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Spectrum and Electromagnetics 101

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Spectrum and Electromagnetics 101

This section contains background and basic educational information on Spectrum Supportability (SS) and electromagnetic environmental effects (E3) issues, some introductory technical material on each and the importance of considering each early in the development and acquisition of systems.  These resources provide acquisition professionals an overview of the technical aspects of spectrum supportability and E3 considerations in military weapons system procurement.

Program management must address SS and E3 throughout the system life cycle when developing an electronic system that will operate compatibly in its fielded environment and must ensure that EMC and SS are designed into the system.  Numerous DoD agencies and organizations can assist the program office with E3 matters.



Definitions

Electromagnetic Environmental Effects (E3):   The impact of the electromagnetic environment upon the operational capability of military forces, equipment, systems, and platforms. Also called E3. (JP 3-13.1).  [From: Department of Defense Dictionary of Military and Associated Terms,  (August 2018)]

Electromagnetic Compatibility:  The ability of systems, equipment, and devices that use the electromagnetic spectrum to operate in their intended environments without causing or suffering unacceptable or unintentional degradation because of electromagnetic radiation or response. Also called EMC. [From: Department of Defense Dictionary of Military and Associated Terms,  (August 2018)]


Electromagnetic Spectrum Management:  Planning, coordinating, and managing use of the electromagnetic spectrum through operational, engineering, and administrative procedures.  [From: Department of Defense Dictionary of Military and Associated Terms,  (August 2018)]

 
Objective for E3 Control:  The objective of establishing E3 control requirements in the acquisition process is to ensure that DoD equipment, subsystems, and systems are designed to be self-compatible and operate compatibly in the operational electromagnetic environment.  To be effective, the program manager should establish E3 control requirements early in the acquisition process to ensure compatibility with co-located equipment, subsystems, and equipment, and with the applicable external electromagnetic environment. [excerpt from DAG Guidebook]


E3 Testing:  E3 can adversely affect the operational effectiveness of military forces, equipment, systems, and platforms. Today's increasingly complex military EM environment is congested and this is coupled with a reduction of spectrum allocation for exclusive military use. The mix of DoD-developed and commercial-off-the-shelf (COTS) electronic equipment increases the importance of the need for effectively managing E3 and spectrum usage in the battle space. It is the responsibility of the program manager (PM) to ensure, and the responsibility of the Developmental and Operational Test Agencies (D&OTA) to validate, the readiness of systems to be fielded into this environment. Historically, failure to verify equipment/platform EMC in the item's intended operational electromagnetic environment have caused costly program delays and reduced operational effectiveness. [excerpt from DAG Guidebook]



 

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ELECTROMAGNETICS - Frequency Allocation Table
 
 
ELF Radio Spectrum Microwaves Terahertz Infrared Visible Ultraviolet   X-Rays   Gamma Rays
3-30 Hz SLF*: 30-300 Hz 300 GHz -300 MHz 300 GHz-3 THz Far Infrared: 300 GHz-30 THz 790 to 405 THz 30 PHz - 790 THz
30 PHz-30 EHz More than 10 EHz
ULF*: 300-3000 Hz
Mid-Infrared: 30-120 THz 8 × 1014 to 3 × 1016 cycles per second (Hz)
VLF*: 3-30 kHz
Near-Infrared: 120-400 THz
LF*: 30-300 kHz
MF*: 300-3000 kHz
HF*: 3-30 MHz
VHF*: 30-300 MHz
UHF*: 300-3000 MHz
SHF*: 3-30 GHz
EHF*: 30-300 GHz

* Super Low Frequency (SLF)   Ultra-Low Frequency (ULF)  Very-Low Frequency (VLF)  Low Frequency (LF)   Medium Frequency (MF)   High Frequency (HF)   Very-High Frequency (VHF)   Ultra-High Frequency (UHF)

  Super-High Frequency (SHF)   Extremely-High Frequency (EHF)



The types of electromagnetic radiation are broadly classified into the following classes:

  • Gamma radiation penetrating EM radiation arising from the radioactive decay of the atomic nuclei. It consists of the shortest wavelength EM waves thereby imparting the highest photon energy.
  • X-ray radiation: a high energy EM wave with a very short wavelength that is able to penetrate and pass through many materials such as clothing, body tissue, and internal organs.
  • Ultraviolet radiationa band of the EM spectrum with wavelength from 10 nm to 400 nm, shorter than visible light but longer than X-rays.
  • Visible radiation:  a form of EM radiation that is visible to most human eyes
  • Infrared radiation known as Infrared and is a region of the EM radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). 
  • Terahertz (THz) radiation: sometimes known as the submillimeter band, consists of EM waves from 0.1 - 30 THz. One THz is 1012 Hz or 1000 GHz. Wavelengths of radiation in the terahertz band range from 1 mm to 0.1 mm
  • Microwave radiationEM radiation between radio waves having frequencies between 300 megahertz (MHz) and 300 gigahertz (GHz) and wavelengths between 1 meter and 1 millimeter.
  • Radio wavesare a type of EM radiation with wavelengths in the EM spectrum longer than infrared light and is used in communications like mobile phones, radio and television 


What is a decibel or dB?

The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities. The decibel is often used to express power or amplitude ratios (gains), in preference to arithmetic ratios or percentages. One advantage is that the total decibel gain of a series of components (e.g., amplifiers) can be calculated simply by summing the decibel gains of the individual components. Similarly, in telecommunications, decibels denote signal gain or loss from a transmitter to a receiver through some medium (free space, waveguide, coax, fiber optics, etc.) using a link budget. The decibel unit can also be combined with a suffix to create an absolute unit of electric power. For example, it can be combined with "m" for "milliwatt" to produce the "dBm". 0 dBm equals one milliwatt, and 1 dBm is one decibel greater (about 1.259 mW).




The following table describes some of the various modulation types with the principle advantages, disadvantages and uses.

Kind Description/Advantages Disadvantages Uses
Amplitude Modulation - double side band plus carrier (AM)The amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal. Simplifies receiver; preserves the waveform of message.  Analog. Doubles bandwidth occupancy; Requires extra signal power. Radio broadcasting, telephony, telegraphy, telemetering.
Single-sideband, suppressed carrier (SSB) Saves bandwidth occupancy; Conserves signal power. SSB transmitters are generally designed to minimize the amplitude of the carrier signal and when the carrier is removed from the transmitted signal, it is called suppressed-carrier SSB. Analog. Unable to handle relatively low frequencies; adds inherent delay; waveform of wanted message is not preserved. Long distance telephony and telegraphy over land and submarine cables.  History - SSB entered commercial service on January 7, 1927 on a transatlantic radiotelephone circuit between New York and London England. The high power SSB transmitters were located at Rocky Point, New York and Rugby, England. The receivers were located in Houlton, Maine and Cupar Scotland (Extremely quiet areas).

​Amplitude Shift Keying
A form of AM that represents digital data as variations in the amplitude of a carrier wave - the binary symbol 1 is represented by transmitting a fixed-amplitude carrier wave and fixed frequency for a bit duration of T seconds. If the signal value is 1 then the carrier signal will be transmitted; otherwise, a signal value of 0 will be transmitted.
ASK is sensitive to atmospheric noise, distortions, propagation conditions. Can be transmitted over fiber optic cable.
ASK was used at radio frequencies to transmit Morse code and was referred to as continuous wave (CW) operation
Angle Modulation - Narrow band Constant signal power Extra bandwidth occupancy Telecommunications, particularly broad-band carrier and TV over microwave radio ready systems.
Wide band Reduces noise in exchange for extra bandwidth occupancy; constant signal power; channel-grabbing property Bandwidth occupancy; sensitive to some forms of transmission impairment; signal power must be adequate to override wideband noise. Telecommunications generally, including such fields as telegraphy, telephony, radio broadcasting, telemetering, mobile communications for military and peacetime services, navigational aids, maritime beacons, and meteorological aids.
Pulse Amplitude Modulation (PAM) Permits multiplexing channels by time division. Sensitive to some forms of transmission impairment. Some versions of Ethernet comm use PAM. 100BASE-T4 and BroadR-Reach Ethernet standard use a three-level PAM modulation (PAM-3), 1000BASE-T Gigabit Ethernet uses five-level PAM-5 modulation. Pulse-amplitude modulation is used for the control of light-emitting diodes (LEDs), especially for lighting applications.
Pulse Duration Modulation (PDM) Permits multiplexing channels by time division; reduces noise in exchange for extra bandwidth occupancy; constant signal power Extra bandwidth occupancy; pulses vary in position. Microwave radio relay systems, telemetering
Pulse Position Modulation (PPM) Permits multiplexing channels by time division; reduces noise in exchange for extra bandwidth occupancy; constant signal power; saves signal power as compared to PDM. Extra bandwidth occupancy; pulses vary in position. Microwave radio relay systems, telemetering
Pulse Code Modulation (PCM) Permits multiplexing channels by time division; in exchange for extra bandwidth occupancy, tolerates considerable noise and serious transmission impairments, and may be repeated again and again without significant distortion; constant signal power. Extra bandwidth occupancy; transmits digital instead of analog signals, thereby introducing quantization noise if the receiver delivers analog signals. Multiplex telephony and telegraphy, TV, data processing, combined transmission and switching systems, telemetering.
Digital Modulation - Phase Shift Keying (PSK) Excellent performance for binary and quaternary systems; good bandwidth efficiency for M-ary systems Poorer performance for M-ary systems when M > 4 Voice, data, and video over satellite and terrestrial channels.
Digital Modulation - Frequency Shift Keying (FSK) Simple receiver structure; excellent performance for M-ary systems when M > 4 Poor performance of binary systems; bandwidth occupancy grows as M/log2 M in M-ary system FSK frequency modulation - digital information is transmitted through discrete frequency changes of a carrier signal.  Voice, data, and video over satellite and terrestrial channels.
FSK PowerPoint Presentation
Multiple Modulation Many depending upon circumstances; often accomplishes what cannot be done in one step. Extra steps usually add extra complexity A feature of all but the simplest transmitters and receivers.




Spread Spectrum Communications Techniques: 

A means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the bandwidth of the unspread bandwidth. Spread spectrum signals are typically transmitted by electromagnetic waves in free space with usage in both no military and military systems.

Motivation for using spread spectrum signals is based on the following facts:

  • These systems have the ability to reject intentional and unintentional jamming by interfering signals so that information can be communicated
  • Spread Spectrum signals have a low probability of being intercepted or detected since the power in the transmitted wave is "spread" over a large bandwidth or frequency extent
  • Since these signals cannot be readily demodulates without knowing the code or cypher, and it's precise timing, message privacy is obtained
  • The wide bandwidth of the spread spectrum signals provides tolerance to multipath (reflected waves that take longer to arrive at the receiver than the direct desired signal)
  • A high degree of precision in ranging (distance measuring) can be obtained by using one type of spread spectrum signal, with applications to navigation
  • Multiple access, or the ability to send many independent signals over the same frequency band.

There are four generic types of spread spectrum signals:

  • Direct Sequence (DSSS) - direct-sequence spread spectrum (DSSS) is used to reduce overall signal interference. This signal spreading makes the resulting wideband channel more noisy, allowing for greater resistance to unintentional/intentional interference.
  • Pseudo-noise (PN) Code -  also known as a pseudo random noise code (PRN code) is a code that has a spectrum similar to a random sequence of bits but is "deterministically" generated. 
  • Frequency Hopping Spread Spectrum (FHSS) a way to transmit radio signals by rapidly switching a carrier signal among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver
  • Linear Frequency Modulation (LFM) - LFM pulse is one in which the "instantaneous frequency" (i.e., change of phase) changes linearly over the duration of the pulse. 
  • Chirp Modulation: This is an older spread spectrum method that was originally developed for radar use. The basic idea of this modulation is to transmit a long rectangular pulse with a carrier frequency that is linearly increased from f1 to f2 (f2>f1). The frequency-modulated (FM) signal returned from the target passes through a filter in the receiver at a velocity of propagation proportional to the frequency. The result is a pulse that is much shorter in time duration than the transmitted pulse with a larger peak power content. Unchirped pulses such as interference or jamming pulses do not "compress" at the receiver, so that this method yields a processing gain or advantage for the chirped signal.
  • Time Hopping: Not normally used alone, is a method in which the transmitted pulse occurs in a manner determined by a pseudorandom code which places the pulse in one of n possible positions per frame. If n is sufficiently large, then the pulse is on only 1/n of the time, and again the transmitted pulse has a processing gain against a full frame jamming pulse of equal energy.
  • Direct Sequence Systems: Direct sequence systems were once the most prevalent method of communicating in spread spectrum communications. Direct sequence modulation is characterized by phase-modulating a sine wave by an unending string of pseudo noise code chips (symbols of much smaller duration than a bit). This unending string is typically based on a pseudo noise code that generates an apparently random sequence of code chips that repeat only after the code period.




For questions regarding any of the information located in the Spectrum and E3 Compliance CoP, please contact the Joint Spectrum Center at: disa.annapolis.dso.list.jsc-j5-training@mail.mil


        
        
        
        
        
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