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What is spread spectrum? • Methods by which energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. • used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference and jamming, and to prevent detection.
Why do we need this? • In some situations it is required that a communication signal be difficult to detect, and difficult to demodulate even when detected. Here the word ‘detect’ is used in the sense of ‘to discover the presence of’. • In other situations a signal is required that is difficult to interfere with, or ‘jam’.
Commercial use • Initial commercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System's very small aperture (VSAT) satellite terminal system for newspaper newswire services, Del Norte Technology's radio navigation system for navigation of aircraft for crop dusting and similar applications, and Qualcomm's OmniTRACS system for communications to trucks
Spread-spectrum telecommunications • a technique in which a (telecommunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information. • a signal structuring technique that employs direct sequence, frequency hopping or a hybrid of these, which can be used for multiple access and/or multiple functions. • This technique decreases the potential interference to other receivers while achieving privacy.
Forms of spread spectrum • Frequency-hopping spread spectrum(FHSS) • direct-sequence spread spectrum (DSSS) • time-hopping spread spectrum (THSS) • chirp spread spectrum (CSS) and combinations of these techniques
Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generators . What is pseudorandom number sequences ? >is a sequence of numbers that has been computed by some defined arithmetic process but is effectively a random number sequence for the purpose for which it is required. To determine and control the spreading pattern of the signal across the allotted bandwidth
Benefits • Resistance to intended or unintended jamming. • Sharing of a single channel among multiple users. • Determination of relative timing between transmitter and receiver.
Uses • The United States GPS, Russian Glonass, and European Galileo satellite navigation systems. • CDMA cellular phones. • Cordless phones operating in the 2.4 and 5.8 GHz bands. • The 802.11 and 802.11b Wi-Fi standards. (The faster modes in 802.11g use OFDM, not spread spectrum, although it can fall back to the slower 802.11b modes.) • ZigBee / 802.15.4 • Automatic meter reading
Spread Spectrum modulation techniques present two major advantages for Wireless Local Area Networks (WLAN):(ranges allocated for use with Spread Spectrum technology is 2.4 GHz to 2.4835 GHz)
Low power density relates to the fact that the transmitted energy is spread over a wide band, and therefore, the amount of energy per specific frequency is very low. The effect of the low power density of the transmitted signal is that such a signal will not disturb (interfere with) the activity of other systems' receivers in the same area.
Redundancy relates to the fact that the message is (or may be) present on different frequencies from where it may be recovered in case of errors. The effect of redundancy is that Spread Spectrum systems present a high resistance to noises and interference, being able to recover their messages even if noises are present on the medium.
A Comparison of Frequency Hopping and Direct Sequence Spread Spectrum
Why we need to compare the performance of this technology? (WLAN’s) • possibility to collocate systems • noise and interference immunity • operation in environments generating radio reflections • data transfer capacity (throughput) • size • power consumption (relevant for battery based note books) • price.
Direct Sequence Spread Spectrum > Modulationtechnique ,Also known as direct sequence code division multiple access (DS-CDMA) > The name 'spread spectrum' comes from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency. >A RF carrier and pseudo-random pulse train are mixed to make a noise like wide-band signal.
Direct sequence contrasts with the other spread spectrum process, known as frequency hopping spread spectrum, in which a broad slice of the bandwidth spectrum is divided into many possible broadcast frequencies. • In general, frequency-hopping devices use less power and are cheaper, but the performance of DS-CDMA systems is usually better and more reliable.
Frequency Hopping spread spectrum • It is the repeated switching of frequencies during radio transmission, often to minimize the effectiveness of "electronic warfare" - that is, the unauthorized interception or jamming of telecommunications. • It also is known as frequency- hopping code division multiple access (FH-CDMA).
Transmitting on one frequency for a certain time, then randomly jumping to another, and transmitting again. • FH-CDMA devices use less power and are generally cheaper, but the performance of DS-CDMA systems is usually better and more reliable. • The biggest advantage of frequency hopping lies in the coexistence of several access points in the same area, something not possible with direct sequence.
Frequency hopping has two benefits. Electrical noise—random electromagnetic signals which are not part of any communications signal—will only affect a small part of the signal. Also, the effects of any other forms of radio communications operating in narrow bands of the spectrum will be minimized. Any such interference that occurs will result in only a slightly reduced quality of voice transmission, or a small loss of data. Since data networks acknowledge successful receipt of data, any missing pieces will trigger a request to transmit the lost data.
Hybrid DS/FH Spread Spectrum • The DS/FH Spread Spectrum technique is a combination of direct-sequence and frequency-hopping. One data bit is divided over frequency-hop channels (carrier frequencies). In each frequency-hop channel one complete PN-code of length is multiplied with the data signal (see figure, where is taken to be 5).
As the FH-sequence and the PN-codes are coupled, an address is a combination of an FH-sequence and PN-codes. To bound the hit-chance (the chance that two users share the same frequency channel in the same time) the frequency-hop sequences are chosen in such a way that two transmitters with different FH-sequences share at most two frequencies at the same time (time-shift is random).
Additional • CDMA use technique Spread Spectrum How spread spectrum generated
Additional • occupies a larger bandwidth than necessary • use of a code which is independent of the transmitted data
Additional Direct Sequence Data signal multiplied by Pseudo Random Noise Code(PN Code) • Low cross-correlation value • Anti-jamming • Main problem: Near-Far effect • In cellular, it can do power control by BS • In non-cellular, it need Frequency Hopping
Additional Frequency Hopping
Additional • Disadvantage: • a high processing-gain is hard. There is need for a frequency-synthesizer able perform fast-hopping over the carrier-frequencies.
