Spread spectrum radio

CDMA trải phổ spread spectrum

COMMUNICATIONS 104 • DECEMBER 2005 • ELECTRONICS FOR YOU WWW.EFYMAG.COM CMYK to integrated bar code scanner/ palmtop computer/radio modem de- vices for warehousing, digital dispatch, digital cellphone communications, and ‘information society’ city-, area-, state- or country-wide networks for transmit- ting facsimile, computer data, e-mail or multimedia data. History of spread spectrum Early spark-gap wireless transmitters actually used spread spectrum, since their RF bandwidths were much wider than their information bandwidth. The first intentional use of spread spectrum was probably by Armstrong in the late 1920s or early 1930s with wide-band frequency modulation (FM). However, the real impetus for spread spectrum came with World War II. Both the allies and the axis powers experimented with simple spread-spec- trum systems. Much of what was done is still shrouded in secrecy, however. The first publicly available patent A dvances in technology have brought to us a new form of digital radio service called ‘spread spectrum.’ First developed by the military as a deterrent to jamming and eavesdropping (espionage), the spread-spectrum technique handles ra- dio signal differently from other forms of digital radio. In spread-spectrum operation, the radio signal is spread across a great bandwidth with the use of a spread- ing algorithm based upon a pseudo- noise (PN) code, or a number that each unit of the system is programmed with. The result is a signal that is es- sentially ‘buried’ in the noise floor of the radio band. The receiver is programmed to ex- amine the bandwidth of the spread sig- nal and correlate the data (despread it). The process of correlation also causes any other signal received to be spread as the wanted signal is despread. This causes unwanted signals which appear as noise. The result is a signal that is extremely difficult to detect, does not interfere with other services and still passes a great bandwidth of data. Spread spectrum, the art of secure digital communications, is now being exploited for commercial and indus- trial purposes. In the future, hardly anyone will escape being involved, in some or the other way, with spread- spectrum communications. Commercial applications for spread spectrum range from wireless PC-to-PC local area networks (LANs)  D. PRABAKARAN on spread spectrum came from Hedy Lamarr, the Hollywood movie actress, and George Antheil, an avant-garde music composer. This patent was granted in 1942, but the details were a military secret for many years. The in- ventors never realised a dime for their invention; they simply turned it over to the US government for use in the war effort, and commercial use was delayed until the patent had expired. Most of the work done in spread spectrum throughout the 1950s, 1960s and 1970s was heavily backed by the military and drowned in secrecy. The global positioning system (GPS) is now the world’s single largest spread- spectrum system. Most of the details on GPS are now public information. Spread spectrum was first used for commercial purposes in the 1980s when Equatorial Communications of Mountain View, CA, used direct se- quence for multiple-access communi- cations over synchronous satellite tran- sponders. In the late 1980s, the US Fed- SPREAD-SPECTRUM TECHNOLOGY AND ITS APPLICATIONS In the future, hardly anyone will escape being involved, in some or the other way, with spread-spectrum—the art of secure digital communications Fig. 1: Spread-spectrum signals are hard to detect on narrow-band equipment because the signal’s energy is spread over a bandwidth of maybe 100 times the information bandwidth COMMUNICATIONS 106 • DECEMBER 2005 • ELECTRONICS FOR YOU WWW.EFYMAG.COM CMYK eral Communications Commission (FCC) opened up the industrial, scien- tific and medicine (ISM) frequency bands for unlicenced spread-spectrum communications. Applications Typical applications for spread-spec- trum radio are: 1. Cellular/PCS base station inter- connect 2. Last-mile obstacle avoidance 3. Private networks 4. Railroads and transportation 5. Utilities like electricity, oil, gas and water 6. Banks, hospitals, universities and corporations 7. Disaster recovery and special- event PSTN extensions 8. TELCO bypass 9. Rural telephony 10. Videoconferencing 11. LAN/WAN/Internet connec- tion How spread spectrum works The term ‘spread spectrum’ describes a modulation technique that makes the sacrifice of bandwidth in order to gain signal-to-noise (S/N) performance. Ba- sically, in a spread-spectrum system, the transmitted signal is spread over a frequency much wider than the mini- mum bandwidth required to send the signal. The fundamental premise is that in channels with narrow-band noise, in- creasing the transmitted signal band- width results in an increased probabil- ity that the received information will be correct. If total signal power is in- terpreted as the area under the spec- tral density curve, signals with equiva- lent total power may have either a large signal power concentrated in a small bandwidth or a small signal power spread over a large bandwidth. Spread signals are intentionally made to be much wider-band than the information they are carrying to make them more noise-like. Because spread- spectrum signals are noise-like, they are hard to detect. They are also hard to intercept or demodulate. Further, spread-spectrum signals are harder to jam (interfere with) than narrow-band signals. The low probability of inter- cept and anti-jam features are the rea- sons why the military has used spread spectrum for so many years. Spread-spectrum signals use fast codes that run many times the infor- mation bandwidth or data rate. These special ‘spreading’ codes are called ‘pseudo random’ or ‘pseudo noise’ codes. They are called ‘pseudo’ be- cause they are not real Gaussian noise. The use of special pseudo-noise codes in spread-spectrum communica- tions makes signals appear wide-band and noise-like. It is this very charac- teristic that makes spread-spectrum signals possess the quality of low prob- ability of intercept. Spread-spectrum transmitters use the same transmit power levels as nar- row-band transmitters. Because spread-spectrum signals are very wide, they transmit at a much lower spec- tral power density, measured in watts per hertz, than narrow-band transmit- ters. The lower transmitted power den- sity characteristic gives spread signals a big plus. Spread and narrow-band signals can occupy the same band, with little or no interference. This ca- pability is the main reason for all the interest in spread spectrum today. Since the total integrated signal density or signal-to-noise ratio (SNR) at the correlator’s input determines whether there will be interference or not, all spread-spectrum systems have a threshold or tolerance level of inter- ference beyond which useful commu- nication ceases. This tolerance or threshold is related to the spread-spec- trum processing gain. Processing gain is essentially the ratio of the RF band- width to the information bandwidth. Direct sequence and frequency hopping are the most commonly used methods for the spread-spectrum tech- nology. Although the basic idea is the same, these two methods have many distinctive characteristics that result in completely different radio perfor- mances. The carrier of direct-sequence ra- dio stays at a fixed frequency. The nar- row-band information is spread out into a much larger (at least ten times) bandwidth by using a pseudo-random chip sequence. Generation of the direct-sequence spread-spectrum signal (spreading) is shown in Fig. 2. The narrow-band sig- nal and the spread-spectrum signal both use the same amount of transmit power and carry the same information. However, the power density of the spread-spectrum signal is much lower than the narrow-band signal. (The power density is the amount of power over a certain frequency.) As a result, it is more difficult to detect the pres- ence of the spread-spectrum signal. In this case, the narrow-band signal’s power density is ten times higher than the spread-spectrum signal, assuming the spread ratio as ‘10.’ At the receiving end, the spread- spectrum signal is despread to gener- ate the original narrow-band signal as shown in Fig. 3. If there is an interference jammer in the same band, it will be spread out during despreading. As a result, its impact is greatly reduced. This is the way the direct-sequence, spread-spec- trum radio fights the interference. It spreads out the offending jammer by the spreading factor, which is at least ‘10.’ In other words, the offending Fig. 2: Generation of the direct-sequence spread-spectrum signal (spreading) Fig. 3: At the receiving end, the spread- spectrum signal is despread to generate the original narrow-band signal COMMUNICATIONS 108 • DECEMBER 2005 • ELECTRONICS FOR YOU WWW.EFYMAG.COM CMYK jammer’s amplitude is greatly reduced by at least 90 per cent. Frequency hopping achieves the same result by using a different carrier frequency at a different time. Its carrier will hop around within the band, so it will avoid the jammer at some frequen- cies. The frequency hopper is more popular and the only way to survive in the 2.45GHz band because the leak- ages from the microwave oven (from 2.4 to 2.5 GHz) sometimes exceed 10W. Frequency hopper is not needed in the 915MHz band because there is no legal big jammer in this frequency. The frequency-hopping technique is shown in Fig. 5. It does not spread the signal, so there is no processing gain. The processing gain is the in- crease in power density when the sig- nal is despread and it improves the received signal’s SNR. In other words, the frequency hopper needs to put out more power in order to have the same SNR as a direct-sequence radio. The frequency hopper is also more difficult to synchronise the receiver to the transmitter because both the time and frequency need to be in tune. Whereas, in a direct-sequence radio, only the timing of the chips needs to be synchronised. The frequency hopper will need to spend more time to search the signal and lock to it. As a result, the latency time is usually longer. Whereas, a direct-sequence radio can lock in the chip sequence in just a few bits. Usually, to make the initial synchronisation possible, the frequency hopper will park at a fixed frequency before hopping or communication be- gins. If the jammer happens to locate at the same frequency as the parking fre- quency, the hopper will not be able to hop at all. And once it hops, it will be very difficult to re-synchronise if the receiver ever lost the sync. The hopper usually costs more and is more complicated than direct-se- quence radio because it needs extra hopping and synchronising circuits to implement the synchronisation algo- rithm. The frequency hopper, however, is better than direct-sequence radio when dealing with multipath. This is because the hopper does not stay at the same frequency and a null at one frequency is usually not a null at another fre- quency if it is not very close to the original frequency. So a hopper can usually survive multipath better than direct-sequence radio. The frequency hopper can usually carry more data than direct-sequence radio because the signal is narrow-band. When two signals collide, the stronger one may survive regardless of the kind of signal. In this band, all radio must not exceed the power den- sity limit set by the FCC. In other words, all radios are equal when in- terfering with one another. The best strategy to prevent interference is to make the important radios close to each other (strengthen the link) and prevent using frequency hopper be- cause they are guaranteed to interfere with other radios. The hopper itself will also suffer when it interferes with other radio. Any system that can suffer more data loss will survive better. In general, a voice system can survive an error rate as high as 10 –2 , while a data system must have an error rate lower than 10 –4 . Voice sys- tem can tolerate more data loss because the human brain can ‘guess’ between the words while a dumb microproces- sor can’t. As a result, the frequency hopper is more popular for voice than data communications. Advantages of spread- spectrum wireless systems Some of the advantages of spread- spectrum wireless systems over con- ventional systems are: 1. No crosstalk interference. Con- ventional cordless phones frequently suffer from crosstalk interference, es- pecially when used in densely popu- lated residential areas (such as apart- ment complexes). This problem disap- pears in spread-spectrum cordless phone systems because: (i) Crosstalk interference is greatly attenuated due to the processing gain of the spread-spectrum system as de- scribed earlier. (ii) The effect of the suppressed crosstalk interference can be essentially removed with digital processing where noise below certain threshold results in negligible bit errors. These negli- gible bit errors will have little effect on voice transmissions. 2. Better voice quality/data integ- rity and less static noise. Due to the processing gain and digital processing nature of spread-spectrum technology, a spread-spectrum based system is more immune to interference and noise. This greatly reduces the static noise induced by consumer electron- ics devices that is commonly experi- enced by conventional analogue wire- less system users. 3. Lowered susceptibility to multipath fading. Because of its inher- ent frequency diversity properties (thanks to wide spectrum spread), a spread-spectrum system is much less Fig. 4: Direct-sequence spread-spectrum signal Fig. 5: Frequency hopping COMMUNICATIONS 110 • DECEMBER 2005 • ELECTRONICS FOR YOU WWW.EFYMAG.COM CMYK susceptible to multipath fading. This makes reception of a spread-spectrum based cordless phone much less sensi- tive to the location and pointing di- rection of the handset than a conven- tional analogue wireless system. 4. Inherent security. In a spread- spectrum system, a PN sequence is used to either modulate the signal in the time domain (direct sequencing) or select the carrier frequency (fre- quency hopping). Due to the pseudo- random nature of the PN sequence, the signal in the air is ‘randomised.’ Only a receiver that has exactly the same pseudo-random sequence and syn- chronous timing can despread and re- trieve the original signal. Conse- quently, a spread-spectrum system provides signal security that is not available to conventional analogue wireless systems. 5. Co-existence. A spread-spectrum system is less susceptible to interfer- ence than other non-spread-spectrum systems. In addition, with proper de- signing of pseudo-random sequences, multiple spread-spectrum systems can coexist without causing severe inter- ference to other systems. This further increases the system capacity for spread-spectrum systems or devices. 6. Longer operating distances. A spread-spectrum device operated in the ISM band is allowed to have higher transmit power due to its non-inter- fering nature. Because of the higher transmit power, the operating distance of such a device can be significantly longer than for a traditional analogue wireless communication device. 7. Hard to detect. Spread-spectrum signals are transmitted over a much wider bandwidth than conventional narrow-band transmissions—20 to 254 times the bandwidth of narrow-band transmissions. Since the communica- tion band is spread, these can be trans- mitted at a low power without suffer- ing interference from background noise. This is because when despreading takes place, the noise at one frequency is rejected, leaving the desired signal. 8. Hard to intercept or demodulate. The very foundation of the spreading technique is the code used to spread the signal. Without knowing the code, it is impossible to decipher the trans- mission. Also, because the codes are so long (and quick), simply viewing the code would still be next to impos- sible to solve the code, hence intercep- tion is very hard. 9. Harder to jam than narrow bands. The most important feature of the spread-spectrum technique is its ability to reject interference. At first glance, it may be considered that spread-spectrum transmission would be most affected by interference. How- ever, any signal is spread in the band- width, and after it passes through the correlator, the bandwidth signal is equal to its original bandwidth plus the bandwidth of local interference. An interference signal with 2MHz bandwidth being input into a direct- sequence receiver whose signal is 10MHz wide gives 12MHz output from the correlator. The wider the in- terference bandwidth, the wider the output signal. Thus the wider the in- put signal, the less the effect on the system because the power density of the signal after processing is lower, and less power falls in the band-pass filter. Conversely, it may be guessed that the most effective interference to a direct-sequence receiver is one with the narrow- est bandwidth (a continuous- wave carrier). This is the most effective because power density in the correlator output due to narrow-band signals is higher than due to wide-band signals. 10. Use for ranging and ra- dar. The spread-spectrum technique can be used to construct precise rang- ing and radar systems. The spread car- rier, modulated with the pseudo noise sequence, permits the receiver to mea- sure very precisely the time when the signal was sent; thus, spread-spectrum technique can be used to time the dis- tance to an object, as in the case of radar reflection. Both applications have been commonly used in the aerospace field for many years. Modulation For direct-sequence systems, the en- coding signal is used to modulate a carrier, usually by phase-shift keying (e.g., biphase or quadriphase) at the code rate. Frequency-hopping systems gener- ate their wide band by transmitting at different frequencies, hopping from one frequency to another according to the code sequence. Typically, such a system may have a few thousand fre- quencies to choose from, and unlike direct-sequence signal, it has only one output rather than symmetrically dis- tributed outputs. The important thing to note is that Fig. 6: Frequency hopping Fig. 7: FHSS spectrum COMMUNICATIONS 112 • DECEMBER 2005 • ELECTRONICS FOR YOU WWW.EFYMAG.COM CMYK both direct sequencing and frequency hopping generate wide-band signals controlled by the code-sequence gen- erator. For direct- sequence systems the code is direct-carrier modulation, while frequency hopping commands the carrier frequency. Considering direct sequencing, bal- ance modulation is an important tool in any suppressed carrier system, used to generate the transmitted signal. Bal- anced modulation helps to hide the signal, and there is no power wasted in transmitting a carrier that would contribute to interference rejection or information transfer. When a signal has poor balance in either code or carrier, spikes are seen in its spec- trum. With these spikes, or spurs, the signal is easily detectable, since once these spikes are noticed above the noise, it is obvious to look for the hidden signal. Information modulation in spread- spectrum systems is possible in most of the conventional ways; both ampli- tude modulation (AM) and angle modulation are satisfactory. Normally, AM is not used for spread-spectrum signals because it tends to be detect- able when examining the spectrum. Frequency modulation (FM) is more useful because it is a constant enve- lope signal, but information is still readily observed. In both AM and FM, no knowledge of the code is needed to receive the transmitted information. Clock modulation is actually fre- quency modulation of the code clock. It is usually avoided (e.g., for fre- quency hopping) because the loss in correlation due to phase slippage be- tween received and local clocks can degrade the performance. For direct sequence, an FM demodulator tuned to radio frequency (RF) carrier plus/ minus the clock could recover the data. Another technique is code modifi- cation, where the code is changed such that the information is embedded in it, then modulated by phase transitions on an RF carrier. Demodulation Once the signal is coded, modulated and then sent, the receiver must de- modulate the signal. This is usually done in two steps: 1. Spectrum-spreading (e.g., direct- sequence or frequency-hopping) modulation is removed. 2. The remaining information-bear- ing signal is demodulated by multi- plying with a local reference identical in structure and synchronised with the received signal. Coding technique in spread spectrum In order to transmit anything, codes used for data transmission have to be considered. However, here we will not discuss the coding of information (like error-correction coding) but codes that act as noise-like carriers for the infor- mation being transferred. These codes are much longer than those for the usual areas of data transfer, as these are intended for bandwidth spreading. Coding can be of three types: 1. Maximal sequences 2. Composite code sequences 3. Error detection and correction codes (EDACs) The properties of codes used in spread-spectrum systems are: 1. Protection against interference. Coding enables a bandwidth trade, for processing gain against interfering sig- nals. 2. Provision for privacy. Coding enables protection of signals from eavesdropping, so even the code is se- cure. 3. Noise-effect reduction. Error-de- tection and correction codes can reduce the effects of noise and interference. One such coding method is maxi- mal sequences. Maximal codes can be generated by a given shift register or a delay element of given length. In bi- nary shift register sequence generators, the maximum sequence length is 2 n -1 chips, where ‘n’ is the number of stages in the shift register. A shift register generator consists of a shift register in conjunction with appropriate logic, which feeds back a logical combination of the state of two or more of its stages to its input. The output, and contents of its ‘n’ stages at any clock time, is a function of the outputs of the stages fed back at the proceeding sample time. Some codes can be 7 to 2 36 –1 chips long. Use of EDACs is mandatory for fre- quency-hopping systems to overcome the high rates of error induced by par- tial band jamming. These codes’ use- fulness has a threshold that must be exceeded before satisfactory perfor- mance is achieved. In direct-sequence systems, EDAC is not advisable because of the effect it has on the code, increasing the appar- ent data transmission rate, and may increase the jamming threshold. Some demodulators can operate error detec- tion with approximately the same ac- curacy as an EDAC, so it may not be worthwhile to include a complex cod- ing/decoding scheme in the system. Applications of spread spectrum Wireless local area network (WLAN). A WLAN is a flexible data communi- cation system implemented as an ex- tension to or an alternative for a wired local area network. WLANs transmit and receive data over the air, minimising the need for wired connec- tions. Thus, WLANs combine data con- nectivity with user mobility and en- able movable LANs. Most WLAN systems use spread- spectrum technique (both frequency hopping and direct sequence). WLANs are being used in health care, retail, manufacturing, warehousing, aca- demic and other arenas. These indus- tries have profited from the produc- tivity gains of using handheld termi- In CDMA spread-spectrum transmission, user channels are created by assigning different codes to different users. This type of system provides privacy by controlling distribution of user-unique code sequences. ELECTRONICS FOR YOU • DECEMBER 2005 • 113WWW.EFYMAG.COM CMYK COMMUNICATIONS nals and notebook computers to trans- mit real-time information to centralised hosts for processing. WLANs offer productivity, conve- nience and cost advantages over wired networks. Space systems. In space stations, which are continuously accessible to interference, spread-spectrum methods have proved effective. This is espe- cially true for communication satellites. In general, satellites do not employ processing on-board as it adds to the complexity and would limit the num- ber of satellite users. A simple repeat- ing satellite is used, so all the spread- spectrum modulation and demodula- tion must be done on the ground. With no on-board processing, the satellite is forced to transmit an uplink interfer- ence signal, which reduces the space- craft transmitter power to send the de- sired signal. Another disadvantage of no on-board processing is that every receiver would have to acquire a spread-spectrum demodulator. Global positioning system (GPS). GPS is a satellite-based navigation sys- tem developed and operated by the US Department of Defense. The idea be- hind GPS is to transmit spread-spec- trum signals that allow range measure- ment from an unknown satellite loca- tion. With knowledge of the transmit- ter location and the distance to the sat- ellite, the receiver can locate itself on a sphere whose radius is the distance measured. After receiving signals and making range measurement on other satellites, the receiver can calculate its position based on the intersection of several spheres. GPS permits users to determine their 3-D position, velocity and time. This service is available for military and commercial users round the clock, in all weather, anywhere in the world. GPS uses NAVSTAR (NAVigation Satellite Timing And Ranging) satel- lites. The constellation consists of 21 operational satellites and three active spares. This provides a GPS receiver with four to twelve usable satellites ‘in view’ at any time. A minimum of four satellites allow the GPS card to com- pute latitude, longitude, altitude and GPS system time. The NAVSTAR sat- ellites orbit the earth at an altitude of 10,898 Nautical miles in six 55-degree orbital planes, with four satellites in each plane. The orbital period of each satellite is approximately 12 hours. The GPS satellite signal contains information to identify the satellite, as also positioning, timing, ranging data and satellite status. The satellites are identified by the space vehicle num- ber or the pseudo-random code num- ber. They transmit on two L-band fre- quencies: 1.57542 GHz (L1) and 1.22760 GHz (L2). The L1 signal has a sequence encoded on the carrier frequency by a modulation technique that contains two codes, a precision (P) code and a course/acquisition (C/A) code. The L2 code contains only P code, which is encrypted for military and authorised commercial users. Personal communications The advantages of using spread spec- trum in data and voice communica- tions are: 1. Spread-spectrum signals can be overlaid onto bands where other sys- tems are already operating, with mini- mal performance impact to or from the other systems. 2. The anti-interference character- istics of spread-spectrum signals are important in environments where sig- nal interference can be harsh, such as networks operating on manufacturing floors. 3. Cellular systems designed with code-division multiple-access (CDMA) spread-spectrum technology offer greater operational flexibility and pos- sibly a greater overall system capacity than systems built on frequency-divi- sion multiple-access (FDMA) or time- division multiple-access (TDMA) methods. 4. The anti-mutipath characteristics of spread-spectrum signaling and re- ception techniques are desirable in ap- plications where multipath is likely to be prevalent. For these reasons, many companies have begun developing spread-spec- trum systems. Voice-orientated digital cellular and personal communication service providers are using CDMA. CDMA implemented with direct- COMMUNICATIONS 114 • DECEMBER 2005 • ELECTRONICS FOR YOU WWW.EFYMAG.COM CMYK sequence spread-spectrum signaling is among the most promising multiplex- ing technologies for cellular telephony services. The advantages of direct-sequence spread-spectrum signaling for these services include superior operation in multipath environments, flexibility in allocation of channels, privacy and the ability to operate asynchronously. Also among the attractive features of CDMA spread-spectrum is the ability to share bandwidth with narrow-band communication without undue degra- dation of either system’s performance. In CDMA spread-spectrum trans- mission, user channels are created by assigning different codes to different users. This type of system provides pri- vacy by controlling distribution of user-unique code sequences. Spread-spectrum systems exhibit unique qualities that cannot be ob- tained from conventional narrow-band systems. There are many research av- enues exploring these unique qualities. Spread-spectrum technology can alleviate the problems of conventional cordless telephones. However, because the technology was initially developed for military applications, it could not be readily applied for commercial use due to its high cost and large size. As this technology and the com- ponents continue to develop, inte- grated circuit (IC) technology has un- dergone drastic advancement. This has made commercial use of spread-spec- trum technology a realistic proposi- tion. In conjunction with increased cordless phone usage, data applica- tions are also increasingly finding their way into homes and small offices. This is due in part to the maturity of multi- media technologies and applications and arrival of the information era where global information through powerful networking vehicles (such as the Internet) is penetrating many homes and offices. These developments have created demands for wireless data for homes and small offices where wiring can of- ten be either very costly or very in- convenient. ISM-band spread-spec- trum devices can be designed to ad- dress the wireless data needs of home and small-office users. The road ahead The factors that will determine the com- mercial success of spread-spectrum technology are its maturity, advance- ment of baseband as well as RF IC tech- nologies, and system integration to of- fer the best value to end users. As these elements continue to ad- vance, spread-spectrum technology will find more and more commercial applications ranging from cordless te- lephony to wireless LAN and wireless data, digital cellular telephony and even personal communication services. The end users will be the ultimate ben- eficiaries as the quality of these prod- ucts improves while the cost contin- ues to decline. z The author is a lecturer in mechanical engineer- ing at N.L. Polytechnic College, Mettupalayam, Tamil Nadu