As the world's standardization bodies set out to define next-generation wireless networks, the 5G vision is forcing researchers to change the way they think. Increasing the spectral efficiency of 4G networks is not sufficient to provide the data rates, delays, and capacity required for three advanced 5G use cases (Figure 1). These three use cases are defined by 3GPP and are expected to provide ubiquitous instantaneous mobile broadband data in the future.
Figure 1. These three 5G use cases are defined by 3GPP and IMT-2020.
The Enhanced Mobile Broadband (eMBB) use case is defined by IMT-2020, which envisages a peak data rate of over 10 Gbps, which is 100 times that of 4G networks. It has been proven that the data rate is directly related to the available spectrum, and according to Shannon's theorem, capacity is a function of bandwidth (ie, spectrum) and channel noise. Spectrum below 6 GHz has been exhausted, and spectrum above 6 GHz, especially millimeter wave frequency, has become a very promising alternative to implementing eMBB use cases. But which millimeter wave frequencies will be used?
Spectrum optionThe International Telecommunication Union (ITU) and 3GPP have reached agreement on plans for the two research phases of the 5G standard. The first phase focuses on frequencies below 40 GHz and is committed to addressing some of the more pressing business needs by September 2018; the second phase begins in 2018 and ends in December 2019, addressing the key to IMT-2020 Performance indicators. The second phase focuses on frequencies up to 100 GHz.
In order to achieve global consensus on millimeter-wave frequency standardization, the ITU announced at the World Radiocommunication Conference (WRC-15) last November a list of globally viable frequencies in the range of 24 GHz to 86 GHz, as shown in Table 1. Shortly after ITU issued the proposal, the Federal Communications Commission (FCC) issued a Notice of Rulemaking Recommendations (NPRM) on October 21, 2015, recommending new flexible service rules for the 28 GHz, 37 GHz, 39 GHz and 64-71 GHz bands. (as shown in picture 2).
Figure 2. The millimeter-wave band proposed by the FCC for mobile communications.
Although standards bodies such as the ITU and 3GPP have set 2020 as the deadline for defining the 5G standard, mobile operators are accelerating the time schedule of 5G services. In the US, Verizon and AT&T plan to test early versions of 5G in 2017. South Korea intends to conduct 5G trials at the 2018 Winter Olympics, while Japan wants to showcase 5G technology at the 2020 Tokyo Olympics. Through the efforts and promotion of various organizations, the candidate frequencies most likely to be used for 5G at present are: 28GHz, 39GHz and 73GHz.
There are several reasons for these three frequency bands. First, the 60 GHz frequency produces about 20 dB/km of attenuation due to oxygen absorption. Unlike 60 GHz, these frequencies have much lower oxygen absorption rates. This makes it possible for long distance communication. These frequencies operate reliably in multipath environments and can be used for non-line of sight (NLoS) communications. By combining beamforming and beam tracking with highly directional antennas, millimeter waves provide a reliable and very secure link. Dr. Ted Rappaport of the New York University School of Engineering and his students have begun research on channel characteristics and potential performance at 28 GHz, 39 GHz and 73 GHz. They have published several papers on propagation measurement and possible service disruption studies at these frequencies. The data and research of these frequencies combined with the availability of the global spectrum make these three frequencies a starting point for millimeter wave prototyping.
Service providers are eager to acquire these large amounts of unallocated millimeter-wave spectrum, and they are the key force in determining which frequencies are used by 5G. In Japan, NTT DoCoMo worked with Nokia, Samsung, Ericsson, Huawei and Fujitsu to test the 28GHz and other frequencies in the field. In February 2015, Samsung conducted channel measurements and proved that 28GHz is a viable frequency for cellular communications. These measurements verify the expected path attenuation in the urban environment: the path attenuation index for non-line-of-sight links is 3.53. Samsung said the data indicates that millimeter-wave communication links can support distances in excess of 200 meters. Its research also includes work on phased array antennas. Samsung has begun to analyze the characteristics of designs that may be suitable for mobile phone phased arrays.
In September 2015, Verizon announced that it will conduct field trials with important partners such as Samsung in 2016. In November 2015, Qualcomm conducted experiments at 28 GHz using 128 antennas to demonstrate millimeter-wave technology in dense urban environments. It demonstrates the application of directional beamforming in non-line-of-sight communications. As the FCC announces that 28 GHz is available for mobile communications, the United States is expected to conduct more in-depth experiments and field trials. Verizon also signed a 28GHz spectrum lease agreement with XO CommunicaTIons to purchase the spectrum by the end of 2018.
However, please note that the 28 GHz band is not included in ITU's list of globally viable frequencies. Whether it will become a long-term frequency choice for 5G millimeter wave applications remains to be determined. Regardless of how global standards are developed, spectrum availability in the United States, South Korea, and Japan, and US service providers' commitment to early field trials, it is possible to introduce 28 GHz into US mobile technology. Before the standards body finalized the 5G standard, South Korea’s desire to showcase 5G technology at the 2018 Winter Olympics will also drive 28GHz into consumer products. In fact, this frequency has not been ignored because it is not on the International Mobile Telecommunications (IMT) spectrum list, but has attracted the attention of the FCC.
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