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The Wireless Channel

The Wireless Channel 46
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Dr.ShivJindal,India,Teacher
Published Date:19-07-2017
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The Wireless Channel Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 1Wireless Channel is Very Different  Wireless channel “feels” very different from a wired channel.  Not a point-to-point link  Variable capacity, errors, delays  Capacity is shared with interferers  Characteristics of the channel appear to change randomly with time, which makes it difficult to design reliable systems with guaranteed performance.  Cellular model vs reality: Cellular system designs are interference-limited, i.e. the interference dominates the noise floor Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 2Basic Ideas: Path Loss, Shadowing, Fading  Variable decay of signal due to environment, multipaths, mobility Shivkumar Kalyanaraman Rensselaer Polytechnic Institute Source: A. Goldsmith book : “shiv rpi” 3Attenuation, Dispersion Effects: ISI Inter-symbol interference (ISI) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute Source: Prof. Raj Jain, WUSTL : “shiv rpi” 4Wireless Multipath Channel Channel varies at two spatial scales: Large scale fading: path loss, shadowing Small scale fading: Multi-path fading (frequency selectivity, coherence b/w, 500kHz), Doppler (time-selectivity, coherence time, 2.5ms) Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 5MultiPath Interference: Constructive & Destructive Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 6Mobile Wireless Channel w/ Multipath Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 7Game plan  We wish to understand how physical parameters such as  carrier frequency  mobile speed  bandwidth  delay spread  angular spread impact how a wireless channel behaves from the cell planning and communication system point of view.  We start with deterministic physical model and progress towards statistical models, which are more useful for design and performance evaluation. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 8Large-scale Fading: Path Loss, Shadowing Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 9Large-scale fading: Cell-Site Planning 2  In free space, received power attenuates like 1/r .  With reflections and obstructions, can attenuate even more rapidly with distance. Detailed modelling complicated.  Time constants associated with variations are very long as the mobile moves, many seconds or minutes.  More important for cell site planning, less for communication system design. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 10Path Loss Modeling  Maxwell’s equations  Complex and impractical  Free space path loss model  Too simple  Ray tracing models  Requires site-specific information  Empirical Models  Don’t always generalize to other environments  Simplified power falloff models  Main characteristics: good for high-level analysis Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 11Free-Space-Propagation  If oscillating field at transmitter, it produces three components: 2 3  The electrostatic and inductive fields that decay as 1/d or 1/d 2  The EM radiation field that decays as 1/d (power decays as 1/d )  Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 12Electric (Far) Field & Transfer Function  Tx: a sinusoid: cos 2ft  Electric Field: source antenna gain ( ) s  Product of antenna gains ()  Consider the function (transfer function)  The electric field is now: Linearity is a good assumption, but time-invariance lost when Tx, Rx or environment in motion Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 13Free-space and received fields: Path Loss (power flux density P ) d Note: Electric Field (E) decays as 1/r, but 2 Power (P ) decays as 1/r d Path Loss in dB: Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 14Decibels: dB, dBm, dBi  dB (Decibel) = 10 log (Pr/Pt) 10 Log-ratio of two signal levels. Named after Alexander Graham Bell. For example, a cable has 6 dB loss or an amplifier has 15 dB of gain. System gains and losses can be added/subtracted, especially when changes are in several orders of magnitude.  dBm (dB milliWatt) Relative to 1mW, i.e. 0 dBm is 1 mW (milliWatt). Small signals are -ve (e.g. -83dBm). Typical 802.11b WLAN cards have +15 dBm (32mW) of output power. They also spec a -83 dBm RX sensitivity (minimum RX signal level required for 11Mbps reception). For example, 125 mW is 21 dBm and 250 mW is 24 dBm. (commonly used numbers)  dBi (dB isotropic) for EIRP (Effective Isotropic Radiated Power) The gain a given antenna has over a theoretical isotropic (point source) antenna. The gain of microwave antennas (above 1 GHz) is generally given in dBi.  dBd (dB dipole) The gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the smallest, least gain practical antenna that can be made. A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. Thus, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd and is used as a standard for calibration. The term dBd (or sometimes just called dB) generally is used to describe antenna gain for antennas that operate under 1GHz (1000Mhz). Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 15dB calculations: Effective Isotropic Radiated Power (EIRP)  EIRP (Effect Isotropic Radiated Power): effective power found in the main lobe of transmitter antenna.  EIRP = P G t t  In dB, EIRP is equal to sum of the antenna gain, Gt (in dBi) plus the power, Pt (in dBm) into that antenna.  For example, a 12 dBi gain antenna fed directly with 15 dBm of power has an Effective Isotropic Radiated Power (EIRP) of: 12 dBi + 15dBm = 27 dBm (500 mW). Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 16Path Loss (Example 1): Carrier Frequency 10m W  Note: effect of frequency f: 900 Mhz vs 5 Ghz.  Either the receiver must have greater sensitivity or the sender must pour 44W of power, even for 10m cell radius Shivkumar Kalyanaraman Rensselaer Polytechnic Institute Source: A. Goldsmith book : “shiv rpi” 17Path Loss (Example 2), Interference & Cell Sizing  Desired signal power:  Interference power:  SIR:  SIR is much better with higher path loss exponent ( = 5)  Higher path loss, smaller cells = lower interference, higher SIR Shivkumar Kalyanaraman Rensselaer Polytechnic Institute Source: J. Andrews et al book : “shiv rpi” 18Path Loss: Range vs Bandwidth Tradeoff  Frequencies 1 GHz are often referred to as “beachfront” spectrum. Why?  1. High frequency RF electronics have traditionally been harder to design and manufacture, and hence more expensive. less so nowadays 2  2. Pathloss increases O(f ) c  A signal at 3.5 GHz (one of WiMAX’s candidate frequencies) will be received with about 20 times less power than at 800 MHz (a popular cellular frequency).  Effective path loss exponent also increases at higher frequencies, due to increased absorption and attenuation of high frequency signals  Tradeoff:  Bandwidth at higher carrier frequencies is more plentiful and less expensive.  Does not support large transmission ranges.  (also increases problems for mobility/Doppler effects etc)  WIMAX Choice:  Pick any two out of three: high data rate, high range, low cost. Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 19Ray Tracing  Models all signal components  Reflections  Scattering  Diffraction  Diffraction: signal “bends around” an object in its path to the receiver:  Diffraction Path loss exceeding 100 dB  Error of the ray tracing approximation is smallest when the receiver is many wavelengths from the nearest scatterer, and all the scatterers are large relative to a wavelength and fairly smooth.  Good match w/ empirical data in rural areas, along city streets (Tx/Rx close to ground), LAN with adjusted diffraction coefficients Shivkumar Kalyanaraman Rensselaer Polytechnic Institute : “shiv rpi” 20