ModulationTechniques.pdf

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Abstract

This article shows some modulation techniques for wireless network which is also called : Spread Spectrum Frequency-Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum(DSSS) Orthogonal Frequency Division Multiplexing(OFDM) and Variant CDMA(Code Division Multiple Access) Comparision All Techniques

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Figure 1: Modulation Techniques for wireless communication In recent days, with the developing technology, data transfer and transmission have become very important for people. The environments where the techniques for sending and receiving data are lacking or are missing have begun to emerge. In this article | mention a few techniques used to make up for the shortcomings. The techniques to be described are data transmission techniques used mostly in wireless communication. These are: " Spread Spectrum-Direct Sequence Spread Spectrum (DSSS) -FHSS (Frequency-Hopping Spread Spectrum), OFDM and variant, Code Division Multiple Access (CDMA)". SPREAD SPECTRUM

In recent years, the rapid development of international and regional communication systems and electronic battles has been done in the development of techniques based on different theories and different theories. The communication systems are most affected by this situation, military communication systems, and the problem of confidentiality has arisen. As a result, intensive studies on theories, techniques and systems that can answer the question of secrecy in military communications have gained much importance both in commercial and academic direction. One of these newly developed systems is the "SPREAD SPECTRUM" techniques. In these techniques, the basic structure is based on using a lot of bandwidth and the information with a code structure which is used by the receiver and the same time.

With these features, the spread spectrum signals are highly resistant tc interference or jamming. Since multiple users can share the same spreac spectrum bandwidth without interfering with one another, these can be callec as multiple access techniques.

Figure 4 : Type of Spread Spectrum

Figure 5: Frequency frequency band used instead of a ((sin x) / x) 2-shaped envelope 6. Direct Sequence Spread Spectrum(DSSS)

7. Direct Sequence Spread Spectrum vs Frequency-Hoppinc Spread Spectrum (FHSS)

if the code is not known to the public. These reasons make DSSS very popular by the military. In fact, DSSS was first used in the 1940s by the military. The information flow in the DSSS is divided into small fragments, each associated with a frequency channel in the spectrum. The data signals at the transmission points are combined with a bit rate higher data rate dividing based on a spreading rate of the data. A chip code in a DSSS is an extra bit pattern associated with each transmitted bit. This helps to increase the intrusion resistance of the signal. If any bits are damaged during transmission, the original data can be restored due to excess transmission. The whole process is performed by multiplying a radio frequency carrier and a pseud noise (PN) digital signal. PN code, quadrature phase shift keying (QPSK), binary phase shift keying (BPSK), etc. Are modulated on an information signal using various modulation techniques. Then, a dual-balanced mixer PN modulates the information signal and the RF carrier.

Figure 10 : Example of Direct Sequence Spread Spectrum

As shown in Figure 11, the spreading code (chipping series) having more than one bit for each transmitted bit is subjected to XOR processing, resulting in as many bits as the number of bits in the chipping series. 2] As the number of bits in the sending chipping series increases for each bit, the bandwidth required to transmit the resulting signal will increase. The receiver will obtain the original signal after removing the chipping series from the incoming signal. The spreading code must be generated in such a way as to carry a random series of features and to be perceived as noise on the backplane. Having random number properties makes it difficult to predict the spreading code. fH Suct numbers are defined as pseudo-noise numbers or series.

1. Multiple Carrier CodeDivision Multiple Access(MC-CDMA) SENA ALTIKULAC

OFDM is a multiplexing and modulating technique designed to overcome the bandwidth problems experienced. The basic principle of OFDM is to provide the best parallel transmission by dividing the existing frequency by the lower channels. Because of perpendicular to the lower channels, the frequencies that are supposed to be lost between the bands experienced in the FDM have been targeted by the loss of most of the losses due to OFDM. The results of this study showed to increase the band width is 50%. As shown in Figure 15,usage of bandwidths between FDM & OFDM and fundamental differences are showed.

2. Advantages and Disadvantages of Orthogonal Frequency Division Multiplexing(/OFDM) shows MC-CDMA receiver data block. After learning what the OFDM is interested in, | would like to mention about a technique in which OFDM is used. This technique, which is a combination of OFDM and CDMA techniques, is referred to as MC-CDMA (Multiple Carrier CodeDivision Multiple Access). It is a technique intended to use the frequencies in the best efficiency. A good result is obtained with the reliability found in CDMA and the frequency subchannel efficiency found in OFDM. In fact, MC-CDMA spreads in the frequency domain in the case of OFDM and CDMA. Others spread in the time domain.|In Figure 17 shows MC-CDMA receiver data block.

b)Disadvantages

€. Wideband Orthogonal Frequency Division Multiplexing(WOFDM) Wideband OFDM is a transmission scheme that is the basis of the IEEE standard 802.11a, which is the foundation of the proposed IEEE standard 802.16. It is a patented technology in the United States under patent number 5,282,222 and in Canada under patent number 2,064,975. W- OFDM overcomes problems with multipath by sending training symbols; the adverse channel effects can then be reduced through a simple division by the channel frequency response. It also employs a spreading forward error correcting code, such as ReedSolomon, to spread the symbols over many frequencies that convert the signal to direct sequence spreadspectrum with the ability to recover the symbols even if some carriers are totally absent. WOFDM's efficiency and noise tolerance unite the best of spreadspectrum and narrowband systems.Recall that spreadspectrum uses excessive bandwidth to compensate for noise and multipath, while narrowband technology is more sensitive to multipath propagation.

Figure 23 : Different code and Different message

Figure 24 : Example of CDMA with more than one bit The following Figure 24 and Figure 25 is taken from a course slide. We have done the above procedure for only one bit. Since the transmitted data contains more than one bit, tr operations are continued until the bit is finished

Figure 27: Receiving Message process in base sitation