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Design and Analysis of an Information Exchange-Based
Radar/Communications Spectrum Sharing System (RCS3)
Alex Lackpour and Dr. Alan Rosenwinkel
Advanced Technology Laboratories
Lockheed Martin
Cherry Hill, NJ, 08002
{alex.lackpour, alan.m.rosenwinkel}@lmco.com
Dr. Joseph R. Guerci Information Systems Laboratories, Inc.
Vienna, VA 22182
Dr. Apura Mody BAE Electronic Systems
Nashua, NH, 03061
David Ryan
Spectrum Effect
Bothell, WA 98011
Abstract— The recent explosion in demand for mobile
broadband wireless communications (comms) services has
created a shortage of Radio Frequency (RF) spectrum
below 6 GHz. The demand for additional spectrum places
pressure upon governments to release wide swaths of RF
spectrum that are exclusively reserved for radar systems.
This paper introduces a new spectrum sharing system
architecture and set of coexistence mechanisms that
mitigate RF interference effects based on the exchange of
internal state information between radar and comms
systems. The exchange of state information enables radar
and comms networks to apply a variety of coordinated
coexistence mechanisms that shrinks the minimum
required standoff range between systems while sustaining
the performance of each system. Our analysis and high
fidelity simulations show that a careful selection of
coexistence mechanisms reduces the minimum required
standoff range up to 8 times compared to the standoff
range when no coexistence mechanisms are used.
Keywords— spectrum sharing, radar interference effects,
dynamic spectrum access, Spectrum Access System (SAS)
I. INTRODUCTION
There is an opportunity to address the explosion in demand for additional spectrum below 6 GHz by finding new methods and approaches to share spectrum between radars and mobile broadband wireless communications (comms) networks. Licensed Long Term Evolution (LTE) cellular networks and unlicensed WiFi networks are densely packed into RF spectrum in urban areas. Similarly, mobile military radio comms networks are experiencing a worsening spectrum shortfall due to a increased number of fielded radios and the high throughput needed to transport data collected by tactical sensors. The growing spectrum deficit for mobile broadband comms cannot be addressed long term by compressing more users into their existing spectrum. Fortunately, there is an enormous opportunity for both radar and comms networks to share the same RF spectrum using new information exchange-based coexistence mechanisms described in this paper. A
related opportunity is that existing radar and radios use software-controllable waveform generators and resource managers that can be updated through minor modifications to support the execution of the coexistence mechanisms.
A major challenge for sharing spectrum between existing radars and comms networks is that spectrum sharing was not a requirement when the systems were originally designed. In our nomenclature, spectrum coexistence describes a scenario where the radar and radios share the same RF spectrum by applying coexistence mechanisms that sufficiently reduces inter-system RF interference. Excessive RF interference from comms will raise the noise floor of a radar and prevent it from receiving weak target returns – leading to a reduction in its maximum target detection range for a fixed probability of detection (Pd) and probability of false alarm (Pfa). Similarly, mobile broadband comms systems are designed to operate robustly in a mobile RF fading channel with interference from other radios; however, a sustained group of high power radar pulses may significantly degrade the comms link quality. The susceptibility of either system to inter-system RF interference and the maximum tolerable performance degradation drives the minimum required standoff range between the systems [1][2].
A near term approach for radar/comms spectrum sharing is the Citizens Broadband Radio Service (CBRS). The Federal Communications Commission (FCC) recently approved the CBRS operating rules in the United States as a shared band from 3.55 to 3.7 GHz [3]. The rules call for the use of a Spectrum Access System (SAS) that contains a combination of spectrum sensing, radio position reporting, and RF physics modeling. When these functions are combined, the SAS can dynamically manage RF interference and dynamic frequency assignments of civilian mobile broadband comms networks operating in the same band as military radars, such as the SPN-43 Air Traffic Control (ATC) radar. While this is a good pathfinder example for our information-based spectrum sharing system, the rules for the CBRS only require a minimal amount of information about the radar to be represented in the SAS for the sake of preserving radar operational security (OPSEC). Therefore, the SAS is limited to dynamically assigning or revoking short-duration spectrum leases as the shipborne radar
This material is based upon work supported by DARPA under Contract No. HR001-13-C-0082. The views expressed are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government.
2016 IEEE Radar Conference (RadarConf)
978-1-5090-0863-6/16/$31.00 ©2016 IEEE
moves away from, or towards, a coastal geographic protection zone [4].
The remainder of the paper is divided into three sections: Section II describes a new approach for dynamically sharing spectrum between radar and mobile broadband comms networks, Section III describes the detailed architecture and control loops of the spectrum sharing system, and Section IV presents quantified performance metrics based on high fidelity modeling and simulation.
II. A NEW APPROACH TO RADAR/COMMS SPECTRUM
SHARING
Fig. 1 is a diagram of the scenario where a radar and a
civilian cellular comms network shares the same RF spectrum
at some standoff range. The objective for maximizing shared
use of the RF spectrum is to minimize the standoff range
between the radar and comms network while simultaneously
maintaining a mutually acceptable minimum performance
level for each system. In terms of the radar, the first order
performance metric is the reduction in radar-to-target
detection range for a fixed Pd and Pfa with respect to the
standalone case (comms not present). For the comms network,
the first order metric is data throughput reduction for a fixed
comms range with respect to the baseline case (radar not
present).
Table 1 contains a partial list of the coordinated spectrum coexistence mechanisms and their associated technical challenges and practical solutions. This table does not address operational challenges such as maintaining military radar OPSEC or civilian user privacy. Different types of coexistence mechanisms may be applied in combination to obtain additional RF isolation and further reduce the minimum system standoff range. Not all radars or radios will be able to apply all of the listed coexistence mechanisms due to their internal limitations. Therefore, the sharing system needs to be
aware of the capabilities and limitations of the RF devices and properties of the Radar/Comms interference channel so that it can intelligently select the set of coexistence mechanisms that maximize sharing performance while meeting other operating goals.
In the case of spectrum sharing with an untrusted civilian comms network, the sharing system can protect the military radar’s OPSEC by using coexistence mechanisms where the radar does not need to provide information about its future dwell schedule (see the description of the Radar Avoiding Comms (RAC) coexistence mechanism in Section IV.A). Alternatively, if the spectrum sharing goals can only be met by increasing RF isolation between systems, the details of the radar’s operation are protected by applying an information obfuscation process that distorts and transforms the parameters of the radar’s future dwell schedule. Obfuscating the true operation of the military radar will lead to degraded radar and/or comms performance in the shared spectrum, but that is an acceptable tradeoff in order to maintain radar OPSEC.
In the case of spectrum sharing with a trusted military comms network, a variety of coordinated and cooperative coexistence mechanisms can be applied when the radar
Fig. 1. Radar/Comms spectrum sharing scenario addressed in this paper
TABLE II. NEW TYPES OF COORDINATED COEXISTENCE MECHANISMS ENABLED BY INFORMATION EXCHANGE
Dimension Description of the Coexistence Mechanism Technical Challenges & Practical Solutions
Space
Radar and/or comms co-schedule their transmissions after exchanging
information about the radar’s position, scheduled mainbeam pointing
angles, and comms node laydown. Measurement and/or prediction of
the radar/comms interference channel is used to identify which radar
dwells angles and comms node locations need to be deconflicted.
Radar and comms need to be time synchronized to interleave their
access to the shared spectrum. In a cluttered environment, RF
multipath can lead to unexpected RF interference. Periodic channel
sounding is used to measure the RF interference channel for better
radar dwell avoidance scheduling.
Frequency
Radar and/or comms exchange future scheduled frequency channel
hopping patterns and use this information to simultaneously access
non-overlapping bands when their Tx/Rx filtering provides sufficient
RF isolation
Sharing spectrum at relatively short standoff ranges requires
improvements to stop band rejection for existing Tx/Rx analog RF
filters. Minimize added cost by combining this technique with other
sharing mechanisms.
Time
Radar and/or comms exchange information about their scheduled
Tx/Rx events so that their spectrum access can be temporally
interleaved and deconflicted.
Stability and limited duration of predicted future radar schedule creates
tight information exchange latency constraints on the systems
Information Exchange Comms Links (See Figure 2)
Code
Radar and/or comms exchange information about their Code Division
Multiple Access (CDMA) waveforms to coordinate selection of code
words that offer interference rejection.
CDMA requires transmit power control between radar and comms to
minimize the near-far interference problem - CDMA comms has been
largely replaced by OFDM/OFDMA for modern civilian comms.
Military Comms are more likely to use CDMA.
Multipath
Precoding the transmitted Multiple-Input-Multiple-Output (MIMO)
radar and comms waveforms enables a MIMO null space projection
interference mitigation technique [5]
Radar and comms need a MIMO capability –imperfect channel
estimates lowers RF isolation between systems – feedback of MIMO
channel coefficients to the transmitter requires a low latency channel
state information (CSI) feedback link for mobile systems. Enable this
mechanism in combination with others when requirements are met.
Polarization
Radar and/or comms exchange information about their current/future
antenna polarizations and the systems select Tx/Rx antenna modes to
maximize RF isolation due to cross-polarization discrimination
RF reflections will depolarize the transmitted waveform and reduces
RF isolation in high clutter environments. Enable this technique in
combination with others when the interference channel supports it.
provides its geolocation and future radar dwells schedule in terms of the sequence of mainbeam angles, carrier frequencies, and waveform pulse description words. As a defensive tactic, military radars with electronically steered beams operate in an intentionally unpredictable manner from the perspective of an outside observer. Explicitly sharing the military radar’s future schedule of dwells with a trusted comms network enables time-synchronized comms nodes to mute their RF transmissions when they know the radar is pointing its mainbeam in a direction that creates a high level of RF interference between the systems (see the description of the Comms Avoiding Radar (CAR) coexistence mechanism in Section IV.B). This is in contrast to sharing spectrum with fixed-frequency mechanically rotating radars. In this case, there is a good opportunity for comms networks to estimate the parameters of the rotating radar’s schedule from its RF emissions. Once estimated, the radar’s parameters are used to predict when periodic high RF interference events will occur and the comms network can schedule to mitigate those events [6].
The RAC and CAR coexistence mechanisms mitigate RF interference between coordinated military radar and comms networks due to the following three system properties:
1. Military phased array radars create highly directional
antenna patterns that rapidly steer across a wide spatial
search area –spatial interference rejection allows a
comms network to simultaneously operate in the
radar’s sidelobe at close range.
2. Frequency hopping radars and comms networks possess
a relatively narrow instantaneous RF bandwidth that
spans a small fraction of their total RF tuning range –
frequency dependent interference rejection allows a
comms network to temporarily tune to a different RF
frequency channel and simultaneously operate at close
range.
3. Radars schedule their future dwells in terms of time,
frequency, and space. If a representation of that future
dwell schedule is shared with a spectrum sharing
comms network, the radios can mitigate severe
interference events before they occur.
III. ARCHITECTURE OF THE SPECTRUM SHARING SYSTEM
A high level architecture diagram of our Radar Comms Spectrum Sharing System (RCS3) is shown in Fig. 1. An authorized and trained Spectrum Manager uses the operator interface to centrally configure and control the sharing system. The benefit of implementing spectrum sharing control functions across a multi-level control plane is that it enables the creation of control loops with different time scales so that the spectrum sharing can robustly scale to a large number of sharing RF devices while experiencing graceful performance degradation in a dynamic operating environment.
A. Slow Time Control Loop via Sharing Policy Composition
At the highest level of the RCS3 control plane, the Global Coordinator (GC) block is responsible for continuously aggregating information about the performance of the spectrum sharing process, information about the location, type, and status of radar and radios, and the operator’s
performance goals for sharing the spectrum. The system’s sharing performance is periodically presented to the operator for their approval or optionally selecting a different course of action. During online operation, the GC iteratively applies an optimization algorithm that composes, or generates, a sharing policy that is designed to meet the operator’s minimum performance constraints for each system while minimizing the required standoff range.
The spectrum sharing policy describes the combination of coordinated coexistence mechanisms that the radar and radios need to apply to attain the operator’s performance goals. The policy also describes the type and update rate of the inter-system information that is needed to execute the coordinated coexistence mechanisms. Finally, the sharing policy contains a description of what spectrum sharing circumstance should trigger the various combinations of coexistence mechanisms to meet the operator’s performance goals. It is intuitive that combining additional coexistence techniques increases the RF isolation between the systems, thereby reducing the minimum required standoff range.
However, our analysis and simulation results prove that the RF device that applies a spectrum sharing coexistence mechanism experiences performance degradation for the sake of reducing RF interference transmitted into the other system. This leads to two important conclusions that drives the design of the RCS3:
1. Coexistence mechanisms should be applied only when
absolutely necessary to avoid unnecessary system
performance degradation.
2. The spectrum sharing system needs to run a continuous
optimization process to identify which coexistence
mechanism should be executed over a time varying
scenario.
B. Fast Time Control Loop via Sharing Policy Enforcement
Once composed, the sharing policy is transferred to the lower level of the control plane where the Radar Local Coordinator (RLC) and Comms Local Coordinator (CLC) rapidly enforce the policy directly with the radar and radios, respectively. The control plane data links between the RLC and CLC are designed to quickly transport delay sensitive information such as a future radar dwell schedule and other
Fig. 2. High-level architecture of the RCS3
rapidly exchanged control messages. If the deployed system does not include low latency data links between the Local Coordinators, then the GC must generate a feasible sharing policy that exclusively uses coexistence mechanisms that are delay tolerant to inter-system information.
Fig 3. contains a detailed architecture diagram of the functions of the RCS3 and how the previously mentioned slow and fast time control loops flow through the system. At system initialization, the GC collects information about the location and type of RF systems from existing authoritative spectrum management databases, as well as dynamically collected information about the spectrum sharing RF devices that register with the RLC and CLC. This situational awareness is then used to build a detailed RF Environment Map that contains the known/planned locations of radar and comms networks and their spectrum operating requirements. The contents of the RF Map is then transformed in a statistical RF system model that describes the spatial RF activity of the radar and comms networks. If the RF propagation pathloss between the radar and comms networks is unknown, a low complexity RF propagation prediction model is automatically invoked to evaluate the site-specific RF pathloss due to natural effects (e.g., terrain elevation masking and atmospheric ducting) and RF shadows created by human-made objects like urban structures. The final stage of the predictive modeling process
takes the statistical inter-system RF interference and passes it through a statistical performance evaluation model that quantifies key performance parameters of the radar and comms networks while applying combinations of feasible coexistence mechanisms.
The number of combinations of coexistence mechanisms that need to be evaluated are reduced by first applying heuristic rules that constrain the solution space. The following are some example rules, presented as questions, which are evaluated to identify the subset of feasible coexistence mechanisms.
Does the radar support MIMO?
Does the radio have a tunable bandpass filter?
Is the comms network a trusted military radio network?
Based on that result, an initial spectrum sharing policy is composed by rank ordering the feasible combinations of coexistence mechanisms by their effectiveness and sharing performance. The highest rank sharing policies are then sent to the RLC and CLC for real-time policy enforcement within the radar and comms networks, respectively. System performance data is periodically collected while the policy is enforced and then forward by the RLC and CLC to the GC so that it can trigger new sharing policy and provide an update on the sharing performance through the operator’s interface.
Fig. 3. Detailed architecture of the RCS3
IV. ANALYSIS OF RESULTS
The remainder of the paper focuses on quantifying the performance of two coexistence mechanisms that work with existing radar and radios: Radar Avoiding Comms (RAC) and Comms Avoiding Radar (CAR).
The RAC coexistence mechanism is designed to provide RF interference reduction between radar and radios when sharing RF spectrum with untrusted civilian comms networks. The RAC concept is that a spectrum sharing radar receives information about the civilian comms network’s frequency hopping pattern and uses that information to determine how to tune the frequency of its radar dwells. The frequency hopping of the civilian comms network ensures that the radar has full access to its entire RF tuning range over relatively short time periods. This allows the radar to maximize its frequency diversity to improve its target detection while maintaining less predictable operation. While LTE and WiFi protocols and radios were not originally designed to support slow frequency hopping, there is no inherent reason why a synchronized network of radios could not be made to slowly frequency hop together through small modifications to their software and their RF carrier tuning circuits. The radios also need additional RF channelization band pass filters to prevent receiver blocking when a strong out-of-band radar pulse is received. As shown in Fig. 4, a practical solution for minimizing the additional hardware cost per radio is to constrain the radio to tune only to two small subbands within the radar’s tuning range. This greatly reduces the complexity and additional device cost by adding two low cost RF channelization filters while still gaining the benefits of the RAC coexistence mechanism.
The CAR mechanism is designed to reduce RF interference experienced by the radar when sharing spectrum with a comms network. For the CAR coexistence mechanism, the radar shares its future schedule of dwells with a network of time synchronized comms so that they can proactively avoid creating RF interference between the systems. Once the comms network receives the radar’s future schedule, each comms device can evaluate its standoff distance, spatial angle offset, and frequency offset per dwell to decide if they need to mute their RF emission on a dwell-by-dwell basis. In order to apply CAR with an untrusted comms network, the radios receive an optimized spectrum sharing policy that specifies the minimum spatial offset angle and/or frequency offset that should cause a radio node to mute its transmission for the duration of the dwell. Alternatively, when applying CAR with a trusted
comms network, a radio could receive information about the spatial and frequency-filtering characteristics of the radar so that it can evaluate its spectrum sharing policy in terms of the minimum RF isolation it experiences per dwell and whether it is sufficiently high for simultaneous transmission with the radar.
A. Simulation Results for Radar/Comms Spectrum Sharing
Table II summarizes the simulation results of our high fidelity models of a high power surface radar sharing spectrum with a dense small cell LTE network and a military MANET radio in two separate scenarios. Recall that the first order performance metric for the radar is target detection range reduction compared to standalone radar case for a fixed Pd and Pfa. The performance threshold for radar is reached when the target range reduction exceeds 5% for any mainbeam angle within 120 seconds. The first order performance metric for the comms network is throughput reduction compared to the standalone case for a fixed comms link range. The performance threshold for comms is reached when the data throughput reduction exceeds 5%. For when a coexistence mechanism is applied, the assumption is that comms performance is further reduced and the standoff range dependent threshold is reached when the comms throughput reduction exceeds 50%. In either case, the minimum system standoff range for both systems is set by either the radar or comms network first crossing their minimum performance thresholds.
TABLE II. SIMULATED SPECTRUM SHARING PERFORMANCE
Radar and Small Cell LTE Radar and MANET
Baseline
Standoff Range 40 km (no coexistence)
52 km (no
coexistence)
RCS3 Enabled
Standoff Range 5 km (using RAC) 7 km (using CAR)
Standoff Range
Reduction Ratio 8x 7.4x
B. Radar Avoiding Comms (RAC): Temporally Coordinated
Slow Frequency Hopping between Radar & Comms
The Radar Avoiding Comms (RAC) information-based coexistence mechanism requires that a comms system be able to slowly frequency hop according to a schedule that is shared with the RLC. The RLC then works with the radar (see Fig. 3) to generate a radar schedule that avoids using the frequency bands when they are scheduled to be used by the comms network. The simulation result are shown in Fig. 5 for a slowly frequency hopping small cell LTE network and high power search radar that achieves a standoff range reduction of 8x.
RAC offers a unique set of benefits:
1. Sensitive radar internal state information does not need
to be shared with an untrusted civilian comms network.
2. The untrusted civilian comms networks can share
spectrum without muting their transmissions so that its
data throughput is not reduced.
Another valuable aspect of the RAC coexistence mechanism is that it preserves the ability of the radar to access its entire RF tuning range in the shared band over a relatively short time period.
Fig. 4. The Radar Avoiding Comms (RAC) mechanism influences
how the radar shares spectrum with LTE & WiFi radio networks
C. Comms Avoiding Radar (CAR): Time Coordinated
Transmit Muting for Military MANETs
Spectrum sharing between high power radar and a military MANET creates significant technical challenges at short standoff ranges. This is because both systems transmit with high RF power and frequency hop in unpredictable patterns over the shared RF band as a self-defense mechanism.
Another technical challenge is how to rapidly disseminate the radar’s future short-term schedule to the interfering MANET radios before the radar executes the schedule. One approach is to transmit the radar schedule to a single gateway MANET radio and rely on the network to forward the schedule to other interfering radios; however, some MANET medium access control (MAC) protocols and ad hoc routing protocols introduce significant message latency over network routes with multiple hops. Our solution is to co-locate a low latency military radio with the radar so that the encrypted schedule can be broadcast directly to all of the interfering MANET radios. Each MANET radio would need to contain two receivers, one for participating in the MANET and a second for receiving the broadcasted radar schedule. This approach does not necessarily increase the cost of the radio equipment because of a new trend to integrate more than one radio receiver in MANET radios.
To share spectrum efficiently, radar and comms need low out-of-band RF emissions or they will cause interference to the other system when their tuned offset frequencies are relatively small. Legacy radars use high power tube amplifiers to amplify the radar pulse on transmit. The problem is that these amplifiers also generate wideband noise during the pulse. Similarly, military MANET radios generate high adjacent band interference when they transmit. Spectrum sharing efficiency will improve with enhanced RF frequency filtering on transmit because the two systems could be tuned to smaller frequency offsets – allowing more simultaneous users in the shared band.
Fig. 6 shows the simulated end-to-end performance of a high power radar and MANET sharing the same spectrum using the CAR coexistence mechanism. The plots show that the minimum standoff range between the MANET and radar is set by the aggregate interference generated by the MANET radios and experienced by the radar. The plot on the left shows the baseline throughput reduction from the standalone case without CAR enabled and then with CAR enabled with three levels of RF isolation between the systems. As the required RF isolation level provide by CAR is increased, the normalized throughput within the MANET will decrease because granting
additional RF isolation requires the radios to mute their transmissions at ever-wider spatial angles and frequency offsets from each radar dwell. Through these simulation results we determined that the standoff reduction between MANET and a high powered radar is 7.4x.
V. CONCLUSION
High power military radars and a variety of mobile broadband comms systems can share RF spectrum at short standoff ranges if their operating capabilities are updated to support information exchange-based coexistence mechanisms. This paper described the preliminary design of our dynamic spectrum access system, the RCS3, and how it uses collected situational awareness to select which set of coexistence mechanisms should be used to mitigate inter-system RF interference. Our high fidelity system simulations show that using the RAC coexistence mechanism between a high power shipborne radar and high density small-cell LTE radio networks will reduce the standoff range by 8x over the baseline system separation range. In addition, our simulation of spectrum sharing between high power shipborne military radar and a military MANET using the CAR coexistence mechanism resulted in a 7.4x reduction of standoff range over the baseline system separation range.
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Fig. 5. Small Cell LTE Throughput as a function of system standoff range
(left) and number of radar interference events in 120 seconds (right)
Fig. 6. Military MANET throughput as a function of system standoff range
(left) and number of radar interference events in 120 seconds (right)
52km
7km Zoomed View
40km
5km
