Direct Sequence Spread- Spectrum with Frequency Hopping

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. 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. 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 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’. 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 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 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. 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. This technique decreases the potential interference to other receivers while achieving privacy.

Forms of spread spectrum Frequency-hopping spread spectrum(FHSS) Frequency-hopping spread spectrum(FHSS) direct-sequence spread spectrum (DSSS) direct-sequence spread spectrum (DSSS) time-hopping spread spectrum (THSS) time-hopping spread spectrum (THSS) chirp spread spectrum (CSS) chirp spread spectrum (CSS) and combinations of these techniques

Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generators. 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. Resistance to intended or unintended jamming. Sharing of a single channel among multiple users. Sharing of a single channel among multiple users. Determination of relative timing between transmitter and receiver. Determination of relative timing between transmitter and receiver.

Uses The United States GPS, Russian Glonass, and European Galileo satellite navigation systems. The United States GPS, Russian Glonass, and European Galileo satellite navigation systems.Glonass CDMA cellular phones. CDMA cellular phones. Cordless phones operating in the 2.4 and 5.8 GHz bands. Cordless phones operating in the 2.4 and 5.8 GHz bands. The and b Wi-Fi standards. (The faster modes in g use OFDM, not spread spectrum, although it can fall back to the slower b modes.) The and b Wi-Fi standards. (The faster modes in g use OFDM, not spread spectrum, although it can fall back to the slower b modes.)Wi-Fi ZigBee / ZigBee / ZigBee Automatic meter reading 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 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. 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. 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 possibility to collocate systems noise and interference immunity noise and interference immunity operation in environments generating radio reflections operation in environments generating radio reflections data transfer capacity (throughput) data transfer capacity (throughput) size size power consumption (relevant for battery based note books) power consumption (relevant for battery based note books) price. price.

Direct Sequence Spread Spectrum > Modulation technique,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. 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. In general, frequency-hopping devices use less power and are cheaper, but the performance of DS-CDMA systems is usually better and more reliable.

DS Modulation DS Modulation

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 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). 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. 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. 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. The biggest advantage of frequency hopping lies in the coexistence of several access points in the same area, something not possible with direct sequence.

FH Modulation FH Modulation

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. 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 Communication System

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). 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). 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).

Hybrid System: DS/(F)FH

Additional CDMA use technique Spread Spectrum CDMA use technique Spread Spectrum How spread spectrum generated

Additional occupies a larger bandwidth than necessary occupies a larger bandwidth than necessary use of a code which is independent of the transmitted data 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 Low cross-correlation value Anti-jamming Anti-jamming Main problem: Near-Far effect Main problem: Near-Far effect In cellular, it can do power control by BS In cellular, it can do power control by BS In non-cellular, it need Frequency Hopping In non-cellular, it need Frequency Hopping

Additional Frequency Hopping

Additional Disadvantage: Disadvantage: a high processing-gain is hard. There is need for a frequency-synthesizer able perform fast-hopping over the carrier- frequencies. a high processing-gain is hard. There is need for a frequency-synthesizer able perform fast-hopping over the carrier- frequencies.