Signal generation & analysis for 5G NR

Author : Dr Patrick Agyapong & Johan Nilsson, Product Managers at Rohde & Schwarz

09 October 2018

Credit: Wikim
Credit: Wikim

5G New Radio promises gigabit connection speeds, increased connection density and improved energy efficiency compared to LTE. In December 2017, 3GPP completed the first set of implementable 5G standards, paving the way for full-scale trials and commercial deployments in 2019. This piece outlines some of the challenges – and solutions – for T&M.

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Two key features in 5G New Radio (5G NR) are especially important for the realisation of higher data rates than in LTE: the use of millimetre wave (mmwave) frequencies (up to 100 GHz) and support for significantly higher signal bandwidths. In particular, 5G NR supports signal bandwidths up to 100 MHz for carrier frequencies below 6 GHz, and up to 400 MHz for frequencies in the mmwave range.

Similar to LTE, 5G NR employs OFDM-based waveforms in both the uplink and downlink to utilise the wide carrier bandwidths efficiently; however, it allows more flexibility in subcarrier spacing for deployment in different frequency bands. It is expected that up to 16 carriers can be aggregated, making it possible to offer bandwidths in the gigahertz range to a single device.

5G NR also employs massive MIMO and beamforming techniques that rely on massive antenna arrays to combat the effects of higher attenuation at higher frequencies.

Major design and testing challenges

These ambitious features bring new challenges for development engineers. Designing power amplifiers that exhibit the necessary characteristics (such as linearity) is not trivial and may necessitate new design approaches (for instance, digital pre-distortion). The circuitry needed to generate frequencies in the mmwave range requires careful design and selection of components in order to reduce the effects of phase noise introduced by mixers and multipliers. Beamforming also requires very good amplitude and phase synchronisation between antenna elements in active antenna systems.

The high frequencies intensify the challenges for test and measurement methods. The short wavelengths and higher losses in circuits necessitate tight integration, making it impractical to supply connector ports for testing. At the same time, the effects of connectors and test fixtures become non-negligible, potentially affecting the validity of conducted measurements. As a result, over-the-air (OTA) testing will play an important role.

Continuously updated test and measurement equipment from Rohde & Schwarz can help the industry quickly bring 5G NR solutions to market. Developers, for example, urgently need suitable signal sources and analysers.

New software for 5G NR signal generation and analysis

The R&S SMW-K144 option for the R&S SMW200A simplifies generation of uplink and downlink 5G NR signals for all testing needs. With this option, the generator produces extremely clean 5G NR signals with a flat frequency response and bandwidths up to 

2 GHz at frequencies up to 40 GHz.

The software supports all waveforms specified in the standards (cyclic prefix OFDM and DFT-s-OFDM), channel bandwidths from 5 MHz to 400 MHz, modulation formats up to 256QAM, subcarrier spacing (15 kHz to 240 kHz) and numerology options. The intuitive GUI allows users to flexibly configure these and many other parameters, including bandwidth parts (BWP).

The R&S FSW-K144 option for the R&S FSW signal and spectrum analyser provides functions for in-depth analysis of 5G NR downlink signals, using standard-compliant parameters such as FFT size, cyclic prefix length, and many more. The option allows configuration of the parameters directly on the instrument or automatic detection of parameters such as the cell ID, which influences other parameters of the signal.

Signal generation with the R&S SMW200A

A signal generator for 5G NR needs to be able to generate extremely clean wideband signals with excellent EVM over a wide frequency range. This requires the signal generator to exhibit outstanding spectral, phase and amplitude characteristics. To overcome high path loss at mmwave frequencies, very high RF output power is also necessary. The R&S SMW200A vector signal generator fulfills all these and other requirements resulting from 3GPP Release 15. It generates signals with a bandwidth of 2 GHz for frequency ranges up to 40 GHz internally, eliminating the need for external mixers and upconverters.

Automatic internal amplitude correction ensures extremely flat signals (< 0.4 dB measured over a 2 GHz bandwidth) over the entire frequency range. With signal output powers up to +18 dBm, it can be used for OTA testing without an external amplifier. The excellent spectral characteristics also ensure that 5G NR signals with measured EVM of less than 0.3 % for a 100 MHz downlink signal can be generated. With the optional integrated fading simulator, the generator also supports complex applications such as real-time fading for all key MIMO scenarios: another unique feature on the market.

Signal analysis with the R&S FSW

With 5G NR, signal analysis bandwidths of at least 1 GHz become necessary in order to capture the interaction of multiple component carriers. The R&S FSW-B1200 option provides 1,200 MHz of internal analysis bandwidth for the 43 GHz and 50 GHz models of the R&S FSW series of signal and spectrum analysers.

The 14-bit A/D converter with its low inherent EVM provides new insight into the design. This internal option also reduces the size of the test setup, reduces the needed cabling between instruments and increases the measurement accuracy compared to systems built with downconverters and separate digitisers.

The new R&S FSW-B2001 option increases the internal analysis bandwidth of the R&S FSW43 and R&S FSW50 to 2 GHz, which enables designers of power amplifiers for 5G NR to develop and evaluate pre-distortion systems, for example. If even larger bandwidths are required, the R&S FSW85 can be combined with the R&S RTO2064 oscilloscope to support analysis bandwidths up to 5 GHz for frequencies up to 90 GHz.

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