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mmWave Spectrum Opens Up 5G

But Which Frequencies Will Be Relevant?
James Kimery, Director of RF Research and SDR Marketing, NI

As the world’s standardization bodies move to define the next generation of wireless networks, the goals and objectives for 5G are forcing researchers to change the way they think. Increasing the spectral efficiency of a 4G-based network is not enough to deliver the step function in data rates, latency, and capacity necessary for the three high-level 5G use cases (Figure 1), as defined by 3GPP to provide ubiquitous, instantaneous mobile broadband data.

 

Figure 1. These three high-level 5G use cases were defined by 3GPP and IMT 2020.

The Enhanced Mobile Broadband (eMBB) use case, as defined by the IMT 2020, envisions peak data rates exceeding10 Gbps, which is 100X faster than 4G. Data rates are empirically linked to available spectrum, according to the Shannon-Hartley theorem, which states that capacity is a function of bandwidth (that is, spectrum) and channel noise. With spectrum below 6 GHz fully allocated, spectrum above 6 GHz, specifically in the mmWave range, presents an attractive alternative to address the eMBB use case. But at what mmWave frequency?

Spectrum Options
The International Telecommunication Union (ITU) and 3GPP have aligned on a plan for two phases of research for 5G standards. The first phase defines a period of research for frequencies under 40 GHz to address the more urgent subset of commercial needs by September 2018. The second phase, slated to begin in 2018 and end in December 2019, addresses the key performance indicators outlined by the IMT 2020. This second phase focuses on frequencies up to 100 GHz.

To globally align the standardization of mmWave frequencies, ITU released a list of proposed globally viable frequencies between 24 GHz and 86 GHz at the World Radio communications Conference this past November (WRC 15):

• 24.25–27.5GHz
• 31.8–33.4GHz
• 37–40.5GHz
• 40.5–42.5GHz
• 45.5–50.2GHz
• 50.4–52.6GHz
• 66–76GHz
• 81–86GHz

Shortly after the ITU proposal, the Federal Communications Commission (FCC) in the United States issued a Notice of Proposed Rule Making (NPRM) on October 21, 2015, that recommended new flexible service rules among the 28 GHz, 37 GHz, 39 GHz, and 64–71 GHz bands.

 

Figure 2. FCC Bands Proposed for Mobile Use

 

Which Are the “Best” Frequencies?
While the ITU, 3GPP, and other standards bodies decided on 2020 as the deadline for the 5G standard to be defined, cellular providers worked on an accelerated schedule for delivering 5G service. In the United States, Verizon and AT&T plan to test an early version of 5G in 2017. Korea is working to conduct 5G trials at the Olympics in 2018, and Japan wants to demonstrate 5G technologies at the Tokyo Olympics in 2020. Through these varying groups and motivations, a set of frequencies is emerging as the candidates for 5G: 28 GHz, 39 GHz, and 73GHz.

These three frequency bands have emerged for several reasons. First, unlike 60 GHz, which has approximately 20 dB/km loss due to oxygen absorption, they have much lower oxygen absorption rates. This makes them more viable for long distance communications. These frequencies also function well in multipath environments, and can be used for non-line-of-sight (NLoS) communications. By combining highly directional antennas with beamforming and beamtracking, mmWave can provide a reliable and very secure link. Dr. Ted Rappaport and his students at NYU Polytechnic School of Engineering have already begun research on the channel properties and potential performance for 28 GHz, 39 GHz, and 73 GHz. They have published several papers with propagation measurements and studies on potential service outages at these frequencies. The data and research at these frequencies combined with the availability of spectrum worldwide make these three frequencies the starting point for mmWave prototyping.

As mentioned previously, service providers are eager to access the extensive unallocated mmWave spectrum. They are the key influencers for the frequencies used in this spectrum. In February 2015, Samsung performed its own channel measurements and showed that 28 GHz is a viable frequency for cellular communications. These measurements validated the expected path loss for urban environments (the path loss exponent is 3.53 in NLoS links), and Samsung claims that this data suggests a mmWave communications link can be supported for over 200 m of distance. Its research also includes work with phased array antennas. Samsung has begun characterizing designs that could fit phased arrays inside cell phones. In Japan, NTT DOCOMO partnered with Nokia, Samsung, Ericsson, Huawei, and Fujitsu to do its own field trials at 28 GHz along with other frequencies.

In September 2015, Verizon announced that it will conduct field trials with key partners including Samsung in the United States in 2016. In November 2015, Qualcomm conducted experiments at 28 GHz with 128 antennas to demonstrate mmWave technology in a dense urban environment. It showed how directional beamforming can be used for NLoS communications. With the FCC announcement that the 28 GHz spectrum can be used for mobile communications, further experiments and field trials in the United States are expected. Verizon has also completed an agreement with XO Communications to lease the 28 GHz spectrum with the option to purchase it by the end of 2018.

Note, however, that the 28 GHz band is not included in ITU’s list of globally viable frequencies. Whether it will be the long-term frequency option for 5G mmWave applications still must be determined. The spectrum’s availability in the United States, Korea, and Japan, along with US service providers’ commitment to early field trials, could push 28 GHz into US mobile technology regardless of the global standards. Korea’s desire to show 5G technology at the 2018 Olympics could also push 28 GHz into consumer products before the standards bodies finalize 5G standards. The fact that this frequency was not on the International Mobile Telecommunications (IMT) spectrum list did not go unnoticed and has drawn some attention from the FCC.

Prototypes Moving mmWave Forward
Though the possible wide adoption of 28 GHz for 5G may not be seen for a while if at all, it is clearly important right now. Mobile communications in the last several years have also focused on 73 GHz, E-band frequencies. Nokia used the channel measurements NYU took at 73 GHz to begin its research at this frequency. In 2014 at NI’s annual user conference, NI Week, Nokia used NI prototyping hardware to demonstrate the first over-the-air demo operating at 73 GHz. The company continued to evolve the prototype with public demonstrations to display new achievements. By Mobile World Congress (MWC)2015, the prototyping system was capable of over 2 Gbps data throughput using a lens antenna and beamtracking. Nokia showcased a MIMO version of this system operating at over 10 Gbpsat the Brooklyn 5G Summit in 2015, and less than a year later at MWC 2016,the company demonstrated a two-way over-the-air link operating at over 14 Gbps. Nokia was not the only company to show a 73 GHz demo at MWC 2016. Huawei also presented a prototype with Deutsche Telekom operating at 73 GHz. This demo, using Multi-user MIMO, displayed high spectrum efficiency and the potential for more than 20 Gbps throughput rates for individual users.

More 73 GHz research is anticipated in the coming years. One of the defining characteristics of this frequency that sets it apart from 28 GHz and 39 GHz is the available contiguous bandwidth (greater than 2 GHz), which is the widest of the proposed frequency spectra. By comparison, 28 GHz offers 850 MHz of bandwidth and the two bands around 39 GHz offer 1.6 GHz and 1.4 GHz bandwidth in the United States. And as mentioned earlier, per Shannon, more bandwidth equates to more data throughput, and this gives 73 GHz a big advantage over the other frequencies mentioned.

The 39 GHz bands are under investigation, but public support and research have not significantly materialized. This frequency features some characteristics that may make it a compromise frequency range for wider adoption. The FCC has proposed 39 GHz for potential mobile use. Verizon, while focusing on 28 GHz for its initial field trials in 2017, has access to 39 GHz via its business relationship with XO Communications, which owns substantial licenses in 39 GHz. Still, public support and acknowledgement of 28 GHz and 73 GHz research are more visible than that for other frequency research.

To capitalize on the promise of mmWave for 5G, researchers must develop new technologies, algorithms, and communications protocols because the fundamental properties of the mmWave channel are different from current cellular models and are relatively unknown. The importance of building mmWave prototypes cannot be overstated, especially in this early timeframe. Building mmWave system prototypes demonstrates the viability and feasibility of a technology or concept in a way that simulations alone cannot. mmWave prototypes communicating in realtime and over the air in a variety of scenarios will unlock the secrets of the mmWave channel and enable innovation, technology adoption, and proliferation.

Challenges
mmWave for mobile access creates several challenges including the availability of commercial off-the-shelf silicon and analog components as well as other elemental building blocks for developing systems. This hinders commercialization. Consider a baseband subsystem capable of processing a multi-gigahertz signal. Most of today’s LTE implementations typically use 10 MHz channels (20 MHz maximally), and the computation load increases linearly with bandwidth. In other words, the computational capacity must increase by a factor of 100 or more to address the 5G data rate requirements. To conduct mmWave system physical-layer computations for infrastructure, FPGAs are an essential technology in developing real-time prototypes. After all, the motivation for moving to mmWave is the existence of large amounts of contiguous bandwidth.

In addition to FPGA boards, a mmWave prototyping system needs state-of-the-art DACs and ADCs to capture up to2 GHz of contiguous bandwidth. Some RFICs on the market today include chips that convert between baseband and mmWave frequencies, but these options are limited and mostly cover the unlicensed 60 GHz band. Engineers can use IF and RF stages as alternatives to RFICs. Once they develop baseband and IF solutions, engineers have a few more vendor-provided options for mmWave radio heads than they do for baseband RFICs, but still not many. Developing a mmWave radio head requires RF and microwave design expertise. This is an entirely different skill set than that used for developing FPGA boards, so pulling together all of the necessary hardware requires a team with diverse experience. FPGAs must be considered core components in a mmWave baseband prototyping system, and programming a multi-FPGA system capable of processing multi-gigahertz channels increases system complexity.

mmWave for 5G Is Inevitable
To address the complexity and software challenges communications researchers face, the NI mmWave Transceiver System provides a configurable set of mmWave prototyping hardware along with a mmWave physical layer in source code. This layer accounts for the fundamental aspects of a mmWave system baseband and provides abstractions for data movement and processing across multiple FPGAs to simplify integration. These tools are designed to accelerate the transition of new prototypes into systems and products that will be crucial to the development of 5G technology.

Figure 3. The NI mmWave Transceiver System is the world’s first software defined radio for the mmWave spectrum.

Though the future of 5G is not yet clear, mmWave will surely be one of the technologies used to define it. The large amount of contiguous bandwidth available above 24 GHz is needed to meet data throughput requirements, and researchers have already used prototypes to show that mmWave technology can deliver data rates above 14 Gbps. The biggest unanswered question is which frequency will be widely adopted for 5G.

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