What is frequency hopping spread spectrum ?

[Anek]

What is the concept of frequency hopping spread spectrum in the networking world ? Is it something to do it with the functioning of Bluetooth software in our mobile phones ? Can anyone provide me the information about this ?

thanks..

[Milton.J]

Definition :
Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequency-hopping code division multiple access (FH-CDMA) scheme.

A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:

1. Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background.
   
2. Spread-spectrum signals are difficult to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if they knew the pseudorandom sequence.
   
3. Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently.

[Spyrus]

Frequency-Hopping Spread-Spectrum (FHSS) is a spread spectrum modulation scheme that uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver.

Properly synchronized, they maintain a single logical channel.

To an unintended receiver, FHSS appears as short-duration impulse noise.

More simply, the data is broken down into packets and transmitted to the receiver of other devices over numerous "hop frequencies" (79 total) in a pseudo random pattern.

Only transmitters and receivers that are synchronized on the same hop frequency pattern will have access to the transmitted data.

The transmitter switches hop frequencies 1,600 times per second to assure a high degree of data security.

[Snake08]

Yes, the functionality of a Bluetooth software is based on the concept or algorithm of FHSS. Typically, the initiation of an FHSS communication is as follows-

• The initiating party sends a request via a predefined frequency or control channel.
• The receiving party sends a number, known as a seed.
• The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single base station to serve multiple connections.
• The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence.
• The communication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time.