In modern 5G commercial cellular technology, the main goals are to provide very high data rates, massive machine-type communications, and ultra-low latency/ultra-reliable communications for consumers and businesses.
In mission-critical communications, such as public safety or defense networks, communication goals are slightly different. While some 5G targets align well with mission-critical networks, the following differentiators are present:
While cellular architecture is prominent in 5G, mission-critical networks rely more on an ad hoc establishment of the network. This mandates a mesh architecture that can be created on-demand, as the bulk of communications is done through “normal” network architecture.
Radio access technology
In 5G, a key concern is adding new bandwidth to existing congested bands to support the ever-increasing needs of new consumer services. In mission-critical systems, the key concern is to transmit messages that are clear and intelligible. While both can use beamforming as radio access technology, for example, the use case scenario is completely different. In 5G, only the data rate is of concern, while in mission-critical communications, key aspects such as intentional jamming is suppressed using a similar radio access technology. Signal design in mission-critical communications is guided by low probability of interception and ease of decoding clear commands.
Use case variety
5G presents well-defined usage scenarios, e.g., an urban environment with a terrestrial cellular network. In mission-critical communications, any means necessary are used to get a message through, including satellites, airplanes, or handheld devices. This means that the propagation environment, and therefore transceiver architectures, are considerably different due to high Doppler and delay. Additionally, the lifespan of a cellular device is years while in mission-critical systems, the lifespan is tens of years, increasing the demands of maintenance and supply of critical components.
Top Five Things When Designing Mission-Critical Communication Systems
While standard cellular propagation test systems are not directly suitable for mission-critical test cases, they can create a cost-efficient, scalable and small footprint solution for propagation testing if properly used and configured. A test designer should consider the following aspects when selecting a propagation test system.
1. No external components
The network architecture of mesh radio systems differs significantly from traditional cellular networks. Thus, the test systems of cellular networks are not directly applicable to mesh radio testing unless many external components (combiners) are used to route signals correctly between the radios.
When the number of radios increases in the test system, the number of external routing components increases, which yields a test system that is prone to both human errors and cabling errors. Therefore, it is mandatory to simplify the test system as much as possible to keep it controllable and maintainable.
Signal routing in the propagation tester must support the full connection of the mesh architecture, including subsets of a full mesh, like loop, convoy or star.
Figure 2 depicts various 16-radio bi-directional mesh topologies (full mesh, star, convoy and loop). As we can see, the number of radio links is easily very large [N*(N-1) in full mesh, where N=number of radios]. Routing of the signal using any external component becomes prohibitive, thus propagation testing requires internal signal routing such that only radios are connected to the test system.
2. Ease of scenario creation
When the number of radios increases in the test system, modeling of the relative movement, signal-to-noise ratio, and any other parameter becomes prohibitive or requires a highly-skilled expert to calculate all parameters for the emulation. It is very important that the test system user interface supports mission-critical scenario creation, with multiple radios in a realistic environment, in such a way that anyone in the organization can create the scenarios needed.
The software tools must be scalable, i.e., they must provide an open interface so that the same scenario can be used in any other test system freely. Equally, the test system must have an open interface to import data from external sources to mimic any desired test requirements.
An example of a user-friendly scenario creation is shown in figure 3.
With this interface, the user places the radios into a network layout and the software then calculates the dynamic radio channel parameters (delay, angular behavior, path loss) automatically and streams the environment to hardware-in-the-loop system. Traditional link-level simulations are insufficient for scenarios where reliability is tested against highly dynamic propagation conditions and dynamically varying network architectures.
3. Real-time controlled system
Modern mission-critical mesh radios are adaptive and deploy LPI (Low Probability of Intercept) signaling. This means that radios can change any parameter of the physical layer signaling on-the-fly. Thus, the test system must reflect this feature to properly address the critical functions of the radio. The environment must be created in real-time, without any recompiling or other time-consuming mathematical processes when changing test system parameters.
For example, the test equipment must have an open interface with the ability to use neural networks and adapt to known jammers and interference. A closed interface or time-consuming process to change the environment can slow down the testing of some modern tactical radios.
4. Wide bandwidth
Even though the radios themselves may use narrowband technology, the test system must support very wide bands with a single hardware resource. One key LPI technique is frequency hopping, where consecutive frequency slots can be separated from each other. However, since the propagation environment is time and frequency selective, the propagation is uncorrelated between the hops. Therefore, the estimate of CSI (Channel State Information) cannot be used in a consecutive hop as it oversimplifies the propagation environment and therefore does not reflect what is happening in the real world.
The second LPI technique is to use DSSS (Direct Sequence Spread Spectrum) technologies, where the signal is coded over very high bandwidth to transmit the information below the noise level. It is beneficial for both techniques to use a single hardware resource that supports wide bands. If the test system is not able to do this, it leads to bandwidth concatenation, which translates to requiring more expensive test systems due to the internal limitations of the test hardware.
Wide bandwidth also allows accurate propagation modeling. Channel models are defined by filter taps and the resolution of the tap spacing is inversely proportional to bandwidth.
5. Extension to MIMO technologies
MIMO radios provide attractive features to mission-critical deployments. They provide reliability and higher data rates on the same frequency band with reasonable small complexity. However, it is critical to understand that MIMO radios deploy the spatial domain of propagation, therefore propagation modeling needs to include dynamic spatial domain behavior, otherwise MIMO benefits are not tested at all.
MIMO configurations span from TX or RX diversity to NxM spatial multiplexing techniques and it is essential that the test system supports MIMO radio testing without any external components, i.e., MIMO is an internal feature of the test platform.
For propagation modeling, all test equipment must support multiple independent propagation environments simultaneously (called channels). A single chassis needs to have enough different frequencies, independent channels and internal MIMO radio connections to sufficiently support comprehensive mesh testing.
For a deeper dive on comprehensively testing mesh networks, reference Spirent’s whitepaper.