June 13, 2019

FAQ: Get Answers to OEMs’ Frequently Asked Questions About OTA Radiated Performance Tests

If you are developing a cellular device, TRP and TIS are the standard test results that measure your product’s wireless performance. BluFlux designs, fabricates, integrates, and tests antennas for cellular devices, so we’ve heard the typical questions that OEM’s have about TRP & TIS. Here are answers to Frequently Asked Questions we’ve heard from OEMs so you can learn what you need to know in order to launch a successful cellular product.

What You’ll Learn

In this tutorial, here’s what you will learn about TRP and TIS:

The Basics

TRP

TIS

When, Where, and Why to Test TRP and TIS

 

The Basics

Why are TRP and TIS called “Over The Air Performance” tests?

Cellular carriers refer to TRP and TIS as “Over The Air Radiated Performance” Tests, or “OTA Performance Tests” because these tests measure the Transmit and Receive performance of your device via a wireless connection, as opposed to requiring a cabled connection to test equipment.

When a cellular device is tested “Over The Air” that means the tests are performed wirelessly using the product’s own radio. OTA testing results are representative of your actual device’s behavior.

OTA testing is different than sending a test signal via a cable to a product’s antenna. Sending a signal from test equipment into an antenna via a cable is a typical method of testing an antenna independently of an actual product device’s radio. Sometimes this test method is referred to as “passive antenna testing.” Cabled, “passive” tests can introduce artifacts so that the results might not represent the actual tuning or performance when antennas are integrated in an actual product device.

To perform OTA Performance (TRP and TIS) tests, specialized test equipment must interact with your device wirelessly in an RF anechoic chamber.

The anechoic chamber minimizes radio wave reflections and prevents signals from the outside world (like signals from actual cell tower base stations) from getting in.

The test equipment includes a so-called “call box” that simulates a cellular base station for the cellular technologies supported by your device (eg LTE CAT-1, LTE CAT-M1, 5G, etc).

As an example, the BluFlux callbox is an Anritsu MT8820C, referred to by Anritsu as a “Radio Communication Analyzer.” Other callbox manufacturers may also call their products “Radio Communication Testers.” The function is the same, regardless of the name.

 

Why is it important for your cellular device to achieve good TRP and TIS results?

It’s critically important that your product achieve good TRP and TIS results because:

  • Good TRP and TIS results mean your product will perform well compared to competing products.
  • If you plan to market your device in the U.S., its TRP and TIS results must exceed minimum levels set by each of your cellular carriers in the U.S. Otherwise the device would not be able to get certified for use on any cellular carrier whose minimum requirements it doesn’t meet.

 

TRP

What is TRP?

TRP stands for Total Radiated Power.

In order for your device to send information to a cellular network, its cellular radio must drive RF power into its cellular antenna.

A TRP test adds up all the power that is transmitted by the device at a particular channel of a cellular transmit band. It is a sum of a complete set of measurements that are performed at discrete spatial intervals in every direction from the device. Think of it as a sum of a “sphere” of measurements around the device.

Antenna gain is the parameter that determines the ability of the device to successfully transmit a signal to a receiver that is far away. For small IoT devices, the actual gain patterns of the antenna are more or less omni-directional. That means that TRP can be considered a single “figure of merit” for transmit performance that represents the overall strength of that transmitted gain pattern.

This makes TRP the preferred way to measure how well the device can get its transmitted signal to cell tower base stations that are far away. The better the TRP, the further the device can be from a cell tower and still have its signal be received by that tower’s base station. Since cellular communications rely on a two-way link, your device’s ability to connect to far-away towers will depend not only on its TRP performance (the device’s transmit/uplink test) but also on its TIS (the device’s receive/downlink test).

How many TRP tests will your cellular IoT device need?

TRP is typically measured at every band that your device supports. 

The number of TRP tests your cellular device will need depends on how many cellular bands it uses.

Each cellular TRP is typically measured for a single channel within a cellular uplink band.

In the U.S., an organization called CTIA decides the standard channels for testing TRP. As of this writing, the current CTIA “Test Plan for Wireless Device Over-the-Air Performance” Version 3.8.1 Table 5-20 specifies 3 channels to be tested for each LTE band. The LTE bands listed in table 5-20 of test plan version are: (2, 4, 5, 7, 12, 13, 14, 17, 25, 26, 30, 41, 66, and 70).

TRP tests for your device will include results for 3 channels at each of those bands that your device supports.

 

What design factors will determine your device’s TRP performance?

TRP is a function of

  • Your product’s cellular module’s conducted output transmit power. This is referred to as Conducted Power or “CP.”
  • The overall radiation efficiency of your primary cellular antenna when it is integrated with your product.

For any actual cellular module part, the CP value will almost always be within in the range shown in your module manufacturer’s datasheet. If a module’s actual CP is toward the low end of that range, that also means the maximum TRP your product could achieve will be reduced accordingly. So it is important to choose a module that will deliver as much CP as possible. It is also advisable to measure the actual CP values of several units of a given module in order to confirm a CP range that represents the transmit power that those modules will deliver.

The overall radiation efficiency of your antenna integrated with your product is a function of

  • The spacing and geometry of the metal parts of your product. In particular, your product’s PCB ground plane length can play a significant role in determining the overall radiation efficiency of your product. This is especially true for antennas that are integrated on or close to your PCB – like PCB trace antennas, ceramic chip antennas, stamped metal antennas, and cabled Flex-PCB antennas.
  • Your product’s primary cellular antenna tuning and matching component network, if any.

In order to get the best possible performance, it is important that you consider all three factors (cellular module, PCB and metal parts, antenna and integration) during your planning and design phases. If you are using off-the-shelf antennas, it is critical to follow the antenna manufacturer’s layout and keepout guidelines so that you prepare for antenna matching and prevent accidentally compromising antenna efficiency.

For all these reasons, BluFlux always recommends that every cellular device OEM conduct RF front end design reviews before releasing a PCB design for fabrication.  Because small deviations in PCB artwork can have significant RF impacts, it is a good idea to have guidance from experienced RF engineers throughout your electronics design process.  The RF engineers supporting your project could me members of your internal team, applications engineers at an OTS antenna supplier, or a third party consultant.

How is TRP Measured?

As explained in “What is TRP,” the Total Radiated Power (TRP) measurement requires adding together all of the individual power measurements at discrete spatial intervals in every direction from the device.  

In most CTIA Authorized Test Labs (CATLs), such as the BluFlux chamber shown in the image shown below, the measurements are received by a series of measurement antennas spaced at regular intervals around the perimeter of the chamber.   There are 23 sets of measurement antennas spaced at 15 degree intervals.

Image: BluFlux CTIA/CATL Test Chamber

Inside the test chamber, in addition to the measurement antennas, there are also two communication antennas in the anechoic testing chamber. These two communication antennas are for maintaining a communications link for exchanging information with the DUT cellular module.

The DUT (for example, your product undergoing TRP tests) is placed on the pedestal at the center of the ring. The pedestal can turn at precise angular intervals. To comply with CTIA requirements, power measurements for TRP must be made at (15) degree intervals.  During a CTIA-compliant TRP test, after measuring each “slice” of powers received by the measurement antennas, the pedestal rotates by 15 degrees. This process continues until all “slices” of power measurements are recorded to account for the entire sphere around the device.

The test instrumentation is located outside the anechoic chamber.  It controls the TRP test process, acquires the individual power measurements,  and combines them to calculate the Total Radiated Power (TRP). 

The test equipment includes a spectrum analyzer.  The spectrum analyzer measures the received power from the measurement antennas.

Image: BluFlux CTIA/CATL Test Chamber Instrumentation Rack

The test equipment also includes a  callbox.  See the section of this FAQ titled “Why are TRP and TIS called Over The Air Radiated Performance tests?” for an explanation of what a callbox is.  For a TRP test, the callbox communicates with the DUT to

  1. command the cellular module in the DUT to transmit a prescribed power at the particular transmit band frequency channel that is being tested
  2. receive confirmation from the cellular module in the DUT that the DUT “thinks” that it is transmitting the correct power for the test

When all of the individual power measurements have been recorded, the test equipment supervisory computer then adds together all the individual measurements in all the slices.  That gives one TRP value for the transmit band channel at which it was transmitting.

 

TIS

What is TIS?

TIS stands for Total Isotropic Sensitivity.

For your device to receive information from a cellular network, its cellular radio has to receive RF power from its main cellular antenna. TIS is a sum over all directions of the minimum power level of a cellular signal that your device can reliably receive via its cellular antennas in all directions.

That’s antennas (plural) because LTE CAT-1 and higher levels all support MIMO receive diversity, which means that the device has more than one antenna that can receive a signal from a cellular tower.

In RF communications, “multipath” describes signals arriving from multiple directions due to reflections of the transmitted signal waves. Multiple receive antennas with complementary abilities to receive signals from different directions can help your device to receive multipath signals.

For low throughput cellular technologies like LTE-CAT-M1 and NB-IOT, only one antenna is used for both transmit and receive.

Having multiple receiving antennas helps the device minimize losses from and even benefit from multipath. Without multiple antennas, your product can suffer from severe degradations in signal and throughput due to multipath fading. With multiple antennas, some or all of this degradation can be recovered.

For LTE CAT-1 and higher with more than one antenna, the additional cellular antennas don’t transmit. They only receive. A cellular product with multiple antennas can undergo TIS testing for each antenna separately or for both of them simultaneously.

For a cellular IOT device, TIS will give you an idea how far away your device can be from a cell tower before your device can no longer receive a signal from that cell tower. If your device can not receive a signal from a cell tower, then no cellular connection or data throughput would be possible.

 

How many TIS tests will your cellular IoT device need?

The number of TIS tests your cellular device will need depends not only on how many cellular bands it uses, but also on exactly which bands it uses.

Cellular TIS is measured on a single channel within a cellular downlink band. Three receive channels are tested per band. But there are also exceptions to that rule. For example, LTE Band 13 is used in the U.S. primarily by Verizon. Band 13 is only tested at one receive channel, which is centered at 751 MHz. For each cellular band that your device uses, every time your device undergoes TIS tests, you will get as many TIS test results per band as that band has channels that need to be tested. So you’ll get three TIS results for most bands, but only one result for Band 13.

For higher categories of LTE, (CAT-4 and CAT-6, for example), your device may support carrier aggregation. Carrier aggregation enables your device to combine one Uplink (Transmit) band with another Receive (Downlink). This adds an additional type of test similar to a TIS test but specific to carrier aggregation. Carrier Aggregation is a whole separate topic that is not relevant for most low-throughput cellular IoT devices.

For TIS, in addition to running TIS for all the bands supported by a device, whenever you fabricate a new design revision of your product, BluFlux typically recommends that you run a full set of TIS tests on at least two separate devices to prove that multiple devices perform as well as each other and that you don’t have any significant unit-to-unit variability.

 

What design factors will determine your device’s TIS performance?

As with TRP, TIS depends, in part, on the performance of your module.  For TRP, the relevant module parameter is Transmit Conducted Power (CP). For TIS, the relevant module parameter is Receive Conducted Sensitivity (CS).

As with TRP, TIS depends, in part, on the performance of your module. For TRP, the relevant module parameter is Transmit Conducted Power (CP). For TIS, the relevant module parameter is Receive Conducted Sensitivity (CS).

CS can vary significantly from one module manufacturer to another and from one particular module to another.

It is therefore particularly important to evaluate candidate modules’ CS specifications when choosing your cellular module.

This is particularly important for small devices for which circuits that generate high frequency periodic signals may be close to the antennas.

High frequency switching or digital signals can interact with nearby metal structures (eg the geometry of your PCB) to produce electromagnetic radiation. If that radiation includes frequencies (spectral content) in your device’s cellular receive bands, then your antennas, your antenna feeds, and your module itself can receive that noise, which would compromise your device’s TIS test results (and its actual ability to receive cellular signals from actual cell towers).

That’s why it’s also critical to perform an EMI and EMC design review of your schematic and layout as early as possible in your development process and before you spin your first prototype that is close to your production-intent form factor.

As explained above, your product’s TRP and TIS will determine how far away from a cell tower it will remain connected to its cellular network. Worse TRP or TIS will probably mean more dropped connections as your device gets further away from the closest cell tower. No connection or a dropped connection means your device cannot send and receive cellular data. Missing data means your customers don’t get the value they are paying you for.

On the other hand, the better your device’s TRP and TIS performance, the more reliable the device’s connections will probably be, and the happier your customers will be.

 

When, Where, and Why to Test TRP and TIS

When should you measure your cellular IoT device’s TRP and TIS?

BluFlux recommends that you run a full set of TRP tests and mid-channel TIS tests on each functional version of your design, starting with the earliest prototype that is representative of the desired form factor of your product.

Shortfalls in TRP or TIS indicate design, fabrication, or assembly issues that would compromise actual wireless performance. Finding and fixing such problems early in your development process will help you prevent the problems from affecting later stages of development and production, when the stakes and costs would be much higher.

 

How many devices should you provide for testing?

For carrier certification OTA Performance testing, many CATL facilities will request that you provide multiple devices for cellular carrier certification. Having spares on hand can help your certification lab stay on schedule in case one of your devices has a problem during setup or during testing itself.

In addition, having spares may enable your certification lab to perform multiple cellular or regulatory tests in parallel, which might help save time in your schedule.

When preparing your product-intent design devices for certification testing, BluFlux therefore often recommends that you be ready to perform pre-certification testing of at least two devices, so that you can be assured that either device will pass certification. This is especially important if prior revisions of your design only passed your cellular carriers’ TRP or TIS test requirements by a small safety margin.

In addition, for earlier prototype designs, BluFlux recommends that at least two device test units for your design undergo TRP and TIS testing when you are evaluating your prototypes.

This is necessary to find out whether multiple devices perform as well as each other, which helps find component variation or assembly issues that cause unit-to-unit variability. Such variability is actually common for early wireless product prototypes.

 

What if your device doesn’t pass your cellular carrier’s official minimum TRP and TIS requirements?

If you’re device doesn’t achieve the TRP and TIS levels required by your cellular carriers, there’s a good chance your device will not be allowed to pass that carrier’s certification process. If your product fails carrier certification, it cannot launch on that carrier’s network.

Occasionally, carriers will grant exception waivers for their OTA performance (TRP and TIS) requirements and still let underperforming devices on the network. However, even if an underperforming device is granted a waiver, that device can be expected to perform worse than competing products that did meet the minimum requirements.

In addition, waivers are not always granted by carriers. You typically won’t find out if a waiver is possible until AFTER your device has attempted to complete certification and has suffered a failing TRP or TIS result. At that point, you would have already invested valuable time and money in your entire development and certification process, and there’s no guarantee that you could get a waiver. Instead, after an OTA performance certification failure, you might need to select different antennas, redesign your product, or both, in order to pass certification on a later attempt.

If you want your device to perform well for your customers and be assured of certification by your cellular carriers, it’s critical that you design and test your product right from the beginning for the best possible TRP and TIS performance for your target form factor.