WIRELESS TRANSMISSION

Transcription

1 COMP 635: WIRELESS NETWORKS WIRELESS TRANSMISSION Jasleen Kaur Fall 205 Outline Frequenc Spectrum Ø Usage and Licensing Signals and Antennas Ø Propagation Characteristics Multipleing Ø Space, Frequenc, Time, Code Spread Spectrum Ø Direct Sequence, Frequenc Hopping Modulation Ø Amplitude, Frequenc, Phase Cellular Sstems Ø Frequenc Planning 2 Jasleen Kaur 205

2 FREQUENCY SPECTRUM Usage and Licensing 3 Carrier Frequenc T 0 t High frequenc Low frequenc Frequenc and wave length ( λ = c / f ) Ø wave length λ, speed of light c 30 8 m/s, frequenc f 4 Jasleen Kaur 205 2

3 Frequencies For Communication VLF = Ver Low Freq HF = High Freq SHF = Super High Freq LF = Low Freq VHF = Ver High Freq EHF = Etra High Freq MF = Medium Freq UHF = Ultra High Freq UV = Ultraviolet Light twisted pair coa cable optical transmission Mm 300 H 0 km 30 kh 00 m 3 MH m 300 MH 0 mm 30 GH 00 µm 3 TH µm 300 TH VLF LF MF HF VHF UHF SHF EHF infrared visible light UV Submarines AM,SW,FM Radio Prof. Dr.- Ing. Jochen H. Schiller MC TV, Mobile, 3G Wireless LANs Directed Links Satellite Comm Directed Laser 5 Frequenc Usage LF: used b submarines Ø Can penetrate water; can follow the earth s surface MF-HF: hundreds of radio stations (AM, SW, FM) Ø Short waves are reflected at ionosphere (amateur radio) VHF-UHF: TV, DAB, mobile, cordless, 3G Ø Allow for small antennas; relativel reliable connections SHF: directed microwave links, fied satellites Ø Small antenna; beam forming; high bandwidth Wireless LANs: UHF-SHF Ø Absorption b water/ogen molecules (fading/loss in rain) Optical transmission: Ø Infra-red for directed links (laser links between buildings) ITU regulates frequenc usage world-wide 6 Jasleen Kaur 205 3

4 SIGNALS & ANTENNAS Propagation Characteristics 7 Signals Signals are phsical representation of data Ø Parameters represent the data values Ø Are functions of time and location Analog vs. digital signals: Ø Analog signals: continuous time and continuous values Ø Digital signals: discrete time and discrete values Periodic signals characteried b parameters: Ø Amplitude A t, period T, frequenc f t =/T, phase shift ϕ t Ø Special case: sine wave: s(t) = A t sin(2 π f t t + ϕ t ) Ø General: g( t) = c + an sin(2πnft) + bn cos(2πnft) 2 n= n= 0 t 8 Jasleen Kaur 205 4

5 Signal Representation A [V] Amplitude Ø Time domain t[s] Frequenc spectrum Ø Frequenc domain ϕ A [V] Ø Fourier transformation Transforms to time domain f [H] Phase state diagram: Ø amplitude M Q = M sin ϕ Ø phase shift ϕ in polar coordinates ϕ I = M cos ϕ 9 Antennas Role: Ø Couple electromagnetic energ to/from space from/to a wire or other conductor Phsicall: Ø An arrangement of one or more conductors (elements) Ø Convert electromagnetic radiation into electric current (and vice versa) Transmission: Ø AC is created b appling voltage at terminals, which causes them to radiate electromagnetic field Reception: Ø Eternal electromagnetic field induces an AC in elements, which induces voltage at the terminals 2 Jasleen Kaur 205 5

6 Theoretical Reference Theoretical reference antenna: isotropic radiator Ø Point in space radiating equal power in all directions (3D) Real antennas ehibit directive effects Ø Intensit not same in all directions 3 Antenna Tpes/Models Simple Hertian Dipole Ø Two collinear conductors of equal length Separated b small feeding gap Ø Length = ½ wavelength to be transmitted Radiation pattern: λ/2 side view (-plane) side view (-plane) top view (-plane) Gain: ma power in the direction of the main lobe Ø Compared to the power of isotropic radiator (with same average power) * image source: wikipedia.org 4 Jasleen Kaur 205 6

7 Directed & Sectoried Antennas Fied preferential transmission/reception directions Ø e.g., satellite dishes Ø Tpicall applied in cellular sstems (radio coverage in a valle) and microwave connections directed antenna side view (-plane) top view (-plane) sectoried antenna top view, 3 sector top view, 6 sector 5 Signal Propagation Ranges (Ideal) Transmission range: Ø Communication possible Ø Low error rate Detection range: Ø Signal can be detected sender Ø No communication Error rate too high Interference range: transmission detection distance Ø Signal can t be detected interference Ø But ma disturb other signals 6 Jasleen Kaur 205 7

8 Path Loss of Radio Signals In free space, signals propagate like light (straight line) Ø Received power proportional to /d² d: sender-receiver distance Ø Received power also depends on atmosphere: Rain absorbs much of radiated energ (microwave oven) Frequenc-dependent propagation behavior: Ø Ground waves: low frequencies (< 2 MH) Follow the earth s surface; can propagate long distances Submarine communication, AM radio Ø Sk waves: 2 30 MH Reflected (refracted) at ionosphere; used for amateur radio Ø Line-of-sight (> 30 MH) Mobile phones, satellites, cordless phones, etc Emitted waves follow straight line of sight 7 Additional Propagation Effects Receiving power additionall influenced b Ø Fading (frequenc dependent) Ø Shadowing Ø Reflection at large obstacles Ø Refraction depending on the densit of a medium Ø Scattering at small obstacles Ø Diffraction at edges blocking / shadowing reflection refraction scattering diffraction 8 Jasleen Kaur 205 8

9 Multi-path Propagation LOS pulses multipath pulses signal at sender signal at receiver Signal can travel different paths (scattering/reflection/ diffraction) Ø Short impulse is smeared onto several weaker impulses Signal distorted according to phases of different parts Ø Causes Inter-smbol Interference (ISI) Receiver can compensate if distortion is known Ø Sender first transmits a known training sequence Ø Receiver then programs an equalier that compensates for distortion 9 Short-term and Long-term Fading Power of received signal varies over time if transmitter and/or receiver are mobile Ø Short-term fading quick changes Ø Long-term fading caused b varing distance power long term fading short term fading t 20 Jasleen Kaur 205 9