Millimeter waves are electromagnetic signals with frequencies ranging from 30 to 300 GHz that correspond to wavelengths of 10 to 1 mm in the free space. Electromagnetic waves in the millimeter-wave band (with frequencies between 30 and 300 GHz, or wavelengths between 10 and 1.0 mm) have attractive characteristics. One of their features is the wider usable frequency band compared with waves in the microwave band or lower bands. Another feature of using the millimeter-wave band is the fact that it becomes possible to design smaller and lighter equipment that utilizes that band. So it is useful to adapt millimeter waves for short-range broadband communication systems, high resolution sensing systems and radio astronomy.
The Fifth-Generation (5G) mobile networks is bringing in the latest wireless revolution, enabling wireless download speed exceeding 10 Gbps for eMBB (enhanced Mobile Broadband) applications, with 100x more wireless connected devices than 4G for mMTC (massive machine type communication) to enable IoE (Internet-of-Everything), and sub-1 ms latency for instant actions with UR/LL, mMTC (ultrareliable machine type communication). It will be extremely challenging to achieve those aggressive 5G performance metrics all at once, and thus the 5G revolution is expected to be happening in stages.
The 5G networks will employ a broad range of spectrum frequencies, including the popular 28 GHz and 39 GHz 5G mmWave bands. Here it’s worth noting that while LTE will be an integral part of 5G networks, the role of mmWave frequencies is likely to grow due to the availability of plentiful spectrum as well as aggregated channel bandwidth of 1 GHz and higher.
“With the advent of smart phones, tablets and connected cameras, mobile data traffic is growing at a very fast pace. The capacity of mobile backhaul network must be increased to deal with this data explosion. In that respect there is at present an increased interest in exploiting the millimeter-wave frequency range (30-300 GHz) for wireless backhauling.
The progress in semiconductor device technology, in particular compound semiconductor transistors such as High Electron Mobility Transistor (HEMT) and Heterojunction Bipolar Transistor (HBT) devices enables the development of wireless communication circuits operating at frequencies well above 100 GHz. The main bottleneck in these mm-wave wireless communication systems today is the power amplifier (PA) which sets the limit on the available transmission range.
The performance of a radio-frequency power amplifier (RF PA) can often dominate the overall transmitter (TX) performance, as its power-added efficiency (PAE) dictates the power and heat dissipation for the entire TX. For enhanced user experience and massive MIMO antennas at cm-Wave/mm-Wave frequencies, the 5G system will require more PAs to be integrated in the RF front-end modules (FEMs), making the design of a 5G PA more critical than that of a 4G PA.
Figure illustrates an example of attractive 5G FEM IC array design in cm-Wave/mm-Wave for phased-array MIMO antennas. The 5G PA, low-noise amplifier (LNA), T/R switches, phase shifter, and various passives are all integrated into the FEM IC . This architecture is rather different from their 3G/4G counterparts and also with a much higher level of IC integration. In some cases the antennas may be directly packaged on top of the FEM IC on the wafer-scale to achieve even higher integration with reasonable performance . The high integration requirement of FEM ICs and massive antenna systems may favor silicon-based technologies for 5G mobile products, even though GaAs or GaN FEMs usually have better performances than their silicon counterparts.
In addition to the high integration requirement, as the TX operation frequency moves to cm-Wave/mm-Wave frequencies, it has been well-recognized as a very difficult task to design a high-efficiency linear PA to overcome the overheating issue for successful massive MIMO realization.
Today, the majority of handset PAs are still designed in III–V semiconductor devices technologies because of their superior frequency responses, breakdown robustness, and faster time-to-market than silicon-based counterparts. Since today’s base station PAs require rather high , they are largely designed in low-cost silicon LDMOS (Laterally Diffused MOSFETs) for sub-3.5 GHz bands, and in GaAs or GaN at higher frequencies, depending on the exact requirements.
For example, GaN devices are capable of operating at a RF power density of 6–8 W/mm of gate periphery at 4G cellular bands and can deliver an impressive power density of 3.6 W/mm at 86 GHz in continuous-wave (CW) operation. In a separate work, of 3.6 Watt at 83 GHz was achieved in pulse mode that silicon-based PA technologies (LDMOS, SiGe, and CMOS) simply cannot match. However, silicon-based RF PAs do have the advantages in offering higher monolithic integration with added functionalities (e.g., on-chip digital control/selection on power detection, adaptive matching, and digital predistortion (DPD)), which can translate to lower cost and smaller sizes attractive for broadband multimode multiband 5G handsets with massive MIMO.
Because of the higher cm-Wave/mm-Wave carrier frequency and the massive MIMO technologies to be deployed, an individual “true-5G” handset or small-cell PA is expected to have lower requirements than those currently used in 4G LTE applications. The 5G PAs used in femtocells and picocells both have fairly low requirements per PA (i.e., <20 dBm), which means they could be realizable by silicon-based PAs. A 5G macrocell, on the other hand, will probably need to utilize GaN or GaAs PAs due to their larger requirements. Power efficiency, robustness, and cost will eventually determine the preferred device technology for a given 5G PA application.
Cost and integration level will also be critical factors in deciding the preferred technology for a given 5G PA implementation. Nakatani et al. recently reported an impressive 15 GHz 5 × 5 mm2 FEM IC that integrates a three-stage PA, a two-stage LNA, and a T/R switch in a 0.15 m GaAs technology for 5G wideband massive MIMO
DARPA awards development of Millimeter wave (mmWave) power amplifiers.
DARPA has awarded a $497,000, one-year study contract to tiny wireless startup MixComm to demonstrate silicon-based millimeter wave (mmWave) power amplifiers. The “exploratory” award was made in response to Open Office Broad Agency Announcement (BAA) from DARPA’s Microsystems Technology Office (MTO).
“The technical goal of MixComm’s work with DARPA under this effort is to try and simultaneously push the bandwidth, efficiency, and linearity of millimeter wave (mmWave) power amplifiers,” said DARPA’s Tim Hancock. “This has direct applicability to 5G.” “These pre-decisional studies are often funded by DARPA to understand if there is sufficient justification to warrant a more formal investigation of a particular technology or research topic,” Hancock said in an email. “The technical goal of MixComm’s work with DARPA under this effort is to try and simultaneously push the bandwidth, efficiency, and linearity of millimeter wave (mmWave) power amplifiers. This has direct applicability to 5G.”
“MixComm develops mmWave Radios,” a company spokesperson elaborated in an email. “These solutions are used in 5G and Satellite Communications and offer tremendous bandwidth, capacity and low latency. mmWave frequencies are being used in cellular communications for the first time in 5G. These frequencies go from 24GHz to 47GHz and are referred to as the FR2 bands. With funding from DARPA, MixComm is able to develop 5G mmWave power amplifiers with groundbreaking output power and efficiency.” All radios actually require an amplifier of some sort to pump up transmitted signals so they can reach distant receivers, Hancock explained.
“A mmWave Power Amplifier is an amplifier that makes that signal larger and often strives to do so with high efficiency to minimize the amount of power that is lost to heat. All signals, including mmWave signals, need to be amplified before they leave an antenna so that they can travel further distances when radiated through the environment, thus providing reliable connections between radios,” he said.
One of the benefits of mmWave radios is that they use “frequencies 10 [times] higher than what we use today,” the MixComm spokesperson said. “With 4G and Wi-Fi, we use frequencies below 6 GHz, but mmWave uses [greater than] 24GHz bands – enabling faster data rates that aren’t possible with other frequencies. For example, most people have seven or eight wireless connections in their home on Wi-Fi. When one person uses Netflix, everyone else gets clogged and wireless speeds dramatically drop. With mmWave, this wouldn’t happen.”