Spread Spectrum L.Tahani Aljehani

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.

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.

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

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.

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

Narrowband vs Spread Spectrum

18-Sep-18 Networks and Communication Department

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

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

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 802.11 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

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

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

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 18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

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 Read more: http://www.differencebetween.com/difference-between-encoding-and-vs-modulation/#ixzz2tS3aEg2e 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: http://www.differencebetween.com/difference-between-encoding-and-vs-modulation/#ixzz2tS3lyzRr 18-Sep-18 Networks and Communication Department

FHSS 18-Sep-18 Networks and Communication Department

FHSS 18-Sep-18 Networks and Communication Department

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 18-Sep-18 Networks and Communication Department

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 18-Sep-18 Networks and Communication Department

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

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

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