Different techniques of spread spectrum
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Published: Mon, 08 May 2017
The idea of this project is to study different techniques of spread spectrum. These techniques include the two processes of direct-sequence spread spectrum and frequency hopping spread spectrum. Both these techniques will be simulated in software called MATLAB. Also a brief mention will be there about the multiple access property of spread spectrum.
This project deals with the implementation of spread spectrum using the techniques- direct sequence spread spectrum and frequency hopping spread spectrum. Spread spectrum is a new method of communication. It provides a jam-free network and prevents any kind on interference. It helps to provide immunity to channels by not allowing any kind of interference or disturbance. We will be discussing more about spread spectrum and its technique in the following chapters.
The project has been divided into six chapters. The second chapter deals with spread spectrum. It starts of with the history of spread spectrum followed by a basic definition of spread spectrum. Certain characteristics of spread spectrum are discussed along with a mathematical explanation of advantage of spread spectrum over narrow band communication. It then describes the working of a spread spectrum and finally mentions how a spread spectrum can be demodulated.
Chapter three describes different techniques of spread spectrum. There are four techniques of spread spectrum namely direct sequence spread spectrum (DSSS), frequency hopping spread spectrum (FHSS), chirp spread spectrum (CSSS) and time hopping spread spectrum (THSS). Each of these techniques is described, with a brief mention about its basic mechanism.A brief comparison is drawn between FHSS AND DHSS. A mathematical explanation is given for DSSS under the conditions of noise (jammer).
Chapter four discusses different multiple accesses of spread spectrum. These are frequency domain multiple access (FDMA), time division multiple access(TDMA) and code division multiple access(CDMA). CDMA is dealt with in a more detail manner and is presented with a mathematical explanation.
Chapter five presents a MATLAB simulation of direct sequence spread spectrum (DSSS), Frequency hopping spread spectrum (FHSS) and Code division multiple access (CDMA). It discusses how a signal when multiplied with a pseudo random noise and put on a frequency wave gets spread. This technique is called direct sequence spread spectrum. Also this chapter presents the MATLAB simulation of Frequency Hopping Spread Spectrum where four randomly carrier frequency waves are generated to form a spread spectrum and multiplied with pseudo random generator to determine the frequencies where the information has to be hopped. The information is sent in the form of a BPSK modulated signal. Also a MATLAB simulation on CDMA is presented .It basically shows how CDMA works. Three stations are taken and how they send codes through a single channel by using the property the CDMA.
Finally, chapter six winds up the project with the conclusion from each chapter. It mentions in short what has been realized from this project.
2.1 HISTORY OF SPREAD SPECTRUM
This chapter talks about spread spectrum. It begins with a brief history behind spread spectrum. It also mentions about the various fields where spread spectrum is being effectively used. Certain characteristics of spread spectrum are also discussed.A basic definition of spread spectrum is discussed in this chapter which basically explains what actually spread spectrum and why is preferred over conventional wireless communication. A brief mechanism of how spread spectrum works is discussed. It also mentions the mathematical reason behind why a spread signal is an effective way to communicate. A comparison is drawn between spread spectrum transmission and fixed frequency transmission. Finally a demodulation technique is discussed mentioning about how demodulation can be achieved.
Spread spectrum has become a new commercial communication technique over the past 8-9 years. However the first intentional use of Spread Spectrum came during the period of 1921-1930 by Armstrong. He had used wideband Frequency Modulation. The real use for Spread Spectrum how ever came in World War II. Both the allies and the Axis powers experimented with simple Spread Spectrum systems. The first publically patent on Spread Spectrum came from Hedy Lamarr, the Hollywood movie actress, and George Antheil, an avant gard composer.
Lamarr had mentioned to Antheil about her idea for a Secret Communications System that could guide torpedoes to their target without being intercepted by the enemy. This could be done by sending messages between transmitter and receiver over multiple radio frequencies in a varied random pattern. The message would be transmitted at such a high rate that no one would be able to decode it. They sent their invention to National Inventors Council. Instead of developing the patent commercially, they gave it away to the government for the war effort. As a result of which the commercial use of Spread spectrum came many years later [1, 3, 19].
Spread Spectrum was first used for commercial purposes in the 1980s when Equatorial Communications of Mountain View used Direct Sequence for multiple access communications over synchronous satellite transponders .
Today, spread spectrum is being used to provide communications in a variety of commercial applications, including mobile communications and interoffice wireless communications. In the coming years hardly anyone will prevent themselves from being involved in one way or the other with spread spectrum communications as it will become an integral part of the communication world. [1, 4].
2.2 DEFINITION OF SPREAD SPECTRUM
Spread-spectrum is a technique by which electromagnetic energy produced over a particular bandwidth is spread in the frequency domain on purpose. Spread spectrum signals are used for transmission of digital information which are characterized by the fact that their bandwidth W is much greater than the information rate R in bits/sec. Hence the bandwidth expansion factor is always greater than unity. This results to provide a signal with a wider bandwidth and a lower power density. These techniques are used for a variety of reasons which include the establishment of secure communications, increasing resistance to natural interference and to limit the power flux density on satellite downlinks. Narrow band signal and spread spectrum use the same amount of power to transmit the same data. However the power density of the spread spectrum is lower as compared to narrow band and it is this property that makes spread spectrum achieve secure and jam free communication.
Spread spectrum is a type of wireless communication in which the frequency of the transmitted signal is intentionally varied and changed to achieve a higher bandwidth. This results to provide a much greater bandwidth than that of the signal if its frequency was not altered [2, 5].
A spread spectrum should full fill the following requirements:
1) The message signal should undergo two modulations that produce the wideband spectrum having bandwidth much in excess of the minimum bandwidth needed to send the message.
2) The spreading ,that is the second modulation is caused by means of a spreading signal
3) The receiver should recover the message by the method of de-spreading the signal that is received by using the same spreading signal .
Spread spectrum is preferred over conventional wireless communication for many reasons; however there are at least a couple of problems associated with conventional wireless communication.
First, a signal whose frequency is constant and not changing is subject to interference. This condition occurs when another signal is sent on, or very near to the frequency of the desired signal. Interference can be accidental (as in the case of amateur-radio communications) or it can be deliberate like during war time.
Second, a constant-frequency signal is very easy to intercept or lock and reveal the information it is carrying and hence is not well preferred to applications where the information must remain confidential between the transmitter and receiver. .
To minimize troubles and to maintain the confidentiality of the information, transmitted signals frequency can be deliberately varied and changed over the large segment of the electromagnetic radiation spectrum. This variation is done by using a specific and complicated mathematical function. This function is the most important part of the spread spectrum for maintaining the confidentiality.
For intercepting the signal, the receiver must be tuned to frequencies that vary exactly to this function. The receiver must have the knowledge of the frequency-versus-time function employed by the transmitter, and must also be provided with the information about the starting-time point of the function. If someone wants to hack or intercept the spread-spectrum signal, that person must possess a transmitter that contains the information about the function and its starting-time point. The spread-spectrum function hence must be well protected for confidential information communication and it this feature of spread spectrum that helps maintaining the secrecy of information. If this function comes in the wrong hands, it can lead to the invasion of secrecy of the information .
2.3 HOW SPREAD SPECTRUM WORKS
Spread Spectrum uses wide band which is a noise-like signals and hence are hard to detect. These signals are also hard to lock on to or be tracked by any intruder. Spread signals are made in such a manner that their bandwidth is larger than the information they are transmitting to give them a noise like appearance, hence protecting the information it is carrying and maintain its secrecy and confidentiality .
Spread Spectrum signals use fast codes that have a high data rate. These codes are called “Pseudo Random” or “Pseudo Noise” codes. These are called Pseudo” for the reason being that they are not real Gaussian noise .
Spread Spectrum transmitters possess similar transmits power levels to that of narrow band transmitters. Spread Spectrum signals are so wide hence they transmit at a much lower spectral power density which is measured in Watts per Hertz as compared to narrowband transmitters. This characteristic (lower spectral power density)gives spread signals a huge advantage in digital communication and it is this capability that is responsible for the rapid use of Spread Spectrum today .
The numerical advantage of wider bandwidth is obtained from the Claude Shannon’s equation describing channel capacity .
C=channel capacity of the signal, B=Bandwidth of the signal, S=signal power, and N=noise power.
On Applying the MacLaurin series development for
is usually low for spread-spectrum applications. Assuming a noise level such that << 1, Shannon’s expression becomes simply:
To send error-free information for a given noise-to-signal (N/S) ratio in the channel, therefore, need to perform the fundamental spread-spectrum signal-spreading operation: increase the transmitted bandwidth. This is the main reason why the band width of spread spectrum is widened .
GP (Process Gain) = BWRF/ RINFO
BWRF= Radio Frequency Bandwidth, RINFO= Information Rate.
Spread spectrum is a band pass communication. At the modulator end, the information signal bandwidth is spreaded by multiplying by spreading signal. The demodulator then multiplies the signal that is received by the same spreading signal and then recovers the original signal by filtering.
Spread spectrum signals are hard to exploit along with being hard to intercept and jam .Signal exploitation is the ability of an intruder from an outside network to invade into the network and use information from that network without actually being a part of that network. Spread spectrum signals are hence more secure than narrowband radio communications. This very nature of spread spectrum allows militarys intelligence levels of privacy and security to be safe guarded without any difficulty.
Spread-spectrum transmission has three main advantages over fixed-frequency transmission:
Spread-spectrum signals have a high resistant to narrowband interference.
Spread-spectrum signals are difficult to intercept or hack and hence the confidentiality of the information can be preserved.
Spread-spectrum transmissions share frequency band with many other types of conventional transmissions that have minimal interference. Hence, much communication information can be send through one channel .
2.4 DEMODULATION OF SPREAD SPECTRUM
For retrieving the information that is sent by the spread spectrum transmitter, the receiver should be able to demodulate the signal. It is possible if we have the knowledge of carrier frequency and phase along with the perfect alignment between the transmitter and receivers pseudo random codes .
The process of despreading or demodulation a signal is called correlation. The spread spectrum signal is de-spread when there is a synchronization of the spreading code between the transmitter and receiver. Synchronization is the biggest challenge faced by the receiver and when achieved makes demodulation possible, hence a lot of time, money and efforts are used to improve this synchronization technique. This problem of synchronization is further broken down into two parts: initial acquisition where the two pseudo random codes are brought into coarse alignment and tracking which continuously maintains a fine alignment of the code[3, 15,22].
A spread spectrum correlator is a special matched filter that is it responds only to those signals that are encoded with a pseudo noise code that matches with its own code. Thus, a Spread Spectrum correlator can be “tuned” to different codes just by changing its local code and hence is very effective in decoding. This correlator does not respond to any manmade or natural or artificial noise or any other kind of interference except to those Spread Spectrum signals with identical matched signal characteristics and pseudo noise code [3,15].
From this chapter we have briefly understood about the mechanism of spread spectrum and why it is being called as the communication technique of tomorrow. It also describes the advantages of spread spectrum. Also the mathematical implementation of using wider bandwidth was discussed. Certain characteristics of spread spectrum too were discussed. We have also discussed about the demodulation of spread spectrum.
In the next chapter, four techniques of spread spectrum will be briefly discussed along with the required diagrams.
TECHNIQUES OF SPREAD SPECTRUM
In the previous chapter we studied the basic definition of spread spectrum along with its advantages over conventional wireless communication and its working mechanism. Also there was a brief mention of its demodulation techniques.
In this chapter, the four types of techniques will be discussed. They are: direct sequence spread spectrum, frequency hopping spread spectrum, chirp spread spectrum and time hopping spread spectrum. Also the mathematical implementations of DSSS will be clearly explained which involves addition of noise(jammers).Also a brief comparison was made between DSSS and FHSS.
All these techniques will be briefly explained with the help of diagrams.
There are four forms of techniques of spread spectrum:
Direct-sequence spread spectrum (DSSS)
Frequency-hopping spread spectrum(FHSS)
Time-hopping spread spectrum (THSS)
Chirp spread spectrum (CSS)
3.1 FREQUENCY-HOPPING SPREAD SPECTRUM
In a Frequency Hopping spread spectrum system, a transmitter “hops” between the available frequencies according to a predefined algorithm or program, which can be either randomly created or planned before being executed. The transmitter and the receiver, both are synchronized to the same centre frequency. A small portion of data is transmitted on a narrowband at a frequency. Then, the transmitter tunes to another frequency and transmits again. Hence the transmitter is capable of hopping its frequency over a given bandwidth many times a second, transmitting on one particular frequency for a certain period of time, then hopping to another frequency and transmitting again. .
The Frequency Hopping Spread Spectrum transmitter is a pseudo-noise PN code controlled frequency synthesizer. The transmitted data from the transmitter hops from one frequency value to another depending on the pseudo-random input from the code generator. Varying this instantaneous frequency gives an output spectrum that is spread over the generated range of frequencies. An important factor in Frequency Hopping Spread Spectrum is the rate at which these hops occur. The minimum time required to change from one frequency to another depends on the information bit rate [3, 7].
A Frequency Hopping Spread Spectrum communication is as follows:
1. The transmitting party sends a request for the algorithm through a predefined frequency channel.
2. The receiving party sends a number, known as a seed.
3. The transmitting party uses seed as a variable in a predefined algorithm. It calculates the sequence of frequencies that will be used.
4. The transmitting party sends a synchronization signal through the first frequency in the calculated sequence and then on the basis of the predefined algorithm the frequency hops to another frequency .
5. The communication begins between both the receiving and the transmitting party and change their frequencies in accordance to the calculated code , starting at the same point in time.
A commonly used data modulation is the M-ary (MFSK) in which case the the message data modulates a fixed frequency carrier.The carrier frequency is made to hop from one frequency value to another.Hence this method involves a 2 step modulation process Data modulation and frequency hop modulation.A large number of carrier frequencies are generated in a frequency synthesizer which is determined with the help of a PN generator.
Here the data is a MPSK modulated with some carrier frequency .The PN generator has an output of PN codes of word size k-bits.Here the modulated data is mixed with the outputs of the frequency synthesizer for hopping and transmission through the channel.
The receiver signal is first developed by using a mixer with the input coming from the same frequency synthesizer.The developed signal is then applied to the demodulator for recovery of data.
There are two types of Frequency hopping
Slow frequency hopping
Fast frequency hopping
In slow-frequency hopping there is one or more symbols per hop while in fast hop there are several frequency hops per message symbol.Slow hop is more susceptible to jamming than fast hopping.The reason is that in slow hopping the jammer has more time to decide which frequency to jam where in fast frequencing hopping by the time the jammer knows about the frequency and is about to jam it,the frequency quickly hops to another frequency and hence jammig becomes tough .
3.2 CHIRP SPREAD SPECTRUM
Chirp spread spectrum (CSS) is a technique of spread spectrum that involves the use of wideband linear frequency modulated chirp pulses to encode the information. A chirp can be defined as a sinusoidal signal whose frequency increases or decreases over a particular amount of time .
Like other methods, Chirp Spread Spectrum makes use of its complete allocated bandwidth to transmit a signal, hence making it strong against channel noise and interference. In this technique, the data signal is spread over a wider frequency band through chirp signals for data transmission. As chirps use a broad band of the spectrum, Chirp Spread Spectrum is resistant to multi-path fading. It does not use the feature of adding any pseudo-random elements to the signal to distinguish it from noise on the channel unlike DSSS and FHSS; rather it relies on the nature of the chirp pulse which is linear. Also Chirp Spread Spectrum provides resistant against Doppler Effect .
Recently CSS techniques have attracted a lot of attention in the field of wireless communications because of its feature to avoid jamming and interference. The CSS technique is classified into two categories, on the basis of the role of the chirp signal. They are the binary orthogonal keying (BOK) and direct modulation(DM).
The Binary Orthogonal Keying scheme uses chirp signals for representing data: for example, bits 1can be used to represent chirp with positive instantaneous frequency while 0 can be used to represent negative instantaneous frequency. In Direct Modulation the chirp signal is used just as a spreading code . Data modulation and demodulation is performed separately from the chirp processing, which adds flexibility to Incorporate various modulation techniques.
3.3 TIME HOPPING SPREAD SPECTRUM
Time-Hopping is one of the techniques of spread spectrum technology. In this technique , a pseudorandom code sequence determines the on and off condition of the carrier.
A time hopping system is a spread spectrum technique where the period and duty cycle of a pulsed Radio Frequency carrier are varied by a code sequence which is generated by a pseudorandom generator. Time hopping is often used effectively in collaboration with frequency hopping to form a much better form of spread spectrum called the hybrid TDMA.
Time hopped spread spectrum systems have not been able to find any commercial use in todays date. But hybrid TDMA which is a combination of frequency hopping and TDMA has a tremendous use in military. However, with the coming of random access memory (RAM) , micro-controller chips and other new such features, time hopping spread spectrum technique will be preferred in the future .
3.4 DIRECT-SEQUENCE SPREAD SPECTRUM
Direct sequence spread spectrum systems are well known and most widely used. This process is achieved by multiplying a radio frequency carrier with a pseudo noise. The (PN-code) is a binary signal which is produced at a higher frequency than the data to be transmitted. Since this has a higher frequency, it is accompanied with a larger bandwidth. The nature of this signal makes it appear that it is random; however it is not random .
Direct-sequence spread-spectrum transmission multiplies the information data that is being transmitted by a “noise” signal which is a pseudorandom code. Basically this PN code is modulated on to the information signal. This noise signal comprises of a pseudorandom sequence consisting of 1 and -1 values. On multiplying the information with PN code it is at a frequency higher than that of the original signal, hence leading to the spreading of the energy of the original signal into a much higher and wider band .
The resulting signal is a mixture of PN coded information signal with the radio frequency. It resembles white noise. This noise-like signal can be used to demodulate the original data at the receiving end, by multiplying the signal with the same pseudorandom sequence (because 1 1 = 1, and -1 -1 = 1). This process is called “de-spreading as it enables to recover the original signal.
For the de-spreading of signal to work correctly, the transmitter and receiver sequences must be synchronized exactly. This requires the need for the receiver to synchronize its sequence with the sequence of the transmitter by any method in order for despreading to take place. However, this drawback can be of quite importance. This concept is used in satellite navigation systems. In this if all the transmitters present are all synchronized which each others sequence, then the relative synchronization that is achieved between the receiver and any one of the transmitters can be used to predict relative timing and hence be used to calculate the receiver’s position provided the transmitters’ positions are known.
The resulting effect of enhancing signal to noise ratio on the channel is called process gain. Process gain can be increased by using a longer PN sequence or more chips per bit. If an unknown transmitter transmits its signal on the same channel but with a different Pseudorandom sequence code as compared to the other transmitter using the same channel, the de-spreading process results in no processing gain for that particular signal. This effect is used in code division multiple access (CDMA) property of DSSS. In contrast, frequency-hopping spread spectrum changes its carrier frequency by multiplying it with pseudorandom code rather than multiplying it with the data.
COMPARISON OF DSSS AND FHSS
Frequency Hopping Spread Spectrum radios are more tolerant and susceptible to narrowband noise and interference than Direct Sequence Spread Spectrum systems, so they will need to retransmit the same packet of information often.
Frequency Hopping Spread Spectrum radio systems appear to do better performance indoors and in severe multipath environments. The reason being that frequency hopping scheme can prevent multipath by just hopping to a new frequency. The wavelength of that new frequency changes bare minimum to change the signal path, and therefore change any multipath interference that might possibly occur. On the other hand Direct Sequence Spread Spectrum are more useful in outdoor and non-cluttered environments.
Frequency hopping spread spectrum does not have processing gain as there is no signal spreading. Processing gain provides the decrease in power density during transmission of a signal and increase during its de-spreading. Processing gain helps to increase the signal to noise ratio. As frequency hopping spread spectrum does not use processing gain, it has to transmit using more power in order to maintain the same signal to noise ratio as compared to direct sequence spread spectrum. As the radio frequency bands possess the same power limit, Frequency hopping spread spectrum cannot reach the same signal to noise ratio as compared to direct sequence spread spectrum.
In Frequency hopping spread spectrum, it is difficult to synchronize the receiver with the transmitter because it involves the tuning of both frequency and time. On the other hand Direct sequence spread spectrum needs only the chips timing to be synchronized which is only a one time affair and then locks on to the frequency. Hence has a lesser latency time as compared to Frequency hoppng spread spectrum .FHSS radios spend more time locking the frequency and hence end up having a larger latency time. The lower latency in Direct sequence spread spectrum is the reason for higher utilization of bandwidth as compared to Frequency hopping spread spectrum.
Frequency Hopping spread spectrum are restricted to a dwell time of 400 microseconds. In FHSS, small amount of information or data is transmitted before they are hopped to another channel. This time element is called Dwell time. After each hop the device must re-coordinate its frequency with the other device before any data transmission can proceed. As Direct sequence spread spectrum does not involve hopping of frequency hence it has no dwell time. Also there is no need to re-synchronize the device each time.
From this chapter we have understood the basic idea about the different techniques of spread spectrum with help of required diagrams. Frequency hopping was studied in detail along with direct sequence spread spectrum where its mathematical implementation was also described. Mathematical implementation of DSSS involved addition of noise in the form of jammer.Techniques like Chirp sequence spectrum and Time hopping spread spectrum were just touched upon. Also a comparison was drawn between FHSS and DSSS.
In the next chapter, multiple access property of spread spectrum will be discussed. How transmission of information by different users can be done over a single channel will be discussed in the following chapter.
SPREAD SPECTRUM MULTIPLE ACCESS
In the previous chapter ,different techniques of spread spectrum were discussed .It included Direct sequence spread spectrum, frequency hopping spread spectrum, Chirp spread spectrum and time hopping spread spectrum. Mechanism of direct sequence spread spectrum and frequency hopping spread spectrum are discussed in details. Mathematical implementation of DSSS was presented as well. Also a comparison is drawn between FHSS and DHSS.
In this chapter, multiple access property of spread spectrum will be included. There are three types of multiple accesses namely FDMA, TDMA and CDMA. A brief idea about how each of these techniques works is the aim of this chapter. CDMA will be dealt with in more detail which is supported with a mathematical explanation as well.
Spread Spectrum can also be used for implementing multiple accesses. So far, three main methods are available:
FREQUENCY DIVISION MULTIPLE ACCESS(FDMA)
TIME DIVISION MULTIPLE ACCESS(TDMA)
CODE DIVISION MULTIPLE ACCESS (CDMA)
4.1 FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)
In FDMA, FDMA assigns a specific carrier frequency to a communication channel. In this the given bandwidth is dived into a specified number of frequencies horizontally. And also one frequency is assigned to an individual user; hence that frequency will remain idle if that user is not transmitting any data. Because of this there is bandwidth wastage and time loss. Also there is a chance of data being stolen by someone. FDMA can only handle voice communications and not any other kind of data. Of the three methods that are Frequency Division Multiple Access, Time Division Multiple Access and Code Division Multiple Access, FDMA is the least efficient of them all in term of frequency-band usage and utilization. Applications of FDMA access include broadcasting of radio and TV and is also applicable in Total access communication system (TACS)[3, 16,20].
4.2 TIME DIVISION MULTIPLE ACCESS (TDMA)
Time Division Multiple Access is a method which comes under digital modulation. It allows a large number of users to access a single RF channel without any kind of interference by allocating each user with an independent time slot. In TDMA, different users communicate with each other that is listen and speak in accordance to a defined allocation of time slots . .
It has a number if uses. It not only increases the efficiency of transmission but also is able to quickly adapt to the transmission of data. It also enables the channel to carry data at a high rate which facilitates the user to communicate through faxes and short service messages (SMS)etc.
A problem with TDMA comes when the user is roaming from one cell to another. As in TDMA each user is assigned a particular time slot, on roaming if the time slots in the next cell are all occupied, then the user will get disconnected. TDMA encounters multipath distortion .
4.3 CDMA (CODE DIVISION MULTIPLE ACCESS)
A basic concept in data communication is permitting a large number of transmitters to transmit information data simultaneously over a single chann
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