W.lilakiatsakun

 Radio Wave Fundamental  Radio Wave Attributes  RF System Component  RF Signal Propagation  RF Mathematics

 A radio wave is a type of electromagnetic signal designed to carry information through the air medium over relatively long distances.  Sometimes radio waves are referred to as radio frequency (RF) signals  RF signals oscillate at very high frequency which allows the wave to travel through the air

 Amplitude of a radio wave indicates its strength  The measure of amplitude is generally power  Power (Electromagnetic) represents the amount of energy necessary to push signal over particular distance (Watts)

 It is possible to use dBm units (decibels referenced to 1 mW) to represent the amplitude of radio waves  0 dBm equals 1 mW (dBm = 10Log10(mW))  1 Watt = ? dBm

 The frequency is the number of times per second that signal repeat itself  The unit for frequency is Hertz(Hz)

 Wavelength ( ) = v/f  v – velocity m/sec, f - frequency (Hz = 1/sec)  - wavelength (m)  n – 2.4GHz and 5 GHz

 Theoretically, higher frequency signal propagates over a shorter range as compared to lower frequency signals  Same transmission power  Shorter wavelength signal will easily be absorbed by objects in the air such as rain, vapor

 The phase of a radio wave corresponds to how far the signal is offset from a reference point  As a convention, each cycle of the signal spans 360 degrees  A signal might have a phase shift of 90 degrees which means that the offset amount is one-quarter (90/360=1/4) of the signal

 It consists of a transmitter and a receiver  Transmitter –  Modulator - convert digital data to RF signal  Amplifier – increase amplitude (signal strength) to desired power  Antenna – transmit signal to the medium

 Receiver  Antenna – receive signal from the medium  Amplifier – increase amplitude of RF signal to appropriate level  Demodulator – convert to digital data

 RF modulation transform digital data (0,1) from the network to RF signal suitable for transmission through the air  Frequency of carrier signal is involved in modulation process  Can we send signal without modulation ?

 Amplitude shift-keying (ASK)  ASK varies the amplitude of a signal to represent data  It does not work well for RF systems because the amplitude of signal would interfere/change by the environment and noise

 Noise and environment effect can have significant impact to ASK

 Frequency Shift-Keying (FSK)  FSK makes slight change of the frequency of the carrier signal to represent data  Suitable for low to moderate data rate  Tolerate to noise

 Phase Shift-Keying (PSK)  PSK makes change of the phase of the carrier signal to represent data (0,1)  Similar to FSK, it used for low to moderate data rate 

 Quadrature Amplitude Modulation (QAM)  QAM causes both amplitude and phase of the carrier to change to represent pattern of data (symbol)  The advantage is to support high data rate

 After modulating the digital signal into analog carrier signal using FSK,PSK or QAM, some WLAN transceiver spreads the modulated carrier over a wider spectrum to comply with the regulatory rules, so called spread spectrum  It reduces the possibility of outward and inward interference the regulatory rules, so called spread spectrum

 Spread spectrum spreads a signal power over a wide band of frequencies

 Direct Sequence Spread Spectrum (DSSS)  Direct sequence modulates a radio carrier by a digital code (Chipping code) with a bit rate (Chip rate) much higher than the information signal bandwidth

 The chipping code is a redundant bit pattern for each bit that is transmitted, which increases the signal's resistance to interference.  If one or more bits in the pattern are damaged during transmission, the original data can be recovered due to the redundancy of the transmission.  The increase in the number of bit sent that represents the data effectively spreads the signal across a wider portion of the frequency spectrum  IEEE b,g

 Frequency Hopping Spread Spectrum (FHSS)  FHSS spreads signal by quickly hopping the radio carrier from one frequency to another within specific range  Bluetooth

 Orthogonal Frequency Division Multiplex (OFDM)  g,n,ADSL  OFDM divides a signal modulated with FSK,PSK or QAM across multiple subcarriers occupying a specific channel  Higher data rate  Minimize multipath propagation problems

 As a radio signal propagates through the transmission medium (air), it experiences a decreasing in amplitude (signal loss) referred to as attenuation  A large part of the decrease in amplitude with attenuation results from what is known as free space loss (FSL)

 The amplitude of a radio wave is proportional to the inverse of the square of the distance from the source  Double distance, the amplitude will be ¼ of intial value  Power  1/d 2  FSL (dB) = 20log 10 (d) + 10log 10 (f)  d –distance f -frequency

FSL (dB) 2.4GHz = approx. 80dB at 100m FSL (dB) 5GHz = approx. 86 dB at 100 m

 As radio waves travel through physical obstacles such as wall and ceiling, they decrease signal strength much more than open air  Depend on the material  However, for outdoor, rain, fog and snow can cause significant attenuation to the propagation of modulated wireless signal

 Multipath propagation occurs when portions of radio wave take different paths when propagating from source to destination

 Signals from different path will arrive at the destination on different time.  Signal distortion

 The capability of the receiver to make sense of the radio wave depends on receiving signal and interfering signal (noise)  It is possible to improve communications by either increasing the transmit power or reducing the noise  In general, an average noise of -95 dBm (noise floor) exists because of the electromagnetic impacts of the atmosphere.  Some area may be -90dBm,-80dBm

 Signal-to-noise ratio (SNR) is the signal power (dBm) of the tradio wave minus the noise power (dBm)

 For example, a signal power of -65 dBm and noise power of -90 dBm yields a SNR of 25 dB  For n, SNR at the receiver is at least dB to provide a safety margin to ensure that noise fluctuation do not cause too many retransmission  May be interfered by other sources such as cordless phones

 > 40dB SNR = Excellent signal (5 bars); always associated; lightening fast.  25dB to 40dB SNR = Very good signal (3 - 4 bars); always associated; very fast.  15dB to 25dB SNR = Low signal (2 bars); always associated; usually fast.  10dB - 15dB SNR = Very low signal (1 bar); mostly associated; mostly slow.  5dB to 10dB SNR = No signal; not associated; no go.  From nets.com/resources/tutorials/define_SNR_values.htmlhttp:// nets.com/resources/tutorials/define_SNR_values.html

 Power of radio signal will be presented both as linear (watts) and logarithmic (dBm) units  The output of an access point is generally given in mW  Most analyzers display output power in dBm  dBm = 10Log 10 (mW)  mw = (10 dBm/10 )

 Multiply by 2 (in mW) equals increase 3 dBm (+)  Divide by 2 (in mW) equals decrease 3 dBm (-)  0 dBm = 1mW  10dBm = 10mW  20dBm = 100mW  30dBm = 1000mW