Understanding The Limitations Of Modern Military Radio Testbeds
In the past 10 years, the use of military radios has increased dramatically, and, in turn, so has the need to test those radios. In part, this increase stems from the advent of such military-based communication applications as the land mobile radio system and tactical networks, as well as the introduction of both software defined radios and cognitive/multi-user Multiple-Input Multiple-Output (MIMO) wireless radios. Typically, these military radios are tested together in a closed mesh network, with programmable attenuators used to vary the attenuation and simulate different distances between the radios. While this test platform (i.e., military radio testbed) enables test engineers to conduct rigorous, transparent, and replicable testing of military radios, it is not without its limitations: namely the size of the mesh network, or, in simpler terms, the number of radios that can be tested together.
JFW Industries, specialists in attenuation and RF switching, designs and manufactures a wide range of solutions for testing military radios. For those test engineers wanting to build their own testbed by piecing together individual components, JFW provides the necessary programmable attenuators, as well as the power divider/combiners to be cabled together. JFW also provides RF test systems with components already packaged together with a computer interface for test engineers in need of an all-in-one solution. These test systems are available in three different configurations or designs, depending on how many military radios test engineers want to test together. The result is a comprehensive approach to enabling today’s test engineers to quickly and successfully test modern military radios.
Military Radio Testbed Options
When testing military radios, a key requirement for the test engineer is to be able to test as many together as possible. Doing so provides the engineer with critical information on how well the military radios will operate and inter-operate in the real world. The problem, however, is that this is not always a straightforward task. The test setup the engineer chooses to use for that testing often has its own set of limitations, and these limitations generally dictate exactly how many radios can be tested together. JFW offers three different test setups for testing military radios: Hub Fan-out, Full Fan-out, and Limited Fan-out.
Hub Fan-Out Design
Most test engineers looking to build their own military radio testbeds use a hub-style design to create a mesh network in which each radio is able to communicate with the entire network at one power level. The other ports then have to adjust their own attenuation to the mesh to receive the input radio at the desired power level. The hub fan-out design utilizes a resistive divider/combiner with a star matrix configuration to combine all ports through a central hub. The star configuration limits the number of paths in the matrix to be equal to the number of ports. Each port has a single programmable attenuator (Figure 1). The attenuators can be remotely controlled to simulate static or dynamic environments.
The hub fan-out design can be used for radio-to-radio communication testing to test up to 18 radios. Because of the lower number of programmable attenuators it uses (no more than 18), it is much lower in cost than the other two design options.
Full Fan-Out Design
For test engineers wanting to test between 19 and 32 radios up to 3 GHz, the simple hub fan-out design does not provide a viable option. Instead, a test system based on a full fan-out design is recommended (Figure 2).
The full fan-out design is constructed as a fully meshed matrix with a path between every pair of ports, with each path having its own individually controlled programmable attenuator. The attenuators enable the test engineer to set a different dB setting for every path through the matrix. Those values can even be faded over a time interval to simulate signal fading between radios. During testing, each port can be connected to a device (e.g., a radio or handset) that can transmit/receive signals. Most JFW test systems utilizing this design cover either 30-3000 MHz or 500-6000 MHz.
In contrast to the hub fan-out design, the full fan-out design uses a reactive, rather than a resistive, combiner/divider. These differences allow the full fan-out design to test more radios — up to 32 radios (32 ports) simultaneously. The design also offers maximum signal fade testing flexibility. And, because it allows every path to be set to a unique attenuation, the full fan-out is ideal for testing radio-to-radio communication. This is the most expensive of the three configurations presented here, as the number of attenuators scales quadratically with the total number of radios.
Limited Fan-Out Design
For test engineers wanting to test more than 32 radios up to 3 GHz, the limited fan-out design offers the best option. With this design configuration, each individual port connects to only “L” number of its neighboring ports, and the number of those neighboring ports varies depending on the application (Figure 3).
The limited fan-out design is particularly useful for reducing the size and cost of designs with a large number of ports. Essentially, by reducing the number of internal paths through the matrix, the size and cost of the test system is reduced. The reduced number of internal paths means a greater number of ports can be offered, and that, in turn, means a greater number of radios can be tested simultaneously.
Whereas a full fan-out 48-port design would require 1,128 programmable attenuators, for example, a limited fan-out 48-port LC16 design would require only 384 programmable attenuators. The difference in cost and size between the full fan-out design and the limited fan-out design is roughly 66 percent in this case. Currently, JFW offers limited fan-out models with up to 40 ports, although designs with up to 64 ports (i.e., 64 radios) are possible.
Identifying the Limitations
While each of the three military radio testbed designs presented offers a number of benefits, they also come with different limitations. Understanding these limitations is critical to determining which option is best for any given radio testing application.
With the hub fan-out design, that limitation stems from its utilization of a resistive divider/combiner, which effectively limits the number of radios that can be tested together. In JFW’s case, for example, two different types of hub fan-out transceiver test systems are offered. The largest resistive divider/combiner with a star configuration that JFW makes to work at DC-3 GHZ is an 18-port model. Consequently, the number of radios that can be tested together up to 3 GHz with the test system is limited to 18. Likewise, the largest resistive divider/combiner with a star configuration that JFW makes to work at DC-6 GHZ is a 12-port model. Here, the number of radios that can be tested together up to 6 GHz is limited to 12.
The hub fan-out design is also limited in its flexibility of not being able to set every possible path to a desired attenuation. Figure 4 is a full fan-out with port-to-port attenuation settings of 40, 10, and 90dB. As you can see in Figure 5, the hub fan-out can not replicate the 40, 10, 90dB path settings of the full fan-out design.
In the case of the full fan-out design, the limitation is the size of the test system. It is limited to just 32 radios or 32 ports. With this design, a connection is required from any given port to any other port. So, for example, for 32 ports, a total of 496 internal paths would be required, with each path containing a programmable attenuator. The 32-port full fan-out model offered by JFW takes up one entire 19-inch rack.
For the limited fan-out design, the limitation stems from the design itself. The “limited” in the phrase “limited fan-out” actually means not every port is internally connected to all other ports, as shown in Figure 3. Here, each port is only connected to its 8 neighboring ports. So, in order for a transmitter (Tx) on port J1 to make it to port J9, it would have to be repeated by one of the radios that can see both J1 and J9 — in this case, J6.
The table below shows the general limitations and abilities of each of the three designs. Relative cost and size can be estimated by the number of attenuators used in each type.
Conclusion
Testing as many military radios together as possible is a key goal for test engineers performing radio testing these days. For this task, there are three different types of test setups or designs that can be employed, each with its own set of limitations. Generally speaking, however, for testing up to 18 military radios, the hub fan-out design is the best option. On the other hand, when testing either up to 32 or 64 military radios, a full fan-out or limited fan-out design, respectively, should be used. For maximum flexibility, some applications may also require every unique path to have its own attenuation. In this case the full fan-out configuration must be used.
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