This paper proposes high-order differential edge modulation signaling (DEMS) for high-speed wireline links. We examine the performance of DEMS and present expressions for the symbol error rate in the presence of additive wideband noise. We find that the signal-to-noise ratio and symbol error rate improve for edge modulation relative to traditional amplitude-modulated signaling. Simulations with scaled supply voltages and lossy wireline channels confirm the improved voltage and timing margins of DEMS. We present expressions for the power spectral density of DEMS to illustrate that it is more spectrally efficient than conventional baseband modulation schemes, including pulse-amplitude modulation, alternate mark inversion, and Manchester-encoded signaling.

Accommodating sustained exponential traffic growth in optical networks requires scaling the spatial dimension using space-division multiplexing. Numerous uncoupled spatial channels may be realized by activating multiple parallel fibers or cores in multicore fibers. A multiplicity of uncoupled spatial channels will render the granularity provided by multiple wavelength channels less essential in enabling optical switching. In this paper, we investigate a possible paradigm shift in optical node architectures, in which nodes based on wavelength-selective switching are replaced by those based on simple spatial switching. We compare spatial switching of full-spectrum superchannels to wavelength switching of uncoupled spatial superchannels, considering the evolution of traffic over time. Our results show that spatial switching may achieve more efficient scaling than wavelength switching in roughly 10 to 17 years, depending on the traffic growth rate assumed.

We evaluate the performance of three direct-detection orthogonal frequency-division multiplexing (OFDM) schemes in combating multipath distortion in indoor optical wireless links, comparing them to unipolar M-ary pulse-amplitude modulation (M-PAM) with minimum mean-square error decision-feedback equalization (MMSE-DFE). The three OFDM techniques are DC-clipped OFDM and asymmetrically clipped optical OFDM (ACO-OFDM) and PAM-modulated discrete multitone (PAM-DMT). We describe an iterative procedure to achieve optimal power allocation for DC-OFDM. For each modulation method, we quantify the received electrical SNR required at a given bit rate on a given channel, considering an ensemble of 170 indoor wireless channels. When using the same symbol rate for all modulation methods, M-PAM with MMSE-DFE has better performance than any OFDM format over a range of spectral efficiencies, with the advantage of (M-PAM) increasing at high spectral efficiency. ACO-OFDM and PAM-DMT have practically identical performance at any spectral efficiency. They are the best OFDM formats at low spectral efficiency, whereas DC-OFDM is best at high spectral efficiency. When ACO-OFDM or PAM-DMT are allowed to use twice the symbol rate of M-PAM, these OFDM formats have better performance than M-PAM. When channel state information is unavailable at the transmitter, however, M-PAM significantly outperforms all OFDM formats. When using the same symbol rate for all modulation methods, M-PAM requires approximately three times more computational complexity per processor than all OFDM formats and 63% faster analog-to-digital converters, assuming oversampling ratios of 1.23 and 2 for ACO-OFDM and M-PAM, respectively. When OFDM uses twice the symbol rate of M-PAM, OFDM requires 23% faster analog-to-digital converters than M-PAM but OFDM requires approximately 40% less computational complexity than M-PAM per processor.

Theories of shaping for lattice codes have been developed for systems (optical or non-optical) using coherent detection with additive white Gaussian noise (AWGN) and for direct-detection optical systems with AWGN. This paper considers shaping for amplified direct-detection optical systems in which signal-spontaneous beat noise, a form of signal-dependent noise, is dominant. An A-dimensional (A-D) signal is formed by modulating the intensities(squares of field magnitudes) of a sequence of N time-disjoint pulses. In field magnitude coordinates, signal energy is represented by a L2norm, and the optimal constellation bounding region is the nonnegative orthant bounded by an N-sphere. Under a continuous approximation, as Nrarrinfin, the ultimate shape gain is 1.53 dB and the induced signaling distribution on the constituent 1-D constellation becomes half- Gaussian. In practice, the ultimate shape gain can be approached when the 1-D constellation follows a truncated half-Gaussian distribution. We investigate the tradeoffs between shape gain and increases in constellation expansion ratio or peak-to-average power ratio. We compare our shaping results with those for coherent detection systems and direct-detection optical systems with AWGN.

Intersymbol interference (ISI) due to modal dispersion is the dominant limitation to the bit rate-distance product in multimode fiber-optic communication systems. If the light launched into the fiber excites only the desired principal modes, modal dispersion can be eliminated. We can achieve this by using spatial light modulators (SLMs) to perform adaptive spatial filtering on the electric fields of the light. In this paper, we develop an optimization framework for setting the SLMs to obtain an upper bound on the achievable performance and develop heuristics that nearly reach this upper bound Using this framework, we show that both a sophisticated semidefinite programming-based algorithm and a simple adaptive algorithm achieve performance close to the upper bound. Performance and system complexity tradeoff curves are constructed, showing that a 20/spl times/20 array of SLM pixels with binary phase control performs within 15% of more complex implementations. Finally, we extend the framework and present preliminary results showing the promise of further increases in the capabilities of multimode fiber by using the fiber as a multiple-input multiple-output (MIMO) transmission medium.

We consider free-space optical communication between a distributed collection of nodes (e.g., a distributed network of sensor nodes) and a central base station with an imaging receiver. This paper studies both synchronous and asynchronous reception of the optical signals from the nodes at the imaging receiver. Synchronous reception is done using a symbol-by-symbol maximal-ratio combining technique. We describe a low-complexity asynchronous reception scheme for the uplink that allows the nodes to transmit at a bit rate slightly lower than the frame rate. Since the two rates are nominally different, the scheme is said to be heterochronous. Our heterochronous detection algorithm uses joint maximum-likelihood sequence detection of multiple trellises, which can be implemented using the Viterbi algorithm, as well as the per-survivor processing technique. We develop an approximate upper bound for the average bit-error probability and compare it to Monte Carlo simulation results.

In free-space optical communication links using intensity modulation and direct detection (IM/DD), atmospheric turbulence-induced intensity fluctuations can significantly impair link performance. Communication techniques can be applied to mitigate turbulence-induced intensity fluctuations (i.e., signal fading) in the regime in which the receiver aperture D/sub 0/ is smaller than the fading correlation length d/sub 0/ and the observation interval T/sub 0/ is smaller than the fading correlation time /spl tau//sub 0/. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, but at the cost of high computational complexity. We introduce a single-step Markov chain (SMC) model for the fading correlation and use it to derive two low-complexity, suboptimal MLSD algorithms based on per-survivor processing (PSP). Simulations are presented to verify the SMC model and the performance improvement achieved using these suboptimal per-survivor processing (PSP) algorithms.

Error-control codes can help to mitigate atmospheric turbulence-induced signal fading in free-space optical communication links using intensity modulation/direct detection (IM/DD). Error performance bound analysis can yield simple analytical upper bounds or approximations to the bit-error probability. We first derive an upper bound on the pairwise codeword-error probability for transmission through channels with correlated turbulence-induced fading, which involves complicated multidimensional integration. To simplify the computations, we derive an approximate upper bound under the assumption of weak turbulence. The accuracy of this approximation under weak turbulence is verified by numerical simulation. Its invalidity when applied to strong turbulence is also shown. This simple approximate upper bound to the pairwise codeword-error probability is then applied to derive an upper bound to the bit-error probability for block codes, convolutional codes, and turbo codes for free-space optical communication through weak atmospheric turbulence channels. We also discuss the choice of interleaver length in block codes and turbo codes based on numerical evaluation of our performance bounds.

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. We describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers.

Previous studies have shown that single-user systems employing n-element antenna arrays at both the transmitter and the receiver can achieve a capacity proportional to n, assuming independent Rayleigh fading between antenna pairs. We explore the capacity of dual-antenna-array systems under correlated fading via theoretical analysis and ray-tracing simulations. We derive and compare expressions for the asymptotic growth rate of capacity with n antennas for both independent and correlated fading cases; the latter is derived under some assumptions about the scaling of the fading correlation structure. In both cases, the theoretic capacity growth is linear in n but the growth rate is 10-20% smaller in the presence of correlated fading. We analyze our assumption of separable transmit/receive correlations via simulations based on a ray-tracing propagation model. Results show that empirical capacities converge to the limit capacity predicted from our asymptotic theory even at moderate n = 16. We present results for both the cases when the transmitter does and does not know the channel realization.

We consider the channel capacity of an optical intensity-modulated direct-detection (IM/DD) system using multiple-subcarrier modulation (MSM) with fixed bias. The channel is modeled as an additive white Gaussian noise (AWGN) channel with nonnegative input waveform. The mean of the nonnegative input waveform is the average transmitted optical power. The mean of the waveform during a symbol period is called the d.c. bias of the symbol. In this work, a fixed bias is used for all symbols. Therefore, the power used for each symbol is constant and equals the average transmitted power. The main result of this correspondence shows that, because the input waveforms during each symbol period are nonnegative and have fixed mean, their Fourier coefficients must lie inside certain trigonometric moment spaces. These moment spaces are characterized both algebraically and geometrically. Through the geometrical characterization, we determine the volumes of these moment spaces. The channel capacities of quadrature amplitude modulation (QAM) and pulse amplitude modulation (PAM) systems are shown to be upper-bounded by sphere-packing Gaussian noise in the respective moment spaces.

We consider the channel capacity of an optical intensity-modulated direct-detected (IM/DD) system using multiple-subcarrier modulation (MSM) with fixed bias. The channel is modeled as an additive white Gaussian noise (AWGN) channel with non-negative input waveform. The mean of the non-negative input waveform is the average transmitted power. The mean of the waveform during a symbol period is called the DC bias of the symbol and in this work is fixed for all symbols. Thus, the power used for each symbol is constant and equals the average transmitted power. The main result of this paper shows that, because the input waveforms during each symbol period are non-negative, their Fourier coefficients must form positive semi-definite (p.s.d.) sequences. Furthermore, these sequences are constrained inside the moment space of trigonometric functions. The volume of the moment space is calculated for both PAM and QAM cases. The capacity of each channel is shown to be upper-bounded by sphere-packing Gaussian noise in the respective moment space.

In free-space optical communication links using intensity modulation/direct detection (IM/DD), atmospheric turbulence can cause signal fading. Error-control codes can help to mitigate this fading and improve system performance. To study the performance of various coding schemes, we derive an upper bound on the pairwise codeword error probability for transmission through channels with correlated turbulence-induced fading. Computing this probability involves complicated multidimensional integration. To simplify the computations, we derive an approximate upper bound under the assumption of weak turbulence. The accuracy of this approximation under weak turbulence is verified by numerical simulation. Its invalidity when applied to strong turbulence is also shown. This simple approximate upper bound to the pairwise codeword error will be useful in the study of coding for free-space optical communication through weak atmospheric turbulence channels.

We describe two classes of simple, effective techniques for reducing the average optical power requirement in intensity-modulated optical systems using multiple BPSK or QPSK subcarriers. The first class of techniques involves block coding between the information bits to be transmitted and the symbol amplitudes modulated onto the subcarriers in order to increase the minimum value of the multiple-subcarrier electrical waveform. The second class of techniques involves replacing the fixed DC bias by a bias signal that varies on a symbol-by-symbol basis. These two classes of techniques can be applied separately or in tandem. The reduction in power requirement increases with the number of subcarriers and, with eight subcarriers, can be as high as about 3.6 dB and 3.2 dB with BPSK and QPSK, respectively. The techniques described are applicable as long as all subcarriers originate from a single transmitter and are symbol-synchronized.

We obtain upper bounds on the bit-error rate (BER) for the performance of linear binary block coded optical pulse position modulation (PPM) code-division multiple-access (CDMA) systems with soft-decision decoding. We use a union bounding technique to obtain these bounds, so our results correspond to the upper bounds. We consider systems using binary PPM (BPPM) and an avalanche photodiode (APD), and treat APD noise, thermal noise, and multi-user interference using a Gaussian approximation. We compare the performance of BCH-coded BPPM CDMA systems with soft-decision decoding with that of BCH-coded BPPM CDMA systems with hard-decision decoding. We show that BCH-coded optical PPM CDMA systems using soft-decision decoding can achieve much lower BER than BCH-coded optical PPM CDMA systems using hard-decision decoding.

In free-space optical links using intensity modulation and direct detection, atmospheric turbulence can cause signal fading. Spatial-diversity with multiple receivers can mitigate this fading, improving the system performance. When the instantaneous fading state is not known, the conventional space-diversity detector must employ equal-gain combining (EGC) and simple threshold detection, but the correlation of fading at different receivers can reduce the diversity gain achieved by EGC. We derive expressions for the fading correlation at different receivers by considering a physical model for optical wave propagation. Using this correlation model, we derive the maximum-likelihood (ML) decision rule under the assumption that the fading correlation properties are known, but the instantaneous fading state is not known. We have performed numerical simulations for the dual-receiver case. Our results show that the ML scheme provides better performance than EGC, particularly when the fading at different receivers is highly correlated and/or when the average SNR is high.

We obtain upper bounds on the bit-error rate (BER) for maximum-likelihood (ML) decoding of binary turbo-codes followed by M-ary orthogonal signal mapping through a uniform interleaver. We use the transfer function bounding techniques to obtain the above bounds. Thus, the upper bounds correspond to the average bound over all interleavers of a given length. We apply the techniques to parallel concatenated coding (PCC) schemes where recursive convolutional codes are used as constituent codes. We present the average bounds on the BER of turbo codes with constraint length 3 for M-ary orthogonal signals on additive white Gaussian noise (AWGN) channels. We show that for a given increase in interleaver length, the improvement in BER is larger for turbo-coded 4-ary orthogonal signals than for turbo-coded binary orthogonal signals. We also show that the coding gains of turbo code for M-ary orthogonal signals becomes smaller as M is increased when the block length N is short.

We obtain upper bounds on the bit-error rate (BER) for turbo-coded optical code-division multiple-access (CDMA) systems using pulse position modulation (PPM.) We use transfer function bounding techniques to obtain these bounds, so our results correspond to the average bound over all interleavers of a given length. We consider parallel concatenated coding (PCC) schemes that use recursive convolutional codes as constituent codes. We consider systems using an avalanche photodiode (APD), and treat APD noise, thermal noise, and multi-user interference using a Gaussian approximation. We compare the performance of turbo coded systems with that of BCH-coded systems with soft-decision decoding. We show that turbo-coded systems have better performance than BCH-coded systems.

We describe two classes of simple, effective techniques for reducing the average optical power requirement in intensity-modulated optical systems using multiple BPSK or QPSK subcarriers. The first class of techniques involves block coding between the information bits to be transmitted and the symbol amplitudes modulated onto the subcarriers, in order to increase the minimum value of the multiple-subcarrier electrical waveform. The second class of techniques involves replacing the fixed d.c. bias by a bias signal that varies on a symbol-by-symbol basis. These two classes of techniques can be applied separately or in tandem. The reduction in power requirement increases with the number of subcarriers, and with eight subcarriers, can be as high as about 3.6 dB and 3.2 dB with BPSK and QPSK, respectively. The techniques described are applicable as long as all subcarriers originate from a single transmitter and are symbol-synchronized.

We demonstrate the capacity growth of multiple-element antenna arrays (MEAs) in a realistic propagation environment using WiSE, an experimental ray tracing tool. WiSE is used to construct the channel response for MEAs operating at 1.9 GHz for two situations: (a) (16-16)-MEAs located inside an office building and (b) (4, 4)-MEAs located in an outdoor fixed wireless loop. We define effective degrees of freedom (EDOFs) as parallel spatial modes of transmission for an MEA. We quantify the increase in both the number of EDOFs and capacity with transmit power, received SNR, and antenna spacing. More EDOFs are present when receiving MEAs are physically closer to the transmitting MEA, regardless of the scattering effect.

Wireless systems using multi-element antenna arrays simultaneously at the both transmitter and receiver promise a much higher capacity than conventional systems. Previous studies have shown that single-user systems employing n-element transmit and receive arrays can achieve a capacity proportional to n, assuming independent Rayleigh fading between pairs of antenna elements. We explore the capacity of dual-antenna-array systems via theoretical analysis and simulation experiments. We present expressions for the asymptotic growth rate of capacity with n for both independent and correlated fading cases; the latter is derived under some assumptions about the fading correlation structure. We show that the capacity growth is linear in n in both the independent and correlated cases, but the growth rate is smaller in the latter case. We compare the predictions of our asymptotic theory to the capacities of channels simulated using ray tracing, and find good agreement even for moderate n, i.e., 1/spl les/n/spl les/16. Our results address both the cases when the transmitter does and does not know the channel realization.

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared (IR) communications systems. Multiple transmitter beams and multiple narrow field-of view receivers reduce the path loss, multipath distortion, and background noise of the channel, which leads to improved range. We also discuss practical considerations for multielement angle diversity systems, including channel characterization and suboptimal detection techniques. Maximal-ratio combining provides nearly optimal performance up to 100 Mb/s for the angle diversity systems considered. The design and performance of a prototype angle diversity IR communication system are discussed. The prototype can maintain 70 Mb/s at a P/sub e/ of 10/sup -9/ over a 4-m range.

We analyze the improvements obtained in wireless infrared (IR) communication links when one replaces traditional single-element receivers by imaging receivers and diffuse transmitters by multibeam (quasi-diffuse) transmitters. This paper addresses both line-of-sight (LOS) and nonline-of-sight (non-LOS) IR links. We quantify link performance in terms of the transmitter power required to achieve a bit error rate (BER) not exceeding 10/sup -9/ with 95% probability. Our results indicate that in LOS links, imaging receivers can reduce the required transmitter power by up to 13 dB compared to single-element receivers. In non-LOS links, imaging receivers and multibeam transmitters can reduce the required transmitter power by more than 20 dB. Furthermore we discuss the use of multibeam transmitters and imaging receivers to implement space-division multiple access (SDMA). In a representative example with two users transmitting at a power sufficient to achieve a BER not exceeding 10/sup -9/ with 95% probability in the absence of cochannel interference, when SDMA is employed, the system can achieve a BER not exceeding 10/sup -9/ with a probability of about 88%.

We investigate the effects of fading correlations in multielement antenna (MEA) communication systems. Pioneering studies showed that if the fades connecting pairs of transmit and receive antenna elements are independently, identically distributed, MEAs offer a large increase in capacity compared to single-antenna systems. An MEA system can be described in terms of spatial eigenmodes, which are single-input single-output subchannels. The channel capacity of an MEA is the sum of capacities of these subchannels. We show that the fading correlation affects the MEA capacity by modifying the distributions of the gains of these subchannels. The fading correlation depends on the physical parameters of MEA and the scatterer characteristics. In this paper, to characterize the fading correlation, we employ an abstract model, which is appropriate for modeling narrow-band Rayleigh fading in fixed wireless systems.

While infrared links can support transmission at high bit rates, it is difficult to maintain a signal-to-noise ratio (SNR) sufficient to support these high bit rates under all conditions using reasonable transmitter power levels. Under adverse conditions, instead of incurring abrupt link failure, it is preferable to achieve graceful degradation by lowering the bit rate until a suitably low error probability can be attained. We describe several coding and decoding schemes for rate-adaptive transmission, and we discuss their performance and implementation complexity. The codes considered are rate-compatible punctured convolutional (RCPC) codes and simple repetition codes. Each code is combined with L-ary pulse-position modulation. We present results of a search for families of good RCPC codes derived from rate-1/5 and rate-1/7 mother codes.

Multiple-antenna systems provide very high capacity compared to single antenna systems in a Rayleigh fading environment. Space-time codes are channel codes designed to exploit this high capacity for multiple-antenna systems without requiring instantaneous channel knowledge at the transmitter. A practical concern for high data rate space-time codes is their decoding complexity. The decoding complexity with ML criterion can be prohibitively large. In this paper, we focus on layered space-time (LST) codes. Two types of LST codes, the horizontally-layered space-time (HLST) codes and the diagonally-layered space-time (DLST) codes, are presented. We analyze the performance of both types of LST codes under slow and fast fading conditions. We conclude that, in a slow fading environment, DLST codes have superior performance over HLST codes. The design criteria for DLST codes are proposed.

Article Next century challenges: mobile networking for "Smart Dust" Share on Authors: J. M. Kahn Department of Electrical Engineering and Computer Sciences, University of California, Berkeley Department of Electrical Engineering and Computer Sciences, University of California, BerkeleyView Profile , R. H. Katz Department of Electrical Engineering and Computer Sciences, University of California, Berkeley Department of Electrical Engineering and Computer Sciences, University of California, BerkeleyView Profile , K. S. J. Pister Department of Electrical Engineering and Computer Sciences, University of California, Berkeley Department of Electrical Engineering and Computer Sciences, University of California, BerkeleyView Profile Authors Info & Claims MobiCom '99: Proceedings of the 5th annual ACM/IEEE international conference on Mobile computing and networkingAugust 1999 Pages 271–278https://doi.org/10.1145/313451.313558Online:01 August 1999Publication History 972citation5,666DownloadsMetricsTotal Citations972Total Downloads5,666Last 12 Months122Last 6 weeks18 Get Citation AlertsNew Citation Alert added!This alert has been successfully added and will be sent to:You will be notified whenever a record that you have chosen has been cited.To manage your alert preferences, click on the button below.Manage my AlertsNew Citation Alert!Please log in to your account Save to BinderSave to BinderCreate a New BinderNameCancelCreateExport CitationPublisher SiteGet Access

Dual antenna-array systems provide very high capacity compared to single antenna systems in a Rayleigh fading environment. If the transmitter does not have the channel state information, to utilize this high capacity, space-time codes must be employed. The diagonally-layered space-time (DLST) architecture is a structure that is capable of providing a high data rate for a low decoding complexity. We analyze the performance of DLST codes with a hard decision-feedback decoder and a soft decision-feedback decoder with iterative decoding. We analyze the error probability performance and propose the criteria for designing the constituent codes.

We examine the performance of two decision-feedback equalizers (DFEs) for pulse-position modulation (PPM) on measured nondirected indoor infrared channels with intersymbol interference. PPM offers high average-power efficiency, but on ISI channels, unequalized PPM suffers severe performance penalties. We have previously examined the performance of the maximum-likelihood sequence detector (MLSD), and found that it yields significant improvements. However, the MLSD often requires such large complexity and delay that it may be impractical. We investigate suboptimal, reduced-complexity equalization techniques for PPM, providing a performance analysis of zero-forcing chip-rate and symbol-rate DFEs. Our results show that a symbol-rate DFE provides performance that closely approaches that of the optimal MLSD.

We analyze the performance of trellis-coded pulse-position modulation with block decision-feedback equalization (BDFE) and parallel decision-feedback decoding (PDFD) on indoor, wireless infrared channels. We show that the reduced complexities of BDFE and PDFD as compared to maximum-likelihood sequence detection allow for better codes whose increased coding gain more than compensates for the penalty due to suboptimal detection. We quantify these net gains in performance over a range of dispersive channels, indicating where BDFE and PDFD provide the best performance. Finally, we present Monte Carlo simulation results to verify our analysis.

We examine the use of differential pulse-position modulation (DPPM) for optical communication systems using intensity modulation with direct detection in the presence of additive white Gaussian noise. We present expressions for the error probability and power spectral density of DPPM. We show that for a given bandwidth, DPPM requires significantly less average power than pulse position modulation (PPM). We also examine the performance of DPPM in the presence of multipath intersymbol interference (ISI). We find that the ISI penalties incurred by PPM and DPPM exhibit very similar dependencies upon the channel RMS delay spread. We discuss the use of chip-rate and multichip-rate equalization to combat ISI. Finally, we describe potential problems caused by the nonuniform bit-rate characteristic of DPPM, and we propose several solutions.

Theory of shaping and nonequiprobable signaling, which has been developed for conventional electrical signals, must be modified to treat intensity-modulated (IM) signals. We show that for IM signals, the optimum shape of the constellation bounding region in N-dimensional (N-D) space is an N-D simplex. As N/spl rarr//spl infin/, the maximum achievable shape gain is 1.33 dB (in terms of transmitted power), and the resulting marginal signaling distribution on the one-dimensional (1-D) constituent constellation is exponential. We also investigate the tradeoffs between shaping and its negative consequences, and find that a 1-dB shape gain can be achieved while incurring reasonable increases in peak-to-average power ratio and constellation expansion ratio.

We outline the benefits and challenges of using angle diversity in nondirected wireless infrared communications systems. Multiple transmitter beams and multiple narrow-field-of-view receivers reduce the path loss, multipath distortion and background noise of the channel, which leads to improved range. The design and performance of a prototype angle-diversity infrared communication system are discussed. The prototype can maintain 70 Mb/s at a BER of 10/sup -9/ over a 4 m range.

We report a non-line-of-sight optical link for short-range, indoor data transmission at 25 Mb/s. The optical link uses a ceiling-directed beam and a wide-field-of-view receiver. The system uses 4-pulse-position modulation (4-PPM) with scalar decision-feedback equalization (SDFE), achieving low bit-error rates with a range of two meters in the presence of background light and multipath-induced intersymbol interference. We observe that the gain in electrical SNR efficiency from equalization is about 0.5 dB larger than theory, which we attribute to timing-offset errors in the unequalized system. We verify that the near-DC noise caused by electronic ballast-driven fluorescent lamps has a negligible impact on the system performance.

We present an algorithm for design of a joint source-channel coder using a channel-optimized quantizer and multicarrier modulation. By changing the power of each subchannel in the multicarrier modulation system, different degrees of error protection can be provided for different bits according to their importance. The algorithm converges to a locally optimum system design. Compared to a Lloyd-Max scalar quantizer or a LBG vector quantizer using single-channel transmission, our optimized code can yield substantial performance improvements. The performance improvements are most pronounced at low channel signal-to-noise ratios.

The use of infrared radiation as a medium for high-speed short-range wireless digital communication is discussed. Available infrared links and local-area networks are described. Advantages and drawbacks of the infrared medium are compared to those of radio and microwave media. The physical characteristics of infrared channels using intensity modulation with direct detection (IM/DD) are presented including path losses and multipath responses. Natural and artificial ambient infrared noise sources are characterized. Strategies for designs of transmitter and receivers that maximize link signal-to-noise ratio (SNR) are described. Several modification formats are discussed in detail, including on-off keying (OOK) pulse-position modulation (PPM), and subcarrier modulation. The performance of these techniques in the presence of multipath distortion is quantified. Techniques for multiplexing the transmissions of different users are reviewed. The performance of an experimental 50-Mb/s on-off-keyed diffuse infrared link is described.

We show that realistic multipath infrared channels can be characterized well by only two parameters: optical path loss and RMS delay spread. Functional models for the impulse response, based on infrared reflection properties, are proposed and analyzed. Using the ceiling-bounce functional model, we develop a computationally efficient method to predict the path loss and multipath power requirement of diffuse links based on the locations of the transmitter and receiver within a room. Use of our model is a simple, yet accurate, alternative to the use of an ensemble of measured channel responses in evaluating the impact of multipath distortion.

We analyze the performance of trellis-coded pulse-position modulation (PPM) on indoor, wireless infrared channels. We show that maximum-likelihood sequence detection (MLSD) of trellis-coded PPM is very effective in mitigating the multipath dispersion on such channels. We present code search results for high constraint-length, rate-2/3, 8-PPM and rate-3/4, 16-PPM codes. We provide bit-error rate curves and intersymbol interference power-penalties for MLSD of PPM with these codes, evaluated on ceiling-bounce channel models with delay spread per bit duration ratios of 0.01-0.3. Finally, we present Monte Carlo simulation results to verify our analysis.

We examine systems of fixed-channel reuse for base stations in an indoor infrared wireless communication system. The following techniques are compared: time-division multiple access (TDMA) using on-off keying (OOK) or pulse-position modulation (PPM); frequency-division multiple access (FDMA) using binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK); code-division multiple access (CDMA) using OOK with direct-sequence spreading by m-sequences or optical orthogonal codes (OOCs). We define a parameter /spl gamma/, which equals the signal-to-noise ratio (SNR) for unit optical path gain and is proportional to the square of the transmitted average optical power. Using measured pathloss data, it is found that in a system using hexagonal cells and a reuse factor of three, for cell radii above 3 m, TDMA with OOK or 2-PPM, and CDMA using OOCs all require approximately the same /spl gamma/ to achieve a worst-case bit-error rate (BER) of 10/sup -9/ within a cell. Using TDMA with 4-PPM results in a 6-dB decrease in the required value of /spl gamma/. CDMA using m-sequences requires an increase in /spl gamma/ of 5 dB over TDMA using OOK, and FDMA with BPSK requires an increase of 12 dB. For a given reuse factor N in the noise-limited regime, the required value of /spl gamma/ decreases in inverse proportion to N/sup 2/ for TDMA schemes and inversely with N for FDMA and CDMA schemes. For cell radii below 3 m, cochannel interference dominates the systems using TDMA, FDMA, and CDMA with an OOC, resulting in an irreducible BER above 10/sup -9/ at cell radii below 1.5 m. Only CDMA with m-sequences does not develop an irreducible BER, making it the only choice for cell radii below 1.5 m.

We analyze the performance of trellis-coded pulse-position modulation (PPM) on indoor, wireless infrared channels. We show that maximum-likelihood sequence detection (MLSD) of trellis-coded PPM is very effective in mitigating the multipath dispersion on such channels. We present code search results for high constraint-length, rate-2/3, 8-PPM and rate-3/4, 16-PPM codes. We provide bit-error-rate curves and intersymbol interference power penalties for MLSD of PPM with these codes, evaluated on ceiling-bounce channel models with delay spread per bit duration ratios of 0.02 to 0.3. Finally, we present Monte Carlo simulation results to verify our analysis.

We examine multiple-subcarrier modulation (MSM) schemes for wireless infrared digital communication in the indoor environment. Intensity modulation with direct detection (IM/DD) is employed, which results in equivalent baseband channels with a nonnegativity constraint on the input. The power efficiencies of modulation schemes are compared at 30 Mb/s and 100 Mb/s over an ensemble of experimentally determined multipath channels. Carrier selection and power shaping are examined as methods for improving MSM performance. It is found that MSM schemes can allow operation at higher data rates than single-carrier modulation schemes without equalization. Moreover, MSM schemes can be more bandwidth-efficient and also can provide a simple and flexible method for multiple access to the channel. However, they are not as power efficient as single-carrier schemes, and this will limit their use to applications which are not power limited.

We examine the performance of pulse-position modulation (PPM) on measured channels with intersymbol interference (ISI). We summarize the bit-error-rate performance of unequalized systems and review the performance of maximum-likelihood sequence detection (MLSD) for PPM over ISI channels with additive white Gaussian noise. We evaluate the performance of PPM links over 46 experimentally measured indoor infrared channels. Detailed results are presented for 2, 4, 8, and 16-PPM at bit rates of 10 Mb/s and 30 Mb/s, and these techniques are compared to on-off keying. Our results show that when MLSD is employed, 16-PPM provides the best average-power efficiency among the modulation techniques considered in this study.

We have proposed and analyzed a combined source-channel coding scheme using multicarrier modulation. By changing the power and modulation of subchannels carrying different bits of the quantized signal, the channel-induced distortion can be minimized. We derive an algorithm for the allocation of power among subchannels. Analog sources quantized by scalar and vector quantizers are considered. For a Gaussian source scalar-quantized to 8 bit, multicarrier transmission yields a 13 dB improvement of signal-to-distortion ratio over a single-carrier system. Quantizers having a smaller number of bits yield smaller improvements. We consider both full-search and binary tree-search vector quantizers using a natural binary coding scheme. For a vector quantizer using 2 b/sample, multicarrier transmission yields an improvement in signal-to-distortion ratio that lies between 0.3 and 0.8 dB, depending on the quantizer design.

For a clipped channel with a Gaussian input, a predistortion/restoration technique to reduce clipping-induced nonlinear distortion is proposed and analyzed. The input signal is processed by a nonlinear predistorter circuit, reducing the probability of clipping. The receiver output signal passes through a restorer having an inverse transfer characteristic, which yields the original signal. For both one-sided and two-sided limiter channels, the optimal predistortion curves are determined analytically. A limiter channel with Gaussian input may be used to model clipping-induced nonlinear distortion in optical-fiber common antenna television (CATV) distribution systems using multiple intensity-modulated subcarriers. When applied to an amplitude-modulated vestigial-sideband (AM-VSB) CATV system, the optimal predistortion curves yield sensitivity improvements of 5.3 and 4.4 dB for oneand two-sided limiter channels, respectively.

We report an experimental nondirected optical link for short-range, indoor data transmission at 50 Mb/s. The system uses on-off keying (OOK) and achieves low bit-error rates (BERs) in the presence of intersymbol interference, background light noise, and shadowing, with a range of 2.9 m in a skylit room. The transmitter produces an eye-safe Lambertian pattern at 806 nm with an average power of 474 mW. The receiver utilizes a hemispherical concentrator with a hemispherical bandpass optical filter, a 1-cm/sup 2/ silicon p-i-n photodiode, and a high-impedance hybrid preamplifier to achieve a high signal-to-noise ratio (SNR). A high-pass filter is used to mitigate fluorescent light noise, with quantized feedback removing the resulting baseline wander. A decision-feedback equalizer provides resistance to intersymbol interference due to multipath. The system and its components are characterized, and compared to theory. We observe that decision-feedback equalization yields a reduction of multipath power penalties that is in good agreement with theory.

We investigate 100 Mb/s wireless nondirectional infrared communication in the indoor environment using baseband nonreturn-to-zero (NRZ) on-off keying (OOK) modulation. We show that intersymbol interference induced by multipath propagation impairs detection efficiency. Analytical and simulation results on specific channels demonstrate that an adaptive decision-feedback equalizer adapted according to the least-mean-squares algorithm recovers most of the performance degradation. We also evaluate the performance of a timing-recovery phase-locked loop operating independent of the adaptive equalizer; showing that it quickly and accurately determines the sampling phase with negligible performance degradation. We discuss effective methods of mitigating low-frequency noise induced by fluorescent lighting. We present a packet-based communication method and describe its features and performance. Our results support the feasibility of communication at 100 Mb/s over the infrared channel.>

We have experimentally characterized nondirected indoor channels that use intensity modulation and direct detection of an infrared carrier at a wavelength of 832 nm. At several locations in each of five different rooms, we have studied line-of-sight and diffuse link configurations, with and without shadowing, amounting to a total of approximately 100 different channels. We have measured channel frequency responses over the 2-300 MHz range by using a swept-modulation frequency technique, and from these data, we have computed channel impulse responses, path losses and r.m.s. delay spreads. Using channel impulse responses, we have calculated power penalties induced by multipath intersymbol interference in baseband on-off-keyed links operating at bit rates of 10, 30 and 100 Mb/s, considering unequalized links and those employing zero-forcing decision-feedback equalization. Unshadowed line-of-sight configurations generally have smaller path losses, r.m.s. delay spreads and power penalties than their unshadowed diffuse counterparts. Shadowed line-of-sight configurations, however, generally exhibit larger values of all three parameters than the corresponding shadowed diffuse configurations. We show that among the channels measured here, there is a strong correspondence between channel r.m.s. delay spread and multipath power penalty. Finally, we provide an analysis indicating why non-directed infrared channels using intensity modulation and direct detection do not exhibit multipath fading, and justifying their representation as linear, time-invariant systems.>

We report an experimental 50-Mb/s free-space infrared link for indoor wireless communication, which achieves low bit-error rates (BERs) in the presence of multipath distortion, shadowing, and ambient natural and artificial lighting. The eye-safe 805-nm transmitter, directed at the ceiling, emits an on-off-keyed signal having 475-mW average power. The receiver employs a 1-cm/sup 2/ Si p-i-n photodiode coupled to a high-index hemispherical concentrator, which is covered by an optical bandpass filter of 30-nm bandwidth. A decision-feedback equalizer mitigates the effect of multipath ISI. In the presence of bright skylight, the link achieves a range of 2.9 m at 10/sup -7/ BER.

A recursive method for evaluating the impulse response of an indoor free-space optical channel with Lambertian reflectors is presented. The method, which accounts for multiple reflections of any order, enables accurate analysis of the effects of multipath dispersion on high-speed indoor optical communication systems. A simple algorithm for computer implementation of the technique and computer simulation results for both line-of-sight and diffuse transmitter configurations are also presented. In both cases, it is shown that reflections of multiple order are a significant source of intersymbol interference. Experimental measurements of optical multipath, which help verify the accuracy of the simulations, are discussed.>