ППРЧ/FHSS -псевдослучайная перестройка рабочей частоты

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ППРЧ/FHSS -псевдослучайная перестройка рабочей частоты

milstar: The Milstar satellite provides enhanced communication security by frequency hopping -- *************************************************************************** a first for communication satellites. ***************************** http://www.spaceflightnow.com/titan/b35/030401milstar.html Programma Milstar -verojatno samaja dorogaja programma sputnikowoj swjazi Tolko w 1981 -1991 bolee 5mlrd $ , w segodnjaschnix cenax po otn VVP eto okolo 15 mlrd $ http://archive.gao.gov/d32t10/146911.pdf The first Milstar satellite was launched Feb. 7, 1994 aboard a Titan IV expendable launch vehicle. The second was launched Nov. 5, 1995. The third launch on April 30, 1999, placed the satellite in a non-usable orbit. The fourth through six satellites have a greatly increased capacity because of an additional medium data rate payload and were launched on Feb. 27, 2001, Jan. 15, 2002, and April 8, 2003 The Milstar system is composed of three segments: space (the satellites), terminal (the users) and mission control. Air Force Space Command's Space and Missile Systems Center at Los Angeles Air Force Base, Calif., developed the Milstar space and mission control segments. The Electronics Systems Center at Hanscom AFB, Mass., developed the Air Force portion of the terminal segment. The 4th Space Operations Squadron at Schriever AFB, Colo., is the front-line organization providing real-time satellite platform control and communications payload management. Inventory: 5 Unit Cost: $800 million http://www.af.mil/information/factsheets/factsheet.asp?fsID=118 Milstar/AEHF -zapuschen 14 awgusta 2010 goda http://www.youtube.com/watch?v=lWvr4mfP6A0 http://www.as.northropgrumman.com/products/aehf/assets/AEHF_datasheet_2010_.pdf uplink 44 ghz s polosoj signala 2000 mgz downlink 20 ghz s polosoj signala 1000 mgz Frequency Hopping Systems ( ispolzuetsja w Milstar) ********************************************* Frequency Hoppers (FH) are a more sophisticated and arguably better family of spread spectrum techniques than the simpler DS systems. However, performance comes with a price tag here, and FH systems are significantly more complex than DS systems. The central idea behind a FH system is to retune the transmitter RF carrier frequency to a pseudorandomly determined frequency value. In this fashion the carrier keeps popping up a different frequencies, in a pseudorandom pattern. The carrier itself amy be modulated directly with the data using one of many possible schemes. The available radio spectrum is thus split up into a discrete number of frequency channels, which are occupied by the RF carrier pseudorandomly in time. Unless you know the PN code used, you have no idea where the carrier wave is likely to pop up next, therefore eavesdropping will be quite difficult. Frequency hoppers are typically divided into fast and slow hoppers. A slow frequency hopper will change carrier frequency pseudorandomly at a frequency which is much slower than the data bit rate on the carrier. A fast frequency hopper will do so at a frequency which is faster than that of the data message. http://www.ausairpower.net/OSR-0597.html

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milstar: W Rossii odno iz izwestenjschix razrabotchikow i polzowatelej sistem s FSS -FGUP Impuls ****************************************************************************** ABSU Perimetr - start raket RWSN pri attake protiwnikom i otsutstwii signala na pusk ot Glawkoma,MO,nach.genstaba http://npoimpuls.ru/ http://npoimpuls.ru/index.php?option=atl2&atl=4 ФГУП «НПО «Импульс» является одной из основных организаций Российской Федерации по созданию новейших автоматизированных систем боевого управления (АСБУ) для ВС РФ и РВСН. Основными заказчиками объединения являются Министерство обороны РФ и предприятия «Федерального космического агентства». Общая численность основного персонала объединения на 1 августа 2009 года составляет 1712 человек. (dlja srawnenija Institut Mintza -razrabotchik Don-2N tolko 880 chel ) МОСКВА, 4 августа. (Корр.АРМС-ТАСС). Санкт-Петербургское НПО "Импульс" завершает поставки инозаказчику не имеющей мировых аналогов адаптивной цифровой системы КВ-радиосвязи с псевдослучайной перестройкой рабочей частоты (система КВ с ППРЧ), которая демонстрируется на выставке МВСВ 2006. Как сообщил начальник отдела конверсии и маркетинга предприятия Александр Запретилин, уникальная система разработана в интересах инозаказчика в одной из азиатских стран. Контрактом предусматривается выполнение НИР и поставка макетов аппаратуры. К настоящему времени выполнены все лабораторные испытания системы и близки к завершению трассовые испытания Система КВ с ППРЧ, по словам А.Запретилина, не была востребована на внутреннем рынке по причине сомнения в возможности обеспечения заявленных уникальных характеристик. Однако завершаемый зарубежный контракт вызвал интерес к КВ с ППРЧ со стороны российских организаций, которые начали переговоры с НПО "Импульс" о дальнейшем развитии системы под новые требования. По словам А.Запретилина, адаптивная цифровая система КВ-радиосвязи с псевдослучайной перестройкой рабочей частоты имеет ряд преимуществ перед традиционными системами связи. Адаптивность, в частности, обеспечивает устойчивую работу коротковолновой связи, остро реагирующей на различные природные явления. Система непрерывно ведет поиск частот, на которых обеспечивается идеальная связь со всеми объектами и адаптирует под эту частоту их работу. Псевдослучайная перестройка рабочей частоты, в свою очередь, предотвращает возможность пеленга или подавления связи между объектами. С этой целью алгоритмом системы предусматривается беспрерывный перевод работы объектов с одной найденной идеальной частоты на другую. В зависимости от трех (по желанию заказчика) разных режимов работы время пребывания системы на одной частоте составляет 50, 100 или 200 мс. При этом обеспечивается скорость передачи информации до 19,2 кбит/с. Как пояснил А.Запретилин, в случае, если система все же попала в невыгодные помеховые условия, что является маловероятным при том алгоритме работы, который обеспечивает НПО "Импульс", она способна перейти с максимальной на более низкие скорости передачи информации, т.е. является адаптивной не только по частоте, но и по скорости. В этом состоит уникальность системы КВ с ППРЧ. По словам А.Запретилина, поставки адаптивной системы - первый и пока единственный зарубежный контракт НПО "Импульс". Очевидно, что для предприятия, занятого в сфере создания средств управления военными объектами страны, в том числе РВСН, существуют определенные ограничения на ведение бизнеса с инозаказчиками. Его деятельность более чем на 90 проц. определяется гособоронзаказом. Помимо РВСН, АСУ "Импульса" эксплуатируются в других родах и видах войск. Тем не менее, петербуржские специалисты непрерывно диверсифицируют свою продукцию и активизируют работу по ее продвижению на внутренний и мировой рынки. На выставке "МВСВ-2006" "Импульс" представил также цифровую аппаратуру обмена закрытой речевой информацией по открытым телефонным каналам связи, устройство для ввода и транспортировки информации и информационную бесконтактную карту для жестких условий эксплуатации. ФГУП "НПО "Импульс" создано на базе Проблемной лаборатории Ленинградского политехнического института. В 1961-1987 гг. оно функционировало как ОКБ ЛПИ, а затем было преобразовано в научно-производственное объединение "Импульс". В конце 60-х годов тогда еще ОКБ ЛПИ выиграло конкурс на создание автоматизированной системы управления РВСН. С тех пор самая надежная в мире АСУ безупречно работает более 30 лет. На смену первому поколению уже приняты на вооружение системы второго, третьего и четвертого поколения. Не имеющие аналогов в мире системы разработки "Импульса" обеспечивают беспрерывное сверхнадежное гарантированное управление, а созданная им аппаратура работает безупречно. По словам А.Запретилина, надежность систем и аппаратуры, созданных предприятием, на порядок выше, чем у конкурентов. Все параметры надежности нарабатываются "Импульсом" до запредельно высоких показателей. В частности, вероятность ошибки одного из уникальных приборов, составила 10 в минус 18-й степени, тогда как приемлемым является показатель минус 8. Такими же сверхнадежными являются и созданные предприятием каналы передачи управляющих команд. Для оружия, рассредоточенного по многим регионам страны, эта проблема особенно актуальна.

milstar: Processing Gain ############ The second major antijamming strategy involves processing gain improvement. The GPS spread-spectrum signal derives some inherent jam protection from the "despreading" process, which converts it from a 20-megahertz bandwidth to a narrower bandwidth. Signal power grows stronger as bandwidth is reduced, so for maximum antijam performance, the narrowest possible bandwidth should be used in the despreading process. ##################################### Just how narrow the bandwidth can be depends in part on the design of the code and carrier tracking loops used by the GPS receiver and the dynamic operating environment. Recall that a GPS receiver gets a signal from a satellite, generates a local copy, and compares the two to derive range and range-rate measurements. The tracking loops try to maintain a "lock" on the satellite signal by driving the difference in the signals (as measured by the signal correlator) to zero. In general, greater antijam performance can be achieved by narrowing the bandwidth of these code and carrier tracking loops. Unfortunately, narrow tracking-loop bandwidths imply sluggish response time, and if a vehicle is undergoing high ######################################################################### acceleration, the narrow-bandwidth tracking loop cannot keep pace. ############################################# If the tracking-loop bandwidth were widened, it would be more responsive to high acceleration, but it would not filter the noise as effectively. In a power-inversion array antenna, the individual elements are geometrically arranged with an interelement spacing of one-half a GPS carrier wavelength. This arrangement is useful for applications where the desired signal is weak and the interference is strong. One solution is to aid the tracking loops by supplying information about the vehicle's acceleration and the motion of the satellite to be tracked. This information could be supplied, for example, by an inertial navigation system and the GPS satellite almanac. With this supplemental information, the receiver's tracking loops can anticipate the dynamics along the line-of-sight to the satellite and use a narrow- bandwidth filter to process the fresh outputs from the signal ############################################################################# correlators. If the aiding information is reasonably accurate, the bandwidth of the tracking loop can be narrowed because it will ########################################################################## only need to track the errors in the aiding information (which vary slowly over time), rather than the absolute motion ########################################################################### of the antenna. ########## The aided tracking loop, with its narrower bandwidth, provides more processing gain and more protection against ######################################################################## jamming; however, it's still not enough to thwart a very strong jammer that may be close to the GPS navigation set. The limitations of aided tracking loops are more practical than theoretical: In actual implementation, the aiding information will contain numerous errors. The most notable errors arise from two sources: imperfect implementation of the aiding data interface, and the inconsistency of the motion between the aiding sensor and the GPS antenna or "lever arm." (In most vehicles, the antenna and the aiding sensors are in different locations, and "lever-arm" compensation must be provided because the GPS antenna is not sensing the same motion as the aiding sensors.) The first error source, the data interface, exists because traditional receivers are designed to use whatever inertial measurement unit is present on the host vehicle. (An inertial measurement unit—or IMU—is a set of gyros and accelerometers that feed the inertial navigation system in an aircraft or missile.) The GPS receiver and the host vehicle communicate over an asynchronous serial bus, and the designer of the GPS receiver usually does not accept the IMU data without "deweighting" it in some manner. This deweighting process can limit the achieved bandwidth reduction below theoretical levels and hence limit the antijam performance. The second error source, lever-arm compensation, is unavoidable if the GPS antenna is not located with the IMU. Unfortunately, many factors—such as vehicle attitude, vehicle rotation, and body flexure—prevent perfect lever-arm compensation, even when the IMU is situated in the same box as the receiver. Hence, the bandwidth of the tracking loops must be wide enough to maintain GPS signal lock despite these factors—and this limits the antijam performance. In some applications, such as small weapons, the antenna is naturally close to the IMU and the body is rigid, so the lever-arm compensation is not as significant an error source as it is in avionics applications. http://www.aero.org/publications/crosslink/summer2002/06.html New Approaches To meet the future challenge of GPS applications that must operate in projected jamming environments, the GPS Joint Program Office is pursuing several promising technologies and a future GPS set architecture that will yield further improvements in antijam performance. Aerospace is actively involved in defining advanced architectures and technologies that will economically provide better antijam performance. Two approaches in particular are generating considerable interest in the field. Microelectromechanics With the recent advances in microelectromechanical systems, new architecture concepts that were unimaginable five years ago have now come within reach. One such technology, the microelectromechanical IMU, will have a significant impact on the future design of user navigation sets. As noted, the best way to reduce the bandwidth of the tracking loops (and thus improve antijam performance) is to keep the GPS antenna and the IMU together, thereby forcing the lever arm to zero. This placement eliminates the need for the lever-arm correction and its associated errors. Of course, when IMUs were first invented, they were very large, and although they've become smaller over the years, they remain large enough to require special attention concerning their placement in a host vehicle or missile. The ability to place an IMU in the same box with the GPS receiver was viewed as a significant step forward. But until recently, no one considered the possibility of embedding the IMU in the antenna itself. A jam-resistant GPS antenna undergoes testing at the Air Force Research Laboratory. That is precisely the thinking now being pursued under the leadership of Aerospace. The cost, size, and performance of microelectromechanical IMUs are improving to the point where they'll soon be good enough to embed in a GPS antenna. This new architecture overcomes many of the factors that prevented the narrowing of tracking-loop bandwidths in older systems. For example, because the IMU would be dedicated to the GPS set, a synchronous interface between the two could be designed with proper attention to interface errors and data latency. In addition, the placement of the IMU with the GPS antenna would make both sensors experience the same motion, so there would be no need for lever-arm compensation with its associated errors. Although the accuracy of microelectromechanical IMUs cannot compete with more traditional technologies (such as those that use ring-laser gyros), accuracy is reaching a level that is adequate for aiding GPS. Extremely high accuracy is not required if the IMU error sources are reasonably stable because the navigation processing algorithm constantly estimates these low-bandwidth error sources and compensates accordingly. In other words, it's the short-term stability of these instrument error sources that's important for aiding GPS. And although short-term stability errors can be sensitive to temperature and acceleration, compensation models whose coefficients are calibrated prior to operation can usually mitigate their effects. So, for short periods of time, errors in the microelectromechanical IMU approach acceptable levels for aiding GPS. It should be noted that the microelectromechanical IMU is not meant to replace the IMU that may be present in the host vehicle. If there is a need for inertial navigation accuracy without GPS, then the microelectromechanical IMU would probably not satisfy that requirement. The microelectromechanical IMU is intended as part of the GPS navigation set (notice that the word "receiver" has not been used), and is present in the GPS antenna regardless of whether there is a need for an IMU by the host vehicle. Ultratight GPS/Inertial Coupling Another technology has recently emerged to address the need for antijam performance. This new technique, called ultratight GPS/inertial coupling, is a different method to jointly process GPS and IMU data (see sidebar, GPS/Inertial Coupling). Several organizations throughout the United States have been performing research in this area, either through independent research and development funds or DOD research contracts. Although each approach is unique in its implementation, they all share certain common traits. For example, they all eliminate the code and carrier tracking operations, which are susceptible to jamming even when aided. All use estimated navigation parameters to generate the local replica signal needed to track the satellite signal. All directly use the correlator outputs (i.e., comparisons of the local and satellite signals) to compute the range and range-rate errors for the navigation processing algorithm. This graph shows the effects of jamming on unprotected GPS performance. For example, at a jammer-to-signal ratio of about 55 decibels, a jammer located about 100 nautical miles from the receiver could jam the GPS signal through a 1-kilowatt signal. At 1000 nautical miles, 100 kilowatts would be required. (View larger image.) Aerospace is an industry leader in ultratight coupling. Four years ago, Aerospace began to develop its formulation of ultratight coupling and filed for a U.S. patent. About the same time, Aerospace became aware of similar research being conducted at other companies and other patents that were pending. When the antijam potential of this processing approach was determined, Aerospace was instrumental in obtaining interest at the various DOD research laboratories to fund development programs. Today, virtually all GPS vendors to DOD have contracts to pursue some sort of ultratight coupling. A milestone was reached in November 2001 when the first official government-sponsored test of an ultratight coupling formulation was conducted at Eglin Air Force Base. The antijam performance was slightly better than predicted. The test results essentially confirmed the performance that had been predicted at Aerospace using simulations. Currently, the Aerospace formulation is being implemented in a real-time computer. One GPS vendor has asked to license the Aerospace formulation, and many other companies are using it for studies. Summary Future GPS systems—particularly for weapon delivery—will benefit from the optimal integration of GPS receivers with inertial measurement units and the use of adaptive processing algorithms and antennas that reject unwanted signal interference while maximizing the power of the desired satellite signal. The combination of all these technologies and the associated system architecture will be the blueprint for DOD GPS sets for the next several decades. Many of the GPS antijam techniques and architectures that will be used in future equipment have roots at Aerospace, which has been the technical conscience of the program since its inception.

milstar: Processing Gain Processing gain is realized at the output of bandpass filter of the sliding carrier based correlator when code alignment is achieved. Here‟s why; When the receiver replica C/A code is not aligned with the transmitted C/A code the received GPS signal power at the output of the bandpass filter is spread over approximately 2mhz of bandwidth ( center lobe of C/A BPSK spectrum is approx 2 Mhz) . When the receiver C/A code is aligned with transmitted code the signal power at the bandpass output is now squished into approximately 100hz of bandwidth ( Center lobe of 50hz random data spectrum). A rule of thumb is to use the ratio of these two bandwidths as the processing gain. Processing gain is remarkable, it is as if the signal is amplified without also amplifying the noise at the same time! Processing Gain = Bandwidth of Uncorrelated Signal / Bandwidth Correlated Signal For a GPS receiver this works out to (in db) Processing Gain GPS Rec.(db ) = 10 log [ 2Mhz / 100Hz] +43dB This number is an estimate of processing gain. A more accurate estimate is the ratio of the SNR before and after correlation. In addition various imperfections in the correlator may also degrade this gain. Regardless the processing gain is large in a GPS receiver and enables the negative SNR environment before correlation to be turned into a positive SNR condition after correlation assuming typical bandwidths before and after. 14 Recovery of signal with Negative SNR The spreading of GPS signal power by the C/A code can lead to the condition where the signal is below the noise floor in one part of the receiver and above it in other sections after processing. When the signal is below the noise floor at a given point in the receiver it has a negative SNR. For a user at the earth‟s surface the received power from the GPS signal is very low. The specified minimum power at the earth‟s surface is approximately – 130dBm. This power is the unmodulated carrier power in one hertz of bandwidth. When the signal is modulated with the C/A code and 50HZ data this power is spread over a larger bandwidth. Once the signal power is spread over 2Mhz the crest of C/A code spectrum is well below the – 130dBm unmodulated carrier power. Ignoring the 50Hz data modulation (data mod off) the carrier power is spread over approximately 2mhz of bandwidth as 1khz equal spaced tones. The power level of these tones in the main lobe is fairly flat at about – 30dB down from the pure carrier level. This puts these lines at approximately – 160dbm. A typical post correlation bandwidth for a analog GPS receiver is 1khz. For a perfect receiver this would put the noise floor at approx. – 143dBm. If we do not have correlation (C/A codes not aligned) the 1khz tones will be well below the noise floor at this point in the receiver resulting in a negative SNR condition. Once we have correlation the signal power is restored to nearly the unmodulated power level and for 1khz bandwidth we would see a SNR of about +13db. So just because a negative SNR condition is present does not mean the GPS signal is “gone”. It is just hiding under the receiver noise floor waiting for the power of the correlation operation to “resurrect” the signal. The exact SNR after correlation will depend on where in the receiver one is talking about and the effective noise bandwidth at that point. In the data demodulator where the data is stripped off the carrier the noise bandwidth can be quite narrow and higher SNR‟s are achieved, typically 20db to 30db. The phenomenon of retrieving a negative SNR signal is hard to swallow. It is the authors feeling that this difficulty can be overcome by understanding what noise is and how it interacts with discrete signals. Noise from a spectral point of view is a density. It is not discrete. Therefore you cannot talk about noise levels or powers quantitatively without specifying the bandwidth that is related to the noise being measured (ie the circuit bandwidth). Noise power goes up as bandwidth widens. A discrete signal is completely different. Discrete signals, such as pure sine waves, have a power (or level) that is independent of the bandwidth of the circuit. Theoretically if one measures the power of a pure sinewave through a bandpass filter whose center frequency equals the sinewave frequency the power measured is independent of filter bandwidth. As we just stated this NOT the case with noise. As stated above the power spectrum of the GPS C/A spectrum is made up of discrete spectral lines spaced at the code repeat rate of 1khz for the case where we have only C/A code modulation (Data =1 or 0 forever). These lines trace out the [sin(x)/x] 2 spectrum. Therefore the spectrum of the C/A modulated signal is not a density but really a collection of discrete signal lines. What would happen if looked at the received signal (before correlation) with an ideal spectrum analyzer that could use extremely narrow bandwidths? Figure A11 shows an ideal spectrum analyzer display of the GPS carrier @1575.42mhz both unmodulated and with C/A code modulation on it (No 50Hz Data Mod). The power levels shown reflect those at the earth‟s surface using a 0db -gain antenna. The unmodulated carrier is shown shaded. Note that once C/A code modulates the carrier, the carrier is suppressed and there would be no signal at that position (in an ideal modulator). Our ideal analyzer does not contribute to the received noise power and has some very narrow Resolution Bandwidths (RBW‟S). For each RBW the theoretical noise floor of the instrument is shown. Note that at a 1khz RBW the C/A tones are well below the noise floor of the instrument. As the RBW is narrowed the noise floor drops by 10Log RBW but the discrete C/A modulation tones remain constant in power/amplitude. At a RBW of 0.1hz we would be able to see the spectral lines of the C/A modulated carrier above the noise floor. So in theory it is possible to see the GPS signal before correlation. It is hoped that this example makes it clear the signal is still there with negative SNR conditions. It is all related to the circuit bandwidth and its effect on noise power. The processing gain of correlation makes it possible to recover the signal and its data by compressing the received signal power into a smaller bandwidth (allowing a reduction in the circuit bandwidth) thereby creating a positive SNR condition.

milstar: processing gain sootw. ( smotri nize adaptivnoe izm .polosi do 10 gerz i nize pri tracking) ywelichiwaetsja To reduce the effects of wideband noise contaminating receiver functions, process base bandwidths should be minimized in receiver designs or be allowed to adaptively reduce in response to detected jamming. Such adaptive bandwidth filtering in the predetection bandwidth and carrier/code tracking loops will minimize the jamming power that is able to influence these receiver processes. The minimum predetection bandwidth is 50 Hz to accommodate the satellite ################################################ ephemeris data carried on the GPS signal. ########################### The tracking loops can be reduced in bandwidth to 10 Hz or less to maintain carrier/code lock ############################################# and maximize the accuracy of satellite range measurements via optimum SIR in the presence of wideband jamming. ############################# Adaptive bandwidth filtering is effective particularly for the signal tracking loops where it can provide on the order of 10 dB of jamming resistance. The down side of narrow tracking loop bandwidths, however, is that they do not accommodate platform dynamics very well, and most PGMs are fairly dynamic platforms. ######################### Accordingly, the narrow bandwidths of AJ GPS receivers require inertial measurement units (IMUs) to rate aid the tracking loops. In this process, velocity measurements from the IMU are provided to the tracking loops, enabling them to follow rapidly time varying Doppler shifts (that are the result of platform dynamics) with their narrow tracking filters. ################################ Without velocity aiding from the IMU, these Doppler effects would shift the GPS signal out of the narrow pass band of the tracking loops, resulting in loss of carrier/ code lock. http://wstiac.alionscience.com/pdf/Vol3Num3.pdf

milstar: Performance Study of Hybrid Spread Spectrum Techniques https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes 19 1.7.1 Types of SS Communication Systems Three important types of spread spectrum techniques are direct sequence spread spectrum (DSSS), frequency hopping spread spectrum (FHSS) and time hopping spread spectrum (THSS). In direct sequence spread spectrum, baseband data is spread by directly multiplying the data pulses with a pseudo noise (PN) sequence that is produced by a pseudo noise code generator. In case of spread spectrum (SS) systems, the pseudo noise sequence is known to the transmitter and intended receiver but appears random, with noise like properties, to other receivers in the system. In frequency hopping spread spectrum, the frequency of the carrier is periodically modified (hopped) following a specific sequence of frequencies obtained using a PN sequence generator. Frequency hopping may be slow frequency hopping (SFH) or fast frequency hopping (FFH).

milstar: Figure 2.11 : Concept of Hybrid Slow and Fast Hopping: (a) BPSK data signal (b) Hybrid DS/SFH with 2 bits/hop and one code sequence per symbol (c) Hybrid DS/SFH with 2 bits/hop and two code sequences per symbol (d) Hybrid DS/FFH with 2 hops/bit and one code sequence per symbol (e) Hybrid DS/FFH with 2 hops/bit and two code sequences per symbol rejection. The DS component of a hybrid signal provides LPI characteristics and helps to suppress the interference, and at the same time the signal is hopping from one frequency to another and the FH part is at work to avoid interference. In a hybrid DS/FH system, a saboteur must know both the hopping sequence and the code used by the signal to accurately jam it. Hence, the probability of a saboteur to block or degrade the desired signal is reduced. The spreading factor for a hybrid DS/FH system is a function of the number of chips per bit, and the hopping frequencies used in the system. In the present thesis, the spreading factor for hybrid systems will be defined by the term “total spreading factor” (TSF) which is equal to the product of the DS spreading factor (SF) and , the number of hopping frequencies used. For example, a TSF of 64 might imply that eight hopping frequencies and a code sequence with SF 8 have been used to generate the hybrid signal. Another combination that will provide a TSF of 64 can be of 4 hopping frequencies and a code with SF equal to 16 and so on. Several of these combinations will be analyzed in this work for both hybrid slow and fast hopping system https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: Many papers were published on spread spectrum systems in the early 1980’s by Pursley and Geraniotis ([32]-[35]). An important work on coherent hybrid systems was published by M. K. Simon and Andreas Polydoros [37]. Their work investigated the performance of coherent FH and hybrid DS/FH systems with QPSK and QASK modulations. They derived expressions for error probability under partial band noise jamming and partial band multitone jamming. An important consequence of using hybrid spread spectrum was the conversion of a FH multitone jammer to an equivalent AWGN jammer (under certain assumptions); due to spectrum spreading associated with the DS modulation part of hybrid DS/FH systems. In other words, the effect of a spread multitone jammer on bit error probability was found to be equivalent to the effect of AWGN interference. The performance of coherent hybrid DS/FH was further examined by Geraniotis in 1985 [34]. Analysis of synchronous and asynchronous hybrid direct sequence/slow-frequency hopped spread spectrum was carried out over an additive white Gaussian noise channel for both deterministic and random signature sequences. Hopping rates much slower than the data rate (i.e. slow frequency hopping) were used in order to implement coherent demodulation with contemporary receivers. Two types of modulation schemes viz. BPSK and QPSK were considered. Expressions for average probability of error were evaluated for both asynchronous and synchronous DS/SFH systems and for quaternary and binary modulation schemes. Two different techniques were developed for the evaluation of the error probability. The first technique, based on the characteristic function method evaluated error probability by integrating the characteristic function of multiple access interference. Another method was based on evaluating the conditional error probability for a given number of hits from other users and then averaging these over the distribution of hits. A summary of pertinent numerical results from [34] is provided below: Nature of code sequences: For binary hybrid systems, synchronous systems with deterministic code sequences performed considerably better than synchronous systems with random signature sequences. This may be attributed to the lower cross correlation functions of well chosen deterministic sequences. However, as the number of active users increases with length of the sequence (which cannot be increased indefinitely due to code acquisition and synchronization requirements) remaining constant, it is difficult to find distinct code sequences with very good cross correlation properties for all users. Thus for hybrid SSMA systems employing random code sequences, asynchronous systems performed better than the corresponding synchronous systems. This improved performance of asynchronous systems is due to the extra averaging with respect to the time delays which takes place in the asynchronous systems. For deterministic code sequences, the synchronous systems outperform the corresponding asynchronous systems Chip waveform: sinusoidal or rectangular Performance of asynchronous binary and quaternary systems was analyzed for rectangular and sine chip waveforms using random code sequences. Results in [34] indicated that hybrid systems with sine chip waveform perform slightly better than rectangular chip waveform for both the binary and quaternary modulation schemes. Overall, results from [34] suggest that hybrid DS/SFH systems compare favorably to the DS and SFH systems which employ identical modulation and demodulation schemes and have the same bandwidth spread. Synchronous hybrid SS systems supported fewer users than asynchronous hybrid SS systems when random code sequences and hopping patterns were used. Also, hybrid systems with QPSK supported fewer users than the corresponding systems employing BPSK modulation. This accounted for the fact that QPSK can support two different data streams (in phase and quadrature) for each user i.e. it has twice the data rate of a BPSK system with the same bandwidth expansion. Since both in phase and quadrature data signals in QPSK employ the same frequency carrier and hopping pattern, whenever a hit occurs due to another user’s signal, both of these interfere with that signal or vice versa. So, all hits from other signals are double hits. Contrary to this, in BPSK system, all the data signals employ different hopping patterns and only single hits occur from the other signals. Another work by E. A. Geraniotis [28], published in 1986, evaluated performance of non-coherent hybrid DS/SFH spread spectrum systems. Both asynchronous and synchronous systems operating through AWGN channels were investigated. Modulation types studied were M-ary FSK and DPSK (Differential Phase Shift Keying). This work extended that of [34], and the main conclusion drawn was that the multiple access capability of non-coherent hybrid spread spectrum was superior to that of non-coherent pure frequency hopped spread spectrum, and inferior to that of non-coherent pure DSSS with the same bandwidth expansion.

milstar: to analyze the performance of a pure phase coherent slow frequency hopped (SFH) receiver with 1 bit/hop in the presence of AWGN and partial band interference and compared the BER performance with non-coherent FSK systems. When the phase distortion in the channels was not excessive, improved performance compared to a coherent system was observed. The importance of fast frequency hopping (FFH) in mitigating follower jamming was emphasized due to its inherent frequency diversity. Moreover, the frequency hops occurring during each symbol could be combined to achieve a reliable decision state. Considering the scenario when a smart receiver is able to track the hopped frequencies, a hybrid system was proposed in which each hopped frequency is spread with a PN sequence. FFH was found to combat both frequency selective fading (because of frequency diversity) and non-selective fading (when hopped frequencies are combined properly). Under severe fading conditions, follower jamming was found to be less effective against a hybrid spread spectrum system. https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: In general, coherent systems (with slow frequency hopping) provided better performance against PBN, Ricean fading, multiple access interference and AWGN. Under severe Rayleigh fading, coherent reception became difficult. Muammar [40] studied the effects of frequency selective Rayleigh fading and log normal shadowing on a DS/FH system with differential phase shift keying (DPSK) modulation. Error probabilities were examined for a Rayleigh fading channel with and without the effects of log normal shadowing. System degradation with log normal shadowing was much smaller than that caused by Rayleigh fading. Byun et al. [42] analyzed a hybrid DS/SFH system subject to a Nakagami fading channel. The bit error probability over a Nakagami fading channel was calculated as a function of the number of jamming tones used by the jammer. Various combinations of number of hopping frequencies and spreading code sequences that satisfied an equal bandwidth constraint were employed. For a low jamming to signal power ratio (JSR) of about 10dB, a pure DSSS system was found to achieve a lower BER than a hybrid DS/SFH system. However, for higher JSR values (20 or 30dB), the hybrid DS/SFH system exhibited superior performance. Also, the worst case performance of a hybrid DS/SFH system was found to be almost equal to the nominal performance of a pure DSSS system. https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: . The proposed transceiver is a slow frequency hopping (SFH) system, meaning the hop rate is slower than the symbol rate. It should be noted, though, that our system still requires a high hop rate. There are many fundamental differences in the transceiver design and implementation between a very low hop rate SFH-TDMA system, which hops to a new frequency every frame (i.e., hop rate 50 Hz), and the proposed SFH-CDMA transceiver. It hops every eight symbols, which results in a hop rate of 20 kHz at a symbol rate of 80 kHz. https://pdfs.semanticscholar.org/c2f9/e8284cbaa43eff5ee97c9fe1d89d38e636bb.pdf

milstar: The effect of the hopping rate was also observed for asynchronous systems. Asynchronous systems with a large number of bits/hop outperformed systems with dwell time equal to symbol period https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes Thus jammed and unjammed frequency lots in a hybrid DS/SFH system differ in their spreading gain requirements and a tradeoff must be devised in order to effectively synchronize both of these. https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: Figure 5.2 : Performance of a Single User System in an AWGN and Rayleigh Fading Environment for Selected SFs. page 79 The BER performance curves for a varying number of users in a Rayleigh fading environment are presented in Figure 5.3 for SF=64. It is clear that the performance degrades gradually as the number of users is increased from 10 to 30. Even in the presence of 30 users, the system is able to provide an acceptable voice BER of 3 10− at a Eb No / value of just 10 dB. Thus the system can accommodate many users even without error correction coding. It can be inferred from Figures 5.2 and 5.3 that a lower spreading factor than 64 will provide a BER greater than 3 10− at 10 dB, while a higher SF than 64 will reduce the BER and thus many more users than 30 will be accommodated at 10 dB for a BER of 10−3 . https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: 5.2 Slow Frequency Hopping: Performance Results For slow frequency hopping systems, 3 sets of simulations were carried out. First, the SFH system was simulated in a Rayleigh fading environment for a single user using 64 hopping frequencies. Following the equal bandwidth constraint, this is equivalent to a SF of 64 in DSSS system. BER performance of a SFH system with 64 hopping frequencies is compared with DSSS system of SF=64 in Figure 5.5. It is evident that the performance of the DSSS system under Rayleigh fading is far better than the SFH system for the same processing gain in a single user system. A second set of SFH system performance results are based on multi-user interference analysis. All the users use two frequencies from a total of 64 frequencies used by the desired user. The SFH system was simulated for a hopping rate equal to 8 bits/hop and 64 hopping frequencies with a varying number of users. Performance results are shown in Figure 5.6 https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: Performance of the FFH system for different numbers of users in a Rayleigh fading environment is plotted in Figure 5.11. A hopping rate of 8 hops/bit and 64 hopping frequencies were used. As with Rayleigh fading performance, FFH multiuser performance is better than SFH but inferior to DSSS performance. A second difference between SFH and FFH performance curves is that FFH performance Figure 5.10 : Comparison of SFH, FFH and DSSS Systems under Rayleigh Fading https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes

milstar: The benefits of frequency hopping spread spectrum (FHSS) are potentially neutralized by a repeater jammer (also known as a follower jammer), which has been investigated for more than ten years. The repeater jamming technique for FHSS has been used in both military communications and commercial communications [1]-[3]. In contrast to this, any power-effective jamming technique used in direct sequence spread spectrum (DSSS) has not been proposed in public literatures. Meanwhile, the current jamming types are ineffective at the current jamming power level when the processing gain is large enough. So it’s necessary to investigate a new power-effective jamming technique for the purpose of both commercial frequency surveillance and military countermeasures. The principal types of jamming on DSSS signals include broadband noise (BBN) jamming, partial-band noise (PBN) jamming, pulsed jamming and tone jamming. The last of these includes both single tone jamming and multiple tones (MT) jamming. The effectiveness of these jamming types is not good, because they are non-correlative jamming types which can not synchronize PN sequences. In order to achieve desired jamming effectiveness, the jammer has to increase power level of jamming signals. Unfortunately the victim receiver will countermine the strong jamming signals with adaptive notch filters, repeat coding and so on [4], [5]. https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4224679&tag=1

milstar: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=140456 BER 10 -6 SNR 56 db fading SNR 74 db shadowing with jamming worster SNR 11.5 db quasi ideal only Gaussian noise

milstar: IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 23, NO. 5, MAY 2005 1113Wireless Field Trial Results of a High Hopping RateFHSS-FSK TestbedDanijela ˇCabric´, Student Member, IEEE, Ahmed M. Eltawil, Member, IEEE, Hanli Zou, Member, IEEE,Sumit Mohan, and Babak Daneshrad, Member, IEEEAbstract—This paper presents a complete study and charac-terization of a real-time frequency-hopped, frequency shift-keyedtestbed capable of transmitting data at 160 kb/s, with hoppingrates of up to 80 Khops/s operating in the 900 MHz band. Thesystem provides the highest hopping rate reported to date and setsa new trend for FHSS communications with superior low proba-bility of interception/detection and anti-jamming (LPI/LPD/AJ)capabilities. https://www.researchgate.net/publication/3236095_Wireless_field_trial_results_of_a_high_hopping_rate_FHSS-FSK_testbed

milstar: https://epdf.pub/spread-spectrum-communications-handbook.html

milstar: https://pdfs.semanticscholar.org/b6fe/00c0a09c08efcb38f39ab01fc7acf86203c8.pdf ORNL Hybrid SS

milstar: https://www.researchgate.net/publication/261119180_Hybrid_DSFFH_spread-spectrum_A_robust_secure_transmission_technique_for_communication_in_harsh_environments Performance Study of Hybrid DS/FFH Spread-Spectrum Systems in the Presence of Multipath Fading and MultipleAccess Interference Mohammed M. Olama Teja P. Kuruganti Steven F. Smith Computational Sciences & Engineering Division Oak Ridge National Laboratory http://cqr2012.ieee-cqr.org/May15/Technical%20Papers/8-Mohammed_Olama.pdf

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