The Spread Spectrum Techniques
Spread Spectrum techniques used in wireless communication systems to combat the impact of the interference from jammers and multipath fading on communication channel. In this project mainly focuses on Frequency Hopping Spread Spectrum in Bluetooth wireless communication. In this project mainly carried out a Matlab/Simulink Simulation of a Bluetooth frequency hopping spread spectrum communication link under the two different channel impairment such as Additive White Gaussian Noise and Rayleigh Fading has been discussed.
Finally results are analysed in terms of BER (Bit Error Rate) Curves under AWGN and Rayleigh fading in order to determine the performance of the system.
Introduction
Wireless communication is a one of the fastest growing sectors in modern communication arena. Over the last decade wireless communication is developed very rapidly and become an essential part of the human life in most countries. Today, "Bluetooth "is the one of the recent development of the wireless communication world.The origin of the Bluetooth wireless communication started in 1994. The development of the Bluetooth industry standard started in the 1998 when Ericssion , IBM , Intel , Nokia and Toshiba formed the Bluetooth Special Industry ( SIG) to develop and promote a global solution for short range wireless communication operating in the unlicensed 2.4GHz ISM ( Industrial Scientific Medical) band[1]. Bluetooth is a wireless technology that designed for the short range communication for both voice and data.
Using Bluetooth technology mobile phones, Portable Computers and Personal Digital Assistant (PDA) can communicate each other without cables and Bluetooth is a perfect and idle solution for short range cable infrastructure. Bluetooth data transfer range is usually approximately 10-100m.In modern communication and Business industry the "Bluetooth Technology" is a hottest topic, because it has low power, very cost effective and simple system. The main features of the Bluetooth wireless communication illustrated follows.Operating frequency: Bluetooth operate in globally available 2.4GHz band and used frequency hopping spread spectrum technique. [2]Ad hoc networking: A device equipped with Bluetooth radio establishes instant connection with another Bluetooth radio as soon as it comes in range [2].
Then Bluetooth supports both point to point and point to multipoint connections to enable ad hoc networking.Security: In Bluetooth short range wireless networks are safer to some extent because a hacker has to be within few meters of the device to break in. Then encryption and authorization can be done at the application level.Speed: Bluetooth wireless communication supports to voice and data transmissions and it works under noisy environments. Specially voice transmissions are also work under noise environment condions.Cost: Bluetooth is very cost effective technology.
According to market analysis Bluetooth supporting devices are very inexpensive, therefore lot of people uses this technology for their purposes.Bluetooth uses a Frequency Hopping Spread Spectrum technique and this spread spectrum technique approach avoid the interference and jamming.Bluetooth Wireless Communication can be used for wide range of applications such as ,File Transferring.Ad-hoc networking.Home Base Station.Enterprise Mobile Solutions.Wireless Hot-Spot Technology.....etc.
Overview of Spread Spectrum in Wireless Communication.
The Spread Spectrum is a significant form of encoding method in wireless communication. Spread Spectrum technology first invented during the second world war and it has dominant military communication applications, where it is attractive because of its resistance to interference and interception as well as its amenability to high resolution ranging. In the later part of 1980 industry was investigated the commercial applications of spread spectrum technology and today spread spectrum technique used for variety of wireless communication applications.Spread Spectrum signals which spread over the wider frequency range and collected in the receiver to their original frequency, so they are not easily seen i.e. almost transparent.
There are two types of Spread Spectrum techniques deployed in wireless communication known as Frequency Hopping Spread Spectrum and Direct Sequence Spread Spectrum, but in this project mainly focused on "Frequency Hopping Spread Spectrum Technique" and this technique is the easiest spread spectrum modulation to use. The Frequency Hopping Spread Spectrum is a type of a spread spectrum when signal is broadcast over a seemingly random series of the radio frequencies hopping from one frequency to another frequency at a predetermined time interval.
Project Aims and Objectives
The main aim of the project is to investigate the spread spectrum for Bluetooth Applications. For this purpose a Bluetooth Frequency Hopping Spread Spectrum Communication link was to be simulated using Matlab/Simulink. This model was to be tested for two different channel impairments such as AWGN (Additive White Gaussian Noise) and Rayleigh Fading . Finally performance analyzed in terms of BER (Bit Error Rate) Curves for two different channel impairments such as AWGN and Rayleigh Fading.
Structure of the Report
This report is organized into five chapters. The following illustrated that each chapter in brief manner.Chapter 2 contains fundamental principles that related to Spread Spectrum Techniques. It describes the types of spread spectrum techniques and properties of the spread spectrum techniques. Afterwards describes the how frequency hopping spread spectrum technique applied to Bluetooth wireless communication. Advantages and Disadvantages of Frequency Hopping Spread Spectrum technique also explained in detail manner.Chapter 3 explains Bluetooth wireless communication channel.
It describes the Noise , Characteristics of Additive White Gaussian Noise and further describes the Multipath propagation , fading and Characteristics of Rayleigh Fading.Chapter 4 contains the Block diagram of the model and gives detail of work done on Matlab/Simulink model and analyze the results that obtained using simulation.Chapter 5 describes the conclusion and suggestions for further work are given.
Summary
In this chapter provides a reader to clear understanding of Bluetooth wireless communication and overview of the spread spectrum in wireless communications were explained. The following chapter provides a clear understanding about the spread spectrum and their properties and how spread spectrum involved to Bluetooth communication.
CHAPTER 2
Introduction
Spread Spectrum is a wireless communication technology it has been used for military applications for basically two reasons. First one is the spread spectrum techniques have a anti-jamming properties and other one is signal can hide from the eavesdropper. Now a day's spread spectrum techniques used for civilian applications like mobile telephony system.
This chapter provides a detail description about spread spectrum starting from the basic concepts and provides clear idea about the subject.
Fundamental Principals of Spread Spectrum.
In Wireless communication systems the bandwidth is a most important and valuable, because in a system increasing a transmit signal bandwidth can sometimes improve the system performance. Therefore spread spectrum is a technique that increases signal bandwidth beyond the minimum necessary for data communication [4]. The following block diagram illustrates the main characteristics of any kind of spread spectrum technique.General Model of Spread Spectrum Digital Communication System [5]According to above diagram input information sequence fed into a channel encoder and encoder produces an analogue signal and it has a very slight bandwidth around some centre frequency value.
Afterwards signal is modulated using spreading sequence and this spreading sequence is generated by using pseudorandom number generator. The major impact of the modulation is increase the bandwidth of the signal to be transmitted. At the receiving end opposite process is used. Same spreading sequence is used to demodulate the spread spectrum signal.
Afterwards using channel decoder the original signal can be recovered.The advantage of the spread spectrum system is that, while the probing of the signal may be wideband, it is possible to detect the transmitted signal using narrowband receiver preceded by a wideband mixer, thus improving the dynamic range of the system as a compared to the direct RF pulse system.Illustration of Power Spectral density of Original signal and spread Spectrum Signal [6]According to above diagram Wide band signal (Spread Spectrum Signal) and Original Signal (Narrow Band Signal) have same power levels, but wide band signal are so wide and possible to transmit the lower spectral power density when comparing with narrowband signals. In Spread Spectrum systems used pseudorandom codes for spreading. These codes are appearing to be random codes but actually they are not truly random because the receiver has to know the pseudorandom code sequence used. The following can be said to any spread spectrum systemsThe bandwidth of the transmitted signal is much higher than the information signal.The pseudorandom code is used to determine the bandwidth of the spread spectrum signal.( need to be write more....)
Types of Spread Spectrum
There are two types of spread spectrum known as Frequency hopping Spread Spectrum and Direct Sequence Spread Spectrum. In this project mainly focused on Frequency Hopping Spread Spectrum.
Frequency Hopping Spread Spectrum
Instead of spreading the signal over a wide band of frequency, Frequency Hopping Spread Spectrum transmits for a short burst of data on one frequency hops to another new frequency. [7] The sequence of frequencies is known as "Hop Sequence" . The transmitter and receiver stations must synchronize to the "sequence ". In frequency hopping spread spectrum possible to have a number of communication taking place at the same time across the same frequency band. Bluetooth wireless communication also used the frequency hopping spread spectrum. The following block diagram illustrates the frequency hopping spread spectrum.Frequency Hopping Transmitter and Receiver [5]According to above initially block diagram binary data sent to the modulator, this can be accomplished using analogue to digital encoding mechanism such as (Frequency Shift Keying) or BPSK (Binary Phase Shift Keying).
After this process the resultant signal is narrowband signal. Then frequency hopping is established according to the hop sequence. Afterwards hopping sequence fed into the frequency synthesiser and generated the carrier frequencies.
Then next step is modulated narrowband signal mixed with carrier frequency and generated the new spread spectrum signal.In the receiver part of the system demodulated the spread spectrum signal. This can be accomplished using same sequence of PN derived frequencies and then demodulated to produce the output data.The Frequency hopping spread spectrum technique further divided into two categories which areFast Frequency Hopping Spread Spectrum.Slow Frequency Hopping Spread Spectrum.The hopping rate (This is known as "Chip rate") is higher than Bit rate, and then it is called "Fast Frequency Hopping. " The major benefit of fast frequency hopping is it can overcome the effects of narrowband interference and frequency selective fading.The hopping rate (This is known as "Chip rate") is lower than Bit rate, and then it is called "Slow Frequency Hopping". Slow Frequency hopping systems are usually have less tolerance and cheaper.
Then slow frequency hopping systems are not as invulnerable to the narrowband interference when compared with fast frequency hopping systems.In Bluetooth wireless communication frequency hopping rate is 1600 hops per second and it is much lower than the bit rate. Hence Bluetooth frequency hopping considered as slow frequency hopping.Comparison between Slow and Fast Frequency Hopping [7]
Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum (DSSS) is a most widely recognized form of spread spectrum. Direct Sequence Spread Spectrum. In Direct Sequence Spread Spectrum the data signal is multiplied by the PN (Pseudo- random Noise) Code with higher chip rate.
Afterwards finally output signal is much wider this is known as Spread Spectrum output.
Properties of Spread Spectrum Techniques
Spread Spectrum techniques are used in communication purposes because it has a attractive properties. The following illustrates the major properties of the Spread Spectrum Techniques.Low probability of InterceptLow probability of Intercept means hide a transmitted signal from detection by an unwanted party. Actually Low probability of Intercept can be realized in various ways such as transmitting at smallest possible power level and restrict the transmission time to as short an interval in time as is possible.According to figure (xxx) the peak amplitude of the spectrum after spreading has been reduced by an amount on the order of the processing gain will result in the spectrum of the signal after spreading falling below the ambient thermal noise level.
Thus there is no easy way for third party to determine that transmission is taking place [9]Protection against multipath interferenceMultipath interference can happened between transmitter and receiver in a communication link. This multipath interference can happened due to reflections and diffractions in the signal path and ultimately a final result is the various components of the signal may interfere with each other. A spread spectrum signal is resist the multipath interference if the spreading codes have good autocorrelation properties.High SecuritySpread Spectrum Systems are very secure, because with help of pseudo-codes are spread spectrum signals can be produced.
Therefore unwanted receivers cannot identify the spread spectrum signals and this capability of the spread spectrum used for the military applications and commercial applications.Immune to jammingThis is a significant benefit of spread spectrum is its ability to resist the strong interference, sometimes generated by an enemy to bock the wireless communication link. This is one reason for broad use of the concepts of spread spectrum in military communication applications.Reduce the CrosstalkCrosstalk interference is deeply attenuated in the spread spectrum systems due to processing gain of the system. The suppressed crosstalk interference can avoid using digital signal processing applications.High resolution and RangingHigh resolution and ranging another attractive property in the spread spectrum techniques.
High resolution and ranging can be used to identify the location and ranging.
Frequency Hopping Spread Spectrum technique applied into Bluetooth Wireless Communication
Bluetooth wireless communication uses frequency hopping with time division duplex ( FH /TDD) is used to guarantee the secure transmission. In this transmission time is divided into 625µs slots and the system bandwidth is divided into 79 channels and each channel has a 1MHz wide. During the each time slot the signal occupies one of the above channels according to pseudo random hopping pattern. Then 1MHz centre frequencies changes at a rate of 1600 hops per second.
Most European countries and USA uses 79 channels are used in frequency hopping but Japan , Spain countries available bandwidth is so smaller and they used 23 hop channels are used. The Bluetooth technology ensures full duplex communication by using Time Division Duplex (TDD). Frequency Hopping Spread Spectrum helps to avoid the interference from other devices that use in the same frequency band. ( need some more )Following diagram illustrated the frequency hopping spread spectrum in the 2.4 GHz ISM Band.Frequency Hopping Spread Spectrum hops the 2.4GHz ISM Band[10]
Advantages and Disadvantages of Frequency Hopping Spread Spectrum
Frequency Hopping Spread Spectrum has variety of advantages and as well as it has disadvantages also. Some of are illustrated below.
Advantages of Frequency Hopping Spread Spectrum
Frequency hopping spread spectrum has a greater advantage of frequency diversity. According to diversity technique same information signal can transmit through different frequencies. If the signal is transmit through different frequencies the probability of the information received at the receiver may correctly, but signal can transmit number of different paths from transmitter to receiver and signal may degrade because of the fading. This fading is the frequency dependent and fading also changes if the transmitter or receiver is moved.Another benefit of the frequency hopping spread spectrum is jamming resistance.
In frequency hopping signal has abundant frequency slots, if the jammer affected to the one of the frequency slots that frequency hopping system having 600 frequency slots , the only 1/600 th signal might be jammed. Hence the lower average power density combined with the pseudorandom frequency hopping make these signals hard to capture.Frequency Hopping Spread Spectrum Systems are having higher degree of security. This frequency hopping spread spectrum systems use pseudorandom hopping pattern this makes hard for other users to decrypt the message.Frequency Hopping Spread Spectrum Systems are resistant to the near far problem.Frequency Hopping spread spectrum systems are having short synchronization time.Frequency hopping spread spectrum systems usually require shorter time to synchronize with other types of systems that having the same bandwidth.
Generally FHSS systems, the receiver has a ability to synchronize with the transmitted signal within a small time.Frequency Hopping Spread spectrum systems are easy to implement.Frequency Hopping Spread Spectrum is a simple system.Another advantage of Frequency Hopping Spread Spectrum is distance effect is less.
Disadvantages of Frequency Hopping Spread Spectrum
The implementation cost of the Frequency Hopping Spread Spectrum System is very expensive.The Frequency Hopping Spread Spectrum system is susceptible to multipath interference and other type's impairments.The Frequency Hopping spread spectrum requires complex frequency synthesizers.Those type of systems require error correction. [11]Frequency Hopping Spread Spectrum system's are not useful for range and range rate measurements. [11]
Summary
Chapter 2 provides a reader to clear understanding about fundamental principals in Spread Spectrum techniques. Afterwards Types of Spread Spectrum were discussed and mainly covers the Frequency Hopping Spread spectrum technique. Because this project mainly focused on Frequency Hopping Spread spectrum.
Then properties of the spread spectrum were discussed. In the later part of this chapter discussed how frequency hopping spread spectrum technique applied into Bluetooth and pros and cons of the frequency hopping spread spectrum.The following chapter covers a Wireless Bluetooth radio channel, Multipath Propagation and characteristics of channel impairments such as Additive White Gaussian Noise and Rayleigh Fading
CHAPTER 3
Wireless Communication Channel
IntroductionIn any real communication system channel plays a significant role. The channel is defined as the transmission medium between transmitter and the receiver. In a wireless communication system, channel is affected on various obstructions and due to this various obstructions wireless communication system performance seriously degraded.
In this project mainly focused on channel impairments that affect on wireless communication channel.
Noise
Any unwanted signal that always present in a communication channel that is known as "Noise ". The noise is superimposed on a signal tends to obscure or mask the signal. Then noise occurs from various types of sources such as both manmade and natural. Noise can be categorized into four types such asThermal NoiseThermal noise is a most fundamental type of noise and it also known as Johnson noise. This thermal noise arises due to thermal motion of electrons in electronic components.
Thermal noise cannot avoid from any communication system. Thermal noise is temperature dependent and it presents all communication channels. Amount of thermal noise in a conductor can be calculated on following equation,No = kT ( W/Hz)Where No = Noise Power Density in watts per 1 Hz of Bandwidth.k = Boltzmann's ConstantT = Temperature in Kelvin's.Thermal noise characteristics can be analyzed in two ways such as statistical characteristics and spectral characteristics. These two forms of characteristics are analyzed in later of this chapter.Impulsive NoiseImpulsive noise is noise spikes that occurred in short duration and quite high amplitude.
This arises due to external electromagnetic disturbances and errors in the communication systems.CrosstalkCrosstalk means interference that causes from other communication channels. There are two types of crosstalk such as near end and far end crosstalk.Intermodulation NoiseIntermodulation noise arises signals that having different frequencies share the same transmission medium.Noise sources of the wireless communication channel can be categorized into two forms such as multiplicative and additive effects. Multiplicative noise causes when transmitted signal come across different processes when signal transmitted on the way from the transmitter antenna to receiver antenna.
The following pointed some of them;ReflectionReflection occurs when radio signal come across a surface that is large relative to the wavelength of the signal. This can be happened when signal impact on earth surfaces or buildings.AbsorptionAttenuation is arises due to absorption of radio signal energy by water vapour and molecular oxygen. This is known as Atmospheric Absorption. [12]ScatteringScattering can be happened when incoming signal strikes an object whose size in order of the wavelength of the signal or less. Scattering occurred from rough surfaces such ground.DiffractionDiffraction arises at the edge of the impenetrable body (eg: building rooftops and hilltops) that is large compared to the wavelength of the radio signal.Transmitter and receiver antenna directional characteristics. [12]Multipath effects arises due to various obstacles [12]Then multiplicative noise effects supplementary categorized into path loss and fading.
These effects explained in the later on this chapter. The additive noise affects causes due to noise produce within the receiver. The example for additive noise effects is thermal noise which generated from active and passive elements in the communication system.
Cosmic radiation, atmospheric effects are also some kind of additive noise effects.
Characteristics of Additive White Gaussian Noise
Additive White Gaussian Noise Chann
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