Thursday, August 11, 2011


Spread Spectrum Multiple Access Techniques
Spread spectrum technique is emitting a signal on the tape frekuensiyang much wider than the bandwidth required on the transmission standard (eg: TDMA, FDMA). An example is the IS-95 CDMA menggunakanlebar 1.25 MHz frequency band, whereas only 30 kHz for AMPS voice menyalurkansinyal. The process of widening the frequency band is called spreading.Spread spectrum signal used to transmit digital information are influenced by the characteristics of the bandwidth W is larger than the information rate R in bits / s, so that little power is required. Another understanding of the spread spectrum technique is a technique that allows multiple users using the same bandwidth at the same time without interference from each other.

A system can be called a spread spectrum system if it meets the following requirements: 1. Signal requires a large bandwidth to transmit information. 2. Spreading signal or code signal is called independent data. 3. At the receiver, despreading is done by adjusting the received spread signal with a replica synchronization of a spread information signal. 
Spread spectrum signal is used for: 1. Eliminate or suppress the detrimental effects of interference on jamming, interference from other users in the channel and interference due to multipath propagation. Jammer should be eliminated because it would interfere with communication because it confuses the receiver. Interference from other users because users share the same channel bandwidth to transmit different information to different destinations at the same time. 2. Hiding a signal by transmitting at low power is obscured by noise. Hiding behind the message is done by spreading the noise bandwidth by coding and transmitting the resultant signal at low power. In these circumstances the probability of another listener to know the content is very low Pean, otherwise known as Low-probability-ofintercept (LPI). 3. Giving private messages to other users without being heard. This is done by superimposing a pseudo-random pattern in the transmission of messages. Messages can be demodulated by the receiver in question, who knows the pseudo-random pattern or code used transmitter, while the other receivers who do not know the code is not able to receive the message. In other applications in addition to communications, spread spectrum signal is used to generate an accurate calculation of time delay and the average speed on the radar and navigation. 

Two types of techniques sektrum spreads: 

1. Frequency Hoping Frequency Hoping obtained by changing the frequency-fox pembawaberdasarkan time with near-random pattern, pseudo random .Whereas CDMA signal obtained by modulating the information with spreadingsequence known as pseudo noise (PN) signal makes the signal information digitalyang ribbon width and shaped like noise (noise)

2. DS (Direct Sequence)-CDMA
Direct Sequence Spread Spectrum is the most popular technique. Data signal is multiplied by a Pseudo Random Noise code (PN-code).
PN-code is a sequence of chips valued -1 and 1 (polar) or 0 and 1 (non-polar). The number of chips in one code is called the period of the code. PN-code is code that resembles the noise (noise-like code) with the properties of certain properties.
There are several classes of binary (two-phase) PN-codes: M-sequences (base), Gold-codes and Kasami-codes. PN-code can be made with one or more shiftregisters. If the length of shiftregister is n, in general the period N can be expressed as follows:
N = 2n -1
In the simplest case, the PN-code is multiplied by one bit of data 
(see figure, in this example N = 7). Bandwidth of the data signal is multiplied by a factor of N, this factor is referred to as processing gain.

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