Link-budget calculations: Needed for 5G OTA testing

5G millimeter wave (mmWave) devices operating above 24 GHz incorporate millimeter-sized patch antenna arrays or dipole antennas that become an integral part of the device module packaging. Testing these assemblies requires over-the-air tests inside a chamber. But, a mmWave test chamber can introduce significant path loss, more than from cables and connectors. Understanding how to calculate the total link budget for over-the-air testing is a critical step in 5G mmWave OTA test. 

Each patch antenna element in a 5G mobile or fixed-access device can transmit or receive electromagnetic waves in either the vertical or horizontal direction. Figure 1 shows the simulated antenna pattern of a 3x3 patch antenna array. The different colors show the intensity of the radiated power, with red indicating the highest power and blue the lowest. 

Figure 1. Simulated antenna pattern of a 3x3 patch-antenna array shows field strength of the radiated waves. 

Because mmWave modules containing patch-antenna arrays and dipole antennas are an integral part of the end-user product, the only way to characterize and test the performance of the antennas is through over the air (OTA) testing. Unlike wireless products operating at sub-6 GHz frequencies, mmWave products introduce a new test challenge related to OTA testing. 

Data path losses
Over-the-air path losses (measured in dB) can be significant at mmWave frequencies relative to contacted cable and connector losses. For example, a 2.92 mm connector-cable assembly can have a path loss of about 2.75 dB/m at 40 GHz, whereas the over-the-air path loss at the same frequency is about 64 dB at an over-the-air distance of 1 m. 

The following Friis equation lets you calculate the OTA path losses when the distance R between the transmitting and receiving antennas are equal or greater to the far-field (FF) region: 

Where λ is the wavelength in meters and R is equal or greater than the far-field region distance as explained next. The far-field distance R is the distance at which the spherical waves can be considered as a “plane” wave at the receiving antenna, thus fulfilling the following mathematical requirement: 

Where D is the largest dimension of the apertures (that is, the maximum effective size of the antenna) of either the transmitting (D₁) or receiving (D₂) antennas as shown in Figure 2. 

Figure 2. The transmitting and receiving antennas as separated by Far-Field distance R. 

The choice of a known-performance horn antenna as the test antenna in Figure 2 illustrates the typical OTA measurement environment in the R&D lab, DVT and manufacturing test setups. Figure 2 shows the device-under-test (DUT) radiating electromagnetic waves energy over-the-air and a test horn antenna receiving the much-attenuated energy which in turn is amplified by its own gain (for example, 15 dBi or 20 dBi of gain). Thus, at 40 GHz, an OTA path loss of 60 dB will become -60 dB + 15 dB = -45 dB at the output of a 15 dBi horn antenna. Thus, the choice of a measuring horn antenna with known gain is a key OTA test set up decision. 

There is, however, a tradeoff between horn antenna gain and the fair field distance R. The higher the horn antenna gain, the larger its aperture D and thus the larger the far field distance R. For example, a typical single polarization 15 dBi-gain antenna has D = 26.1 mm, which translates to a far field distance R = 182.3 mm; whereas a 20 dBi-gain antenna has D = 51.5 mm, which translates to a far field distance R = 706.6 mm, an increase of about half a meter in distance. This half-meter increase in far field distance will result in an additional 11.8 dB of OTA path losses thus more than offsetting the higher horn antenna gain. 

Thus, the larger the measuring horn antenna gain, the larger the D, the larger the R and thus the larger the OTA chamber required and, the larger the R distance, the larger the OTA path losses. Thus, clearly, various technical tradeoffs must be decided when designing OTA chambers as the over-the-air path losses can be significant. 

Figure 3 shows a 60-cm far-field distance (R) OTA chamber for testing of mmWave devices. 

Figure 3. 60-cm far-field OTA Chamber isolates mmWave signals from the ambient electromagnetic environment. 

OTA Link Budget Calculations
Calculating the total OTA test chamber link budget is of critical importance for making accurate DUT antenna measurements. The resulting link budget net loss needs to be combined with the measurements made by the mmWave tester instrument to determine the actual radiated power and phase being generated by the each of the DUT antenna array elements, or, likewise, when generating mmWave signals from the test horn antenna into the DUT. 

Once the choices have been made for a DUT antenna array size with aperture D₁, a chamber test horn antenna with aperture D₂, and a chamber with a far-field distance R (such as the chamber shown in Fig. 3), Friis transmission equation can be used to calculate the overall link budget of the OTA test set up. 

Figure 4 shows the setup variables required to calculate the OTA link budget using the Friis equation. 

Figure 4. Setup variables required to calculate the OTA link budget using Friis equation.

Where:

P_R = Power at the receiving antenna
G_R = Gain of the receiving antenna
P_T = Power at the transmitting antenna
G_T = Gain of the transmitting antenna
λ = Wavelength of the operating frequency
R = Far-field distance between the two antennas

You must also take into account the insertion loss of cables and connectors when calculating. Whereas connector insertion losses are relatively small—in the range of 0.3 dB—typical 2.92 mm cable losses can add up at a rate of 2.75 dB per meter at 40 GHz. For a total cable length of 1.5 m, the cable insertion loss is in the range of 4 dB, which is only about 6% of the total link losses at 40 GHz. The OTA path loss represents 94% of the total path losses as seen in the example of Table 1. 

Table 1. OTA Chamber Link Budget Calculation Example.

Table 1 shows an example of a link budget calculation for a 60-cm far-field chamber at 40 GHz with a total cable length of 1.5 m, a test horn antenna with 15 dBi of gain, and a DUT antenna with 5 dBi of gain. In the TX Case, Friis transmission equation yields an adjusted P_R = -44.2 dBm at the tester instrument input, taking into account the cable loss. Table 1 also shows the reverse case when the tester is transmitting through the test horn antenna into the DUT. In the RX Case, the DUT would be receiving -34.2 dBm at its patch antenna array or dipole antenna when the tester output power is set at 10 dBm. Having a good understanding of 5G link budget calculations is a critical step toward ensuring a proper OTA test set up as it drives the selection of the appropriate test horn antennas, cables, connectors and tester power settings. 

Having a good understanding of 5G link budget calculations is a critical step toward ensuring a proper OTA test set up as it drives the selection of the appropriate test horn antennas, cables, connectors and tester power settings. 


—Jeorge Hurtarte is RF Market Segment Manager at LitePoint.