Institute for Systems Research
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Item Optimal Strategies for Admitting Voice and Data Traffic in Networks of Leo Satellites Using CDMA(1995) Geraniotis, Evaggelos A.; Chang, Yu-Wen; Yang, Wen-Bin; ISR; CSHCNEfficient policies are derived for admitting voice and data traffic into networks of low-earth-orbit (LEO) satellites using code-division multiple-access (CDMA) with direct-sequence spread- spectrum (DS/SS) signaling. The satellites act as bent-pipes; no on-board processing or inter-satellite links are present. Dual satellite diversity is used to mitigate the effects of shadowing. The policies assume a movable boundary form, allocate optimally the CDMA capacity (PN codes) to voice and data users, and can increase significantly the number of users served while satisfying their bit error rate (BER) requirements. A modified version of our policies can handle two classes of data users: one with high priority which requires real-time delivery and another with low priority that can be queued; the BER requirements of the two data types may differ.Item Voice, Data, and Video Integration for Multi-Access in Broadband Satellite Networks(1993) Ghaffari, Behzad; Geraniotis, Evaggelos A.; ISR; CSHCNMulti-media integration of broadband services in a broadband satellite network is considered. Voice, data, video teleconferencing, and television with broad range of service (bit) rates are multiplexed through a broadband satellite, channel in a multiple-access fashion. Large (but finite) population sizes are considered with arrivals modeled by binomial distributions. A two-state minisource model is used for voice signals. For video, variable rate interframe coding is utilized to reduce the bandwidth requirements, and Markov phase processes model the modulation of the rates of the video teleconferencing and television signals.Among these services, video and voice are real-time signals and can not tolerate large random delays. In our attempt to satisfy this, video and voice use the Synchronous Transfer Mode (STM) with a frame structure, while the data users (with their bursty traffic) send (and retransmit, if necessary) their packets randomly within a frame. The video and voice users make their schedules in advance by using a pre- assigned slot (status slot). The first portion of a frame is assigned to the variable rate video users, while the variable rate voice users fill up the last portion of the frame. Data packets fill up the remaining slots between these two movable boundaries in a random-access fashion. In this protocol, the delay introduced by the satellite is taken into consideration. This multiple-access integration protocol is optimized with respect to performance measures, such as the blocking probabilities for voice and video, the average delay for data, and the average throughput for voice, video, and data.
Item Comparison of Coherent WDMA and Hybrid WDMA/CDMA for the Multiplexing of Optical Signals(1992) Ghaffari, Behzad; Geraniotis, Evaggelos A.; ISRIn this paper, we provide an accurate analysis of the performance of coherent dense wavelength-division multiple-access (WDMA) schemes introduced for use in high-capacity optical networks. In our analysis, the effects of interference from other signals due to the frequency overlap caused by the instability of the carrier frequency of laser, or to mistakes in frequency coordination and assignment, are taken into account. Phase noise and thermal noise are also taken into consideration. Dense WDMA is then coupled with spread-spectrum direct-sequence modulation in order to mitigate the effect of interference from other signals. The performance of this hybrid of WDMA and code-division multiple- access (CDMA) scheme is also analyzed and compared to that of pure WDMA.The average bit error probability of dense WDMA and WDMA/CDMA schemes is evaluated in integrating the characteristic function of other-user interference at the output of the matched optical filter. Gaussian approximation techniques are also employed. Time-synchronous and as asynchronous systems are analyzed in this context. Binary phase-shift-keying (BPSK) data modulation is considered. Our analysis quantifies accurately for first time the multiple-access capability of dense WDMA schemes and the advantages offered by employing hybrids of WDMA and CDMA.
Item Analysis of Coherent Random-Carrier Code-Division Multiple- Access for High-Capacity Optical Networks(1992) Ghaffari, Behzad; Geraniotis, Evaggelos A.; ISRIn this paper we provide an accurate analysis of the performance of a random-carrier (RC) code-division multiple-access (CDMA) scheme recently introduced for use in high-capacity optical networks. According to this scheme coherent optical techniques are employed to exploit the huge bandwidth of single-mode optical fibers and are coupled with spread-spectrum direct-sequence modulation in order to mitigate the interference from other signals due to the frequency overlap caused by the instability of the carrier frequency of the laser, or to the mistakes in the frequency coordination and assignment.The average bit error probability of this multiple-access scheme is evaluated by using the characteristic function of the other-user interference at the output of the matched optical filter. Both phase noise and thermal noise are taken into account in the computation. Time- Synchronous as well as asynchronous systems are analyzed in this context. Binary phase-shift-keying (BPSK) and on-off-keying (OOK) data modulation schemes are considered. The analysis is valid for arbitrary values of the spreading gain and the number of interfering users. The performance evaluation of RC CDMA established the potential advantage in employing hybrids of wavelength-division multiple-access )WDMA) and CDMA to combat inter-carrier interference in dense WDMA systems.
Item Multi-Sensor Correlation and Quantization in Distributed Detection Systems(1991) Chau, Yawgeng A.; Geraniotis, Evaggelos A.; ISRQuantization and fusion schemes are derived for multi-sensor correlation in distributed K- sensor systems that are used for the detection of weak signals or general signal discrimination from dependent observations. The dependence in the observations across time and sensors is modeled via stationary m - dependent, f - mixing, or r - mixing processes. The observation sequences of the various sensors have identical individual statistics and identical pairwise statistics (symmetric conditions). Each sensor observation is passed through a memoryless non-linearity or quantizer (the same one for all sensors) to form the sensor test statistic; the decision statistics of the various sensors are then passed to the fusion center in an unquantized or binary quantized manner to form the final decision statistic of the fusion center. Based on a common large sample size for each sensor that is necessary for achieving high-quality performance, an asymptotic analysis is applied for the error probabilities of the fusion center. This provides design criteria for the optimal memoryless nonlinearity and quantizer. Optimization of these design criteria yields the optimal nonlinearity or quantizer as solutions to linear integral equations involving the first - and second-order pdfs of the sensor observations describing the individual and pairwise dependence. the analytical results obtained are valid for any number of sensors K. Numerical results based on the simulation of the performance of our schemes with different number of sensors are presented. The performance of the optimal nonlinearities and quantizers is shown to outperform that of nonlinearities or quantizers obtained by ignoring the dependence in sensor observations and to improve as the number of sensors increases.Item Quantization and Fusion for Multi-Sensor Discrimination from Dependent Observations(1991) Chau, Yawgeng A.; Geraniotis, Evaggelos A.; ISRSchemes for quantization and fusion in multi-sensor systems used for discriminating between two sequences of dependent observations are introduced and analyzed. The observation sequences of each sensor under the two hypotheses are arbitrary stationary dependent sequences that can not be modeled as signal in additive noise; the objective of the fusion center is to discriminate between the two hypotheses. These observation models are well motivated by practical multi-sensor target discrimination problems. Two cases are considered: in the first, the observation sequences of the sensors are individually dependent but jointly mutually independent; in the second case, the observation sequences are dependent across both time and sensors. The dependence in the observations across time and/or sensors is modeled by m - dependent, j - mixing or r - mixing processes. The following four quantization/fusion schemes are considered: (a) forming test statistics at the sensors by passing the observations through memoryless nonlinearities, summing them up, and fusing these test statistics without previous quantization; (b) quantizing uniformally (with equidistant breakpoints) each sensor observation and then fusing; (c) quantizing optimally each sensor observation and then fusing; and (d) using the sensor test statistic of (a) to make binary decisions and then fusing the binary decisions. To guarantee high-quality performance, a common large sample size is employed by each sensor and an asymptotic analysis is pursued. Design criteria are developed from the bayesian cost of the fusion center for deriving the optimal memoryless nonlinearities of the sensor test statistics and the sensor quantizer parameters (quantization levels and breakpoints). These design criteria are shown to involve an extension of the generalized signal-to-noise ratio used in single-sensor detection and quantization. The optimal nonlinearities and quantizers are obtained as the solutions of linear coupled or uncoupled integral equations involving the univariate and bivariate probability densities of the sensor observations. Numerical results based on simulation are presented for specific cases of practical interest to compare the relative performance of the four quantization/fusion schemes described above and to establish their superiority to schemes that ignore the dependence across time and/or sensors in the observations.