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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
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SPREAD SPECTRUM 9 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 18 -Oct-21 Networks and Communication Department to recover the data.
Narrow Band vs Spread Spectrum Narrow Band 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 11 Spread Spectrum Use In the 1980 s 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) 18 -Oct-21 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 -Oct-21 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 -Oct-21 Networks and Communication Department
General layout for FHSS 14 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 -Oct-21 Networks and Communication Department
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FHSS 16 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 18 -Oct-21 Networks and Communication Department
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FHSS 19 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 -Oct-21 Networks and Communication Department
Dwell time 20 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 • 18 -Oct-21 Longer dwell time = greater throughput. Shorter dwell time = less throughput Networks and Communication Department
Hopping Sequence 21 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 -Oct-21 Networks and Communication Department
Hop Time 22 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 -Oct-21 Networks and Communication Department
Disadvantages of FHSS 23 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 and is Communication prohibitively 18 -Oct-21 Networks Department expensive.