L 3 WIRELESS TRANSMISSION FUNDAMENTALS WEIJIA WIRELESS MOBILE

L 3: WIRELESS TRANSMISSION FUNDAMENTALS WEIJIA

WIRELESS MOBILE COMMUNICATION LAYER Application layer Transport layer Network layer Data link layer Physical layer 2 u

• You are mobile phone and mobile Internet users • You may ask: • “Why are mobile communications so expensive, and the performance so poor and no guarantee mostly? ” • Mobile phone operator Co. replies: 3 • “Data Transmission through a wireless network is totally different from transmission through a wired network. …”

COMMUNICATION CHANNELS Transmitter sender Channel Medium (channel) transmits signals from the transmitter to the receiver Receiver Wired / Wireless channels • What is communication? What is a channel? • Transmitter converts data into signals (i. e. , coding and modulation) and sends them to the receiver through a channel (medium). Receiver receives the signals and converts the signals back to the original data 4 • What are the channels in a wired/wireless network?

WIRED CHANNELS Every user accesses the network by means of a dedicated channel Switching Center or Dedicated Channels Network Access Point Channel length ↑ signal strength↓ New user is served by a new wireline circuit Access capacity is “unlimited”. Agree? NO. Not a big problem. For wired systems, we can simply install new cables to increase the capacity 5

WIRED CHANNELS Transmitter Wired Channel, e. g. copper wire Too many noises? Large signal attenuation? Data speed too low? Data speed still too low? Receiver Shielded against electromagnetic noise Use repeaters Upgrade to coaxial cable Upgrade to optical fiber 6 More easy for controlling the signals for transmission

HOW ABOUT WIRELESS NETWORKS? Sh are d. C Base Stations (Senders) han nel How to make the sharing? Receivers Wireless users access the network by means of a shared channel Access capacity is inherently limited. For wireless systems, the channel are shared by ALL users. Capacity cannot be increased by adding more channels 7 A shared transmission medium

The FIRST main reason is … A SINGLE medium SHARD by ALL users 8 What is the medium?

Wired Channels vs. Wireless Channels Dedicated Channels vs. 9 Shared Channels

10 TO SHARE

• What are the problems resulting from shared medium? • Some observations in mobile communications: • Low bandwidth, high error rate, • Error-rate is location-dependent, • frequent disconnection, … 11 • Why do we have such problems?

• How do you talk (communicate) with your friend? • Wireless communication: • If he is next to you, you may talk with him directly • Wired communication: • If he is far from you, you may use a (wired) telephone • Why do not talk directly through the air? • Too far and unclear to hear 12 • Speak louder => Create noises to others

• Sharing the SAME MEDIUM creates … • NOISE problem • Propagation in ALL directions creates the signal STRENGTH problem 13 • NOISE problem + SIGNAL STRENGTH problem => High Error Rates (Low Bandwidth), frequent disconnections, …

WHERE DO NOISES COME FROM? • Interference => received signals are unclear due to noises • Where do noises come from? • Why do we have these three types of interferences 14 • Other transmitters: Shared medium (channels) for data transmissions from different transmitters • The same transmitter: Interferences of the signals from the same transmitter (inter-symbol Interference) • The signal itself : the same signal received by a receiver may be from different directions as a result of multi-path propagation (delay spread)

ELECTROMAGNETIC (EM) SIGNAL Function of time Can also be expressed as a function of frequency 15 • Signal consists of components of different frequencies

FREQUENCIES FOR RADIO TRANSMISSION twisted pair coax cable 1 Mm 300 Hz 10 km 30 k. Hz VLF LF optical transmission 100 m 3 MHz MF HF 1 m 300 MHz VHF UHF 10 mm 30 GHz SHF 100 m 3 THz EHF infrared 1 m 300 THz visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency and wave length: = c/f 16 wave length , speed of light c 3 x 108 m/s, frequency f

TIME-DOMAIN CONCEPTS • Analog signal - signal intensity varies in a smooth fashion over time. • No breaks or discontinuities in the signal • Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level • Periodic signal - analog or digital signal pattern that repeats over time 17 • s(t +T ) = s(t ) - < t < + • where T is the period of the signal

TIME-DOMAIN CONCEPTS 18 • Aperiodic signal - analog or digital signal pattern that doesn't repeat over time • Peak amplitude (A) - maximum value or strength of the signal over time; typically measured in volts • Frequency (f )：Rate, in cycles per second, or Hertz (Hz) at which the signal repeats

TIME-DOMAIN CONCEPTS • Period (T ) - amount of time it takes for one repetition of the signal • T = 1/f • Phase ( ) - measure of the relative position in time within a single period of a signal 19 • Wavelength ( ) - distance occupied by a single cycle of the signal or, the distance between two points of corresponding phase of two consecutive cycles

SINE WAVE PARAMETERS General sine wave • s(t ) = A sin(2 ft + ) Effect of varying each of the three parameters note: 2 radians = 360° = 1 period 20 (a) A = 1, f = 1 Hz, = 0; thus T = 1 s (b) Reduced peak amplitude; A=0. 5 (c) Increased frequency; f = 2, thus T = ½ (d) Phase shift; = /4 radians (45 degrees)

21 SINE WAVE PARAMETERS

TIME VS. DISTANCE • When horizontal axis is time, graphs display the value of a signal at a given point in space as a function of time • With the horizontal axis in space, graphs display the value of a signal at a given point in time as a function of distance 22 • At a particular instant of time, the intensity of the signal varies as a function of distance from the source

FREQUENCY-DOMAIN CONCEPTS • Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it is referred to as the fundamental frequency • Spectrum - range of frequencies that a signal contains • Absolute bandwidth - width of the spectrum of a signal 23 • Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in

24 EXAMPLE

25 EXAMPLE

FREQUENCY-DOMAIN CONCEPTS • Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases 26 • The period of the total signal is equal to the period of the fundamental frequency

DATA COMMUNICATION TERMS • Data - entities that convey meaning, or information • Signals - electric or electromagnetic representations of data 27 • Transmission - communication of data by the propagation and processing of signals

EXAMPLES OF ANALOG AND DIGITAL DATA Analog • Video • Audio Digital 28 • Text • Integers

SIGNALS ØSignals: representation of data ØWhat are signals? üFunction of time and location/value (position in a curve, i. e. , amplitude) ØTypes üSine wave as special periodic signal for a carrier: s(t) = At sin(2 ft t + t) üChanging the wave to represent data which are a sequence of bits Time 1 0 1 t 1 0 1 29 üAnalog signals: continuous time and continuous values üDigital signals: discrete time and discrete values ØSignal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift

ANALOG SIGNALS • A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency • Examples of media: • Copper wire media (twisted pair and coaxial cable) • Fiber optic cable • Atmosphere or space propagation 30 • Analog signals can propagate analog and digital data

31 ANALOG SIGNALING

DIGITAL SIGNALS • A sequence of voltage pulses that may be transmitted over a copper wire medium • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation 32 • Digital signals can propagate analog and digital data

33 DIGITAL SIGNALING

HOW TO REPRESENT DATA IN EM WAVES? 34 • Assign a certain frequency band (channels) to a transmitter for sending signals • Carrier/base frequency (each channel) is a periodic sine/cosine wave • Input: a sequence of bit stream (0/1) • Output: adjust the periodic wave based on the values (0/1) of the input stream

SIGNAL REPRESENTATIONS ØMapping methods (what to be adjusted in the carrier function? ): üAmplitude (amplitude x time) • Changing amplitude with time üFrequency spectrum (frequency x amplitude)) • Amplitude of a certain frequency part of the signal vs. amplitude • Fourier transformation translates the time domain into the frequency domain üPhase state diagram (amplitude x phase) • Amplitude M and phase in polar coordinates equals to zero => in phase Q = M sin • A [V] t[s] f [Hz] Fr. Schiller 35 I= M cos

MODULATION 36 ØModulation: conversion of data into signals for sending ØBasic schemes üAmplitude Modulation (AM) üFrequency Modulation (FM) üPhase Modulation (PM) ØTwo steps üDigital modulation • Digital data are translated into analog signals • Differences in spectral efficiency, power efficiency, robustness üAnalog modulation • Shifts center frequency of baseband signals up to the radio carrier (Why? ) ØMotivations üSmaller antennas (e. g. , /4. 1 MHz wavelength => hundred meters) üFrequency division multiplexing (different senders use different frequencies) => baseband frequency different from carrier frequency üMedium characteristics (i. e. , path loss depends on frequencies)

MODULATION AND DEMODULATION 101101001 digital modulation Digital signal => Analog signals Antenna analog demodulation analog baseband signal analog modulation radio transmitter radio carrier synchronization decision digital data 101101001 radio receiver radio carrier Known 37 digital data analog baseband signal Shifting center freq. to radio carrier

DIGITAL MODULATION ØAmplitude Shift Keying (ASK) üvery simple ülow bandwidth requirements üvery susceptible to interferences 1 0 1 t ØFrequency Shift Keying (FSK) üBinary FSK: • Assign one frequency f 1 to binary 1 • Assign another frequency f 2 to binary 0 üNeeds more bandwidth 1 0 1 t 1 0 1 ØPhase Shift Keying (PSK) t 38 üI. e. , shifting 1800 each time the value changes ümore complex but robust against interference

FOURIER REPRESENTATION OF PERIODIC SIGNALS 1 1 0 0 Fr. Schiller t ideal periodic signals t real composition (based on harmonics) • Representation of the carrier (the basic function) => Fourier equation (time x freq. ) • In reality, the bandwidth of any medium is limited and the upper frequencies are ignored mostly 39 • The equation shows that an infinity number of sine and cosine waves (harmonies) are needed to construct arbitrary periodic functions

FREQUENCIES FOR RADIO TRANSMISSION ØWireless communication üUsing EM (electromagnetic) waves üA single medium for all => channels are logical instead of physical ØChannels are different types of transmission signals => frequencies ØA range of frequencies (frequency band) => one channel ØFrequency division: radio transmission can take place using many different frequency bands => closer frequency => higher interferences üThe receiver tunes in the right frequency for receiving signals üSignals in other frequencies are ignored 40 ØWhy can frequency band reduce interferences?

FREQUENCIES FOR RADIO TRANSMISSION ØHigher frequency => lower wavelength (same speed but different propagation characteristics, = c/f ) üWavelength vs. absorption/penetration/diffraction üI. e. , Compare light with sound in propagation üWhich penetration power is higher? Light or Receiver sound? 41 Objects

FREQUENCIES FOR RADIO TRANSMISSION • Low frequency (LF): are used by submarines for communication since they can penetrate water better and can follow the earth’s surface • Medium frequency (MF) and high frequency (HF): for transmission of radio stations as amplitude modulation (AM) and frequency modulation (FM) • VHF-/UHF-ranges: hundreds MHz 42 • For mobile radio and TV station broadcast • SHF and higher for directed microwave links and satellite communication.

FREQUENCIES FOR RADIO TRANSMISSION 43 • Wireless LANs use frequencies in UHF to SHF range • Some systems planned up to EHF • Limitations due to absorption by water and oxygen molecules • Weather dependent fading, signal loss caused by heavy rainfall etc. • Infrared: for direct transmission (line-ofsight), i. e. , between mobile phones, PADs etc.

FREQUENCIES FOR RADIO TRANSMISSION twisted pair coax cable 1 Mm 300 Hz 10 km 30 k. Hz VLF LF optical transmission 100 m 3 MHz MF HF 1 m 300 MHz VHF UHF 10 mm 30 GHz SHF 100 m 3 THz EHF infrared 1 m 300 THz visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency and wave length: = c/f 44 wave length , speed of light c 3 x 108 m/s, frequency f

FREQUENCIES AND REGULATIONS Values in MHz 45 ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

DATA TRANSMISSION • PROBLEMS OF DATA TRANSMISSION IN WIRELESS NETWORKS • INTERFERENCE, FADING AND MULTI -PATH PROPAGATION • FREQUENCY ASSIGNMENT • SIGNALS AND MODULATION 46 • MULTIPLEXING

SIGNAL PROPAGATION ØSignals (EM waves) propagates in free space always like light (straight line) üNo wire to determine the propagation directions üReceiver may require to be in the line-of-sight (LOS) of the sender. But radio waves normally can penetrate objects and the loss in power depends on the frequency. Higher freq. , greater interference? ØPath loss üReceiving power inversely proportional to the distance from the sender, i. e. , 1/d² in vacuum üMuch more in real environments due to other factors resulted from the environment. What are the factors? i. e. , Moisture ØHow about the situation in wired communication? 47 üIn perfect medium (i. e. , copper wire), the path loss is zero in principles (decreases in a much lower rate)

SIGNAL PROPAGATION RANGES ØTransmission range ü Detection of the signal possible ü No communication possible ØInterference range ü Signals may not be detected ü Signal adds to the background noises sender transmission distance detection interference Fr. Schiller 48 ü Communication possible ü Low (acceptable) error rate ØDetection range

REASONS FOR CHOOSING DATA AND SIGNAL COMBINATIONS • Digital data, digital signal • Equipment for encoding is less expensive than digital-to-analog equipment • Analog data, digital signal • Conversion permits use of modern digital transmission and switching equipment • Digital data, analog signal • Some transmission media will only propagate analog signals • Examples include optical fiber and satellite • Analog data, analog signal 49 • Analog data easily converted to analog signal

ANALOG TRANSMISSION • Transmit analog signals without regard to content • Attenuation limits length of transmission link • Cascaded amplifiers boost signal’s energy for longer distances but cause distortion 50 • Analog data can tolerate distortion • Introduces errors in digital data

DIGITAL TRANSMISSION • Concerned with the content of the signal • Attenuation endangers integrity of data • Digital Signal • Repeaters achieve greater distance • Repeaters recover the signal and retransmit • Retransmission device recovers the digital data from analog signal • Generates new, clean analog signal 51 • Analog signal carrying digital data

SIGNAL PROPAGATION Ø Propagation affected by environment ü Shadowing (different frequencies have different penetration power) ü Reflection at large obstacles ü Refraction depending on the density of a medium ü Scattering at small obstacles ü Diffraction at edges Ø Compare the propagation of light with sound? Fr. Schiller reflection refraction scattering diffraction 52 shadowing

MULTI-PATH PROPAGATION • Signals can take many different paths between sender and receiver due to reflection, scattering, diffraction multipath LOS pulses signal at sender signal at receiver Fr. Schiller • Time dispersion: a signal is dispersed over time (delay spread) • Distorted signal depending on the phases (i. e. , out of phase cancel each other) of the different parts 53 • Interference with “neighbor” symbols, inter symbol interference • The signal reaches a receiver directly and phase shifted

FADING EFFECT • Fading: decreases and frustrates in received signal strength • Typical indoor wireless environment (similar distance) • Signal strength fluctuates significantly. Why? • Wireless channels are difficult to be engineered (improved) • You can only improve your transmission and reception techniques • Increase the transmission power (What are the tradeoffs? ) 54

EFFECTS OF MOBILITY ØChannel characteristics change over time and location (location-dependent) ü Signal paths to a receiver change ü Different phases of signal parts ØQuick changes in the power received (short term fading) power ü Distance to sender ü Obstacles further away ØSlow changes in the average power received (long term fading) short term fading ü Increase the sending power ØThus, many factors may affect the strength of signals received => No single solution for solving the problem long term fading t Fr. Schiller 55 ØAdditional changes in

SOME IMPLICATIONS OF HIGH ERROR RATE ØBit error rate üOptical fiber: 10 -11 or 10 -12 üMobile channels: • Good quality: 10 -6 • Actual condition: 10 -2 or worse ØFor wired systems, it is assumed that the channel is basically error free ØImplication => Many communication protocols are designed with this assumption ØThese protocols do not work well in a wireless environment üe. g. TCP (why? ) ØThe design of communication protocols need to make different assumptions on the underlying networks ØHow to improve the quality of data transmission in wireless channels? 56 üLower error rate => higher bandwidth üAny suggestion? Re-design the communication architecture, signal representation process and controlling the transmissions

Receiver A Receiver B Receiver 57 A

GRAVITATIONAL WAVES （引力波） Detection on Sept 14, 2015 Thus, even waves from extreme systems like merging binary black holes die out to very small amplitude by the time they reach the Earth. Astrophysicists expect that some gravitational waves passing the Earth may be as large as h ≈ 10− 20, but generally no bigger. [2] 1. Thorne, Kip S. (1995). "Gravitational Waves". ar. Xiv: gr-qc/9506086. 58 2. David G. Blair (Ed. ) (1991). The detection of gravitational waves. Cambridge University Press.

GRAVITATIONAL WAVES （引力波） Gravitational waves are not easily detectable. When they reach the Earth, they have a small amplitude, meaning that an extremely sensitive detector is needed, and that other sources of noise can overwhelm the signal. [50] Gravitational waves are expected to have frequencies 10− 16 Hz < f < 104 Hz. [1] Even waves from extreme systems like merging binary black holes die out to very small amplitude by the time they reach the Earth. Astrophysicists expect that some gravitational waves passing the Earth may be as large as h ≈ 10− 20, but generally no bigger. [2] 1. Thorne, Kip S. (1995). "Gravitational Waves". ar. Xiv: gr-qc/9506086. 59 2. David G. Blair (Ed. ) (1991). The detection of gravitational waves. Cambridge University Press.

60 HTTPS: //EN. WIKIPEDIA. ORG/WIKI/GRAVITATIONAL_WAV E

REFERENCES ØSchiller: Ch. 2. 1, 2. 2, 2. 4, 2. 5, 2. 6. 1 -2. 6. 4 ØSchiller: Ch 3. 1, 3. 2, 3. 3, 3. 4. 1, 3. 4. 8 , 3. 4. 9, 3. 4. 10, 3. 5, 3. 6 61 ØWilliam Stallings. com/Student. Support. html Ch 2