Institute for Systems Research

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Author: search.filters.author.Geraniotis, Evaggelos A.End date: 1989

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Now showing 1 - 10 of 26
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    Multireception Probabilities for FH/SSMA Communications.
    (1989) Geraniotis, Evaggelos A.; ISR
    Exact expressions for the probabilities P (I,m-l1k) of I correct packet receptions and m-l erroneous ones, out of total k packets contending in a slot, are presented for the case of frequency- hopped spread-spectrum random-access slotted networks employing random frequency hopping pattems. These expressions are difficult to evaluate numerically for values of m > 3. However, our numerical analysis indicates that under light traffic conditions these probability values are very close to the ones provided by the independent receiver operation assumption, under which, the distribution of multireception obeys the binomial law.
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    Performance Evaluation of Multi-Access Strategies for an Integrated Voice/Data CDMA Packet Radio Network.
    (1989) Soroushndad, M.; Geraniotis, Evaggelos A.; ISR
    The problem of voice/data integration in a random access radio network employing the ALOHA protocol in conjunction with recursive retransmission control is investigated. Code-division multiplexing (CDMA) is used as a suitable modulation in a radio environment to decrease the effect of multiple-access interference. Multi-access control strategies are introduced that take advantage of the multipleaccess capability of the CDMA channel to accommodate several voice calls simultaneously, while the data users contend for the remaining (if any) multiple-access capability of the CDMA channel. The retransmission probabilities of the backlogged data users are updated based on estimates of data backlog and number of established voice calls which are obtained from the side information about the state of channel activities. A two-dimensional Markovian model is developed for the voice and data traffic, with the data backlog and number of established voice calls representing the state of the system. Based on this model, the voice-call blocking probability, the throughput of both traffic types, and the delay of the data packets are evaluated and the tradeoffs between the parameters of different traffic types are quantified.
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    Analysis of Coherent Random-Carrier CDMA and Hybrid WDMA/CDMA Multiplexing for High-Capacity Optical Networks.
    (1989) Ghaffari, Behzad; Geraniotis, Evaggelos A.; ISR
    In 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 frequeng 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 multiplexing 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 (AWGN) are taken into account in the computation. Both synchronous and asynchronous systems are analyzed in this context. The analysis is valid for any spreading gain and any number of interfering users and makes very limited use of approximations. The performance evaluation of RC CDMA establishes the potential advantage in employing hybrids of WDMA (wavelength- division multiple-access) and CDMA multiplexing to combat inter- carrier interference in dense WDMA systems.
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    Optimal Quantization and Fusion In Multi-Sensor Systems for the Detection of Weak Signals in Dependent Noise.
    (1989) Chau, Yawgeng A.; Geraniotis, Evaggelos A.; ISR
    Two problems of memoryless quantization and data fusion for the detection of a weak signal in stationary dependent noise are addressed: (i) fusion from sensors with mutually independent observations across sensors but dependent across time and (ii) fusion from sensors with correlated observations across time and sensors. For each problem, we consider four distinct schemes (a) fusing the test statistics formed by the sensors without previous quantization (b) quantizing suboptimally each observation and then fusing, (c) quantizing optimally each observation and then fusing, and (d) quantizing optimally each test statistic of the sensors and then fusing the observation sequence of each sensor consists of a common weak signal disturbed by an additive stationary m-dependent, f-mixing or p-mixing noise process. To guarantee high-quality performance, a common large sample size is employed by each sensor. Design criteria are developed from the Neyman-Pearson test in the fusion center for the optimal memoryless sensor test statistics and the sensor quantizer parameters (quantization levels and breakpoints); these design criteria are shown to involve an extension of the asymptotic relative efficiency used in single-sensor detection and quantization. Numerical results in support of the analysis are given for the case of dependent p=mixing Cauchy noise.
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    Distributed Detection From Multiple Sensors with Correlated Observations.
    (1989) Chau, Yawgeng A.; Geraniotis, Evaggelos A.; ISR
    We address two problems of memoryless distributed dependent observations across time and sensors. In the first problem, the observation sequence of each sensor consists of common weak signal in additive dependent noise with stationary univariate and second-order joint densities; here the objective of the sensors is to cooperatively detect the presence of a weak signal. In the second problem, the observation sequence of each sensor is characterized by its stationary univariate and second-order pint densities; here the objective of the sensors is to cooperatively discriminate between two arbitrary such sequences of observations. For both problems, the analysis and design are based on a common large sample size. The dependence acms time and sensors is modeled by m-dependent, f-mixing, or p-mixing processes. The performance of the two-sensor configuration for each problem is measured by an average cost, which couples the decisions of the sensors. The design criteria for the test statistics of the sensors, which consist of sums of memoryless nonlinearities, are established by using two-dimensional Chemoff bounds on the associated error probabilities involved in the average cost. The optimal nonlinearities are obtain as the solutions of linear coupled or uncoupled integral equations. Numerical results for specific cases of practical interest show that the performance of the proposed scheme is superior to the one that ignores the dependence across time and/or sensors for each of the two problems.
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    Distributed Estimation of a Location Parameter in Dependent Noise.
    (1989) Chau, Yawgeng A.; Geraniotis, Evaggelos A.; ISR
    We address the problem of distributed estimation from dependent observations involving two sensors that collect observations of the same nonrandom location parameter THETA in additive noise. We consider two cases of interest, the case of independent observations across sensors and the case of correlated observations across sensors. The estimation schemes of the sensors are chosen so as to minimize a common cost function consisting of the weighted sum of the mean square errors of the estimates from the two sensors and the mean square of their difference. The observations of the two sensors are modeled as two MU - dependent or PHI mixing sequences. The correlation between the two observation sequences is also characterized by an p-dependent or PHI mixing sequence. Because high-order statistics of dependent observations are generally difficult to characterize, maximum-likehood estimates may be impossible to derive or implement; instead, suboptimal estimates which use memoryless nonlinearities g_k (DOT) (i.e. nonlinear functions of observations,) for k = 1,2, are employed by the two sensors. With this structure in each sensor, minimizing the above cost function with respect to the estimates is equivalent to minimizing it with respect to the nonlinearities g_k (DOT), which results in linear integral equations. If we solve these integral equations, we obtain optimal nonlinearities within this suboptimal scheme. Examples for m - dependent Cauchy noise are provided in support of our analysis.
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    Optimal and Robust Memoryless Discrimination from Dependent Observations.
    (1989) Sauder, D.; Geraniotis, Evaggelos A.; ISR
    In this paper we consider discrimination between two possible sources based on observations of their output. The discrimination problem is modeled by means of a general binary hypothesis test, the main emphasis being on situations that cannot be modeled as signals in additive noise. The structure of the discriminator is such that the observations are passed through a memoryless nonlinearity and summed up to form a test statistic, which is then compared to a threshold. In this paper we consider only fixed sample size tests. Four different performance measures, which resemble the signal-to-noise ratios encountered in the signal in additive noise problems, are derived under different problem formulations. The optimal non-linearities for each of the performance measures are derived as solutions to various integral equations. For three of the four performance measures, we have successfully obtained robust nonlinearities for uncertainty in the marginal and the pint probaWlity density functions of the observations. Computer simulation results which demonstrate the advantage of using our non-linearities over the i.i.d. nonlinearity under the probability of error criterion are presented.
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    Broadcast Capability of Direct-Sequence and Hybrid Spread Spectrum.
    (1989) Geraniotis, Evaggelos A.; Ghaffari, Behzad; ISR
    Two forms of spread-spectrum signaling: direct-sequence and hybrid (direct-sequence/ frequency-hopped) are shown to provide high broadcast capability especially when used in conjunction with forward-error-control coding schemes. The broadcast capability is defined as the maximum number of simultaneous distinct messages that can be transmitted to distant receivers from a single transmitter at a given bit-error-rate. This quantity provides a useful measure of the capacity of hub-to- mobile or satellite-to-earthstation links of communication networks. When bursty data or voice traffic is dominant in such networks, the above forms of spread-spectrum code-division multiple-access (CDMA) provide a viable alternative to frequency- division (FDMA) or time-division (TDMA) multiple-access.
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    Throughput and Packet Error Probability for Cellular Direct- Sequence and Hybrid Spread-Spectrum Radio Networks.
    (1988) Geraniotis, Evaggelos A.; Soroushnejad, M.; ISR
    In this paper multiple-access interference is characterized for cellular mobile networks, in which users are assumed to be Poisson-distributed on the plane and to employ direct-sequence or hybrid (frequency-hopped/direct-sequence) spread-spectrum signaling with a transmitter-oriented assignment of signature sequences and/or frequencyhopping patterns. Approximations for the bit- and packet-error probabilities are derived for data modulation schemes like binary phase-shift-keying (BPSK) with noncoherent demodulation and forward-error-control codes (like Reed-Solomon codes). In all cases, the effect of varying interference power of the desired signal and the other interfering signals and of Rayleigh nonselective channel fading is taken into account accurately. The throughput in the mobile- to-base transmission mode is then evaluated for the aforementioned data modulation, demodulation, and forward-error- control coding schemes. Furthermore, a comparison of the performance of frequence-hopped, direct-sequence, and hybrid spread-spectrum signaling schemes with the same bandwidth is carried out, to show that, under the varying interference power model, the frequence-hopped system performs best among them.
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    Multiple-Access Capability of Frequency-Hopped Spread-Spectrum Revisited: An Exact Analysis of the Effect of Unequal Power Levels.
    (1988) Geraniotis, Evaggelos A.; ISR
    In this paper we present a new method for the evaluation of the probability of error of uncoded frequency-hopped spread-spectrum multiple-access (FH/SSMA) communications. For systems with binary FSK modulation the method provides an accurate approximation and a tight upper bound to the bit error probability; for systems with m-ary FSK modulation it provides tight upper bounds to the symbol error probability. This method relies on the integration of the product of the characteristic function of the envelope of the branch of the BFSK demodulator, which carries the desired signal, and of the derivative of the characteristic function of the envelope of the other branch; it can achieve any desirable accuracy and the computational effort required for its evaluation grows linearly with the number of interfering users. In the M-ary case tight upper bounds based on the union bound and the results of the binary case are derived. The new method allows us to quantify accurately the effect of unequal power levels on the otheruser interference for the first time. Comparison of the multiple-access capability of FH/SS systems as predicted by the bounds available in the literature and by the new method indicates that FH/SS systems without error-control can support (at a given error rate) considerably many more simultaneous users than previously thought when the relative received powers of the users are not significantly different. This trend is amplified further for systems with error-control. Our results indicate that the FH/SSMA systems also suffer from the near-far problem although less seriously than the direct-sequence SSMA systems.