In this article, we propose a molecular communication system to localize an abnormality in a diffusion-based medium. We consider a general setup to perform joint sensing, communication, and localization. This setup consists of three types of devices, each for a different task: mobile sensors for navigation and molecule releasing (for communication), fusion centers (FCs) for sampling, amplifying, and forwarding the signal, and a gateway (GW) for making decision or exchanging the information with an external device. The sensors move randomly in the environment to reach the abnormality. We consider both collaborative and noncollaborative sensors that simultaneously release their molecules to the FCs when the number of activated sensors or the moving time reaches a certain threshold, respectively. The FCs amplify the received signal and forward it to the GW for making a decision using either an ideal or a noisy communication channel. A practical application of the proposed model is drug delivery in a tissue of the human body, to guide the nanomachine-bound drug to the exact location and so to eliminate the adverse effects of the drug on normal cells. Further applications are health-care, treatments of localized disease (e.g., tumors and inflammations), immune system triggering, and nanosurgery. The decision rules and probabilities of error are obtained for two considered sensor types in both ideal and noisy communication channels.

In this paper, we study the degrees of freedom (DoF) region and sum DoF of the time-selective$K$-user interference channel in the presence of intelligent reflecting surfaces (IRSs). We consider both active and passive IRSs. While both types of IRS can attenuate the amplitude and change the phase of a reflected electromagnetic wave, active IRSs are also capable of amplifying the wave. We derive inner and outer bounds for the DoF region and lower and upper bounds for the sum DoF of the$K$-user interference channel in the presence of an active IRS and prove that the maximum value of$K$for the sum DoF can be achieved if the number of IRS elements exceeds a certain finite value. The analysis framework developed for active IRSs forms the basis for our analysis of passive IRS-assisted systems. In particular, we present probabilistic inner and outer bounds for the DoF region and probabilistic lower and upper bounds for the sum DoF of the$K$-user interference channel in the presence of a passive IRS and prove that the lower bound for the sum DoF asymptotically approaches$K$as the number of IRS elements grows large.

One of the main challenges in diffusion-based molecular communication is dealing with the non-linearity of reaction-diffusion chemical equations. While numerical methods can be used to solve these equations, a change in the input signals or the parameters of the medium requires one to redo the simulations. This makes it difficult to design modulation schemes and practically impossible to prove the optimality of a given transmission strategy. In this paper, we provide an analytical technique for modeling the non-linearity of chemical reaction equations based on the perturbation method. The perturbation method expresses the solution in terms of an infinite power series. An approximate solution can be found by keeping the leading terms of the power series. The approximate solution is shown to track the true solution if either the simulation time interval or the reaction rate is sufficiently small. Approximate solutions for long time intervals are also discussed. An illustrative example is given. For this example, it is shown that when the reaction rate (or the total time interval) is low, instead of using a continuous release waveform, it is optimal for the transmitters to release molecules at two time instances.

One of the main challenges in diffusion-based molecular communication is dealing with the non-linearity of reaction diffusion chemical equations. While numerical methods can be used to solve these equations, a change in the input signals or in the parameters of the medium requires one to redo the simulations. This makes it difficult to design modulation schemes and practically impossible to prove the optimality of a given transmission strategy. We provide an analytical technique for modeling non-linearity of chemical reaction equations based on the perturbation method. It is shown that at low reaction rates, instead of using a continuous transmission waveform, it is optimal for each transmitter to release molecules in a sequence of spikes.

In this paper, we study the zero error capacity of the molecular delay channel when multiple molecule types are available at the transmitter. In the molecular delay channel, each transmitted molecule (of any type) is received by a delay of at most $k$ time slots. Depending on the number of molecules that the transmitter is allowed to release in each time slot, we consider the following three cases: (i) when the maximum number of the released molecules of each type in each time slot is restricted (ii) when the total number of the released molecules (regardless of their type) in each time slot is restricted, and (iii) when the transmitter can use only one molecule type (of its choice) in each time slot. We derive lower bounds on the zero-error capacity of the delay channel for each case, by proposing zero-error codes that are based on the results by Kovačević and Popovski. We also derive upper bounds on the zero-error capacity of the delay channel. In the first case, these bounds match and yield the exact capacity, while in the other two cases, the bounds are shown to be close numerically. Our numerical results show that as the number of available molecule types increases, the capacity of the system increases substantially, compared to using only one molecule type. Furthermore, it is shown that the lower and upper bounds on the zero-error capacity of the delay channel in the second case are generally close to the lower and upper bounds in the third case, respectively, indicating the closeness of the zero-error capacities of the two cases. This result enables one to design a simpler system by employing a high rate code that has only one molecule type in each slot (designed for the third case) in the channel of the second case, without much rate loss.

This paper develops a stochastic geometry-based approach for the modeling, analysis, and optimization of wireless cloud caching networks comprised of multiple-antenna radio units (RUs) inside clouds with coordinated multi-point transmissions and guard zones. We consider Poisson cluster processes to model RUs and users, and the probabilistic content placement to cache files in RUs. Accordingly, we study the exact hit probability for a user of interest for two strategies; closest selection, where the user is served by the closest RU that has its requested file, and best power selection, where the serving RU having the requested file provides the maximum instantaneous received power at the user. As key steps for the analyses, the Laplace transform of out of cloud interference, the desired link distance distribution in the closest selection, and the desired link received power distribution in the best power selection are derived. Also, we approximate the derived exact hit probabilities for both the closest and the best power selections in such a way that the related objective functions for the content caching design of the network can lead to tractable concave optimization problems. Solving the optimization problems, we propose algorithms to efficiently find their optimal content placements. Finally, we investigate the impact of different parameters on the caching performance.

Massive multiple-input multiple-output (M-MIMO) is recognized as a promising technology for the next generation of wireless networks because of its potential to increase the spectral efficiency. In initial studies of M-MIMO, the system has been considered to be perfectly synchronized throughout the entire cells. However, perfect synchronization may be hard to attain in practice. Therefore, we study a M-MIMO system whose cells are not synchronous to each other, while transmissions in a cell are still synchronous. We analyze an asynchronous downlink M-MIMO system in terms of the coverage probability and the ergodic rate by means of the stochastic geometry tool. For comparison, we also obtain results for the corresponding synchronous system. In addition, we investigate the effect of the uplink power control and the number of pilot symbols on the downlink ergodic rate, and we observe that there is an optimal value for the number of pilot symbols maximizing the downlink ergodic rate of a cell. Our results also indicate that, compared to the synchronous system, the downlink ergodic rate is more sensitive to the uplink power control in the asynchronous mode.

Chemical reactions are a prominent feature of molecular communication systems, with no direct parallels in wireless communications. While chemical reactions may be used inside the transmitter nodes, receiver nodes, or the communication medium, we focus on its utility in the medium in this paper. Such chemical reactions can be used to perform computation over the medium as molecules diffuse and react with each other (physical-layer computation). We propose the use of chemical reactions for the following purposes: 1) to reduce signal-dependent observation noise of receivers by reducing the signal density; 2) to realize molecular physical-layer network coding (PNC) by performing the natural XOR operation inside the medium; and 3) to reduce the inter-symbol interference (ISI) of other transmitters by canceling out the remaining molecules from previous transmissions. To make the ideas formal, we consider an explicit two-way relaying example with a transparent receiver (which has a signal-dependent noise). The proposed ideas are used to define a modulation scheme (which we call the PNC scheme). We compare the PNC with a previously proposed scheme for this problem, where the XOR operation is performed at the relay node (using a molecular logic gate). We call the latter, the straightforward network coding (SNC). It is observed that in addition to the simplicity of the proposed PNC scheme, it outperforms the SNC scheme especially when we consider ISI.

In this paper, we consider target detection in suspicious tissue via diffusive molecular communications (MCs). If a target is present, it continuously and with a constant rate secretes molecules of a specific type, so-called biomarkers, into the medium, which are symptomatic for the presence of the target. Detection of these biomarkers is challenging since due to the diffusion and degradation, the biomarkers are only detectable in the vicinity of the target. In addition, the exact location of the target within the tissue is not known. In this paper, we propose to distribute several reactive nanosensors (NSs) across the tissue such that at least some of them are expected to come in contact with biomarkers, which cause them to become activated. Upon activation, an NS releases a certain number of molecules of a secondary type into the medium to alert a fusion center (FC), where the final decision regarding the presence of the target is made. In particular, we consider a composite hypothesis testing framework where it is assumed that the location of the target and the biomarker secretion rate are unknown, whereas the locations of the NSs are known. We derive the uniformly most powerful (UMP) test for the detection at the NSs. For the final decision at the FC, we show that the UMP test does not exist. Hence, we derive a genie-aided detector as an upper bound on performance. We then propose two sub-optimal detectors and evaluate their performance via simulations.

This paper develops a stochastic geometry-based approach for the modeling and analysis of single- and multi-cluster wireless networks. We first define finite homogeneous Poisson point processes to model the number and locations of the transmitters in a confined region as a single-cluster wireless network. We study the coverage probability for a reference receiver for two strategies; closest-selection, where the receiver is served by the closest transmitter among all transmitters, and uniform-selection, where the serving transmitter is selected randomly with uniform distribution. Second, using Matern cluster processes, we extend our model and analysis to multi-cluster wireless networks. Here, two types of receivers are modeled, namely, closed- and open-access receivers. Closed-access receivers are distributed around the cluster centers of the transmitters according to a symmetric normal distribution and can be served only by the transmitters of their corresponding clusters. Open-access receivers, on the other hand, are placed independently of the transmitters and can be served by all transmitters. In all cases, the link distance distribution and the Laplace transform (LT) of the interference are derived. We also derive closed-form lower bounds on the LT of the interference for single-cluster wireless networks. The impact of different parameters on the performance is also investigated.

While ISI is a common issue in classical communications, it is more challenging and prominent in the context of molecular communication, because one cannot readily combat ISI with classical channel equalization techniques. This is due to the fact that transmitter can only release a positive amount of concentration of a specific molecule into the medium. Previous works have proposed use of chemical reactions to remove molecules from the environment, and to effectively simulate negative signals. However, the differential equation describing a diffusion-reaction process is non-linear. This precludes the possibility of using Fourier transform tools. In this paper, a solution for simulating negative signals based on the diffusion-reaction channel model is proposed. While the proposed solution does not exploit the full degrees of freedom available for signaling in a diffusion-reaction process, but its end-to-end system is a linear channel and amenable to Fourier transform analysis. Based on our solution, a modulation scheme and a precoder are introduced and shown to have a significant reduction in error probability compared with previous modulation schemes, such as concentration shift keying (CSK), pre-equalization, depleted-molecule shift keying (D-MoSK), and molecular concentration shift keying (MCSK). The effects of various imperfections (such as quantization error) on the communication system performance are studied.

In some applications, the variance of additive measurement noise depends on the signal that we aim to measure. For instance, additive signal-dependent Gaussian noise (ASDGN) channel models are used in molecular and optical communication. Herein, we provide lower and upper bounds on the capacity of additive signal-dependent noise (ASDN) channels. The first lower bound is based on an extension of majorization inequalities, and the second lower bound utilizes the properties of the differential entropy. The lower bounds are valid for arbitrary ASDN channels. The upper bound is based on a previous idea of the authors (“symmetric relative entropy”) and is applied to the ASDGN channels. These bounds indicate that in the ASDN channels (unlike the classical additive white Gaussian noise channels), the capacity does not necessarily become larger by reducing the noise variance function. We also provide sufficient conditions under which the capacity becomes infinite. This is complemented by some conditions implying that the capacity is finite, and a unique capacity achieving measure exists (in the sense of the output measure).

This paper introduces a new metric, referred to as delay-sensitive area spectral efficiency (DASE), to analyze the performance of delay-constrained wireless networks. We study DASE in different point-to-point, spectrum sharing, interference, and Poisson-point process-based dense network configurations and with different levels of channel state information (CSI) at the transmitters. Also, we determine the optimal rates/powers optimizing DASE. Finally, we use some recent results on finite block-length codes to analyze the effect of the codewords length on the network DASE. As demonstrated, DASE is a useful metric for analyzing delay-sensitive green networks. Moreover, adaptive power allocation and partial CSI feedback are powerful tools for performance improvement in green networks.

This paper investigates the outage probability and the throughput of relay networks with joint information and energy transfer where the relay harvests energy from transmitted radio-frequency (RF) signal of the source. Considering different power consumption models, we derive the outage probability of the systems for both adaptive and non-adaptive power allocations at the relay. With a total energy consumption constraint at the source, we provide closed-form expressions for the optimal time sharing and power allocation between the source energy and information transfer signals. The analytical and simulation results demonstrate the efficiency of joint energy and information transfer systems in different conditions.

In this paper, a simple memory limited transmitter for molecular communication is proposed, in which information is encoded in the emission rate of the molecules. Taking advantage of memory, the proposed transmitter reduces the ISI problem by properly adjusting its emission rate, which can be interpreted as water-filling on the expected interference. The error probability of the proposed scheme is derived and the result is compared with the error probability of the optimal transmitter obtained by dynamic programming methods. Furthermore, for the special case of channel with one symbol memory, a tight lower bound on error probability is derived. Numerical results show that the performance of introduced transmitter is near optimal. Simplicity is the key feature of the presented communication system: the transmitter follows a simple rule, the receiver is a simple threshold decoder, and only one type of molecule is used to convey the information.

The LTI-Poisson model is a natural extension of the conventional memoryless Poisson channel to include memory, and can model the ISI effect in diffusion based molecular communication networks. In this paper, we exploit prior art on linear ISI channels to provide a computable finite-letter characterization of the capacity of single-hop LTI-Poisson networks. Then we find more explicit single-letter lower and upper bounds on the capacity in the point to point case. Further, an approach for bounding mutual information in the low SNR regime using the symmetrized KL divergence is introduced and its applicability for Poisson channels is demonstrated. This leads to a non-trivial upper bound on the capacity of Poisson channel with a maximum transmission constraint in the low SNR regime, which to best of our knowledge is the first such bound.

In this paper, a simple memory limited transmitter for molecular communication is proposed, in which information is encoded in the diffusion rate of the molecules. Taking advantage of memory, the proposed transmitter reduces the ISI problem by properly adjusting its diffusion rate. The error probability of the proposed scheme is derived and the result is compared with the lower bound on error probability of the optimum transmitter. It is shown that the performance of introduced transmitter is near optimal (under certain simplifications). Simplicity is the key feature of the presented communication system: the transmitter follows a simple rule, the receiver is a simple threshold decoder and only one type of molecule is used to convey the information.

In this paper, we consider the bacterial point-to-point communication problem with one transmitter and one receiver by considering the ligand receptor binding process. The most commonly investigated signalling model, referred to as the Level Scenario (LS), uses one type of a molecule with different concentration levels for signaling. An alternative approach is to employ multiple types of molecules with a single concentration level, referred to as the Type Scenario (TS). We investigate the trade-offs between the two scenarios for the ligand receptor from the capacity point of view. For this purpose, we evaluate the capacity using numerical algorithms. Moreover, we derive an upper bound on the capacity of the ligand receptor for a Binomial Channel (BIC) model, using symmetrized Kullback-Leibler (KL) divergence. A lower bound is also derived when the environment noise is negligible. Finally, we analyse the effect of blocking of a receptor by a molecule of a different type, by proposing a new Markov model in the multiple-type signalling.

In this study, a new two‐way relaying scheme based on compute‐and‐forward (CMF) framework and relay selection strategies is proposed, which provides a higher throughput than the conventional two‐way relaying schemes. Two cases of relays with or without feedback transmission capability are considered. An upper bound on the computation rate of each relay is derived, and based on that, a lower bound on the outage probability of the system is presented assuming block Rayleigh fading channels. Numerical results show that while the average sum rate of the system without feedback, named as max‐CMF (M‐CMF), reaches the derived upper bound only in low SNRs, that of the system with feedback, named as aligned‐CMF (A‐CMF) reaches the bound in all SNRs. However, both schemes approach the derived lower bound on the outage probability in all SNRs. For the A‐CMF, another power assignment based on applying the constraint on the total powers of both users rather than on the power of each separately, is introduced. The numerical results show that the outage performance, average sum rate, and symbol error rate of the proposed schemes are significantly better than those of two‐step and three‐step decode‐and‐forward and amplify‐and‐ forward strategies for the examples considered.

This paper studies the energy-limited performance of multi-relay networks. Taking the properties of the power amplifiers (PAs) into account, we derive closed-form expressions for the optimal power allocation, the outage probability and the throughput in the cases with a sum consumed energy constraint. Moreover, we analyze the diversity and the multiplexing gains of the PA-aware systems. Finally, we investigate the performance of large-scale multi-relay networks and develop efficient multi-relay systems with low cooperation overhead. The numerical and the analytical results show that the inefficiency of the PAs affects the outage probability and the throughput of the multi-relay systems substantially.

Spectrum mobility in cognitive radio networks not only enables the secondary users to guarantee the desired QoS of the primary users but also grants an efficient exploitation of the available spectrum holes in the network. In this paper, we propose a probabilistic approach in determining the initial and target channels for the handoff procedure in a single secondary user network. To characterize the network, a queuing theoretical framework is introduced, and “stay” and “change” handoff policies are both addressed. The performance of the secondary user in terms of average sojourn and extended service times for secondary connections is analyzed, and convex optimization problems with the objective of minimizing those times as well as analytical solutions are presented. Simulation results confirm the validity of our analytical approach.

In wireless body area networks (WBANs) it is necessary to devise an energy-efficient MAC scheduling mechanism which is capable of meeting strict QoS requirements. In this paper, special characteristics of WBAN channels, namely, the slow fading, the periodicity of fading, and the correlation among the channels have been exploited to formulate the sensor scheduling problem as a partially observable Markov decision problem (POMDP). Specific algorithms based on value iteration and pruning have been proposed in order to reduce the computational burden associated with the corresponding POMDP. The proposed scheduling mechanism is compared against a TDMA scheduling for a three-sensor network, and an average 22-32% improvement in energy consumption and slight improvement in reliability is reported.

Femtocell has appeared as a solution to increase both the coverage and the capacity of cellular networks. However, interference problem between the macrocell and the femtocell must be solved before any deployment. Since femtocells are deployed by home users in random locations, decentralised interference management schemes remain worth of investigation. In this study, the author deal with the problem of aggregate interference modelling and static resource allocation in orthogonal frequency division multiple access (OFDMA)‐based two‐tier femtocell networks for both closed and open access femtocells. It is assumed that for macrocell users (MUs), spectrum allocation is accomplished through fractional frequency reuse. The author's objective is to maximise the femtocell user's throughput while maintaining as low as its possible impact on the MUs’ performance. Their proposed algorithm is decentralised, and based on some measurements, needs to be performed only when femtocell access point is plugged in. Simulation results show the accuracy of their model and the effectiveness of the proposed scheme compared to the previous methods.

In this paper, a diffusion-based molecular communication channel between two nano-machines is considered. The effect of the amount of memory on performance is characterized, and a simple memory-limited decoder is proposed; its performance is shown to be close to that of the best possible decoder (without any restrictions on the computational complexity or its functional form), using genie-aided upper bounds. This effect is adapted to the case of Molecular Concentration Shift Keying; it is shown that a four-bit memory achieves nearly the same performance as infinite memory for all of the examples considered. A general class of threshold decoders is considered and shown to be suboptimal for a Poisson channel with memory, unless the SNR is higher than a computed threshold. During each symbol duration (symbol period), the probability that a released molecule hits the receiver changes over the duration of the period; thus, we also consider a receiver that samples at a rate higher than the transmission rate (a multi-read system). A multi-read system improves performance. The associated decision rule for this system is shown to be a weighted sum of the samples during each symbol interval. The performance of the system is analyzed using the saddle point approximation. The best performance gains are achieved for an oversampling factor of three for the examples considered.

We propose a novel cooperative transmission scheme called Convolutional Network-Coded Cooperation (CNCC) for a network including N sources, one M-antenna relay, and one common destination. The source-relay (S-R) channels are assumed to be Nakagami-m fading, while the source-destination (S-D) and the relay-destination (R-D) channels are considered Rayleigh fading. The CNCC scheme exploits the generator matrix of a good (N + M', N,v) systematic convolutional code, with the free distance of dfreedesigned over GF(2), as the network coding matrix which is run by the network's nodes, such that the systematic symbols are directly transmitted from the sources, and the parity symbols are sent by the best antenna of the relay. An upper bound on the BER of the sources, and consequently, the achieved diversity orders are obtained. The numerical results indicate that the CNCC scheme outperforms the other cooperative schemes considered, in terms of the diversity order and the network throughput. The simulation results confirm the accuracy of the theoretical analysis.

In this study, the average throughput maximisation of a secondary user (SU) by optimising its spectrum sensing time is formulated, assuming that a priori knowledge of the presence and absence probabilities of the primary users (PUs) is available. The energy consumed to find a transmission opportunity is evaluated, and a discussion on the impacts of the number of PUs on SU throughput and consumed energy are presented. To avoid the challenges associated with the analytical method, as a second solution, a systematic adaptive neural network‐based sensing time optimisation approach is also proposed. The proposed scheme is able to find the optimum value of the channel sensing time without any prior knowledge or assumption about the wireless environment. The structure, performance and cooperation of the artificial neural networks used in the proposed method are explained in detail, and a set of illustrative simulation results is presented to validate the analytical results as well as the performance of the proposed learning‐based optimisation scheme.

The compute‐and‐forward (CMF) method has shown a great promise as an innovative approach to exploit interference towards achieving higher network throughput. The CMF was primarily introduced by means of information theory tools. Although there have been some recent works discussing different aspects of efficient and practical implementation of CMF, there are still some issues that are not covered. In this study, the authors first introduce a method to decrease the implementation complexity of the CMF method. They then evaluate the exact outage probability of the authors proposed simplified CMF scheme, and hereby provide an upper bound on the outage probability of the optimum CMF in all signal‐to‐noise ratio (SNR) values, and a close approximation of its outage probability in low SNR regimes. They also evaluate the effect of the channel estimation error (CEE) on the performance of both optimum and their proposed simplified CMF by simulations. Their simulation results indicate that the proposed method is more robust against CEE than the optimum CMF method for the examples considered.

Effective spectrum sensing strategies enable cognitive radios (CRs) to identify and opportunistically transmit on under-utilized spectral resources. In this paper, sequential channel sensing problems for single and multiple secondary users (SUs) networks are effectively modeled through finite state Markovian processes. More specifically, a model for single user case is introduced and its performance is validated through analytical analysis. Then, in order to address multiple SUs case, this model is extended to include the modified p-persistent access (MPPA) protocol. Since the scheme utilized experiences a high level of collision among the SUs, to mitigate the problem appropriately, p-persistent random access (PPRA) protocol is considered, which offers higher average throughput for SUs by statistically distributing their loads among all channels. The structure and performance of the proposed schemes are discussed in detail, and a set of illustrative numerical results is presented to validate and compare the performance of the proposed sense-access strategies.

Femtocell has appeared as a solution to increase both coverage and capacity of cellular networks. However, interference problem between the macrocell and the femtocell must be solved before any deployment. In this paper, we deal with the problem of joint subband, rate, and power allocation in OFDMA based two-tier femtocell networks. It is assumed that for macrocell users, spectrum allocation is accomplished through Fractional Frequency Reuse (FFR). Our objective is to maximize the femtocell user's throughput while maintaining as little as possible reduction in the macrocell users' performance. Our proposed algorithm is decentralized, and needs to be done only when femtocell access point is plugged in, based on some measurements. Simulation results show superior performance of the proposed scheme compared to the other methods.

Powerful spectrum decision schemes enable cognitive radios (CRs) to find transmission opportunities in spectral resources allocated exclusively to the primary users. One of the key effecting factors on the CR network throughput is the spectrum sensing sequence used by each secondary user (SU). In this study, SUs’ throughput maximisation through finding an appropriate sensing matrix (SM) is investigated. First, the average throughput of the CR network is evaluated for a given SM. Then, an optimisation problem based on the maximisation of the network throughput is formulated in order to find the optimal SM. As the optimum solution is very complicated, three novel suboptimal solutions are proposed for various cases including perfect and non-perfect sensing. Despite having less computational complexities as well as lower consumed energies, the proposed solutions perform quite well compared to the optimum solution. The structure and performance of the proposed SM setting schemes are discussed in detail and a set of illustrative numerical results is presented to validate their efficiencies.

In this paper, we consider a broadband secondary transmitter-receiver pair which interferes with N narrowband primary users and study the effect of cognition and cooperation on the maximum stable throughput. In our study we focus on four transmission protocols as well as two channel types, i.e., flat fading and frequency selective fading. In the cooperative protocols, the broadband transmitter relays the packets of the primary users which have not correctly decoded at the primary receiver. The analysis includes random packet arrivals at the transmitters which may impact on the maximum stable throughput. Moreover, sensing errors at the secondary user are considered. In this paper, we derive the exact stability region for non-cooperative protocols and inner bounds for cooperative ones. The results reveal that depending on the channel states, the cooperative protocols may provide significant performance gains over non-cooperative protocols. Numerical results indicate that in the frequency selective channel, independent and simultaneous parallel transmissions in the available subbands are preferred, while in the flat fading channel, a non-parallel transmission in the total available bandwidth is recommended. In part II of our paper, we investigate the impact of cognition and cooperation on the delay performance of the protocols introduced in this paper.

In this paper, we present the bit error probability (BEP) of a relay network employing UWB links between its nodes. Since UWB systems can resolve many paths and are thus rich in multipath diversity, the use of the Rake diversity combining is very effective. However, because of the complexity, it is impractical in the scenarios with portable terminals or in the networks with multiple relays. Time-reversal technique, in which the transmitting nodes can do the pre-diversity filtering of UWB signal before transmission, is known as an approach to achieve the performance equivalent to the Rake combining without increasing the receiver complexity. The performance of such a network with simple correlators for each link at the receiver side is analyzed considering some realistic channel estimation errors. The results show that there is a great potential in combining time reversal and UWB techniques with respect to both reducing the receiver complexity and improving the system performance. The system robustness to imperfect channel estimation is also shown in the numerical analysis and compared to traditional UWB system with All-rake receiver. The analytical expressions are also validated by computer simulations which confirm the accuracy of the approximations used in obtaining these results.

In this paper, we propose a novel cognitive cooperative relaying scheme using multi-carrier transmission in which cognitive users assist primary users by relaying their information using Decode and Forward (DF) strategy. The best cognitive user is selected as a relay for a Primary User (PU). Outage probability of the PU is investigated. Our analyses show substantial improvements in the outage probability of the PU when using the proposed cooperative scheme compared to non-cooperative one. We also perform several simulations to corroborate our theoretical results.

This paper proposes efficient cooperative cyclostationary spectrum sensing schemes in which each secondary-user (SU) performs single-cycle (SC) cyclostationary detection for fast and simple implementation, while collaboration between SUs in final decision on the presence or absence of the primary-user (PU) is explored to improve its performance. As the SUs simultaneously measure the spectral correlation functions at different cycle-frequencies (CF) and exchange their information regarding the measured results, a sufficient number of CFs are effectively examined with parallel searching, which makes the proposed cooperative spectrum sensing more reliable. This paper presents another look at performance evaluation of cyclostationary detectors in terms of deflection coefficients. Outage probability of deflection coefficient is defined as a measure to compare the performance of different cyclostationary detectors in a fading channel. Furthermore, performance of the proposed schemes in terms of false-alarm and detection probabilities is evaluated by analysis and simulation in AWGN and fading channels. Illustrative and analytical results show that the proposed schemes outperform both SC and multi-cycle (MC) cyclostationary detectors, especially in fading channels.

The paper proposes efficient cooperative cyclostationary spectrum sensing schemes in which each secondary user (SU) performs single-cycle cyclostationary detection for fast and simple implementation, while collaboration between SUs in final decision on the presence or absence of the primary user (PU) is explored to improve its performance. As the SUs simultaneously measure the spectral correlation functions at different cycle frequencies (CF) and exchange their information regarding the measured results, a sufficient number of CFs are effectively examined in a short period of time because of parallel searching, which makes the proposed cooperative spectrum sensing more reliable and faster. Performance of the proposed schemes in terms of false-alarm and detection probabilities and deflection coefficients is evaluated by analysis and simulation in AWGN, fading and shadowing channels. Illustrative results show that the proposed schemes outperform both single-cycle (SC) and multi-cycle (MC) cyclostationary detectors, especially in fading and shadowing channels.

The authors investigate the achievable rates of a multiple-access channel with one cognitive transmitter in a fading channel. The cognitive transmitter uses dirty-paper coding to mitigate the known interference. They assume partial transmit channel state information which is obtained through a quantised limited feedback. In this model, the receiver sends as feedback only a finite number of bits describing the fading coefficients. Numerical results show that different quantisation schemes must be used for different transmitters (cognitive and non-cognitive) in order to maximise their rates. Also, the number of quantisation bits allocated to each channel coefficient (i.e. the channels from the non-cognitive and cognitive transmitters to the receiver), for a fixed total feedback bits, can affect the achievable rates of the transmitters, and even for each channel coefficient, how to dedicate the considered quantisation bits between the coefficient magnitude and phase can have substantial effects on the rate.

A novel orthogonal frequency division multiple access-based cognitive cooperative relaying scheme is proposed in which cognitive users assist primary users (PUs) by relaying their information. Both PUs and cognitive users communicate to a common base station. Based on the received feedback, cognitive users relay data of PUs. Two different schemes of relay assignment and power allocation are considered. Probability of error of PU and outage probabilities of both primary and cognitive users are investigated. Our analyses show substantial improvements in the probability of error and outage probability of PU while the outage probability of the cognitive user does not change significantly. We also perform several simulations to corroborate our theoretical results.

Despite their attractive features, free-space optical (FSO) communications suffer from several challenges in practical deployment; the major of them is fading or scintillation. To overcome such limitations, multiple input multiple output (MIMO) and cooperative techniques have been proposed. Although the promising effects of cooperative transmissions in radio frequency (RF) communications have greatly been investigated, there have not been remarkable researches on cooperative transmissions in FSO communications. In this study, the authors consider a relay-assisted FSO communication and investigate the outage performance of two well-known cooperative protocols, namely amplify-and-forward (AF) and decode-and-forward (DF), for this communication system. By a new definition of the order of diversity, the authors derive an analytical expression for the performance comparison of different cooperative and non-cooperative protocols. The authors also evaluate the performance of a FSO communication with two transmitter telescopes (MIMO systems) and compare its outage probability with those of cooperation strategies. The authors investigate the best place for the relay for both the AF and DF strategies and show that relay-assisted techniques can be used for FSO communication to achieve spatial diversity. Their performance can surpass that of the two-transmitter case by properly choosing the location of the relay.

In this paper, we consider the problem of spectrum sensing by using multiple antenna in cognitive radios when the noise and the primary user signal are assumed as independent complex zero-mean Gaussian random signals. The optimal multiple antenna spectrum sensing detector needs to know the channel gains, noise variance, and primary user signal variance. In practice some or all of these parameters may be unknown, so we derive the generalized likelihood ratio (GLR) detectors under these circumstances. The proposed GLR detector, in which all the parameters are unknown, is a blind and invariant detector with a low computational complexity. We also analytically compute the missed detection and false alarm probabilities for the proposed GLR detectors. The simulation results provide the available traded-off in using multiple antenna techniques for spectrum sensing and illustrates the robustness of the proposed GLR detectors compared to the traditional energy detector when there is some uncertainty in the given noise variance.

In this paper, the extension of cooperative communication to the context of TH-UWB is investigated. In particular, the average bit error probability (BEP) is provided for cooperative TH-UWB systems with decode-and-forward relaying protocol. In the considered relay network, UWB links among the nodes are modeled according to IEEE 802.15.4a standards. Our methodology is based on computing the characteristic function (CF) of the decision variable at the destination terminal. We use Gaussian quadrature numerical method to approximate the CF of interference component appeared in decision variable term. This technique permits to predict the system performance in different IEEE 802.15.4a defined channels with high accuracy and reasonable complexity. Numerical results show that significant improvement in the BEP of impulse radio UWB system is obtained by adding a relay node. The analytical expressions are also validated by computer simulations which confirm the accuracy of the approximations used in obtaining the BEP.

A multistage decoder for the internally convolutionally coded fibre-optic time-hopping code division multiple access system recently introduced is considered. In this system, the decoder consists of several stages. The first stage is implemented using one of the single-user decoders introduced. The following stages are maximum likelihood (ML) decoders each of which use the decisions made by the previous stage. The performance of the proposed decoder is evaluated by a Monte Carlo simulation. Numerical results show that a multistage decoder with only two stages greatly outperforms the single-stage decoder with negligible increase in complexity. The authors also derive the Chernoff bound for the ML decoder with known interference, which is the ultimate performance of the multistage decoder.

The internally coded time-hopping coherent ultra-short light pulse code division multiple access (CULP CDMA) scheme (recently introduced) with an optical amplifier is described and its performance in fibre-optic communication systems is analysed. In accordance with the important role of optical amplifiers in optical communication systems, a preamplifier at the input of the receiver is used in order to compensate the losses because of the spectral encoder, spectral decoder and optical fibre path. The authors evaluate the bit error rate of the system considering the effects of the multiple access interference, noise because of the optical amplifier and thermal noise using saddle point approximation, and compare the results with those of the conventional CULP CDMA system with and without an amplifier. The numerical results indicate a substantial improvement in the performance of the coded system in comparison with the uncoded one. In addition, the negative effect of amplifier noise in the proposed scheme is much less than that of the conventional CULP CDMA system.

In part I, a new internally coded time hoping optical code division multiple access (TH/OCDMA) scheme for fiber optic communication systems has been proposed and its multiple access performance has been evaluated using optical orthogonal code (OOC). Due to low cardinality of OOCs with a correlation value of 1, the capability of the proposed scheme could not be utilized effectively for an increasing number of simultaneous users. In this part, we consider applying the internally coded technique introduced in part one to the frame time hopping/OCDMA scheme. We evaluate the multiple access performance of the system for the three detectors introduced in part I. Our results demonstrate the capability of the internally coded TH/CDMA scheme to substantially increase the maximum number of simultaneous users when using FTH codes instead of OOCs.

We derive bounds on the expected capacity and outage capacity of a three-node relay network for UWB communications. We also provide a simple tight approximation for the derived upper bound on the capacity and then using this bound we obtain the outage probability of the network. Numerical results show that a significant improvement in the system capacity and outage probability is obtained by adding a relay node. Moreover, our theoretical results reveal that the diversity gain of a relay channel substantially increases by using UWB links instead of NB links. We also derive these bounds when we have a constraint on the total transmitted power of the source and the relay nodes.

In this paper, we divide a wide frequency range into multiple subbands and in each subband detect whether in a primary user (PU) is active or not. We assume that PU signal at each subband and the additive noise are white zero-mean independent Gaussian random processes with unknown variances. We also assume that at least a minimum given number of subbands is vacant of PU signal and propose an invariant generalized likelihood ratio (GLR) detector. The concept of the grouping of subbands allows faster spectrum sensing of a subset of subbands which may be occupied by a specific PU. Also, we evaluate trade-offs involved in the proposed algorithms by simulation.

In this paper, we consider the internally coded time-hopping spread-time multiple access scheme introduced in. We evaluate the performance of the correlator receiver followed by the Viterbi algorithm in fading channel. We analyze the effect of multiple access interference (MAI) and narrowband interference (NBI). We also provide simulation results and further evaluate the performance of the system for the ultra wide band (UWB) application with the UWB channel model specified by the IEEE 802.15.4a group. For comparison, we consider the conventional ST-CDMA system. The results indicate that our method has much better performance while it has the same bandwidth and spectral efficiency as the conventional ST-CDMA system.

We introduce a new communication model consisting of two interfering broadcast channels with one cognitive transmitter, who knows the message transmitted by the other non-cognitive transmitter, in a non-causal manner. An achievable rate region is derived for this model. The region is computed based on the Marton region for the general broadcast channel and its extension by Steinberg to the broadcast channel with the states known at the transmitter. By using the region evaluated, an achievable rate for the Gaussian broadcast channel is derived.

A new internally coded scheme that combines frequency hopping (FH) and multicarrier (MC) code division multiple access (CDMA) techniques using a super-orthogonal encoder is proposed. In this method, the total bandwidth is partitioned into Ns1 disjoint bands and each band is also segmented into Ns2 subbands. On the basis of a super-orthogonal encoder output and a pseudorandom sequence, one of the Ns1 bands is selected. The data bit is then transmitted in the band in the form of the multicarrier FH (MC-FH) CDMA scheme, that is, Ns2 carriers are transmitted in the Ns2 subbands of the selected band. At the receiver, both hard and soft detectors are used. The performance of the proposed method in the additive white Gaussian noise (AWGN) and fading channels is evaluated. The results indicate that the proposed method outperforms the previously presented uncoded and coded MC-FH-CDMA systems, where the data are transmitted over the whole bandwidth, keeping the same bandwidth (spectral efficiency). Further, in the new method, the carriers hop in part of the total bandwidth, and hence coherent detection is more feasible.

The spectrum sensing of a wideband frequency range is studied by dividing it into multiple subbands. It is assumed that in each subband either a primary user (PU) is active or absent in a additive white Gaussian noise environment with an unknown variance. It is also assumed that at least a minimum given number of subbands are vacant of PUs. In this multiple interrelated hypothesis testing problem, the noise variance is estimated and a generalised likelihood ratio detector is proposed to identify possible spectrum holes at a secondary user (SU). Provided that it is known that a specific PU can occupy a subset of subbands simultaneously, a grouping algorithm which allows faster spectrum sensing is proposed. The collaboration of multiple SUs can also be considered in order to enhance the detection performance. The collaborative algorithms are compared in terms of the required exchange information among SUs in some collaboration methods. The simulation results show that the proposed detector outperforms the energy detector in the presence of noise variance mismatch above 2.3 dB. Some involved trade-offs in the spectrum sensing using the proposed detector are discussed.

In this paper, we will combine the frequency hopping (FH) and the multi-carrier code division multiple access scheme (MC-CDMA) through an internal coding. The total bandwidth will be divided to a number of sub-bands with equal bandwidth. In each signalling interval, the output of a super orthogonal encoder selects one of the sub-bands for data transmission. Sub-carriers are modulated by a user specific PN sequence, and are transmitted over the selected sub-band similar to the MC-CDMA communications. We will derive the exact bit error rate of the MC-CDMA system and the new coded system. It will be shown by analysis and simulation that the proposed system performs much better than the MC- CDMA system for the same bandwidth and spectral efficiency.

This paper considers a cooperative space-frequency (SF) coded OFDM system and evaluates its performance over quasi-static frequency selective Rayleigh fading channels. An expression for exact outage error probability is derived and its tight closed form lower bound is presented. The tightness of the lower bound is demonstrated through the Monte Carlo simulations. The asymptotic analysis indicates that the proposed protocol achieves full spatial and frequency diversities available in the cooperative communication system. The theoretical analysis of the proposed space-frequency coded cooperation protocol is further confirmed by computer simulations.

Recent measurements suggest the possibility of sharing spectrum among different parties subject to interference-protection constraints. In order to enable access to an unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. In this paper, we study spectrum-sharing between a primary licensee and a group of secondary users. The structure of an asymptotically optimum detector based on the measurements of all secondary users is derived and the effect of the quantization error in such a system is evaluated. The results show the superiority of the proposed detector to other schemes.

A cooperative space–frequency (SF) coded orthogonal frequency division multiplexing system is considered and its performance over quasi-static frequency-selective Rayleigh fading channels is evaluated. An expression for exact outage error probability is derived and its tight closed-form lower bound is presented. The tightness of the lower bound is demonstrated through Monte Carlo simulation. Asymptotic analysis indicates that the proposed protocol achieves full spatial and frequency diversities available in the cooperative communication system. The theoretical analysis of the proposed SF coded cooperation protocol is further confirmed by computer simulation using a previously introduced SF block code that is capable of achieving full spatial and frequency diversities.

A new time-hopping ultrawideband (TH–UWB) CDMA scheme for indoor wireless communications is presented. In the proposed method, the duration of each bit is divided into Ns1 frames, each one containing Ns2 subframes. Two pseudorandom sequences are assigned to each user. During each bit interval, based on the output of a super-orthogonal encoder and the user's first dedicated pseudorandom sequence, the transmitter selects one of the Ns1 frames and then transmits Ns2 narrow pulses in that frame, one in each of the Ns2 subframes. The location of the pulse in each subframe is determined by the user's second dedicated PN sequence. Four different detection techniques are considered at the receiver front end, namely thresholded hard decision, strict hard decision, soft decision and chip-based decision. Their performances are analysed and the results are compared with those of the previously introduced coded and uncoded TH–UWB systems. The results indicate that the proposed scheme has the best performance without requiring any extra bandwidth. It is also shown that the chip-based decoding technique works better in moderate and high SNRs while the soft decision method has better performance in low SNRs.

Traditionally, the frequency spectrum is licensed to users by government agencies in a fixed manner where the licensee has exclusive right to access the allocated band. However, with increasing demand for the spectrum and scarcity of vacant bands, a spectrum policy reform seems inevitable. Meanwhile, recent measurements suggest the possibility of sharing spectrum among different parties subject to interference-protection constraints. In order to enable access to an unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. Spectrum-sharing between a primary licensee and a group of secondary users has been studied. The structure of an asymptotically optimum detector based on the measurements of all secondary users is derived and the effect of the quantisation error in such a system is evaluated. Also, it is shown that by using the proposed detector in a sequential detection structure, it is possible to shorten the decision time needed by the detector. The results show the superiority of the proposed detector to other schemes.

In this paper, a new internally coded time-hopping optical code-division multiple-access (TH/OCDMA) scheme for fiber-optic communication systems is proposed, and its multiple-access performance is evaluated using optical orthogonal codes (OOCs). In the proposed method, the duration of each bit is divided into Nsframes, each one containing L chips. Two signature codes, namely, one pseudonoise (PN) sequence and one OOC, are assigned to each user. During each bit interval, based on the output of an channel encoder and the user's dedicated PN sequence, one of the Nsframes is selected, in which W pulses are transmitted in W chips, marked by the user's OOC. We consider three detectors at the receiver front end, namely, correlation, soft chip-level, and hard chip-level detectors. We evaluate the multiple-access performance of the system for each detector considering the effects of shot noise, dark current, and thermal noise. We compare the results with those obtained for the conventional OCDMA systems. Our numerical results indicate that, for the same bit rate and bandwidth, our proposed method substantially outperforms the conventional OCDMA system. Our results also show that the soft chip-level detector outperforms the other detection techniques in all cases considered

A class of generalized optical orthogonal codes (OOCs), namely, frame time-hopping (FTH) patterns with an extremely large cardinality, are studied for implementing multirate and multiservice (MR/MS) optical CDMA (OCDMA) networks. Conventional MR/MS methods, namely variable spreading rate and parallel mapping, are considered. Using FTH patterns, the problem of low OOC code cardinality in conventional MR/MS schemes is removed. Moreover, several new multicode methods, using subcode concatenation scheme, are proposed for MR/MS OCDMA. The proposed multicode schemes present flexibility for supporting MR/MS applications such as lower implementation complexity and cost, less need for service synchronization, and finally lower link power budget. Multiple-access performances of the systems are evaluated using saddle-point approximation methods considering photodetector shot-noise, dark current, and circuit thermal noise. The results show that the conventional parallel mapping outperforms the other schemes in high received powers, and the proposed multicode method, using Walsh subcode along with difference modulation, presents the best performance in low received powers for the cases considered.

We evaluate and compare the multiuser performance of the multicarrier frequency-hopping (MC-FH) CDMA system using coherent and non-coherent detection in AWGN and slowly frequency-selective Rayleigh fading channels in a downlink application. For fair comparison, we use BFSK modulation for both cases. Our numerical results indicate that in both AWGN and fading channels, coherent detection substantially outperforms non-coherent detection. Furthermore, it has been realized that the performance enhancement due to the diversity order is only observable for coherent detection.

In this paper, we introduce new unblind and blind multiuser detectors for an optical code-division multiple-access system. The detectors have two soft and hard stages. In the soft stage, a soft estimation of the interference is obtained by solving an unconstrained maximum-likelihood problem via the iterative expectation-maximization (EM) algorithm. Then, the hard stage detects the user information bit by solving a one-dimensional Boolean constrained problem conditioned on knowing the interference. Our results reveal that the proposed detectors have very low complexity, and are robust against changes in parameters. Moreover, the numerical results illustrate that despite of their simplicities, our detectors substantially outperform other well-known suboptimum detectors, such as multistage and decorrelating detectors.

In this paper, we consider a low-complexity iterative receiver for decoding multi-user information data for the bandwidth efficient convolutionally coded UWB TH-CDMA system, in a multipath channel. The proposed receiver structure consists of a bank of soft interference cancellers (SIC), followed by a block of MMSE filters. The outputs of these filters, are fed to a bank of likelihood calculators (LC), each followed by a soft-input soft-output (SISO) convolutional decoder. Information is fed back from SISO decoders to SICs for interference cancellation. This receiver requires the knowledge of the attenuation and delay of the strongest path of each active user. We evaluate the performance of the proposed receiver in multipath channels. Our numerical results show that the new receiver significantly improves the system performance.

In this paper, we consider the application of low-density parity-check (LDPC) codes in multi-carrier frequency-hopping (MC-FH) CDMA systems. We evaluate the performance of the coded system in a slowly Rayleigh fading frequency-selective channel using different construction methods of regular LDPC codes. We then compare the results with those of the super-orthogonal coded system. Our simulation results show that the LDPC coded scheme significantly outperforms the uncoded and the super-orthogonal coded scheme. Finally, we propose a new semi-random construction of regular LDPC code and evaluate its performance in MC-FH-CDMA system. Our numerical results indicate that this new construction outperforms other regular constructions of LDPC codes.

Recently, there have been several schemes on concatenated space-time codes. Previous works that do not assume perfect CSI at the receiver suffer from exponential complexity of decoder in rate and 3 dB performance loss of differential detection or the complexity of channel estimation [A. Grant et al., 2001],[A.V. Nguyen et al., 2000] due to using trellis structure for differential codes. In this paper, we propose an iterative joint channel estimation and data selection scheme for a concatenated orthogonal space-time block code, which is very simple and reaches coherent performance without assuming perfect CSI available at the receiver.

In this paper, we provide the exact multiuser performance analysis of the MC-FH system introduced in [E. Lance and G.K. Kaleh, 1997] in the presence of partial-band interference in both AWGN and fading channels. For comparisons, we also provide the performance analysis of both MC-FH and FFH systems based on Gaussian distribution assumption. The partial-band interference is assumed as a Gaussian zero-mean random process with flat power spectrum on a fraction of the signal bandwidth. Our numerical results indicate that in both channels, the MC-FH system significantly outperforms the FFH system in the applications considered. Further, they indicate that the Gaussian analysis works quite well at high number of users.

In this paper, we consider a low complexity iterative multiuser receiver for the coded MC-FH-CDMA system, introduced in M. Ebrahimi T.M. and M. Nasiri-Kenari (May 2002). The structure of the proposed receiver consists of a multiuser likelihood calculator (MULC) and a bank of SISO channel decoders one for each active user. Based on the received signal and a priori information about code bits provided by SISO decoders in the previous iteration, MULC computes extrinsic information of coded symbols of active users in the form of log-likelihood ratio (LLR). This information is used by channel decoders as a priori information to compute an update of LLRs. The processing proceeds in an iterative manner similar to the decoding of turbo codes. We evaluate the performance of the proposed receiver in AWGN channels. Our numerical results show that the new receiver significantly improves the system performance. After a few iterations, the receiver reaches its ultimate performance, which is very close to the single-user performance.

We study and examine the effect of fading channels on the performance of spread-time/code division multiple access (ST/CDMA). We consider two different models for fading channel and obtain the average signal to interference plus noise ratio (SIR) for ST/CDMA in both cases. We also obtain the SIR for spread-spectrum/CDMA (SS/CDMA) and compare the results with that of ST/CDMA. Depending upon the transmitted chip pulse shape, and in the absence of thermal noise, spread-time (ST) performance is equal to or greater than spread spectrum (SS). For example ST/CDMA has 0.62 dB higher SIR value compared to SS/CDMA with root raised cosine pulse shape with rolloff factor 0.22 used in W-CDMA. The spread-time technique was first proposed by Crespo et al. (1991, 1995), and US Patents no. 5177768, 5175744, 5175743, 5173923, and is considered as the time-frequency dual of spread spectrum technique.

We consider using a soft-input decoder for internally bandwidth efficient convolutionally coded fiber-optic code division multiple access (CDMA) communication systems using optical orthogonal codes (OOC). The convolution codes that are used for demonstrating the capabilities of the soft-input decoder are super orthogonal codes (SOC). These codes are near optimal and have a relatively low complexity. We evaluate the upper bounds on the bit error probability of the internally coded fiber-optic CDMA system using soft-input and hard-input decoders, assuming both on-off keying (OOK) and binary pulse position modulation (BPPM) schemes. It is shown that the soft-input decoder significantly outperforms the hard-input decoder.

We evaluate the performance, in multipath Rayleigh fading channels, of both the uncoded and coded multiple access TH-UWB systems introduced by R.A. Scholtz (see Proc. Milcom 93, 1993), M.Z. Win (see Ph.D. dissertation, Electrical Engineering, University of Southern California, 1998) and A.R. Forouzan et al. (see Proc. PIAMC 2000, vol.2, p.1555-8, 2000). The receiver is a selective diversity combining receiver, known as SRAKE. Based on a Gaussian distribution assumption for multiple access interference at the output of the SRAKE receiver, and by using a virtual branch technique as introduced by M.Z. Win and J.H. Winters (see Proc. 49th Annual Int. Veh. Technol. Conf, vol.1, p.215-20, 1999; Proc. IEEE Int. Conf. on Commun., vol.1, p.6-10, 1999), the bit error rates for uncoded and coded schemes are derived. Our analysis shows that the effective order of diversity achieved by the coded scheme is the product of the number of branches of the SRAKE receiver and the Hamming distance of the code applied. Furthermore, it indicates that the coded scheme significantly outperforms the uncoded scheme.

One of the most prominent features of code division multiple-access (CDMA) cellular networks is their ability for applying soft-handoff. This type of handoff results in a better support of seamless communication services, capacity enhancement, QoS improvement, etc. Because of the existence of more parameters and flexibilities compared to hard-handoff, an optimum solution for soft-handoff management and admission control for new and handoff calls is necessary. We propose a new algorithm with two thresholds for admission of new and handoff calls, and with two decision boundaries for controlling of handoff traffic. We compare this algorithm with a simpler one with only one decision boundary, by employing a general and flexible 3-dimensional framework with respect to important traffic parameters including loss probability, carried traffic, blocking and dropping probabilities, and show that our algorithm can result in less loss probability as well as less blocking and dropping probabilities with more control over traffic growth in the cell.

A receiver structure, which his linear computational complexity with the number of users, is proposed for decoding multiuser information data in a convolutionally coded asynchronous DS-CDMA system in multipath fading channels. The proposed receiver architecture consists of a multiuser likelihood calculator followed by a bank of soft-input soft-output (SISO) channel decoders. Information is fed back from SISO channel decoders to multiuser likelihood calculator, and the processing proceeds in an iterative fashion analogous to the decoding of turbo codes. A simplification to the above receiver structure is given also. Simulation results demonstrate that, for both receiver structures, at high signal-to-noise ratios (SNR), both multiple-access interference (MAI) and intersymbol interference (ISI) are efficiently suppressed, and single-user performance is approached. Furthermore, the proposed iterative receivers are near-far resistant.

We compute the outputs of matched filters for spread-time CDMA with and without the effect of window which is used for truncating the resulting pulse. We obtain the signal to interference plus noise ratio (SIR) in both cases and compare the results. We examine the effect of different multiuser detectors used for spread spectrum (SS) in spread-time (ST). It is shown that ST and SS have the same performance while ST has the ability of matching the transmitted spectrum to the channel. Furthermore we consider the effect of synchronization error in the absence of any window. It is shown that synchronization error in timing causes infinite ISI terms that do not converge.

Spread-time code division multiple access (ST-CDMA) was proposed as an alternative CDMA scheme to spread spectrum (SS) (see Crespo, P.M. et al., Proc. IEEE Globecom, p.836-40, 1991; US Patents No. 5177768, 5175744, 5175743, 5173923; IEEE Trans. Commun., vol.43, no.6, p.2139-48, 1995). ST-CDMA can be considered as the time-frequency dual of SS-CDMA. In ST-CDMA, pseudorandom (PN) sequences are assigned to each user, and the Fourier transform of the transmitted pulse for a given user is determined by modulating the amplitude of the desired spectrum by the user's PN sequence. The main advantage of ST-CDMA is the flexibility with which power-limited pulses can be designed. We examine the effect of fading channels on the performance of spread-time. We compute the average signal-to-interference-plus-noise ratio in two different fading channels both for ST-CDMA and SS-CDMA. We show that ST-CDMA has 2.1 dB better performance than SS-CDMA with a rectangular pulse chip.

In this paper, we investigate the use of M-ary infrared CDMA technique in wireless infrared networks. It is shown that this method is highly capable in providing the multiple access facility for downlinks of infrared wireless networks. The results show that the M-ary CDMA technique, which fully utilizes the properties of optical orthogonal codes, achieves better BER performance in comparison with ordinary infrared CDMA networks. The main advantages are the reduction in bandwidth, power, and the elimination of threshold-setting at the receiver.

We consider the multiuser capacity of a multicarrier frequency-hopping (MC-FH) CDMA system as first introduced by Lance and Kaleh (1997). We propose to use a practical low-rate convolutional error correcting code in this system which does not require any extra bandwidth further than needed by the frequency-hopping spread-spectrum modulation. We provide a multiuser performance analysis of the system for both uncoded and coded schemes in AWGN and fading channels based on a Gaussian distribution assumption for the multiuser interference term. We also derive the exact bit error rate (BER) of the system in an AWGN channel. Our numerical results first indicate that the coded scheme significantly outperforms the uncoded scheme in both AWGN and fading channels. Moreover, it shows that the Gaussian analysis in some cases does not predict accurately the number of users supported by the system.

Matsufuji and Imamura (see IEEE Trans. on Info. Theory, vol.46, no.4, p.1691-95, 2000) proposed a new method to generate an optimal set of bent sequences as a set of signature sequences for a CDMA system has been introduced. This method is designed to make the CDMA system secure against interception, for both an insider intruder (a user) and an outsider intruder. The security analysis of this system of Matsufuji et al. was based on computing the cardinality of the search space. In this paper after reforming the bent function decomposition, we reapply the above analysis to the system. Then for binary sequences, we consider another analysis to this system, which is based on the fact that all the sequences in the search space do not have equal worthiness for the intruder. Moreover, another application of the signature assignment algorithm to provide multi-level security is presented.

In this paper, we consider the problem of multiuser detection in the presence of users with unknown signature codes for a synchronous coded CDMA system. Interference suppression for unknown users is done by projecting the received signal on the null space of known users and then using eigenvalue decomposition and subspace techniques to derive a detector that cancels the known interference and successfully mitigates the unknown interference. Significant performance gains over detectors that ignore unknown users and treats them as noise is achieved.

We propose a new scheme to reduce the peak-to-average power ratio (PAPR) in a MC-CDMA system. In this scheme, some codes among the family of signature codes used in the system are utilized only for PAPR reduction. A set of spreading codes and only one despreading code are assigned to each user. The user selects its own spreading code among the assigned set based on which one provides lower PAPR. The numerical results show that despite of its simplicity, our proposed approach substantially reduces the PAPR of MC-CDMA system at the expense of not capturing some portions of the transmitted energy in the receiver for some users.

We propose using a soft-input decoder for the decoding of internally convolutional coded Poisson noise-dominated fiber-optic code-division multiple-access (CDMA) communication systems using optical orthogonal codes. We first compute the coded symbol log-likelihoods at the output of the Poisson noise-dominated channel, which is then used by a soft-input maximum-likelihood decoder, for a fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. Furthermore, we develop a discrete soft-output channel model for a Poisson noise-dominated channel, with which we evaluate the upper bound on the bit error probability of the internally coded Poisson noise-dominated fiber-optic CDMA system using a soft-input decoder. It is shown that the soft-input decoder significantly outperforms the hard-input decoder. Furthermore, the performance of the soft-input decoder is also evaluated in the presence of different values of dark current.

A new spreading technique for intensity modulation direct detection fiber-optic code-division multiple-access (FO-CDMA) communication systems is proposed. This new spreading technique is based on generalized optical orthogonal codes (OOC) with large cardinality and minimal degradation in performance when compared with a more optimum system, namely, an optical CDMA system using OOC with autocorrelation and cross-correlation value bounded by one, i.e., OOC (/spl lambda/=1). To obtain the performance of such systems, we use a communication scheme, namely, frame time-hopping (FTH)-CDMA with random coding. Systems with generalized OOC patterns and random time-hopping coding are close in structure and performance. Furthermore, the performance of such systems is near the performance of optical CDMA with optimum but low cardinality OOC (/spl lambda/=1), which further renders the practicality of the proposed technique with very large cardinality. Two new receiver structures for FO-CDMA, namely, chip-level detector with optimum comparator threshold and correlation receiver with an electrical hard limiter, are also proposed. The performance analysis for a binary pulse position FTH-FO-CDMA network is considered for the correlation receiver, chip-level detector, correlation receiver with an optical hard limiter, optimum receiver, and the two newly proposed receiver structures. The results also show that a chip-level detector with optimum comparator threshold is superior to a chip-level detector for received low signal powers, and predict that the performance of the correlation receiver with an electrical hard limiter is superior to all other considered receiver structures, e.g., requiring one third of transmission power to achieve a desired bit error rate when compared with other receiver structures.

An ultra-wide bandwidth time-hopping spread-spectrum code division multiple-access system employing a binary PPM signaling has been introduced by Scholtz (1993), and its performance was obtained based on a Gaussian distribution assumption for the multiple-access interference. In this paper, we begin first by proposing to use a practical low-rate error correcting code in the system without any further required bandwidth expansion. We then present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates for the uncoded scheme. Furthermore, it indicates that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate, the coding scheme increases the number of users by a factor which is logarithmic in the number of pulses used in time-hopping spread-spectrum systems.

In a previous paper by R.A. Scholtz (see Proc. IEEE MILCOM '93, p.447-50, Oct. 1993), an ultrawide bandwidth time-hopping spread-spectrum code division multiple access system employing a binary PPM signaling was introduced, and its performance was obtained based on a Gaussian distribution assumption for the multiple access interference. A coded scheme of this system was proposed by the authors (see Proc. PIMRC '00, vol.2, p.1555-8, Sept. 2000), which showed much better performance in comparison to the uncoded scheme under the above Gaussian assumption. In this paper, we present a more precise performance analysis of the system for both coded and uncoded schemes. Our analysis shows that the Gaussian assumption is not accurate for predicting bit error rates at high data transmission rates.

In this paper we consider two multistage multiuser DS/CDMA detectors in multipath fading channels. The first detector is a modified version of detector proposed in Hui and Letaief (1998), and the second detector is a combination of the first one used as a PIC (parallel interference cancellation) detector and a detector called SIC (serial interference cancellation). These detectors take the advantage of multipath diversity to improve the signal to noise ratio (SNR). We also use an adaptive method to compute the threshold levels. We obtain an analytical equation for BER (bit error rate). The results show that the mentioned methods achieve much better performance than taht of Hui and Letaief.

We propose to use a practical low rate error-correcting code in a time-hopping spread spectrum multiple access system employing a binary PPM signaling. To this end, superorthogonal codes, which are near optimal and have relatively low implementation complexity, are considered. The performance analysis shows that the proposed coded scheme outperforms the uncoded scheme significantly, or more importantly, at a given bit error rate the proposed scheme increases the number of users by a factor which is logarithmic in the number of pulses used in a time-hopping spread spectrum system.

Indoors wireless infrared transmission is severely impaired by ambient noise. In addition, in a multiple user network, since many users share the same physical channel, considerable amount of multi-user interference will also be introduced. Infrared CDMA techniques are proposed to combat the effect of ambient and multi-user interference. Our analysis is based on photon counting approach and the bit error rate is given versus signal power. The results show that using infrared CDMA techniques, transmission can be performed well below the ambient noise power with transmission power that satisfies eye-safety power regulations.

We describe a class of DC-free subcodes of convolutional codes that satisfy certain runlength constraints and that also possess error-correcting capability. The running disparity and the maximum runlength of these codes are bounded by quantities that are independent of the free distance. They can be decoded using a Viterbi decoder for the underlying convolutional code.

Data transmission in a binary partial-response channel is often accomplished using a concatenated code consisting of an inner modulation code and an outer error-correcting code (ECC). We consider two inner decoders for such a code, each consisting of a reduced-complexity sequence detector modified to provide an estimate of the reliability of each bit. These reliability values are then used by the outer decoder to achieve improved performance. Although one of these decoders is considerably simpler than the other, their performances were comparable in the cases we considered. Both achieve considerable improvement over a decoder that uses hard-decision decoding of the inner code.

Considers the concatenation of an inner matched-spectral-null (MSN) code with an outer error-correcting code to achieve improved performance on a partial-response channel in the presence of additive white Gaussian noise. The authors show how to modify the reduced-complexity Viterbi algorithm of Karabed and Siegel [1991] to produce reliability values for each bit using the procedure proposed by Hagenauer and Hoeher [1989]. These reliability values are then used by the outer decoder to achieve improved performance. The authors simulated a number of codes and observed improvements due to soft-decision decoding of 1-2 db.>

Considers three methods for constructing (d,k) codes with guaranteed error-correcting capability. All three methods begin with state-dependent modulation codes to satisfy the (d,k) constraints. The first is an ad hoc method that uses the state-splitting procedure of Adler et al. (1983), together with additional constraints to ensure that the required minimum distance is achieved. The second method partitions the state-dependent modulation code into a part with memory and a part without, and then combines the part with memory with a matched convolutional code to provide error protection. The third method is obtained by modifying of the procedure of Lee and Wolf (1989) to accommodate state-dependent modulation codes. All three methods have yielded codes whose rates, computational complexities, and error-correcting capabilities compare favorably with codes designed using other methods.>