Wireless networks - Lecture 2: Introduction to Wireless communication. The main topics covered in this chapter include: wireless transmission; digital data analog signals; noises; atmospheric noise like thunderstorms; attenuation and other impairments;...
Wireless Networks Lecture Introduction to Wireless Communication Dr Ghalib A Shah Outlines Review of previous lecture #1 Wireless Transmission Encoding/Modulation Noises Losses/Gain Summary of today’s lecture 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 Trans mis s io n in Wire le s s Do main Bas e band S ig nal ► o btaine d by c o nve rting analo g o r dig ital data into analo g o r dig ital s ig nal, bandwidth = [0, fmax) Bandpas s S ig nal ► bandlimite d s ig nal who s e minimum fre que nc y is diffe re nt fro m ze ro , bandwidth = [f1, f2) 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 ► MHz signal few 100 m-s high antenna; ► 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 ► not propagate very far 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 Encoding Each pulse in digital signal is a signal element Binary data are transmitted by encoding each data bit into signal elements 1 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 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 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 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 10 Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise 24 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 25 Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N0 kT W/Hz • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803 ´ 1023 J/K • T = temperature, in kelvins (absolute temperature) 26 Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): N kTB or, in decibelwatts N 10 log k 10 log T 10 log B 228.6 dBW 10 log T 10 log B 27 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 28 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 29 Natural Noise Naturally occurring noise has two main sources ► Atmospheric noise, such as thunderstorms, from to 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 30 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 31 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 32 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 33 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 34 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 35 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 36 Summary Wireless Transmission ► Why baseband signal can not be transmitted? ► Need bandpass signals whose minimum frequency is higher than ► 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 37 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 38 ... by encoding each data bit into signal elements 1 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... Review of previous lecture #1 Wireless Transmission Encoding/Modulation Noises Losses/Gain Summary of today’s lecture Last Lecture Review Objectives of course Course syllabus Wireless vision... frequency 16 16 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