Spectrum Sharing Innovation Test-Bed Pilot Program
Phase II/III Test Plan
Institute for Telecommunication Sciences
National Telecommunications and Information Administration
U.S. Department of Commerce
Boulder, CO 80305-3328
Table of Contents
1.5DUT to LMR Interference Mechanisms
1.6DUT Operating Channel Restrictions
1.7DUT Fixed Station Operations
1.8LMR Operating Parameters
1.9DUT Operating Parameter Restrictions
1.10Test Location and Pre-qualification
1.11Test equipment disclaimer
2Phase II: DUT sensor behavior
2.6Data Analysis and Interpretation
3Phase III: LMR hidden node test
4Phase III: DUT sharing with Conventional LMR system
5Phase III: DUT sharing with TRunked LMR system
6Phase III: DUT Geolocation characteristics
7.1Acronyms and abbreviations
The National Telecommunications and Information Administration (NTIA), in coordination with the Federal Communications Commission (FCC) and federal agencies, has established a Spectrum Sharing Innovation Test-Bed pilot program to examine the feasibility of increased sharing between federal and non-federal users. The pilot program is evaluating the ability of dynamic spectrum access (DSA) devices employing spectrum sensing and/or geo-location technologies to share spectrum with land mobile radio (LMR) systems operating in the 410-420 MHz Federal band and in the 470-512 MHz non-Federal band.
Since NTIA published the Test-Bed Phase I test plan on its website in February 2009, it has been engaged in testing DSA devices at its laboratory, the Institute for Telecommunications Sciences (ITS), in Boulder, CO. The Phase I tests assess the performance and behavior of DSA devices in response to simulated LMR environment signals within a laboratory setting.
The Phase I test plan document provided a notional definition of subsequent Phase II and III field testing. Phase II will assess the spectrum sensing capabilities of DSA devices in a live LMR environment with sufficient DSA transmitter attenuation to prevent interference to LMR systems. In Phase III, the DSA devices’ transmitter attenuation will be removed and the devices will be allowed to operate freely in a variety of live LMR radiofrequency signal environments.
This test plan covers Phases II and III of the test-bed and is organized as follows. Section 2 describes the Phase II test. This test assesses the devices’ behavior in a live LMR environment with their transmissions isolated from the LMRs. NTIA will examine the Phase II test data for the frequency and severity of potential co-channel interference with LMR systems. Provided the DSA devices detect and avoid LMRs, they will proceed to the less-controlled Phase III test conditions. The remainder of the test plan encompasses Phase III.
Phase III testing will follow a coexistence analysis approach as defined in IEEE 1900.2-2008. A coexistence analysis—as opposed to an interference analysis—is a system-level test intended to evaluate the ability of an incumbent system, i.e., LMR, to perform its intended function in the presence of emissions from a secondary system, i.e., DSA. Sections 3.1 and 3.2 examine the performance of DSA devices in the presence of a hidden node on the downlink and uplink, respectively. These tests will provide, under repeatable test conditions, an estimate of the minimum separation distance required to prevent DSA-to-LMR interference, i.e., the interference range. The results may be used to adjust DSA policy databases or test procedures in preparation for subsequent Phase III tests. For example, a DSA device may be reconfigured to lock out critical LMR channels or a drive test route modified to guarantee a minimum separation between DSA devices and critical LMR systems. Devices that do not exhibit anomalous behaviors in the Section 3 tests will proceed to Section 4.
Section 4 tests DSA devices operating under less-controlled conditions within the coverage area of a live conventional voice LMR network. Similarly, Section 5 examines the potential impact of DSA devices on a trunked LMR system with particular emphasis on compatibility with LMR control channel signaling. Section 6 describes a geolocation test for suitably equipped devices. Section 7 completes the test plan with lists of acronyms and abbreviations, definitions, and references.
Two different types of radio systems are involved in these tests. Land mobile radio is the largest primary use within the band. DSA transceivers are the secondary use being evaluated for compatibility with the primary one. Hereafter this document refers to DSA transceivers as the device under test (DUT). Some DUTs employ communications architectures where the access point controls the remote. Wherever this functional distinction may be significant, the master will be termed the DUT base and the slave termed the DUT subscriber.
Similarly, LMR transceivers are divided into LMR base stations and LMR subscriber units. An LMR base station is a fixed station with an elevated antenna(s). They may be deployed in multichannel configurations and operate in either trunked or conventional configurations. LMR subscriber units are divided into mobiles and portables. LMR mobiles are typically vehicle mounted while LMR portables are typically handheld. An LMR subscriber unit can support both conventional and trunked mode operation.
This document describes NTIA’s plan to assess the ability of DUTs to share spectrum with LMR systems in the 410-420 MHz frequency band. The test procedures and resultant test data are intended to complement the Phase I test results. The Phase I lab tests methodically characterized several key operating parameters of each DUT type for spectrum sensor performance, channel abandonment, and emissions characteristics. Numerous test trials were required to derive statistically valid measurements of these parameters. Assessing these parameters in a dynamic field test environment would be logistically infeasible and would introduce unmanageable measurement uncertainties. Instead, the Phase II and III tests will focus on DUT to LMR system interactions that could not be replicated in a controlled laboratory test environment. Also, this plan will test hypotheses that emerged from the interpretation of the Phase I test results, in particular, a field assessment of DUT and LMR responses to hidden node conditions.
The DUTs in the test-bed employ a variety of sensor-based detect and avoid technologies. They use unique RF sensors, modulations, channel access schemes, and time division duplex schemes. Based on Phase I test results, it is anticipated that there will be wide variations in equipment behavior. Therefore, the type and depth of testing described herein is of a general nature. NTIA will tailor to each DUT detailed test procedures derived from these general plans.
NTIA will limit testing to the assessment of DUT interactions with LMR systems. Limited performance characteristics of DUTs were observed during Phase I testing with two transceiver systems. However, with DUTs in limited availability, it is not feasible to conduct a meaningful assessment or characterization of DUT system performance in the field. Also, assessment of the compatibility between different types of DUTs is also outside the scope of this plan.
The tests described in this document will assess particular phenomena such as the hidden node condition or characteristics unique to particular DUTs. Therefore, some of the tests described in this document will not be applicable to all DUTs. Furthermore, additional test procedures to examine unexpected DUT behaviors may be required.
Nothing in this test plan should be construed as a final rule, interference criteria, or acceptable use policy. While the results of these field tests will inform the rulemaking process, they will not necessarily establish specific precedents or pass/fail criteria. Further system modeling and testing may be required to establish a complete set of criteria required for establishing rules.
1.5 DUT-to-LMR Interference Mechanisms
Physical layer interactions between LMR transmitters and LMR receivers are well understood. Conformity assessment criteria are documented in standards TIA-603-C, TIA-102.CAAA-B, and TIA-102.CAAB-B. Methods to model interactions between various types of narrowband emissions and different LMR receiver types are described in depth in TSB-88.1-C. LMR-to-LMR interactions can provide a model for assessing DUT-to-LMR interactions.
The simplest interaction is a co-channel transmission. The corresponding standard LMR test procedure is the co-channel rejection test. This test measures the performance of a receiver when unwanted signals are present on the operating frequency and are thus passed without attenuation through the receiver’s intermediate frequency (IF) stage filter. In the LMR-to-LMR case, the test is performed with a steady state unwanted signal, which yields a carrier to interference (C/I) ratio on the order of 8-9 dB. DUT-to-LMR co-channel interference is similar, except that meaningful measures of C/I must consider the power spectral density of the DUT emission within the equivalent noise bandwidth (ENBW) of the LMR receiver. In addition, a DUT waveform’s lower duty cycle should have less of an impact on an LMR receiver.
An adjacent channel DUT-to-LMR interaction may also occur. The standard LMR test for this is the adjacent-channel rejection measurement. When an LMR receiver is in the proximity of an adjacent-channel transmitter, some of the modulated signal passes through the receiver’s IF filter. Adjacent-channel interference is more difficult to examine in the context of a DUT transmitter in a field environment. Since DUTs are not constrained to operate on a fixed channel, the same test trial may contain co-channel, adjacent-channel and possibly other interactions. The test conditions cannot be controlled to the degree necessary to separate and quantify each interaction.
Another interference mechanism is out of band emissions (OOBE). LMRs have very low OOBE, so there is no standard test for the interaction of out of band LMR emissions with LMR receivers. Some DUT transmitters are broadband and may have significant out of band emissions. Phase I testing suggests that the impact of OOBE on an LMR can be determined analytically by calculating the amount of energy that passes through the ENBW of the LMR IF stage filter at a given frequency offset from the center of the DUT channel.
Finally, LMR receiver desensitization is possible in the presence of strong DUT transmissions. Practically speaking, in a field test it will be impossible to distinguish the effects of DUT OOBE from LMR receiver desensitization due to the DUT carrier itself. Even so, incidences of DUT impairment to LMR receivers will be fully investigated.
1.6 DUT Operating Channel Restrictions
The DUTs participating in the test-bed employ unique methods of opportunistically selecting and, in some cases, aggregating narrowband segments of spectrum. Different types of carrier aggregation are supported. One approach is to aggregate adjoining subcarriers to form a single broadband carrier. Another approach is to notch out selected subcarriers from what is otherwise a contiguous broadband transmission. Finally, some systems aggregate disconnected narrowband carriers into a coordinated set of transmissions. The subcarriers do not necessarily align with the LMR channelization scheme or match the LMR channel bandwidths. DUT transmissions may consist of one or more subcarriers that span all or part of the 410-420 MHz test-bed band. For each DUT there is a minimum set of subcarriers that is required for the device to function normally, which was defined in Phase I as the minimum DUT operating channel. From the LMR channelization perspective the minimum DUT operating channel intersects a corresponding set of LMR channels. The DUT may use one or more sets of LMR channels to span the 410-420 MHz band.
During Phase III testing, NTIA will restrict the DUTs to operate on one or two sets of LMR channels. LMR systems that are actively involved in the test-bed will be aggregated in one set of channels. A second set of channels available for DUT operation may contain non-participating LMR systems. All other LMR channels in the band will be Locked-Out LMR Channels that the DUTs cannot use. The purpose of this restriction is to constrain DUT operation to a reduced number of LMR channels to improve spectrum monitoring of measurement events.
1.7 DUT Fixed Station Operations
Some of the DUTs in the test-bed normally operate with fixed base and subscriber stations. These DUTs will remain stationary during data collection. The base station and subscriber unit may be repositioned to simulate various separation distances from LMR transmitters. If necessary, the link between the base station and subscriber unit will be reestablished after a DUT is repositioned.
1.8 LMR Operating Parameters
This test plan assesses the performance of DUT detection of LMR downlink transmissions separately from LMR uplink transmissions. While some DUTs implement policies that ensure the DUTs avoid operation on one frequency based on the detection of a paired transmission on a different, paired frequency, it is assumed throughout this test plan (with the exception of the Section 6 test procedure) that frequency assignment data of this nature are not generally available. Such a capability is predicated on routine DUT access to data abstracted from the government master file (GMF), the Federal government’s list of frequency assignments. Access to the GMF is outside the scope of this test plan.
The Telecommunications Industry Association has documented interference protection criteria for LMRs in TIA TSB-88.1-C. TSB-88.1-C catalogs the delivered audio quality (DAQ) for a variety of LMR modulation and vocoder combinations including the Project 25 configuration considered in this test plan. It indicates that a given carrier-to-noise (C/N) ratio will provide a perceived audio quality and intelligibility that meets public safety requirements. This corresponds to a DAQ of 3.4, which is present when a Project 25 LMR has a mean bit error rate (BER) of 2% and a C/N of approximately 18 dB in a noise limited mobile environment. The 2% BER value is defined in TSB-88.1-C as a pass/fail threshold for system acceptance testing when a DAQ of 3.4 is stipulated. Under this condition, test locations where BER values are just below the 2% threshold represent the edge of coverage. This test plan uses this criterion throughout, since these locations are the most susceptible to interference.
1.9 DUT Operating Parameter Restrictions
Consistent with the methodology adopted for Phase I testing, prior to Phase II and III testing, the test-bed participant will be permitted to evaluate its DUTs to ensure that they are functioning properly. After this point, the test-bed participant will not be allowed to change them, including the operational parameters, without prior coordination with NTIA.
1.10 Test Location and Pre-qualification
NTIA will select test sites in the greater Denver metropolitan area for the Phase II and III tests, using spectrum surveys to determine the suitability of proposed test locations. The spectrum surveys will be conducted using ITS’s Radio Spectrum Measurement System (RSMS). RSMS employs custom-built low noise preselectors with FFT-based spectrum analyzers. Customized signal processing of the RSMS data reduces in-band impulsive noise for an accurate determination of usage profiles for each LMR channel in the band. NTIA Report TR-07-448A provided a detailed description of the test system.
The spectrum surveys will permit NTIA to confirm the source of all emissions in the 410-420 MHz band, characterize their amplitudes and temporal characteristics, and assign one or two sets of LMR channels for each DUT appropriate to the various test procedures. Prior to testing each DUT, NTIA will obtain an updated spectrum survey.
1.11 Test equipment disclaimer
This test plan identifies certain commercial equipment and materials to accurately describe the technical aspects of the test equipment. In no case does such identification imply recommendations or endorsement by NTIA, nor does it imply that the material or equipment identified is the best available for this purpose.
2 Phase II: DUT sensor behavior
In this stage of testing NTIA will assess the DUT sensor behavior in the presence of live LMR systems of varying complexity. The DUTs will be isolated from the LMR systems using a custom test system that will simultaneously log DUT and LMR transmissions. Test engineers will review the logs and assess the spectrum access behavior of the DUTs. Knowledge of the spectrum signature of each DUT type will differentiate DUT and LMR transmissions.
This test will act as a coarse filter of DUT behavior in a complex RF environment. It will record and identify any serious interference events and provide an indication of the DUTs’ ability to operate in a congested LMR environment. Assessment of the DUTs’ ability to detect weak LMR signals is reserved for Section 3.