Design & verification solutions for a 5G future
01 February 2019
As 5G momentum grows, and standards solidify, global expectations continue to increase. While the 3GPP releases updated 5G specifications, designers must still address issues to overcome 5G mmWave technical challenges such as large path loss, wide bandwidth & limited instrument accessibility.
This article was originally featured in the February 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy.
Coexistence challenges and unavoidable levels of intermodulation (IMD) power both arise when anchoring new 5G service to existing 4G core networks. And more challenges lie on the horizon as device manufacturers implement mmWave components into mobile devices. As technical challenges continue to arise, Kaelly Farnham, PathWave Marketing Manager at Keysight Technologies explains how system-level simulation and circuit design are necessary to successfully pass 3GPP tests, such as the over-the-air-test (OTA) for 5G.
Accounting for differences between 5G NR & 4G LT
Fundamentally, sub-6 GHz 5G and 4G RFFE requirements look the same. However, there are several differences between 5G NR and LTE that the RF designer needs to consider. The first of these differences is increased bandwidth. 5G NR has a maximum channel bandwidth of up to 100 MHz, and wide spectrum allocated in new bands. The n77 NR band, for example, has a 900 MHz wide frequency range, from 3.3 GHz to 4.2 GHz, that is too wide to be supported by a single component. Thus, designers need to test and verify more components than previous LTE designs required.
Waveform is the next big difference. The uplink carrier for 5G NR uses both CP-OFDM and DFT-S-OFDM waveforms. There is also a PAPR (peak-to-average power ratio) difference on the uplink signal, when compared to LTE. The use of new waveforms to overcome restrictions and problems with the current 4G waveform (OFDM) requires changes to the existing architecture. The new architecture needs to support the new 5G waveforms and the legacy 4G waveforms. There are also challenges related to how those types of waveforms will coexist. These complex interactions and coexistence scenarios need exploration in simulation before the rollout of 5G designs.
Finally, 5G brings more challenges related to multi-band and multi-RAT (Radio Access Technology) implementations. 4G was already complex, with more than 100 band combinations. 5G brings a dramatic increase in this number, due to multiple bands, multiple frequency ranges and new channel-bonding schemes. Intermodulation issues are more significant, due to 5G’s multi-band requirements, and will be extremely difficult to troubleshoot, even for experienced designers. Solving intermodulation challenges requires designers to run simulations and tests to address IMD issues, such as interference in cell communication, that would reduce receiver sensitivity, or even completely obstruct communication.
With the switch to 5G, it will take considerable time to create thousands of required test cases. Those include tests to verify support for more frequency bands, tests that validate various carrier aggregation (CA) scenarios, as well as calculating IMDs and harmonics with different combinations of aggressor bands and victim bands. Experienced 4G component development engineers are confident that their product will meet system level performance metrics when integrated into the target system. However, in 5G DC cases, many RF engineers will need to do more research and simulation to achieve the required performance at the system level.
mmWave components for mobile devices
Figure 1 shows how the architecture for the mmWave front-end drastically differs from that of sub-6 GHz, due to substantial antenna and beamforming gain. mmWave’s high frequency causes a large conducted path loss at the front-end, and therefore, very low tolerances for the trace length between antenna element and active circuitry. The increase in frequency also comes with a reduction in wavelength. It allows the antenna array to be made small enough to integrate into the same package that contains the active transceiver circuits operating at mmWave.
RF engineers will design mmWave component circuits and characterise their models to capture the fluctuation of the individual device. They can then verify the component level design for system level performance evaluation, as shown in Figure 2. Figure 2a displays the schematic and graph of a mmWave phase shifter circuit design using a TriQuint pHEMT process. The graph shows the circuit simulation result of the phase and amplitude variation. It displays unwrapped phases in red traces with frequency sweeps from 20 GHz to 30 GHz. From component level characterisation to system level beam quality analysis, engineers can extract s-parameter data from the circuit design and put them into the system level simulator. Looking at the beam quality analysis in Figure 2b, the 3D beam plot using the s-parameters shows an increased side-lobe level, compared to the ideal beam. The beam shape also differs, with less sharp nulls in side-lobes with the modulated signal because the phase can vary across the wideband signal.
Once the circuit and system level design are complete, the designer can proceed with verification and testing of the prototype hardware. This remains one of the most challenging tasks for all 5G mmWave device developers. The limitation of physical access to test points on mobile devices has necessitated the radiated measurement, or over-the-air (OTA) test.
Problem solving using over-the-air simulation
3GPP defined an over-the-air (OTA) test standard for 4G and aggressively worked to establish a radiated performance specification for 5G. An OTA test system helps engineers perform antenna measurements, RF parametric, function and protocol testing. A complex integration of software and hardware controls this system. When problems occur, running simulations of an OTA environment helps in modeling the individual building blocks and creates better performance for root cause analysis. Figure 3 shows an OTA analysis example such as this.
The building blocks used in the simulation provide great flexibility when modelling various components in an OTA system, and they are compliant with the 3GPP standard for reference baseband modem IP. These building blocks can be used for RF behavioural models with non-linear characteristics, antenna patterns, beam controllers inside the chamber and even virtual instruments.
The abundance of new frequency band configurations in sub-6GHz, combined with LTE and 5G NR dual connectivity and the complexity of non-instrument-accessible mmWave mobile devices makes it exceptionally difficult for design and verification engineers to develop commercial 5G devices.
Packages integrate more devices that require different types of design and simulation tools. Modern EDA tools are needed from component level implementation, all the way through to system level performance verification.
5G’s complexity continues to increase, and solutions like Keysight’s circuit simulator (ADS) and communication system simulator (SystemVue) provide component level characterisation and system level evaluation in one solution environment. Finding effective solutions like these continues to be one of the greatest challenges for wireless technology experts in the 5G journey.
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