What does the ideal waveform for 5G communications look like?
By Gent Paparisto, Joel Kirshman, and David Vye
5G New Radio (5G NR) is the wireless standard defining the next generation of mobile networks. 5G will offer higher capacity than current 4G, enabling a higher density of mobile broadband users and supporting device-to-device and massive-machine communications. 5G research and development will support lower latency, improved reliability, and lower battery consumption for implementation of the Internet of Things (IoT), a network of devices, vehicles, home appliances, and more that will connect and exchange data.
5G overview
The ideal waveform for 5G communications will be capable of supporting high data rates and wide bandwidth communications. It will enable energy efficient operation, have low latency for long and short burst transmission modes, and will be capable of fast switching between the uplink and downlink. This article examines several existing and proposed waveforms as well as MIMO technology being considered for 5G and how system simulation software will enable designers with the ability to accurately simulate their overall system performance.
To put things in perspective, whereas 3G and 4G connected people, 5G will connect everything—smartphones, cars, utility meters, wearables, and much more. To achieve 5G performance requirements, numerous technologies advancing spectral and spatial efficiencies are in development. New candidate modulation waveforms, massive multiple-in-multiple-out (MIMO) antennas, millimeter-wave (mmWave) spectral allocations (Figure 1), and higher density cells are among the technologies being investigated. The use of the mmWave frequency range is being explored for wide bandwidths, exploiting available spectrum once thought impractical for commercial wireless applications.
Figure1: 5G radio access and long-term evolution (LTE) (image courtesy of Ericsson).
Improved spatial efficiency is being addressed through cell densification to increase access point density across a geography that will reduce power consumption while improving spectrum reuse for increased data rates. In addition, MIMO and Massive MIMO antenna technologies are being developed to dramatically increase over-the-air efficiency in existing cells.
These technologies will, in turn, impact individual radio components. For instance, front-end device design may need to support new modulation techniques being considered for 5G. While orthogonal frequency-division multiplexing (OFDM) has worked well for 4G, the advances in processing capabilities that will be available by 2020 when initial 5G deployment is expected has system designers considering other waveforms that may offer a number of advantages.
Early 5G candidate waveforms
Several existing waveforms have been under consideration for 5G. Whereas OFDM has performed well in 4G systems, it requires the use of a cyclic prefix (CP) which occupies space within the data streams, decreasing spectral efficiency. The modulation technique based on filter-bank multicarrier (FBMC) does not have a cyclic prefix and as a result can provide higher levels of spectral efficiency.
Instead of filtering the whole band, as in the case of OFDM, FBMC filters each subcarrier individually. The subcarrier filters in FBMC are very narrow with long time constants—as long as four times a typical multicarrier symbol. Thus, symbols overlap with each other and offset quadrature amplitude modulation (QAM) is used to ensure orthogonality. The number of subcarriers belonging to one sub-band is a design specification; the more subcarriers in one sub-band, the less load on baseband processing, but final performance degrades as more subcarriers are put into one sub-band.
NI AWR design software, in particular Visual System Simulator (VSS), supports analysis of systems using this proposed waveform with dedicated FBMC modulation/demodulation blocks, which can control parameters such as the number of subcarriers, subcarrier spacing, and center frequency. In the example in Figure 2, the impact of convolutional coding on bit error rate (BER) performance is determined for an FBMC modulated signal passed through an additive white Gaussian noise (AWGN) channel, demodulated, and decoded by soft and hard decision generators.
Figure 2: Standard FBMC modulation/demodulation blocks are used to analyze BER performance.
Another modulation scheme under consideration is the generalized frequency-division multiplexing (GFDM) technique (Figure 3). In this waveform, the carriers are not orthogonal to each other, allowing better control of out-of-band emissions and reducing the peak-to-average power ratio (PAPR). Both of these issues are major drawbacks of OFDM technology.
GFDM is based on traditional filter bank multi-branch multi-carrier concepts that are implemented digitally. The GFDM approach exhibits benefits that are particularly attractive in systems with a high degree of spectrum fragmentation, as would be the case with carrier aggregation called for in LTE-A and 5G systems. Overall, GFDM features offer a lower PAPR compared to OFDM, an ultra-low out-of-band radiation (due to adjustable Tx-filtering and block-based transmission using cyclic prefix insertion), and efficient fast Fourier transform (FFT)-based equalization.
Figure 3: GFDM waveform.1
In addition to FBMC and GFDM, other new waveforms being considered include non-orthogonal multiple access (NOMA) and universal filtered multicarrier (UFMC), both of which can increase physical layer (PHY) flexibility. VSS enables system designers to understand the benefits and shortcomings of these proposed waveforms through simulation, providing critical information to front-end component designers.
Figure 4 illustrates how VSS can be used to evaluate and compare the performance of 5G modulation techniques when they are subjected to nonlinear devices. In this example four types of waveforms are generated and then passed through identical power amplifiers (PAs). The signal sources can be configured via adjustable parameters such as carrier frequency, subcarrier spacing, number of subcarriers, and subcarrier mapping, which accommodate particular configurations that have been proposed for 3GPP NR specification.
Figure 4: Spectral regrowth for various 5G waveform candidates at different drive levels shows the rise of out-of-band emissions as the nonlinear PA is driven into saturation.
Each modulation source block is followed by a linear pre-amplifier, a nonlinear PA, and the corresponding demodulator. A tuner can be used to adjust the gain of the pre-amplifiers to look at the impact of the nonlinear PA at different input power levels. Transmit spectrum before and after the nonlinear PA shows the spectral characteristics of these waveforms. This example demonstrates how the spectral purity advantages (low adjacent channel power) of some 5G waveforms is degraded by PA nonlinearities.
With VSS, the power statistics of the signal can be completely characterized by the complementary cumulative distribution function (CCDF), which shows the probability that the power is equal to or above a certain PAPR for different probabilities. The CCDF curve, which was simulated in VSS, can be used to calculate the headroom required in PAs and other components. Signals with different peak-to-average statistics can stress components in a transmitter and lead to distortion. The CCDFs of each generated waveform are shown to compare their PAPR characteristics. CCDF measurements can also be performed across different points in the transmitter to pinpoint potential trouble spots, as shown in Figure 5.
Figure 5. Simulated CCDF curves as a function of PAPR.
CP-OFDM
The 3GPP standards ratified in December 2017 converged towards using cyclic prefix OFDM (CP-OFDM) as the waveform of choice for 5G non-standalone (NSA) NR. CP-OFDM is used by LTE and ranks best on the performance indicators that matter most: compatibility with multi-antenna technologies such as MIMO, high spectral efficiency, and low implementation complexity. It is less susceptible to phase noise and Doppler effects than other multicarrier systems. On the downside, CP-OFDM has a high PAPR like other OFDM signals, which adversely impacts power amplifier linearity (and efficiency). Verizon 5G Technical Forum has also chosen CP-OFDM.2
Error vector magnitude (EVM) is another metric used to determine how Tx/Rx links and receiver architecture affect system performance. Figure 6 shows the impact on the EVM measurement and the actual demodulated constellation as the result of phase noise introduced into the system for a 16-QAM signal with CP-OFDM (blue trace) versus OFDM (red trace) modulation. In this case, the cyclic prefix improves the EVM metric by more than 3 dB.
These results highlight why CP-OFDM is the waveform of choice for 5G. It is compatible with LTE and far easier to implement than the other signals. VSS enables designers to generate standard 5G signals, superimpose phase noise and other impairments, and look at the metric of choice to determine if the overall system requirements can be met for a given component specification.
Figure 6: EVM measurements and demodulated constellation for OFDM and CP-OFDM system with phase noise introduced into the system.
References
Gent Paparisto is a product manager for RF Systems at NI’s AWR Group.
Joel Kirshman is a marketing researcher for NI’s AWR Group.
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Leave a Reply