Millimeter wave (mmWave): the battle of the band

The amount of data consumed by wireless devices and the amount of data they process is exponentially increasing each year (53% compound annual growth rate [1]). As the amount of data generated and processed by these devices increases, the wireless communication infrastructure connecting these devices must continue to evolve to meet demand. As shown in Figure 1, the increase in spectral efficiency of 4G networks is not sufficient to provide a step function for the data rates required by the three advanced 5G use cases defined by 3GPP [2], which are designed to provide ubiquitous real-time mobile broadband data. Recognizing this, researchers began looking for higher frequencies as a possible solution. The positive results of early channel sounding efforts have led global wireless standardization organizations to shift their focus to the next generation of 5G wireless systems and how they can benefit from new frequencies and wider bandwidth.

All use cases are designed to enable future wireless standards to handle new applications that are not adequate for existing wireless standards, each requiring a whole new set of key performance (KPI). The Enhanced Mobile Broadband (eMBB) defined by the IMT 2020 use case is expected to have a maximum data rate of 10 Gb/s, which is 100 times that of 4G [3]. According to the relationship between channel capacity and bandwidth (spectrum) and channel noise summarized by Shannon's theorem [4], the data rate is closely related to the available spectrum. Since the spectrum of less than 6 Ghz has been fully allocated, spectrums above 6 Ghz, especially in the millimeter-wave range, are an excellent choice for eMBB use cases.

Figure 1: Three high level 5G use cases as defined by 3GPP and IMT 2020

To serve customers, service operators around the world spend billions of dollars on the spectrum. The high price of the auction price also highlights its market value and the scarcity of this precious resource. The development of new spectrum enables service operators to accommodate more users and deliver a higher performance mobile broadband data experience. Compared to the spectrum of less than 6Ghz, the millimeter wave is more abundant and requires less license, which means that service operators all over the world can use it. Advanced silicon manufacturing processes significantly reduce the price of millimeter-wave devices and are fully available for consumer electronics. The main problem currently affecting millimeter wave applications is that many aspects of this spectrum have not been studied and technical questions need to be answered.

Service operators have begun to study millimeter-wave technology to evaluate the best candidate frequencies for mobile applications. The International Telecommunication Union (ITU) and 3GPP have reached a consensus on the two-phase planning of the 5G standard study. The first phase studies frequencies below 40 GHz to meet more urgent business needs and will be completed in September 2018. The second phase is planned to begin in 2018 and be completed in September 2019 to address the KPPI outlined in IMT 2020, which focuses on frequencies up to 100 Ghz.

To achieve global uniform standardization of millimeter-wave frequencies, the ITU has released a list of globally available frequency recommendations from 24 Ghz to 86 Ghz after the recent World Radiocommunication Conference (WRC)[5]:

Shortly after the ITU issued its recommendations, the Federal Communications Commission (FCC) issued a Notice of Rulemaking Recommendations (NPRM) on October 21, 2015, recommending the 28 GHz, 37 GHz, 39 Ghz and 64-71 Ghz bands [6] Flexible new service rules.

Figure 2: FCC Recommended Bands for Mobile Applications [6]

While the ITU, 3GPP and other standards bodies have decided to use 2020 as the deadline for the 5G standard, handset vendors are working hard to accelerate the delivery of 5G services. Verizon and AT&T in the US plan to launch an early version of 5G in 2017. South Korea plans to launch a 5G trial at the 2018 Olympics, and Japan hopes to showcase 5G technology at the 2020 Tokyo Olympics. Through the continuous efforts of various groups driven by various factors, some frequencies have begun to become candidates for 5G: 28 GHz, 39 Ghz and 72 GHz.

There are reasons why these three bands stand out. First, unlike 60 Ghz, which produces about 20 dB/km loss due to oxygen absorption [7], the oxygen absorption rates of these bands are much lower, as shown in the figure below, and are more suitable for long-distance communication. These frequencies also perform well in multi-channel environments and can be used for non-line-of-sight communications. Millimeter waves combine highly directional antennas with beamforming and beam tracking to provide an extremely safe and reliable link. Dr. Ted Rappaport of New York University's Polytechnic and his students have begun to study the channel characteristics and potential performance of 28GHz, 38GHz and 73Ghz. They published several papers on propagation measurements at these frequencies and potential service disruption studies. Through the existing data and research of these frequencies plus the globally available spectrum, prototypes of millimeter waves can be made from these three frequencies.

Figure 3: Atmospheric Absorption Rate (dB/km) at millimeter wave frequency [7]

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