Spread Spectrum Modulation
🛑 ALL INFORMATION CLICK HERE 👈🏻👈🏻👈🏻
Spread Spectrum Modulation
Sitemap
Privacy & Security
Terms of use
Access denied under U.S. Export Administration Regulations.
© 1995 - 2022 Analog Devices, Inc. All Rights Reserved
From troubleshooting technical issues and product recommendations, to quotes and orders, we’re here to help.
Electronic Test and Instrumentation
Electronic Test and Instrumentation
Understanding Spread Spectrum for Communications
Please enter your information below and we'll be intouch soon.
Please enter your information below and we'll be intouch soon.
© 2022 NATIONAL INSTRUMENTS CORP. ALL RIGHTS RESERVED.
Distributed Measurement and Control
You can request repair, RMA, schedule calibration, or get technical support. A valid service agreement may be required.
Provides support for NI data acquisition and signal conditioning devices.
Provides support for Ethernet, GPIB, serial, USB, and other types of instruments.
Provides support for NI GPIB controllers and NI embedded controllers with GPIB ports.
This tutorial covers an introduction to RF, wireless and high-frequency signals and systems.
Spread Spectrum refers to a system originally developed for military applications, to provide secure communications by spreading the signal over a large frequency band.
Figure 1 represents a narrow band signal in the frequency domain. These narrowband signals are easily jammed by any other signal in the same band. Likewise, the signal can also be intercepted since the frequency band is fixed and narrow (i.e. easy to detect).
Figure 1: Narrow band signal, relatively easy to jam or intercepted.
The idea behind spread spectrum is to use more bandwidth than the original message while maintaining the same signal power. A spread spectrum signal does not have a clearly distinguishable peak in the spectrum. This makes the signal more difficult to distinguish from noise and therefore more difficult to jam or intercept. This concept is illustrated in Figure 3.
This document will explore basics concepts of spread spectrum for the remaining of the introduction and then it will explore the supporting concepts of the most used technique in spread spectrum systems. The last section will give the reader some insight of more advance topics but will not deeply explore them. We encourage the reader to seek the references for advance knowledge of spread spectrum systems.
We present now the block diagram of a typical communication system with the difference that the modulator/demodulator has as input the spreading generator. This piece will be explored in following sections.
Figure 2: Block diagram of the spread spectrum communication system
There are two predominant techniques to spread the spectrum:
1) Frequency hoping (FH), which makes the narrow band signal jump in random narrow bands within a larger bandwidth.
2) Direct sequence (DS) which introduces rapid phase transition to the data to make it larger in bandwidth.
We will focus on Direct Sequence Spread Spectrum technique since it is the mostly used in the industry (CDMA, UMTS, 802.11, GPS).
Direct sequence spread spectrum (DSSS) introduces rapid phase transition to the data making it larger in bandwidth. As the period T of a signal gets shorter in time (or rate R increases), the bandwidth B of the signal increases: R = 1/T = 2B (Nyquist Rate)
The following figures explain it:
Figure 3: Rate and period are related to bandwidth by when pulse shaping is used.
The rapid phase transition (chip rate ) signal has a larger bandwidth given that the rate is greater (without changing the power of the original signal) and behaves similar to noise in such a way that their spectrums are similar for bandwidth in scope. In fact, the power density amplitude of the spread spectrum output signal is similar to the noise floor. The signal is “hidden” under the noise.
To get the signal back, the exact same high bandwidth signal is needed. This is like a key, only the demodulator that “knows” such a key will be able to demodulate and get the message back. This “key” is in fact a pseudo random sequence (rapid phase transition) also known as pseudo noise (PN). These sequences are generated by m-sequences.
m-Sequences
These codes (DSSS codes) will all be treated as pseudonoise (PN) sequences because resembles random sequences of bits with a flat noiselike spectrum.
This sequence appears to have random pattern but in fact can be recreated by using the shift register structure in Figure 4 with M=4, polynomial and initial state ‘1 1 0 0’.
Figure 4: Shift register structure for m-sequence
Where ‘ ’ represent modulo 2 addition.
Using this scheme, the initial state is only needed to generate exactly the same sequence of length (the only forbidden state is all zeros since the register will lock in this state).
Take for example:
The final sequence will look like this,
1 1 -1 -1 -1 1 -1 -1 1 1 -1 1 -1 1 1
After the fifteenth shift, the values on the registers will be again the starting seed.
Properties of m-Sequences
Period:
After this number of ‘1’ and ‘-1’ the sequence will start to repeat since the starting symbols will be the same.
Autocorrelation:
The formal definition of discrete autocorrelation is:
If we perform the following operation:
= 15 which is multiply each value by itself and add them all ( ).
Now take ,
Performing the same operation:
= -1 =
This is the autocorrelation for each shift point. If we take them all and plot them so that there are 15 points before 0 and 15 after:
a)
b)
Figure 5: Correlation of a) example sequence and b) other sequence with polynomial created with LabVIEW and MathScript
As seen, only if the end user having the exact sequence is able to demodulate the message when the sequence is synchronized (peak at correlation = 1). Other users will have very little amplitude of the original signal. This is the principle of Code Division Multiple Access (CDMA) cellular systems, in other words, share the same frequency and time with multiple users with different codes.
Spreading
The block diagram of the DSSS communication system for QPSK is presented in Figure 6. Notice that the PN sequence is introduced here to both in-phase (I) and quadrature (Q) components.
Figure 6: Block diagram of the spread spectrum QPSK modulator
The sequence should be long enough (with respect to the message signal) to have the noise-like spectrum. This is the relation between spreading sequence rate and message rate :
In practical systems, is an integer number and it is the number of phase shift of the PN sequence for each message bit. For example, for GPS systems N = 1024.
Figure 7: Spreading the message, each bit of the message will contain the entire PN sequence
The new message has now and therefore
The output combined baseband sequence is:
Where is the sent baseband waveform, is the PN waveform and is the bit sequence.
Despreading
Received baseband waveform is the combination of the transmitted waveform and noise in the channel.
Figure 8: Simple additive white Gaussian noise (AWDG) channel model. LabVIEW Vi
The received signal will be combined again with the spreading sequence. Notice that the noise is also going to be processed on the same procedure but correlation properties will not increase the noise power.
The received signal will be the combination of the transmitted signal plus noise:
We can substitute the sent waveform by the combination of the PN sequence and the bit sequence.
The modulator will multiply it by the PN sequence :
If is synchronized, and is like multiplying noise times noise which gives other kind of noise (similar in amplitude).
Consider the ideal example from Figure 9.
Figure 9: Recuperate original signal for two different users. User 1 has the correct sequence. User 2 has different sequence and therefore the output message will have many errors (no information)
M-sequences are the basics of PN sequences and they are used in real systems (GPS) but these are not the only PN sequences. Since Spread Spectrum is the basis of CDMA, we will highlight the basis of two of the most used sequences involved in the system: Gold sequences (WCDMA) and Walsh-Hadamard sequences (IS-95).
Gold Sequences
Gold sequences help generate more sequences out of a pair of m-sequences giving now many more different sequences to have multiple users. Gold sequences are based on preferred pairs m-sequences. For example, take the polynomials and :
Figure 10: Example of gold sequence generator using one preferred pair of m-sequences: and
Remember m-sequences gave only one sequence of length . By combining two of these sequences, we can obtain up to 31 ( ) plus the two m-sequences themselves, generate 33 sequences (each one length ) that can be used to spread different input messages (different users CDMA).
The m-sequence pair plus the Gold sequences form the available sequences to use in DSSS. The wanted property about Gold codes is that they are balanced (i.e. same number of 1 and -1s).
Walsh-Hadamard Sequences
Other common sequences are Walsh-Hadamard sequences currently used in CDMA systems. These sequences are orthogonal (i.e. where is a row of the matrix), convenient properties for multiple users. The sequences are the rows of the Hadamard matrix defined for as:
For larger matrices use the recursion:
Example for
Orthogonal codes have perfect properties of cross correlation (if no shift is implemented).
We have now explored different sequences that can spread the signal like noise would behave. Spread spectrum systems expand beyond this point to many different paths: modulation schemes, performance under fading, under interference, capacity in CDMA systems, etc.
To finalize, we would like to point out the advantages and disadvantages of using spread spectrum:
Advantages:
[1] G. L. Stüber, “Principles of mobile communication,” Kluwer Academic, Boston 1996.
[2] J. G. Proakis, “Digital Communications,” 4th Edition, McGraw-Hill Higher Education.
[3] T. S. Rappaport, “Wireless communications: principles and practice,” Pentrice Hall PTR, N.J., 1996.
[4] T. Pratt, C. W. Bostian and J.E. Allnutt, “Satellite Communications,” 2nd ed. John Wiley publication, 2002.
[5] R: Prasad, T. Ojanperä, “An Overview of CDMA Evolution Toward wideband CDMA,” IEEE communications surveys, Vol. 1, No. 1 Q4 1998.
PXI Vector Signal Analyzer
PXI Vector Signal Generator
NI RF Switches
The National Instruments RF switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz to bandwidths as high as 26.5 GHz.
LabVIEW
National Instruments LabVIEW is an industry-leading graphical software tool for designing test, measurement, and automation systems.
This site uses cookies to offer you a better browsing experience. Learn more about our privacy statement and cookie policy.
What do you need our team of experts to assist you with?
What do you need our team of experts to assist you with?
News the global electronics community can trust
eetimes.com
The trusted news source for power-conscious design engineers
powerelectronicsnews.com
Supply chain news for the electronics industry
ebnonline.com
The can't-miss forum engineers and hobbyists
elektroda.pl
Product news that empowers design decisions
electronicproducts.com
Design engineer' search engine for electronic components
datasheets.com
The electronic components resource for engineers and purchasers
eem.com
The design site for hardware software, and firmware engineers
embedded.com
Where makers and hobbyists share projects
electroschematics.com
The design site for electronics engineers and engineering managers
edn.com
The learning center for future and novice engineers
electronics-tutorials.ws
The educational resource for the global engineering community
techonline.com
Where electronics engineers discover the latest toolsThe design site for hardware software, and firmware engineers
eeweb.com
Circuit simulation made easy
partsim.com
Brings you all the tools to tackle projects big and small - combining real-world components with online collaboration
schematics.com
Hardware design made easy
pcbweb.com
A free online environment where users can create, edit, and share electrical schematics, or convert between popular file
formats like Eagle, Altium, and OrCAD.
schematics.io
Find the IoT board you’ve been searching for using this interactive solution space to help you visualize the product selection
process and showcase important trade-off decisions.
transim.com/iot
Transform your product pages with embeddable schematic, simulation, and 3D content modules while providing interactive user
experiences for your customers.
transim.com/Products/Engage
A worldwide innovation hub servicing component manufacturers and distributors with unique marketing solutions
aspencore.com
SiliconExpert provides engineers with the data and insight they need to remove risk from the supply chain.
siliconexpert.com
Transim powers many of the tools engineers use every day on manufacturers' websites and can develop solutions for any company.
transim.com
Figure 1: Propagation characteristics in free space
Spreading the spectrum (i.e. dispersing the signal energy over a wide band) yields a variety of characteristics:
Figure 2: Spread spectrum modulation and demodulation using PSK for primary modulation.
Figure 4: Spread spectrum modulation
Figure 5: 2.4GHz Spread spectrum module
All contents are Copyright © 2022 by AspenCore, Inc. All Rights Reserved.
Contact Us
About Us
Privacy Policy
Terms of Use
Site Map
Newsletters
California Do Not Sell
We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. By clicking “Accept All”, you consent to the use of ALL the cookies. However, you may visit "Cookie Settings" to provide a controlled consent.
This website uses cookies to improve your experience while you navigate through the website. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the ...
Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features.
Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors.
Analytical cookies are used to understand how visitors interact with the website. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc.
Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. These cookies track visitors across websites and collect information to provide customized ads.
Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet.
Editor's Note: Wireless 101: Alan Lowne of Saelig Company Inc. gives an introduction to what causes weakened radio waves in wireless transmissions – from industrial control to wireless microphones, and he explains spread-spectrum and antenna
Xxx Sex School Girl
Delyno Private
Www Evil Angel Com