonstrated that the older model underestimates the potential of the overhead lines for broadband transmissions, significantly.

A channel model suitable for multi-wire overhead medium-voltage lines is proposed. The model, incorporating ground admittance, is more appropriate for broadband communications. This model is then employed in order to evaluate the multipath channel impulse response and the associated transmission capacity limit in actual overhead medium-voltage power distribution networks for broadband power-line communications applications.

Recently, IEEE is interested in specifying the next generation 10GBASE-T Ethernet network. Due to the limitations of the channel characteristics of Category 6 (CAT-6) unshielded twisted pair (UTP) cables, capacity of the channel is only slightly above the proposed 10 Gbps data rate. This poses a great challenge to the design of equalizers. This paper addresses the problem of designing a minimum mean-square error (MMSE) maximal shortening equalizer with decision feedback cancellation. Unlike traditional approaches such as the decision feedback equalizers (DFEs), the shortening equalizer optimally focuses the channel impulse response to a small number of taps. If a subsequent maximum likelihood sequence estimation (MLSE) equalizer is used, the performance loss caused by symbol-by-symbol hard decision of traditional equalizers can be reduced. However, due to the complexity involved in the shortening equalizers is generally prohibitive; it is difficult to apply them to systems using modulation with large constellations. To overcome this difficulty, we propose an equalizer that combines DFE and the shortening equalizer. Thus, with acceptable complexity, the shortened signal-to-noise ratio (SSNR) can be significantly improved.

An iterative turbo receiver is proposed for the uplink of a MIMO CDMA system employing layered space-time transmission. The proposed receiver consists of a low-complexity layered space-time multiuser detector using linear minimum mean squared error (MMSE) filtering with a-priori information and parallel a-posteriori probability (APP) computing soft-input soft-output (SISO) decoders. Turbo trellis-coded modulation (TTCM) is used as error-correcting tool to achieve bandwidth efficient transmission. Computer simulation results show that the proposed receiver using an iterative operation along with the detector and decoders outperforms an iterative receiver based on parallel interference cancellation as well as a non-iterative MMSE receiver. Furthermore, while providing an increase in system capacity, the proposed receiver offers an error performance similar to that of the single-user case at high SNR, in a multiuser environment.

The IEEE is interested in specifying the next generation 10 Gbps copper cable Ethernet network, termed 10GBASE-T. In the design of a 10GBASE-T transmission system over up to 100 meters of a standard category 6 cable, the equalization of widely spread channel impulse responses is one of the most challenging problems. In this paper, we investigate various turbo equalization techniques, applicable to this problem, and present performance for links using a 1/2 rate recursive systematic convolutional code in BPSK modulation.

IEEE is interested in specifying the next generation 10 Gbit/s copper cable Ethernet network, termed 10GBASE-T. In the design of a 10GBASE-T transmission system up to 100 meters of a standard category-6 (CAT-6) copper cable, equalization and decoding play a significant role in achieving reliable data communication. In this paper, we focus on the design of the equalization and decoding processes and propose a vector transmission system of a 4-dimensional trellis coded-modulation (TCM) with 10-PAM signaling. At the receiver, a joint multiple-input multiple-output equalization and decoding with a survival path feedback equalizer (SPFE) is employed. Based on the design, simulation results show that reliable transmissions at less than 10/sup -12/ BER can be achieved by the proposed system.

Optical (infrared) wireless communications links offer an attractive solution for indoor applications. To enable terminal mobility and reduce temporal dispersion, a configuration known as multispot-diffusing (MSD) is used, which is a multiinput, multioutput (MIMO) architecture. In this configuration, a transmitter generates multiple narrow beams that get uniformly spread over a reflection surface accessible to a service area. A multibranch receiver with each branch having access to a diffusing spot (generated by transmitter) combines signals on its branches. This scheme creates multiple virtually ideal communications channels between a base station and terminals. This paper considers issues involved in the design of a transmitter-based holographic spot array generator. The generator produces equally-spaced diffusing spots on the room ceiling and/or walls. To overcome power limitation set by eye safety requirements, a receiver optical concentrator is proposed. Furthermore, to improve receiver signal-to-noise ratio, an optical filter that rejects optical noise is needed. Thus, functionally receiver branch optical front-end consists of an optical concentrator and an optical band-pass filter. A single holographic optical element, capable of performing both functions is proposed. Link performance is investigated by providing equivalent link model and comparing probability of error for a bare and a holographic receiver. From performance evaluations, our results show that it is possible to achieve an increase of 11 dB in the SNR and improve power budget by reducing the path loss by over 6 dB.

A theory of equalization and cancellation in digital data transmission over a MEMO multipath fading channel is presented. The theory extends the classic equalization and cancellation theory into matrix forms to mitigate intersymbol interference (ISI) and cross-polarization interference (CPI) by means of minimizing the overall mean-square error (MSE). We evaluate the performance of several configurations for a sample propagation model with several snapshots of fading events in computer simulations. Both MSE and average error probability are evaluated and compared. We find that the decision feedback structure demonstrates a better performance than the linear equalizer structure. Furthermore, less amount of MSE can be translated into smaller error probability values only in cases of relatively large signal-to-noise ratios.

We propose a wireless optical-mesh network based on optical code-division multiple-access (OCDMA)-coded free-space optical links. The performance of the proposed network under intermediate and strong turbulence channel conditions is investigated for synchronous and asynchronous OCDMA. We show that synchronous OCDMA using complementary Walsh-Hadamard codes operating at a raw bit rate of 622 Mb/s per user can achieve a bit-error rate of 10/sup -9/. This is achievable in channels with strong turbulence and high temporal correlation, or in channels with intermediate turbulence conditions; reasonable conditions based on channel characteristics inferred from the refractive index spectrum. For asynchronous OCDMA using optical orthogonal codes (OOCs), using a lower bound, we show that asynchronous OCDMA using OOCs cannot achieve acceptable performance for the same channel conditions, due to interference limitations.

Spatial confinement of the paths traveled by infrared signals enables reuse of bandwidth and resources in adjacent indoor environments. This property can also result in undesirable shadowing that occurs whenever an obstacle blocks signal paths between transceivers. Shadowing compromises link reliability by increasing the error rate, and can result in service interruption. We propose spatial coding to improve link performance and alleviate the shadowing effect in multi spot diffusing, a multi-input, multi-output architecture. Spatial coding performance is compared to receiver multi-level combining. Our results show that when a channel is shadowed, 4-level combining at the receiver fails to maintain a reliable link irrespective of transmitted power. Using spatial coding over 6 channels, a reliable link is possible in the presence of a shadowed channel with an optical power penalty of 4.5 dB.

In this paper, we investigate vulnerability of a so called "cellular" and that of a multi-spot diffusing (MSD) multi-input-multi-output (MIMO) architecture to shadowing and its effect on communications cell size and the required transmit power. Because signal path obstruction by furniture is easier to predict and avoid, we are mainly concerned with shadowing and blockage caused by people either sitting or standing. To avoid blockage of communication link by a user of portable units, maximum radiation angle at the transmitter in both configurations must not exceed 45/spl deg/. This restricts communication cell size. We show the probability of blockage of a cellular link depends almost linearly on the distance of the portable unit from the communication cell center. Unlike cellular links, MSD-MIMO links can be designed to be robust against blockage, though still vulnerable to shadowing. In a typical office having a height of 3 m, probability of shadowing is less than 2% and in majority of cases shadowing causes less than 50% reduction in the received signal power. Power penalty due to shadowing is insignificant (less than 0.1 dB) with a 1% outage.

IEEE is interested in specifying the next generation 10 Gbps copper cable Ethernet network, termed 10000BASE-T, in the not too distant future. In search of a proper solution, we are investigating the involved tradeoffs at every design step. The paper presents conceptual designs of a 10000BASE-T copper transmission system for distribution of a digital signal over /spl sim/100 meters of a standard CAT-5 copper cable. These designs make it possible to achieve this at a target average bit error rate (BER) of 10/sup -10/ after equalization and decoding, using standard CAT-5 cable, if we adopt vectored transmission on 4 pairs. This is done in the presence of insertion-loss dispersion, NEXT, FEXT, alien NEXT, echo and all the associated noise. To achieve this, the following tasks have been defined: MIMO cancellation of all crosstalk terms; iterative vector equalization and decoding of the received signals.

Traditionally, diffused infrared configuration has been studied with the assumption that the illuminated region is placed at room ceiling. This is done to ensure minimum path loss to a maximum number of receiver locations. In many practical situations, this arrangement might be difficult to implement, either due to the ceiling structure or the difficulty in illuminating the entire ceiling uniformly without introducing an intrusive transmitter. In this paper, we propose a new configuration, in which the upper regions of the walls are illuminated. Path loss and delay spread are used as performance measures to assess the feasibility of the proposed configuration.

Performance of a diffuse optical link may potentially be degraded by temporal dispersions resulting from surface reflections. In order to devise techniques to alleviate the adverse effect of dispersion, an accurate channel model is needed. Obtaining the impulse response (IR) for a given receiver location requires consideration not only of the direct path, but also of reflections up to nth order. The IR is only valid for a specific location and specific receiver parameters. If a receiver moves, the IR has to be recalculated. We propose a new approach for characterizing diffuse links that results in a tremendous saving in calculation and also gives more insight into the channel characteristics. The new approach is based on consolidating the dependence of receiver parameters, transmitter parameters, and the indoor environment into independent components. Thus, changing one of the parameters of the link requires recalculation of one of these components. The new model is utilized to obtain an accurate profile of delay spread and received power throughout a room.

While use of power-efficient signaling schemes appears to be effective at compensating for the inherent high path-loss associated with pure diffuse infrared links, it begins to lose its effectiveness as the data rate is increased. At very high data rates, intersymbol interference (ISI) can result in a very high and sometimes irreducible power penalty, preventing the system from operating at a low bit-error probability. We use a link design employing a multibeam transmitter in conjunction with a narrow field-of-view (FOV) direction diversity receiver. The design goal is to eliminate the effect of ISI so that power-efficient signaling schemes such as pulse-position modulation (PPM) can be employed at very high data rates. We also use high-rate Reed-Solomon codes to further increase the power efficiency of PPM signals. The proposed system can be made rate-adaptive through varying modulation level L and/or code rate R without increasing the complexity significantly. This provides a dynamic range large enough to allow efficient utilization of available bandwidth, i.e., to allow portable terminals to communicate at their highest permitted data rate, without sacrificing the quality of service. It is shown that a bit error rate not exceeding 10/sup -9/ can be achieved within the link coverage area with 99% probability at bit rates up to a few hundreds of megabits per second, using transmitted power levels well below 1 W.

For pt.I see ibid., vol.50, no.7, p.1098-1109 (2002). We examine an infrared link composed of a multibeam transmitter and a direction-diversity receiver, employing code combining. The latter represents an added dimension to the conventional diversity concepts, which are limited to combining the individual received symbols. Rate-compatible punctured convolutional codes are used to encode intensity-modulated on-off keying (OOK) optical power, to create an adaptive environment for efficient utilization of channel spectral bandwidth, to provide a means for accurate channel estimation, and to maintain a guaranteed bit error rate (BER) performance at all receiver positions. It is shown that a BER not exceeding 10/sup -9/ with 99% probability can be achieved at bit rates up to a few hundreds of megabits per second, at very low transmitted power levels.

In this paper, we consider a fixed wireless cellular network that uses dynamic time division duplex (D-TDD). We analyze the signal-to-interference ratio (SIR) outage performance of a D-TDD fixed cellular system, and propose a scheme to improve the outage probability performance. First, outage probability is evaluated using an analytical model, when omnidirectional antennas are deployed at a base-station site and a subscriber site. Our model is verified, using Monte Carlo simulations. According to our investigation, the outage performance of the D-TDD system is severely limited by a strong interference from the cochannel cell on the downlink, while the reference cell is in the uplink cycle. To improve the outage performance during uplink receptions, we introduce two time-slot allocation methods, combined with sector antennas: max min{SIR} and max{SIR}. Max min{SIR} is an exhaustive search algorithm for assigning subscribers to a few extra uplink time slots, so as to maximize the minimum SIR expectation value over the extra uplink time-slots region. It is used as a performance benchmark in our analysis. Meanwhile, the max{SIR} is a simpler and efficient algorithm for improving the outage performance. It is established that the performance difference between the two algorithms is not noticeable. Especially, the difference is negligible, when the dynamic range of the traffic pattern between uplink and downlink is small. Also, the outage performance of a system that employs the max{SIR} algorithm combined with sectored antennas is compared to that of a system employing adaptive-array antennas. The proposed system shows promise, and offers a compromise between system complexity and network guaranteed availability.

Dynamic TDD (D-TDD) has gained attention as a means to deliver high density two-way traffic. However, the outage performance of D-TDD system is severely limited by the strong interference from co-channel cells in a downlink cycle, while the reference cell is in its uplink duty cycle. To suppress the co-channel interference during uplink reception, we consider two kinds of time slot allocation strategies, combined with sector antennas: MaxMin (SIR) and Max (SIR). MaxMin (SIR) is an optimum time slot allocation strategy, in the sense that it maximizes the minimum signal to interference ratio expectation value over extra uplink time slot region. It is used as a performance benchmark in our analysis. Meanwhile, Max (SIR) is a simple and efficient algorithm to improve outage performance. It is established that the performance difference between the two algorithms is not noticeable, especially when the dynamic range of traffic pattern between the uplink and downlink is small. Also, outage performance comparison between Max (SIR) algorithm and adaptive array antennas is provided. The proposed system shows promise and offers a compromise between system complexity and the desired availability.

We suggest applying multi-carrier spread spectrum (MC-SS) techniques to fixed wireless access applications. In these applications, cross-polarization discrimination and directional subscriber antennas can be used to reduce interference in multi-cellular systems. Power control is applied on both uplink and downlink. The suggested system has a high capacity and uses a simple multi-access layer. It is robust against multipath effects and can provide service coverage not only to line-of-sight (LOS) subscribers but also to nearly LOS subscribers. Only a single carrier frequency is used in the entire network.

The growth of digital cellular telephony has created demand for rich multimedia services similar to what we have come to expect of wire-line communications. The applications are far reaching such as video conferencing, emergency medical consultation and directions to m-commerce, remote site surveying and embedded navigational systems. Global standards bodies such as 3GPP, ETSI, ARIB and ITU-T have converged upon the next generation infrastructure capable of providing broadband services to mobile devices in the form of packet switched WCDMA. The focus of our research is on error resilient coding earmarked to reach data rates ranging from 144 Kbps to 2 Mbps on the uplink with the aim of substantial performance improvements. Toward this end we present a two-fold approach, focusing on error-resilient video coding and smart antennae for robust transmission. An accurate simulation of the end-to-end system was implemented using SPW, with performance measured in terms of PSNR and bit error rates.

This paper addresses one of the most promising candidates for high-speed in-house wireless communications, namely, the multi-spot diffusing configuration (MSDC). Since it uses the optical infrared medium for data transmission, it has the inherent potential for achieving very high capacities. The channel characteristics in MSDC are simulated and the causes for channel distortion are analyzed. Then, conditions for creation of a virtually ideal channel are derived. It is shown that the 3-dB channel bandwidth can be extended up to beyond 2 GHz. This bandwidth comes at the cost of a poor power efficiency. In order to compensate for that, a novel receiver optical front-end design is proposed and its performance is analyzed. Taking advantage of the unique properties of the holographic optical elements, the conventional optical front-end, consisting of a concentrator and a filter, is replaced by a single holographic curved mirror. The utilization of such a holographic optical element improves the signal-to-shot noise ratio by up to 18.5 dB.

While use of a power efficient signaling scheme appears to be effective in compensating for inherent high path-loss associated with pure diffuse infrared links, it begins to lose its effectiveness as data rate is increased. At very high data rates, intersymbol interference (ISI) can result in a very high and sometimes irreducible power penalty, preventing the system from operating at a low bit-error probability. In this paper, we investigate a link design employing a multi-beam transmitter in conjunction with a narrow field-of-view (FOV) direction diversity receiver. The design goal is to eliminate the effect of ISI, so that power efficient signaling schemes such as pulse-position modulation (PPM) can be employed at very high data rates. It is shown that an average bit-error rate (BER) not exceeding 10/sup -9/ can be achieved within the link coverage area with more than 99% probability at bit rates up to a few hundreds of megabits per second, using transmitted power levels well below one Watt.

This paper presents a theoretical analysis of throughput and transmission efficiency for a rapidly deployable data distribution network in UNII-bands. To enhance the network throughput, all radio links are considered to exercise a collision avoidance frequency hopping technique. Such a technique is extremely attractive for efficient utilization of unlicensed bands by a group of uncoordinated radio links. Performance analysis shows that the network can maintain a very high throughput and a low outage even when it is heavily loaded.

In order to combine the advantages and to overcome the drawbacks of the direct line-of-sight and diffuse configurations for indoor wireless optical LANs, a multi-spot diffusing concept utilizing holographic spot array generator is suggested. Simulation results for the link characteristics are presented and comparison is made with a pure diffuse configuration.

In this paper, we propose a multi-access scheme based on OFDM/orthogonal-CDMA/sectored antenna for the last mile LMDS applications. Power control is adopted on both the uplink and downlink. Compared to TDMA/FDMA systems, the proposed configuration has a higher capacity and it offers a simpler multi-access scheme. It is more robust against multipath effects and makes it possible to increase the service coverage. Since only a single carrier frequency is used at 28 GHz, the hardware cost is lower than that for TDMA/FDMA systems.

This paper presents a solution to the problem of providing quality-of-service (QoS) guarantees in an ATM network with wireless links. The suggested resolution to this problem is through selectable-rate channel coding. In particular, rate-compatible punctured convolutional (RCPC) codes are employed due to their several attractive features that make them favorable for ATM-based applications. In this adaptive scheme, the RCPC code rate is intelligently varied in response to the channel's nonstationary behavior according to the channel state information (CSI). This yields an equivalent stationary channel with a fixed and predetermined bit-error rate (BER) in which QoS can be guaranteed. In order to demonstrate the performance of RCPC codes in providing and maintaining the QoS, a lognormal fading channel model is assumed. An architecture is proposed for the wireless link. This system assumes that accurate CSI is available at the transmitter as well as at the receiver. Bandwidth utilization of the proposed system under various channel conditions is then evaluated, plotted, and discussed.

This investigation has been motivated by a recent increased interest in orthogonal frequency division multiplexing (OFDM) CDMA systems for broadband wireless services. One of the major problems in OFDM systems is nonlinear distortion caused by high-power amplifiers. In this paper, the effect of non-perfect power amplification on OFDM-CDMA systems is examined. Simulation results show that, with an ideally linearized amplifier, the performance of OFDM-CDMA systems can be improved over an AWGN channel. In general, OFDM-CDMA systems perform better than OFDM systems with non-ideal power amplification, by using a simple code word assignment scheme.

A set of measurements for indoor infrared channel are reported. These measurements have been performed to investigate the effects of rotation on the parameters of the indoor infrared channel. A large database containing more than 5000 frequency responses has been created. The measurements have been performed in rooms with different shapes and physical properties to address all kinds of operating environments. This paper addresses parts of the results obtained from these measurements. The channel path loss and its delay spread are two important channel parameters for indoor infrared channel. The effects of receiver rotation on these two parameters have been studied. It is shown that the delay spread of the channel and its path loss are linearly correlated on the log scale when the receiver is rotated. The correlation between these two variables in different rooms and also different transceiver configurations remains an almost constant value close to unity. Therefore, we conclude that in general, when the receiver rotates, the logarithm of the delay spread of the channel changes linearly with the log of the channel path loss.

This paper presents an efficient method for simulating the performance of multicarrier modulations. The method is based upon the Gauss quadrature rules and offers a significant computational advantage over commonly used Monte Carlo techniques. It takes into account the effects of intersymbol interference, interchannel interference and noise. BER performance curves of representative FFT based multicarrier systems for ADSL and VDSL are obtained using the simulation method described.

The deployment of the local multipoint distribution systems (LMDS) is not too far off in the future. Challenging milestones are now being tackled, such as the design of the hub and end user transmitter amplifier. The design goals placed on this amplifier at 28 GHz make linearization an inevitable choice. Of the published linearization techniques, the feedback+feedforward design shows the most promising solution for an LMDS implementation. This design is adaptive, offers fairly simple implementation, and does not decrease the overall gain of the amplifier as a typical feedback system does. Since designing an amplifier for linearity and gain is difficult at 28 GHz, sacrificing gain will only make problems worse. This technique is a solution to the feedback dilemma and is effective in reducing distortion.

A new correlator receiver architecture based on a modified version of unipolar-bipolar correlation is proposed for noncoherent optical fiber code-division multiple-access (CDMA) networks. For this architecture, the receiver average bit-error rate (BER) performance is numerically evaluated as a function of the received optical power for noncoherent transmission and direct detection with the number of simultaneous users as a parameter. The BER performance is also evaluated by a closed-form formula that is developed in this paper. Comparison of the results from the latter with numerical results show that, the formula provides a good approximation to the system performance. Furthermore, the closed form solution suggests that the system can achieve the same capacity as a CDMA system using coherent detection.

There has been a growing interest in using infrared light for broadband indoor wireless communications. There are two major limitations for establishing a wideband infrared communications link. The first and most important limit is the power requirements of such a link. The second important impairment is the intersymbol interference caused by multipath dispersion. We address the issue of designing an optimized transmission system to provide the best power efficiency for an infrared link. Important parameters that should be considered for this design and some examples of efficient link designs are discussed. The suggested designs can improve the power efficiency of an indoor infrared link up to 4 to 8 dBo while providing a smaller RMS delay spread, more robustness to shadowing and more uniform distribution of power.

Degradation in carrier-to-interference ratio (CIR) due to optical beat interference (OBI) in subcarrier multiplexed wavelength division multiple Access (SCM/WDMA) networks using direct photodetection is evaluated. The subcarriers are assumed to directly intensity modulate separate lasers. A large signal model for the modulated laser light is used. The discrepancy in CIR between this model and the small signal model commonly used in the literature is important at high modulation indices (4.5 dB at about unity modulation index). This justifies the introduction of the large signal model for analyzing OBI. It is found from analytical results that OBI is lower for higher intensity modulation indices.>

A hybrid scheme is proposed for the purpose of suppressing the effects of external modulation and/or laser nonlinearities in subcarrier multiplexing (SCM) fiber optic communications systems. Hybrid CDMA/FDMA combines two schemes in such a way that the resulting hybrid network is robust against interference and is much more spectrally efficient than a CDMA system. Several possible architectures are introduced for the hybrid CDMA/FDMA subcarrier fiber optic local area network (LAN). The networks utilize CDMA and SCM, an asynchronous multiple access scheme. Direct sequence spread spectrum multiple access (DS/SSMA), the most common form of CDMA is employed. It is shown that by using the code sequence sets for which the shift-and-add property holds, intermodulation products (IMPs) and harmonics have a similar interference-like effect as nonmatching sequences do. Owing to the fact that the shift-and-add property holds for conventional spreading sequences, suppression of nonlinear distortions is examined. An average error probability performance evaluation of the selected configuration is presented for a transceiver pair. In analysis of the system, the authors assume the interference term arising from other users is Gaussian distributed. The results are compared to that obtained by exact evaluation of interference distribution using Gauss quadrature rule integration method. The authors compare the performance of this scheme for two different code sequence lengths. They also present some preliminary experimental results on the proposed LAN implementation as well as its measured transmission performance.>

The path diversity improvement inherent within direct sequence spread spectrum (DS/SS) signalling in multipath propagation environments is investigated for mobile/personal radio communications systems that employ DPSK modulation. The bit error rate (BER) performance of post-demodulation selection diversity reception is theoretically analyzed in the presence of noise-only-paths in the time window for diversity combining. Results of laboratory experiments conducted to evaluate the BER performance are also presented. It is shown that the experiment results agree well with the theoretical BER.>

A protocol for a multiuser very high-speed optical data network based on a passive star topology is presented and analyzed. The users' traffic is assumed to be bursty as in the case of computer communications. Tunable transmitters and receivers which are capable of tuning to N+1 relatively stable wavelengths are required to be cross-connected via a passive optical star coupler to the other users of the network. One of the N+1 wavelengths is used to provide the switching information to the receivers and the others are used for actual data transmission. The protocol employs the pure ALOHA random access scheme on the control channel and the slotted-CSMA (carrier sense multiple access). Numerical results for different values of wavelength number, propagation delay time and data packet length show that for large ratios of the length of a data packet to a control packet and for heavy traffic conditions, the protocol achieves a higher throughput and causes much less delay.>

Experimental results on fiber-optic transmission of microwave 64-level quadrature amplitude modulated (QAM) signals at a 1.3 mu m wavelength at a 90 Mb/s transmission rate are presented. Two important methods of improving the system performance are discussed and demonstrated: laser intensity noise minimization and error-correction coding using a self-orthogonal convolutional code. The applicability of this transmission technique to the distribution of digital TV services is assessed.>

The effects of capture on the average system throughput and delay performance of slotted ALOHA were analyzed for slow and fast Rayleigh fading radio channels. A short-range multipoint-to-base station packet radio network is considered. It is shown that larger capture effects and thus improved network performance can be achieved with proper choice of modulation. It is also shown that the use of simple error-correcting codes improves capture. The use of selection diversity also improves the capture effect both for fast and slow fading. It is concluded that the inverse distance variability of the received signal is the main reason for the capture effect. The Rayleigh fading alone yields a very small contribution in terms of throughput; nonetheless, it helps to stabilize the system. Numerical results are presented for a slotted ALOHA system with 50 users. It is found that the maximum average throughput can be increased from about 36% to almost 60% by using channel coding and space diversity.>

The problem of interconnecting many high-speed terminal users via an optical local area network (LAN) is addressed. Space-division multiplexing (SDM) is used to provide point-to-point connectivity, so simple light sources and receivers are all that is required. The call setup between a source and a destination is based on the broadcasting of a short address packet called flooding, which is a simple topology-independent routing method that alleviates the need to have intelligent nodes (cross points). A simple protocol is used to establish an end-to-end path using flooding. Once a source/destination path is established, the actual call starts. The established path is not interrupted by other call setup flooding attempts and/or other cells. A performance analysis for a simple tree network indicates that a capacity of 66% can be achieved at reasonable average blocking delays. The network users can each access full electronics speeds, and the total throughput of the network is a multiple of full electronics speed, with concurrency achieved by SDM.>

We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.

We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.

Direct-sequence spread spectrum with differential phase shift-keying (DPSK) modulation and code-division multiple-access is a promising approach for wireless communications in an indoor environment, which is characterized in this paper by a Rayleigh-fading multipath channel. In this study, we consider two specific channel models having different path-delay distributions and average path power profiles. A star configuration, in which each user exercises average power control in transmitting to a central station, is the basic communication unit, which could be one cell in a cellular hierarchy. We obtain the performance of a single link between a user and its receiver in the central station, and consider two types of diversity, selection diversity and predetection combining to exploit the multipath. A similar system with coherent PSK (CPSK) modulation has been studied previously for one of the channel models considered here. For the same channel model, we show that the irreducible error probability with selection diversity is about half an order of magnitude higher when DPSK is used instead of CPSK. With predetection combining, the performance improves significantly in comparison with selection diversity as the diversity order increases. DPSK modulation with predetection combining is akin to coherent PSK with optimal maximal-ratio combining, but is simpler to implement. The performance with selection diversity for a second channel model, which is based on measurements in an office building, is not significantly different. This indicates that the spreadspectrum approach is rather robust to the path-delay distribution and average path-power profile.

Convolutionally encodedM-ary quadrature amplitude modulation (M-QAM) systems operated over multidimensional channels, for example dual-polarized radio systems, are considered in this paper. We have derived upper bounds on the average bit-error probability for 4QAM (QPSK) with conventional convolutional coding by means of a truncated union bound technique and averaging over the cross-polarization interference by means of the method of moments. By modifying this technique, we have found approximate upper bounds on the average biterror probability for bandwidth efficient trellis-coded QAM systems. This is an extension of our previous work [1] that was based on one dominating error event probability as a performance measure. Our evaluations seem to indicate that bandwidth efficient trellis-codedMQAM schemes offer much larger coding gains in an interference environment, e.g., a cross-coupled interference channel, than in a Gaussian noise channel. In general, our findings point out that optimum codes for a Gaussian channel are not optimum when applied in an interference environment. We note that a rate 1/2 convolutional code for example, with a code memory greater than two, if applied to two of the bits in each signal point representation, can be utilized with a simple decoder to greatly improve the performance of a QAM signal in interference. Also, we have introduced a new concept referred to as dualchannel polarization hopping in this paper which can improve the system performance significantly for systems with nonsymmetrical interference.

The maximum entropy criterion for estimating an unknown probability density function from its moments is applied to the evaluation of the average error probability in digital communications. Accurate averages are obtained, even when a few moments are available. The method is stable and results compare well with those from the powerful and widely used Gauss quadrature rules (GQR) method. For test cases presented in this work, the maximum entropy method achieved results with typically a few moments, while the GQR method required many more moments to obtain the same, as accurately. The method requires about the same number of moments as techniques based on orthogonal expansions. In addition, it provides an estimate of the probability density function of the target variable in a digital communication application.

The performance of cross-coupled,M-ary quadrature amplitude modulation (QAM) systems is determined when bandwidth efficient trellis codes are used to combat interference. Performance with and without compensation for cross-coupled interference is presented. It is found that simple trellis codes can maintain the error probability at an acceptable level for cross-coupling parameters that render uncoded systems unusable. Up to two-dimensional trellis codes are considered for four-dimensional QAM signals, and possibilities of obtaining diversity advantages in the form of higher total system throughput by prolonged availability of the two signals are explored. This is accomplished through joint coding over two different constellations. The probability of the most likely error events is calculated by using the method of moments. The results are applicable to any digital communication system using multidimensional quadrature amplitude modulation, e.g., voiceband modems, cross-polarized radio systems and, to some extent, optical systems. In the paper the analysis is restricted to nondispersive cross-coupling models. In most cases the coding gain is larger than in the absence of cross-coupling interference. Specifically, it is found that simple codes have coding gains increased by at least 2 dB with cross-coupling interference relative to that obtained on the additive white Gaussian noise channel.

For long haul transmissions using frequency-division multiple-access methods, several frequency-modulated carriers are transmitted over a common communications channel consisting of saturating amplifiers operated near saturation to maximize the system power efficiency. One of the problems caused by saturating high power amplifiers is intermodulation distortion, which is the subject of this paper. A simple computation method is introduced which is potentially useful in calculating intermodulation distortion effects of FDM/FM equal power carriers. Based on the results of the study, new and simple empirical expressions for the amplitudes of the single third-order intermodulation products are then presented and compared to previous theoretical and experimental results.