Challenges for Radar Spectrum Sharing
Challenges for Radar Spectrum Sharing
Summary Description:
From the Joint E3 Bulletin, Volume 17, Issue 1, Febrary 2010.
Introduction: Below 3.5 GHz, only 12% of the RF spectrum is allocated for exclusive use by Federal Government Users, including the DoD. For more than a decade now, however, the DoD has been challenged by advances in Broadband Wireless Access (BWA) technologies and spectrum encroachment decisions. As today's trends continue, EMI problems in DoD operations can be expected to increase in proportion to spectrum sharing and relocation congestion. This article will address one example of challengs being faced by the DoD in efforts to maintain the performance of vital sensor systems in the face of mandated requirements to share spectrum with industry.
The Changing Landscape: Today's 3G networks and services are all based on a global ITU-R framework of standards known as International Telecommunications (IMT)-2000. With an estimated 800 million subscribers worldwide, further rapid growth is anticipated to see the market swell to 1.6 billion users within the next five years. Thus, lobbying efforts by industry for Federal spectrum is feverish and can be expected to continue for the foreseeable future.
The Competition for Frequencies: Internationally, the DoD must press its positions in NATO, via country-to-country alliances, and develop international support for its requirements at WRCs organized by the ITU. Nationally, the FCC has overarching responsibility for coordination of the many existing policies, but critical spectrum is also controlled by other parts of the Federal government, each with its own area of responsibility.
Many frequencies used by DoD are those that work best for the intended purpose. The statement could not be truer than for sensitive fixed, airborne, and maritime sensor systems whose operation within select frequency bands is essential for obtaining acceptable performance and the tactical edge. The technical characteristics of radars, such as high transmitter peak power levels and sensitive receiver designs have typically resulted in primary (exclusive) spectrum allocations for their operation. In recent years, however, spectrum over crowding has led to proposals for reductions of available spectrum for primary radar operations.
S-Band, which occupies roughly the 2 to 4 GHz range, consists of spectrum optimally suited for radar design. Dtection and track accuracy and maximum possible bandwidths are achieved using these frequencies. In addition, S-Band antennas enhance targets through greater angular resolution, greater transmitter effective radiated power, and greater receiver signal strength. S-Band radars compared to other radars also perform significantly better in rain. Rain attenuation of approximately 0.005 dB per mile represents the upper limit of what radar can tolerate at ranges exceeding 200 miles. This requires long-range radars to operate at frequencies below 5 GHz.
Enter WiMAX...: Worldwire Interoperability for Microwave Access (WiMAX) is a BWA. high-speed technology based on the IEEE 802.16 and European Telecommunications Standards Institute (ETSI), High Performance Radio Metroplitan Area Network (HiperMAN) standards. It uses advanced radio technology with Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) and smart antennas to deliver triple play services, i.e., voice, video and data over fixed and mobile networks without the need for direct line-of-sight with a base station.
The Mobile WiMAX standard, IEEE 802.16e of 2005, is an IMT-Advanced standard representing 4G technology, offering peak data rates of 128 Mbps downlink and 56 Mbps uplink over 20 MHz wide channels. The newest vaersion of the IEEE 802.16 standard, 802.16m or Mobile WiMax 2.0, could push data transfer speed up to 1 Gbps. The technology offers flexible and economically viable solutions for different deployment scenarios in urban, suburban and rural environments.*
Spectrum Sharing and Degraded Operations: There are many US military radar systems currently occupying the bands from 2.9-3.6 GHz. By NTIA footnote S5.433, ITU Regions 2 and 3 still provide a primary allocation to radars in the 3.4-3.6 GHz band. In other regions of the world where radars are deployed, however, WiMAX devices are permitted to operate using these radar frequencies. And, in fact, it is in this band that many countries are currently deploying fixed IMT and WiMAX networks. Under the WiMAX standards, most vendors are now manufacturing equipment that operate in the 3.5 GHz band, a significant threat for radar systems operating in costal (littoral) areas, subject to EMI from shore-based WiMAX networks.
It stands to reason that sharing of bands by traditional exclusive users with new dissimilar technologies probably increases the likelihood of inteference since traditional EMI mitigation efforts for these systems would have focused on other exclusive users of the band. This is certainly borne out in available data concerning radars.
Testing performed by the NTIA in 2006 on a variety of fixed, airborne and maritime radars demonstrated conclusively that radars can be severely impacted by WiMAX type communication/mobile devices. WiMAX devices use Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulations (QAM), and Binary Phase Shift Keying (BPSK) modulation techniques with coding schemes, the transmissions of which would appear as noise interference to a radar receiver. Noise in radar receiver circuits presents a significant limiting factor to system performance. Not suprisingly, results of the NTIA study noted a big distinction between the ways in which radar receivers can tolerate interference from other radars (pulsed, low duty cycle emissions) as opposed to non-radar type continuous wave (CW) and wide-band communications transmissions. Generally, radars were found to be extremely susceptible to the communictions signals, but not so much to other radars in the same band. Specifically, the study determined that interference from communication-type signals degrade radar performance at interference-to-noise (I/N) levels as low as -9 dB (well below the internal noise of radar receivers). At -6 dB, all but one of the radars in the study lost targets. The report categorized the target-loss effects from this type of low EMI as insidious; in that during testing no indication that interfeence was occurring. It was further noted that targets can be lost at any range in the presence of the interference, making use of EMI prediction metrics based on range reduction measures alone unreliable. Conversely, radar signal interference was found to be tolerated at I/N ratios as high as +30 to +63 dB. Radar interference rejection (IR) circuitry was found to be very effective at mitigating the effects of pulsed EMI.**
The NTIA study findings have since been supported by a number of independent analyses and test events conducted by the DoD and others, including one commissioned by the WiMAX Forum itself, an organization of more than 400 leading WiMAX industry operators, component and equipment manufacturers.
The DoD is currently conducting intensive research and in cooperation with industry is investigating EMI mitigation techniques for current deployed capabilities. Advanced concepts in radar design do hold promise for the future. One such effort is the Advanced Multifunctional RF Concept (AMRFC) being explored by the Naval Research Lab (NRL); intended to impact mutual signal interference, as well as ship's signature, manning, spares and life cycle costs. The idea centers on software defined functions combining communications, electronic warfare, and radar that drive the electronics of a single antenna suite to maximize interactive performance in a highly congested spectrum. Sharing is a relatively new DoD challenge, the concepts of which will mature through applications of advanced technology.
Conclusion: Today, few would argue against the DoD case as addressed here. A look at BWA growth projections suggests, however, that ther will soon be a few places to conduct radar operations away from these networks. While this raises concerns as to implications of such an increase in spectrum congestion, it also brings into focus two facts that define today's reality and the way forward for the DoDs response: Future radar designs must be tolerant of spectrum congestion and employ software that assesses frindly, neutral, and hostile use of the spectrum in a way that maximizes our spectrum dominance in war, but provides spectrum tolerance in times of peace. Secondly, EMC, the control of EMI at equipment, system, platfrm and force levels will become increasingly critical as technology and the cited trends progress. For future robust systems, adherence to and enforcement of DoD policies for spectrum supportability and E3 control is essential. Failure to do so may well relegate the effectiveness of critical National Security Systems to the auction block.
* What is WiMAX: A WiMAX Overview, Agilent Technologies, 9 October 2009.
** NTIA Report TR-06-444, Effects of RF Inteferene on Radar Receivers, US Department of Commerce, September 2006.
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