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Wireless Communications Engineering

Wireless Communications Engineering

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Wireless Communications Engineering

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  1. Wireless Communications Engineering Cellular Fundamentals

  2. Definitions – Wireless Communication • What is Wireless Communication? • Ability to communicate via wireless links. Mobile Communication = + ?

  3. Wireless Communication • Wireless Communication are of two types: • Fixed Wireless Communication • Mobile Wireless Communication.

  4. Mobile Wireless Communication • Mobile Wireless Communication (Infrastructured Network) Single Hop Wireless Link to reach a mobile Terminal. Mobile Communication = + ?

  5. Mobile Ad Hoc Networks • Infrastructureless or Adhoc Network Multihop Wireless path from source to destination.

  6. Mobile Radio Environment

  7. Mobile Radio Environment • The transmissions over the wireless link are in general very difficult to characterize. • EM signals often encounter obstacles, causing reflection, diffraction, and scattering. • Mobility introduces further complexity. • We have focused on simple models to help gain basic insight and understanding of the wireless radio medium. • Three main components: Path Loss, Shadow fading, Multipath fading (or fast fading).

  8. Free Space loss • Transmitted signal attenuates over distance because it is spread over larger and larger area • This is known as free space loss and for isotropic antennas Pt = power at the transmitting antenna Pr = power at the receiving antenna λ = carrier wavelength d = propagation distance between the antennas c = speed of light

  9. Free Space loss • For other antennas Gt = Gain of transmitting antenna Gr = Gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna

  10. Thermal Noise • Thermal noise is introduced due to thermal agitation of electrons • Present in all transmission media and all electronic devices • a function of temperature • uniformly distributed across the frequency spectrum and hence is often referred to as white noise • amount of noise found in a bandwidth of 1 Hz is N0 = k T N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzman’s constant = 1.3803 x 10-23 J/K T = temperature, in Kelvins N = thermal noise in watts present in a bandwidth of B = kTB where

  11. Free Space loss • Transmitted signal attenuates over distance because it is spread over larger and larger area • This is known as free space loss and for isotropic antennas Pt = power at the transmitting antenna Pr = power at the receiving antenna λ = carrier wavelength d = propagation distance between the antennas c = speed of light

  12. Free Space loss • For other antennas Gt = Gain of transmitting antenna Gr = Gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna

  13. Thermal Noise • Thermal noise is introduced due to thermal agitation of electrons • Present in all transmission media and all electronic devices • a function of temperature • uniformly distributed across the frequency spectrum and hence is often referred to as white noise • amount of noise found in a bandwidth of 1 Hz is N0 = k T N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzman’s constant = 1.3803 x 10-23 J/K T = temperature, in Kelvins N = thermal noise in watts present in a bandwidth of B = kTB where

  14. Data rate and error rate • Bit error rate is a decreasing function of Eb/N0. • If bit rate R is to increase, then to keep bit error rate (or Eb/N0) same, the transmitted signal power must increase, relative to noise • Eb/N0 is related to SNR as follows B = signal bandwidth (since N = N0 B)

  15. Doppler’s Shift • When a client is mobile, the frequency of received signal could be less or more than that of the transmitted signal due to Doppler’s effect • If the mobile is moving towards the direction of arrival of the wave, the Doppler’s shift is positive • If the mobile is moving away from the direction of arrival of the wave, the Doppler’s shift is negative

  16. Doppler’s Shift S where fd =change in frequency due to Doppler’s shift v = constant velocity of the mobile receiver λ = wavelength of the transmission θ X Y

  17. Doppler’s shift f = fc + fd where f = the received carrier frequency fc = carrier frequency being transmitted fd = Doppler’s shift as per the formula in the previous slide.

  18. Multipath Propagation • Wireless signal can arrive at the receiver through different paths • LOS • Reflections from objects • Diffraction • Occurs at the edge of an impenetrable body that is large compared to the wavelength of the signal

  19. Multipath Propagation (source: Stallings)

  20. Mobile Radio Channel: Fading

  21. Limitations of Wireless • Channel is unreliable • Spectrum is scarce, and not all ranges are suitable for mobile communication • Transmission power is often limited • Battery • Interference to others

  22. Advent of Cellular Systems • Noting from the channel model, we know signal will attenuated with distance and have no interference to far users. • In the late 1960s and early 1970s, work began on the first cellular telephone systems. • The term cellular refers to dividing the service area into many small regions (cells) each served by a low-power transmitter with moderate antenna height.

  23. Cell Concept • Cell A cell is a small geographical area served by a singlebase station or a cluster of base stations • Areas divided into cells • Each served by its own antenna • Served by base station consisting of transmitter, receiver, and control unit • Band of frequencies allocated • Cells set up such that antennas of all neighbors are equidistant

  24. Cellular Networks

  25. Cellular Network Organization • Use multiple low-power transmitters • Areas divided into cells • Each served by its own antenna • Served by base station consisting of transmitter, receiver, and control unit • Band of frequencies allocated • Cells set up such that antennas of all neighbors are equidistant

  26. Consequences • Transmit frequencies are re-used across these cells and the system becomes interference rather than noise limited • the need for careful radio frequency planning – colouring in hexagons! • a mechanism for handling the call as the user crosses the cell boundary - call handoff (or handover) • increased network complexity to route the call and track the users as they move around • But one significant benefit: very much increased traffic capacity, the ability to service many users

  27. Cellular System Architecture

  28. Cellular Systems Terms • Mobile Station • users transceiver terminal (handset, mobile) • Base Station (BS) • fixed transmitter usually at centre of cell • includes an antenna, a controller, and a number of receivers • Mobile Telecommunications Switching Office (MTSO) /Mobile Switch Center (MSC) • handles routing of calls in a service area • tracks user • connects to base stations and PSTN

  29. Cellular Systems Terms (Cont’d) • Two types of channels available between mobile unit and BS • Control channels – used to exchange information for setting up and maintaining calls • Traffic channels – carry voice or data connection between users • Handoff or handover • process of transferring mobile station from one base station to another, may also apply to change of radio channel within a cell

  30. Cellular Systems Terms (Cont’d) • Downlink or Forward Channel • radio channel for transmission of information (e.g.speech) from base station to mobile station • Uplink or Reverse Channel • radio channel for transmission of information (e.g.speech) from mobile station to base station • Paging • a message broadcast over an entire service area, includes use for mobile station alert (ringing) • Roaming • a mobile station operating in a service area other than the one to which it subscribes

  31. Steps in an MTSO Controlled Call between Mobile Users • Mobile unit initialization • Mobile-originated call • Paging • Call accepted • Ongoing call • Handoff

  32. Frequency Reuse • Cellular relies on the intelligent allocation and re–use of radio channels throughout a coverage area. • Each base station is allocated a group of radio channels to be used within the small geographic area of its cell • Neighbouring base stations are given different channel allocation from eachother

  33. Frequency Reuse (Cont’d) • If we limit the coverage area within the cell by design of the antennas • we can re-use that same group of frequencies to cover another cell separated by a large enough distance • transmission power controlled to limit power at that frequency to keep interference levels within tolerable limits • the issue is to determine how many cells must intervene between two cells using the same frequency

  34. Radio Planning • Design process of selecting and allocating channel frequencies for all cellular base stations within a system is known as frequency re-use or frequency planning. • Cell planning is carried out to find a geometric shape to • tessellate a 2D space • represent contours of equal transmit power • Real cells are never regular in shape

  35. Two-Dimensional Cell Clusters • Regular geometric shapes tessellating a 2D space: Square, triangle, and hexagon. • ‘Tessellating Hexagon’ is often used to model cells in wireless systems: • Good approximation to a circle (useful when antennas radiate uniformly in the x-y directions). • Also offer a wide variety of reuse pattern • Simple geometric properties help gain basic understanding and develop useful models.

  36. Coverage Patterns

  37. Cellular Coverage Representation

  38. Geometry of Hexagons Hexagonal cell geometry and axes

  39. Geometry of Hexagons (Cont’d) • D = minimum distance between centers of cells that use the same band of frequencies (called co-channels) • R = radius of a cell d = distance between centers of adjacent cells (d = R√3) • N = number of cells in repetitious pattern (Cluster) Reuse factor • Each cell in pattern uses unique band of frequencies

  40. Geometry of Hexagons (Cont’d) • The distance between the nearest cochannel cells in a hexagonal area can be calculated from the previous figure • The distance between the two adjacent co-channel cells is D=√3R. • (D/d)2 = j2 cos2(30) + (i+ jsin30)2 = i2 + j2 +ij = N • D=Dnorm x √3 R =(√3N)R • In general a candidate cell is surrounded by 6k cells in tier k.

  41. Geometry of Hexagons (Cont’d) • Using this equation to locate co-channel cells, we start from a reference cell and move i hexagons along the u-axis then j hexagons along the v-axis. Hence the distance between co–channel cells in adjacent clusters is given by: • D = (i2 + ij + j2)1/2 • where D is the distance between co–channel cells in adjacent clusters (called frequency reuse distance). • and thenumber of cells in a cluster, N is given by D2 • N = i2 + ij + j2

  42. Hexagon Reuse Clusters

  43. 3-cell reuse pattern (i=1,j=1)

  44. 4-cell reuse pattern (i=2,j=0)

  45. 7-cell reuse pattern (i=2,j=1)

  46. 12-cell reuse pattern (i=2,j=2)

  47. 19-cell reuse pattern (i=3,j=2)

  48. Relationship between Q and N

  49. Proof

  50. Cell Clusters since D = SQRT(N)