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Direct sequence spread spectrum: history, principles and modern applications
Nenad M. Stojanovica, Branislav M. Todorovicb, Vladimir B. Risticc, Ivana V. Stojanovicd
a Serbian Armed Forces, General Staff, Department for Telecommunication and Informatics, Center for Applied Mathematics and Electronics, Belgrade, Republic of Serbia,
e-mail: nivzvk@hotmail.com, corresponding author, ORCID ¡D: https://orcid.org/0000-0001-9328-5348 b Institute for Artificial Intelligence of Serbia, Novi Sad, Republic of Serbia, e-mail: branislav.todorovic@ivi.ac.rs, ORCID iD: https://orcid.org/0000-0003-1932-8332 c University of Defence in Belgrade, Military Academy, Department of Telecommunications and Informatics, Belgrade, Republic of Serbia, e-mail: vladarist@gmail.com, ORCID iD: https://orcid.org/0000-0003-0422-9737 d „Telekom Srbija" a.d., Belgrade, Republic of Serbia, e-mail: ivnvukanic@gmail.com, ORCID iD: https://orcid.org/0009-0002-4216-935X
doi https://doi.org/10.5937/vojtehg72-49325
FIELD: telecommunications ARTICLE TYPE: review paper
Abstract:
Introduction/purpose: Direct sequence spread spectrum modulation is widely used in many radiocommunications systems. At the first time, this modulation technique was used in military communications and navigation systems. Later, applications became diverse in civil communication systems as well. Today, there are many systems where direct sequence spread spectrum modulation is implemented as a part of the system. This article aims to sublimate knowledge about the direct sequence spread spectrum modulation technique and its applications. Methods: The article presents a review of the historical development of the direct sequence spread spectrum modulation technique, its principles and the most important current applications.
Results: Based on a large number of references, this article summarizes the historical development, basic principles and modern applications of the direct sequence spread spectrum modulation in military and commercial communication systems.
Conclusion: Direct sequence spread spectrum modulation is widely used in modern wireless and satellite radiocommunications. It is expected to be part of future global communication systems.
Key words: radiocommunications, military communications, cellular mobile systems, spread spectrum, direct sequence, global positioning system, underwater acoustic communications, unmanned aerial vehicles, wireless fidelity, ZigBee.
Introduction
Spread Spectrum (SS) modulation has been developed in the middle of the 20th century. In the beginning, this modulation was developed for military purposes. Today, there are many reasons for commercial applications of SS modulation, such as: anti-jamming capability, interference rejection, low probability of intercept, multiple access capability, improved multipath and diversity reception, high resolution ranging, and accurate universal timing (Fazel & Kaiser, 2003). Also, these systems cause relatively minor interference with other systems operating in the same frequency band (Torrieri, 2015). There are also promising possibilities for its implementation in future global communication systems (Todorovic et al, 2024).
In radiocommunication systems with SS modulation, the transmitted signal requires a much wider frequency bandwidth than the minimum bandwidth needed to transmit information in classical communication systems. The pseudonoise (PN) sequence is used for signal spreading at the transmitter and signal despreading at the receiver. The PN sequence, or code sequence as it is also called in SS, is independent of the data signal.
The two most commonly used SS modulations are: Direct Sequence Spread Spectrum (DS-SS) and Frequency Hopping Spread Spectrum (FH-SS) (Torrieri, 2015).
In the transmitter, a data signal arriving at the input of the transmitter is multiplied by the PN sequence generated locally. The PN sequence must have a much higher rate than the data signal. In this way, the data signal is spread. At the receiver, the incoming spread signal is multiplied by the same locally generated PN sequence which is synchronized with the PN sequence contained in the incoming signal. In that way, the spectrum of the transmitting signal is despread. In other words, the removal of the PN sequence at the receiver results in spectrum despreading and it is exploited by appropriate filtering to remove a large portion of the interference.
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This paper presents: (1) the history of the DS-SS modulation technique and its application to radiocommunication systems, (2) the operating principles of the DS-SS modulation technique, and (3) an overview of modern systems, both military and commercial, using the DS-SS modulation technique. At the end of this paper, some conclusions are given.
History of DS-SS modulation technique
The first system which looks like today's DS-SS modulation appeared in 1935. Two Telefunken engineers, Paul Kotowski and Kurt Dannehl, made a device for masking voice signals. The voice was masked by combining it with a broadband noise signal produced by a rotating generator. The U.S. version of this patent (Kotowski & Dannehl, 1940), which was published in 1940, was considered as the forerunner of the DS-SS modulation technique. Kotowski and Dannehl had an idea to create a device that used key-stream generators for discrete data encryption. This patent did not mention the spreading spectrum, although there were some concepts of DS-SS modulation. These principles were implemented in radio devices in Very High Frequency (VHF) and Ultra High Frequency (UHF) bands during World War II (Scholtz, 1982).
The occurrence of DS-SS modulation is connected with the occurrence of Shannon's theory of information (Shannon, 1948). The idea was how to suppress interference and jamming signals (Scholtz, 1994).
In August 1954, a transcontinental system project called F9C (Green, 1954) commenced. The transmitter was located in Davis, California, and the receiver in Deal, New York, and based on DS-SS modulation. The system operated in a High Frequency (HF) band. A further test of the F9C system was performed in February 1955. The improvement of the system was tested in situations with and without jammers. In December 1956, the manuals for the new advanced F9C system named F9C-A were published. It was planned to make 16 pairs of transmitters and receivers and put them in final testing, but because of the limited funds of the project, only six were produced. The devices were installed in a couple of places around the world, and the testing lasted until 1959. The last version of this system was F9C-A/Rake. It had the additional implementation of a rake receiver, as a solution for ionospheric multipath propagations. The F9C-A/Rake was tested until 1962, between Hawaii and Tokyo. That system is no longer on-site, operational and supported by the United States Army (Scholtz, 1982).
The COded DOppler, RAnging, and Command (CODORAC) system was developed by Eberhardt Rechtin, Richard Jaffe, and Walt Victor
(Scholtz, 1982). In 1952, the Jet Propulsion Laboratory (JPL) of the California Institute of Technology was attempting to construct a radio command link for remote control of rocket systems. In 1953, DS-SS modulation began to be used in this project. In the period from 1954 to 1958, the radio guidance system for the Sergeant and Jupiter space programs (Mudgway, 2001) was developed from that project. In October 1958, the National Aeronautics and Space Administration (NASA) was officially established to consolidate the separately developing space-exploration programs of the United States Army, United States Navy, and United States Air Force into one civilian organization. That was the beginning of the deep space network (Scholtz, 1982).
In the NASA Apollo program, which lasted from 1969 to 1974, it was necessary to determine the precise position of the spacecraft during the journey to the Moon. Due to a great distance, frequently used pulse radars were not good enough due to excessive attenuation. To overcome that problem, a PN sequence was generated using a linear shift register with a length of up to 127 chips (the symbol of the data signal is called a bit, while a symbol of the PN sequence is called a chip). This method of signal transmission was chosen because it was easy to extract a significantly attenuated signal from the noise on the receiver, which is a very useful feature of the DS-SS modulation technique. The basic principles applied in this system are applied today in satellite communications, more precisely in the Global Positioning System (GPS) (Shirriff, 2022).
For the communication between fighter planes, the ARC-50 project was launched in 1953. The researchers who were on the CODORAC project proposed the use of DS-SS modulation for that communication. Before that time, the tactic of aero units was to keep planes in radio silence when they were in the range of radars or radio eavesdroppers. Then it was realized that upon entering the radar range, it was not necessary for aircraft to be in radio silence and therefore it was important to avoid possible radio signal interference, where the good characteristics of DS-SS modulation could be expressed. This system initially used PN sequences generated by linear shift registers with 31 registers. Later, Bob Gold proposed a way to generate PN sequences with very good cross-correlation characteristics, which was finally implemented (Scholtz, 1982).
The Joint Tactical Information Distribution System (JTIDS) was founded in 1973. It was created by the Department of Defence of the United States as a joint program office with the Air Force as an executive agent. In the beginning, their work on communication systems for military purposes was based on DS-SS modulation. Later, they also often used FH-SS in their radiocommunication solutions. The JTIDS solutions are
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also used by allies of the United States. Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) are used both in their network radio systems and in a combination with SS to reduce susceptibility to noise, jamming, and interception. Their equipment is highly survivable radio and meets the most stringent requirements of modern combat, providing reliable situational awareness for fast-moving forces. Great performances were shown during the Desert Storm operation. Even today, JTIDS solutions are implemented in the radio infrastructure of the North Atlantic Treaty Organization (NATO) (DTIC, 1988).
A project called SECRAL, which was realized during the 1950s, was based on DS-SS modulation and was intended for missile guidance. This project was terminated as a failure. This is only one of the known examples, and there were certainly others that were unsuccessful, but that information never reached the general public (Scholtz, 1982).
Major advances in the field of sS systems were made during the 1950s, 1960s and 1970s. Almost all researchers from that period were shrouded in secrecy because these systems were exclusively for military purposes. Some of these systemic solutions were later fully disclosed to the general public, while some never were (Malik, 2001).
Operating principles of the DS-SS modulation technique
The DS-SS modulation technique has great resistance to interference and multipath propagation. Also, it can operate in the environment of low Signal to Noise plus Interference Ratio (SNIR). Using the orthogonal code, sequences can create a system with multiple access, named Code Division Multiple Access (CDMA) in which all channels operate on the same frequency bandwidth simultaneously (Todorovic, 2021).
Figure 1 shows a block diagram of the DS-SS transmitter. The spectrum is spread by the direct multiplication of the data signal, um(t), and the PN sequence, c(t), generated in the PN generator. The data signal in this case is assumed to be polar. The product of multiplication of the data signal and the PN sequence is labeled as uc(t), and that signal is further fed to the classic modulator. In the classic modulator, some of the classic digital modulation techniques are applied. PSK modulation techniques are most often used in applied systems. After the classic modulation, a DS-SS signal is obtained, and it is denoted with uos(t) (Todorovic, 2021).
Since the interval of the PN sequence chip is much shorter than the interval of the data bit, the chip rate defines the transmitting bandwidth.
Hence, the transmitted signal needs a much wider bandwidth than it would be necessary when only classical modulation is used.
Figure 2 shows the waveforms of the signals during the processing in the transmitter. In the example, Binary PSK (BPSK) modulation is applied.
Figure 1 - Block diagram of the direct sequence spread spectrum transmitter
The processing gain or spreading ratio is an important system parameter in DS-SS systems. The processing gain is defined as:
T
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PGb = 10 log ( PG ).
(3)
The processing gain determines the level of interference protection. In modern devices, the processing gain value varies between 20 and 60 dB (Todorovic, 2021).
Figure 2 - Signals waveforms in the direct sequence spread spectrum transmitter
The block diagram of the DS-SS receiver is shown in Figure 3. At the receiver, the received spread spectrum signal is despread by the multiplication of signals uDs(t) and c(t). The locally generated PN sequence has to be synchronized with the PN sequence contained in the received signal. If the two PN sequences are synchronized, the original data signal after correlation can be recovered. The aim of a bandpass (BP) filter is to remove the undesirable frequency components. Finally, classic demodulation is applied (Todorovic, 2021).
Figure 4 presents the power spectral densities of the DS-SS data signal and interference before and after signal despreading at the receiver. In Figure 4 (a), the data signal is spread and its power spectral density is below the power spectral density of the narrowband interference. In the receiver, the data signal is despread by multiplying with the locally generated and synchronized PN sequence. In addition to the data signal, interference is also multiplied by the same PN sequence, thereby spreading the interference signal. The result is shown in Figure 4 (b). The data signal is despead to the bandwidth before spreading, while the interference is spread and its impact on the data signal is significantly reduced.
Figure 3 - Block diagram of the direct sequence spread spectrum receiver
-Data signal ■ Interference
---Noise
(a)
(b)
Figure 4 - Data signal and interference in the frequency domain at the receiver: (a) before and (b) after spectrum despreading
Modern applications
DS-SS modulation has been implemented in many modern radiocommunication systems. In the text which follows, eight applications of DS-SS modulation in commercial and military radiocommunication systems are described.
Interim standard 95
The Interim Standard 95 (IS-95) is the first commercial cellular radiocommunication system based on the DS-SS modulation technique and Frequency Division Duplex (FDD). This system was developed and implemented in the United States just after the introduction of the Global System for Mobile Telecommunications (GSM) in Europe (Fazel & Kaiser, 2003). It is also known as CDMAOne.
The IS-95 operates in two frequency bandwidths. The first frequency bandwidth is from 824-849 MHz for uplink and from 869-894 MHz for
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downlink. The second uses a frequency bandwidth from 1850-1910 MHz for uplink and from 1930-1990 MHz for downlink (Fazel & Kaiser, 2003).
The channel bandwidth is 1.25 MHz. Without guard bands, the channel bandwidth is 1.23 MHz. The processing gain of the system is 19.3 dB. The modulation scheme for the downlink is coherent Quadrature Phase Shift Keying (QPSK). The convolutional code is used for error protection with a code rate of 1/2 and nine memory elements. In the uplink, noncoherence Offset QPSK (OQPSK) is used. For error protection, the convolutional code with a code rate of 1/3 and nine memory elements is used after which the Walsh-Hadamard (6, 64) code is implemented. Spectrum is spreading using the short and long codes. For the short code, the Walsh-Hadamard orthogonal code is used and for the long code, the PN code is used. Timeshift offset of the same PN sequence is used. The maximal throughput is 14.4 kbit/s for data and 9.6 kbit/s for voice. The system chip rate is 1.2288 Mchip/s (Fazel & Kaiser, 2003), (Garg, 2007).
CDMA2000
The CDMA2000 was created as the evolution of the CDMAOne (IS-95) standard. It represents the family of third Generations (3G) of mobile communications. The common feature of these two technologies is that they are based on the same type of short (Walsh-Hadamard orthogonal code) and long (PN code) codes for spreading. The same channel width of 1.25 MHz and the same chip rate of 1.2288 Mchip/s are used to enable backward compatibility with CDMAOne systems. Some transmission modes in CDMA2000 use the same chip rate and carrier spacing as CDMAOne, but different spreading factors and channel coding schemes. In contrast to CDMAOne, the Walsh codes are not fixed but of variable length to offer a variety of data rates (Schullze & Luders, 2005).
Some modifications have been introduced to offer more flexibility with the data rate and the chip rate. In the uplink, the CDMA2000 uses the DS-SS modulation technique. The chip rate is a multiple of 1.2288 Mchip/s. That multiple factor can be 3, 6, 9 or 12. Also, the channel width is a multiple of 1.25 MHz. The QPSK modulation scheme and fast closed loop power control are used here. For the downlink, a multicarrier concept has been defined. The number of carriers is also a multiple that can have values of 3, 6, 9 or 12. The width of each subchannel is 1.25 MHz and the chip rate is 1.2288 Mchip/s for each subcarrier (Schullze & Luders, 2005; Garg, 2007).
Wideband CDMA
The wideband CDMA (WCDMA) is the heart of 3G Universal Mobile Telecommunication Systems (UMTS). This system is very similar to the CDMA2000 and it is implemented in Europe and Japan (Prasad & Ojanpera, 1998).
Compared to GSM, the improvement refers to higher data rates transmission as well as the ability to send better quality multimedia content and to enable video calls. Transmission rates reach up to 2 Mbit/s. Frequency bandwidths are defined for the uplink and downlink: for uplink from 1920 to 1980 MHz, and downlink from 2110 to 2170 MHz. The DS-SS modulation technique is used in both links. Possible channel bandwidths are 1.25 MHz, 5 MHz, 10 MHz and 20 MHz, depending on the transmission rate and the number of users at the moment (Garg, 2007).
Three-structured orthogonal variable spreading factor codes as short codes and PN codes as long codes are used for spectrum spreading. For user separation, contrary to CDMA2000, different PN sequences are used instead of time shifts of the same PN sequence. The chip rate of the system is 3.84 Mchip/s (Prasad & Ojanpera, 1998).
A coherent QPSK modulation scheme is used for both the downlink and uplink. Several types of correction codes are used to control errors. While transmitting voice, convolutional codes with a code ratio of 1/3 and nine memory elements are used. A combination of the concatenated Reed Solomon code and the convolutional code is used for data transmission. For a high rate of data transmissions and services that require high quality, convolutional turbo codes are used (Fazel & Kaiser, 2003).
Global positioning system
The Global Positioning System (GPS) is the first satellite navigation system. The GPS program started in the early 1970s and was put into operation status in 1995. This system nominally has 24 satellites in the Medium-altitude Earth Orbit (MEO), (Hegarty, 2017).
The GPS consists of three segments: (1) satellite constellation, (2) ground-control/monitoring network, and (3) user receiving equipment (Kaplan & Hegarty, 2006). The GPS was developed for military purposes by the Department of Defense of the United States. Later, it was opened to civil applications. The GPS offers two types of services: (1) Precise Positioning Service (PPS) and (2) Standard Positioning Service (SPS) which is less accurate than the PPS. The positioning is based on trilateration. A GPS device measures the propagation delay of the signal received from the three satellites. As the positions of the satellites are
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known, the position of the GPS terrestrial device is obtained as the intersection of the three spheres. If more than three satellites are used during the determining of the position, a more precise position is obtained (Schullze & Luders, 2005).
The GPS uses two frequency bandwidths. The first frequency bandwidth is labeled as L1 and its carrier frequency is 1575.42 MHz. The other frequency bandwidth is labeled as L2 with a carrier frequency of 1227.6 MHz. The SPS uses only the first carrier, while the PPS requires both carriers. The modulation scheme that is used in the GPS is BPSK. The system processing gain is around 60 dB.
Two types of codes are used for the signal spreading. The first is Coarse/Acquisition or Clear/Access codes (C/A code) whose chip rate is 1.023 Mchip/s. The second is Precision or Protected code (P code) with a chip rate of 10.23 Mchip/s.
The C/A codes are Gold sequences generated in two 10-stage shift registers. These codes are used only with the first frequency carrier for SPS. These codes also divide the users who consume this service at the same time in nearby locations. The P code is used on both frequency bandwidths mainly for military applications. The determination of the user position with this code is more precise than with C/A codes (Schullze & Luders, 2005).
The accuracy of the determined position became higher during the time and after the enhancements and modernizations of the system. The specified accuracy of the SPS is 13 m in 95% of cases for horizontal positioning and 22 m in 95% of cases for vertical positioning. These specification data are valid in an ideal environment. The performance of the system is typically better than the specification. Errors can be greater than specification due to atmosphere processes, multipath propagation and different user equipment. The PPS gives much better results than the SPS. In 95% of cases, the position precision is 1-2 m for both horizontal and vertical positioning. Advanced user equipment and modern positioning techniques can achieve accuracies better than 1 cm.
GPS transceivers are embedded in everyday devices such as smartphones. The performance of some smartphones' GPS and some sophisticated GPS devices were compared. Sophisticated GPS devices give better results than GPSs on smartphones, but the performance of smartphones is accurate enough for everyday life for fast positioning requirements (Zandbergen & Barbeau, 2011).
Wireless Fidelity
Wireless Local Area Network (WLAN) is a network type that is developed based on the IEEE 802.11 standard. This technology is also known as Wireless Fidelity (Wi-Fi). WLAN connects a small group of devices on short distances via the wireless access point. During the evolution of the IEEE 802.11 standard, several types of that standard were created (Schullze & Luders, 2005). In the early beginning of Wi-Fi, DS-SS and FH-SS were planned to be used for transmission, but only products using DS-SS modulation have been established on the market. Today, there are several releases of IEEE 802.11 standards such as: IEEE 802.11a, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, IEEE 802.11ac, etc. (IEEE802.11, 2024).
The DS-SS modulation technique is implemented in the IEEE 802.11b and IEEE 802.11g standards. IEEE 802.11b was released in 1999. The system operates on a frequency of 2.4 GHz with a channel bandwidth of 22 MHz. Transmission data rates are going from 1 to 11 Mbit/s. The maximum ranges from the access point are from 35 m in the indoor environment to 140 m in the outdoor environment (Abdelrahman et al, 2015).
The available frequency bandwidth of 2.4 GHz has actually 83.5 MHz and it is from 2.4 GHz to 2.4835 GHz. In the United States in this bandwidth, there are thirteen channels, and in Europe, there are fourteen channels. According to the bandwidth of one channel, in order to avoid inter-channel interference, it is recommended to use three non-overlapping channels at the same time: in the United States, the first, sixth and eleventh channel, and in Europe the first, seventh and thirteen channel (Al Agha et al, 2016). In Figure 5, channel allocations are illustrated with the markation of non-overlapped channels in the United States. IEEE 802.11b is the longest, well-supported, stable, and cost-effective technique, but security is the main disadvantage. It has a limited number of access points. Sometimes the IEEE 802.11b devices are affected by interference from other products that operate in the 2.4 GHz bandwidth (Garg, 2007). Devices operating in the 2.4 GHz frequency are microwave ovens, bluetooth devices, baby monitors and cordless telephones.
Four years later, in 2003, IEEE 802.11g was released. In this release, the transmission data rates are significantly higher relative to IEEE 802.11b and it can be from 6 Mbit/s to 54 Mbit/s. This system also operates on a 2.4 GHz frequency range and has a narrower bandwidth of 20 MHz. The maximum ranges from the access point are from 38 m in the indoor
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Figure 5 - Channel allocation of the IEEE 802.11b standard
Wi-Fi found a place in many applications like Internet distribution, Voice over Internet Protocol (VoIP) phone access, gaming and connecting consumer electronics devices. It is used as a radio interface for the sensor devices in the Internet of Things (IoT), especially in smart home solutions (Guo et al, 2012).
The latest IEEE 802.11 standards use Orthogonal Frequency Division Multiplexing (OFDM) instead of DS-SS modulation.
ZigBee
ZigBee is a set of specifications for Wireless Personal Area Networking (WPAN) or short-distance wireless networks. ZigBee is an often used standard in Wireless Sensor Networks (WSN). WSN request node devices with low power, low data rate, low cost and short time delay characteristics. The ZigBee standard got its name from the zigzagging patterns of honey bees between flowers which represent the communication between nodes in a mesh network (Ramya et al, 2011).
ZigBee is a standard based network protocol developed by the ZigBee Alliance that uses the transport services of the IEEE 802.15.4 network specification (ZigBee Alliance, 2024). The reasons why ZigBee is used are: low cost, easy to deploy, secure and reliable communication, open standards protocol, low power, low maintenance, very long battery life, supports a large number of nodes, etc (Ramya et al, 2011).
ZigBee found applications in areas like home automation, telecommunication services, healthcare, remote control, etc. (Challoo et al, 2012).
The physical layer of the communication process is based on the DS-SS modulation technique. It operates in three frequency bandwidths. The first frequency bandwidth is between 868-868.6 MHz. Only one channel
labeled with 0 operates in this frequency bandwidth. This frequency bandwidth is used in Europe. The maximum throughput is 20 kbit/s. The modulation sheme is BPSK. The chip rate for spectrum spreading is 300 kchip/s.
The second frequency bandwidth is used in America. It encompasses a band from 902-928 MHz. Ten channels operate in this frequency bandwidth, labeled from 1 to 10. The maximum throughput is 40 kbit/s. The modulation scheme is BPSK and the chip rate is 600 kchip/s.
The third frequency bandwidth is used globally and that is the frequency bandwidth from 2400-2483.5 MHz. This frequency bandwidth is for industrial, scientific and medical (ISM) purposes. In this frequency bandwidth, there are 16 channels labeled from 11 to 26. The modulation type that is used is OQPSK. The maximum throughput in this frequency bandwidth is 250 kbit/s. The chip rate for spectrum spreading is 2000 kchip/s.
The transmission range can be between 10 meters and 75 meters, depending on power output and environmental characteristics, but in ideal conditions, it can achieve a distance of 150 meters in an outdoor environment (Ramya et al, 2011).
There are also some other standards similar to ZigBee which are used in WPAN and WSN based on DS-SS modulation such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPAN) (Mulligan, 2007), (Ma & Luo, 2008). However, ZigBee is still the most popular choice.
Unmanned aerial vehicles
The Unmanned Aerial Vehicle (UAV) represents an aircraft without a crew. These aircraft are also called drones. UAVs come in two varieties: some are controlled from a remote location while others fly autonomously based on pre-programmed flight plans. If UAVs are controlled remotely, wireless communication can be direct, which can significantly affect the control range, or by using the infrastructure of some of wireless technologies, such as mobile cellular systems, satellite communications, etc. If a UAV flies autonomously based on the pre-programmed flight plan, it uses the GPS or computer vision, where the camera or appropriate sensors detect obstacles and, if necessary, bypass them (Zhao et al, 2018).
The very beginning of using UAVs was for military purposes. Later, UAVs began to be used in various civilian applications (Milicevic & Bojkovic, 2021). UAVs are extremely important in modern military operations (Milic et al, 2019; Radovanovic et al, 2022).
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UAVs use two types of communication channels. One is used for the transmission of command and control data. Control data is sent in both directions, from the control station to the UAV (uplink) and from the UAV to the control station (downlink). The data sent through this channel is related to the control of the aircraft, whether it is generated by the operator by issuing commands or collected by the UAV during flight using the sensors it is equipped with. The data that the UAV sends to the control station is intended to provide a flight as reliable as possible. The second communication channel serves to collect the main data depending on the purpose of the UAV itself. It is mainly used for the transmission of video signals in the downlink (Milicevic & Bojkovic, 2021).
Due to the need for high reliability and resistance to interference, the use of DS-SS modulation was suggested (Todorovic & Orlic, 2010). Different vendors use DS-SS and FH-SS modulation techniques because of difficult interception and high resistance to interference. The most commonly used frequency bandwidth is the unlicensed band around 2.4 GHz. A brief overview of the vendors and the SS modulation techniques used is given in (Ristic et al, 2022).
Underwater acoustic communications
Underwater acoustic communications are used in military, scientific and civilian applications. Some of the applications are control of underwater vehicles and military facilities under the sea and on the seabed, instrument monitoring, pollution control, climate monitoring, prediction of natural changes, search and rescue missions, etc. (Song et al, 2019).
Underwater acoustic channels are time-frequency and spatially variable channels with limited bandwidth and interference that occurs due to multiple propagations (Du et al, 2019; Singer et al, 2009). These channels are bandwidth limited due to the increased attenuation at higher frequencies (Yang & Yang, 2008).
In this specific kind of communication, the DS-SS modulation technique is commonly used because of its excellent anti-interference ability, antichannel fading ability, and low Signal to Noise Ratio (SNR) performance. However, DS-SS modulation is limited by the narrow bandwidth of the underwater acoustic channel and has a low transmission rate (Chitre et al, 2008; Yang et al, 2021).
The biggest influence on the gain of an SS modulation is the fluctuation of the carrier phase. Fluctuation of the carrier phase directly affects the receiver to, for example, detect and decide which symbol was sent, which can lead to the inability to despread the signal. Carrier
fluctuation is caused by the appearance of the Doppler effect. Multiple signal propagation in underwater communications can lead to the loss of orthogonality characteristics of the PN sequence, thus reducing decoding performance. The core of DS-SS underwater acoustic communication is to obtain the matched filter processing gain and the fast carrier phase fluctuation interference caused by the Doppler effect is the most important factor affecting the matched filter processing gain (Du et al, 2019).
Underwater acoustic communications are used to send control signals and commands to Autonomous Underwater Vehicles (AUVs) and underwater sensors. Due to their nature, controlling signals and short commands often require high reliability (Qu et al, 2009).
Since the very beginning of the use of underwater acoustic communications, dS-SS modulation has been used at low frequencies and small channel bandwidths. These communications were first tested for military purposes (Loubet et al, 1997).
In (Loubet et al, 1997), tests of DS-SS signal transmission at carrier frequencies from 1.5 kHz to 2 kHz are described. Channel bandwidths are very narrow, up to 500 Hz with the maximum transmission rates of 80 bit/s. For the purposes of spread spectrum, Gold and Kasami sequences are used with the system process gains of 9 dB and 18 dB, depending on the sequence duration.
In (Yang & Yang, 2008) the DS-SS modulated signals had a carrier frequency of 17 kHz with a channel bandwidth of 4 kHz. The transmitted symbols are spread with an m-sequence with 511 chips. The sequence of chips is then transmitted using BPSK modulation. The obtained processing gain is around 27 dB.
Other realizations using DS-SS modulation for underwater acoustic communications are presented in (Yang et al, 2021) and (Qu et al, 2009).
Conclusion
The DS-SS modulation technique is based on direct spectrum spreading by multiplying a data signal with the PN sequence defined in the PN sequence generator. The DS-SS modulation technique has been successfully implemented in many military and commercial applications, due to its high protection against interference, making communication difficult for reconnaissance and eavesdropping, and its ability to provide code division multiple access. The history, principles and modern applications of the DS-SS modulation technique are presented in this article.
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Historically, the first developed DS-SS systems were used in military radiocommunications, since it requires a technique that has good resistance to various forms of electronic attacks. Later, DS-SS modulation found its applications in commercial radiocommunication systems.
Mobile phones, wireless sensor networks, wireless local area computer networks, radio-relay and satellite communications are just some of the applications. Communication links for UAVs control are very important, so they need to use technology that is difficult to interfere with. Applications to control signals of UAVs and underwater acoustic communications are promising in the near future.
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Espectro ensanchado de secuencia directa: historia, principios y aplicaciones modernas
Nenad M. Stojanovic3, Branislav M. Todorovicb, Vladimir B. Risticc, Ivana V. Stojanovicd
a Fuerzas Armadas de Serbia, Estado Mayor, Departamento de telecomunicaciones e Informática, Centro de Matemática Aplicada y Electrónica, Belgrado, República de Serbia, autor de correspondencia
b Instituto de Inteligencia Artificial de Serbia, Novi Sad, República de Serbia
c Universidad de Defensa de Belgrado, Academia Militar, Departamento de Telecomunicaciones e Informática, Belgrado, República de Serbia
d "Telekom Srbija", Belgrado, República de Serbia
CAMPO: telecomunicación
TIPO DE ARTÍCULO: artículo de revisión
Resumen:
Introducción/objetivo: La modulación de espectro ensanchado de secuencia directa se utiliza ampliamente en muchos sistemas de radiocomunicaciones. Al principio, esta técnica de modulación se utilizó en sistemas de navegación y comunicaciones militares. Posteriormente, las aplicaciones también se diversificaron en los sistemas de comunicación civiles. Hoy en día, existen muchos sistemas en los que se implementa modulación de espectro ensanchado de secuencia directa como parte del sistema. Este artículo tiene como objetivo sublimar el conocimiento sobre la técnica de modulación de espectro ensanchado de secuencia directa y sus aplicaciones.
Métodos: El artículo presenta una revisión del desarrollo histórico de la técnica de modulación de espectro ensanchado de secuencia directa, sus principios y las aplicaciones actuales más importantes.
Resultados: Basado en una gran cantidad de referencias, este artículo resume el desarrollo histórico, los principios básicos y las aplicaciones modernas de la modulación de espectro ensanchado de secuencia directa en sistemas de comunicaciones militares y comerciales.
Conclusión: La modulación de espectro ensanchado de secuencia directa se utiliza ampliamente en las radiocomunicaciones modernas inalámbricas y por satélite. Se espera que forme parte de futuros sistemas de comunicación globales.
Palabras claves: radiocomunicaciones, comunicaciones militares, sistemas móviles celulares, espectro ensanchado, secuencia directa, sistema de posicionamiento global, comunicaciones acústicas submarinas, vehículos aéreos no tripulados, fidelidad inalámbrica, ZigBee.
Спектр распространения прямой последовательности: история, принципы и современные применения
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Ненад М. Стояновича, Бранислав М. Тодорович6, ^
Владимир Б. Ристичв, Ивана В. Стояновичг а.
' Вооруженные силы Республики Сербия, Генеральный штаб, Управление информатики и телекоммуникаций ^-6), Центр прикладной математики и электроники, .у
го
г. Белград, Республика Сербия, корреспондент
6 Институт искусственного интеллекта Сербии, г. Нови-Сад, Республика Сербия ф
в Университет обороны в г. Белград, Военная академия, кафедра телекоммуникаций и информатики, г. Белград, Республика Сербия
г Телеком Сербия, а.д., г. Белград, Республика Сербия
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РУБРИКАГРНТИ: 49.27.00 Система передачи,
49.43.00 Радиосвязь и радиовещание ВИД СТАТЬИ: обзорная статья
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Резюме: £
Введение/цель: Модуляция расширение спектра методом прямой последовательности широко используется во многих системах радиосвязи. Впервые этот метод модуляции был использован в "ё военных системах управления, связи и навигации. Затем применение распространилось и на системы гражданской связи. В современном мире существует множество систем, в которых модуляция расширенного спектра методом прямой последовательности стала частью системы. Целью данной 8 статьи является обобщение знаний о методе модуляции с расширенным спектром методом прямой последовательности и его применении.
Методы: В статье представлен обзор исторического развития модуляции с расширенным спектром методом прямой последовательности, его принципов и наиболее важных в настоящее время приложений.
Результаты: В данной статье, основанной на большом количестве ю литературы, обобщаются историческое развитие, основные принципы и современное применение модуляции с расширенным Д спектром методом прямой последовательности в военных и 55 коммерческих системах связи.
Вывод: Модуляция с расширенным спектром методом прямой последовательности широко используется в современной беспроводной и спутниковой радиосвязи. Можно предположить, что он станет частью будущих глобальных коммуникационных систем.
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Ключевые слова: радиосвязь, военная связь, сотовые мобильные системы, расширенный спектр, прямая последовательность, глобальная система позиционирования, подводная акустическая связь, беспилотные летательные аппараты, Wi-Fi, ZigBee.
Пренос у проширеном спектру - директна секвенца: истори]а,
принципи и савремене примене
Ненад М. Стс^анови^3, Бранислав М. Тодорови^6,
Владимир Б. Ристи^в, Ивана В. Сто]анови^г
а Во]ска Ср6и]е, Генералштаб, Управа за телекомуникаци]е и информатику (J-6), Центар за приме^ену математику и електронику, Београд, Република Ср6и]а, аутор за преписку
6 Институт за вештачку интелигенци]у Срби]е, Нови Сад, Република Срби]а
в Универзитет одбране у Београду, Во]на академи]а, Катедра телекомуникаци]а и информатике, Београд, Република Срби]а
г Телеком Срби]а, а. д., Београд, Република Срби]а
ОБЛАСТ: телекомуникаци]е КАТЕГОРИJА (ТИП) ЧЛАНКА: прегледни рад
Сажетак:
Увод/цил>: Многи системи радио-комуникаци}а користе систем преноса у проширеном спектру помогу директне секвенце. На почетку, ова техника се користила у во]ним и навигационим системима, док се касни}е примере за различите сврхе и у цивилним телекомуникационим системима. Данас посто]е многи системи где jе техника директне секвенце имплементирана у неком ъиховом делу. Цил> овог рада }есте да сублимира знаъа о техници проширеног спектра са директном секвенцом и ъеним применама.
Методе: Представъени су истори]ски развоj технике проширеног спектра са директном секвенцом, принципи рада и на]важни]е савремене примене.
Резултати: На основу доступне литературе, рад сумира истори]ски развоj, основне принципе и савремене примене проширеног спектра са директном секвенцом у во]ним и комерци]алним телекомуникационим системима. Закъучак: Систем преноса проширеног спектра са директном секвенцом jе у широкоj примени у модерним бежичним и сателитским радио-комуникаци]ама. Очекуе се да буде део будущих глобалних телекомуникационих система.
Кл>учне речи: радио-комуникаци}е, во}не телекомуникаци}е, Яелирки мобилни системи, проширени спектар, директна секвенца, глобални систем позиционира^а, подводне акустичне ® комуникаци}е, беспилотне летелице, Wi-Fi, ZigBee. g-
Paper received on: 18.02.2024. J
Manuscript corrections submitted on: 07.06.2024. g:
Paper accepted for publishing on: 08.06.2024. ®
© 2024 The Authors. Published by Vojnotehnicki glasnik / Military Technical Courier -g
(www.vtg.mod.gov.rs, BTr.MO.ynp.cp6). This article is an open access article distributed under the ° terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/rs/).
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