GaN on SiC Meets 5G Power & Wideband Needs
5G communication standards are the manifestation of our need for higher speed to support the ever-increasing amount of data we want to
move across the growing number of devices or internet of
things nodes in the shortest possible time.
While the wideband and high-power requirements of 5G go beyond the capabilities of technologies like LDMOS, they are easily met
through careful power amplifier
(PA) design using GaN on SiC components.
3GPP Requirements
The Third Generation Partnership Project (3GPP), which reviews solutions to meet the IMT-2020 (5G) specifications, requires 5G-compliant networks to meet the following conditions:
Peak data rates of more than 20 Gb/s downlink and 10 Gb/s uplink
Latency of less
than 1 ms
A minimum connection density of 1 million connections/km2
The high data rates are achieved using two key techniques.
Increase
Bandwith
Use Massive
MIMO
Evolving Channel Capacity
Channel capacity in bits per second is proportional to the bandwidth of the channel as described by the Shannon–Hartley theorem, which says that maximum error-free digital data that can be transmitted over a channel of a given bandwidth in the presence of noise is given by the equation:
5G systems aim to increase bandwidth, the number of channels, and transmit power while trying to keep noise low in order to increase the channel capacity.
SISO & 4G
LTE/5G with MISO
Full mMIMO for 5G
PA Design Meets Linearizability Challenge
Wolfspeed’s high-power multi-chip asymmetrical Doherty PA module (PAM), designed using a GaN on SiC HEMT, meets 4G/LTE/5G standards with a 28-V supply voltage, 50-Ω input, and output matching. Two-stage internally matched Doherty PAs can deliver up to 30-dB gain and high saturated power of up to 46 dBm over 3.4 to 3.6 GHz.
Tested using Analog Devices Inc.’s (ADI’s) DPD system with 200-MHz iBW and 8-dB PAPR signal, the PAM provides excellent linearizability of –50-dBc ACPR at average output power of 37.5 dBm.
Erik Dahlman: 5G NR: The Next Generation Wireless Access Technology. Academic Press 2018, ISBN-13: 978-0128143230.
3GPP Roadmap (2020). Proposed 3GPP releases and milestones (https://www.3gpp.org/specifications/releases)
3GPP standard: https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3389
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Learn About 5G Enabling Products
LTE/5G with MISO
Increase Bandwidth
To achieve higher data rates, the range of frequencies or bandwidth occupied by a modulated carrier signal must also be increased.
Data rate = 2 × bandwidth log (N) bits per second (b/s)
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Where:
N = the number of coding states
Bandwidth = (Upper frequency – lower frequency)/center frequency
Today’s 4G allows up to 20-MHz bandwidth per channel, while 5G will give us 100 MHz in the bands below 6 GHz and 400 MHz in the bands above 6 GHz. The change in bandwidth from 20 MHz to 400 MHz offers a 20× increase in channel capacity.
Multiple-input/multiple-output (MIMO) is a set of technologies that multiplies the capacity of a wireless connection without requiring additional spectrum. It does so by transmitting and receiving more than one data signal simultaneously.
The enabling technology for a 5G network is massive MIMO (mMIMO), which refers to a system comprising an array of at least 16 transmit × 16 receive antennas. mMIMO base stations with 64T64R antenna arrays have been commercially deployed since 2018.
Use Massive MIMO
SISO & 4G
4G narrowband applications usually operate in single-input/single-output (SISO) configurations, called macro systems, that utilize single antennas at the base station and handset. However, the handset may switch across a pair of antennas to overcome signal loss due to hand placement.
Its throughput is limited by channel bandwidth and propagation conditions. Using the Shannon–Hartley equation, the SISO channel capacity is given by:
3GPP Releases 13 and 14 (LTE Advanced Pro) and 15 (5G) offer many of the technologies that rely on increasing bandwidth by parallelizing channel carriers. This includes channel aggregation (CA) and dynamic spectrum sharing (DSS) in the sub-6-GHz space. Both technologies increase throughput by using extra channels of up to 20 MHz in bandwidth. 3GPP Release 15 also allows use of both LTE and 5G channels simultaneously with DSS to increase throughput.
The use of multiple antennas at the transmit (tower) end of the link leads to linear increases in data rates as the system operates in a multiple-input/single-output (MISO) mode. While the handset may switch across a pair of antennas to overcome signal loss due to hand placement, the antenna tower adjusts its phase and amplitude components for that handset to beamform or peak throughput in a multipath environment.
While 4G LTE Advanced Pro chipsets were operating in the 1-GSPS range, the use of mMIMO has allowed sub-6-GHz handsets to get data rates in the 2.5- to 3-GSPS range. The channel capacity, as a function of base station antennas (N) for such contemporary 5G systems, is given by the Shannon–Hartley equation:
Full mMIMO for 5G
Full mMIMO solutions, with the multitude of antennas on both base station and handsets, will unleash an exponential throughput increase edging upwards of 10 Gb/s at 28.8 GHz. The mMIMO channel capacity description will then become a complex equation determined by the channel matrix between many users and the base station antenna array size.
The channel capacity matrix will have to take into account a large number of channels of differing mismatches due to the multipath environment as each effective (SISO) signal travels within the larger array. Such a mMimo array can be described by a MT x NR matrix (Transmit and Receive channel/antenna numbers) whose x,y (denoted as I,J) characteristics describe the path between the Jth transmit and the Ith receive antenna. Therefore, the channel capacity for full mMimo reduces to:
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Full mMIMO systems will likely use higher spectrum allocations to achieve the bandwidth necessary for speed improvement. Another advantage at higher frequencies is the reduction in size of the necessary antennas for both handsets and base stations.
The 3GPP will continue developing the wireless communication specification beyond 5G through additional releases to finally get to 6G. The 3GPP will emphasize higher (millimeter-wave, or mmWave) frequency allocations in Releases 16 and 17 and aim to freeze Release 18 by 2022.
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View Chart
Source: 3GPP TSG SA#87e, 17-20 March 2020, e-meeting document SP-200222
3GPP-2020
The static amplitude modulation to amplitude modulation (AM/AM) and amplitude modulation to phase modulation (AM/PM) characteristics serve as metrics for wideband linearizability.
Metrics for
Wideband Linearizability
Wideband linearizability problems arise at various points in the system. Therefore, design optimization should not only focus on the final-stage PA but the entire amplification lineup, keeping in mind the transceiver and the DPD systems.
Deliver the required linear response
Operate at high efficiency
Meet high-power requirements
Transmit multi-carrier, multi-mode signals over the
full frequency span of a given communication spectrum
To meet the demands of growing data rates that 5G base stations must deliver, PAs must be able to:
Challenges to Power Amplifiers
<1°–2° AM/PM inflection in any static AM/PM shape within the frequency band
Static AM/PM dispersion across the frequency band. Low dispersion leads to good linearizability.
Low peak-power capability of the amplifier at any frequency
For wideband with >60-MHz instantaneous bandwidth (iBW), linearizability is indicated by:
Input source/pre-driver: Nonlinearity, harmonics, noise level, and bandwidth
Measurement setup: Calibration, isolation, filtering, load, thermal management, and dynamic range
Memory effects of the amplifier: Bias network, trapping effect, mismatch of even harmonics, and power supply modulation
Transceiver: Nonlinearity of up/down converters, LO leakage/spurs, image/harmonics, dynamic range, and/or noise level
DPD: For wideband applications (>20 MHz), the DPD should include memory. Because increasing algorithm complexity increases power consumption, it is not feasible with mMIMO.
To reduce nonlinearity, the PA can be operated at a lower power, or “backed off.” But 3GPP LTE and 5G’s high peak-to-average power ratio (PAPR) means that the
PA would have to be backed off well below saturation, which would result in very low efficiency.
Because linearity and efficiency are trade-off parameters, the PA should be designed with the minimum required linearity.
The high-efficiency asymmetrical Doherty architectures used for PAs are highly nonlinear. DPD adds a nonlinearity in the baseband that complements the compressing characteristic of the PA. This linearizes the PA.
An indicator of linearity is the adjacent channel power ratio (ACPR), which 3GPP requires to be <–45 dBc.
Deliver the required linear response
GaN-on-SiC offers excellent high-frequency performance because of its lower terminal capacitances and lack of a body diode with reverse-recovery loss. This lowers operational costs of base stations.
If the PA is easy to linearize with the digital predistortion (DPD) system, it leads to higher system efficiency.
Operate at high efficiency
Today’s sub-6-GHz mMIMO applications already require 5–10 W.
GaN-on-SiC’s significantly higher breakdown voltage and higher thermal conductivity than LDMOS allows much higher power densities to be achieved.
Higher power densities address size and weight issues with base stations by bringing down both installation and real-estate rental costs.
Meet high-power requirements
In today’s sub-6-GHz mMIMO applications, the required bandwidth is in the 200-MHz range.
The wideband requirements are met by using GaN-on-SiC devices. GaN’s higher electron mobility than LDMOS provides higher gain at higher frequencies and better efficiency.
Transmit multi-carrier, multi-mode signals over the
full frequency span of a given communication spectrum
