S. R. Zinka srinivasa_zinka@daiict.ac.in October 16, 2014
First of all... Which frequencies we can use for wireless communications?
Atmospheric Attenuation of EM Waves 100 % Gamma rays, X-rays and ultraviolet light blocked by the upper atmosphere (best observed from space). Visible light observable from Earth, with some atmospheric distortion. Most of the infrared spectrum absorbed by atmospheric gasses (best observed from space). Radio waves observable from Earth. Long-wavelength radio waves blocked. Atmospheric opacity 50 % 0 % 0.1 nm 1 nm 10 nm 100 nm 1 µm 10 µm 100 µm 1 mm 1 cm 10 cm 1 m 10 m 100 m 1 km Wavelength
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
FDMA Shared Bandwidth Subchannels F1 F2 F3 Frequency F10 F11 F12
TDMA 1 Input Channels 2 3 4 Scanner Output 1 2 3 4 5 Time 5
TDMA... Contd Data stream divided into frames 1 2 3 4 Frames divided into time slots. Each user is allocated one slot. 101010000100011101 Time slot contain data with a gaurd period if needed for syncronisation. Gaurd Periods
CDMA T b Data Signal Pseudorandom Code T c Transmitted signal: Data Signal XOR with the Pseudorandom
FDMA vs TDMA vs CDMA Carrier Frequency 1 Carrier Frequency 2 FDMA Carrier Frequency 3 Carrier Frequency 4 Carrier Frequency 5 Carrier Frequency 6 TDMA T S 1 T S 1 T S 2 T S 2 T S 3 T S 3 Carrier Frequency 1 Carrier Frequency 2 CDMA Code 1 Code 2 Code 3 Code 4 Code 5 Code 6 Carrier Frequency 1
SDMA R = Cell Radius N = Cluster Size f6 f7 f5 f1 f2 f4 f3 Frequency Reuse Distance
SDMA... Contd F1 F2 F3 F4
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
Frequency Reuse R = Cell Radius N = Cluster Size f6 f7 Public Switched Telephone Network f5 f4 f1 f3 f2 Uplink Base Station Base Station Downlink Switching Center Frequency Reuse Distance Base Station Base Station
Classification of Cells Global Coverage Marcro Cell Sub Urban Coverage Micro Cell Neighbhourhood Coverage Pico Cell In-Building Coverage
Calling from a Mobile Scan control channels request authorization Switching Center Switching Center Step 1: Scan for the strongest control channel and establish authorization Step 2: Switching center assigns an available traffic channel Switching Center PSTN Step 3: Switching center setups audio connection through PSTN
Calling a Mobile Paging Switching Center Step 1: Switching center sends page alert message signal to last known mobile location Step 2: Switching center assigns an available traffic channel Switching Center Switching Center PSTN Step 3: Switching center setups audio connection through PSTN
Handoff / Handover + BS 1 BS 2 BS 1 BS 2
Cell Splitting
Cell Sectoring
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
Friss Transmission Equation Transmitter Reciever P t R ( ) λ 2 P r = P t G t G r 4πR
Inverse Square Law S r A A A 2r 3r
Multipath Propagation Diffraction Scattering Direct LOS Path Reflection
Multipath Propagation... Contd Scattering Reflection Diffraction Transmitter Direct path Receiver Refraction
Fading 10 12 14 Slow fading Fast fading 16 Received power 18 20 22 24 26 28 30 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Distance (km) Fading is defined as the waxing and waning of the received signal strength.
Delay Spread D Diffraction A Delay Spread Direct LOS Path Scattering C B Reflection A B C D Time
Delay Spread... Contd Direct Signal Time Reflected and Delayed Signal Time Resulting Recieved Signal Time
Doppler Spread Scattering object is: Recieved signal Moving towards Transmitted pulse Moving away Stationary or moving across
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
Equalization Transmitted signal x(t) h c (t) y(t) h eq (t) ˆx(t) Channel impulse response Equalizer impulse response Received signal H eq (f ) = 1 H c ( f )
Adaptive Equalization y(t) Equalizer xˆ ( t) h eq(t) e(t) Error signal r(t) Reference signal
Diversity Space Diversity Polarization Diversity Time Diversity Frequency Diversity
Channel Coding Channel coding deliberately introduces redundancies (e.g., parity bits) into the data to allow for correction of the errors caused by dispersion. These redundant symbols can be used to detect errors and/or correct the errors in the corrupted data. Thus, the channel codes can fall into two types: Error detection codes (automatic repeat request (ARQ) codes) Error correction codes (forward error correcting (FEC) codes)
MIMO Transmitter 1 2 M-antenna elements s M 1 M Channel H 1 2 x N 1 N Receiver N-antenna elements x = H s + n
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
Types of Satellites Astronomical satellites (Hubble Space Telescope) Communications satellites (INSAT, GSAT) Navigational satellites (GPS, GLONASS, IRNSS) Reconnaissance satellites Earth observation satellites (IRS, Cartosat, Oceansat, Megha-Tropiques) Space stations (Skylab, Mir, ISS)
Types of Orbits GEO MEO LEO Van Allen Outer Belt (13000-60000 km above the surface of the Earth) HEO Van Allen Inner Belt (1000-6000 km above the surface of the Earth)
Van Allen Radiation Belts Rotational Axis Outer Radiation Belt Inner Radiation Belt Inner Radiation Belt Outer Radiation Belt Magnetic Axis
Disadvantages of GEOS Two-way propagation delay on the order of 0.6 s encountered by a signal makes it unacceptable for voice communication. It is difficult to cover the area of the globe far south and far north of the equator using GEOS. GEOS requires high-power transmitters with large antennas to overcome the propagation loss suffered by the signal due to a large distance.
Disadvantages of LEOS A large number of satellites are required for global coverage. There is a requirement of fast handoffs, as the satellites move rapidly and are in view only for a short time. There is also a concern of limited lifetime (five-ten years) due to orbital decay, requiring regular replacement..
Shaped Beam vs Spot Beams
Outline 1 Multiple-Access 2 Cellular Communications 3 Propagation Characteristics 4 Improving Signal Quality 5 Satellite-Mobile Communications 6 Use of Arrays
Adaptive Beams
Dynamic Cells
Adaptive Beams in Satellite Communications (TX)
Adaptive Beams in Satellite Communications (RX)
Satellite-to-Satellite Communication Jammer