1. Spread Spectrum
L.Tahani Aljehani

2. SPREAD SPECTRUM
Spread spectrum is an important form of encoding for wireless communications.
The use of spread spectrum makes jamming and interception more difficult and provides improved reception.
The basic idea of spread spectrum is to modulate the signal so as to increase the bandwidth (spread the spectrum) of the signal to be transmitted.

3. SPREAD SPECTRUM
The spread spectrum technique was developed initially for military and intelligence requirements.
The essential idea is to spread the information signal over a wider bandwidth to make jamming and interception more difficult.
The first type of spread spectrum developed is known as frequency hopping.
A more recent type of spread spectrum is direct sequence.
Both of these techniques are used in various wireless communications standards and products.

4. SPREAD SPECTRUM
Spread spectrum achieves its goals through two principles :
1. The bandwidth allocated to each station needs to be, larger than what is needed.
2. The expanding of the original bandwidth B to the bandwidth Bss must be done by aprocess that is independent of the original signal.
In other words, the spreading process occurs after the signal is created by the source

5. SPREAD SPECTRUM
After the signal is created by the source, the spreading process uses a spreading code and spreads the bandwidth
spreading code is a series of numbers that look random, but are actually a pattern.

6. SPREAD SPECTRUM Spread spectrum is a communication technique that
spreads a narrowband communication signal over a wide range
of frequencies for transmission then de-spreads it into the
original data bandwidth at the receive.
Spread spectrum is characterized by:
wide bandwidth and
low power
Jamming and interference have less effect on Spread
spectrum because it is:
Resembles noise
Hard to detect
Hard to intercept

7. Narrowband vs Spread Spectrum

8. 18-Sep-18
Networks and Communication Department

9. SPREAD SPECTRUM
Input is fed into a channel encoder that produces an analog signal with a relatively narrow bandwidth around some center frequency.
This signal is further modulated using a sequence of digits known as a spreading code or spreading sequence.
On the receiving end, the same digit sequence is used to demodulate the spread spectrum signal.
Finally, the signal is fed into a channel decoder to recover the data.
18-Sep-18
Networks and Communication Department

10. Narrow Band vs Spread Spectrum
Uses only enough frequency spectrum to carry the signal
High peak power
Easily jammed
Spread Spectrum
The bandwidth is much wider than required to send to the signal.
Low peak power
Hard to detect
Hard to intercept
Difficult to jam

11. Spread Spectrum Use Spread Spectrum Use
In the 1980s FCC implemented a set of rules making Spread Spectrum available to the public.
Cordless Telephones
Global Positioning Systems (GPS)
Cell Phones
Personal Communication Systems
Wireless video cameras
Local Area Networks
Wireless Local Area Networks (WLAN)
Wireless Personal Area Network (WPAN)
Wireless Metropolitan Area Network (WMAN)
Wireless Wide Area Network (WWAN)
The FHSS modulation technique uses the available channels to transmit and receive data, but rather than staying on any one channel, it rapidly switches between channels using a pseudorandom pattern that is based on an initial key; this key is shared between the participants of the communication session.
If interference affects only a few of the channels, this interference is minimized because each channel is used only briefly. If the interference is broad, it can still affect all the channels that are in use. This modulation technique requires that the initial seed or key be shared, but after that has happened, it is very difficult to eavesdrop on.
IEEE wireless networks use this technique for modulation, while Bluetooth uses an adaptive version of this technique that stops using channels where interference or weak signals exist.
18-Sep-18
Networks and Communication Department

12. frequency-hopping spread spectrum FHSS
With frequency-hopping spread spectrum (FHSS), the signal is broadcast over random series of radio frequencies
Hopping from frequency to frequency at fixed intervals.
A receiver, hopping between frequencies in synchronization with the transmitter, picks up the message.
18-Sep-18
Networks and Communication Department

13. Frequency hopping spread spectrum
rather than staying on any one channel, it rapidly switches between channels using a pseudorandom pattern that is based on an initial key
this key is shared between the participants of the communication session.
18-Sep-18
Networks and Communication Department

14. General layout for FHSS
A pseudorandom code generator, called pseudorandom noise (PN), creates a k-bit pattern for every hopping
The frequency table uses the pattern to find the frequency to be used for this hopping
passes it to the frequency synthesizer
The frequency synthesizer creates a carrier signal of that frequency.
source signal modulates the carrier signal
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Networks and Communication Department

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Networks and Communication Department

16. FHSS Suppose we have decided to have eight hopping frequencies.
In this case, M is 8 and k is 3
code generator will create eight different 3-bit patterns
These are mapped to eight different frequencies in the frequency table
What is Encoding?
Encoding is mainly used in computers, and the process includes arranging a sequence of characters such as letters, punctuation, numbers and certain other symbols into a specialized format for the purpose of efficient transmission and storage. This is a common operation done in most wireless communication systems.
Generally, encoded data can be easily reversed by using the technique called decoding. ASCII (American Standard Code for Information Interchange, pronounced ASK-ee) is the encoding scheme widely used by computers for text files. Here, all the characters are encoded using numbers. For example, ‘A’ is represented using number 65, ‘B’ by number 66, etc. ASCII is also used to represent all the uppercase and lowercase alphabetic characters, numerals, punctuation marks, and other common symbols
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What is Modulation?
Modulation can be simply defined as a way of facilitating the transfer of information over a certain medium. For example, sound generated from our lungs, transmitted through the air can only travel for a limited distance depending on the amount of power we consume.
In order to extend the distance, a proper medium is required such as phone line or radio (wireless). This conversion process of voice to travel in such a medium is known as modulation. Modulation can be divided into two sub categories based on the modulation process.
Read more: 
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17. FHSS
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18. FHSS
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19. FHSS
FHSS systems include characteristics that are not included in any of the other modulation and communication technologies
These characteristics include:
dwell time,
hopping sequences,
and hop time
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Networks and Communication Department

20. Dwell time
The amount of time spent on a specific frequency in an FHSS hopping sequence is known as the dwell time.
The Dwell time per frequency is around 100 ms
Longer dwell time = greater throughput.
Shorter dwell time = less throughput
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Networks and Communication Department

21. Hopping Sequence
The hopping sequence is the list of frequencies through which the FHSS system will hop according to the specified dwell time.
This hopping sequence is also known as a hopping pattern or hopping set.
Every station in a Basic Service Set must use the same hopping sequence.
Every station must also store a table of all the hopping sequences that are used within the system.
18-Sep-18
Networks and Communication Department

22. Hop Time
Since hopping from one frequency to another is an action, it takes some duration of time. The duration of time required to hop from one frequency in the hopping sequence to the next is called the hop time.
Hop times are measured in microseconds (μs) and are commonly rated at 200−300 μs.
18-Sep-18
Networks and Communication Department

23. Disadvantages of FHSS Not as fast as a wired Lan or the newer WLAN
Standards
Lower throughput due to interference.
FHSS is subject to interference from other frequencies in
the ISM band because it hops across the entire frequency
spectrum.
Adjacent FHSS access points can synchronize
their hopping sequence to increase the number of colocated systems, however, it is prohibitively expensive.
18-Sep-18
Networks and Communication Department