Wireless Networks - CS718 Power Point Slides Lecture 02.ppt
- 1. 1 Wireless Networks Lecture 2 Introduction to Wireless Communication Dr. Ghalib A. Shah
- 2. 2 Outlines Review of previous lecture #1 Wireless Transmission Encoding/Modulation Noises Losses/Gain Summary of today’s lecture
- 3. 3 Last Lecture Review Objectives of course Course syllabus Wireless vision ► Driving factors • Tetherless connectivity • VLSI technology • Success of 2G systems ► Wireless Prons/Cons ► EM Signal • Time domain concept: analog, digital, periodic, aperiodic, Amplitude, frequency, period, wavelength • Frequency domain concept: fundamental frequency, spectrum, absolute bandwidth, effective bandwidth • Channel capacity: Nyquist formulation, SNR, Shannon formula ► EM Spectrum
- 4. 4 Transmission in Wireless Domain Baseband Signal ► obtained by converting analog or digital data into analog or digital signal, bandwidth = [0, fmax) Bandpass Signal ► band-limited signal whose minimum frequency is different from zero, bandwidth = [f1, f2)
- 5. 5 Wireless Transmission virtually impossible to transmit baseband signals in wireless domain. single transmission medium (air) for all users and applications. in wired networks, new wiring can be added to accommodate new applications/users – one wire for telephone, one for cable, one for LAN, etc. antenna size must correspond to signal’s wavelength ► 1 MHz signal few 100 m-s high antenna; ► 1 GHz signal few cm-s high antenna characteristics of wireless-signal propagation heavily depend on signal’s frequency low-frequency signals ‘tilt downwards’ and follow the Earth’s surface ► do not propagate very far
- 6. 6 Signal Encoding/Modulation We are concerned with transmitting digital data. Some transmission media will only propagate analog signals e.g., optical fiber and unguided media Therefore, we will discuss transmitting digital data using analog signals. The most familiar use of this transformation is transmitting digital data through the public telephone network.
- 7. 7 Encoding Each pulse in digital signal is a signal element. Binary data are transmitted by encoding each data bit into signal elements. There can be one-to-one correspondence between data elements and signal elements or one-to-multiple/multiple-to-one Data rate: the rate in bits/sec that data are transmitted Modulation rate: the rate at which signal element is changed and is expressed in baud i.e. signal elements/second. 1 0 1 0 1 0
- 8. 8 Encoding The duration or length of bit is the amount of time it takes for the transmitter to emit the bit. For data rate R, bit time is 1/R. At receiver ► the bit time must be known i.e. start and end time of bit. ► The encoding must be known i.e. high (1) and low (0) ► These tasks are performed by sampling each bit position at middle of interval and comparing the value to threshold.
- 9. 9 Carrier and Information Signals carrier signal: In radio frequency systems an analog signal is always used as the main airborne signal Information Signal: On top of this signal another signal, analog or digital, is added that carries the information Modulation: This combination of signals is called the modulation 9
- 10. 10 Modulation Modulation is how an information signal is added to a carrier signal This is the superimposing of the information onto the carrier In an RF system a modulator generates this information signal Then it is passed to the transmitter and out the antenna 10
- 11. 11 Modulation Then at the other end the signal is demodulated The way to think of this is like a letter ► The envelope is the carrier and the letter is the information ► The envelope is only needed during transmission Three types ► AM ► FM ► PM 11
- 13. 13 Types of Encoding There are three forms of Encoding ► ASK – Amplitude-Shift Keying ► FSK – Frequency-Shift Keying ► PSK – Phase-Shift Keying 13
- 14. 14 Amplitude Shift-Keying (ASK) ASK changes the height of the sine wave as time goes by The two binary values are represented by two different amplitudes of the carrier frequency. One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier 14
- 15. 15 ASK Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical fiber
- 16. 16 Binary Frequency Shift-Keying (BFSK) FSK changes the frequency of the sine wave as time goes by, without changing the height Two binary digits represented by two different frequencies near the carrier frequency 16
- 17. 17 Binary Frequency-Shift Keying (BFSK) Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio transmission Can be used at higher frequencies on LANs that use coaxial cable
- 18. 18 Multiple Frequency-Shift Keying (MFSK) More than two frequencies are used More bandwidth efficient and less susceptible to error To match data rate of input bit stream, each output signal element is held for: Ts=LT seconds • where T is the bit period (data rate = 1/T) f i = f c + (2i – 1 – M)f d • f c = the carrier frequency • f d = the difference frequency • M = number of different signal elements = 2 L • L = number of bits per signal element
- 19. 19 Multiple Frequency-Shift Keying (MFSK)
- 20. 20 Phase-Shift Keying (PSK) PSK changes the phase of successive sine waves Two-level PSK (BPSK) ► Uses two phases to represent binary digits 20 t s t f A c 2 cos t f A c 2 cos 1 binary 0 binary t f A c 2 cos t f A c 2 cos 1 binary 0 binary
- 21. 21 PSK In general when you see phase modulation schemes explained B stands for binary, which is only 2 points. Q stands for quadrature, which is 4 points and 16 and 64 represent the higher number of points in the modulation schemes 21
- 22. 22 PSK Every time the number of points is increased the speed is increased, but interference tolerance is reduced This is one of the reasons for automatic speed reduction in the face of interference Going from binary - 2 to 64 requires a really clean signal 22
- 23. 23 Noise Noise consists of all undesired radio signals, whether manmade or natural Noise makes the reception of useful information difficult The radio signal’s strength is of little use, if the noise power is greater than the received signal power This is why the signal to noise ratio is important 23
- 24. 24 Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise
- 25. 25 Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and transmission media Cannot be eliminated Function of temperature Particularly significant for satellite communication
- 26. 26 Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803 ´ 10-23 J/K • T = temperature, in kelvins (absolute temperature) W/Hz k 0 T N
- 27. 27 Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts TB N k B T N log 10 log 10 k log 10 B T log 10 log 10 dBW 6 . 228
- 28. 28 Noise Terminology Intermodulation noise – occurs if signals with different frequencies share the same medium ► Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies Crosstalk – unwanted coupling between signal paths ► Nearby twisted pairs, unwanted signals are picked by antennas Impulse noise – irregular pulses or noise spikes ► Short duration and of relatively high amplitude ► Caused by external electromagnetic disturbances, or faults and flaws in the communications system
- 29. 29 Manmade Noise Manmade noise is part of modern life It is generated almost anywhere that there is electrical activity, such as automobile ignition systems, power lines, motors, arc welders, fluorescent lights, and so on Each occurrence is small, but there are so many that together they can completely hide a weak signal that would be above the natural noise in a less populated area 29
- 30. 30 Natural Noise Naturally occurring noise has two main sources ► Atmospheric noise, such as thunderstorms, from 0 to 5 MHz ► Galactic noise, such as stars, at all higher frequencies Both of these sources generate sharp pulses of electromagnetic energy over all frequencies The pulses are propagated according to the same laws as the desirable signals being generated by the radio frequency equipment The receiving systems must accept them along with the desired signal 30
- 31. 31 Noise Remedy Increasing receiver amplification cannot improve the signal to noise ratio since both signal and noise will be amplified equally and the ratio will remain the same 31
- 32. 32 Loss All components exhibit one of two properties ► Loss ► or ► Gain If the signal coming out is smaller than the signal going in, it is loss that appears as heat Attenuators produce loss 32
- 33. 33 Attenuation Causes of loss or attenuation in RF systems and the environments through which they transmit include ► Water, regardless of how it appears or where it is found including inside connections ► When water is encountered in the air as the signal passes through, the form of the moisture matters ► At frequencies above 10 GHz attenuation from rain becomes significant ► When the raindrop’s size matches the wavelength attenuation occurs 33
- 34. 34 Attenuation ► Examples of the affect outside include • Rain causes about .08 dB of loss per mile for 2.4 GHz and 5.8 GHz • Fog causes about .03 dB per mile for 2.4 GHz • For 5.8 GHz the loss is about .11 dB per mile • Ice changes the effective design of an antenna, therefore changing its performance 34
- 35. 35 Other Impairments Atmospheric absorption – water vapor and oxygen contribute to attenuation Multipath – obstacles reflect signals so that multiple copies with varying delays are received Refraction – bending of radio waves as they propagate through the atmosphere
- 36. 36 Gain If the signal gets larger before it exits the device, it is gain RF amplifiers produce gain Gain is an active process in most cases, in other words it requires a power source Gain can also be the combination of signals from different directions appearing together, such as the main signal and a reflected signal However, the total gain cannot exceed the original level transmitted from the antenna in such a case 36
- 37. 37 Summary Wireless Transmission ► Why baseband signal can not be transmitted? ► Need bandpass signals whose minimum frequency is higher than 0 ► Modulator produces bandpass by superomposing basband signal over higher frequency signals • AM, FM, PM Digital data analog signals ► Some transmission media like optical fibers and unguided propagate only analog signals ► For example public telephone network ► Requires data encoding • ASK, FSK, PSK
- 38. 38 Summary Noises ► Thermal/white noise ► Intermodulation noise ► Crosstalk ► Impulse noise ► Natural noise • Atmospheric noise like thunderstorms • Galatic noise such as stars ► Manmade noise • Ignition systems, power lines, motors arc welders, flourscent lightd etc Attenuation and other impairments