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)

802.11n – 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 802.11 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)

802.11g,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/d2

FSL (dB) = 20log10(d) + 10log10(f)-147.56 d –distance f -frequency

FSL (dB) 2.4GHz = approx. 80dB at 100mFSL (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 802.11n, SNR at the receiver is at least 15-20 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 http://www.wireless-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 = 10Log10 (mW) mw = (10dBm/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