One of the most significant challenges in cognitive radio networks is to design a framework in which the selfish secondary users are obliged to interact with each other truthfully. Moreover, due to the vulnerability of these networks against the jamming attack, providing the security defense is also challenging. In this paper, we investigate a dynamic stochastic medium consisting of some selfish secondary users and a malicious user. Each secondary user participates in an auction and wish to use the unjammed spectrum, and the malicious user aims at jamming a channel by corrupting the communication link. We design a truthful game amongst the secondary users. Furthermore, we propose a zero sum game between the whole set of secondary users and the malicious user. We prove that this problem can be converted to the randomized two-level auctions in which the first auction allocates the vacant channels, and then the second one assigns the remaining unallocated channels. This solution can be changed to a trustful distributed fashion. Finally, simulation results show that the distributed algorithm with much less complexity has almost similar performances to the centralized one.

We consider a K user Gaussian multiple access system with impulse radio ultra-wideband signaling. The receiver is assumed to treat the Multi-Access Interference (MAI) as additive noise and does not enjoy multiuser detection. It is well known that for such a system the MAI is neither Gaussian nor white. Although, the non-Gaussianity of the MAI has been extensively studied, little attention has been directed to the non-whiteness of it. In this paper, we show that by employing a new front-end filter at the receiver, the non-whiteness of the MAI can be exploited to improve the performance of the single-user detector. We use the signal-to-interference-plus-noise ratio (SINR) as a merit to evaluate the performance improvement brought to the system. The SINR improvement depends on the interference level as well as pulse waveform. The bit error rate of the system is also simulated and compared with that of conventional matched filter receiver.

This paper proposes a synchronization method with a preamble for IEEE 802.16 and other systems that use Orthogonal Frequency Division Multiplexing (OFDM). The preamble for synchronization is composed of two identical periods, which are polyphase codes, and have an autocorrelation function with one main peak and very small sub peaks. Using this signal repetition and autocorrelation, the correlation for symbol timing detection is calculated with a new metric. The proposed metric uses a filtering structure to find the accurate start point. Proposed method drastically improves the performance of timing synchronization, even with a very low SNR, in both AWGN and multipath fading Rayleigh channels.

In this paper, we investigate the average capacity and its scaling law of the user scheduling for the multiuser MIMO systems exploiting multiuser and antenna diversities. We show that the limiting distribution of maximum SNR is of the Gumbel-type and average capacity scaling law follows log log K form. We also consider the effect of scheduling delay on the performance of scheduling scheme. Then, we propose two limited feedback scheduling and obtain their average capacity and show that they match the simulation results very well. In the first approach, the base station (BS) always collects feedback from the user with the best channel condition. We derive the normalized feedback load as well as the threshold value that minimizes it. The second approach employs quantization for each user, so that few bits of SNR are sent to the BS. We show that even with 1-bit quantization, the proposed quantized feedback scheduling can exploit multiuser diversity at the expense of trivial decrease of average capacity.

In many applications of OFDM systems it is necessary to use a transmitting window, with a proper roll-off, in order to reduce out of band spectral leakage. In this case, i.e. windowed OFDM, the conventional cyclic prefix based maximum likelihood (ML) estimator of timing and frequency offset is not optimal and suffers from an error floor. In this paper we derive the optimum ML estimator for the windowed OFDM systems. Furthermore we have shown that by adding an appropriate receiving window to conventional ML estimator an excellent approximation to the optimal ML estimator can be achieved. This window is independent of SNR, unlike the windows needed in the optimum estimator, and therefore highly reduces the complexity.

Ultra-wideband (UWB) communication systems are used in indoor environments with dense multi-path characteristics. Therefore channel estimation has an important role in the receiver of these systems. A new approach for data-aided (DA) and non-data-aided (NDA) channel estimation is proposed, which is called the pulse compression (PC) method. This method is useful for UWB systems employing time-hopping pulse position modulation. The PC method requires only some basic operations such as sampling, overlap-add and finite impulse response filtering. The PC method, in both DA and NDA scenarios, in spite of its low complexity, outperforms the maximum-likelihood (ML) method in channel parameters estimation. The bit error rate (BER) of the DA method, in single-user scenario, performs as well as the ML method, and in multi-user scenario, in the worst case, there is only 0.5 dB loss compared with the ML method. In the case of NDA scenario, the proposed method outperforms the NDA-ML method, that is, in the single-user scenario about 4 dB gain at the BER of 10−3 is observed. In multi-user scenario, it outperforms significantly the NDA-ML method, and its performance loss in comparison with the perfect channel knowledge scenario is about 3 dB at the BER of 10−3.

In this paper, novel routing tree construction algorithms for emerging multi-channel WiMax networks are presented. Both centralized and distributed schemes are explored. Simulation results show the improvement achieved compared to similar networks with a mesh topology.

Two iterative cross layer schemes are introduced for throughput maximization in multi channel WMN's where each link has a single designated channel. For both schemes, a two phase algorithm is developed where each phase is composed of routing and channel assignment steps. Results extracted from the channel assignment carried out in the first phase are fed back into phase 2 and appropriately utilized to improve aggregate network throughput.

In CDMA systems, turbo multi user detection for multiple access interference cancellation and channel decoding in CDMA systems has an exponentially growing complexity with number of users. In these systems, when bandwidth efficient modulations like M-ary PSK are used another exponential factor that is relates to the levels of modulation prohibitively increases the computational complexity of the receiver. In this paper we have examined a symbol based reduced complexity multi user detector, for coded and un-coded CDMA system, with a performance very near to optimal detector while having extremely low complexity. We have introduced a design formula for the required Hamming distance value for no loss in performance. Simulation results of coded and un-coded CDMA system verify the performance of our reduced complexity detector that we expect.

Ultra-wideband (UWB) is a very promising wireless technology for short ranges, because of its many benefits. A common practice in UWB is time hopping (TH)-PAM impulse radio system, in which a number of extremely short pulses are transmitted for each bit and the polarity of pulses determines the bit. In multi-user environments, the interference from other users [multiple access interference (MAI)] may well dominate the Gaussian noise which makes the matched filter receiver be a suboptimal receiver. A model for the statistical structure of MAI in an additive white Gaussian noise channel is presented. Then, using this model, the authors propose a nonlinear pulse combining scheme for the receiver of TH-PAM systems. The bit error rate (BER) of our receiver is compared with that of matched filter receiver and blinking receiver, via simulations. The simulation results show that our receiver always outperforms the matched filter receiver. Blinking receiver performs better than our receiver only when the probability of collision is low and the energy of interfering users is high. In all other cases, our receiver has a lower BER than the blinking receiver. Furthermore, the blinking receiver needs to know the locations of all the interfering pulses, while in our method such information is not required.

A simple and efficient bit loading algorithm for orthogonal frequency division multiplex-based systems in frequency selective environments is presented here. The proposed algorithm minimises the total transmitted power for a given bit rate and probability of error. Simulation results show that the proposed algorithm has approximately the same performance as the optimal algorithms with much less complexity. When compared with suboptimal algorithms, the computational complexity is comparable while the overall performance is much closer to the optimum solution.

As the UWB systems are designed to operate in environments having dense multipath characteristics (i.e., indoor environments) the channel estimation task is much more involved than the conventional systems. In this paper, we present a new blind channel estimation technique for Time-Hopping Pluse-Position-Modulation (TH-PPM) systems. Our proposed method is based on the Pulse Compression (PC) technique introduced in [1]. In spite of its low complexity, the proposed blind channel estimation method outperforms the Non-Data Aided Maximum Likelihood (NDA-ML) technique. Our simulation results show the promising gain of 4 dB around the Bit Error Rate (BER) of 10-3, compared to NDA-ML. The performance loss, compared to the Perfect Channel Knowledge (PCK) is only 1.5 dB at this BER.

The local oscillator phase noise can severely affect the performance of OFDM systems. In this paper a new method is presented for compensation of this phase noise. The method is based on the fact that the scattered pilots, which are used for channel estimation, are subject to the same phase error as the OFDM symbols. Least square approach is used to exploit the phase noise information that exists in the scattered pilots and compensate the phase noise on the OFDM symbols. Simulation results show that the method is very effective, especially in high SNR.

In many engineering applications, specially in communication engineering, one encounters a bandpass non-Gaussian random process, with a slowly varying envelope. Among the available models for non-Gaussian random processes, spherically invariant random processes (SIRPs) play an important role. These processes are of interest mainly due to the fact that they allow one to relax the assumption of Gaussianity, while keeping many of its useful characteristics. In this paper, we have derived a simple and closed-form formula for the expected number of maxima of a SIRP envelope. Since Gaussian random processes are special cases of SIRPs, this formula holds for Gaussian random processes as well. In contrast with the available complicated expression for the expected number of maxima in the envelope of a Gaussian random process, our simple result holds for an arbitrary power spectrum. The key idea in deriving this result is the application of the characteristic function, rather than the probability density function, for calculating the expected level crossing rate of a random process.

There are various cases in physics and engineering sciences (especially communications) where one requires the envelope probability density function (PDF) of the sum of several random sinusoidal signals. According to the correspondence between a random sinusoidal signal and a random vector, the sum of random vectors can be considered as an abstract mathematical model for the above sum. Now it is desired to obtain the PDF of the length of the resulting vector. Considering the common and reasonable assumption of uniform distributions for the angles of the vectors, many researchers have obtained the PDF of the length of the resulting vector only for special cases. However in this paper, the PDF is obtained for the most general case in which the lengths of vectors are arbitrary dependent random variables. This PDF is in the form of a definite integral, which may be inappropriate for analytic manipulations and numerical computations. So an appropriate infinite Laguerre expansion is also derived. Finally, the results are applied to solve a typical example in computing the scattering cross section of random scatterers.