In cognitive radios, energy detector is considered for spectrum sensing in the literature. However, its performance deteriorates rapidly if the noise power is not known exactly. Moreover, due to the presence of a colored channel interferer or some other reasons, the conventional white Gaussian noise may become colored. In order to solve these problems, this paper proposes several multicumulant based spectrum sensing methods: generalized likelihood ratio test (GLRT) based multicumulant (GLRTMC) based detection method and multiantenna-assisted multicumulant (MAMC) based detection method. GLRTMC detection method is derived from generalized likelihood ratio test and assumed to be near optimum in theory. MAMC detection method, on the other hand, by using multiple antennas, is a complexity-reduced detector and allows us to make a compromise between performance and complexity. It is well known that cumulants higher than second order are zero for Gaussian distributions. Thus, GLRTMC detection method and MAMC detection method can extract a non-Gaussian signal from Gaussian noise even when the noise is colored. In addition, the proposed methods are nonparametric in the sense that they do not require any exact prior knowledge about the signal or the noise, such as noise power or cyclic frequencies. Hence they are immune from noise uncertainty. Simulation experiments are provided to show the validity and the superiority over single-cumulant based detector of the proposed multicumulant based detectors.

The authors prove that multi-path diversity improves the detection performance of primary users for cognitive radio users. They analyse the relationship between the performance and the diversity order. The result shows that robust performance gain is hard to achieve by choosing fixed diversity order in unknown and varying channel states. Therefore they propose here a blind multi-path diversity auto-correlation ratio detection scheme that can enable dynamic diversity order to adapt to the channel state. Further, they extend this scheme to multi-antenna scenario where the cross-correlation information of the received signal at each pair of antennas is exploited. Simulations show that the proposed schemes perform as well as or even better than the ideal energy detection. One can use these schemes to detect all types of wireless signal without having the knowledge of the targeted signal, the channel and the noise power. Besides, unlike in other robust detection schemes, the thresholds are asymptotically independent of the sample size, which facilitates the flexible sensing period adjustment.

In cognitive radios, energy detector is considered for spectrum sensing in the literature. However, its performance deteriorates rapidly when noise power is fluctuating. In order to solve this problem, this paper proposes a spectrum sensing method based on multiple cumulants. Although cumulant-based detection methods have been extensively studied, they usually consider a single cumulant of a certain order and a certain lag. Therefore, they only make limited use of the rich statistical information contained in the signal. The proposed detection method is based on multiple cumulants of arbitrary orders and arbitrary lags hence it is superior and more flexible. In addition, the proposed detector is nonparametric in the sense that it does not require any exact prior knowledge about the signal or the noise, such as noise power or cyclic frequencies. Thus it is immune from noise uncertainty. Simulation experiments are provided to show the validity and the superiority over single-cumulant based detector of the proposed multicumulant based detector.

Sensing the weak signals of primary users by cognitive devices greatly challenges the capability of energy detector under noise uncertainty. This paper proposes three schemes for detecting the primary signals of OFDM type. They are based on the auto-correlation ratio detection and exploit the multi-path diversity gain and multi-user cooperation. The detection and false alarm probabilities are then calculated and analyzed. The simulation results show that They effectively improve the performance of the ratio detection.

Cognitive radio (CR) is a promising wireless communications technology. When using NC (non contiguous)-OFDM to transmit data, CU (cognitive user) promptly evacuates the channel reused for communication once PU's (Primary User's) reappearance is detected, and needs to acquire new channel. One way is to increase modulation rate on remainder uninterfered channels, while the other is to acquire new channels. Acquiring new subcarriers takes a great portion of the link maintenance time. This paper proposes using increased modulation mode to keep the same transmission data rate so that CU can save the time of acquiring new channel. Numerical result shows that the goodput of increased modulation rate method is 14 Mb/s advantageous over that of acquiring new channel.

Sensing the weak signals of primary users by cognitive UWB devices greatly challenges the capability of energy detector under noise uncertainty. This paper presents a simple and robust statistic for detecting the primary signals of OFDM type. The statistic, constructed from the auto-correlation function of the received signal, is unsusceptible to the noise uncertainty since it avoids estimating the noise level. The detection and false alarm probabilities are then calculated. The comparison with energy detection demonstrates its superiority to the energy detector in case of low SNR and noise uncertainty.

A bandwidth efficient three-user cooperative diversity scheme is proposed. Each user has two partners who receive its symbols and rotate the phases of these symbols. Then, two partners retransmit the real parts and imaginary parts of these rotated symbols, respectively. This scheme can offer diversity order of three, and the bandwidth efficiency is only decreased by 1/2 compared to a noncooperative diversity scheme. The optimal rotation angles are derived when QPSK and 8PSK modulations are employed, respectively. Then, a new symbol mapping for 8PSK modulation is proposed, and it is a simple way to improve the system performance.

The limited bandwidth and the finite battery power of mobile devices represent one of the greatest limitations of current WPANs. In this paper, an analytical model for the energy efficiency of the IEEE 802.15.4 slotted CSMA/CA which can be studied as slotted non-persistent CSMA was proposed and validated. Key to the accuracy of the model is a careful study of the idle period after last collision, which has been studied by Zhang et al. (2006). Specifically, a distributed mechanism to improve the energy efficiency for contention control in IEEE 802.15.4 was also present and evaluated and show that power saving and throughput maximization can be jointly achieved. Simulation results indicate that our mechanism is very effective and robust. This mechanism can be used to extend the standard 802.15.4 access mechanism without requiring any additional hardware.

In this paper, a multicarrier time division multiple access (MC-TDMA) technique is presented for downlink broadband wireless communications. It combats multipath interferences by OFDM modulations and distinguishes multiple users through different chip-level time slots. In order to gain insights on its performance and complexity, the relationship between this MC-TDMA scheme and a special MC-CDMA technique with maximum multipath diversity gain is emphatically investigated, which reveals that the former can be viewed as an equivalent but simplified implementation approach of the latter and furthermore, the former offers more flexibility in the system design and reconfiguration than the latter.

Based on the soft-input soft-output (SISO) algorithm that is used to iteratively decode serially concatenated codes, we present a new modular iterative decoding algorithm for woven convolutional codes. The proposed algorithm takes the log-likelihood ratio of a priori probability as inputs and produces log-likelihood ratio of post a priori probability as outputs. The justification and the performance are demonstrated by numerical simulation.

In this paper, a new simplified soft decision metric, without noise variance knowledge, is derived. By recognizing that it is simply a distance metric in Euclidian space, we further generalize it to p-norm. Performance comparisons are presented for LDPC coded 8PSK/16QAM with different parameters. Simulation results show that, when compared with the simplified soft decision metric reported in [F. Tosato et al. 2002], there is no performance loss with our proposed generalized soft decision metric. Moreover, it is unnecessary to estimate noise variance at the channel output, which greatly facilitates practical implementation. Further simulation indicates that, at high signal to noise ratio, a small performance gain can be obtained with p a bit less than 2.

The two well-known on-demand protocols for ad hoc networks - DSR and AODV show degraded performance with traffic load increase. In this paper, we propose a traffic-balanced scheme, called TB scheme, to solve this problem. The TB scheme adopts MAC layer information for routing decisions. It avoids involving heavy loaded nodes in a new route and thus balances network traffic. Simulation results show that the new scheme significantly improves DSR and AODV's performance under heavy load conditions.

This paper presents a class of complex orthogonal wavelet packets through optimization and proposes a complex wavelet packet based OFDM system (CWPOFDM). Besides, a serially implemented multistage MLSE/SIC (successive interference cancellation) detector is developed to retrieve the transmitted CWPOFDM symbols. By comparison, it is shown that the performance of CWPOFDM without cyclic prefix (CP) is close to or superior to that of DFT based OFDM (DFTOFDM) with CP, at the price of a little increased computational complexity.

In this paper, the approach to produce the complex wavelet packet basis function is given. A complex wavelet packet basis function based frequency-hopping multiple access communication system, which employs antenna diversity and decision feedback equalization, is proposed. The simulation results show that the new multiple access system outperforms the analogy system based on real wavelet packet basis function. In addition, it has a high spectrum efficiency, high security and good narrow band interference rejection capability. Furthermore, the developed system is also easy to support multirate voice and data services.

In this paper we introduce a design of a phase-modulation spreading sequence for a spread spectrum multiple access communication system with full response continuous phase modulation. The continuous phase modulation with the phase-modulation spreading sequences in the CDMA communication system is shown to provide a desirable spectral property and lower bit-error-rate in comparison with BPSK over Rayleigh fading channel.

In this paper we present a reduced complexity soft-output detection algorithm called the adaptive SO-M-algorithm for turbo-equalization, which is a suboptimum version of the Lee algorithm based on the principle of the M-algorithm. In the adaptive SO-M-algorithm, the breadth and depth parameters are adaptively adjusted according to the convergence process of turbo-equalization, and a good performance/complexity ratio is obtained for channels with severe ISI.

Integrating an adaptive antenna array (AAA) with multiuser detection (MUD) is a new research area aimed at improving the system performance and capacity. A new dynamic groupwise successive interference cancellation array receiver scheme for DS-CDMA systems is proposed and compared with other schemes. In this scheme, AAA beamforming based on direction of arrival (DOA) estimation is integrated with multistage interference cancellation (IC). Simulation results illustrate that our method outperforms existing interference cancellers for DS-CDMA systems in a multipath Rayleigh fading channel environment.

In this paper the performance of the uplink in the cellular CDMA network, based on the reception of a smart antenna, is analyzed in the frequency-selective fading channel. The analysis is under the asynchronous cellular CDMA system of multiple cells, in which BPSK modulation and RAKE combination based on smart antenna reception are employed. The closed-forms of the average error-probability are derived in this paper. The numerical results show that the performance of the cellular CDMA system based on a smart antenna is improved greatly; in addition, the final formula describes the inter-relations among BER, the user number of the system, the number of cells and fading model.

Multidimensional trellis-coded modulation (multi-D TCM) codes are applied to direct sequence spread spectrum multiple access (DS/SSMA) communication system in an urban multipath fading channel. Based on the characteristics of the channel under DS/SS and the models of the transmitter and receiver corresponding to multi-D TCM codes, an new way for analyzing the properties of multi-D TCM DS/SSMA systems over such channel is proposed. Simulation results show that the performance of the DS/SSMA system with TCM of a lower coding rate is better than that of system with traditional Ungerboeck's TCM over the multipath fading channel, and the performance of the DS/SSMA systems can be improved greatly by adopting multi-D TCM codes, compared with that of systems with one dimensional TCM codes.

It is expected that the available spectrum for the proposed personal communication networks will soon be at a premium as the user population increases, and using multilevel quadrature amplitude modulation (MQAM), an bandwidth efficient transmission method for digital signals, may significantly ease the problem. A new lattice polynomial perceptron (LPP), with faster and less input signal-dependent convergence behavior, is presented and applied to frequency-selective slow fading channel equalization and adjacent-channel interference (ACI) suppression in a 16-QAM system. Computer simulation results are given, which shows that, in a 16-QAM system, the performance of LPP is clearly superior to that of the other structures.

The concept of a complex neuron and its model are described. The complex neural-network-based adaptive decision feedback filter (CNNDFF) for M-QAM digital communication reception systems is put forward. The fast adaptive learning algorithm, called mixed-gradient-based fast learning algorithm with variable learning gain and selective updates, is adopted to train the CNNDFF. Experimental results indicate that the CNNDFF can simultaneously overcome the performance degradations due to multipath fading of channels and reject the non-Gaussian cochannel interferences efficiently. By using the fast learning algorithm, improved convergence and tracking ability can be obtained for the CNNDFF with moderate computational complexity.>

A method for rejection of narrowband interference in pseudonoise spread-spectrum systems is proposed. The method is based on the quadratically constrained least-mean-square-error (QC-LMSE) filtering. It is shown that perfect rejection of the single-tone interference can be achieved through appropriate choice of the constraint parameters. This method does not require any a priori knowledge about the frequency and the amplitude of the interference, and hence the filter can be made adaptive. An adaptive algorithm is developed and its learning characteristics are discussed with simulations on computer.>

Performance of the threshold coherent frame sync acquisition (TCFSA) algorithm on the AWGN channel is analyzed and compared with the performance of the algorithm for the BSC. Cases both with and without binary phase ambiguity are considered. Analytical and simulation results are presented and the agreement between the two is shown to he very close. It is shown that the threshold frame sync acquisition algorithm on the BSC outperforms the TCFSA algorithm on the AWGN channel whenP_{e} \leq 0.1, while it is inferior whenP_{e} \geq 0.13in cases of practical interest. A modified acquisition algorithm on the AWGN channel is then proposed. Analytical and simulation results for the modified algorithm show that it is superior to conventional algorithms on the BSC and the AWGN channel.