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Electromagnetic compatibility of wireless communications and sensor networks with mission critical electronic equipments is assessed for below-deck spaces on ships and submarines. With the proliferation of RF wireless technologies currently being deployed in enclosed, reflective spaces, such as found in aircraft, ships and buildings, it is critical to assess the resultant EME in these spaces, especially where potentially disruptive effects to critical electronic equipment systems may exist. Reverberation chamber test methods and their associated statistical analyses are well established [4],[5].
A sufficiently large sample of measured maximum received power values is collected over a given frequency bandwidth from each random-walk trial. Once the cavity calibration factor has been determined for a space, the maximum electric field component amplitude in that space can be predicted to the given confidence level for any input power Pinput. The random-walk technique is expected to report maximum values that would be generated from a large number of independent received power measurements obtained as the volume is source stirred and spatially sampled during the walk-arounds.
In addition, the cumulative distribution function in (3), estimated from the 108 maximum power data samples, is also obtained. Wireless communications and sensor networks emit intentional radiated RF energy into the reverberant environment encountered in ship and submarine below-deck spaces. The total radiated power (TRP) limit of 550 mW for multiple emitters in command and control spaces in surface ships is established in MIL-STD-464C to bound the total aggregate electric field that can build up in a reverberant space. The electromagnetic environment created in ship and submarine below-deck spaces by wireless network RF emissions can be predicted from a cavity calibration factor (CCF) derived from in-situ maximum received power measurements and statistical complex cavity theory. DoD Interface Standard MIL-STD-464C: Electromagnetic Environmental Effects Requirements for Systems, Department of Defense, Dec.
DoD Interface Standard MIL-STD-461F: Requirements for the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment, Department of Defense, Dec.
Gregory Tait is currently an electrical engineer with the Electromagnetic and Sensor Systems Department at the Naval Surface Warfare Center Dahlgren Division in Dahlgren, VA.
Paul Opperman is an electrical engineer with the Submarine Electromagnetic Environmental Effects (E3) Branch at the Naval Undersea Warfare Center in Newport, RI. The 7th Asia-Pacific International Symposium on Electromagnetic Compatibility & Signal Integrity and Technical Exhibition (APEMC 2016) will be held in Shenzhen, China, May 18 to 21, 2016. The IEEE MTT International Microwave Symposium (IMS) is the premier annual international meeting for technologists involved in all aspects of microwave theory and practice. With intentional wireless emission of radio-frequency (RF) energy into the reverberant spaces, it is necessary to measure and characterize the statistical electromagnetic environment (EME) and to predict maximum electric field strengths to a specified confidence level.
Examples of these spaces include below-deck compartments in ships and submarines, and in aircraft cabins and bays [1]-[3]. Such complex cavities are characterized by a chaotic electric field standing wave pattern of maximums and minimums whose locations are very sensitive to small changes in boundary conditions, such as occur from changes in physical structure (mechanical stirring), frequency (frequency stirring), and loading (material, personnel, equipment) over a period of time.

It was necessary to modify and adapt these techniques to address the special circumstances of field-operational spaces. The maximum fractional bandwidth around the center frequency f that can be utilized for frequency stirring is limited by the fact that insertion loss (and hence Q) is frequency dependent, and insertion loss bias in the data must be avoided. The walk-around process exploits both source stirring and volume sampling as the transmit and receive antennas are moved throughout the space. A visual check on the statistical distribution function fit to the measured maximum received power data is provided. Input power is 0 dBmA unit that defines a signal level by comparing it to a reference level.
The resultant increase in the ambient electromagnetic environment (EME) has been identified as a cause of electromagnetic interference (EMI) to mission critical electronic equipments.
A random-walk test technique has been developed and applied to measurements conducted in these reverberant spaces. Arandom-walk test technique has been developed and applied to measurements conducted in these reverberant spaces.
Of particular note is the introduction of wireless communications and sensor network systems into ship and submarine below-deck spaces. The dominant multi-path nature of such coupled reflective spaces creates a significant Rayleigh amplitude fading of signals in the communications channel.
This is a necessary consideration if these multiple frequencies are within the susceptibility bandwidth of the equipment.
This is confirmed by the very small variations of ±1-2 dB observed in the measured maximum power values in the laboratory setting of the reverberation chamber [6]. Lastly, the maximum electric field is predicted to a specified confidence level as a function of power emitted into the space.
Accordingly, interface controls are required to assure total system electromagnetic compatibility.
Modern spread-spectrum digitally-modulated RF signals possess spectral and temporal properties that can present a more stressful waveform for interfering with electronic components and systems than can waveforms of CW or 50%-duty cycle 1-kHz pulsed signals.
The maximum radiated power values from all emitters are summed to give the TRP value, giving consideration to available collision avoidance protocols, such as the carrier sense multiple access (CSMA) scheme. With maximum field strengths predicted to a specified confidence level, it becomes possible to assess the compatibility of the wireless network with mission critical electronic equipments based on the MIL-STD-464C standard. Areas of current interest include stochastic electromagnetics in complex cavities and wireless signal propagation in coupled reflective spaces. His current efforts include platform level E3 design of submarines, wireless system integration, E3 modeling, and intelligent data processing.

Maximum electric field strengths versus input power in several submarine spaces, ranging from high to low reverberant properties, are demonstrated.
In assessing the potential impact associated with using intentional emitters in reverberant spaces, we must address the cumulative build-up of the RF energy.
Therefore, it is vitally important to characterize the full statistical properties of the electromagnetic environment in assessing compatibility.
This number can be augmented by including maximum power data points at adjacent frequencies, a process associated with frequency stirring of the field distribution. In the very complex environment of the submarine below deck, the variation in the collected maximum power data samples is expectedly larger, on the order of ±3-4 dB. The predicted electromagnetic environment is compared to a maximum level specified for below-deck spaces in MIL-STD-464C [8]. MIL-STD-464C establishes electromagnetic environmental effects (E3) requirements for shipboard internal EME [8]. The wide power spectral density bandwidth, low temporal coherence time (rapid signal fluctuations), and large amplitude spikes of filtered white Gaussian noise RF emissions serve as upper-bound conditions for digitally modulated wireless RF emissions.
Such assessments rely on knowledge of radiated power characteristics of all emitters and need to be informed by specific wireless network configuration and operating protocols.
This effect is demonstrated in the max-hold traces for the 12 walk-around trials in the submarine. In Figure 5, the maximum electric field environment is predicted at the 95% confidence level as a function of total radiated power (TRP) into the space. In Figure 8, the maximum electric field environment is predicted at the 95% confidence level as a function of total radiated power (TRP) into the space.
Prior compatibility testing has been successful, but is costly and time-consuming, and verifies compatibility at only a single frequency and for a specific communications protocol. The use of band-limited white Gaussian noise as a stressful modulation waveform for robust susceptibility testing of electronic equipment and systems is also advocated as an up-front approach to risk mitigation. First, the 12 maximum power data points at any given frequency are spatially uncorrelated, as it is virtually impossible that any two maximum data points will originate under identical transmit and receive antenna positioning given the 6 degrees of freedom for orienting the antenna mounting pole in the space during the 12 random walk-arounds. The development of a band-limited white Gaussian noise emission source capable of testing a wide swept frequency range, while providing a conservative worst-case test of digital wireless signal spectra and waveforms, enables a cost-effective electromagnetic compatibility test that can be used across all platforms.

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