Slotted ALOHA is a simple and straightforward random multiple access technique, which has been used extensively in data and cellular networks as the protocol for random access. The complexity of state space-based analysis methods for finite user finite buffer systems increases exponentially with buffer size and number of users. The presence of multipath frequency selective fading channel further adds to the complexity, making the analysis practically intractable. This paper uses an approximate analysis technique called tagged user analysis TUA to analyze the performance parameters of slotted ALOHA over multipath and frequency selective fading channels for finite user finite buffer systems. In TUA, the steady state system performance is evaluated from the analysis of a single user. Moreover, the state flow graph of TUA has just four states, thus reducing the complexity of the analysis. Simulation results confirm the validity of the TUA analysis. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents the performance analysis of IEEE 802.11 DCF (distributed coordination function) using tagged user analysis (TUA). Performance of IEEE 802.11 DCF has been widely studied, mostly using Markov chains. Markov analysis becomes practically intractable for systems with finite users/stations (STAs) each having a finite buffer capacity. The analysis becomes even complicated under realistic channel models where multipath fading is encountered. This paper uses TUA, an approximate technique, for the analysis of quality of service (QoS) parameters like system throughput, wait delay, response time, packet blocking probability and average queue size. Simulation results show that TUA accurately predicts the performance of IEEE 802.11 DCF.

This study presents statistical properties of the vehicle‐to‐vehicle Nakagami–Hoyt channel model with diffused line of sight (LOS) under isotropic condition. The envelope and phase probability density functions, spatial‐time autocorrelation function, the power spectral density, level crossing rate, the average duration of fade, squared time autocorrelation function and mean‐square error of time autocorrelation function of the Nakagami–Hoyt channel have been derived under the assumption that both the transmitter and receiver are mobile having omnidirectional antennas and a diffused LOS component is present. The model has been validated using the inverse fast Fourier transform‐based simulator. The simulator and analytical results corroborate.

Recently several algorithms have been developed to exploit the distributive nature of an ad hoc wireless sensor network to estimate a certain parameter of interest. However, no work has been done to specifically address the issue of estimation of a sparse parameter. Recently, a sparse estimation algorithm based on the LMS algorithm was proposed. This work proposes a new sparse LMS algorithm for parameter estimation in adaptive networks. Two different schemes are used for incorporating the sparse LMS algorithm into the adaptive network framework. Simulation results show that in a sparse environment the proposed algorithms perform better than the LMS algorithm.

Slotted ALOHA (S-ALOHA) is a simple random multiple access technique used in many data and cellular networks for random access phase. We use tagged user analysis (TUA), an approximate analysis technique, for evaluation of performance parameters of finite user finite buffer (FU-FB) S-ALOHA system over frequency selective fading channels. We also derive analytical expressions for several performance metrics. TUA does not suffer from the complexity issues normally associated with Markov model, where a state space of complexity in the exponential order of buffer size and the number of users must be solved. On comparison of analytical results with simulations, it is observed that TUA is able to accurately analyze the system for a much lower computational cost.

Abstract In this work, we present a new multiuser detection (MUD) structure which results from incorporating the tabu search (TS) heuristic algorithm with the local search (LS) heuristic algorithm. The new proposed structure brings much improvement when compared to both the conventional (matched filter) detector and the decorrelating detector. Moreover, the new TS‐LS‐detector proposed here approximates well the performance of the optimal MUD detector but with a very low computational complexity. Copyright © 2008 John Wiley & Sons, Ltd.

Abstract In this paper, we analyze a homogeneous finite‐users finite‐buffer (FUFB) CSMA system operating in a Rayleigh fading channel using tagged user analysis. Assuming that the system has reached a steady‐state condition in a symmetric channel, we select arbitrarily one representative user, called tagged user, and analyze its behavior in the presence of interference from other contending users. The packet service time distribution of this user is found using a state flow graph. This information is then used for the queueing analysis of the tagged user and thus different performance measures, like throughput and packet transmission delay are easily obtained. This approach decouples the channel contention from queueing behavior and greatly simplifies the problem. As shown in the paper, the approach is also transparent to channel model and spatial distribution of users. The analytical results are also verified by comparing with simulation values using a number of examples. Copyright © 2006 John Wiley & Sons, Ltd.

Sharing of 3G network infrastructure among operators offers an alternative solution to reducing the investment in the coverage phase of WCDMA. For radio access network (RAN) sharing method each operator has its own core network and only the RAN is shared. Without an efficient RRM, one operator can exhausts the capacity of others. This paper proposes and analyzes an efficient uplink-scheduling scheme in case of RAN sharing method. We refer to this new scheme as multi-operators code division generalized processor sharing scheme (M-CDGPS). It employs both adaptive rate allocation to maximize the resource utilization and GPS techniques to provide fair services for each operator. The performance analysis of this scheme is derived using the GPS performance model. Also, it is compared with static rate M-CDGPS scheme. Numerical and simulation results show that the proposed adaptive rate M-CDGPS scheduling scheme improves both system throughput and average delays.

Tagged user analysis (TUA) is a generic approximate method of analyzing random access protocols for finite-user finite-buffer systems. This technique decouples the channel contention behavior from the user queuing behavior and allows the use of classical queuing theory results to be directly applicable to the analysis of finite-user finite-buffer random access methods. In this paper, we extend TUA to analyze finite buffer S-ALOHA operating over flat fading radio channels and derive expressions for system performance indices like throughput, average packet delay, blocking probability and queue length. It is shown that for a moderate number of active users, the simulation and analytical results fit closely

Abstract In this paper, we consider a linear group polynomial expansion successive interference cancellation (GPE‐SIC) detector in a synchronous CDMA system. It is a hybrid detector, which combines parallel and successive cancellation techniques in order to extract the advantages of both schemes. We use the fact that even if the cross‐correlation matrix of the system is not diagonal dominant, the sub‐matrices corresponding to different groups can be forced to be diagonal dominant by suitable grouping of users to approximate the decorrelator/MMSE detector by a low‐complexity polynomial expansion detector. The latter is computationally very efficient if the cross‐correlation matrix of users within the same group is diagonal dominant. Simulation results showed that the (GPE‐SIC) detector has the same performance as the linear group decorrelator successive interference cancellation (GDEC‐SIC) detector but with low‐computational complexity. Copyright © 2006 John Wiley & Sons, Ltd.

In Direct Sequence Code Division Multiple Access (DSCDMA) all users transmit at the same time and at the same frequency thus causing mutual interference. In such situation when the powers of the interfering signals are large compared to the desired signal, the performance of the matched filter receiver degrades. This is due to the near-far effect. One way to combat this effect is to use stringent power control, as is done in most commercial systems. Another approach is to use multiuser detection (MUD), which are near-far resistant. MUD has the more fundamental potential of raising capacity by canceling Multiple Access Interference (MAI). MUD has now developed into an important, full-fledged field in multipleaccess communication systems. In this paper, we present advances in algorithms for MUD.

In this paper we propose application of heuristic algorithms in multiuser detection (MUD). The proposed algorithm combines a tabu search heuristic algorithm with a local search heuristic algorithm. The new proposed structure brings several improvements when compared to both the conventional (matched filter) detector and the decorrelating detector. Additionally, the algorithm proposed here fairly approximates the performance of the optimal MUD detector with much reduced computational complexity.

Multiple access interference and near-far effect cause performance limitation in the conventional single-user detector used in direct sequence/code division multiple access (DS-CDMA)-systems. In this paper, we present a novel multiuser detector (MUD) based on the new heuristic algorithm known as particle swarm algorithm (MUDPSO). We evaluate the BER performance of the proposed algorithm, by means of Monte Carlo simulation technique, and compare it to the BER performances of both the matched filter detector and the decorrelator multiuser detectors. We show that the new algorithm outperforms the other two. Moreover, the performance under near-far scenario, the system capacity and the computational complexity of the proposed detector are also investigated

A new linear hybrid multiuser detection scheme is proposed. The structure cascades a linear successive interference cancellation detector and a linear parallel interference cancellation detector in order to extract the advantages of both. The convergence behavior of the hybrid scheme is investigated and moreover, the condition of convergence is determined. The proposed multiuser detector is compared to the other multiuser detectors in terms of complexity, delay, and other factors. Simulation results illustrate the superiority of the proposed.

In this work, we consider a linear group polynomial expansion successive interference cancellation (GPE-SIC) detector in a synchronous CDMA system. It is a hybrid detector, which combines parallel and successive cancellation techniques in order to extract the advantages of both of the schemes. Benefiting from the fact that even if the cross-correlation matrix of the system is not diagonal-dominant, we can force the cross-correlation matrix of users within the same group to be diagonal-dominant by suitable grouping and approximate the decorrelator/MMSE detector by a low-complexity polynomial expansion detector. This approximation is very accurate if the cross-correlation matrix of users within the same group is diagonal-dominant. Simulation results showed that the (GPE-SIC) detector has the same performance as the linear group decorrelator successive interference cancellation (GDEC-SIC) detector but with lower computational complexity.

The study of statistical distribution of errors is a prerequisite in the design of appropriate coding techniques to effectively control errors. An architecture, V-BLAST, that promises very high spectral efficiency is analyzed and compared for two transmitter-receiver arrangements. We analyzed the system performance over Rayleigh and Nakagami channels along with statistical analysis of errors. Average error burst lengths, average error tree lengths and BER are determined. We also evaluate the effects of different interleaver depths. It is shown that increasing the number of transmitters and receivers, and uncorrelated channel paths give improved performance.

The idea of multi-split adaptive filtering is applied to the DFE. The feedforward and feedback sections in the DFE are divided into parallel sub-filters by imposing separately symmetry and antisymmetry conditions on the impulse responses of the filters by using appropriate and distinct sets of linear constraints. The purpose of this is to obtain a new structure that gives better performance than the conventional structure. The performance of multi-split DFE is tested with different algorithms (LMS, RLS, DCT-LMS) and compared to that of a multi-split linear transversal equalizer.

IMT-DS system is an approved terrestrial radio interface standard for 3G mobile communication based on direct sequence code division multiple access (DS-CDMA). It employs a RAKE receiver to exploit multipath diversity. This paper discusses a new dimension in DS-CDMA channel estimation i.e., chip-level adaptive channel estimation. The DL channel of an IMT-DS system consists of time-multiplexed pilot and data symbols to facilitate coherent detection. To obtain channel estimates during pilot symbols, we propose a chip level adaptive channel estimation which performs better than the conventional method. For slow fading channels, like a pedestrian channel, zero order interpolation provides satisfactory performance. However, for fast fading channels, a common decision directed algorithm is applied whose performance is limited due to error propagation. The proposed schemes are assessed over the IMT-DS system by performing simulations.

In conventional channel models, the AoA's at the BS, are assumed to be uniformly distributed. However, measurements in urban areas have shown non-uniform AoA distributions. In this paper, the spatial characterization of wideband urban mobile radio channels is investigated and the resulting and spatial correlations at the BS receiver are computed. A geometrically based modeling is used to spatially characterize both TU and BU environments. The clustered scatterers are assumed to have bivariate Gaussian densities. The AoA pdf's are derived. The spatial correlations are computed for tooth environments and the effects of the channel parameters such as the scatterer's variance, the BS-MS distance, the mean angle of arrival on these correlations are investigated.

Abstract This paper presents an application of LMS algorithm with optimum step size (µopt‐LMS) for fading channel estimation. The expression for optimum step size of the LMS algorithm is modified for use in the fading channel estimation problem. The robustness of the proposed µopt‐LMS algorithm is demonstrated via simulation in different channel conditions. Copyright © 2002 John Wiley & Sons, Ltd.

The IMT-DS system is one of the approved 3G radio interface standards which employ RAKE receivers to exploit multipath diversity. This paper discusses a new dimension in DS-CDMA channel estimation, i.e., chip-level adaptive channel estimation. A novel despreader-respreader based channel estimator (DRCE) is proposed to obtain uplink (UL) channel estimates at chip-level which resolves the deficiencies of conventional methods that work at symbol level. The DRCE employs an adaptive filter whose weights are adapted by using an LMS algorithm. The performance of the RAKE receiver with DRCE for an IMT-DS system is evaluated in terms of BER by simulations for pedestrian and vehicular channels.

IMT-DS, formerly known as the UTRA system is one of five radio interface standards adopted for the third generation IMT-2000 systems. We present results of our investigation on the sensitivity of IMT-DS performance to variations in the system parameters. In particular, the performance of a multi-rate IMT-DS system is investigated for different RAKE structures, interleaving depths, and Doppler frequencies. An approximate expression for the BER of a RAKE receiver mismatched to the channel is derived. It is shown that the performance is dependent on the inter-path interference. An improvement in performance is obtained if antenna diversity is combined with traditionally used path diversity available in RAKE receivers.

An adaptive combined array and multiuser detector (spatial and temporal) structure for direct sequence code division multiple access (DS-CDMA) has been proposed in Legnain et al. (1998) to combat co-channel interference and increase the capacity for cellular CDMA. The structure uses a L-sensor (element) linear array. Each element is followed by a chip matched filter (CMF) and a fractionally spaced adaptive equalizer. The proposed structure outperforms the conventional receiver and the structures proposed in Abdulrahman et al. (1994) and Miller (1995) in-terms of capacity and near far resistance. In this paper we propose a new centralized decision feedback space-time receiver, which feeds back the detected symbols for all users through a FIR filter without respreading the detected symbols. The focus in this paper is the co-channel interference rejection capability of this structure. Analysis of the optimum tap weights and the minimum mean square error in multipath (no fading) channels are provided, and comparison between this structure and other structures are also provided.

In this paper, DS/CDMA S-ALOHA systems with finite buffer capacity and finite user population are analyzed using the tagged user approach (TUA) proposed in Wan and Sheikh. The data are assumed to arrive in the user buffer in the form of a message containing one or more packets. An arriving message is accepted into the user buffer if the buffer has enough space to hold the whole message, otherwise it is rejected. The queueing analysis for the system is developed in this paper. The analysis is verified by simulation.

An adaptive combined space-time detector for DS-CDMA system has been proposed by Legnain, Falconer and Sheikh (see IEEE GLOBECOM 98, Sydney, Australia, 1998) to combat cochannel interference and enhance the capacity of the adaptive MMSE multiuser detection. In this paper we study the effect of time varying user population (sudden death and birth) on the performance of the adaptive space-time multiuser detector proposed by Legnain et al. in frequency selective fading channels. Sudden birth of interferers degrades the performance of the adaptive MMSE detector, and the receiver needs to be readapted each time new interferers appear. We show that the adaptive space-time techniques alleviate the effect of sudden birth of interferers and there is no need to retrain the detector each time a new interferer appears or disappears.

The International Telecommunication Union is currently in the process of standardizing IMT-2000, the third generation mobile radio systems. Specifications of IMT-2000 have been defined and in 1997 eleven alternative systems have been proposed. A majority of these systems are based on wideband CDMA technology. The Universal Terrestrial Radio Access (UTRA) system proposed by ETSI and NTT is considered to be a leading candidate for standardization. We present results of our investigation on parametric sensitivities of UTRA. In particular, results of our studies on the performance of UTRA for different data rates, interleaving depths and RAKE receiver mismatch with the number of channel paths are presented. Investigations on the influences of these parameters on the system performance are also presented.

In mobile radio systems, antenna diversity is used to combat fading and reduce the impact of cochannel interference. We derived a new expression for probability density functions of the signal-to-interference-plus-noise ratio and apply it to analyze the outage probability (OTP) for a maximal ratio combining diversity system when multiple cochannel interferers are present. Numerical results showing the impact of the number of antenna elements, the number of cochannel interferers, and signal-to-noise ratio on the OTP are presented. Simulation results validating the analytical results are also presented.

The acquisition-based capacity of a DS/CDMA system is defined as the maximum number of simultaneous users that can achieve PN code acquisition with certain reliability. This paper introduces a novel PN code acquisition scheme utilizing an LMS adaptive filter structure to estimate the delay offset of the desired user in the presence of multiple access interference. It is shown that the adaptive filter significantly outperforms conventional matched filter acquisition schemes. It improves the acquisition-based capacity because it takes into consideration the presence of the multi-user interference while finding the optimum tap-weight setting as opposed to the matched filter which completely neglects the interference. For a PN code length of 127 chips, the matched filter can support only 6 users while the adaptive filter supports up to 40 users at a probability of acquisition failure of 1%. This is essentially the same as the capacity achieved at 10/sup -3/ bit-error-rate.

In this paper, a new approach is proposed to analyze packet reservation multiple access (PRMA) protocols that integrate voice and buffered data users. Unlike the available analysis methods, the system state information is carried by the state equilibrium probabilities. As a result, the classical queueing theory results are used to analyze the behaviour of both voice users and data users which have a finite buffer capacity. Compared to equilibrium point analysis (EPA) or Markov analysis, the proposed method is much simpler and therefore can be applied to systems with greater complexity.

With the development of wireless and personal communication systems, there is considerable interest in using antenna array technology to improve the system capacity in both present and future generation wireless systems. A novel and simple algorithm is proposed for blind adaptive extraction of a binary phase-shift keying (BPSK) signal in the presence of interference by cyclostationary signal processing using an antenna array. The algorithm operates in an interference-limited system in which the desired and interfering signals have identical symbol rates, but are modulated on slightly different carrier frequencies. Compared to existing blind algorithms which also exploit the cyclostationarity of the received signal, the new algorithm provides a simpler and faster converging means to estimate the channel phase for diversity combining. Analytical and simulation results are presented and performances are compared with direct matrix inversion (DMI) and existing blind algorithms. Based on the proposed algorithm and the result obtained by Gardner, Schell and Murphy (1992), a time-division multiple-access (TDMA) system with the new channel assignment scheme called cyclic TDMA (CTDMA) is proposed. The simulation results also show that the proposed algorithm is relatively simple and very promising in applications to indoor wireless communication where interference rejection and increased spectrum efficiency are the objectives. Analysis and simulation results are presented to confirm the interference rejection capabilities. The robustness of existing and proposed algorithms to the perturbation of cyclostationarity is also discussed.

This paper investigates tracking of direct-sequence spread-spectrum (DS/SS) signals based on an adaptive filtering technique. It is shown that a previously proposed hardware for code acquisition is also capable of code-tracking, and, hence, by performing both acquisition and tracking with the same circuitry, a significant simplification in the overall DS/SS receiver structure is gained. Analytical results show that the proposed scheme has a good tracking performance, as measured by the hold-in time and the false alarm penalty time, and is less sensitive to variations in the signal-to-noise ratio (SNR) compared to conventional delay-locked loops (DLLs). Moreover, simulation results show that the proposed adaptive filter tracking scheme has a smaller residual tracking error than that produced by a conventional maximum-likelihood estimator (MLE).

Antenna diversity is an important technique to combat fading and reduce co-channel interference (CCI). When evaluating the system performance, such as the BER, Monte Carlo simulation is widely used due to the difficulty of analysis. This paper presents the analysis of BER for optimum combining (OC) with two cochannel interferers and maximal ratio combining (MRC) with arbitrary number of interferers. For a DPSK signal, an exact BER expression is derived. The paper also provides an upper bound for BPSK or QAM signals based on results of Foschini and Salz (1983).

This paper investigates the tracking of DS/SS signals based on an adaptive filtering technique. It is shown that a previously proposed system for code acquisition is also capable of code tracking and, by performing both acquisition and tracking with the same circuitry, significant simplification in the overall DS/SS receiver is gained. Analytical results show that the proposed scheme has a good tracking performance, as measured by the hold-in time and the false alarm penalty time, and less sensitivity to variations in the received SNR compared to conventional delay lock loops (DLLs).

The decorrelating detector is a near-far resistant linear joint detector for a code-division multiple-access (CDMA) system. It consists of a bank of matched filters followed by a decorrelating matrix. For proper operation, both the matched filter bank and the decorrelating matrix require knowledge of the delays of all users. The delays are due to the different propagation times from each user to the base station. Delay estimation is a weak link in this system since it is complex and prone to error. The proposed decorrelating detector does not require exact knowledge of the user delays, but instead requires that the delays be bounded to a fraction of a symbol interval. The delays are naturally bounded in this way in many microcell and picocell systems where the round trip propagation time is small compared to the symbol interval. The new delay independent decorrelating detector is shown to be near-far resistant and, through appropriate spreading code selection, to experience a modest 3 dB signal-to-noise ratio (SNR) loss relative to orthogonal access schemes. It is also shown to limit capacity to a maximum of 50% of the spreading gain when the delays are bounded by a single chip interval. The complexity is similar to the conventional correlating receiver which is far less complex than the joint detection schemes proposed to date.

In emerging integrated wireless personal communications systems, the users are given freedom to choose a service out of many available. The integration of voice, data and video services to meet the user needs on to a common mode of transmission is a difficult task. An overlaid cellular CDMA system is proposed to carry different rate traffic on the same carrier frequency. It is shown that different rate services can coexists in a DS-CDMA-DFE system without any noticeable impact on the system capacity.

This paper presents a novel Kalman filtering approach to the suppression of narrowband interference from direct sequence spread spectrum communications systems. The approach is based on the digital phase-locked loop Kalman filter. Because the interference is assumed to be much stronger than either the signal or noise, the Kalman filter locks onto a function related to the interference. The net result is an estimate of the phase of the interference and its envelope. Though more optimum for interferer's of constant envelope, the algorithm is characterized as a function of the Kalman filter parameters through computer simulation for narrowband Gaussian noise interference. Examples of the phase- and envelope-tracking capabilities of the algorithm are presented, followed by bit error rate curves for interference bandwidths ranging from 0.5% to 5.0% of the chip rate of the spread spectrum signal.>

The paper concerns the feasibility and achievable performance of adaptive filtering in an interference-limited multipath fading environment as encountered in indoor wireless communications. In a typical cellular radio application, the performance-limiting impairment is interference due to synchronous data streams from other co-channel and adjacent channel users (CCI and ACI). The receiver under consideration employs an adaptive fractionally spaced decision feedback equalizer (DFE) Which exploits the correlation of the cyclostationary interference to achieve superior performance relative to the worst case when the interference is stationary noise. The paper presents ideal calculations which confirm that significant DFE performance gains are potentially achievable by explicitly accounting for the cyclostationary CCI. Two adaptive DFE strategies are considered. One approach is to adapt the DFE directly using iterative algorithms such as least mean square (LMS) or recursive least squares (RLS). Another approach is to compute the minimum mean square error DFE using an RLS channel impulse response (CIR) estimate and a sample estimate of the CCI autocorrelation obtained from the CIR estimation error during training. The best approach for adaptive equalization, in terms of adaptation speed and system performance, is to employ an RLS DFE which does not explicitly estimate the CIR or the CCI autocorrelation.>

Narrowband indoor communication channels can be characterised by statistical distributions. The Rayleigh and Ricean distributions are commonly used to describe measured channels, but in some instances, other distributions may be found to be more appropriate for the task. Determining the reason for one particular distribution being more appropriate for one environment over another may be a non-trivial task, In order to assist in this process, a channel simulation based on the physical structure of the environment is presented, along with narrowband results, obtained both from measurement and simulation experiments. Using these results, the basis for a Nakagami distributed channel, observed in some indoor communication environments, is be shown.

Investigates the average bit error rate (EER) and outage performance of a TDMA indoor wireless cellular system employing an adaptive fractionally spaced decision feedback equalizer (DFE) in the presence of adjacent and co-channel interference (ACI and CCI) and additive Gaussian noise. There exist performance gains in the presence of strong interference and negligible ambient noise relative to the equivalent stationary noise case. Moreover, a directly adapted RLS DFE performs better than a computed MMSE DFE which employs estimates of the channel impulse response (CIR) and interference plus noise autocorrelation. However, these comparative performance gains are compromised by the presence of appreciable noise power and/or more interferers than the receiver can process effectively, The use of a wide receiver bandwidth yields a performance improvement for channel spacings which allow for sufficient spectral overlap of the ACI with the desired signal bandwidth. Thus, a reduction in channel spacing increases the radio capacity while maintaining a desired average EER or outage performance.>

Commercial interest in Code Division Multiple Access (CDMA) systems has risen dramatically in the last few years. It yields a potential increase in capacity over other access schemes, because it provides protection against interference, multipath, fading, and jamming. Recently, several interference cancellation schemes for CDMA have been proposed but they require information about all interfering active users or some channel parameters. The authors present an adaptive fractionally spaced decision feedback equalizer (DFE) for a CDMA system in an indoor wireless Rayleigh fading environment. This system only uses information about the desired user's spreading code and a training sequence. An analysis on the optimum performance of the DFE receiver shows the advantages of this system over others in terms of capacity improvements. A simulation of this system is also presented to study the convergence properties and implementation considerations of the DFE receiver. Effects on the performance because of sudden birth and death of users in the CDMA system and bit error rate performance of the DFE receiver is also presented.>

Investigates the performance of direct-sequence spread-spectrum packet radio networks in the presence of the near/far problem. It is found that the maximum throughput of the network suffers degradation due to the near/far problem. However, analysis also shows that, under high traffic conditions, a network with the near/far problem delivers higher throughput than another without the near/far problem. This suggests that the direct-sequence spread-spectrum packet radio network with the near/far problem retains stability for heavier traffic conditions. Following these findings, a new adaptive power control scheme is suggested to enhance network throughput under both low and heavy traffic conditions. The mean and variance of the packet delay are derived and network stability and deadlock avoidance issues are discussed. The impact of channel fading on the network behavior is also studied.>

Data communication at rates near or above 2 kbits/s on 3 kHz-baadwidth HF radio channels is subject to impairment from severe linear dispersion, rapid channel time variation, and severe fading. In this investigation, recorded 2.4 kbit/s QPSK signals received from HF channels were processed to extract a time-varying estimate of the channel impulse response. From the estimated channel impulse responses, performance-related parameters were computed for ideal matched filter reception, maximum-likelihood sequence-estimation (MLSE), and decision feedback equalization (DFE). The results indicated that the simpler DFE receiver suffered only a small theoretical performance degradation relative to the more complex MLSE receiver. Other HF channel impulse response statistics were also obtained to shed light on equalization and filter adaptation techniques.