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Multichip Modules Provide the Performance Needed for 5G Massive MIMO

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by Nathan Glaza, Integrated Power Solution Portfolio Manager, Radio Power BL at NXP Semiconductors

The 5G collection of standards provides numerous means for dramatically improving the data rates that users on the move can enjoy through their handsets and other devices. The applications utilized to support 5G include small cell, millimeter wave (mmWave), massive multiple input/multiple output (mMIMO), and macrocell antennas. The desired application is dependent on the density of the area. For instance, in urban/suburban areas with higher density, it is vital to utilize a mixture of small cell, mmWave, and mMIMO applications to support the large quantity of connections in the most efficient manner. When in rural, less dense areas, a macrocell provides the needed capacity to support the required connections.

One of the more crucial technologies required to support 5G is massive multiple input and multiple output antennas, also known as mMIMO. Massive MIMO antennas provide a variety of enhanced attributes to antenna technology. These enhanced attributes improve the ability to detect usable data signals at the base station and increase the signal rate that the RF channel can handle while in the presence of a variety of interference conditions.

Figure 1: NXP 5G Solutions

One of the enhanced attributes utilized in mMIMO antennas is the ability of spatial multiplexing. Spatial multiplexing relies on multiple antennas being mounted at different points of the base station or array.  Each antenna transmits a modified version of the signal stream that is intended for each active user in range. The signals will arrive at the receiving antenna with differing time delays and levels of attenuation. With the ability to pick up signals from multiple beams, the mobile user’s device has a higher probability of detecting a usable signal. Furthermore, signal processing techniques provide the ability to derive information from all the detected sources to improve the overall signal-to-noise ratio.

The increased number of antennas within mMIMO systems introduces the concept of beamforming. This enhanced attribute creates the ability to harness constructive interference between closely spaced antennas and direct the signal to a point where the mobile user is most likely to benefit. The signal can be directed in 3 dimensions both horizontally and vertically to the specific users and is highly effective in dense urban areas with high-rise buildings. In addition, the advanced signal processing makes it possible to tune the beams individually in range. This guarantees the maximum data rate within the current channel conditions even if those conditions change rapidly.

The shift to massive MIMO transmission is leading to major changes in the design of the base station. In traditional macrocell architectures, the RF control circuitry was at the base of the tower. Within this architecture, the signals were relayed to the antenna array at the top of the base station with long, inefficient coaxial cables. Massive MIMO transmission utilizes the remote radio head (RRH) architecture. Within this architecture, the RF components have been moved closer to the antenna, at the top of the tower. Fiber optic cables are now used to pass baseband signals from the RRH at the top of the tower to the processing electronics underneath. Furthermore, these fiber optic cables are a large improvement over the coaxial cables used in traditional macrocell architectures due to the increased efficiencies driven by less power losses from the fiber optic cables. 

The integration of the RF amplifier circuitry directly into the antenna unit has improved both efficiency and installation procedures, but it has also placed increasingly stringent demands on the electronics. The power amplifier and control circuitry need to be both compact and energy efficient while satisfying the need for responsiveness to allow the use of advanced signal processing schemes. A key requirement is support for high peak-to-average power ratio (PAPR) and multiple carrier aggregation. This, in turn, calls for high amplifier efficiency over a wide bandwidth and power levels that are significantly backed off from saturation.

The key element that allows the amplifier to operate under these restrictions is the Doherty Combiner Architecture. This architecture consists of 2 separate amplifiers that include the carrier and peaking. Generally, the carrier amplifier should be saturated at the back-off input power with the peaking amplifier used to vary output power rapidly, up to its own saturation point. The combination of the two provides high efficiency while satisfying the need to have instantaneous power levels that are significantly below maximum.

Though the Doherty combiner provides a conceptually simple structure for satisfying the modulation needs of 5G NR systems, achieving the performance required has called for advances in semiconductor technology. Wideband performance is essential, which can be satisfied by using high electron mobility transistors based on gallium nitride (GaN) materials.

As a wide-bandgap semiconductor, GaN can support the high dielectric field strength needed to handle the power output levels of 5G NR antennas with relatively small on-die transistor structures. Though GaN has a lower electron mobility than older-generation transceivers, its ability to support high carrier densities means that it can handle higher switching frequencies at higher peak output power levels. The ability to support higher peak power means that GaN readily lends itself to Doherty amplifier designs, especially those that need compact electronics, such as the integrated antennas that are required by massive MIMO architectures.

However, it is not enough to simply substitute GaN for older silicon technologies. Making full use of GaN requires attention to aspects of its operation that, if not controlled, reduce its overall performance. An issue with earlier GaN technologies lies in charge trapping, which is commonly encountered in pulsed operation under high electron densities. Some of the carriers remain trapped in the transistor channel at interfaces and crystal boundaries which causes small deviations in characteristics such as threshold voltage before the carriers are released later than expected in the switching cycle. This leads to phenomena such as the electrical memory effect and unwanted decreases in linearity. Other results of this charge trapping are gate lag and drain lag in time domain, the latter being where the drain current reaches its final value after some delay as the bias voltages are abruptly changed.

Figure 2: Hybrid LDMOS and GaN Power Amplifier

NXP’s GaN 4.5 generation uses several techniques to control the doping and crystal structure conditions that exacerbate charge trapping. The result is a Hybrid LDMOS and GaN power amplifier transistor structure that supports higher linearity than previous generations. In the context of the Hybird LDMOS and GaN series of power amplifier modules, the result is a lineup efficiency of 52% at 2.6 GHz, 8% higher than the previous generation of technology.

Multichip module integration rather than monolithic integration provides the best way forward because it makes it possible to employ the advantage of specific process technologies instead of having to make tradeoffs with a single technology, such as GaN. The Hybrid LDMOS and GaN family of 5G modules couples the GaN-based Doherty combiner elements with LDMOS driver ICs. The design of the driver circuitry is critical as it provides the means to operate GaN devices at peak performance. LDMOS provides the best combination of characteristics to handle this function; integration on a multichip provides further improvements by helping to minimize inductance and other parasitics that can compromise high-frequency operation.

Figure 3: RapidRF Module showcasing NXP’s full line-up of solutions

The multichip module approach provides ease of integration for the design team. Each NXP module includes a 50Ω in-out matching network, which reduces complexity for the PCB design team. Furthermore, the use of pin-compatible modules with different characteristics makes it easier for vendors to produce variants of their massive MIMO products and tune them for different markets and applications. It is possible to cover a wide range of deployment scenarios with the NXP module, including towers with 32T32R or 64T64R antenna configurations, outdoor small cells, transmitters intended for proprietary radio access networks and novel designs that take advantage of the shift towards open RAN architectures.

5G NR and the shift towards massive MIMO architectures bring new challenges to the designers of many different types of cellular base station transceivers. Through the combination of high-integration multichip packaging and the use of semiconductor technologies tuned to specific tasks, NXP is easing the path to building efficient transmitters for this new generation.

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