The paper considers the problem of deep-learning-based classification of digitally modulated signals using I/Q data and studies the generalization ability of a trained neural network (NN) to correctly classify digitally modulated signals it has been trained to recognize when the training and testing datasets are distinct. Specifically, we consider both a residual network (RN) and a convolutional neural network (CNN) and use them in conjunction with two different datasets that contain similar classes of digitally modulated signals but that have been generated independently using different means, with one dataset used for training and the other one for testing.

Secret key generation based on channel measurements has emerged as a meaningful approach to enhance the security of wireless links. The key generation mechanism takes advantage of wireless channel reciprocity and uses the mutual information between legitimate users' channel measurements. However, the rate at which key bits are generated depends on the channel coherence time. Furthermore, eavesdroppers may obtain information to generate the secret key bits if their channels become correlated to the legitimate channels. In this paper, we propose a new approach for key generation that uses MIMO precoding and role switching by legitimate users during the key generation process in order to prevent an eavesdropper whose channel is correlated with the legitimate channel to acquire information that can be used to generate the encryption key, while also ensuring that the rate at which uncorrelated key bits are generated does not depend on the channel coherence time. The proposed approach is supported by an information theoretic analysis and is illustrated with numerical results obtained from simulations showing that, even when the legitimate and eavesdropper channels are correlated, the bit mismatch rate at the eavesdropper does not improve with increasing SNR, while that at the legitimate users improves.

Secret key generation using channel measurements has emerged as a meaningful approach to enhance the security of a wireless link at the physical layer. This paper presents a novel approach to key generation based on precoding of the MIMO channel measurements, that does not require information on the channel statistics and has the ability to generate key bits at a rate that is independent of the channel coherence time. The proposed approach is illustrated with numerical results obtained from simulations showing the probability of bit disagreement between keys generated at transmitter/receiver side and Slepian-Wolf encoding with LDPC codes is used for key reconciliation.

Physical layer approaches for generating secret encryption keys for wireless systems using channel information has attracted increased interest from researchers in recent years. This paper presents a new approach for secret key generation that is based on one-bit quantization of channel measurements along with log-likelihood ratios (LLRs) that are calculated based on the difference between channel estimates at legitimate reciprocal nodes. The proposed approach, which implements advantage distillation along with information reconciliation using Slepian-Wolf low-density parity check (LDPC) codes, is discussed and illustrated with numerical results obtained from simulations. These show the probability of bit disagreement for keys generated through the proposed approach and compare it with that of alternative methods for key generation based on channel state information.

The paper studies waveform and receiver design for MIMO radars through a greedy procedure based on maximization of the signal-to-interference-plus-noise ratio (SINR). The procedure enables joint design of estimation waveforms for multiple extended targets and maximizes also the sum of radar mutual information measures. Numerical results obtained from simulations are presented to illustrate the proposed approach. Different scenarios with various levels of interference among targets are considered and characteristics of the waveforms designed in each scenario are discussed. Results indicate that the designed waveforms overlap in fewer frequency bands when interference between target reflections is stronger.

Steady-state visual evoked potentials (SSVEP) are a class of signals obtained from the electroencephalogram (EEG) that are used in conjunction with brain-computer interfaces (BCIs). Inducing SSVEP signals requires flickering lights as stimuli, typically in the range of 5-45 Hz. However, due to low signal-to-noise ratio (SNR), SSVEP signals generated in certain frequency ranges can be difficult to detect. This paper studies cyclostationary-based detection for SSVEPs, which is a popular method for signal detection in low SNR environments, but whose application in the context of BCI systems has received only limited attention in the BCI research community. The results presented in the paper demonstrate that cyclostationary-based detection of SSVEP using spectral correlation density (SCD) performs as well as canonical correlation analysis (CCA), which is the most widely used method of SSVEP classification.

Cognitive radio (CR) is an emerging concept that aims at enabling more efficient utilization of the frequency spectrum in future generations of wireless networks through the reuse of licensed frequency bands. Specifically, in CR networks, unlicensed transmitters are allowed to access licensed frequencies as secondary users (SU) if the primary users (PU) holding the spectrum license are not active. When multiple SUs with no central control or coordination are trying to access a band of licensed frequencies that is not actively used by PUs, the available spectrum is shared in a horizontal manner, where all SUs compete for spectrum access. In such a scenario, greedy approaches lead in general to increased interference in the system, which negatively affects the transmission rates for all users. To alleviate this problem, in this paper we propose a new approach for distributed allocation of SU transmit powers is based on avoiding interference while seeking to maximize SU transmission rates. The proposed approach is illustrated with numerical results obtained from simulation, which show that the resulting power allocation for SUs implies a more efficient use of the available spectrum than alternative approaches available in the literature.

Orthogonal Frequency Division Multiplexing (OFDM) is at the heart of current wireless standards and is the main modulation scheme envisioned for use in future generations of wireless communication systems. In this paper we study the performance of precoded OFDM systems in the presence of intentional jamming. Specifically, we consider OFDM systems with sub-band Hadamard precoding for which we derive the expression of the signal-to-jammer+noise-ratio (SJNR) and study how jamming signals with different bandwidths affect the bit error rate (BER) performance of the system.

One of the recent research directions in Intelligent Transportation System (ITS) focuses on Automatic Incident Detection (AID) techniques. Using advances in wireless networking and sensor technologies, modern vehicles have the ability to communicate with each other as well as with roadside infrastructure units in order to increase road safety. These innovations in transportation technology provide traffic managers with the ability to use data collected from the vehicles on the road to detect congestion and traffic incidents. Lately, many techniques were developed to alert drivers in advance about traffic incidents and to enable them to avoid congestion. In this paper, we propose two new techniques for automatic detection of traffic incidents in a highway scenario that are based on the use of distance and time for changing lanes, respectively vehicle speed changes over time. The proposed methods are illustrated with numerical results obtained from simulations that show higher incident detections rate those obtained with alternative methods.

In this paper, we study joint beamforming and power control for downlink multiple-input multiple-output systems with multiple users and target values for signal-to-interference plus noise ratios SINRs. We formulate this as a constrained optimization problem of minimizing total interference subject to constraints on the beamforming vector norms, target SINRs, and total transmit power. Necessary and sufficient conditions satisfied by the optimal beamformer and power allocation are presented, and a new algorithm for joint beamforming and power control is proposed. This adapts the beamforming vectors and transmit powers incrementally, and it stops when the specified SINR targets are achieved with minimum powers. The proposed algorithm is illustrated with numerical results obtained from simulations, which study its convergence and compare it with other similar algorithms. Copyright © 2013 John Wiley & Sons, Ltd.

Cognitive Radio (CR) is a new concept in wireless communication systems that aims at enabling dynamic access to the frequency spectrum for unlicensed secondary users (SU) through the reuse of licensed frequencies. Specifically, CR systems are expected to detect idle licensed frequencies (spectrum holes) by applying a spectrum sensing procedure, to take advantage of them for providing service to the SU, and to release them to the licensed primary users (PU) upon request. The duration of the spectrum sensing procedure is an overhead factor that affects the actual utilisation of the spectrum by the CR system and whose adaptation may enhance spectrum utilisation: the less time the CR spends for spectrum sensing and identifying spectrum holes, the more time the spectrum is available for secondary use by the CR. In this paper, we define a formal measure for spectrum hole utilisation in dynamic scenarios and propose a new method for adapting the duration of energy‐based spectrum sensing that improves the detection of PU signals as well as the utilisation of spectrum holes by the SU. The proposed method is illustrated with numerical results obtained from simulations which confirm its effectiveness.

Cognitive Radio (CR) is a new concept in wireless communication systems that aims at enabling opportunistic spectrum access (OSA) to licensed frequencies by secondary users (SU). Specifically, CR systems are expected to detect idle periods when licensed spectrum is not used by primary users (PU), to take advantage of them for providing service to SU, and to release them when PU become active. Thus, OSA performance relies on the ability of the SU to accurately detect the PU activity, and this paper examines the relationship between changes in the PU activity and the performance of energy-based spectrum sensing. Analytical expressions for the probabilities of detection and false alarm are derived by explicitly considering the probability that the PU state switches while the SU is sensing the spectrum, and are corroborated with numerical results obtained from simulations.

Cognitive radios (CR) that sense spectrum usage and adjust their transmissions to meet specified spectral constraints have been proposed for use in future generations of wireless systems to enable dynamic access to the frequency spectrum. In cooperative spectrum sensing, a fusion center (FC) receives local sensing information from multiple CRs and combines it to provide better decision on the spectrum use. In dynamic scenarios where occurrence of spectrum opportunities is intermittent and their durations have limited time, the performance of cooperative sensing is affected by the size of the sensing window at local CR and by the time delay introduced by the FC for processing of the local sensing information. In this paper we study how these parameters influence the performance of cooperative sensing by introducing a new metric which incorporates both probability of detection along and spectrum utilization, and which can be used to optimize the performance of cooperative sensing. Numerical performance studies in dynamic scenarios where the PU activity changes in time and/or is affected by fading are also presented.

Cooperative sensing has been shown to improve the performance of spectrum sensing in cognitive radio (CR) networks where multiple secondary users are sending local sensing information to a CR fusion centre (FC) which makes the final determination on the occupancy of a given frequency band by licensed primary users. In this study, the authors observe the use of a hidden Markov model for evaluating the performance of cooperative sensing at the FC and propose a method that uses the history of FC sensing decisions to estimate the cooperative probabilities of detection and false alarm. The proposed method enables the FC to become aware when the performance of cooperative spectrum sensing degrades without requiring knowledge of the local sensing statistics. Numerical results obtained from simulations confirm the effectiveness of the proposed method for both soft and hard combining schemes in practical scenarios with noise and/or multipath fading.

Cognitive Radio (CR) is a new concept in wireless communication systems that aims at enabling dynamic access to the frequency spectrum for unlicensed secondary users (SU) through the reuse of licensed frequencies. Specifically, CR systems are expected to detect idle frequency bands that are not actively used by licensed primary users (PU) by applying a spectrum sensing procedure, to take advantage of them for providing service to SU, and to release them to the PU upon request. The duration of the spectrum sensing procedure is an overhead factor that affects the actual utilization of the spectrum by the CR system, and in this paper we propose a new method for adapting the duration of energy-based spectrum sensing window that improves the detection of PU signals as well as the utilization of spectrum opportunities. We present also numerical results obtained from simulations which confirm the effectiveness of the proposed method.

In this paper we study the use of a beaconing mechanism for vehicle-to-infrastructure communication and information exchange in intelligent transportation systems. We provide analytical expressions for the probability of successful connection between roadside infrastructure and passing vehicles showing the dependence on the beacon frequency and distribution of the vehicle speed, as well as on the amount of information to be exchanged between vehicle and roadside infrastructure and the data rate of the wireless link established between the vehicle and roadside infrastructure. The performance of the beaconing mechanism is illustrated for specific scenarios which show that it can be used successfully used in conjunction with wireless systems using the Dedicated Short Range Communications (DSRC) standard as well as other types of systems.

Cognitive radios (CR) are regarded as a viable solution to enabling flexible use of the frequency spectrum in future generations of wireless systems by taking advantage of spectrum holes, that is idle frequency bands not actively used by primary users (PU) for licensed transmissions. The actual utilization of a spectrum hole is determined by its duration (implied by the time interval between two PU transmissions) as well as by the duration of the spectrum sensing window. In this paper we propose a new method for adapting the duration of the spectrum sensing window that improves the probability of detection of the spectrum holes as well as their actual utilization. The proposed method is illustrated with numerical results obtained from simulations which show that both the probability of detection and the spectrum hole utilization are improved when compared to a scheme with fixed sensing window length.

Cognitive radio (CR) technology is an emerging technique for dynamic spectrum access, which attempts to share licensed spectrum with licensed users opportunistically and dynamically to provide high bandwidth and efficient spectrum utilisation. In this study, the authors address the problem of finding combined precoder and power of CRs for dynamic spectrum sharing in interference networks. First, the authors present the precoder and power optimisation problem in spectrum underlay by considering both power and interference constraints where CR users coexist and transmit simultaneously with primary users. Then the authors particularise the problem of precoder and power adaptation to spectrum overlay where CRs operate only in idle frequency bands obtained from spectrum sensing. In the proposed approach, bandwidth use is expressed in terms of a finite set of discrete resources whose use is optimised by the CR transmitters in terms of achievable rate corresponding to individual link under operating constraints that restrict the operation of unlicensed secondary CR transmitters to protect licensed primary user transmissions. The authors formulate the optimal resource allocation for CR links as a constrained optimisation problem and solve using the Lagrange method. The authors illustrate the proposed algorithm with the help of numerical results obtained from simulations.

In this paper we consider orthogonal frequency division multiplexing (OFDM) based cognitive radio systems that operate as secondary users (SUs) in a spectrum overlay approach and study joint spectral shaping and power control subject to specified target signal-to-interference+noise ratio (SINR) as a constrained optimization problem. We discuss necessary and sufficient conditions for the optimal solution of this problem where the spectrum overlay constraints are satisfied and the specified target SINR value is achieved with minimum transmit power. We also present an algorithm that incrementally adapts the OFDM transmitter to reach the optimal solution and study the convergence speed of the proposed algorithm through simulations.

Cognitive radio (CR) technology represents a promising solution for solving the problem of spectrum scarcity and underutilization in wireless communications. While orthogonal frequency division multiplexing (OFDM) is regarded as a suitable technology for the CR physical layer, it can suffer from out-of-band radiation, which hinders its spectrum sharing capabilities. In this paper we consider a turbo-coded OFDM based CR system, and propose three methods for shaping the spectrum of the CR signal to reduce interference affecting primary users (PUs) that operate in adjacent bands. The proposed methods use multiple choice sequences (MCS) generated at the bit level by employing different pseudo random bit sequences, by using bit interleavers with different depths, or by a combination of both, respectively. In order to reduce interference to adjacent bands, the sequence with the best spectral shape is transmitted along with its index. Performance of the proposed methods is illustrated with numerical results obtained from simulations, and the trade off between a better spectral shape and the reduction in the spectral efficiency of the systems is discussed.

In this paper we study distributed transmitter adaptation with limited feedback where a B-bit codebook known at both transmitter and receiver is used in the update process. Specifically, we discuss predictive vector quantization (PVQ) where the transmitter is updated incrementally using the codebook element which results in smallest distortion in sum capacity from among the available 2Bcodebook entries. The performance of the proposed PVQ method is compared with that of random vector quantization (RVQ) with numerical results obtained from simulations.

Spectrum awareness is one of the most challenging requirements in cognitive radio (CR). To adequately adapt to the changing radio environment, it is necessary for the CR to be able to perform joint detection and classification of low signal-to-noise ratio (SNR) signals. In this paper we propose a joint detection and classification algorithm for the mobile Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE) signals, which exploits the second-order signal cyclostationarity. Simulation results are presented, which show the efficiency of the proposed algorithm under diverse scenarios. Furthermore, we provide new analytical findings related to the second-order cyclostationarity of the signals of interest.

In this paper, we present a new scheme for dynamic adaptation of transmission power and contention window (CW) size to enhance performance of information dissemination in Vehicular Ad-hoc Networks (VANETs). The proposed scheme incorporates the Enhanced Distributed Channel Access (EDCA) mechanism of 802.11e and uses a joint approach to adapt transmission power at the physical (PHY) layer and quality-of-service (QoS) parameters at the medium access control (MAC) layer. In our scheme, transmission power is adapted based on the estimated local vehicle density to change the transmission range dynamically, while the CW size is adapted according to the instantaneous collision rate to enable service differentiation. In the interest of promoting timely propagation of information, VANET advisories are prioritized according to their urgency and the EDCA mechanism is employed for their dissemination. The performance of the proposed joint adaptation scheme was evaluated using the ns-2 simulator with added EDCA support. Extensive simulations have demonstrated that our scheme features significantly better throughput and lower average end-to-end delay compared with a similar scheme with static parameters.

In this paper we present a new algorithm for detecting mobile Worldwide Interoperability for Microwave Access (WiMAX) signals in cognitive radio systems, which is based on preamble-induced second-order cyclostationarity. We derive closed form expressions for the cyclic autocorrelation function (CAF) and cyclic frequencies (CFs) due to the preamble, and use these results in the proposed algorithm for signal detection. The algorithm is illustrated with numerical results obtained from simulations, which demonstrate its efficiency under diverse scenarios.

In this paper, a distinctive cyclostationarity-based feature of the Long Term Evolution (LTE) Orthogonal Frequency Division Multiplexing (OFDM) signals used in the Frequency Division Duplex (FDD) downlink transmission is proved, and further employed for their detection. This relates to the existence of the reference signals (RSs) used for channel estimation and cell search/acquisition purposes. The analytical closed form expressions for the RS-induced cyclic autocorrelation function (CAF) and cyclic frequencies (CFs) are derived. Based on these findings, a signal detection algorithm is then developed. Simulation results show that the proposed algorithm achieves a good detection performance for low signal-to-noise ratios (SNRs), short sensing times, and under diverse channel conditions.

Next generation wireless systems will be required to support heterogeneous services with different transmission rates that include real time multimedia transmissions, as well as non-real time data transmissions. In order to provide such flexible transmission rates, efficient use of system resources in next generation systems will require control of both data transmission rate and power for mobile terminals. In this paper we formulate the problem of joint transmission rate and power control for the uplink of a single cell CDMA system as a non-cooperative game. We assume that the utility function depends on both transmission rates and powers and show the existence of Nash equilibrium in the non-cooperative joint transmission rate and power control game (NRPG). We include numerical results obtained from simulations that compare the proposed algorithm with a similar one which is also based on game theory and it also updates the transmission rates and powers simultaneously in a single step.

Game theory has emerged as a new mathematical tool in the analysis and design of wireless communication systems, being particularly useful in studying the interactions among adaptive transmitters that attempt to achieve specific objectives without cooperation. In this paper, we present a game-theoretic approach to the problem of joint transmitter adaptation and power control in wireless systems, where users' transmissions are subject to quality-of-service requirements specified in terms of target signal-to-interference-plus-noise ratios (SINRs) and nonideal vector channels between transmitters and receivers are explicitly considered. Our approach is based on application of separable games, which are a specific class of noncooperative games where the players' cost is a separable function of their strategic choices. We formally state a joint codeword and power adaptation game, which is separable, and we study its properties in terms of its subgames, namely, the codeword adaptation subgame and the power adaptation subgame. We investigate the necessary conditions for an optimal Nash equilibrium and show that this corresponds to an ensemble of user codewords and powers, which maximizes the sum capacity of the corresponding multiaccess vector channel model, and for which the specified target SINRs are achieved with minimum transmitted power.

In this paper we present a new approach for joint precoder and power adaptation applicable in multi-user spectrum sharing cognitive radio (CR) networks. In the proposed approach bandwidth use is expressed in terms of a finite set of discrete resources whose use is optimized by the CR transmitters in terms of system sum-rate under operating constraints that restrict the operation of secondary (unlicensed) CR transmitters to protect licensed primary user-transmissions. We formulate the optimal resource allocation for CR users as a constrained optimization problem and illustrate the proposed approach with numerical results obtained from simulations.

In this paper, we propose an algorithm for joint adaptation of transmission power and contention window to improve the performance of vehicular network in a cross layer approach. The high mobility of vehicles in vehicular communication results in the change in topology of the Vehicular Ad-hoc Network (VANET) dynamically, and the communication link between two vehicles might remain active only for short duration of time. In order for VANET to make a connection for long time and to mitigate adverse effects due to high and fixed transmission power, the proposed algorithm adapts transmission power dynamically based on estimated local traffic density. In addition to that, the prioritization of messages according to their urgency is performed for timely propagation of high priority messages to the destination region. In this paper, we incorporate the contention based MAC protocol 802.11e enhanced distributed channel access (EDCA) mechanism to implement a priority-based vehicle-to-vehicle (V2V) communication. Simulation results show that the proposed algorithm is successful in getting better throughput with lower average end-to-end delay than the algorithm with static/default parameters.

In this paper, likelihood-based algorithms are explored for linear digital modulation classification. Hybrid likelihood ratio test (HLRT)- and quasi HLRT (QHLRT)- based algorithms are examined, with signal amplitude, phase, and noise power as unknown parameters. The algorithm complexity is first investigated, and findings show that the HLRT suffers from very high complexity, whereas the QHLRT provides a reasonable solution. An upper bound on the performance of QHLRT-based algorithms, which employ unbiased and normally distributed non-data aided estimates of the unknown parameters, is proposed. This is referred to as the QHLRT-Upper Bound (QHLRT-UB). Classification of binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) signals is presented as a case study. The Cramer-Rao Lower Bounds (CRBs) of non-data aided joint estimates of signal amplitude and phase, and noise power are derived for BPSK and QPSK signals, and further employed to obtain the QHLRT-UB. An upper bound on classification performance of any likelihood-based algorithms is also introduced. Method-of-moments (MoM) estimates of the unknown parameters are investigated and used to develop the QHLRT-based algorithm. Classification performance of this algorithm is compared with the upper bounds, as well as with the quasi Log-Likelihood Ratio (qLLR) and fourth-order cumulant based algorithms.

In this paper we present a new algorithm for joint codeword and power adaptation in the uplink of code division multiple access (CDMA) systems, in which codewords are adjusted using gradient-descent interference avoidance, and powers are updated to match target signal-to-interference plus noise-ratio (SINR) values. The proposed algorithm is analyzed and illustrated with numerical examples obtained from simulations.

In this paper we present an algorithm for joint codeword and power adaptation applicable in the uplink of a Code Division Multiple Access (CDMA) system in which multipath channels between users and the base station receiver are explicitly considered. The proposed algorithm uses incremental updates for codeword and power that allow a smooth transition of the system to a steady state configuration where specified targets for the signal-to-interference+noise-ratio (SINR) are achieved with minimum powers. The algorithm is illustrated with numerical results obtained from simulations.

Game theory has emerged as a new mathematical tool in the analysis and design of wireless communication systems, being particularly useful in studying the interactions among adaptive transmitters that attempt to achieve specific objectives without cooperation. In this paper we present application of separable game theory to the development of adaptive algorithms for joint codeword optimization and power control in code division multiple access (CDMA) systems.

The NOTICE architecture is a new concept in vehicular ad-hoc networking (VANET) that aims at providing automated notification of traffic incidents on highways in order to reduce congestion and improve overall traffic safety. The basic component of NOTICE is a system of sensor belts that collect information from passing cars through a wireless radio link. An indicator of performance for the NOTICE system is the incident detection time which depends on various parameters among which we note the vehicle speed, the time required by communicating radios for connection setup and information exchange, the amount of information to be transmitted, or the radio technology employed. In this paper we present a numerical performance study of the NOTICE system with realistic values of these parameters in an attempt to provide some basic specifications for the physical layer of the system.

An alternative approach is presented for centralised algorithms that design optimal sequences and powers under quality of service constraints in the uplink of a code division multiple access systems. The authors propose a distributed algorithm, where each user designs its optimal codeword in such a way to transmit minimum power, based on certain feedback information sent from the base station. The authors define the user cost function, which is the user power written as a convex function of user codewords for a defined signal-to-interference plus noise ratio target. For the constrained optimisation problem, optimal codewords are designed based on feasible direction method and the optimal user powers are the minimum values of the user cost functions. For the optimal configuration, the matched filter employed at receiver will have the same performance as the minimum mean squared error filter. Even if the optimal user powers are unique, the optimal codewords do not correspond to a unique allocation, but rather to an entire class of codewords that can be related by unitary transformations. The algorithm works properly in the presence of multiple access interference with white or coloured additive noise and requires no ordering. The proposed algorithm is analysed and illustrated with numerical examples obtained from simulations.

The paper presents an interference avoidance al- gorithm for codeword and power adaptation in the uplink of a Code Division Multiple Access (CDMA) system with variable number of active users. The algorithm tracks changes in the system configuration, and transitions incrementally from one optimal point corresponding to a given number of active users to a new optimal point corresponding to the new number of active users. The transition uses gradient-based adaptive interference avoidance in which users adapt their codewords to optimize their corresponding spectral efficiency subject to constraints on the signal-to-interference plus noise-ratio (SINR). The algorithm is illustrated with numerical examples obtained from simulations. I. INTRODUCTION Interference avoidance has emerged in the literature as a new technique by which transmitters in a wireless communication system are optimized in response to changing patterns of interference to better suit the environment in which they operate. Currently, interference avoidance algorithms are static in the number of users (1), and each time this changes the algorithms must be reiterated in order to determine the socially optimal solution for the new number of users. We note that other related algorithms for codeword adaptation for uplink CDMA systems (2)-(7) have the same characteristic, and are not adaptable to changing numbers of active users in the system. In order to overcome this limitation, recent research (8) proposes using Grassmannian signatures for use in dynamic systems with variable number of users in the system. These are designed to support a maximum number of active users in the system subject to a given interference level, and have the nice property that interference among users does not change when less users are active in the system. As noted in (8) the disadvantage associated with equiangular Grassmannian signatures is that they may not exist for any desired system configuration specified by given number of users and processing gain value. In this paper we present an alternative approach to dealing with variable number of active users in the uplink of a CDMA system, and propose an adaptive algorithm that moves the system incrementally from an optimal configuration with a given number of active users, to a new optimal configuration with a different number of active users. The transition between the two optimal configurations is based on an adaptive inter- ference avoidance procedure: when a change in the system status occurs this translates in a change of the SINR of active users which will employ a greedy gradient-based technique to optimize their corresponding spectral efficiency subject to

The paper presents a distributed algorithm that allocates the minimum user powers for the optimal codewords in the uplink of a code division multiple access (CDMA) system. The user cost function will be the user power written as a convex function of user codeword for a defined signal-to-interference plus-noise ratios (SINR) target. The optimization problem is treated as a constrained problem over a convex set, for which the optimal codewords are designed based on feasible direction approach and the optimal user powers are the minimum values of the user cost functions. Even if the optimal user powers are unique, the optimal codewords don't correspond to a unique allocation, but rather to an entire class of codewords that can be related by unitary transformations. For the optimal point the user codewords maximize the uplink sum capacity (Csum) for the minimum power allocation. The proposed algorithm is analyzed and illustrated with numerical examples obtained from simulations.

In this paper we present application of interference avoidance in the context of a general multiple access vector channel model. We show that this monotonically increases sum capacity, and discuss algorithms for code division multiple access (CDMA) codeword optimization based on this procedure. A greedy interference avoidance algorithm for multiaccess vector channels is presented in the paper, for which we discuss convergence to a class of codeword ensembles that satisfy a simultaneous water filling solution and maximize sum capacity. Numerical results obtained from simulations that corroborate our analytical results are also presented.

In this letter we present a new method for mitigating narrowband interference in orthogonal frequency-division multiplexing-based ultra-wideband communication systems. The proposed narrowband interference avoidance (NBIA) method performs spectral shaping of the transmitted signal using binary signature sequences with minimum total squared correlation (TSC) to avoid the narrowband interfering signal. We illustrate the proposed method with plots that show the spectrum of the transmitted signal with and without NBIA, and also present numerical results obtained from simulations showing improvement in the bit error rate (BER) performance of the system when NBIA is employed.

In this paper we investigate properties of simultaneous water filling for a wireless system with two mutually interfering transmitters and receivers with non-cooperative coding strategies. This is slightly different from the traditional interference channel problem which assumes that transmitters cooperate in their respective coding strategies, and that interference cancellation can be performed at the receivers. In this noncooperative setup, greedy capacity optimization by individual transmitters through various algorithms leads to simultaneous water filling fixed points where the spectrum of the transmit covariance matrix of one user water fills over the spectrum of its corresponding interference-plus-noise covariance matrix, and in our paper we study the properties of these fixed points. We show that at a simultaneous water filling point the eigenvectors of transmit covariance matrices at each receiver are aligned, and identify three regimes which correspond to simultaneous water filling that depend on the interference gains: a) complete spectral overlap, b) partial spectral overlap, and c) spectral segregation. These imply that the transmit covariance matrices will be white in regions of both overlap and segregation, but not necessarily white overall. We also consider performance as a function of interference gain and show that complete spectral overlap is a strongly suboptimal solution over a wide range of gains. Overall, our results suggest that for strong mutual interference, an effort should be made to do joint decoding over receivers since such collaboration can provide large capacity increases. For moderate interference, distributed and/or centralized conflict resolution algorithms would be most effective since more complex collaborative methods do not afford much improvement and strictly greedy methods such as water filling perform poorly, while for weak interference a laissez faire approach seems reasonable

Abstract — Efficient use of available resources in next gener-ation wireless systems require control of both data rate and transmitted power for mobile terminals. In this paper the prob-lem of joint transmission rate and power control is approached from the perspective of non-cooperative game theory, and an algorithm for joint rate and power control is presented. A new utility function for mobile terminals is defined, and a detailed analysis of the existence and uniqueness of Nash equilibrium for the non-cooperative joint transmission rate and power control game is presented. The utility function depends on the signal to interference ratio (SIR), and can be adjusted to provide the desired Quality of Service (QoS) requirement. Numerical simulations that compare the proposed algorithm with alternative algorithms developed using game theory are also presented in the paper. I.

In this paper we present an algorithm for codeword adaptation in the downlink of a code division multiple access (CDMA) system where the base station transmits information to multiple wireless terminals. The algorithm is based on interference avoidance and updates user codewords such that the corresponding signal-to-interference plus noise-ratio (SINR) at the receiver is maximized. Numerical examples obtained from simulations are also presented to illustrate the algorithm.

The paper presents a distributed algorithm for joint codeword and power adaptation in the uplink of a code division multiple access (CDMA) system, in which users sequentially adjust codewords and powers using gradient-based updates to maximize their individual spectral efficiency subject to constraints on the signal-to-interference plus-noise ratios (SINR). Codeword adaptation is based on greedy interference avoidance which decreases the effective interference seen by users, and is followed by incremental power adaptation. The proposed algorithm is analyzed and illustrated with numerical examples obtained from simulations

Next generation wireless systems is required to support heterogeneous services with different transmission rates that include real time multimedia transmissions, as well as non-real time data transmissions. In order to provide flexible transmission rates to each terminal, efficient use of system resources requires transmission rate control in addition to power control. In this paper, we present an algorithm for joint transmission rate and power control based on a non-cooperative game theoretic approach. A new utility function that includes pricing is defined for joint transmission rate and power control, and a detailed analysis of the existence and uniqueness of Nash equilibrium for the non-cooperative joint transmission rate and power control game with pricing is presented. Numerical results obtained from simulations that compare the proposed algorithm with alternative algorithms on joint rate and power control are also presented in the paper

In this paper we present a new method for mitigating narrowband interference in OFDM-based ultrawide band (UWB) communication systems. This is based on spectral shaping of the transmitted signal using binary signature sequences with minimum total squared correlation (TSC) to avoid the narrowband interfering signal. When spectral shaping is employed, the spectrum of the transmitted signal has a gap in the frequency band of the narrowband interfering signal, and therefore avoids transmitting in this band. The proposed method is illustrated with plots that show the spectrum of the transmitted signal with and without spectral shaping. Numerical results obtained from simulations showing the bit error rate (BER) performance of the system with and without spectral shaping are also presented

We present the application of greedy interference avoidance methods to codeword optimization in the uplink of a code division multiple access (CDMA) system in which the channel between a given user and the base station receiver is assumed known and stable for the duration of the transmission. Repeated application of greedy interference avoidance monotonically increases sum capacity and yields an optimal codeword ensemble that satisfies a simultaneous water-filling distribution. However, algorithms for codeword optimization based on the greedy interference avoidance procedure are in general different from water-filling schemes. We illustrate the algorithms with examples and look at properties of optimal codeword ensembles.

We consider a communication system with multiple independent user-base pairings in a white Gaussian noise environment, and for which a simultaneous water filling condition is satisfied by users at their respective bases. We focus on the weak mutual interference case for which the simultaneous water filling solution is unique and corresponds to complete user overlap in signal space. We show that when users at other bases are treated as Gaussian noise, simple separation of users in signal space usually offers better performance than simultaneous water filling, and present a distributed algorithm which separates users in signal space.

We present the application of interference avoidance methods to multiuser systems with multiple inputs and multiple outputs. A general signal space formulation is used which makes the approach applicable to any MIMO system model regardless of the choice of basis functions. Information is transmitted via multicode CDMA where symbols that comprise the data frame from a given user are "spread" over the available dimensions using a precoding matrix. Optimal precoding matrices that maximize signal-to-interference-plus-noise ratio for all symbols/users are then obtained by application of distributed greedy interference avoidance methods. Numerical simulations have been performed and the signal-to-noise ratio distribution for receiver antennas and complementary cumulative distribution functions for sum capacity with optimal precoding matrices are also presented.

Programmable radios offer a new perspective on wireless communications since the modulation method is no longer fixed. Adaptive methods where user signatures and corresponding receiver filters are iteratively adapted can be used to improve performance. However, since codeword adjustments must be fed back to the transmitter, compact representation of codewords is extremely important. This issue is important for systems which employ interference avoidance since as opposed to current CDMA systems where uniform-amplitude codeword chips are used, interference avoidance employs real-valued "chips"-real-valued coefficients for a set of orthonormal basis functions of the signal space used by the transmitter and receiver. The paper represents a simple investigation of how codeword quantization affects the performance of interference avoidance algorithms. Results indicate that using 4-5 bits per chip for codeword representation is sufficient to maintain performance close to optimal values.

A method for applying Kalman filtering to speech signals corrupted by colored noise is presented. Both speech and colored noise are modeled as autoregressive (AR) processes using speech and silence regions determined by an automatic end-point detector. Due to the non-stationary nature of the speech signal, non-stationary Kalman filter is used. Experiments indicate that non-stationary Kalman filtering outperforms the stationary case, the average SNR improvement increasing from 0.53 dB to 2.3 dB. Even better results are obtained if noise is considered also non-stationary, in addition to being colored, achieving an average of 7.14 dB SNR improvement.