Free space optic communications has received extensive interest over the past decade, and many potential applications for this technology have been suggested. Among various digital modulation formats for wireless optical links like OOK, PPM, DPIM and MDPIM, DPIM has got advantages in terms of transmission capacity، bandwidth requirement, power requirement and receiver design. DPIM displays a higher transmission capacity by eliminating all the unused time slots from within each symbol in PPM, and decreased bandwidth requirement and afford the possibility to introduce some redundancy into the code for security. Design of PPM receiver has more complexity compared to OOK and DPIM since both slot and symbol synchronizations are required at the receiver. In this study, DPIM Modem is implemented on FPGA and its Functionality is compared with PPM Modem.

In this research, a survey of the sacred symbols of the naturere surrection has been done. The researcher has investigated and analyzed the categories such as the ritual death of plants (being ritually honored,a symbol of human), the season shift (a symbol of the change in human characters), the rebirth of the sun inside the petals of morning glolry flower (the rebirth symbol of man), the metamorphosisin the from of a plant in the oppressed murder of mankind, etc. in the poetical masterpieces of the great Persian poets such as Roodaki, Ferdowsi, Naser Khosro, Khaghani, Nezami, Sana"i, Mowlawi, and Hafez.

The Best way of recognizing of every new phenomena specially in field of culture and literature is knowing the quality and relationship with its past and history. That means how it was created through history and we must not study it separately. So look at the first and the most important works of those who study the principal and important differences between modem prose and traditional one can help to recognize it better.

Multicarrier methods have long been identified for optimizing data transceivers on band limited channels. Discrete Multitone is a multicarrier transmission method which can be implemented using DSP techniques. Transmission rate in a simple DMT system can be increased by adding a full duplex echo canceller. So far, there have been several problems with the proposed echo cancellers regarding their high computational complexity and the relatively low stability. In this paper, a new high speed technique based on neural network with a good stability is represented. By using of this method, the echo of DMT modulation in ADSL system is cancelled. This technique optimizes the learning factor and consequently increases the echo cancellation rate in comparison with LMS method in about 2.37dB. In this method, only the operational phase of neural network is applied to DMT system. After training network, the weights are frozen and the computational cost is reduced to an acceptable level.

Since a secure voice or data session is very important in military HF communication, it needs a HF modem designed with cipher capability. Whereas multipath and Doppler affection are very adverse in HF sky-wave mode, so the production of a robust modem is essential for military application to compensate channel distortion. Then channel estimation block is very important and this paper proposes an enhanced method for channel estimation. The OFDM pilot based channel estimation with wiener filtering is a robust method that estimates channel excellently with minimum pilot density. The result show that proposed method can increase bit rate with decrement in pilot density, which allows use of a stronger channel coding and subsequently a lower bit error rate. Simulation shows that the bit-error rate of the proposed method passes the American military standards.

The increase in the number of ADSL users over recent years has encouraged different companies to enter to this context. For this reason, the current article aims to study Internet ADSL users' characteristics that affect their usage rate as one of the factors of organization’s achievement. Therefore, the data were collected using simple random method and questionnaire, the validity and reliability of which have been tested by professional's opinions and Cronbach's Alpha (0.83). The samples included 35392 persons selected from the community of interest (all ADSL users in country). After analyzing gathered data, it was understood that most of the users are young, loyal university students who use internet more than three hours a day. After determining the above characteristics, a significant difference was shown to exist between usage rates of the users based on their characteristics using one way ANOVA technique.

With respect to the dramatic increasing use of electronically communicators and wireless modems, concerns have been raised about the possible effects of emitted electromagnetic radiation on human brain. In this paper, the effects of high-frequency wireless modem waves on the brain signal are investigated. To this end, the Electroencephalograph (EEG) recording of 15 volunteers is examined in four different bands. The experiments are designed to cover in four steps. Average power statistical analysis in different frequency bands and magnitude-squared coherence function shows significant changes in some bands.

In this paper, we introduce a new constellation scheme to have better performance (in terms of BER) with respect to the conventional rectangular QAM constellation. We compared the new one and two new kinds of circular constellations with QAM. They were chosen so that they have the same average power. Also we compared the effects of using FHT and FFT for all constellations. Based on the results, the new constellations to provide slightly better performance in channels which have maximum SNR=20 dB. Also it is shown that the new constellation has better performance for the noisy channels (AWGN and burst) with ISI and low SNR. Also we have shown the effect of channel length with respect to cyclic prefix in this system.