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A soft iterative multisensor array receiver for coded multiuser wideband code-division multiple-access wireless uplink channels is proposed, such channels are typically both frequency- and time-selective. A new equivalent discrete-time synchronous representation is used to model asynchronous multiuser dispersive channels that employ, in general, random spreading. The proposed scheme suppresses multiuser interference over a wide range of user signal powers, by iteratively exchanging soft information between a minimum mean-square-error (MMSE) multiuser demodulator and a bank of single-user decoders, feeding back the outputs of the latter to aid in soft multiuser multisensor MMSE-RAKE demodulation and subtractive interference cancellation, in the second and subsequent iterations. It displays near-far resistance since it behaves like a successive interference subtracter across iterations. The array responses are obtained via a multipass estimation scheme that uses both (temporal) pilot symbols and soft estimates fed back from the decoders to effectively provide multisymbol pilot signals and thence successively refined estimates with increasing iterations, while seeking neither to rely on the array geometry nor to estimate the directions from which users' signals arrive at the receiver. Simulation studies indicate that this scheme performs close to the single-user case with a two-sensor receiver array, and perfect channel state information, after four iterations; alternatively, it allows significantly increased user capacity compared with conventional receivers, and suffers only a modest loss with estimated array responses.

This paper introduces a code-division methodology into switching applications. The proposed method is applied in satellite-switched code-division multiple-access (SS/CDMA) systems for routing CDMA traffic channels on board the multibeam satellites. We present code-division switch (CDS) architectures, analyze the CDS performance, and assess its complexity. The CDS has been shown to route CDMA user channels without introducing interference. The proposed CDS architecture is nonblocking, and its hardware complexity and speed are proportional to the size of the switch. We also examine the amplitude distribution of the combined signal in the CDS bus and the interference evaluation of the end-to-end link in the proposed applications. Then we consider the problem of switch control under an optimum or a random algorithm and compare its complexity with the equivalent problem in time-multiplexed switching methods.

A new iterative receiver for joint detection and decoding of code division multiple access (CDMA) signals is presented. The new scheme is based on a combination of the minimum mean square error (MMSE) criterion and the turbo processing principle by Hagenauer (see Proc. Int. Symp. Turbo Codes and Related Topics, Brest, France, p.1-9, 1997). The complexity of the new scheme is of polynomial order in the number of users. The new scheme is applicable to two situations: (a) when the receiver is capable of decoding the signals from all users and (b) when the receiver is only capable of decoding the signals from a subset of users. In the first scenario, we establish that the proposed receiver achieves superior performance to the iterative soft interference cancellation technique under certain conditions. On the other hand, in the second scenario, we argue that the proposed receiver outperforms both the iterative soft interference canceler and the iterative maximum a posteriori (MAP) receiver because of its superior near-far resistance. For operation over fading channels, the estimation of the complex fading parameters for all users becomes an important ingredient in any multiuser detector. In our scheme, the soft information provided by the decoders is used to enhance this estimation process. Two iterative soft-input channel estimation algorithms are presented: the first is based on the MMSE criterion, and the second is a lower-complexity approximation of the first. The proposed multiuser detection algorithm(s) are suitable for both terrestrial and satellite applications of CDMA.

Satellite-switched code-division multiple access is a system proposed for geostationary fixed satellite service. SS/CDMA provides both multiple access and switching to a multibeam satellite. In this article we present the system architecture and performance. The SS/CDMA consist of the common air interface (CAI) and the onboard code switching mechanism. The CAI provides signaling control and traffic channels. Traffic channel access and modulation is based on a spectrally efficient CDMA scheme. The satellite code-division switch (CDS) routes calls from an uplink to a downlink beam without onboard demodulation and channel decoding. This system operates with demand assignment control; that is, channel bandwidth and switch connections are only assigned per user request. The system can offer a wide variety of bit rates with wireline quality of service. The services provided are circuit-switched calls for voice data and video, and packet-switched for data. As shown, the proposed SE-CDMA can achieve high capacity and very low bit error rates at low signal-to-noise ratio. Also, the innovative switching technique can provide low complexity and achieve high throughput for all services.

A commonly used method of calculating TDOA and FDOA is the cross ambiguity function (CAF). The narrowband model assumes that the Doppler is constant over the bandwidth of the signal. When time companding, caused by relative motion, becomes large enough, the CAF performance is degraded. One method of compensation is to resample the signal, but this can be computationally intensive, especially if there is a wide range of time companding. A second method is to reduce the time the CAF is computed over, but the processing gain is reduced. If the short-term CAFs are recombined after the time companding has been compensated, much of the processing gain can be recovered. In this paper, three novel methods of recombining short-time CAFs are developed and analyzed. The first method is to estimate the TDOA and FDOA for each short-time CAF. The second method is to compensate and noncoherently add the CAFs together. This improves the peak detection over the first method but not the TDOA and FDOA accuracy. The third method is to compensate for both time and phase and then coherently add the short-time CAFs together. This last method improves the FDOA accuracy over the first two methods. The effects of sampled data and frequency dual are analyzed. Simulation results are provided to verify analytical results.

We consider the application of multicarrier modulation in a wireless cellular network in order to enable high-data rate communication and alleviate the multipath induced intersymbol interference (ISI). In this scenario, power control becomes crucial in enhancing the spectral and power efficiency. A conventional approach of maintaining the same link quality for all the subchannels, in other words, disregarding any possible post-demodulation processing, is considered first. This approach appears to have increasing power consumption as the number of subchannels increases. It also deteriorates the power control stability and convergence properties in a multicell network. We attribute this phenomenon to lack of frequency diversity exploitation, and thus, we propose to use channel coding and soft decoding as vehicles to profit from the (frequency) diversity advantage in addition to the coding advantage. Based on the soft decoding performance bound, a power allocation and control algorithm is proposed. It is shown through simulations that the proposed algorithm improves the power efficiency as the number of subchannels increases. It also provides a better convergence property and is able to "detect" and eliminate ill-conditioned subchannels. The advantages of using multicarrier modulation are thus reassured. Besides these enhancements, the proposed algorithm is simple and feasible in that it consists of only the traditional closed-loop power control algorithm and a target signal-to-interference ratio (SIR) reassignment at the receiver. Detailed channel information feedback from receiver to transmitter is not required.

The paper examines protocols for voice/data integration in hybrid packet radio networks consisting of satellite and terrestrial components. The same protocols can be useful in the context of mixed-media packet radio networks employing HF ground radio links and EHF SATCOM radio links. As part of the integration protocol the ground subnetwork employs code-division multiple-access with movable boundary in the code domain to serve both the voice traffic and the retransmitted data traffic; on the satellite subnetwork framed ALOHA with movable boundary is used for both data and voice. A complete analysis of the channel-access protocols for both traffic types is provided based on a multi-dimensional Markovian model of the integrated voice/data system. The performance of the hybrid network is evaluated in terms of voice and data throughput, voice blocking probability, and data delay. It is observed that appropriate splitting of the retransmission traffic between the satellite and ground subnets may increase the overall data throughput when the voice load is high. >

Multiple-access schemes are introduced and analyzed for the integration of voice and data traffic in packet radio networks using code-division multiple-access (CDMA). The multiple-access capability of the CDMA channel is used to accommodate several voice calls simultaneously, while the data users follow the ALOHA protocol with retransmission control and contend for the remaining (if any) multiple-access capability of that channel. The retransmission probabilities of the backlogged data users are

We provide methods to evaluate the probabilities P(l, m-l|K), l=0, 1, ..., m and m/spl les/K of exactly l correct packet receptions in a group of m receivers, given that K packets are transmitted simultaneously from users employing direct-sequence spread spectrum (DS/SS) signalling schemes. This quantity is useful for the design and performance evaluation of protocols for admission control and dynamic code allocation in multiple-access spread spectrum packet radio networks intended for terrestrial or satellite applications. The evaluations are carried out for DS/SS networks employing BPSK modulation with coherent demodulation and convolutional codes with Viterbi decoding. Systems with geographically dispersed receivers and systems with colocated receivers are considered. Approximations based on the independent receiver operation assumption (IROA) and the Gaussian multivariate distribution are developed, and their accuracy is checked against the exact expressions derived for synchronous systems. The joint first error event approximation (JFEEA) is also developed for coded systems and compared to the IROA. >

The authors derive optimal admission policies for integrated voice and data traffic in packet radio networks employing code division multiple access (CDMA) with direct-sequence spread spectrum (DS/SS) signaling. The network performance is measured in terms of the average blocking probability of voice calls and the average delay and packet loss probability of data messages. The admission scheme determines the number of newly arrived voice users that are accepted in the network so that the long-term blocking probability of voice calls is minimized. In addition, new data arrivals are rejected if the mean delay or the packet loss probability of data exceeds a desirable prespecified level. A semi-Markov decision process (SMDP) is used to model the system operation. Then, a value iteration algorithm is used to derive the optimal admission control. Two models for the other-user interference of the CDMA system are considered: one based on thresholds and another based on the graceful degradation of the CDMA system performance, and their performance is compared. These admission policies find application in emerging commercial CDMA packet radio networks including cellular networks, personal communication networks, and networks of LEO satellites for global communications. >

Detectors based on one-step memory nonlinearities are introduced. Problems of discrimination between two arbitrary stationary m-dependent or mixing sequences of observations and problems of detecting a weak signal in additive stationary m-dependent or mixing noise are considered in this context. For each problem, the nonlinearity is optimized for performance criteria such as the generalized signal-to-noise ratio (SNR) and the efficacy, and is obtained as the solution to an appropriate linear integral equation. The schemes considered can be made robust to statistical uncertainties determined by two-alternating capacity classes, for the second-order joint PDFs (probability distribution functions) of the observations, and by bounds on the correlation coefficients of time shifts of the observation sequence for the third- and fourth-order joint PDFs. Evaluation of the performance of the schemes via simulation reveals significant gains over that of detectors employing memoryless nonlinearities or the independent and identically distributed nonlinearity. >

An adaptive multichannel radiometer designed to detect frequency-hopping (FH) signals in complex signal environments is presented. Each channel updates its hop threshold to reflect the current environment and excise any persistent hop activity inconsistent with an FH signal from subsequent processing. This strategy allows the receiver to discriminate FH signals from any random noise or interference activity with relatively small degradations as compared to operation in stationary additive white Gaussian noise. Two data collection schemes are considered for the proposed receiver, both of which attempt overall decisions using fixed-length blocks of data. In the first scheme, block detection, successive decisions are based on consecutive, nonoverlapping blocks of data, whereas in the second, block-sequential detection, decisions are made each time a new datum is collected. The block-sequential scheme is shown to offer greatly reduced average signal detection times and, thus, is the preferred approach. >

Exact expressions for the probabilities P(l,m-l/k) of l 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 patterns. These expressions are difficult to evaluate numerically for values of m>3. However, their 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. >

Discrimination is considered between two possible sources based on dependent 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 observations are modeled as stationary m-dependent or rho -mixing processes. The structure of the discriminator is such that the observations are passed through a memoryless nonlinearity summed up to form a test statistic, which is then compared to a threshold. Only fixed sample size tests are considered. 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 nonlinearities for each of the performance measures are derived as solutions to various integral equations. For three of the four performance measures the authors have successfully obtained robust nonlinearities for uncertainty in the marginal and the joint probability density functions of the observations. Computer simulation results that demonstrate the advantage of using these nonlinearities over the i.i.d. nonlinearity under the probability of error criterion are presented. >

The specific problem of detecting the presence of either binary-phase-shift-keyed (BPSK) signals or BPSK direct-sequence spread-spectrum (DS/SS) signals with a prefilter-delay-and-multiply (PFDM) device is considered. Using stationary process theory and Fourier analysis, the optimum PFDM structures for signal presence detection of BPSK signals with known bit rates and carrier frequencies and BPSK DS/SS signals with known chip rates and carriers in additive colored Gaussian noise are derived. The structures are optimum in the sense that they maximize the spectral signal-to-noise ratio (SNR) of an output periodic waveform which has fundamental frequency equal to the bit or chip rate of the signal. Two of the optimum structures that are derived and analyzed are the optimal prefilter-square device and the optimal PFDM with delay set to one half of the signal's bit or chip duration. >

The author characterizes and evaluates the effect of simultaneous multiple partial-band noise or tone jammers and other user interference on a single communication link employing frequency-hopped spread-spectrum (FH/SS) signaling, M-ary frequency-shift keying (FSK) modulation with noncoherent demodulation, and Reed-Solomon coding. For the symbol error probability of these systems, the author derives exact expressions in the absence of multiple-access interference and tight upper bounds in the presence of other-user interference. Although the analytical methods are valid for any number of multiple jammers, the numerical study is restricted to the cases of two and three-partial-band noise and tone jammers. For fixed values of the spectral densities of noise jammers, or the energies per symbol of tone jammers, the worst-case fraction of the band that each jammer should use in order to maximize the error probability of the FH/SS or FH/SSMA system is evaluated. For the range of the signal-to-jammer power ratios examined, multiple-noise or multiple-tone jammers appear to have no advantage over single-tone jammers of equivalent spectral density or energy per symbol, but achieve approximately the same worst-case performance by jamming smaller fractions of the band. >

Two forms of spread-spectrum signaling, namely 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. The quantity provides a useful measure of the capacity of hub-to-mobile or satellite-to-earth-station links of communication networks. When bursty data or voice traffic is dominant in such networks, the above forms of spread-spectrum CDMA provide a viable alternative to FDMA or TDMA. Ways of multiplexing the direct-sequence and hybrid signals are presented that use, respectively, distinct carriers, distinct pairs of orthogonal carriers, and only two orthogonal carriers for broadcasting the different messages. Systems with chip-synchronous signals and systems with random delays between the signals are considered. The average error probability of all systems is evaluated using the characteristic-function and Gaussian-approximation techniques. Besides the uncoded systems, systems using Reed-Solomon and convolutional codes are analyzed. A comparison of the broadcast capability of the different schemes is presented. >

The problem of designing matched filters that are robust against uncertainty in the statistics of a noise process modeled by two alternating Choquet capacities is addressed. The robust design is based on the maximum signal-to-noise ratio. The problem is formulated and solved for both discrete-time and continuous-time matched filters with uncertainty in either the autocorrelation function or the spectral measure of the noise. Explicit solutions that are characterized by the Hyber-Strassen derivative of the capacity generating the class with respect to a Lebesgue-like measure on a suitable interval are obtained. >

Minimax robust data fusion schemes for multisensor detection systems with discrete-time observations characterized by statistical uncertainty are developed and analyzed. Block, sequential, and serial fusion rules are considered. The performance measures used, and made robust with respect to the uncertainties, include the error probabilities of the hypothesis testing problem in the block fusion case and the error probabilities and expected numbers of samples or sensors in the sequential and serial fusion cases. For different sensor observation statistics, the minimax robust fusion rules are derived for two asymptotic cases of interest: when the number of sensors is large and when the number of times the fusion center collects the local (sensor) decisions is large. Moreover, for the case of identical sensor observation statistics and a large number of sensors, it is shown that there is no loss in optimality, if local tests using likelihood ratios and equal thresholds are used in the sequential fusion rule. In all situations, the robust decision rules at the sensors and the fusion center are shown to make use of likelihood ratios and thresholds that depend on the least-favorable probability distributions of the uncertainty class describing the statistics of sensor observations. >

We consider the performance analysis of generalized selection combining (GSC) for BPSK signals in time-correlated Rayleigh fading channels. For the coherent detection of BPSK signals, we employ pilot symbol aided modulation (PSAM) to provide channel side information (CSI) and the optimum GSC follows the maximal signal to noise ratio (SNR) criterion. To analyze the bit-error-rate (BER) of GSC for BPSK signals, we propose a novel method based on the Monte-Carlo integration technique to circumvent the high computational complexity occurred in the evaluation of analytical BER expression.

A soft iterative multisensor array receiver for coded dispersive multiuser code division multiple access (CDMA) wireless uplink channels is proposed. The proposed scheme suppresses multiuser interference over a wide range of user signal powers, by iteratively exchanging soft information between a minimum mean square error (MMSE) multiuser demodulator and a bank of single-user decoders, feeding back the outputs of the latter to aid in soft multiuser multisensor MMSE-RAKE demodulation and subtractive interference cancellation. It displays remarkable near-far resistance, since it functions like a successive interference subtracter across iterations. The array responses are also obtained using soft-decoded feedback estimates and (temporal) pilot symbols, seeking neither to rely on the array geometry nor to estimate the directions from which users' signals arrive at the receiver. Simulation studies indicate that this scheme performs remarkably close to the single-user case with a two-sensor receiver array, and perfect channel state information, after four iterations; alternatively, it allows significant overload capability compared with conventional receivers, and suffers only a modest loss with estimated array responses.

Iterative multiuser detection in coded CDMA uplink channels is based on the iterative exchange of soft information between a (multiuser) demodulator and a bank of single-user soft decoders to mutually benefit them. From a practical perspective, using the MMSE multiuser interference suppressor for demodulation yields good performance over a wide range of received signal powers. However, the progressively increasing correlation between estimates (with an increasing number of iterations) limits the improvement that can be obtained via further iterations, implying that the suboptimality of the MMSE demodulator plays a role in the performance levels that can be achieved. An immediate enhancement of the demodulator's performance is obtained by introducing multiple sensors at the receiver; the resulting space-time demodulator now operates on received signal mixtures with components whose spatial separation is accounted for, by virtue of their different directions of arrival. Since this separation is limited in practice, by restrictions on sensor count, array geometry, and the receiver's inability to control arrival directions, a multisensor block modulation scheme is proposed. This scheme transmits cyclically block-shifted versions of bit sequences from additional sensors using different random spreading sequences, and exploits the variation in the channel response between various transmission instances of a given bit. An iterative multiuser channel state estimator is also developed. Performance studies indicate significant reductions in the decoded bit error rate-up to that of a single user at moderately low received signal powers-and hence vastly increased spectral efficiencies.

Existing narrowband jammer suppressors for DS-CDMA, typically function independently of the multiuser interference cancellation strategy used. Besides, certain high-performance jammer suppression techniques, e.g., linear MMSE suppression, assume that the powers and frequencies of the interfering signals are known, or that they can be adaptively estimated. The former is never true in practice and the latter is not feasible when long spreading codes are used. The present work eliminates these restrictions, by applying the idea of iterative MMSE multiuser interference suppression, introduced earlier by the authors, to this context. The first iteration in this procedure is effectively MMSE multiuser interference cancellation; the parameters of the jammers are unknown at this stage. The outputs of all users' soft decoders (applied to the demodulator outputs of this iteration) are then fed back to subtract their (estimated) cumulative contribution from the received signal vector; the residue comprises the narrowband jammers embedded in wideband noise, so that the former can be piecewise interpolated over chip intervals of appropriate duration, and thence subtracted from the received signal for the succeeding iteration. The soft-decoded feedback estimates of the interfering signals are also used in each iteration to perform soft MMSE multiuser interference suppression, in conjunction with subtractive interference cancellation. The approach is extended to exploit the enhanced separation (in the spatial sense) that is afforded by employing multisensor receivers.

An iterative minimum mean square error (MMSE) cochannel interference suppressor is proposed for coded narrowband time division multiple access (TDMA) uplink channels. This scheme discriminates the desired user from interferers, and the latter from each other, based on the differences in their propagation channel responses which serve as their spatial signatures. Assuming a protocol that provides for synchronisation between cochannel base stations, each receiver decodes its desired user and interferers (that are served by neighbouring cochannel receivers) whose channel responses can be estimated. It feeds these soft-decoded values back to aid (in the next iteration) a channel state estimator and a multiuser demodulator in MMSE suppression and subtractive cancellation of these interferers. The fact that this scheme displays the successive cancellation property across iterations, enables it to exploit the near-far nature of cochannel interferers in a narrowband cellular system. Practical realisations of this technique depend on the accuracy of the channel estimation procedure; with sufficiently diverse and accurately estimated channel responses, almost universal frequency reuse seems possible.

The performance of inhibit sense multiple access (ISMA) in a wireless CDMA environment for short burst data service with multiple codes is investigated. Data users are allowed to transmit a short message up to several frames in each burst in a contention manner using the reverse common channels. Several such channels are allocated for this data service. Data users contend for the traffic channels using either the ISMA or slotted Aloha protocol. The capture probability with multiple-codes is derived. Simulations are used to compare the performance of the protocols. The results show that for this short data messages scenario ISMA has a higher throughput than slotted Aloha. A slightly different version of ISMA is also proposed to reduce the access delay for real-time messages.

A new iterative receiver for joint detection and decoding of code division multiple access (CDMA) signals is proposed. The new scheme is based on the minimum mean square error (MMSE) criterion. The new scheme is applicable to two situations (a) when the receiver is capable of decoding the signals from all users, and (b) when be receiver is only capable of decoding a subset of the users, either due to limited processing power or the unavailability of information about the other users. For operation over the AWGN channel we establish that the proposed receiver achieves a much better performance compared to other iterative receivers in highly loaded networks.

The performance of inhibit sense multiple access (ISMA) in a wireless CDMA environment for short burst data service is analyzed.. Data users are allowed to transmit short messages (up to several frames at a time) in a contention manner using the reverse common channels. In this paper, we assume that one such common traffic channel is allocated for this data service. Data users contend for the traffic channel using either the ISMA or slotted Aloha protocol. Markovian analysis is given and the throughput as well as the delay are computed assuming a fixed number of voice users in the system. Numerical results show that for this short data message scenario ISMA has a higher throughput than slotted Aloha.

This work presents a soft iterative minimum mean square error (MMSE) multiuser interference suppressor for coded direct sequence code division multiple access (CDMA) uplink channels that employ space-time processing at both the mobile transmitters and the base station receiver. With a new equivalent discrete-time synchronous model for randomly spread asynchronous multiuser CDMA channels, we develop an iterative scheme that exchanges soft information between an MMSE multiuser demodulator (that also concurrently performs subtractive interference cancellation) and a bank of single-user decoders. It performs well both at low and high received signal powers, and displays the successive cancellation property across iterations, exhibiting good near-far resistance. Introducing multisensor arrays at the receiver relaxes the limitations imposed by the inherent suboptimality of MMSE multiuser demodulation, in so far as it enhances interuser separation, now in the spatial sense, and thereby allows significant overload capability. Further, this work explores new territory by studying multisensor transmission in multiuser channels. A new multisensor block modulation scheme called space-time spreading, for mobile transmitters is proposed; it is observed to provide significant benefits especially when the spatial separation afforded by the use of multisensor receiver arrays, is otherwise limited.

A general spectrum-analysis-based approach to the problem of detecting the presence of unknown weak signals in noisy observations is developed. The weak signal assumption enables exploitation of familiar results from the theory of local optimality, from which correlation-based detectors for the presence of known weak signals in additive independent and identically distributed (IID) noise have been derived. The development here follows from the perception that a spectrum analyzer is nothing more than a number of correlators in parallel. The approach has application to any observations that are not IID Gaussian processes. Some practical situations treated here, in which signal presence detection might be better accomplished using the proposed approach, are unknown weak signals: (1) in additive non-Gaussian interference, (2) in additive colored Gaussian noise, and (3) at the output of a nonlinear system, such as a square-law device. Items (2) and (3) required the extension of correlations-based locally optimum detection theory to cover observation processes that are not necessarily signals in additive noise. >

The authors present an adaptive multichannel radiometer designed to detect frequency hopping (FH) signals in complex signal environments. This is accomplished by having each channel update its hop threshold to reflect the current environment and excise any persistent activity inconsistent with an FH signal from subsequent processing. This strategy allows the receiver to discriminate FH signals from any random noise or interference activity with relatively small degradations as compared to operation in additive white Gaussian noise. Two data collection schemes are considered for the proposed receiver, both of which attempt overall decisions using fixed-length blocks of data. In the first scheme, 'block detection', successive decisions are based on consecutive, nonoverlapping blocks of data, while in the second, block-sequential detection, decisions are made each time a new datum is collected. The block-sequential scheme is shown to offer greatly reduced average detection times. >

In this paper, the tradeoff between call blocking and rate dropping in multicasting networks is analyzed. Specifically, we study the differences in network performance under the following two scenarios: admitting more high class users and admitting more low class users. Closed-form expressions for the two end-to-end performance measures are given. However, since the blocking probability requires prohibitive computational efforts even for small-size networks, an accurate and time-efficient approximation is developed through the reduced load method. The results of this paper can be used to coordinate the access bandwidths of the receivers to maximize the throughput of the network.

This paper considers the optimal reverse-link power control law for multimedia multirate DS/CDMA systems which offer services with different transmission rates (VBR or CBR) and quality of service (QoS) requirements. In the multimedia multirate CDMA system, the transmission power of a mobile station (MS) is proportional to a power control law, which is a function of its distance to the base station (BS), the transmission rates and interference level (number of active users). Each transmitted signal from the MS will be affected by path loss, Rayleigh fading and log-normal shadowing in the reverse link. The bit error probability of the asynchronous multirate CDMA system that takes the MS locations into account is derived first in order to derive the optimum power control functions. By applying the optimal power control functions to mobile stations, the QoS (bit error probability) requirements can be met, the system capacity (number of simultaneously transmitted users) can be maximized, and the power consumption can be minimized. Since the difficulty of deriving the optimal power control law in a closed form, we discretize the power control function so that dynamic programming (DP) can be utilized to obtain the optimal solution. The numerical results show that the optimal power control law can increase the system capacity and decrease the power consumption compared with other power control laws (eg. equal received bit energy) in the multimedia multirate CDMA system.

The use of adaptive arrays in a multi-rate multi-media CDMA network is analyzed, and simulation results are provided for a candidate two-rate system. Our proposed approach uses reference signal-based adaptation (LMS) for antenna weight control. Simulation results show adaptive arrays can significantly enhance the multi-media services that can be provided by a CDMA network. In addition, preliminary simulation results for RLS and LMS weight control algorithms are presented.

Outage probabilities for CDMA hard and soft handoffs are derived to compare their cell coverage areas. Rayleigh fading and log-normal shadowing are included in the propagation model. Signal-to-interference-plus-noise ratio is used to compute the outage probabilities for any mobile location distributions. In the soft handoff system, mobiles choose the base station with the least attenuation among nearest neighbors. Coverage areas for reverse and forward links and different diversity combining techniques are considered.

We develop a maximum likelihood optimum demodulation scheme for direct optical detection of PPM signals used in airborne/underwater communications. We use the irradiance of an optical pulse propagating through clear air and sea water derived using radiative transport theory. This hybrid optical channel is characterized by intersymbol and interslot interference. >

Circuit-switched networks of video users with different grades of service are considered. The authors consider admission control schemes that result in an increase to network throughput at the expense of some fraction of the information (rate) transmitted by the video sources being dropped. The end-to-end probability of blocking video requests and the associated probability of rate dropping of video sources are evaluated along each network path for the above control schemes and for an arbitrary circuit-switched network architecture. Instead of the exact expressions, which require prohibitive computational effort, the stochastic knapsack approximation and the Monte Carlo summation method are employed. >

Perceptron neural networks are applied to the problem of discriminating between two classes of radar returns. The perceptron neural networks are used as nonlinearities in two threshold sequential discriminators which act upon samples of the radar return. The neural network's training phase eliminates the impractical task of estimating high-order probability density functions when designing a discriminator; consequently discriminators with memory are easily obtained. The discriminators using neural networks for their nonlinearities significantly outperform the optimal memoryless discriminators of Geraniotis (1989). The discriminators constructed with neural networks made no classification errors in 10000 trials from each hypothesis. These discriminators also used a significantly smaller expected number of samples to make their decisions than did known discriminators. >

Assuming that a binary phase-shift-keyed (PSK) waveform plus Gaussian noise is input to a delay-and-multiply or squarer, a memoryless locally optimum scheme for detecting the generated rate line is reported. The locally optimum formulation is used because the rate line is a signal in non-Gaussian noise, and it is typically weak. It is found that if the double-frequency output of a bandpass squaring detector is used, the locally optimum spectral technique outperforms standard spectral analysis. >

This paper presents throughput evaluation of a satellite switched code division multiple access (SS/CDMA) system which operates under demand assignment control. The SS/CDMA provides both multiple access and switching to a geostationary multibeam satellite. Multiple access is resolved by space, frequency and code division. Space division is introduced by the multibeam antennae that provide frequency reuse in each beam. The spectrum is then channelized into frequency bands where each band is accessed by code division for both the uplink and the downlink. The satellite on-board performs the switching function which is also based on compatible code multiplexed switching. The switch may route both circuit calls and data packets which are assigned upon request. The on-board code division switch operates under the control of a channel assignment algorithm. We provide channel assignment algorithms for optimum, sub-optimum and random switch operation. Computer simulation has been used to evaluate the performance of the three algorithms. It is shown that their circuit call blocking probabilities are almost the same. The optimum algorithm achieves the minimum data packet delay, while the performance of the sub-optimum algorithm is slightly better than the random one. Furthermore, data packets may be routed via the switch with small delay, even with heavy load of circuit calls.

We present a linear-complexity fast-convergence blindly adaptive space-time processing which maximizes the received signal to interference and noise ratio (SINR) in wideband direct sequence code division multiple access (W-CDMA) networks. Compared to the other famous array processing algorithms, the proposed blind space-time processing technique outperforms the least mean square (LMS) algorithm. Both the aforementioned algorithms operate at the same order of complexity and, furthermore, the proposed one need no training sequence or feedback training. The convergence speed of the low-complexity algorithm almost competes which of the recursive least square (RLS) algorithm which is well-known for its fast convergence by using a training sequence and high computational complexity.

We investigate the application of joint decoding and channel estimation for DS/CDMA transmission over slow Rayleigh fading channels. Some known symbols are inserted in the encoded data stream to enhance the channel estimation process. An iterative algorithm that uses the decoding information, in addition to the information contained in the known symbols, to improve the channel parameters estimate is proposed. A is shown that the optimum scheme has a complexity which grows exponentially with the channel estimation filter length. Hence, we propose some alternative sub-optimum schemes with polynomial complexity. The different techniques that provide a tradeoff between simplicity of implementation and BER performance are compared.

In this paper, we present an overlapped channel transmission (OCT) multiple access scheme which increases the uplink capacity of the slow FHMA systems. By using antenna array techniques, subscribers can transmit data through time-frequency overlapped slots and the proposed system increases the spectrum efficiency. A practical system that uses 2-fold OCT to increase system capacities with the same total bandwidth are proposed along with a fast processing and numerically stable beamforming algorithm and VLSI architecture. The beamforming weights can be calculated in one symbol's duration and thus coherent combining can be achieved to further increase the beamforming performance.

We examine the applicability of symbol-aided demodulation (SAD) to Rayleigh and Rician fading channels, which are the channels most frequently encountered in terrestrial mobile communications and satellite communications, respectively. The effects of filtering and interpolation on the bit error rate (BER) performance of the SAD system are evaluated. The BER comparisons for optimum Wiener filtering and linear interpolation are carried out. The BER of SAD systems in the presence of Rician fading are compared with those of the fully coherent and differentially coherent schemes. It is established that SAD performance is close to that of perfectly coherent demodulation and significantly better than differentially coherent demodulation for a wide range of system and channel parameters.

We evaluate the capacity of the reverse link of a synchronous FH/SSMA multi-cell network in the presence of partial band jamming. A set of orthogonal frequency hopping (FH) patterns is assigned to the users in each cell. Cells are differentiated by concatenating the user FH pattern with a shuffling sequences of the same hop rate. The use of guard times enables synchronous operation for the range of hopping rates of interest. The capacities of the above FH/SSMA network with either coherent BPSK modulation or non-coherent BFSK modulation are first evaluated for the AWGN channel. The effect of power control errors on the system performance is accounted for in each case. Then the effects of Rayleigh fading are factored in. Similar results for asynchronous DS/SS cellular network (reverse link) are also reported for comparison purpose.

The BER of DS/CDMA systems that use symbol-aided demodulation (SAD) over Rayleigh and Rician fading channels is evaluated. Known symbols are transmitted with a certain frequency together with the data symbols and are used by the receiver to extract a phase reference for demodulation. Different schemes for filtering of the known symbols to obtain the phase reference needed for demodulation are considered and BER comparisons are made. The BER of these SAD DS/CDMA schemes is also compared with that of fully coherent and differentially coherent DS/CDMA systems operating over Rician and Rayleigh fading channels. The important role of filtering in SAD for DS/CDMA networks is studied in detail and it is shown that under some channel and system conditions (such as terrestrial mobile with low data rates) optimum (Wiener) filtering is needed while for other channel and system conditions (such as GEO satellite with higher data rates) simpler low pass filters are sufficient.

In this paper, we present a blindly adaptive beamforming algorithm which is based on the second order interference estimation to maximize the received SINR. Using the desired signature code and the code orthogonal to it, a new code filter is introduced to decompose the received signals into two parts: desired information and interference. The above method motivates us to develop the QR-decomposition based dominant eigenmode search (QRD-DMS) algorithm which is more numerically stable than the DMS one. The corresponding wavefront systolic architecture is also proposed for the VLSI implementation. Compared to the families of minimum mean square error (MMSE) algorithms which need training sequences, we have completed the maximum SINR families by proposing the QRD-DMS method which not only blindly updates the beamforming weights but converges as fast as the QRD-RLS method.

A joint frequency compensation and angle estimation array processing technique is proposed to combat the broadband jammers and the performance drop during the hop-to-hop transient time in slow frequency hopping multiple access (FHMA) networks. We use the phase shifting technique, which is based on the direction of arrival (DOA) estimation, to modify the data matrices before the hop happens. A good beamforming weight is thus obtained during the transient period and also helps to find the arriving angle at the end of this hop. The DOA is obtained cheaply in the process of power iterations but as accurately as the costly subspace method. By using these techniques, we can rescue the data at the beginning of each dwell time and allow faster hopping to escape from following jammers. The method also has a good property: as the broadband jammers get closer to the desired signal in azimuth, which results in low received signal to interference and noise ratio (SINR), our proposed frequency compensation method works more efficiently in proportion. When jammers are away, the receiving signals will have high enough SINRs for low bit error rate decoding. Thus, we provide a robust approach in the most difficult communication situations.

This paper describes a type-II adaptive hybrid FEC/ARQ protocol using turbo codes. Both voice and data services are considered. Recently, a powerful family of error correcting codes using parallel concatenated convolutional codes is introduced. Near-Shannon-limit is achievable by such codes. For example, a bit error rate of 10/sup -5/; can be achieved at E/sub b//N/sub o/ 0.7 dB using a rate 1/2 code or at about 2.5 dB if fewer iterations are used in decoding. Given the promising performance of the codes, it is expected that the performance of an adaptive FEC/ARQ protocol employing turbo codes will be better than protocols using other error correcting codes. For real-time services, the number of retransmissions is limited to one so as to limit the delay. For data services, the number of retransmissions is unlimited to provide a reliable communication link. Results in terms of throughput are obtained through analysis and approximation using the performance curves.