DC@UNIT 5 ppt.ppt
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This document discusses a digital communication course at Matrusri Engineering College. It outlines the course objectives, which include familiarizing students with digital communication systems, information theory, and channel coding techniques. It also lists the course outcomes, such as classifying modulation techniques and distinguishing error control codes. The document provides information on topics that will be covered, including spread spectrum communication, pseudorandom noise sequences, direct sequence spread spectrum, and frequency hopping spread spectrum.
DC@UNIT 5 ppt.ppt
- 1. MATRUSRI ENGINEERING COLLEGE DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING SUBJECT NAME:DIGITAL COMMUNICATION(PC601EC) FACULTY NAME: Mr.A.ABHISHEK REDDY MATRUSRI ENGINEERING COLLEGE
- 2. DIGITAL COMMUNICATION (PC601EC) COURSE OBJECTIVES: 1. Familiarize the students with elements of digital communication system and waveform coding techniques like PCM, DPCM, DM and ADM. 2. Introduce the concepts of information theory and source coding 3. Familiarize the students with channel coding techniques such as LBC, BCC and convolution codes 4. Introduce the concepts of baseband digital data transmission and analyze the error performance of different digital carrier modulation schemes like ASK, FSK, PSK etc. 5. Familiarize the students with the concepts of spread spectrum communication with emphasis on DSSS and FHS. MATRUSRI ENGINEERING COLLEGE
- 3. DIGITAL COMMUNICATION(PC601EC) COURSE OUTCOMES: 1. Classify the different types of digital modulation techniques PCM, DPCM, DM and ADM and compare their performance by SNR. 2. Illustrate the classification of channels and Source coding methods. 3. Distinguish different types of Error control codes along with their encoding/decoding algorithms. 4. Examine the Performance of different Digital Carrier Modulation schemes of Coherent and Non-coherent type based on Probability of error. 5. Generation of PN sequence using Spread Spectrum and characterize the Acquisition Schemes for Receivers to track the signals. MATRUSRI ENGINEERING COLLEGE
- 4. INTRODUCTION: Understand the concept of spread spectrum communication and Generation and characteristics of PN sequences. Direct sequence spread spectrum and their applications. Frequency hopping spread spectrum systems and their applications. Acquisition and tracking of DSSS and FHSS signals UNIT-V OUTCOMES: Generation of PN sequence using Spread Spectrum and characterize the Acquisition Schemes for Receivers to track the signals. MATRUSRI ENGINEERING COLLEGE
- 5. CONTENTS: ADVANTAGES OF SPREAD SPECTRUM. GENERATION AND CHARACTERISTICS OF PN SEQUENCES. OUTCOMES: Design a PN sequence and understand the properties. MODULE-I MATRUSRI ENGINEERING COLLEGE
- 6. A MAJOR CONCERN IN THE STUDY OF DIGITAL COMMUNICATIONS IS TO PROVIDE EFFICIENT UTILIZATION OF BANDWIDTH AND POWER. HOWEVER, THERE ARE SITUATIONS WHERE IT IS REQUIRED TO SACRIFICE EFFICIENT USE OF THESE TWO RESOURCES TO MEET CERTAIN DESIGN OBJECTIVE. FOR EXAMPLE THE SYSTEM MAY BE REQUIRED TO PROVIDE A FORM OF SECURE COMMUNICATION IN A HOSTILE ENVIRONMENT SUCH THAT THE TRANSMITTED SIGNAL IS NOT EASILY DETECTED BY UNWANTED LISTENERS THIS REQUIREMENT IS CATERED TO BY A CLASS OF SIGNALING TECHNIQUES KNOWN AS SPREAD-SPECTRUM (SS) MODULATION. What is Spread Spectrum Communication? MATRUSRI ENGINEERING COLLEGE
- 7. SPREAD SPECTRUM – TECHNIQUE WHEREBY AN ALREADY MODULATED SIGNAL IS MODULATED A SECOND TIME, PRODUCING A WAVEFORM THAT INTERFERES IN A BARELY NOTICEABLE WAY WITH ANY OTHER SIGNAL OPERATING IN THE SAME FREQUENCY BAND – INTERFERING SIGNALS ARE TRANSPARENT TO SPREAD SPECTRUM SIGNALS –SPREAD SPECTRUM SIGNALS ARE TRANSPARENT TO INTERFERING SIGNALS THE MAIN ADVANTAGE OF A SS SIGNAL IS ITS ABILITY TO REJECT UNINTENTIONAL INTERFERENCE BY ANOTHER USER SIMULTANEOUSLY SHARING THE CHANNEL, OR TO REJECT INTENTIONAL INTERFERENCE BY A HOSTILE TRANSMITTER ATTEMPTING TO JAM THE TRANSMISSION. SS is a means of transmission in which the data sequence occupies bandwidth in excess of the minimum bandwidth necessary to send it. The spectrum spreading is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used in the receiver (operating in synchronism with the transmitter) to despread the received signal so that the original data sequence may be recovered MATRUSRI ENGINEERING COLLEGE
- 10. BLOCK DIAGRAM OF SS MATRUSRI ENGINEERING COLLEGE
- 12. A pseudonoise (PN) sequence/MAXIMUM LENGTH SEQUENCE(ML) may be defined as a coded sequence of 1’s and 0’s with certain autocorrelation properties. The PN sequence is a deterministic, periodic signal that is known to both the transmitter and receiver. Even though the signal is deterministic it appears to have the statistical properties of sampled white noise. Hence, it appears to be a truly random signal, to an unauthorised listener. PN SEQUENCE(PSEUDO NOISE/PSEUDO RANDOM) MATRUSRI ENGINEERING COLLEGE
- 13. Basic Block diagram: PN SEQUENCE(PSEUDO NOISE/PSEUDO RANDOM) MATRUSRI ENGINEERING COLLEGE
- 14. A 15-stage PN generator using shift register PN SEQUENCE(PSEUDO NOISE/PSEUDO RANDOM) MATRUSRI ENGINEERING COLLEGE
- 16. Properties of PN sequence MATRUSRI ENGINEERING COLLEGE
- 17. 1. What is the need of spread-spectrum communication? 2. What is a PN sequence? 3. What are the properties of PN sequence? Answer! MATRUSRI ENGINEERING COLLEGE
- 18. CONTENTS: Direct sequence spread spectrum and their applications OUTCOMES: Learn DSSS spread Spectrum system. MODULE-II MATRUSRI ENGINEERING COLLEGE
- 19. The most important advantage of spread spectrum modulation is that it provides protection against externally generated interfering signals such as jamming signals. The direct sequence spread spectrum (DS-SS) technique can be used in practice for such interference suppression. For this transmission of information signal is carried over a band pass channel (e.G. Satellite channel). For such an application, the coherent binary phase shift keying (BPSK) is used in the communication system. In the direct sequence spread spectrum (DS-SS) systems, the use of a PN sequence to modulate a phase shift keyed signal achieves instantaneous spreading of the transmission bandwidth. Direct Sequence Spread Spectrum MATRUSRI ENGINEERING COLLEGE
- 22. MODEL FOR DS SPREAD-SPECTRUM BPSK COMMUNICATION SYSTEM: MATRUSRI ENGINEERING COLLEGE
- 24. PROCESSING GAIN=GP= (FC/FB), The ratio of the chip frequency to the bit frequency Jamming margin may be defined as the ratio of average interference power J and the average signal power Ps MATRUSRI ENGINEERING COLLEGE 10 10 (indB) 10log 10log ( / ) p b j JM G E N b p c b b s T G T J E E P 1 2 b e c E P erfc JT
- 25. RANGING USING DS-SPREAD SPECTRUM SIGNALS. MATRUSRI ENGINEERING COLLEGE
- 26. DSSS-CODE DIVISION MULTIPLE ACCESSES (CDMA) MATRUSRI ENGINEERING COLLEGE
- 27. DSSS-CODE DIVISION MULTIPLE ACCESSES (CDMA) MATRUSRI ENGINEERING COLLEGE
- 28. DSSS-CODE DIVISION MULTIPLE ACCESSES (CDMA) MATRUSRI ENGINEERING COLLEGE
- 29. ADVANTAGES OF DS-SS SYSTEM 1. This system combats the intentional interference (jamming) most effectively. 2. This system has a very high degree of discrimination against the multi path signals. Therefore, the interference caused by the multipath reception is minimized successfully. 3. The performance of DS-SS system in the presence of noise is superior to other systems. Disadvantages of DS-SS system 1. The PN code generator output must have a high rate. The length of such a sequence needs to be long enough to make the sequence truly random. 2. With the serial search system, the acquisition time is too large. This makes the DS-SS system be slow. 3. Synchronization is affected by the variable distance between the transmitter and receiver. 4. The DS-SS signal is not very effective against broadband interference. Major applications of DS-SS system 1. Providing immunity against a jamming signal – anti-jamming application. 2. Low detectability signal transmission – the signal is purposely transmitted at a very low power level. Hence the signal has a low probability of being intercepted (LPI) and it is called an LPI signal. 3. Accommodating a number of simultaneous signal transmissions on the same channel, i.E. Code division multiple access (CDMA) or spread spectrum multiple access (SSMA). MATRUSRI ENGINEERING COLLEGE
- 30. 1. What is meant by DSSS? 2. Define the terms processing gain and jamming margin? 3. Advantages and disadvantages of DSSS? 4. Applications of DSSS? Answer! MATRUSRI ENGINEERING COLLEGE
- 31. CONTENTS: Frequency Hopping spread spectrum and their applications OUTCOMES: Learn FHSS spread Spectrum system. MODULE-III MATRUSRI ENGINEERING COLLEGE
- 32. The frequency hopping spread spectrum (FH-SS) system is an alternative method. In FH-SS, the spectrum of the transmitted signal is spread sequentially by randomly hopping the data modulated carrier from one frequency to the next. Hence, the type of spread spectrum in which the carrier hops randomly from one frequency to another is called frequency-hopped spread spectrum (FH-SS) system. In a FH-SS communication system the available channel bandwidth is subdivided into a large number of contiguous frequency slots. In any signaling interval, the transmitted signal occupies one or more of the available frequency slots. The selection of the frequency slot(s) in each signaling interval is made pseudo randomly according to the output from a PN generator. The figure illustrates a particular FH pattern in the time-frequency plane. FHSS MATRUSRI ENGINEERING COLLEGE
- 34. Using the M-ary FSK system, M symbols can be transmitted, where M=2K. Here k is the number of bits of the input binary data that form one symbol. The M-ary FSK modulator will assign a distinct frequency for each of these M symbols. The synthesizer output at a given instant of time is the frequency hop. The output bits of the PN generator change randomly. Hence the synthesizer output frequency will also change randomly. Each frequency hop is mixed with the MFSK signal to produce the transmitted signal. If the number of successive bits at the output of PN generator is n, then the total number of frequency hops will be 2n. The total bandwidth of the transmitted FH-MFSK signal is equal to the sum of all the frequency hops. Therefore, the bandwidth of the transmitted FH-MFSK signal is very large of the order of few GHz. MATRUSRI ENGINEERING COLLEGE
- 35. The received signal is applied as input to the Mixer. The other input to the mixer is obtained from the digital frequency synthesizer. The frequency synthesizer is driven by a PN code generator. This generator is synchronized with the PN code generator at the transmitter. Therefore, the frequency hops produced at the synthesizer output will be identical to those at the transmitter. The mixer produces two outputs of the sum frequency and the difference frequency. The band pass filter selects only the difference frequency, which is the MFSK signal. Thus the mixer removes the frequency hopping. The MFSK signal is then applied to a non-coherent MFSK detector. MATRUSRI ENGINEERING COLLEGE
- 36. A bank of M, non-coherent matched filters is used for non-coherent MFSK detection. Each matched filter is matched to one of the tones of the MFSK signal. An estimate of the original symbol transmitted is obtained by selecting the largest filter output. MATRUSRI ENGINEERING COLLEGE
- 39. If the symbol rate Rs of MFSK is an integer multiple of hop rate Rh i.e several symbols are transmitted in each frequency hop. MATRUSRI ENGINEERING COLLEGE
- 40. If the hop rate Rh is an integer multiple of symbol rate Rs of MFSK i.e carrier frequency will hop several times during the transmission of one symbol. MATRUSRI ENGINEERING COLLEGE
- 41. Advantages of FH-SS system: The processing gain PG is higher than that of DS-SS system. Synchronization is not greatly dependent on the distance. The serial search system with FH-SS needs shorter time for acquisition. Disadvantages of FH-SS system: The bandwidth of FH-SS system is too large (in GHz). Complex and expensive digital frequency synthesizers are required. Applications of FHSS system: CDMA systems based on FH spread spectrum signals are particularly attractive for mobile communication. Wireless local area networks (WLAN) standard for Wi-Fi. Wireless Personal area network (WPAN) standard for Bluetooth. MATRUSRI ENGINEERING COLLEGE
- 42. 1. Why FHSS is advantages over DSSS? 2. Differences between slow-hop and Fast-hop FHSS? 3. Advantages and disadvantages of FHSS? 4. Applications of FHSS? Answer! MATRUSRI ENGINEERING COLLEGE
- 43. CONTENTS: TRACKING AND ACQUISITION OUTCOMES: LEARN THE IMPORTANCE OF SYNCHRONIZATION TECHNIQUES. MODULE-IV MATRUSRI ENGINEERING COLLEGE
- 44. ACQUISITION: The first step, called acquisition, consists of bringing the two spreading signals into coarse alignment with one another. TRACKING: Once the received spread spectrum signal has been acquired, the second step, called tracking, takes over for fine alignment. BOTH ACQUISITION AND TRACKING MAKE USE OF THE FEEDBACK LOOP. ACQUISITION: SYNCHRONIZATION TECHNIQUES MATRUSRI ENGINEERING COLLEGE
- 45. MATRUSRI ENGINEERING COLLEGE Serial search scheme for Direct Sequence spread spectrum systems. There is always an initial timing uncertainty between the receiver and the transmitter. Let us suppose that the transmitter has N chips and the chip duration isTc. If initial synchronization is to take place in the presence of additive noise and other interference, it is necessary to dwell for Td=NTc in order to test synchronism at each time instant. We search over the time uncertainty interval in(coarse) time stepsof ½Tc.
- 46. serial search scheme for frequency hopping spread spectrum systems. Here the non-coherent matched filter consists of a mixer followed by a bandpass filter (BPF) and a square law envelope detector. The PN code generator controls the frequency hopper. Acquisition is accomplished when the local hopping is aligned with that of the received signal. MATRUSRI ENGINEERING COLLEGE
- 47. The received DS spread spectrum signal is applied simultaneously to two multipliers. One of the multipliers is fed with PN code delayed by , a fraction of thechip interval. The other multiplier is fed with the same PN code advanced by . Theoutput from each multiplier is fed to a BPF centred on f0. MATRUSRI ENGINEERING COLLEGE The commonly used tracking loop for a direct sequence spectrum signal is the Delay-locked loop (DLL)
- 48. MATRUSRI ENGINEERING COLLEGE A typical tracking technique for FH spread spectrum signals is illustrated in Although initial acquisition has been achieved, there is a small timing error between the received signal and the receiver clock. The BPF is tuned to a single intermediate frequency and its bandwidth is of the order of 1/Tc .Its output is envelope detected and then multiplied by the clock signal to produce a three-level signal. This drives the loop filter.
- 50. APPLICATIONS OF SS: Combating or suppressing the detrimental effects of interference due to jamming (Intentional interference). It can be used in military applications also. Accommodating multiple users to transmit messages simultaneously over the same channel bandwidth. This type of digital communication in which each user (transmitter-receiver pair) has a distinct PN code for transmitting over a common channel bandwidth is called as Code Division Multiple Access (CDMA) or Spread Spectrum Multiple Access (SSMA). This technique is popularly used in digital cellular communications. Reducing the unintentional interference arising from other users of the channel. Suppressing self-interference due to multipath propagation. Hiding a signal by transmitting it at low power and, thus, making it difficult for an unintended listener to detect in the presence of background noise. It is also called a Low Probability of Intercept (LPI) signal. Achieving message privacy in the presence of other listeners. Obtaining accurate range (time delay) and range rate (velocity)measurements in radar and navigation. MATRUSRI ENGINEERING COLLEGE
- 51. 1. What is meant by tracking and acquisition? 2. What are the applications of spread spectrum modulation technique? 3. working principle of “camp-and-wait” circuit used in FHSS? Working principle of “delay locked loop “ circuit used in DSSS? Answer! MATRUSRI ENGINEERING COLLEGE