In the present age of digital communication, two parameters are of paramount importance-volume of data and the associated Bit Error Rate (BER).MIMO technique is a big help here.However, the phenomenon of multipath signals leads to Inter Symbol Interference (ISI).The pre-shaping of the pulse at the transmitter mitigates the ISI.Interestingly, the BER is dependent on the modulation order, the Eb/N0 ratio and the roll-off rate of the filters.In this paper, we have studied the effects of the three mentioned parameters on the BER for a given system.The results do show some trends.
The wireless communication system are facing the spectrum shortage and interference problem for the latest developments in the cellular industry. These problems can be solved by using a new technology known as Cognitive Radio Network (CRN) where dynamic utilization of spectrum takes place. Cognitive Radio is an intelligent transceiver system that continuously monitors its radio environment in search of the unused frequency slots (known as spectrum holes) which can be seen in the environment, analyze and take decisions from threshold. This frequency slots are used up by secondary user in the absence of primary user. In this paper, we have analyzed four different types of spectrum sensing techniques, challenges faced by spectrum sensing method and strategies to solve this problem.
Cognitive radio technology makes efficient use of the valuable radio frequency spectrum in a non-interfering manner to solve the problem of spectrum scarcity. This paper aims to design a scheme for the concurrent use of licensed frequencies by Underlay Cognitive Users (UCUs). We develop a new receiver-initiated Medium Access Control (MAC) protocol to facilitate the selections of alternative reliable carrier frequencies. A circuit is designed to establish reliable carrier selections based on the Received Signal Strength Indicator (RSSI) at the receiving end. Based on both packet-level simulations and various performance parameters, a comparison is carried out among conventional techniques, including the Multiple Access with Collision Avoidance (MACA) and MACA by invitation(MACA-BI) techniques, and our scheme. The simulated results demonstrate that when conventional techniques are used, the system overhead time increases from 0.5 s on the first attempt to 16.5 s on the sixth attempt. In the proposed scheme under the same failure condition, overhead time varies from 0.5 s to 2 s. This improvement is due to the complete elimination of the exponential waiting time that occurs during failed transmissions. An average efficiency of 60% is achieved with our scheme while only 43% and 34% average efficiencies are achieved with the MACA and MACA-BI techniques, respectively. The throughput performance of our scheme on the fourth attempt is 7 Mbps, whereas for the MACA and MACA-BI protocols, it is 1.9 Mbps and 2.2 Mbps respectively.
The rapid growth in the ubiquitous wireless service has imposed increasing stress on the fixed and limited radio spectrum. Cognitive Radio (CR) is promising solution to the problem of overcrowded and inefficient use of licensed spectrum. In this paper, Interference Temperature (IT) model is used to define the occupancy and availability of a channel. Interference temperature has been proposed by the FCC as a metric for interference analysis. Our goal is to find out the useable receiver bandwidth (BW) portion of available BW. Here we have applied the principle of Hill-Climbing algorithm. By the random selection of portion of a frequency band we have performed necessary experiment and calculated the value of Received signal strength indicator (RSSI) for a given sub-band. We have subsequently used this value to maximize the effective bandwidth of a given system.
A regular diffraction grating produces intensity patterns that combine waves coming through equispaced slits, so that the waves emerging from any two neighboring slits have identical phase differences. In this paper, we calculate the degradation in the intensity pattern when the grating has irregular spacing. The model of randomness considers the grating spacing and openings as being created by a "random walk." The resolving power of the grating is evaluated in relation to the D lines of sodium. It is shown that as the number of rulings increases, the uniformity of their spacing becomes more important in precision spectroscopic measurements, such as in astrophysical spectroscopy.
Purpose : Attenuation of x-rays in any object is a combined effect of scattering and absorption of x-ray photons by the atoms in the material. While the scattering part dependents linearly upon the material’s electron density (ρe) and is weakly dependent on the energy (E) of the photon, the photoelectric part varies as [ρe Zx eff /Ey] where Zeff is the effective atomic number of the material. We aim to determine the exponents (x,y) which are crucial for many radiological studies. Methods : In order to obtain exponent ‘y’, we find an equation in which the exponent x does not appear and the dependence on ‘y’ appears only linearly. Having thus reduced the problem to that of ‘one parameter fit’ in ‘y’, we generate the ‘data’ numerically from the numerical values of physical constants given in the NIST (The National Institute of Standards and Technology, or NIST) tables and determine ‘y’ from linear regression. The effect of the source spectrum on the effective value of ‘y’, denoted by ‘ym’ is studied for different low Zeff substances in different energy ranges. Results : It is seen that with x-ray sources operating at 80, 100,120,140 kVp, the effective exponent ‘y’ progressively decreases, as the x-ray source spectrum is pushed to higher energy side. However, for most practical purposes y=2.99 may be used for a wide variety of low Zeff substances. For practical cases with different source spectra, the effective energy of the source and the effective photoelectric exponent are seen to increase as the thickness of the aluminum filter increases. Conclusion : It thus follows that for applications such as DECT inversion, the appropriate values of ‘ym’ ought to be used that takes into account the appropriate source spectrum.
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