Chapter 3 -Wireless_Networks_Principles_Lec.pptx

Chapter 3 -Wireless_Networks

1. Wireless Network Principles CHAPTER THREE Wireless Networks Principles By Mikiyas . A 1

2. Chapter Contents Wireless Network Principles 2 Wireless networks basics, Wireless communications, Applications, Simple reference model, Wireless transmissions, Frequencies allocation, Antennas, Signal, Signal Propagation Multiplexing, Modulation Media Access Control

3. Wireless Network Basics Wireless Network Principles 3 A wireless local-area network (LAN) uses radio waves to connect devices to the Internet and to business network and its applications. A wired network connects devices to the Internet or other network using cables. The most common wired networks use cables connected to Ethernet ports on the network router on one end and to a computer or other device on the cable's opposite end.

4. Wireless Communications Wireless communication is the  transfer of information  between two or more points that are not connected by an electrical conductor. Wireless communication generally works through electromagnetic signals that are broadcast by an enabled device within the air, physical environment or atmosphere. The way of accessing a network or other communication partners i.e without a wire . The wire is replaced by the transmission of electromagnetic waves through ‘the air’ Wireless Network Principles 4

5. What Is a Wireless Network?: The Benefits 5 Convenience: Access network resources from any location Productivity: Get the job done and encourages collaboration. Easy setup: No cables, so installation can be quick and cost effective. Expandable : Easily expand wireless networks with existing equipment Security: Provide robust security protections. Cost: Eliminate or reduce wiring costs, less cost to operate Wireless Network Principles

6. Applications Vehicles Emergencies Business Replacement of wired networks Infotainment Mobile and Wireless devices Sensor (Sensing the Door) Embedded controllers (keyboard, washing machines) Pager (Display short message) Mobile Phones (Migrate, Color graphic display touch screen) Personal Digital assistant (Accompany calendar, notepad) Pocket computer Notebook / Laptop Wireless Network Principles 6

7. Simple Reference Model Application Transport Network Data Link Physical Medium Data Link Physical Application Transport Network Data Link Physical Data Link Physical Network Network Radio Wireless Network Principles 7

8. Wireless Transmission The frequencies used for transmission are all regulated. Why multiplexing used in wireless communication Multiplexing is used because the medium is always shared. Multiplexing schemes have to ensure low interference between different sender. Modulation is needed to transmit digital data via certain frequencies. Wireless Network Principles 8

9. 9 Frequencies - Spectrum Allocation 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 Relationship between frequency ‘f’ and wave length ‘ ’ :  = c/f where c is the speed of light  3x10 8 m/s 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz visible light VLF LF MF HF VHF UHF SHF EHF infrared UV optical transmission coax cable twisted pair Wireless Network Principles

10. 10 Frequencies Allocated for Mobile Communication VHF & UHF ranges for mobile radio allows for simple, small antennas for cars deterministic propagation characteristics less subject to weather conditions –> more reliable connections SHF and higher for directed radio links, satellite communication small antennas with directed transmission large bandwidths available Radar systems Wireless LANs use frequencies in UHF to SHF spectrum 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. Wireless Network Principles

11. Band chart Wireless Network Principles 11 Common amateur activity falls into three major bands:  HF ,  VHF , and  UHF  bands.  MF (Medium Frequency) 300-3000 kilohertz HF (High Frequency) 3 - 30 megahertz VHF (Very High Frequency) 30-300 megahertz UHF (Ultra High Frequency) 300-3000 megahertz SHF (Super High Frequency) 3-30 gigahertz EHF (Extremely High Frequency) Anything above 30 gigahertz

12. Antennas Wireless Network Principles 12 Antennas are used to radiate and receive EM waves (energy) Antennas link this energy between the ether and a device such as a transmission line Antennas consist of one or several radiating elements through which an electric current circulates Electromagnetic waves propagate along transmission lines and through space. The antenna is the interface between these two media and is a very important part of the communication path. First, antennas are passive devices. Therefore, the power radiated by a transmitting antenna cannot be greater than the power entering from the transmitter.

13. Cont’d Wireless Network Principles 13 In fact, it is always less because of losses. We will speak of antenna gain, but remember that gain in one direction results from a concentration of power and is accompanied by a loss in other directions. Antennas achieve gain the same way a flashlight reflector increases the brightness of the bulb: by concentrating energy. The second concept to keep in mind is that antennas are reciprocal devices; that is, the same design works equally well as a transmitting or a receiving antenna and in fact has the same gain. In wireless communication, often the same antenna is used for both transmission and reception.

14. Cont’d Wireless Network Principles 14 Essentially, the task of a transmitting antenna is to convert the electrical energy travelling along a transmission line into electromagnetic waves in space. At the receiving antenna, the electric and magnetic fields in space cause current to flow in the conductors that make up the antenna.

15. Types of Antennas Wireless Network Principles 15 Types of antennas: Omnidirectional Directional Phased arrays Adaptive Optimal Principal characteristics used to characterize an antenna are: Radiation pattern Directivity Gain Efficiency

16. Omnidirectional Antennas Wireless Network Principles 16 Omnidirectional antennas, such as simple dipoles, are antennas that radiate or receive electromagnetic waves equally in all directions along a single plane. They are often used in applications where communication needs to occur in multiple directions without requiring the antenna to be pointed towards a specific target.

17. Omnidirectional Antennas: simple dipoles Wireless Network Principles 17 Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4, or Hertzian dipole: /2 (2 dipoles)  shape/size of antenna proportional to wavelength Example: Radiation pattern of a simple Hertzian dipole Gain: ratio of the maximum power in the direction of the main lobe to the power of an isotropic radiator (with the same average power)

18. Half-wave dipole antenna Wireless Network Principles 18 on the other hand, is a simple, practical antenna which is in common use. An understanding of the half-wave dipole is important both in its own right and as a basis for the study of more complex antennas.

19. Cont’d Wireless Network Principles 19 The word dipole simply means it has two parts, as shown. A dipole antenna does not have to be one-half wavelength in length like the one shown in the figure, but this length is handy for impedance matching, as we shall see. A half-wave dipole is sometimes called a Hertz antenna, though strictly speaking the term Hertzian dipole refers to a dipole of infinitesimal length.

20. Cont’d Wireless Network Principles 20 Typically the length of a half-wave dipole, assuming that the conductor diameter is much less than the length of the antenna, is 95% of one-half the wavelength measured in free space. the free- space wavelength is given by λ = c/ ƒ where λ = free-space wavelength in meters c = 300 × 10^6 m/s ƒ = frequency in hertz

21. Cont’d Wireless Network Principles 21 the frequency is given in megahertz, this equation becomes L=142.5/f ------------------ equation(1) where L = length of a half-wave dipole in meters ƒ = frequency in megahertz For length measurements in feet, the equivalent equation is L=468/f --------------------- equation(2) where L = length of a half-wave dipole in feet ƒ = operating frequency in megahertz

22. Cont’d Wireless Network Principles 22 Example : Calculate the length of a half-wave dipole for an operating frequency of 200 MHz. Solution From equation: L=142.5/f =142.5/200 =0.7125 m

23. Directional Antennas Wireless Network Principles 23 Directional antennas are antennas designed to focus radio frequency (RF) energy in a particular direction or pattern. Unlike omnidirectional antennas, which radiate or receive RF signals equally in all directions, directional antennas concentrate the RF energy in specific directions, allowing for longer range communication, increased signal strength, and reduced interference. Gain is a measure of the antenna's ability to focus energy in a particular direction compared to an isotropic radiator (an idealized omnidirectional antenna).

24. Cont’d Wireless Network Principles 24

25. Isotropic Antennas Wireless Network Principles 25 Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna It serves as a benchmark for comparing the performance of real antennas. Real antennas always have directive effects (vertical and/or horizontal) Radiation pattern: measurement of radiation around an antenna z y x ideal isotropic radiator

26. Array Antennas Wireless Network Principles 26 Simple antenna elements can be combined to form arrays resulting in reinforcement in some directions and cancellations in others to give better gain and directional characteristics Arrays can be classified as broadside or end-fire Examples of arrays are: The Yagi Array The Log-Periodic Dipole Array The Turnstile Array The Monopole Phased Array Other Phased Arrays

27. Array Antennas Wireless Network Principles 27 Grouping of 2 or more antennas to obtain radiating characteristics that cannot be obtained from a single element Antenna diversity switched diversity, selection diversity receiver chooses antenna with largest output diversity combining combine output power to produce gain cophasing needed to avoid cancellation

28. Mobile & Portable Antennas Wireless Network Principles 28 The portable and mobile antennas used with cellular systems have to be omnidirectional and small, especially in the case of portable phones. Easier to achieve at 1900 MHz than at 800 MHz. Many PCS phones must double as 800-MHz cell phones, however, so they need an antenna that works well at 800 MHz.

29. Cont’d Wireless Network Principles 29 The simplest suitable antenna is a quarter-wave monopole, and these are the usual antennas supplied with portable phones. For mobile phones, where compact size is not quite as important, a very common configuration consists of a quarter-wave antenna with a half-wave antenna mounted collinearly above it.

30. Reflectors Wireless Network Principles 30 It is possible to construct a conductive surface that reflects antenna power in the desired direction The surface may consist of one or more planes or may be parabolic Typical reflectors are: Plane and corner Reflectors radar systems The Parabolic Reflector Satellite Dishes Telescopes ,etc.

31. Signal Wireless Network Principles 31 physical representation of data function of time and location signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values

32. Signal Wireless Network Principles 32 Some types of signal includes: Digital signal Analog signal Radio signal Audio signal Visual signal signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift .

33. Cont’d Wireless Network Principles 33 P eriod (T) : The period of a periodic signal is the time it takes for one complete cycle of the signal to occur. It is usually denoted by T . Frequency (f) : The frequency of a periodic signal is the number of cycles that occur per unit time. It is the reciprocal of the period and is denoted by f . The relationship between period and frequency is given by f =1/ T ​. Amplitude (A) : The amplitude of a periodic signal is the maximum absolute value of the signal. It represents the strength or intensity of the signal. It is typically denoted by A . Phase Shift (  ) : The phase shift of a periodic signal represents a horizontal shift in the waveform. It indicates the displacement of the signal from a reference point on the time axis. It is usually denoted by  .

34. Cont’d Wireless Network Principles 34 These parameters help characterize and analyze periodic signals in various applications, including communication systems, signal processing, and control systems. Understanding these parameters is crucial for manipulating, transmitting, and interpreting periodic signals effectively sine wave as special periodic signal for a carrier: the angular frequency ω( omega ) =2 πf s(t) = A t sin(2  f t t +  t )

35. Wireless Network Principles 35 Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude A and phase  in polar coordinates) time domain frequency domain phase domain Composite signals mapped into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect representation modulation with a carrier frequency for transmission (->analog signal!) Signal f [Hz] A [V]  I= A cos  Q = A sin   A [V] t[s]

36. Signal propagation Wireless Network Principles 36 Signal propagation refers to the transmission or spread of signals through a medium or space. Propagation in free space is always like light (straight line) Receiving power is proportional to 1/d² in vacuum – much more in real environments (d = distance between sender and receiver) Radio wave propagation (Receiving power) is influenced by Shadowing induced by obstacles in path between the transmitted & receiver

37. Cont’d reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges pathloss due to distance covered by radio signal (frequency dependent, less at low frequencies) fading (frequency dependent, related to multipath propagation) shadowing reflection scattering diffraction refraction

38. Signal propagation ranges distance sender transmission detection interference Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise Wireless Network Principles 38

39. Wireless Network Principles 39 Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time  interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted  distorted signal depending on the phases of the different parts Positive effects of multipath: enables communication even when transmitter and receiver are not in LOS conditions - allows radio waves effectively to go through obstacles by getting around them thereby increasing the radio coverage area Multipath Propagation signal at sender signal at receiver

40. 40 Cont’d Negative effects of multipath: Time dispersion or delay spread: signal is dispersed over time due signals coming over different paths of different lengths  Causes i nterference with “neighboring” symbols, Inter Symbol Interference (ISI) multipath spread (in secs) = (longest 1 – shortest 2 )/c For a 5ms symbol duration a 1ms delay spread means about a 20% intersymbol overlap. The signal reaches a receiver directly and phase shifted (due to reflections)  Distorted signal depending on the phases of the different parts, this is referred to as Rayleigh fading , due to the distribution of the fades. It creates fast fluctuations of the received signal (fast fading). Random frequency modulation due to Doppler shifts on the different paths. Doppler shift is caused by the relative velocity of the receiver to the transmitter, leads to a frequency variation of the received signal. Wireless Network Principles

41. Wireless Network Principles 41 Effects of Mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts  quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away  slow changes in the average power received (long term fading) short term fading long term fading t power

42. Wireless Network Principles 42 Thanks , See You Tuesday class !!!

43. Wireless Network Principles 43 Multiplexing Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. Multiplexing is the set of techniques that allows users simultaneous transmission of multiple signals across a single data link. The users are mobile and the transmission resource is the radio spectrum. Sharing a common resource requires an access mechanism that will control the multiplexing mechanism. As data and telecommunications use increases, so does traffic. A Multiplexer (MUX) is a device that combines several signals into a single signal. Demultiplexer (DEMUX) is a device that performs the inverse operation.

44. Wireless Network Principles Multiplexing can be carried out in 4 dimensions space (s i ) – Space Division Multiplexing (SDM) time (t) – Time Division Multiplexing (TDM) frequency (f) – Frequency Division Multiplexing(FDM) code (c) – Code Division Multiplexing (CDM) Types Multiplexing 44

45. SDM, (3D) space si is represented via circles indicating the interference range Goal: multiple use of a shared medium The channels k1 to k3 can be mapped onto three ‘spaces’ s1 to s3 which clearly separate channels and prevent interference ranges from overlapping. The space between the interference ranges is called guard space . Three spaces are required for remaining channels k4 to k6 Important: guard spaces needed! Example: FM Radios Disadvantages: if channels established within same space(radio stations in the same city) s 2 s 3 s 1 Space Division Multiplexing(SDM) f t c k 2 k 3 k 4 k 5 k 6 k 1 f t c f t c channels k i Wireless Network Principles 45

46. Wireless Network Principles Frequency Division Multiplexing(FDM) FDM is an analog multiplexing technique that combines analog signals. Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination necessary works also for analog signals Low bit rates – cheaper, delay spread Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces k 2 k 3 k 4 k 5 k 6 k 1 f t c 46

47. Wireless Network Principles f t c k 2 k 3 k 4 k 5 k 6 k 1 Time Division Multiplexing(TDM) TDM is a digital multiplexing technique for combining several low-rate digital channels into one high-rate one. A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high - supports bursts flexible – multiple slots no guard bands Disadvantages: Framing and precise synchronization necessary high bit rates 47

48. Wireless Network Principles f Hybrid - Time and Frequency Multiplexing Combination of both methods (TDM and FDM) A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: better protection against tapping protection against frequency selective interference higher data rates compared to code multiplex but: precise coordination required t c k 2 k 3 k 4 k 5 k 6 k 1 48

49. Wireless Network Principles Code Division Multiplexing Each channel has a unique code All channels use the same spectrum at the same time Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology k 2 k 3 k 4 k 5 k 6 k 1 f t c 49

50. Wireless Network Principles Modulation Digital modulation digital data is translated into an analog signal (baseband) Analog modulation shifts center frequency of baseband signal up to the radio carrier Modulation and Demodulation 50 synchronization decision digital data analog demodulation radio carrier analog baseband signal 101101001 radio receiver digital modulation digital data analog modulation radio carrier analog baseband signal 101101001 radio transmitter

51. Wireless Network Principles Areas of research in wireless & mobile communication Wireless Communication transmission quality (bandwidth, error rate, delay) modulation, coding, interference media access, regulations ... Mobility location dependent services location transparency quality of service support (delay, jitter, security) ad-Hoc & sensor Portability power consumption limited computing power, sizes of display, ... usability 51

52. Wireless Network Principles 52 Media Access Control MAC stands for M edia A ccess C ontrol . A MAC layer protocol is the protocol that controls access to the physical transmission medium on a LAN. It tries to ensure that no two nodes are interfering with each other’s transmissions, and deals with the situation when they do.

53. Wireless Network Principles 53 Motivation Can we apply media access methods from fixed networks? Example CSMA/CD CSMA/CD, or Carrier Sense Multiple Access with Collision Detection, is a protocol used in Ethernet networks to regulate access to the network medium and handle collisions that may occur when multiple devices attempt to transmit data simultaneously C arrier S ense M ultiple A ccess with C ollision D etection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)

54. Cont’d Wireless Network Principles 54 Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot “hear” the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden”

55. Wireless Network Principles 55 Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B Motivation - hidden and exposed terminals B A C

56. Wireless Network Principles 56 Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A’s signal C cannot receive A If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! Motivation - near and far terminals A B C

57. Wireless Network Principles 57 Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time The multiplexing schemes are used to control medium access!

58. Wireless Network Principles 58 Access method CDMA CDMA (Code Division Multiple Access) all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel each sender has a unique random number, the sender XORs the signal with this random number the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function Disadvantages: higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) all signals should have the same strength at a receiver Advantages: all terminals can use the same frequency, no planning needed huge code space (e.g. 2 32 ) compared to frequency space interferences (e.g. white noise) is not coded forward error correction and encryption can be easily integrated

59. Wireless Network Principles Comparison SDMA/TDMA/FDMA/CDMA 59

60. Wireless Network Principles 60 CSMA/CD MAC MAC schemes from wired networks, CSMA/CD as used in original specification of IEEE 802.3 (aka Ethernet). CSMA/CD architecture used in Ethernet is a common MAC layer standard. It acts as an interface between the Logical Link Control sublayer and the network's Physical layer. A sender senses the medium (a wire or coaxial cable) to see if it is free. If the medium is busy, the sender waits until it is free. If the medium is free, the sender starts transmitting data and continues to listen into the medium. If the sender detects a collision while sending, it stops at once and sends a jamming signal. Contention-based protocols CSMA — Carrier Sense Multiple Access Ethernet (CSMA/CD) is not enough for wireless (collision at receiver cannot detect at sender). Hence hidden terminal problem.

61. Wireless Network Principles 61 MACA - Collision Avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address, receiver address, packet size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) MACA Protocol solved hidden and exposed terminal problems: Sender broadcasts a Request-to-Send ( RTS ) and the intended receiver sends a Clear-to-Send ( CTS ). Upon receipt of a CTS, the sender begins transmission of the frame. RTS, CTS helps determine who else is in range or busy ( C ollision A voidance).

62. Wireless Network Principles 62 MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A MACA examples A B C RTS CTS CTS A B C RTS CTS RTS

63. Diagram how MACA Works Wireless Network Principles 63

64. Wireless Network Principles 64 Thanks A lot ! See You Next Class!!