NETEX UWB

Arthur H. Light SYColeman Corporation

Test Results

ABSTRACT

The goal of the Defense Advanced Research Projects Agency's (DARPA) Networking in Extreme Environments (NETEX) program is to create a wireless networking technology for the military user that enables robust connectivity in harsh environments and support its integration into new and emerging sensor and communication systems. Phase 1, resulted in a thorough understanding of the effects of Ultrawide Band (UWB) system operation on existing military spectrum users based on modeling, simulation, and measurements. In order to accomplish this task, the DARPA Advanced Technology Office (ATO) procured a set of UWB emitters and broadband antennas to use as interference sources and contracted with the Naval Air Warfare Center Aircraft Division (NAWC AD) Electromagnetic Environmental Effects (E') Division to provide candidate victim systems from the existing (legacy) US naval aircraft and shipboard inventory for testing. Testing was conducted on seventeen legacy systems during October 2002 through March 2003. The purpose of this paper is to provide a brief overview of the results of these tests.

This paper will provide a brief discussion of the UWB emissions as described by the US Federal Communications Commission (FCC) and describe the generic UWB emitter used for these tests. It will then provide a discussion of the results as they apply to the purpose of the NETEX program.

INTRODUCTION

In the spring of 2002, the Defense Advanced Research Projects Agency's (DARPA) Advanced Technology Office

Distribution Statement "A" (Approved for Public Release, Distribution Unlimited).

~ ~ ~~~~~

Authors' Current Addresses: A.H. Light. SYColeman Corporation, Fairfax. VA, USA. Based on a presentation at The 2003 Ultrawideband Conference. 0885/8985/04/ $17.00 8 2004 IEEE

(ATO) initiated the Networking in Extreme Environments (NETEX) Program to create a wireless networking technology for the military user that enables robust connectivity in harsh environments and support its integration into new and emerging sensor and communication systems. Phase lmask 1 resulted in a thorough understanding of the effects of Ultrawide Band (UWB) system operation on existing military spectrum users based on modeling, simulation, and measurements. As part of this program, DARPA AT0 contracted with the Naval Air Warfare Center Aircraft Division (NAWC AD) Electromagnetic Environmental Effects (E3) Division to provide candidate victim systems from the existing (legacy) US naval aircraft and shipboard inventory for testing. These systems were subjected to conducted electromagnetic interference (EMI) testing similar to the tests of MIL-STD-462 [l] procedure CS04 and MIL-STD-462D [2] and MIL-STD-461E [3] procedure CS104. The results of this investigation provided the information necessary to evaluate the potential for UWB devices to interfere with existing military radio communication and sensing systems and help to understand how UWB systems could be implemented in a manner that makes optimum use of their unique capabilities.

The tests discussed were conducted at the NAWC AD Patuxent River and St. Inigoes, Maryland facilities. The primary testers at NAWC AD were A. Light and E. McNett of SYColeman Corp., and T. Carney of Systems Planning Corporation. They were supported by numerous US Navy employees and contractors who are expert in the operation and maintenance of the various units under test (UUTs). The testing was conducted during the period of October 2002 through March 2003.

In order to keep this report unclassified, the actual systems tested are not named. The DARPA NETEX Program will be publishing individual test reports for each system and an overall final report detailing the composite results.

The Generic UWB Generator The test program used a set of generic UWB generators

designed to generate essentially the same pulse but over a large range of pulse repetition frequencies (PRFs) and pulse groupings to represent different types of UWB generators considered operational today and in the near future. The peak output power of the generators was approximately 1 Wakk (W) with a frequency occupancy above -50 decibels with respect to

IEEE A&E SYSTEMS MAGAZINE, JUNE 2004 35

0 1 2 3 4 5 6 7 Fiequency IGHrl

Fig. 1. Time Domain Representation of the Most Commonly Used Pulse from the Generic

UWB Generator, the Positive Double Exponential (Pos DE) Pulse

the highest measurable frequency (dBc) to be approximately 7 GHz. Figure 1 shows a single pulse of the most commonly used UWB output pulse in the time domain. Figure 2 shows the frequency occupancy of the UWB generators for a number of PRFs. The spectra in Figure 2 present the average power in a 3 MHz bandwidth for each sample window within the displayed frequency span, 7 GHz. In addition to the internally generated pulse waveforms, the UWBs would also accept an external modulation pulse with a PRFequaI to or less than 100 Mpps for either the Pos DE or Neg DE. Because the external triggers for the Pos DE and the Neg DE were independent, a combination of Pos DE and Neg DE pulses could be generated to produce very specialized waveforms. In order to test potential victims with assigned frequency bands above 4 GHz, the test team used one or a combination of amplification techniques to produce sufficient UWB power. The primary amplifier was a c,ommercial-off-the-shelf (COTS) 1 - 26 GHz, 40 dB amplifier with a peak output power of about 1 W. The amplified output spectra of these devices are shown in Figure 3.

OVERVIEW OF THE TEST PLAN

The generic test plan for UWB UUTs is laid out in the TEST MASTER PLAN FOR THE NETEX PROGRAM, hereinafter called the Test Master Plan (TMP). The TMP provides for a progressive suite of tests, each of which builds on preceding tests:

The initial test for each UUT is a standard system sensitivity test, based on the published UUT system sensitivity requirements (SSR), such as minimum detectible signal (MDS); n dB signal to interference plus additive noise (SINAD), where n is specified in system documentation;, bit error

Fig. 2. Frequency Occupancy of the Positive Double Exponential Pulse for PRFs of

100 Mpps, 50 Mpps, 10 Mpps, and 2.5 Mpps

10

0

8 do

!a i I ,

40

9)

80 2 3 4 5 6 7 8 B 10 11 12 13 14 15 16 17 18 l B 20

F m q u o n o ~ ~ ~ I

Fig. 3. Spectra of Unamplitied UWB, UWB Amplified by Solid State Amplifier, EW TWTA, and Both

rate (BER) equal to or greater than some predetermined number, m; or some other mutually agreed upon system sensitivity specification (SSS). This SSR then became the interference criterion for all subsequent tests on that UUT.

Following the system sensitivity test, the W T was tested for susceptibility to white noise in the UUT’s receiver radio frequency (RF) passbandhandwidth at system sensitivity plus 6 dB (AT + 6 dB).

Following the white noise tests, the UUT was tested for susceptibility to each of the U W B test waveforms in the UUT’s receiver RF passband. The TMP describes a set of seven (7) UWB pulse

36 IEEE A&E SYSTEMS MAGAZINE, JUNE 2004

waveforms to be used against each mode of the victim units at each test frequency. Each test waveform (TW) is described in abbreviated form in Table 1.

SUMMARY OF RESULTS

Altogether seventeen (17) different receivers, operating in a total of thirty-two (32) modes and at a total of fifty-nine (59) frequencies spanning frequency bands from 30 MHz to 16 GHz, were tested. Altogether over 1,000 individual tests were conducted over a period of five (5) months. Receivers tested included communications, aircraft guidance systems, and radars. The results of these tests were a set of observations about the interference potential of UWBs including: types of UWB waveforms which probably would or would not cause interference to an UUT; UWB signal strength necessary to cause interference to an UUT; and an estimation of the

Table 1. Descriptions of UWB Test Waveforms

Test Pulse Modulation

(TW) Frequency (PRF) Waveform Repetition of PRF

1 Test Frequency

2 TFIm (TF)/n

( m 2 n )

3 RBW

4 RBW

5 RBWI10 6 RBW*lO 7 RBW/lOO

Not Applicable (NlA)

Dithered at greatest available percentage (%)

which was less than the full receiver

RBW. Dithered to fill all

or a portion of RBW

Modulation designed to cause

maximum interference to selected victim.

NIA N/A NIA

effectiveness of the FCC mask to protect potential victim systems.

8 0

BO

f -loo B

8 -130

f -140 a

Fig. 4. Comparison of Onset of UWB Interference to Thermal and Background Noise and the FCC Mask

What UWB Waveforms Do and Do Not Cause Interference ?

The initial answer to the above question is that all waveforms tested caused interference to at least some of the UUTs. However, certain waveforms are less likely to cause interference than others. Two very general answers are:

High PRFs cause interference at the frequencies of their spectral lines, but there is a significant amount of spectral space between the lines where there is very little or no interference. If the PRF is not steady, the energy in the lines will decrease but the interference levels between the lines will increase and may eliminate the possibility of all other forms of RF operations within some limited area around the U W B transmitter.

Very low PRFs are very unlikely to cause interference at any frequency. An exception to this is peak sensitive, low P@ victim systems, such as dumb radars, which may suffer high levels of interference

Other waveform generation and frequency management techniques are also available to help reduce the probability of the occurrence of interference to legacy receivers which may be operating in the vicinity of a UWB system.

At What Level Does an UWB Source Cause Interference?

variables for a general answer. There is no easy answer to this. There are just too many

IEEE A&E SYSTEMS MAGAZINE, JUNE 2004 37

How Does UWB Interference Compare to Broadband Noise Interference?

There is no good generalization to be made about the probability of UWB interference as compared to white noise.

How Well Does the FCC Mask Protect Potential Interference Victims?

As shown in Figure 4, many of the systems tested were susceptible to UWB interference at UWB emission levels below those allowed by the FCC mask.

REFERENCES

[I] MIL-STD-462, Military Standard Measurement of Electromagnetic Interference Characteristics.

[2] MIL-STD-462D. Military Standard Measurement of Electromagnetic Interference Characteristics, 13 January 1993.

[3] MIL-STD-461E, Military Standard Requirements for the Control of Electromagnetic Interference Emissions and Susceptibility.

[4] The Defense Advanced Research Projects Agency (DARPA) Networking in Extreme Environments (NETEX) Program,

Test Master Plan For The Netex Program, unpublished.. A

38 IEEE A&E SYSTEMS MAGAZINE, JUNE 2004