Fifth-generation (5G) cellular networks are expected to exhibit at least three primary physical-layer differences relative to fourth-generation (4G) ones: millimeter-wave propagation, massive antenna arrays, and densification of base stations. As in 4G systems, such as LTE, 5G systems are likely to continue to use single-carrier frequency-division multiple-access on the uplink due to its advantageous peak-to-average power ratio. Moreover, 5G systems are likely to use frequency hopping on the uplink to help randomize interference and provide diversity against frequency-selective fading. In this paper, the implications of these and other physical-layer features on uplink performance are assessed using a novel millimeter-wave propagation model featuring distance-dependent parameters that characterize the path-loss, shadowing, and fading. The analysis proceeds by first fixing the location of the mobile devices and finding the performance conditioned on the topology. The spatially averaged performance is then found by averaging with respect to the location of the mobile devices. The analysis allows for the use of actual base-station topologies and the propagation model can leverage empirical millimeter-wave measurements. The benefits of base-station densification, highly directional sectorization, frequency hopping, a large available bandwidth, and a high code rate are illustrated. The minor importance of fractional power control is shown.

Geographic routing protocols greatly reduce the requirements of topology storage and provide flexibility in the accommodation of the dynamic behavior of mobile ad hoc networks. This paper presents performance evaluations and comparisons of two geographic routing protocols and the popular AODV protocol. The tradeoffs among the average path reliabilities, average conditional delays, average conditional numbers of hops, and area spectral efficiencies and the effects of various parameters are illustrated for finite ad hoc networks with randomly placed mobiles. This paper uses a dual method of closed-form analysis and simple simulation that is applicable to most routing protocols and provides a much more realistic performance evaluation than has previously been possible. Some features included in the new analysis are shadowing, exclusion and guard zones, distance-dependent fading, and interference correlation.

When a spread-spectrum receiver in a network discovers that it is processing a jamming signal transmitted by a compromised node, its first response is to attempt to identify the compromised node. In this paper, an adaptive array is used to find the direction to the jamming source despite the presence of interference signals transmitted by both legitimate network nodes and external sources. Unlike other direction-finding algorithms, the desired-signal classification (DESIC) algorithm requires no information or special assumptions about the interference signals to effectively cancel them and find the desired direction. Simulation experiments show that the DESIC algorithm provides an excellent performance in many scenarios, even when the received signals cannot be resolved by the widely used MUSIC algorithm.

Multi-cell cooperation (MCC) mitigates intercell interference and improves throughput at the cell edge. This paper considers a cooperative downlink, whereby cell-edge mobiles are served by multiple cooperative base stations. The cooperating base stations transmit identical signals over paths with non-identical path losses, and the receiving mobile performs diversity combining. The analysis in this paper is driven by a new expression for the conditional outage probability when signals arriving over different paths are combined in the presence of noise and interference, where the conditioning is with respect to the network topology and shadowing. The channel model accounts for path loss, shadowing, and Nakagami fading, and the Nakagami fading parameters do not need to be identical for all paths. To study performance over a wide class of network topologies, a random spatial model is adopted, and performance is found by statistically characterizing the rates provided on the downlinks. To model realistic networks, the model requires a minimum separation among base stations. Having adopted a realistic model and an accurate analysis, the paper proceeds to determine performance under several resource-allocation policies and provides insight regarding how the cell edge should be defined.

Digital network coding improves the throughput of the two-way relay channel by allowing multiple sources to transmit simultaneously to the relay. This work considers the development of a relay receiver applying a specific modulation and channel coding technique - turbo-coded noncoherent orthogonal FSK in the two-way relay channel operated with digital network coding. The relay receiver supports any modulation order which is a power of two, and iterative channel decoding with information feedback from decoder to demodulator, using bit interleaved coded modulation with iterative decoding (BICM-ID). The performance of the receiver is investigated in fading channels through error-rate simulations and capacity analysis, and results show an energy efficiency improvement of 0.5-0.9 dB over similar systems which do not utilize BICM-ID.

A new analysis is presented for the direct-sequence code-division multiple access (DS-CDMA) cellular uplink. For a given network topology, closed-form expressions are found for the outage probability and rate of each uplink in the presence of path-dependent Nakagami fading and log-normal shadowing. The topology may be arbitrary or modeled by a random spatial distribution for a fixed number of base stations and mobiles placed over a finite area with the separations among them constrained to exceed a minimum distance. The analysis is more detailed and accurate than existing ones and facilitates the resolution of network design issues, including the influence of the minimum base-station separation, the role of the spreading factor, and the impact of various power-control and rate-control policies. It is shown that once power control is established, the rate can be allocated according to a fixed-rate or variable-rate policy with the objective of either meeting an outage constraint or maximizing throughput. An advantage of the variable-rate policy is that it allows an outage constraint to be enforced on every uplink, whereas the fixed-rate policy can only meet an average outage constraint.

This paper considers ad hoc networks that use the combination of coded continuous-phase frequency-shift keying (CPFSK) and frequency-hopping multiple access. Although CPFSK has a compact spectrum, some of the signal power inevitably splatters into adjacent frequency channels, thereby causing adjacent-channel interference (ACI). The amount of ACI is controlled by setting the fractional in-band power; i.e., the fraction of the signal power that lies within the band of each frequency channel. While this quantity is often selected arbitrarily, a tradeoff is involved in the choice. This paper presents a new analysis of frequency-hopping ad hoc networks that carefully incorporates the effect of ACI. The analysis accounts for the shadowing, Nakagami fading, CPFSK modulation index, code rate, number of frequency channels, fractional in-band power, and spatial distribution of the interfering mobiles. Expressions are presented for both outage probability and transmission capacity. With the objective of maximizing the transmission capacity, the optimal fractional in-band power that should be contained in each frequency channel is identified.

The central issue in direct-sequence code-division multiple-access (DS-CDMA) ad hoc networks is the prevention of a near-far problem. This paper considers two types of guard zones that may be used to control the near-far problem: a fundamental exclusion zone and an additional CSMA guard zone that may be established by the carrier-sense multiple-access (CSMA) protocol. In the exclusion zone, no mobiles are physically present, modeling the minimum physical separation among mobiles that is always present in actual networks. Potentially interfering mobiles beyond a transmitting mobile's exclusion zone, but within its CSMA guard zone, are deactivated by the protocol. This paper provides an analysis of DS-CSMA networks with either or both types of guard zones. A network of finite extent with a finite number of mobiles and uniform clustering as the spatial distribution is modeled. The analysis applies a closed-form expression for the outage probability in the presence of Nakagami fading, conditioned on the network geometry. The tradeoffs between exclusion zones and CSMA guard zones are explored for DS-CDMA and unspread networks. The spreading factor and the guard-zone radius provide design flexibility in achieving specified levels of average outage probability and transmission capacity. The advantage of an exclusion zone over a CSMA guard zone is that since the network is not thinned, the number of active mobiles remains constant, and higher transmission capacities can be achieved.

A new analysis is presented for the direct-sequence code-division multiple access (DS-CDMA) cellular uplink. For a given network topology, closed-form expressions are found for the outage probability and rate of each uplink in the presence of path-dependent Nakagami fading and shadowing. The topology may be arbitrary or modeled by a random spatial distribution with a fixed number of base stations and mobiles placed over a finite area. The analysis is more detailed and accurate than existing ones and facilitates the resolution of network design issues including the influence of the minimum base-station separation, the role of the spreading factor, and the impact of various power-control and rate-control policies. It is shown that once power control is established, the rate can be allocated according to a fixed-rate or variable-rate policy with the objective of either meeting an outage constraint or maximizing throughput. An advantage of variable-rate power control is that it allows an outage constraint to be enforced on every uplink, which is impossible when a fixed rate is used throughout the network.

An ad hoc network with a finite spatial extent and number of nodes or mobiles is analyzed. The mobile locations may be drawn from any spatial distribution, and interference-avoidance protocols or protection against physical collisions among the mobiles may be modeled by placing an exclusion zone around each radio. The channel model accounts for the path loss, Nakagami fading, and shadowing of each received signal. The Nakagami m-parameter can vary among the mobiles, taking any positive value for each of the interference signals and any positive integer value for the desired signal. The analysis is governed by a new exact expression for the outage probability, defined to be the probability that the signal-to-interference-and-noise ratio (SINR) drops below a threshold, and is conditioned on the network geometry and shadowing factors, which have dynamics over much slower timescales than the fading. By averaging over many network and shadowing realizations, the average outage probability and transmission capacity are computed. Using the analysis, many aspects of the network performance are illuminated. For example, one can determine the influence of the choice of spreading factors, the effect of the receiver location within the finite network region, and the impact of both the fading parameters and the attenuation power laws.

A channel-coded physical-layer network coding strategy is refined for practical operation. The system uses frequency-shift keying (FSK) modulation and operates noncoherently, providing advantages over coherent operation: there are no requirements for perfect power control, phase synchronism, or estimates of carrier-phase offset. In contrast with analog network coding, which relays received analog signals plus noise, the system relays digital network codewords, obtained by digital demodulation and channel decoding at the relay. The emphasis of this paper is on the relay receiver formulation. Closed-form expressions are derived that provide bitwise log-likelihood ratios, which may be passed through a standard error-correction decoder. The role of fading-amplitude estimates is investigated, and an effective fading-amplitude estimator is developed. Simulation results are presented for a Rayleigh block-fading channel, and the influence of block length is explored. An example realization of the proposed system demonstrates a 32.4% throughput improvement compared to a similar system that performs network coding at the link layer. By properly selecting the rates of the channel codes, this benefit may be achieved without requiring an increase in transmit power.

Space-time block codes with orthogonal structures typically provide full-diversity reception and simple receiver processing. However, rate-1 orthogonal codes for complex constellations have not been found for more than two transmit antennas. By using a genetic algorithm, rate-1 space-time block codes that accommodate very simple receiver processing at the cost of reduced diversity are designed in this paper for more than two transmit antennas. Simulation results show that evolved codes combined with efficient outer codes provide better performance over fading channels than minimum-decoding-complexity quasiorthogonal codes at typical operating signal-to-noise ratios. When the fading is more severe than Rayleigh fading, the spectral efficiency is specified, and an efficient outer code is used, evolved codes outperform orthogonal space-time block codes.

A system involving the multisymbol noncoherent reception of coded continuous-phase frequency-shift keying is developed, optimized, and analyzed. Unlike coherent systems, the modulation index of the waveform does not need to be rational with a small denominator, and the oscillator only needs to be stable for the duration of a small block of symbols. The achievable performance over AWGN and Rayleigh block-fading channels is determined by computing the average mutual information, which is the capacity of a channel using the given modulation format and receiver architecture under the constraint of uniformly distributed input symbols. The code rate and modulation index are jointly optimized with respect to average mutual information under a bandwidth constraint. For binary and quaternary signaling, the information-theoretic results are corroborated by bit-error-rate curves generated using a standardized turbo code in conjunction with iterative demodulation and decoding. It is shown that, while more robust than a system with coherent reception, the proposed system offers superior energy efficiency compared with conventional single-symbol noncoherent reception.

In this paper, we describe direct-sequence code-division multiple-access (DS-CDMA) systems with quadriphase-shift keying in which channel estimation, coherent demodulation, and decoding are iteratively performed without the use of any training or pilot symbols. An expectation-maximization channel-estimation algorithm for the fading amplitude, phase, and the interference power spectral density (PSD) due to the combined interference and thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate codes. After initial estimates of the fading amplitude, phase, and interference PSD are obtained from the received symbols, subsequent values of these parameters are iteratively updated by using the soft feedback from the channel decoder. The updated estimates are combined with the received symbols and iteratively passed to the decoder. The elimination of pilot symbols simplifies the system design and allows either an enhanced information throughput, an improved bit error rate, or greater spectral efficiency. The interference-PSD estimation enables DS-CDMA systems to significantly suppress interference.

The symmetric information rate of a modulation-constrained transmission system is the information-theoretic limit on performance under the assumption that the inputs are independent and uniformly distributed. The symmetric information rate for continuous-phase frequency-shift keying (CPFSK) over an AWGN channel may be estimated by considering the system to be a finite-state Markov channel and executing a BCJR-like algorithm. In this paper, the estimated symmetric information rate is used along with the exact expression for the 99% power bandwidth to determine the information-theoretic tradeoff between energy and spectral efficiency for CPFSK modulation. Using this tradeoff, the code rate and modulation index are jointly optimized for a particular spectral efficiency and alphabet size. Codes are then designed for the optimized system. The codes are comprised of variable nodes (which represent irregular repetition codes), check nodes (which represent single parity-check codes), and an interleaver connecting the variable and check nodes. The degree distributions of the code are optimized from the system's EXIT chart by using linear programming. Additional details of the code design, including labeling and interleaver design, are also discussed. Simulation results show that the optimized coded systems achieve bit error rates within 0.4 dB of the information-theoretic limits at BER = 10-5.

A robust frequency-hopping system with noncoherent detection, iterative turbo decoding and demodulation, and channel estimation is presented. The data modulation is the spectrally compact nonorthogonal continuous-phase frequency-shift keying, which strengthens the frequency-hopping system against multiple-access interference and multitone jamming. An analysis based on information theory provides the optimal values of the modulation index when there is a bandwidth constraint. The channel estimator, which is derived by applying the expectation- maximization algorithm, accommodates both frequency-selective fading and interference. Simulation experiments demonstrate the excellent system performance against both partial-band and multiple-access interference.

Strong directional interference can prevent code acquisition in a direct-sequence system, thereby rendering the system inoperable. This paper presents a computationally simple adaptive-array algorithm that cancels directional interference prior to code acquisition when the direction of arrival of the desired signal is unknown, no pilot symbols are available, and long spreading sequences must be accommodated. The preacquisition algorithm is shown to dramatically reduce the interference level relative to that of a desired direct-sequence signal before code acquisition has been achieved. As a result, the code acquisition process is greatly facilitated.

A constellation labeling map is the assignment of a bit pattern to each symbol in a signal-set constellation. In a system with iterative decoding and demodulation, the error floor of the bit error rate is highly dependent on the labeling map. A simple class of labeling maps that significantly lower the error floors is presented. Examples show the applications of the proposed mapping to multiple phase-shift keying (MPSK), quadrature amplitude modulation (QAM), and continuous-phase frequency-shift keying (CPFSK). Simulation results indicate that the proposed labeling maps are comparable to or better than other labeling maps in providing a low error floor. A major advantage of the proposed labeling maps is that they are easily generated even when the alphabet size is large.

A robust frequency-hopping system with noncoherent detection, iterative turbo decoding and demodulation, and channel estimation is presented. The data modulation is the spectrally compact nonorthogonal continuous-phase frequency-shift keying, which strengthens the frequency-hopping system against multiple- access interference and multitone jamming. An analysis based on information theory provides the optimal values of the modulation index when there is a bandwidth constraint. The channel estimator, which is derived by applying the expectation-maximization algorithm, accommodates both frequency-selective fading and interference. Simulation experiments demonstrate the excellent system performance against partial-band interference.

The authors of this paper proposed an iterative expectation-maximization (EM) channel estimation based on a low-density parity-check (LDPC) code-division multiple access receiver in their previous work. The receiver algorithm could efficiently estimate both channel coefficients and noise power spectral density (PSD). However, their previous work considered only a single user channel environment, so they tested only for a constant noise PSD. In the current paper, we extend the single user to a more realistic multiuser environment for the testing of a time-varying interference plus noise PSD estimation. Also our proposed adaptive PSD estimation scheme will be compared with a conventional system in which a constant PSD is assumed. It will show that the proposed iterative EM scheme can provide accurate interference plus noise PSD estimates to the LDPC decoder.

An iterative noncoherent receiver is developed for bit-interleaved coded orthogonal multiple frequency-shift keying (FSK) over block fading channels. The receiver uses the Expectation Maximization (EM) algorithm to jointly estimate the received amplitude and noise spectral density of each block. The received symbols and their corresponding channel estimates are passed through a soft-output demapper, deinterleaved, and decoded. Soft-outputs from the decoder are passed back to the channel estimator and demapper to refine estimates of the channel and bit likelihoods, respectively. Several techniques for reducing the estimator's complexity are discussed, and the performance is assessed through simulation.

In cellular direct-sequence CDMA networks, each base station attempts to control the received power of signals from mobiles associated with the station. A simple upper bound on the uplink capacity per unit bandwidth is derived from consideration of the effects of fast fading and the limitations of power estimation. This bound decreases rapidly with increasing carrier frequency or vehicular speeds and indicates that a bandwidth expansion is necessary to maintain a specified uplink capacity.

Proposed and existing power-control systems for direct-sequence CDMA cellular networks measure instantaneous power, whereas theoretical analyzes almost always assume long-term-average or local-mean power control. In this paper, the distinction between the two types of power control is clarified. The outage probability for direct-sequence CDMA systems using instantaneous power control is derived. The results of the analysis of local-mean power control are shown to be similar to those provided by a far more elaborate theoretical analysis.

A simple alternative formula for the error probability of a RAKE demodulator for noncoherent, square-law detected, binary orthogonal signals and Rayleigh fading is presented. The formula is more compact and considerably easier to evaluate than the classical formula while providing identical numerical results. The derivation of the alternative formula is shown to be much simpler than the derivation of the classical formula.

Through analysis and simulation, a unified evaluation of the potential performance of both mobile peer-to-peer and sectorized cellular frequency-hopping code-division multiple-access (CDMA) systems is presented. The equivalent number of frequency channels and the minimum area-mean signal-to-noise ratio are defined and shown to be important parameters in understanding and predicting network capacity. The effects of spectral splatter are analyzed. Separated orthogonality is defined and shown to be useful in completely eliminating intracell or intrasector interference. Spatial diversity by postdetection rather than predetection combining is proposed and shown to be invaluable. Noncoherent demodulation by a frequency discriminator rather than parallel matched filters and envelope detectors is proposed and shown to be effective. It is shown that even without exploiting either its natural bandwidth advantage or power control, frequency-hopping CDMA provides a reverse-link capacity nearly the same as direct-sequence CDMA with realistic power-control imperfections.

A major problem for a detection algorithm is the vast amount of false alarms normally generated. This amount of false alarms has to be substantially reduced so that a typical target classifier in the subsequent stage may work reasonably. We use the bipolar eigenspace separation transformation (BEST) and neural network techniques to improve the clutter rejection performance of an automatic target detector. Experiments have been conducted on huge and realistic datasets of forward looking infrared (FLIR) imagery. Compared to the performance of the unipolar EST and principal component analysis (PCA) with the same datasets, significant improvement in clutter rejection rates has been achieved with BEST.

An effective clutter rejection scheme is needed to distinguish between clutter and targets in a high-performance automatic target recognition (ATR) system. We present a clutter rejection scheme that consists of an eigenspace transformation and a multilayer perceptron (MLP). We use either principal component analysis (PCA) or the eigenspace separation transform (EST) to perform feature extraction and dimensionality reduction. The transformed data is then fed to an MLP that predicts the identity of the input, which is either a target or clutter. We devise an MLP training algorithm that seeks to maximize the class separation at a given false-alarm rate, which does not necessarily minimize the average deviation of the MLP outputs from their target valves. Experimental results are presented on a huge and realistic dataset of forward-looking infrared imagery.

The detection and disposal of antipersonnel land mines is one of the most difficult and intractable problems faced in ground conflict. This paper presents detection methods which use a separated-aperture microwave sensor and an artificial neural network pattern classifier. Several data-specific preprocessing methods are developed to enhance neural network learning. In addition, a generalized Karhunen-Loeve transform and the eigenspace separation transform are used to perform data reduction and reduce network complexity. Highly favorable results have been obtained using the above methods in conjunction with a feedforward neural network.

A performance analysis of direct-sequence systems with long pseudonoise sequences is presented. The assessment of the symbol error probability in the presence of multiple-access interference is stressed, but other types of interference are also considered for completeness. Both binary and quaternary spreading waveforms are treated. The analysis, which uses a nonstandard Gaussian approximation, makes transparent the answers to a number of technical issues. Approximate error probabilities that are relatively simple computationally are derived. The approximations are accurate to within at least one decibel and, more typically, a few tenths of a decibel.>

An easily implemented matched filter scheme for acquiring hopping code synchronization of incoming frequency-hopping (FH) signals is analyzed, and its performance is evaluated for two types of jamming: partial-band noise jamming and partial-band multitone jamming. The system is designed to reduce jammer-induced false alarms. The system's matched filter output is compared to an adaptive threshold that is derived from a measurement of the number of acquisition channels being jammed. Example performance calculations are given for the frequency coverage of the jamming either fixed over the entire acquisition period or hopped, that is, changed for each acquisition pulse. It is shown that the jammer's optimum strategy (the worst case) is to maximize the false alarm probability without regard for the effect on detection probability, for both partial-band noise and multitone jamming. It is also shown that a significantly lower probability of false acquisition results from using an adaptive matched filter threshold, demonstrating that the strategy studied here is superior to conventional nonadaptive threshold schemes.>

The motives for seeking a set of short disjoint paths in a communication network are explained. A sequentially constructed optimal set is defined. Three efficient algorithms, one that constructs an optimal set and two that construct approximations, are presented. One of the latter algorithms not only constructs a larger set of short disjoint paths than an iterated version of the standard Dijkstra algorithm, but also offers a major reduction in computation time for large networks.>

Fundamental limitations on the effectiveness of repeater jamming are derived. These limitations arise because of the geometry and the need for frequency estimation and signal sorting. Limitations on the hopping rate, which might be increased to counter a repeater-jamming threat, are derived from consideration of practical requirements.>

There are two types of bounded-distance decoders for linear block codes: erasing decoders that discard uncorrectable received words, and reproducing decoders that reproduce uncorrectable received words. Exact expressions for the information-symbol and decoded-symbol error rates are derived for both types. Necessary and sufficient conditions are derived for the quality of the information-symbol and decoded symbol error rates, It is formally proved that these two error rates are equal for cyclic codes with either erasing or reproducing decoders. For reproducing decoders, two approximations to the information-bit error rate and their applicability are examined.>

Metrics that are effective against white Gaussian noise are not necessarily effective against pulsed jamming that is optimized by an opponent. This paper analyzes and compares the information-bit error rates that result from using the hard-decision, maximum-likelihood, white-noise, automatic gain control, and erasure metrics against pulsed jamming. Practical implementation problems and limitations are identified and discussed.

The relations between the word error probability and the decoding algorithms for block codes are reviewed. A simple approximation that does not depend upon the code weight structure or the decoding details is derived for the information-bit error rate in terms of the channel-symbol error probability.

The performance of frequency-hopping systems with multiple frequency-shift keying and hard decisions is determined for operation against optimal partial-band jamming. A receiver that eliminates the potential advantage of optimally placed jamming tones is proposed. The effects of Reed-Solomon, binary block, and convolutional codes are analyzed. Concatenated codes with hard and soft decisions available to the outer decoder are examined. Continuous-phase frequency-shift keying with limiter-discriminator demodulation is shown to offer potentially improved performance in frequency-hopping systems.