Radio Frequency Basic

Radio Frequency Basic


Every communication technology must have a medium to travel along in order to pass information between two devices.

Ethernet network pass this information along coaxial cable such as UTP,telephone use wiring as the medium.

Wireless network specially Wi-Fi,Wimax,Bluetooth,Infra Red use radio frequency (RF) waves as the medium to pass the information to the network

RF Defined

It Should be immediately clear that there at least two important words related to RF that need to defined;however there is a third word that also needs to defined to fully understand RF.These three words are :

 Radio
 Frequency
 Signal

Radio

Radio is the devices that generates and receives signal known as an RF signal,and this signal is transmitted on a specific frequency

Frequency

the frequency is what distinguishes one RF signal from another

Signal

There are two general categories of signals :

 Digital
 Analog














The Sine Wave


Digital signal differ from analog signal

Digital signal are used to communicate and relay information within computer networks,but are not used as carriers of information in RF networks

Analog signal are used as a carriers of information in RF networks





RF Range and speed Factors

To effectively implement an RF based network,such as Wi-Fi or Wimax,you need to understand the factors that impact the distances or range RF signals can travel and bandwith available for communication

1. Line of Sight


Line of Sight ( LOS ) is the seemingly straight line from the object in view ( Transmitter ) to the observer eye ( receiver ) / or in general we can summarize Line of Sight is a Distances between Transmitter and object





















2. Fresnel Zone

The Fresnel ( Pronounced fra-nel ) zone is an area centered on the visible LOS between the transmitting and receiving antenna and was discovered by Augustin Jean Fresnel

This fresnel zones comes about because an antenna does not transmit RF waves in a laser type beam, but rather the RF Waves propagate ( Or Spread ) as they travel through the air


Interference Devices

In addition to the interference caused by objects in the environment where RF communication are taking place,device can cause interference by generating RF noise or signals.many objects use RF technology as a form of communications and other use RF energy for alternative purposes

The most common of these include the following :

 Microwave Ovens
 Elevators
 Baby Monitors
 Spread Spectrum phone
 Bright Sunlight
 Weather

Gain and losses

Radio frequency communication occur within many different environments,and these various environments introduce problem and advantages for the RF signals

what is generally considered an advantages in RF communication is called gain and that which is usually seen as a problem is called loss .

Gain

Amplitude is the measurement of the change in RF energy caused by a passing RF Wave.this is much like the voltage level in an electrical signal is more likely to have a strong signal strength at the receiver.An increase in amplitude of the RF signal is known as GAIN


Gain is either or passive.active gain is achived by adding an amplifier in-line between the RF signal generator,such as wireless access point,and the propragatin antenna.adding an amplifier increase the signal strength in a literal way in that the signal is actually stronger.

Passive gain does not actually increase the signal strength,but directs it.passive gain is achieved by using semidirectional or highly directional antennas to focus the energy of the signal in a more specific direction

Loss

When a signal strength is weakend,we refer to it as loss.loss is decrease in the signal amplitude.this signal loss can occur within cabling of the wireless device and infrastructure and when antenna sends the signal through the air as RF waves.

When this loss occurs in the cabling and connector the temperature of the cables will increase generally

The AC Eletrical signal is absorbed as it travels on the wire ( the signal travels the wire as an AC electrical signal and is converted to an RF wave by antenna) because of resistance

RF Behaviour

1. Reflection











2. Refraction



3. Diffraction















4. Scattering


5. Absorption

RF and Infrared Technologies

There are many different technologies that utilize RF Communication

The major differences among them are the frequencies and encoding techniques they use

These technologies Include the following :

 DSSS
 OFDM
 FHSS
 INFRARED
 MIMO

DSSS (Direct sequence spread spectrum ) is the technology used in 802,11 and 802.11b devices,providing data rates of 1,2 , 5,5 , and 11 Mbps

Because backward compatability is provided,802.11b devices operating at 5,5 or 11 Mbps can communicate with 802.11 devices running at 1 or 2 Mbps

Of course these devices are limited to the speed of the slower device in the link.the 802.11 802.11b standards specify that DSSS is to be used in the 2,4 Ghz spectrum of RF technologies

DSSS system are assigned channel for communications.these channel are blocks of frequency space or frequency changes.

DSSS systems are configured to use the same frequencies permanently,so frequencies do not change during communications


OFDM

802.11a utilizes a different technology for communications known as Orthogonal Frequency Division Multiplexing or OFDM to avoid the mouth full.

OFDM is also used in 802.11g with modifications for backward compatability with 802.11b use of DSSS.

Unlike DSSS,which uses the full channel as a single “Pipe”,OFDM divides a channel into subcarriers or subchannel and sends data streams on the “Separate pipe”.this provides for greater resistance to multipath interference and allows for greater bandwith

FHSS

Frequency Hopping spread spectrum (FHSS) is another spread spectrum technologyu,such as DSSS,that is used by older 802.11 devices and bluetooth devices as well as some RFID Units.

FHSS is limited to 1 or 2 Mbps specification,and as you might guess by the name,FHSS systems hop from one frequency to another during communications.these
This helps to avoid the problem caused by narrow-band interference

Infrared

The 802.11 standards includes a PHY ( Physcal Layer ) specification for infrared communications.

MIMO ( Multiple Input,multiple Output )

Devices allow the use more than one anthena at the same time by using multiple data streams in the same channel via “ Smart Antennas”

MIMO is expected to be a part of the new 802.11n standard for high throuhput communications