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5G Drama stage

Time:2018-08-20     Click:loading...   【Print】  【Close
With the use of advanced modulation schemes and technologies such as carrier polymerization for cellular base stations, the early application of commercial GaN PA may decline. After that, however, with the rise of millimeter wave applications, especially large-scale MIMO, the market prospect of GaN is still strong, because there is likely to be no other candidate technology to meet the power density requirements of large-scale active electronic scanning array.The race for 5G appears to be accelerating, especially in the United States, where major telecom carriers such as AT&T and Verizon have announced plans to roll out 5G services by the end of 2018, according to myms consulting. Advanced LTE (lte-a) has been rapidly upgraded and extended to the current base station (BS). Field trials of the lte-pro (also known as 4.5g) are in full swing, with download speeds reaching 1 Gbps. Fixed wireless access (FWA) technology has also passed a number of field trials, demonstrating the early successful application of mmWave spectrum.

The strict requirements for 5G are not only reflected in the density densification of base stations at the macro level, but also the enhancement of power density at the device level. GaN (gallium nitride) will significantly penetrate two major markets - defense and wireless communications - in the next few decades at a compound annual growth rate of 20 percent (CAGR), according to myms consulting. While many other compound semiconductors and processes will also play an important role in 5G development, it is clear that GaN will play a key role in high-performance wireless solutions with its power/efficiency level and high-frequency performance.

With the development of cellular technology, the modulation scheme used is usually defined by an unsteady envelope with a peak-value average power ratio (PAPR, the ratio of peak power to signal average power). As shown in the figure below, PAPR has dramatically increased from about 2:1 of 3G (w-cdma) to 7:1 of 4G (LTE/OFDM). Moreover, although advanced modulation schemes such as OFDM achieve very high speeds with limited network resources, the improvement of spectral efficiency comes at the expense of the reduction of power amplifier (PA) energy efficiency.To avoid signal distortion, the high PAPR waveform must be linearly amplified. If the signal passes through the nonlinear PA, in-band distortion will occur, which will increase the bit error rate (BER) and out-of-band radiation, resulting in adjacent channel interference. As a result, these high-power amplifiers often require a trade-off between linearity and efficiency.

In addition to the growing PAPR design constraints, you need to run on a much wider bandwidth than traditional PA. Mobile network operators (MNO) are already facing the need to achieve higher data rates, but are severely limited by bandwidth below 20 MHz. Carrier convergence is to increase the effective bandwidth greatly in the spectrum sparse operating region. Carrier convergence combines radio channels within the same band (in-band) or different band (inter-band) to increase wireless data rates and reduce latency.Lte-a allows carrier units to have up to 20MHz of bandwidth, up to five of which can be assembled into up to 100MHz of bandwidth for aggregation. In the past, mobile network operators could also use systems that cover a single 20mhz band, but in the future network capacity would have to be significantly increased to support the upcoming surge in mobile traffic. Current technologies require up to 20 times the bandwidth needed to handle these multi-band and multi-carrier systems.

To support these advanced modulation schemes, there are many problems to be faced, and several known solutions have been developed. Some include digital predistortion (DPD) to improve linearity, Doherty and envelope tracking (ET) techniques for greater efficiency. GaN high electron mobility transistor (HEMT), with its inherent high breakdown voltage, high power density, high bandwidth and high efficiency, has become a strong candidate technology for base station PA. For Johnson quality factor (FoM), which measures the semiconductor suitability of high-frequency power transistors, GaN devices are several orders of magnitude higher than silicon (Si), gallium arsenide (GaAs), silicon carbide (SiC) and indium phosphide (InP).Compared with existing silicon LDMOS and GaAs solutions, GaN devices can provide the power and efficiency required for the next generation of high frequency telecommunications networks. Moreover, the broadband performance of GaN is also one of the key factors to realize multi-band carrier polymerization and other important new technologies. Since LDMOS can no longer support higher frequencies, GaAs is no longer the optimal solution for high-power applications, and it is expected that GaN devices will be used in most macro network unit applications below 6GHz in the future.

As mentioned earlier, gan-on-sic HEMT is the primary candidate technology for base station PA, as they are able to achieve higher power additional efficiency (PAE) in Doherty configurations with greater bandwidth and higher frequencies than silicon LDMOS transistors. GaN HEMT technology can also be very rugged and durable, operating at high power loads that are severely mismatched, with minimal performance degradation. This inherent high operating voltage and output impedance results in low loss, broadband matching and large output power.In addition, its larger safe operating area (SAO) can mitigate any thermal or electric field breakdown problems caused by power fluctuations, thus minimizing the need for maintenance of base station equipment. The low noise factor performance of GaN MMIC combined with its high power density makes them potential ideal choices for PA substrate in the emitter chain and low noise amplifier (LNA) in the receiver chain. Several existing GaN lna implementations can meet the requirements of low noise and can withstand high incident power without damage.

Millimeter wave (mmWave) spectrum is the key to realize 5G. Its vast available bandwidth is a strong choice for supporting high data rate applications (such as 4K/8K video streaming) as well as augmented and virtual reality (AR/VR) applications. Small base stations are ideal for utilizing millimeter wave bands because they can be tightly arranged in urban environments, reducing the lossy propagation characteristics of high frequency signals. For practical purposes, these small base stations must be easy to install on high size, weight, and power-limited structures.The size problem has, in fact, been solved to some extent as the size of transistors and antennae gradually decreases at higher frequencies. Smaller components, however, generally have poorer thermal management characteristics because a larger surface area is better at dispersing heat on the device. The SiC substrate has relatively high thermal conductivity (~120 W/MK), so it is easier to transfer heat from the transistor to the radiator. Chemical vapor deposition (CVD) diamond (~1800 W/MK) has higher thermal conductivity than SiC for small base station applications at lower cost.

GaN PA is already used in Ka - band transponders in advanced satellite communications. The upcoming high-throughput satellites (HTS) and low-earth orbit (LEO) small and medium-sized satellites require components that are smaller in size in order to achieve a high degree of integration in an extremely power-constrained environment. The technology can be used in 5G millimeter bands above 24 GHz. The current GaN process of 0.2um, 0.15um and 0.1um enables the cutoff frequency to enter the W band, and the power density is about 2W/mm.GaN PA shows high power density, wide running bandwidth, good PAE and linearity, as well as low noise performance at lower frequencies, and the same performance performance at MMW frequency. AlGaN/GaN heterostructure is particularly suitable for high frequency performance. Unlike AlGaAs/GaAs based devices, AlGaN/GaN heterostructure with large spontaneous and piezoelectric polarization effects can generate electron channels without any modulation doping.

Current base station technology involves MIMO configurations with up to eight antennas to control signals through simple beamforming algorithms, but large-scale MIMO may require the use of hundreds of antennas to achieve the data rate and spectrum efficiency required for 5G. The high-power active electronically scanned array (AESA) used in large-scale MIMO requires a separate PA to drive each antenna element, which presents significant size, weight, power density and cost (swap-c) challenges.This will always involve a radiofrequency PA capable of meeting the power, linearity, thermal management and size requirements of 64 components and beyond the MIMO array with minimum deviation on each transmit/receive (T/R) module. Since GaN chips make a leap in power density and packaging every year, GaN is likely to become a natural choice when large-scale MIMO systems are commercially viable.

GaN substrates have been used in military radar for decades, but the confidentiality of such applications has, to some extent, hindered its growth and maturity in the commercial sphere. GaN devices originated from the us department of defense and have been widely used in the new generation of space and ground radar systems. The high power performance of GaN improves the detection distance and resolution of the radar, and the application of this new technology has become increasingly mature. However, military-related technologies are always very sensitive. With the increasingly favored GaN devices in the field of defense applications, non-military applications may be affected, especially market mergers and acquisitions for this technology. When it comes to military applications, the government is bound to intervene, such as the FGC Investment Fund's acquisition of Aixtron and Infineon's takeover of Wolfspeed.

Still, as Yole and other researchers predict, the demand for such wideband gap materials is shifting, which would essentially eliminate the exclusivity of military and integrated equipment manufacturers (IDM) to independent foundries and design firms.In addition, the development of cellular communication technology and industry provides a very promising niche market for GaN's application. This demand in the commercial sector is likely to drive the manufacture of GaN based devices and ultimately reduce the bulk price of GaN based devices.With the use of advanced modulation schemes and technologies such as carrier polymerization for cellular base stations, the early application of commercial GaN PA may decline. After that, however, with the rise of millimeter wave applications, especially large-scale MIMO, the market prospect of GaN is still strong, because there is likely to be no other candidate technology to meet the power density requirements of large-scale active electronic scanning array.

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