Showing papers on "Phase-shift keying published in 1996"

CD3-OFDM: a novel demodulation scheme for fixed and mobile receivers

Abstract: This paper describes a novel channel estimation scheme identified as coded decision directed demodulation (CD3) for coherent demodulation of orthogonal frequency division multiplex (OFDM) signals making use of any constellation format [e.g., quaternary phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), 64-QAM]. The structure of the CD3-OFDM demodulator is described, based on a new channel estimation loop exploiting the error correction capability of a forward error correction (FEC) decoder and frequency and time domain filtering to mitigate the effects of noise and residual errors. In contrast to the conventional coherent OFDM demodulation schemes, CD3-OFDM does not require the transmission of a comb of pilot tones for channel estimation and equalization, therefore yielding a significant improvement in spectrum efficiency (typically between 5-15%). The performance of the system with QPSK modulation is analyzed by computer simulations, on additive white Gaussian noise (AWGN) and frequency selective channels, under static and mobile reception conditions. For convolutional coding rate 1/2, the results indicate that CD3-OFDM allows one to achieve a very fast adaptation to the channel characteristics in a mobile environment (maximum tolerable Doppler shift of about 80 Hz for an OFDM symbol duration of 1 ms, as differential demodulation) and an E/sub b//N/sub 0/ performance similar to coherent demodulation (e.g., E/sub b//N/sub 0/=4.3 dB at bit-error rate (BER)=2/spl middot/10/sup -4/ on the AWGN channel). Therefore, CD3-OFDM can be suitable for digital sound and television broadcasting services over selective radio channels, addressed to fixed and vehicular receivers.

Parallel concatenated trellis coded modulation

Abstract: In this paper, we propose a new solution to parallel concatenation of trellis codes with multilevel amplitude/phase modulations and a suitable bit by bit iterative decoding structure. Examples are given for throughput 2 and 4 bits/sec/Hz with 8 PSK, 16 QAM, and 64 QAM modulations. For parallel concatenated trellis codes in the examples, rate 2/3 and 4/5, 8, and 16-state binary convolutional codes with Ungerboeck mapping by set partitioning (natural mapping), a reordered mapping, and Gray code mapping are used. The performance of these codes is within 1 dB from the Shannon limit at a bit error probability of 10/sup -7/ for a given throughput, which outperforms the performance of all codes reported in the past for the same throughput.

Multiuser ML sequence estimator for convolutionally coded asynchronous DS-CDMA systems

Abstract: The optimal multiuser sequence estimator is formulated for an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system where each user employs convolutional coding to improve its performance on a nondispersive additive white Gaussian noise (AWGN) channel. It is shown that the decoder may be implemented efficiently using a Viterbi algorithm which operates on a time-varying trellis with a number of states which is exponential in the product of the number of users in the system and the constraint length of the codes used (for the rate 1/2 code case). The asymptotic efficiency of this receiver relative to an uncoded coherent binary phase shift keying (BPSK) receiver (termed asymptotic multiuser coding gain, or AMCG) is then upper and lower bounded. The AMCG parameter unifies the asymptotic coding gain parameter and the asymptotic multiuser efficiency parameter which are traditional figure of merit parameters for single-user coded systems and multiuser uncoded systems, respectively. Finally, some simulations are presented to illustrate the performance of the maximum likelihood sequence estimator (MLSE) at moderate and low bit error rates.

Wideband indoor channel measurements and BER analysis of frequency selective multipath channels at 2.4, 4.75, and 11.5 GHz

Abstract: Results from propagation measurements, conducted in an indoor office environment at 2.4, 4.75, and 11.5 GHz, are presented. The data were obtained in small clusters of six measurements, using a coherent wideband measurement system. The channel characteristics for the three frequencies are compared by evaluating path loss, rms delay spread, and coherence bandwidth. An analytical model for evaluation of the bit-error rate (BER) of the stationary frequency selective indoor channel is developed for a coherent binary phase shift keying (BPSK) receiver, based on the complex impulse response of the channel. Computational BER results are obtained for data rates up to 50 Mb/s, using the measured multipath channel impulse responses. The BER results for a number of clusters are presented and compared for the maximum reliable data rate as inferred by the measured rms delay spread of the channel.

FMOD transceivers including continuous and burst operated TDMA, FDMA, spread spectrum CDMA, WCDMA and CSMA

Abstract: Binary and Quadrature Feher's Modulation (F-Modulation, or FMOD) Transmitter-Receiver systems and circuits exhibit reduced envelope fluctuation and peak radiation, and increased efficiency. A subclass of these systems has a constant envelope. They advantageously provide lower power operation with improved performance including robust BER performance, and compatibility with both linearly and nonlinearly amplified narrow spectrum, and without disadvantages of conventional BPSK, DBPSK QPSK and π/4-QPSK. Feher's BPSK (FBPSK) is an improved efficiency transmitter which is compatible with conventional BPSK receivers. FBPSK modems are based on using quadrature structure where Q-channel data is inserted in quadrature with I-channel data for certain applications. The Q-channel data may be "offset" from the I-channel data by an amount selectable between zero and a specified time. Further improvement in the spectrum is attained using correlation between I and Q channels. FBPSK modem is shown to meet the IEEE 802.11 specified spectral direct sequence spread spectrum mask (-30 dB point) for wireless LAN, and leads to an output power gain of 6.5 dB over conventional BPSK modems. The cross-coupled quadrature FMOD structure is also suitable for continuous mode and for burst operated TDMA, FDMA, CDMA, WCDMA and CSMA Frequency Modulation Quadrature AM (QAM), QPSK and offset QPSK, as well as π/4-shifted QPSK modems/processors. Reduced modulation index Gaussian FSK (GFSK), multilevel FM and cross-coupled Quadrature Amplitude Modulated (QAM) transmitters and combinations of these modulations and corresponding coherent demodulators are disclosed. Controlled rise and fall time descriptions of burst operated systems are included.

Variable length burst transmission over the physical layer of a multilayer transmission format

Abstract: Apparatus is provided for communicating data packets in variable length bursts over a physical layer in a multilayer data communication scheme. Each burst contains information data (40, 50, 60, 64, 74, 78, 82) and overhead (30, 32, 34, 36, 38, 42, 44, 62, 66, 76, 80, 84). The overhead includes forward error control (FEC) data (42, 62, 66, 76, 80, 84). Different burst modes (Figs. 2, 3, 4, 5) are provided to enable a trade-off to be made between bandwidth efficiency and data transmission robustness. The burst modes provide different combinations of modulation (such as QPSK and 16-QAM), symbol rates, FEC coding levels and frame and preamble structure. The apparatus is particularly suitable for use in upstream communications over hybrid fiber coax cable television plants.

Transmitter for encoded data bits

Abstract: Quadrature Amplitude Modulated signals are generated from data bits by using a first Quadrature Phase Shift Keying (QPSK) modulator for encoding a first pair of the data bits into one of four carrier signal phases, thereby producing a first QPSK signal. A second QPSK modulator encodes a second pair of the data bits into one of four carrier signal phases, thereby producing a second QPSK signal. The first QPSK signal is amplified to a first power level, and the second QPSK signal is amplified to a second power level. The first and second amplified signals are then combined to produce a signal in which four data bits are encoded. In another aspect of the invention, a new type of modulation, called Offset Quadrature Phase Shift Keying (OQPSK), is used in place of the first and second QPSK modulators, so that an Offset Quadrature Amplitude Modulation (OQAM) transmitter is formed. An OQPSK modulator encodes data bits by encoding a first sub-group of the data bits into a real part of a complex signal at an odd instant of a clock, and by encoding a second sub-group of the data bits into an imaginary part of the complex signal at an even instant of the clock. OQPSK modulation provides the benefit of having all signal transitions being constrained to trajectories around constant radius circles, thereby producing spectral efficiency.

A novel bandwidth efficient coding scheme employing turbo codes

Abstract: We have investigated a novel bandwidth efficient channel coding scheme that has a code structure similar to binary turbo codes but employs Ungerboeck codes as component codes. The combination of turbo codes with powerful bandwidth efficient component codes leads to a straightforward encoder structure and allows iterative decoding in analogy to the binary turbo decoder. However, certain special conditions need to be met at the encoder, and the iterative decoder needs to be adapted to the decoding of the component Ungerboeck codes. The scheme has been investigated for 8 PSK and 16 QAM modulation, for which a first attempt at component code optimization was performed. Simulation results are presented and the scheme is compared with Ungerboeck codes on their own and also turbo codes with Gray mapping. The results show that the novel scheme is markedly superior at comparable complexity.

Orthogonal frequency division multiplexing transmission system and transmitter and receiver therefor

Abstract: A transmission system is disclosed which permits the receiving end to demodulate multi-valued modulated symbols successfully under fading conditions and reduces the amount of transmitted reference data to improve data transmission efficiency. At the transmitting end, a multiplexing section, a modulating section, and a transmitting section are provided. In transmitting an OFDM transmission frame, null symbols and reference symbols are placed in the beginning portion of the frame and QPSK symbols are placed in an information symbol data region in the frame with equal spacings in time and frequency. At the receiving end, a receiving section, a demodulation section, an equalizing section, and a demultiplexing section are provided. An error detector detects amplitude and phase errors of each carrier from the reference symbols, and a variation detector detects variations in the amplitude and phase of a received signal from the QPSK symbols. The carrier amplitude and phase errors are corrected by a correction information producing section on the amplitude and phase variations of the received signal detected by the variation detector to produce correction information. The equalizing section equalizes the demodulated symbol data according to the correction information.

Transmitter antenna diversity and fading-resistant modulation for wireless communication systems

Abstract: The present invention provides a bandwidth-efficient fading-resistant transmission scheme where a base station implements transmitter diversity using L antennas or L carrier frequencies or L time slots, regardless of the use of frame oriented power control. When the antennas or carriers are spaced sufficiently far apart, or when a different power is used for each power control frame, the transmission from each antenna or carrier or time frame undergoes independent fading. These transmissions are coordinated to mitigate the effects of Rayleigh fading and the mobile receiver can recover the entire L-dimensional transmitted vector as long as the signal energy of at least one coordinate is large enough. L-dimensional fading-resistant signal constellations are generated by maximizing a figure of merit for the Rayleigh fading channel. This scheme offers a significant performance improvement over a conventional single-antenna or single-carrier narrowband BPSK scheme when coding is ineffective due to slow fading. When there is background white Gaussian noise, the fading-resistant scheme has a significant energy savings advantage over an uncoded BPSK scheme, for a given bit error rate. In the forward link of a cellular network, where cochannel interference is the dominant source of noise, the fading-resistant scheme results in a significant capacity increase over uncoded BPSK, for a given bit error rate. Both coherent and differentially coherent systems are disclosed.

Frequency-diversity spread-spectrum communication system to counter bandlimited Gaussian interference

Abstract: A spread spectrum system to counter bandlimited Gaussian interference is proposed. The optimum receiver for this system is easy to build. Frequency diversity which allows the receiver to distinguish unjammed signal replicas from their jammed versions is used. The system can also resist bandlimited partial-time jamming. The only choice left to a smart jammer to maximize the error probability is to spread its signal like the communicator. The optimum receiver, which jointly performs symbol detection and interference rejection, is derived. Side information needed by this receiver can easily be estimated. However, if the interference bandwidth is narrow compared to the signal bandwidth, side information on noise and interference levels is not needed by a simpler and near-optimum receiver. Bit-error probability is evaluated for quaternary phase shift keying (QPSK) modulation and compared to that of direct sequence spread spectrum. We also propose the use of polyphase filters for simple system implementation.

Communication signal processors and methods

Abstract: A digital signal processor is provided which is compatible with a large variety of modulation processes (e.g., BPSK, QPSK,π/4 QPSK, M-ary FSK and M-ary PSK). The processor has a transmit section which can convert input data streams into baseband I and Q signals and a receive section which can recover data streams from input baseband I and Q signals. The transmit section includes a direct I/Q modulator and a common phase modulator and the receive section includes an M-FSK to M-PSK converter and a common phase demodulator. The processor is particularly suited for realization as an application-specific integrated circuit (ASIC) which can be integrated in multiband, multimode transceivers.

The error probability of M-ary PSK block coded modulation schemes

Abstract: A tight upper bound on the decoding error probability is derived for block-coded modulation structures where an M-ary phase shift keying (M-PSK) signal constellation is employed. This bound, called a tangential sphere bound, is tight for very low (as well as for high) signal-to-noise ratios (SNRs). Berlekamp's tangential union bound, previously derived for binary codes, can be derived for an M-PSK block coded modulation structure as well. However, it is proven that our tangential sphere bound is tighter than Berlekamp's (1980) tangential bound. For particular schemes, it is shown that for low SNRs our bound is considerably tighter than the tangential bound. As one of the examples, a multistage decoder is considered.

Gigabit/s indoor wireless systems with directional antennas

Abstract: The design of a high speed (>150 Mb/s) wireless local area network (WLAN), requires that many factors be considered, including technical, economic, and regulatory. A major technical factor is the channel response behavior (multipath) in the indoor environment as a function of the frequency band, building type and radio system architecture. The consequences of designing indoor wireless systems with directional antennas at one or both ends of a line-of-sight (LOS) link are investigated. We determine how narrow the beamwidth must be so that the maximum data rate is not limited by multipath effects. For such beamwidths, simple unequalized two-level frequency shift keying (FSK) or phase shift keying (PSK) modems can be used in place of the more costly and complex "anti-multipath" modems, and data rates above 1 Gb/s may be achieved. The channel impulse response in an empty room is estimated using geometrical optics, observing that with directional antennas, multipath rays must arrive from the same direction as the LOS ray. The link outage probability is then estimated as a function of the antenna beamwidth, and guidelines are established for the selection of the frequency band and antenna placement. Experiments using a 19-GHz 622-Mb/s binary phase shift keying (BPSK) link and 15/spl deg/ beamwidth horn antennas in an office building with plaster walls and large metallized windows have demonstrated error-free performance on both LOS and non-LOS (NLOS) links.

Error rate analysis of differentially encoded and detected 16APSK under Rician fading

Abstract: Mobile radio systems require highly bandwidth efficient digital modulation schemes because of the limited resources of the available radio spectrum. A theoretical analysis of bit error rate (BER) is presented for the differential detection of differentially encoded 16-level amplitude/phase shift keying (16DAPSK) under Rician fading in the presence of Rayleigh faded co-channel interference (CCI) and additive white Gaussian noise (AWGN). Differential detection comprises eight-level differential phase detection (DPD) and two-level amplitude ratio detection (ARD). Exact expressions for probability distributions of differential phase noise and amplitude ratio are derived for the BER calculation. The calculated BER performance of 16DAPSK is presented for various values of Rician fading K factor, Doppler spread of diffused component, and Doppler shift of the specular component, and is compared with that of 4-16DPSK. It is shown that 16DAPSK is superior to 16DPSK and requires 1.7 (1.6) dB less E/sub b//N/sub 0/ (SIR) at BER=10/sup -3/ in Rician channels with K=5 dB.

Channel reuse strategies for indoor infrared wireless communications

Abstract: 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), and code-division multiple access (CDMA) using OOK with direct-sequence spreading by m-sequences or optical orthogonal codes (OOCs). TDMA using PPM is found to offer superior performance in terms of complexity and average optical power required to cover a room.

Decision feedback differential detection of differentially encoded 16APSK signals

Abstract: Multiple-symbol differential detection (DD) with reference signal estimation based on the feedback of past detected symbols is presented for differentially encoded 16-level amplitude/phase shift keying (16DAPSK). A suboptimal decision is derived. The approximate analysis of the bit-error rate (BER) performance taking into account the decision error propagation effect is presented in an additive white Gaussian noise (AWGN) channel and the BER performance is compared with those of 16DPSK and 16 quadrature amplitude modulation (16QAM).

On the sensitivity of multicarrier transmission over multipath channels to phase noise and frequency offset

Abstract: The sensitivity of multicarrier (MC) transmission over a multipath channel, to the phase impairment is addressed. The degradation of the performance of the system with the BPSK and QPSK signaling due to the variance of phase error, carrier frequency offset, delay spread of the multipath channel as well as the number of carriers is investigated. The results can be used in the design and evaluation of realistic nonideal multicarrier systems for high speed communications over multipath mobile/indoor radio channels.

Adaptive modulation in a slow Rayleigh fading channel

Abstract: A novel, uneven protection phase shift keying technique is proposed for the encoding of the required modulation scheme in an adaptive modem arrangement, which exhibits an improved performance in comparison to previously proposed schemes. The performance is derived numerically and a system dependent optimisation is presented. A benefit of 5 dB is achieved in a Rayleigh channel.

Blind subspace identification of a BPSK communication channel

Abstract: This paper considers the problem of blind estimation of multiple FIR channels. When a subspace algorithm is applied to the blind identification problem, incorporating information about the symbol constellation is in general not possible. However, by exploiting special properties of one dimensional symbol constellations (BPSK), it is shown that it is possible to improve or simplify a class of algorithms for blind channel identification. It is also shown that in the case of one dimensional symbol constellations there is a third way, apart from multiple antennas and oversampling, of arriving at a multichannel representation of the communication system.

Nonlinear equalization of multipath fading channels with noncoherent demodulation

Abstract: A nonlinear decision-based adaptive equalizer compatible with differentially coherent phase shift keying (PSK) is proposed for frequency-selective fading channels. This equalization scheme is appropriate whenever conventional equalizers are not capable of tracking phase variations in selective fading channels. The received signal is first converted to a baseband signal and then sent through a differential detector. A nonlinear processor before the equalizer generates the needed nonlinear terms that are weighted and summed in the equalizer. Nonlinear intersymbol interference at the output of the differential detector is dealt with by minimizing an error signal between the output of the equalizer and the detected data. The adaptation algorithm can be any algorithm currently used for conventional equalizers. Our simulation results confirm that for channels with spectral nulls, equalization is achieved successfully with the proposed scheme, whereas, linear equalizers, either with coherent or noncoherent detection, fail.

Cyclic spectral features based modulation recognition

Abstract: Modulation recognition of an intercepted communication signal is a fundamental problem of electromagnetic signal monitoring task arising in many fields, such as electronic surveillance and broadcasting control. A cyclic spectral features based neural network modulation recognition method is proposed. Because of the use of cyclic spectral features and the application of neural network classifier, the proposed method can efficiently recognize almost all currently used modulation types. Some computer simulation results are also reported.

Signal structure for multicarrier modulation, which reduces the overhead requirement

Abstract: A transmission system is disclosed which permits the receiving end to demodulate multi-valued modulated symbols successfully under fading conditions and reduces the amount of transmit reference data to improve data transmission efficiency. At the transmitting end, a multiplexing section (101), a modulating section (102), and a transmitting section (103) are provided. In transmitting an OFDM transmission frame, null symbols and reference symbols are placed in the beginning portion of the frame and QPSK symbols are placed in an information symbol data region in the frame with equal spacings in time and frequency. At the receiving end, a receiving section (111), a demodulation (112) section, an equalizing section (113), and a demultiplexing section (117) are provided. An error detector (114) detects amplitude and phase errors of each carrier from the reference symbols, and a variation detector (115) detects variations in amplitude and phase of a received signal from the QPSK symbols. The carrier amplitude and phase errors are corrected by a correction information producing section (116) on the amplitude and phase variations of the received signal detected by the variation detector to produce correction information. The equalizing section equalizes the demodulated symbol data according to the correction information.

Digital modulator with compensation and method therefor

Abstract: In a digital modulator with compensation, a digital bit stream from a microphone (705), a speech coder (706), and a channel encoder (707) is sent to an encoder (720) which translates the bits into I/Q digital pulses with phase shift keying. A symbol correlater (730) notes when predetermined target phase symbol sequences, which contribute to a high peak-to-average power ratio, enter the encoder. If a target symbol sequence is encountered, the correlater directs compensation amplifiers or compensation filters (734, 737) of the I/Q signals. Next, I/Q pulse shaping filters (764, 765) filter the I/Q pulses according to communication system specifications. The filtered I/Q signals are then sent to a quadrature modulator (790) for RF modulation. Compensation of target symbol sequences reduces the negative impact they have on peak-to-average power ratio and increases the efficiency of the power amplifier (795) which extends the battery life of a communication device (700).

Double-filtering receivers for PSK signals transmitted over Rayleigh frequency-flat fading channels

Abstract: This paper describes novel detection algorithms for coded and uncoded M-ary phase shift keying (M-PSK) signals transmitted over frequency-flat fading channels. Fading distortion is estimated with per-survivor methods using the sampled outputs of two receive filters. Some of the proposed algorithms do not require information on the fading statistics and are dubbed blind algorithms. The error rate performance of blind and nonblind algorithms is assessed by simulation for uncoded and trellis-coded phase shift keying (PSK) and is compared to the performance of other detection schemes proposed in the literature.

A novel CDMA multiuser interference cancellation receiver with reference symbol aided estimation of channel parameters

Abstract: A novel multistage successive interference cancellation scheme is proposed which operates on the reverse code-division multiple-access (CDMA) link using binary phase shift keying (BPSK) modulation, coherent detection and reference symbols to obtain channel estimates. The results of a single cell multiuser investigation demonstrate increased traffic capacity. However, this increase is initially very moderate due to corruption of the channel estimates by interference from symbols not yet demodulated and cancelled. A modification in the transmitted signal structure addressing this problem is proposed. The results of a single cell analysis of the modified cancellation scheme demonstrate that the system's traffic capacity reaches approximately 80% of that of a multistage successive interference cancelling receiver operating with the perfect knowledge of channel parameters. A subsequent multicell investigation shows that for a hexagonal cell geometry with a path loss exponent of four and without any forward error correction (FEC) coding or cell sectorization, the system capacity compares very favorably with that of the IS-95 system, which employs powerful error control coding. Capacities with other path loss exponents and cell geometries are also investigated. The results show a substantial traffic capacity increase over that of a comparable receiver without interference cancellation.

N‐branch diversity reception of mary DPSK signals in slow and nonselective nakagami fading

Abstract: The problem of diversity reception of M-ary differential phase shift keying modulated, signals in the presence of additive white Gaussian noise and Nakagami-distributed slow and non-selective fading is addressed. N-branch selection diversity and maximal ratio combiners are considered. The average symbol error rate is analytically evaluated in terms of integral expressions, which can be easily computed via numerical integration routines. Numerical results are presented that allow one to point out those operational situations where diversity systems can operate successfully to mitigate the degrading effects of short-term fading.

Error probability for block codes over channels with block interference

Abstract: A methodology is presented to evaluate analytically the average decoded error probability for block codes over block interference channels. The proposed analysis is based on the knowledge of the moments of the bit error probability over the fading channel, thus allowing, e.g., fast performance evaluation of block coded slow frequency hopping (SFH) systems over Rayleigh fading channels.

Differential detection with IIR filter for improving DPSK detection performance

Abstract: This paper presents a method for improving the performance of differential detection of differentially encoded phase shift keying (DPSK). The structure of the proposed detection scheme consists of a conventional differential detection circuit equipped with an infinite impulse response (IIR) filter combined with decision feedback. The results of theoretical analysis and computer simulation show that performance of the proposed detection method can approach that of coherent detection of DPSK under additive white Gaussian noise (AWGN) conditions without any increase in architectural complexity. Furthermore, a parameter of the proposed detector can be varied to optimize performance for static or fading conditions. An adaptive scheme suitable for time varying Rician-fading channel conditions is presented, and performance results obtained by computer simulation are given.

Error compensation via signal correlation in high-precision closed-loop fiber optic gyros

Abstract: Fiber optic gyroscopes (FOGs) are preferably driven as closed-loop controlled systems, if linearity and dynamic range are of major concern. Proper modulation of the Sagnac interferometer (SIF) feedback signal is necessary to minimize low frequency signal perturbation and to reliably detect luminance intensity in the linear regions of the sinusoidal Sagnac phase to intensity mapping. Deterministic modulation however, is accompanied by well known 'dead zones' and bias errors due to unavoidable crosstalk between the modulator and the optical detector. In the paper we propose a high precision closed-loop FOG system with deadbeat control and pseudorandom modulation of the SIF feedback signal. The random modulation principle completely eliminates 'dead zones' in the detection of small rotation rates, and bears an inherent potential for compensation and control of several error sources encountered in non-ideal systems by means of signal correlation. The principle of correlation based control is introduced in a general context and applied to a set of dedicated control loops within the proposed closed-loop FOG. Results obtained form several prototype realizations of the correlation controlled high precision FOG indicate a potential for bias error reduction by two orders of magnitude and considerable decrease in random walk.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.