The application of a blanking nonlinearity to cope with impulsive interference is a common approach in OFDM systems. The choice of the amplitude threshold to decide whether a received sample is blanked, heavily affects the performance of the entire transmission. As a matter of fact, the perfect blanking threshold strongly depends on the characteristics of the interference. In previous publications this perfect threshold has been derived, given knowledge about the interference statistics. In general, however, no reliable information about the interference statistics is known at the receiver side. In this paper a practical method for calculating an adaptive blanking threshold to maximize the signal-to-interference-and-noise ratio after the blanking nonlinearity is proposed. The calculation is based on the distribution of the amplitude of the received signal at the OFDM receiver. Simulation results show only a negligible performance degradation when comparing the proposed method to the case where the theoretically derived perfect threshold is applied for blanking.

In this paper, we analyze the influence of a blanking nonlinearity onto OFDM systems in detail. We will derive closed-form expressions for the signal degradation and the interference, which is introduced by the blanking nonlinearity. The theoretical results will be validated by simulations, which illustrate the detrimental blanking influence in a descriptive way. In addition it will be shown how the results affect an OFDM receiver and how the receiver can be adapted to the blanking nonlinearity. Simulation results confirm a gain of 3 dB at BER=1·10-3for a 16QAM transmission, when adapting the receiver accordingly.

The objective of this paper is to characterize the scattered components in an aeronautical channel in terms of delay and Doppler frequency. Geometric considerations make it possible to locate the scatterers on the ground and derive analytical expressions of their shape in the Doppler-delay domain. The delay and Doppler frequencies are at first described as a function of the horizontal distance to the specular reflection point. Subsequently, the spatial relationship is used to directly link delay and Doppler frequencies in one formula. The results show a very good match of the theoretical expressions and the measured data. The presented results are currently used to develop a three-dimensional model of the scatterers that enables us to define Doppler power profiles for arbitrary delays.

When pulsed interference is mitigated by means of pulse blanking, interference is reduced significantly. However, at the same time certain fractions of the useful orthogonal frequency-division multiplexing (OFDM) signal are erased. Consequently, only moderate improvements are achieved with pulse blanking. When representing the pulse blanking operation as multiplication with a rectangular window exhibiting notches at the positions where blanks are inserted, it becomes obvious, that pulse blanking mainly leads to inter-carrier interference (ICI). In this paper, the compensation of the impact of pulse blanking is proposed by reconstructing and subtracting ICI. The required shape of the subcarrier spectra is derived from the pulse blanking window. For an estimation of the transmitted data symbols and the channel coefficients of each subcarrier an iterative receiver structure is proposed. Simulation results at hand of a realistic interference scenario show that with perfect channel estimation and known data symbols the impact of pulse blanking can be reduced to a small loss in signal-to-noise ratio resulting from the energy loss due to erasing a certain fraction of the OFDM signal. With real channel estimation and estimated data symbols, the ideal case with perfect channel estimation and known data symbols is approached by 1.2 dB after only three iterations.

With respect to the ever-growing demand for mobile communications on the one hand and spectral scarcity on the other hand it is necessary to increase spectral efficiency. In this paper, we focus on OFDM (Orthogonal Frequency-Division Multiplex) based overlay systems, which share a frequency band with already existing systems by adaptively filling the spectral holes. In order to enable such a co-existence, on the physical (PHY) layer promising methods for minimising mutual interference between the overlay system and the licensed systems are presented. The medium-access control (MAC) layer protocols are designed such as to be able to efficiently signal the available subcarriers as well as to handle the changing available bandwidth. A third issue is the joint optimisation of PHY and MAC parameters which is essential in order to optimally exploit the available spectrum with minimum signalling overhead.

The “Ball-in-tube” experiment can be applied in some industrial field such as oil industry .etc. It is more of being used in the university laboratory for researching non-linear control technology. The control object for “ball in a tube” is to regulate the speed of a fan blowing air into a tube so as to keep a Ping-Pang ball suspended at some predetermined position in the tube. Although conceptually simple, this is a difficult control problem due to the non-linear effects on the ball and the complex physics governing its behavior. Furthermore, the ball is extremely sensitive to any external disturbances from the fan. Taken together, it is difficult to be controlled by the traditional mathematics, and not easily captured in simulated or mathematical comparisons of control algorithms. The Fuzzy-PID can be built to regulate the ball’s position automatically. The parameters in the PID controller are treated as fuzzy variables and the controller is implemented and tuned. We use this device to experimentally compare a traditional PID controller and Fuzzy-PID controller. The results show significant differences in the performance characteristics of the controllers.

This paper proposes two frequency offset estimation algorithms for the uplink of an interleaved frequency-division multiple-access (IFDMA) system. One algorithm performs estimation in the frequency domain and the other in the time domain. Both algorithms are based on the maximum likelihood estimation (MLE) principle and use knowledge about pilot symbols. IFDMA utilizes a block-interleaved frequency allocation scheme to exploit the frequency diversity of the channel. In the presence of frequency offsets between users, multiple-access interference (MAI) appears, which has a negative impact on existing frequency offset estimation algorithms. The proposed algorithms are robust, since a special construction of pilot symbols allows to exclude a large amount of MAI in the presence of frequency offsets between users. As a result, the proposed time domain frequency estimation algorithm outperforms the frequency domain algorithm and all other known schemes.

In this paper, superimposed packet allocation for orthogonal frequency-division multiple-access code-division multiplexing (OFDMA-CDM) is presented, where each transmitted packet is associated with one spreading code. An iterative algorithm which is a combination of parallel interference cancellation and hybrid automatic repeat request (H-ARQ) based on soft value combining (SVC) is proposed, and its performance is studied and compared with other existing H-ARQ schemes. The proposed algorithm exploits the reliability information of erroneously received copies of the same data packet to improve the performance of interference cancellation. The interference of correctly received packets is ideally reconstructed and subtracted. Thus, the overall system performance improves iteratively. As a result, the proposed algorithm outperforms conventional H-ARQ based on SVC, as well as H-ARQ based on maximum ratio combining.

In this paper, we consider wireless multicarrier communications systems applying different transmit-antenna diversity techniques. Whereas the transmitter has available multiple antennas, we assume a single antenna at the receiver. Thus, we consider multiple-input single-output multicarrier transmission systems which are of special interest for downlink mobile radio applications. For these systems, we investigate the matched-filter bounds (MFBs) which represent the performance in an environment free of any interference and, thus, are lower bounds on the achievable performance. Especially, we focus on the analytical determination of the MFBs for selection diversity (SD), since this transmit-antenna diversity technique requires the least amount of preprocessing at the transmitter. Based on the derived MFBs, SD is investigated in detail, and compared with other transmit-antenna diversity techniques which are based on the maximum-ratio and the equal-gain transmission principles, respectively

In this paper, a new technique for sidelobe suppression in orthogonal frequency-division multiplexing (OFDM) systems is proposed and investigated. This technique can be applied to both pure OFDM and OFDM based multiple-access systems to improve spectral efficiency. Moreover, it can be utilized within OFDM based overlay systems to avoid interference towards the legacy systems operating in the same frequency band as the overlay system. The proposed sidelobe suppression technique is based on subcarrier weighting. The real-valued subcarrier weights are determined in such a way that the sidelobes of the transmission signal are minimized using an optimization algorithm which is capable to take several optimization constraints into account. Results show that sidelobes can be easily suppressed by more than 10 dB with the proposed technique. These results are achieved by allowing only a moderate loss in bit-error rate performance.

In this letter, we propose the concept of combined equalization for uplink multicarrier code-division multiple access (MC-CDMA) and perform a theoretical analysis which shows that better single-user bounds than the classical matched-filter bounds are achieved with this new concept. Moreover, we illustrate how to properly design an uplink MC-CDMA transmitter and receiver for combined equalization, and show by Monte Carlo simulations that the improved single-user bounds are closely approached, even in the case of a fully loaded system.

In this paper, uplink time division duplex (TDD) multi-carrier code-division multiple-access (MC-CDMA) systems applying pre-equalization to combat channel impairments are considered. Especially, the performance degradation caused by the Doppler induced frequency offsets among the different mobile users is analyzed in detail. Based on this analysis different frequency interleaving schemes are proposed and their potential to reduce the performance degradation is investigated. It turns out, that a frequency interleaver which is allowed to skip a few, well-defined subcarriers is capable of reducing the influence of Doppler induced frequency offsets significantly causing a very little loss in bandwidth efficiency.

"Interleaved frequency-division multiple-access" (IFDMA) has previously been introduced as a new spread-spectrum multiple-access scheme for mobile communications. In this paper, performance results for uncoded and coded mobile radio transmission taking into account different equalization techniques are presented. As a result, equalization reduces the effects of intersymbol interference significantly. Uncoded IFDMA applying optimum equalization performs even better than uncoded multicarrier code-division multiple-access applying maximum likelihood multi-user detection. Moreover, in the case of coded IFDMA transmission coding gains up to 6.8 dB at a bit-error-rate of 10/sup -4/ are achieved using a standard convolutional code.

A new spread-spectrum multiple-access scheme applicable for both the uplink and the downlink of a mobile radio communications system is proposed, which is termed "interleaved frequency-division multiple-access". This scheme combines the advantages of spread-spectrum and multicarrier transmission. Moreover, it can be viewed as a special kind of a multicarrier spread-spectrum scheme where user discrimination is carried out using frequency-division multiple-access. The multiple-access capability is achieved by assigning to each user a different set of orthogonal subcarriers. The orthogonality between the different users is maintained at the receiver, even in time-dispersive channels resulting in no multiple-access interference. However, the cost to avoid multiple-access interference is to allow for intersymbol interference and therefore equalization is required at the receiver.

The other-user interference of a multi-carrier spread-spectrum multiple-access system applying conventional detection is considered. The investigations refer to the synchronous uplink, i.e. the synchronous transmission from the mobile stations to the base station in a mobile communications system. Under the assumption of independent Rayleigh fading the probability density function of the interference signal is calculated analytically and it is shown that the resulting probability density function can be well approximated by a Gaussian distribution. Moreover, refering to the bit error rate formula for the single-user case a closed-form expression for the bit error rate in the case of other-user interference is derived using the Gaussian approximation method. The analytical results are validated by Monte-Carlo simulations.