Mobile and Wireless Communication

1. FHCC
MOBILE & WIRELESS NETWORKS
FREQUENCY HOPPING SPREAD SPECTRUM

2. OUTLINE
 CONVENTIONAL WIRELESS COMMUNICATION
 PROBLEM WITH CONVENTIONAL WIRELESS COMMUNICATION
 SPREAD SPECTRUM
 SPREAD SPECTRUM METHODS
 FREQUENCY HOPPING SPREAD SPECTRUM
 WORKING OF FHSS
 TYEPS OF FHSS
 ADVANTAGES OF FHSS
 ITS USES

3. CONVENTIONAL WIRELESS COMMUNICATION
 A conventional wireless signal has a frequency, usually specified in
megahertz (MHz) or gigahertz gigahertz), that does not change with time
(except for small, rapid fluctuations that occur as a result of modulation).
When you listen to a signal at 103.1 MHz on an FM stereo receiver, for
example, the signal stays at 103.1 MHz. It does not go up to 105.1 MHz or
down to 99.1 MHz. The digits on the radio's frequency dial stay the same
at all times. The frequency of a conventional wireless signal is kept as
constant as the state of the art will permit, so the bandwidth can be kept
within certain limits, and so the signal can be easily located by someone
who wants to retrieve the information.

4. PROBLEM WITH CONVENTIONAL WIRELESS
COMMUNICATION
There are at least two problems with conventional wireless communications
that can occur under certain circumstances.
 First, a signal whose frequency is constant is subject to catastrophic
interference. This occurs when another signal is transmitted on, or very
near, the frequency of the desired signal. Catastrophic interference can be
accidental (as in amateur-radio communications) or it can be deliberate
(as in wartime).
 Second, a constant-frequency signal is easy to intercept, and is therefore
not well suited to applications in which information must be kept
confidential between the source (transmitting party) and destination
(receiving party).

5. SPREAD SPECTRUM
 Spread spectrum is a form of wireless communications in which the
frequency of the transmitted signal is deliberately varied.
 This results in a much greater bandwidth than the signal would have if its
frequency were not varied.
 The advantages of spectrum spreading include
 Security
 Resistance to jamming and interception
 Coexistence of several signals – the receiver can
separate each user based on code
 No need of frequency planning as all user uses
same BW

6. SPREAD SPECTRUM METHODS
 Frequency hopping spread spectrum (FHSS)
 Direct sequence spread spectrum (DSSS)
 Time hopping spread spectrum (THSS)

7. FREQUENCY HOPPING SPREAD SPECTRUM
(FHSS)
 Hopping - very active or lively.
 Frequency-hopping spread spectrum (FHSS) transmission is the repeated
switching of frequencies during radio transmission to reduce interference
and avoid interception. It is useful to counter eavesdropping, or to
obstruct jamming of telecommunications. And it can minimize the effects
of unintentional interference.
 Switching Frequencies – changing of frequencies within a range
 Interception – action taken to prevent someone or something from continuing
to a destination.
 Eavesdropping - secretly listen to a conversation.

8. WORKING OF FHSS
 In FHSS, the transmitter hops between available narrowband frequencies
within a specified broad channel in a pseudo-random sequence known to
both sender and receiver. A short burst of data is transmitted on the
current narrowband channel, then transmitter and receiver tune to the
next frequency in the sequence for the next burst of data. In most systems,
the transmitter will hop to a new frequency more than twice per second.
Because no channel is used for long, and the odds (chances) of any other
transmitter being on the same channel at the same time are low, FHSS is
often used as a method to allow multiple transmitter and receiver pairs to
operate in the same space on the same broad channel at the same time.

9. FHSS ILLUSTRATION

10. FHSS ILLUSTRATION

11. FHSS ILLUSTRATION

12. TYPE OF FHSS
THERE ARE TWO TYPES OF FREQUENCY HOPPING:
1. SLOW FREQUENCY HOPPING SPREAD SPECTRUM
2. FAST FREQUENCY HOPPING SPREAD SPECTRUM

13. DIFFERENCET BETWEEN SLOW & FAST FHSS
TC – FREQUENCY SHIFT TIME
TS – DURATION OF SIGNLA ELEMENTS

14. ADVANTAGES
 IMPROVED INTERFERENCE REJECTION
 CODE DIVISION MULTIPLEXING FOR CDMA
 SECURE COMMUNICATION
 INCREASED CAPACITY AND SPECTRAL EFFICIENCY
 LOWER COST OF IMPLEMENTATION

15. USES
 IN MILITARY COMMUNICATION SYSTEMS
 UNAUTHORISED RECEIVERS CANNOT DETECT MESSAGE
 MILITARY RADIOS USES CRYPTOGRAPHIC TECHNIQUES TO GENERATE THE
CHANNEL SEQUENCE
 TRANSMISSION SECURITY KEY (TRANSEC)
 IN SATELLITE COMMUNICATION
 IN LOCAL AREA NETWORKS (LAN)