Lecture 3: Wirelss Data Transmission

Question 1

What is the general application of Wireless Communication ?

Answer 1

• Any time you listen to the radio, talk to your friends over the the celluar phone, watch broadcast TV, or communicate on a Wi-Fi network, you are using eletromegnatic waves (signals)
• Electromagnatic waves carry information without wires and are used in most major wireless communications today.
• So the medium is really the atmosphere (free space) for wireless communications.

Question 2

What are Signals ?

Answer 2

Electric energy can be made to vary over time. This varince in electrical energy is known as a signal.

To be transmitted, information data must be transformed to eletromagnetic signals.

Question 3

Electromagnetic Radiation

Answer 3

• Wireless divices communicate through the transmission and reception of electromegnetic waves through space at speed of light:
  
  • 186,000 miles per second
• Like a wave in the ocean or a ripple effect in a pond, the same concept of disturbance-based wave forms is also for the electromagnetic wave.
• Antennas are used to transmit receive an electromagnetic wave
• The sending of the electromagnetic wave out through the antenna is known as radiation.

Question 4

RF Transmission

Answer 4

Wireless communications, specifically Wi-Fi, RFID, Bluetooth, WiMax, cellular network, and satellite system, use radioi frequency (RF) transmission

Question 5

RF transmitting (Cont.)

Answer 5

• All types of eletromagnetic radiation have waveforms, including: radio waves, microwaves, infrared light, visible light, ultravilent light, and gamma rays
• A transmitted RF wireless signal (including radio wave and microwave) begins as an eletrical current ona wire that represents voice, music, images, or data
• The eletrical currtent is created by the back and forth movement of eletrons in the wire in an alternating and contious pattern, which also creates an eletromagnetic field around the wire.

Question 6

RF Transmitting (Cont. Part 2)

Answer 6

• AN antenna is a length of copper wire, or similar material, with one end free and the other end connected to a receiver or transmitter.
• When transmittign, the electrimagnetic field created by the transmitter along the wire with the current into an antenna
• The sending antenna is designed to radiate the eleromagnetic field
• A receiving antenna converts the sensed eletromagnetic field into a current in a wire

Question 7

Data Transmission

Answer 7

• In order for a computer network or a data/voice communication system to transmit data, the data must first be converted (encoded) into the appropriate carrier signals
• Computer networks and data/voice communicatrionsystems thus transmit data using (carrier) signals.
• Data and signals can be analog or digital
• Wireless communications use analog signals to carry analog or digital data

Question 8

Analog and Digital Data

Answer 8

Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states.

Analog data are continuous and take continuous values

Digital data have discrete states and take discrete values.

Question 9

Analog/Digital – Data/Signal

Answer 9

• Data
  
  • Analog: Continuous value data (sound, image, light, temperature)
  • Digital: Discrete value (text, intergers, symbols, MP3 audio, JPEG images AVI video)
• Signal
  
  • Analog: Continuously varying electromagnetic wave
  • Digital: Series of voltage pulses (square wave)

Question 10

Digital Signal

Answer 10

• Consists of discrete or separate pulses
• Has numerous starts and stops throughout the signal stream
• Signal intensity maintains a constant level for some period of time and then changes to another constant level.

Question 11

PC Input Concerted to Digital Signal

Answer 11

• Computers operate using digital signal
• Analog signal must be converted into a digital format before it can be stored and processed or interpreted by a computer.

Question 12

Wireless Communication Carrier Signal

Answer 12

• In data communication, we commonly use periodic analog signals and non-periodic digital signals.
• Digital signals are used to communicat and relay information within computer networks. but are not used as carriers of information in wireless networks.
• Wireless communications commonly use periodic analog signals to carry analog or digital data.

Question 13

Periodic Signal

Answer 13

• The simplest sort of signal is a periodic signal.
• Periodic signal: analog (top) or digital (bottom) signal pattern that repeats over time.
• T is the period. f is the repeating frequency.

Question 14

Periodic Analog Signal Components

Answer 14

• In wireless communication, we commonly use periodic analog signals, since wave, as data carriers.

Question 15

Amplitude (A)

Answer 15

• The height of the wave above or below a given reference point, typically measured in volts.
  
  • In sound it relates to loundness
  • In radio wave it relates to strength

Question 16

Frequency ( f )

Answer 16

• Frequency is the rate at which a radio circut creates the waves
• It is the number of times a signal makes a complete cycle within a second, usually measured in Hertz (Hz)
• For example, the average human voice has a frequency range of roughly 300 Hz to 3400 Hz

Question 17

Frequency (cont.)

Answer 17

Frequency si the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency.

Question 18

Period and Frequency

Answer 18

• Period ( T ) is the amount of time it takes a wave to complete one cycle,
  
  • Frequency and period are the inverse of each other. They are just one characteristic defined in two ways:

Question 19

Phase

Answer 19

• The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero.
• Measured in number of degrees the wave is either ahead of (lead) or behind (lag) the reference wave.
• A change in phase can be any number of angles between 0 and 360 degress in a cycle.
• Phase changes often occur on common angles, such as 45, 90, 135, etc.

Question 20

Wavelength

Answer 20

The wavelength of a radio wave is measured as the distance between waves

Question 21

Wavelength and Frequency

Answer 21

• The lower the frequency, the fewer waves over a given time, resulting in a greater distance between waves (longer wavelength).
• The higher the frequency, the short the distance between waves ( shorter wavelength)

Question 22

Wavelength and Frequency ( Cont. )

Answer 22

• Wavelength x frequency = speed of wave (c)
• Electromagnetic waves travel at the speed of light in a vaccum, 3 x 10⁸ meter per second.
• Atmospheric conditons and the frequency of a wave affect its speed, but only by a small amount
  
  • For radio waves or light waves, the speed of the wave is essentially the speed of light.

Question 23

Various Propagatin Speeds on the Medium

Answer 23

• When electromagnetic waves enter a medium from the air, their propagation speed is reduced.
  
  • Their wavlength is reduced by a factor, but the frequency of the wave is unchanged.

Question 24

Wavelength and Antenna Size

Answer 24

• Wavelength determines the size of antenna
  
  • ​The size of an antenna is proportional to the wavelength of the signal it is designed to transmit or receive.
• Full-wave antenna
  
  • ​Antenna transmits and receives a signal most effciently at a specific frequency when it is as long as the wavelength of the signal
  • In most cases, this is not practical
• ​For practical reasons, antenna sizes are more commonly designed as exact fraction of the wavelength:
  
  • ​Half-wave antennas, quarter-wave antennas, or eigth-wave antennas
  • The most common length of an antenna is usually about 1/2 of the wavelength

Question 25

Frequency and Antenna Size

Answer 25

• In fact, for any electromagnetic wave, its frequency times its wavelength should equal the speed of light.
• Hence, antenna size is inversely proportional to the frequency it is designed to transmit or receive.
  
  • ​Lower frequency signals require larger antennas.
  • A shorter antenna is used for higher frequencies.

Question 26

Modulation

Answer 26

• The information data being carried by the RF waves (radio waves, microwaves) mignt be analog (e.g., AM, FM radio) or digital (e.g., wifi and cellular systems), but it is encoded (carrier) on analog (carrier) signals in a wireless connection.
• An analog carrier signal can carry data represented by analog or digital techques.
• This carrying of an information data on a carrier signal is known as modulation.

Question 27

Bandwidth of Signal

Answer 27

• The modulated wave (electromagnetic signal) for transmission is a composite signal, a signal made of many periodic analog, signals (sine wave) at different amplitudes, frequencies, and phrases.
• The difference between the highest and the lowest frequencies contained in the signal is known as the bandwidth.

Question 28

AM Radio Channel Bandwidth

Answer 28

• An example of a non-periodic composite signal is the signal propagated by an AM radion station.
• In the United States, each AM radio station is assigned a 10-kHz bandwidth.
• The total bandwidth dedicated to AM radio ranges from 530 to 1700 kHz, or 0.53 to 1.7 MHz.

Question 29

FM Radio Channel Bandwidth

Answer 29

• Another example of non-periodic composite signal is the signal propagated by an FM radio station.
• In the United States, each FM radio station is assigned a 200-kHz bandwidth.
• The total bandwidth dedicated to FM radio ranges from 88 to 108 MHz.
• Note: FM radio channel has much higher bandwidth than AM radio.

Question 30

Shannon Equation

Answer 30

• Specifies the connection between channel bandwidth in Hertz and the channel’s maximum digital transmission speed (data rate) in bits per second (bps)
  
  • ​C = B [Log₂(1+S?N)]
    
    • ​C = Maximum possible transmission speed in the channel (bps)
    • B = Bandwidth (Hz)
    • S/N = Signal-to-Noise Ratio

Question 31

Relationship between Channel Data Rate and Channel Bandwidth

Answer 31

• Note that doubling the bandwidth will double the maximum possible data transmission speed.
  
  • ​Increasing the bandwidth by X increases the maximum possible speed by X
• ​Wide bandwidth of the channel is the key to fast tranmission.
  
  • ​Higher data rate of the transmitted signal requires more channel bandwidth.
  • The greater the bandwidth, the higher the information-carrying capacity (data rate)
• ​Increasing S/N helps slightly, but usually cannot be done to any significant extent.

Question 32

Electromagnetic Spectrum

Answer 32

• Any electromagnetic signal can be shown to consist of a collection of periodic analog signals ( sine waves) at different amplitudes, frequencies, and phases.
• Spectrum- the range of frequencies that a signal spans from minimum to maximum.
  
  • ​The spectrum of the voice signal would thus be 300 - 340 Hz
• ​Electromagnetic Spectrum
  
  • ​the frequency range of electromagnetic radiation with wavelengths from the thousands of kilometers down to fractions of the size of an atom.

Question 33

RF Bands Allocation

Answer 33

• Within the U.S., the RF spectrum is divied by the FCC into a series of bands, which are allocated to specfic usages or purposes.
• The Federal Communications Commission (FFC) is responsible for the allocation, licensing, registration, and mangement of the RF bands not used for government and military purposes.

Question 34

RF Bands Allocation (Cont.)

Answer 34

• International spectrum allocations are established by the ITU (International Telecommunication Union)
• License exempt spectrum
  
  • ​Unregulated bands
    
    • ​Radio spectra avaiable without charge to any users without a licence
  • ​Devices from different vendors may attempt to use the same frequency (disadvantage)

Question 35

Radio Frequency Spectrum

Answer 35

• Wireless data signals travel on electromagnetic waves
• Three basic types of waves
  
  • ​Infrared light
  • Radio waves
  • Microwaves
• ​Radio Frequency Spectrum
  
  • ​Entire range of all radio frequencies that exist
  • Range extends from 3 KHz to over 40 GHz
  • Spectrum is divided into 450 different sections (bands)

Question 36

Radio Frequency Spectrum (cont.)

Answer 36

• Radio Frequencies of other common devices include:
  
  • ​Garage door openers, alarm systems: 40 MHz
  • Baby monitors: 49 MHz
  • Radio-controlled airplanes: 72 MHz
  • Radio-controlled car: 75 MHz
  • Wildlife tracking collar: 215 MHz - 220 MHz
  • Global positioning system (GPS): 1.227 GHz and 1.575 GHz

Question 37

Infrared

Answer 37

• Frequency range: 300 GHz to 400 THz
• Wide bandwidth allows for transmitting digital data with a very high data rate
• Used for short-range communication a closed area using line-of-sight propagation or diffused transmission such as communication between devices like TV/DVD player remote control, keyboards, mice, PCs, and printers, etc

Question 38

Infrared Light

Answer 38

• It is easy to transmit information with light
  
  • ​Because computers and data communiction equipment use binary code
  • A 1 in binary code result in a light quickly flashing on
• Infrared light​
  
  • ​Adjacent to vidible light (although invisible)
  • A much better medium for data transmission
  • Less susceptible to interference

​

Question 39

Infrared Light Transmission

Answer 39

• Infrared wireless systems require:
  
  • ​Emitter that transmitt a signal (LED)
  • Detector that receives the signal
• ​Infrared wireless systems send data by the intensity of the light wave
  
  • ​Detector senses the higher intensity pulse of light and produces a proportional electircal current
• ​Infrared wireless transmission types
  
  • ​Directed transmission (called line-of-sight or LOS)
  • Diffused transmission

Question 40

Infrared Communicartion Advantages and Disadvantages

Answer 40

• Advantages
  
  • It does not interfere with other types of communication signals
  • Infrared light does not penetrate walls
    
    • ​Signals are kept inside a room
• ​​Limitations
  
  • ​Lack of mobility
  • Range of coverage
    
    • ​Can cover a range of only 50 feet ( 15 meters)
    • Diffused infrared can only be used indoors
  • ​Speed of transmission

Question 41

Infrared Light vs. Radio Waves or Microwaves

Answer 41

• Some specialized wireless local area networks are based on the infared method
  
  • ​Used in situations where radio signals would interfere with other equipment
• ​Radio waves and microwaves
  
  • ​Do not have the distance limitations of light or infared
  • Most common and effective means of wireless communication today

Question 42

Radio Waves and Microwaves

Answer 42

• Radio waves and Microwaves are electromagnetic waves that propagated (transmitted or broadcasted) through the air and received by using antenna
• Energy travels through space or air in electromagnetic waves
• Intensity is a measure of how much energy is propagated along the wave
  
  • ​Expressed in volts or watts
• ​Receivers are tuned to receive a particular range of frequencies or a specific frequency

Question 43

Advantages of Radio Waves and Microwaves

Answer 43

• Can travel great distance
• Can penetrate nonmetallic objects
• Invisible

Question 44

Radio Waves

Answer 44

• Frequency range: 30 MHz to 1 GHz
• Suitable for omni-directional applications
• Used for multicast communications, such as AM and FM radio. television, cordless phone, and paging systems

Question 45

Microwaves

Answer 45

• Frequency range: 1 GHz to 300 GHz
• Relatively wide frequency band, almost 299 GHz. Therefore wider subbands can be assigned, and a high data rate is possible.
• However, very high-frequency microwaves cannot penerate walls: a disadvantage if receiver are inside buildings.
• Used for unicast communication such as bluetooth, cellular telephones, satellite networks, and wireless LANs, etc.

Question 46

Radio Frequency Behavior

Answer 46

• RF signal behavior has an impact upon the speed of a transmission and distance achieved between two devices
  
  • ​Classified as propagation behaviors

Question 47

Propagation Behaviors

Answer 47

• Common misconception: an RF signal goes out from an antenna is a single signal that takes a direct path to a receiver
  
  • ​Incorrect in two ways:
    
    • ​There is not just one RF signal - multiple copies may reach the receiver (known as multipath)
    • Signal may “bounce” off of walls and other objects
      
      • ​Wave propagation: the way in which the signal travels
• ​​​Several different behaviors the wave will take:
  
  • ​absorption, reflection, scattering, refraction, and diffraction

Question 48

Absorption

Answer 48

Materials that will “absorb” the RF signal: Concrete, wood, and asphalt

Question 49

Reflection

Answer 49

• Reflection: when a signal is bounced back
  
  • ​Opposite of absorbtion
  • Caused by objects that are very large and smooth (walls. buildings, and the surface of the earth)
  • Object made of metal will reflect a signal

Question 50

Scattering

Answer 50

• Caused by small objects such as: foliage, rocksm and sand
  
  • ​Can also occur when RF signal comes in contract with airbone substances such as rain or hevy dust

Question 51

Refraction

Answer 51

• When an RF signal moves from one medium to another of a different density the signal bends instead of traveling in a straight line
  
  • ​Bending behavior is known as refraction

Question 52

Diffraction

Answer 52

• Diffraction occurs when an object with rough surfaces is in the path of the RF signal and causes it to bend

Question 53

Impact of Behaviors

Answer 53

• Wave propagation (absorption, reflection, scattering, refraction, and diffraction) and multipath can all have an impact upon the RF signal reaching its destination
• Attenuation: loss of signal strength
• Two phenomena that can result in loss of an RF signal:
  
  • ​Free Space Path Loss (FPSL): natural loss of signal strength through space
    
    • ​Not a result of absorption, reflection, scattering, or diffraction
  • ​Delay Spread: The difference in time of multipath signals that reach the receiver

Question 54

Signal Strength

Answer 54

• Strength of the signal in a radio system
  
  • ​Must be sufficient for the signal to reach its destination
    
    • ​With enough amplitude to be picked up by the antenna
    • And for the information to be extracted from it
• ​​Electromagnetic interference (EMI)
  
  • ​One factor that affects radio signal strength
  • also called noise
• ​Noise: unwanted interference that impacts the RF signal
  
  • ​Compares signal strength with background noise

Question 55

Signal-to-Noise Ratio (SNR)

Answer 55

• Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission/
• Typically mesured at a receiver.
• A high SNR means a high-quality signal, low number of required intermediate repeaters.
• SNP sets upper bound on achievable data rate.

Question 56

Attenuation and Multipath Distortion

Answer 56

• When signal strength falls near the level of noise
  
  • ​Interference can take place
• To reduce the interference of noise
  
  • ​Boost the strength of the signal
  • Use of filters when receiving the signal
• ​Attenuation
  
  • ​A loss of signal strength
• Mutipath distortion
  
  • As a radio signal is transmitted, the electromagnetic waves spread out

Question 57

Attenuation

Answer 57

If propagation problems are too large, the receiver will not able to read the received signal

Question 58

Multipath Propagation

Answer 58

• Multipath propagation happens when an RF signal cannot take a clear and direct path between a transmitter and a receiver
  
  • ​A portion of the original may make it to the receiver unimpeded, while other parts of the signal may take an indirect route by way of the floor or ceiling causing delay.
  • Inner-symbol interference (ISI) occurs when the data of one signal overlaps those of another.

Question 59

Reduce Multipath Distortion

Answer 59

• Directional antenna
  
  • ​Used to minimize multipath distortion
  • Radiate electomagnetic waves in one direction only
• ​Other methods to reduce multipath distortion
  
  • ​Use an amplifier in front of receiver to increase SNR
  • Transmit the same signal on separate frequencies

Question 60

Simplex Transmission

Answer 60

• Occurs in only one direction
• Rarely used in wireless communication today
  
  • ​Except for broadcast radio and television

Question 61

Half-Duplex Transmission

Answer 61

• A radio system for wireless networking device typically includes both a transmitter and receiver, called a transceiver.
• Transmission are called half-duplex, meaning you can both send and receive but not at the same time.
• Used in consumer devices such as citizens band (CB) radios or walkie-talkies

Question 62

Full-duplex Transmission

Answer 62

• Allows data to flow in both directions simultaneously
• Example: A phone system
• If the same antenna is used for wireless transmission and reception
  
  • ​A filter (either passes or rejects a signal based on frequency) can be used to handle full-duplex transmissions
• ​Full-duplex wireless commnunication equipment
  
  • ​Sends and receives on different frequencies

Question 63

Baseband vs. Broadband

Answer 63

• Broadband transmission
  
  • ​carry several channels at different frequencies simultaneously over a single transmission medium
    
    • ​Cable TV is a good example
• ​​Baseband transmission
  
  • ​Use the entire communications medium to transmita single data stream
  • Only one signal (channel) can be set at a time
  • Used by wired local area network (Ethernets)