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Solving 5G Antenna Design Challenges

As the cellular industry continues to move forward to fulfill the need of higher data rates, lower latency and greater reliability, RF system design has again become the bottleneck for any cellular devices or networks that aim to deliver more data to more users in more demanding use cases. 

While 3rd Generation Partnership Project (3GPP) continues to release more specifications addressing the emerging demands and pushing the industry deeper into the era of 5G, confusion and misunderstanding also emerge, casting doubts among device OEMs on plans to launch their next-generation 5G products.

The antenna design is by far the most confusing part of this process as it is almost entirely depended on the end device form factor and OEMs’ preferences.

From the beginning, Digi Wireless Design Services has been on the forefront of emerging technologies. It has a long list of partners in the wireless device design industry and a proven record of success in designing wireless devices. In this blog, we offer insights on the 5G User Equipment (UE) device antenna design.
 

New 5G Features and How They Differ from Current 4G LTE

To understand the reason for 5G being able to deliver a much higher data rates than the current 4G technology, it might be helpful to look at Shannon-Hartley theorem first:
 
C = M * B log2(1 + S/N)
  • C is the channel capacity in bit/second
  • M is the number of channels
  • B is the bandwidth of each channel
  • S/N is signal to noise ratio
It is actually intuitive based on the theorem that in order to have a higher channel capacity, improvements must be done to adjust the system M, B and S/N. 5G, which evolves from 4G, implements some well-known and long-existing techniques in its architecture to improve its channel capacity:
  • Carrier Aggregation (CA) > Increased bandwidth (B)
  • Multiple-in-multiple-out architecture (MIMO) > increase the number of channels (M)
  • Allocating new frequency bands > Increased bandwidth (B)
  • Adaptively adopt higher-order modulation schemes > S/N and B
Compared to 4G, 5G pushes the same set of techniques to the next level of capability and complexity. This inevitably pushes antenna design for 5G devices to the next level to fit the ever-increasing requirements for greater bandwidth, more frequency bands and better interference immunity.
 

How the New 5G Features Create New Antenna Design Challenges

To plan and design antennas for the functionality of 5G, it's important to understand the challenges and how to address them.  Here we review those considerations.
 

Actively Tunable Antenna System

Due to stringent size constraints, modern wireless devices typically use active antenna tuners as an effective mean to shrink antenna size. It can tune antenna smartly based on the changing operating environment, frequency band, and bandwidth coverage. With a potentially higher order of CA in 5G and additional cellular bands, the antenna tuning system must be able to support more tuner state as well as wider frequency bandwidth per tuner state.
 

New Frequency Bands

Based on 3GPP Release 15, two basic frequency ranges (FR1 and FR2) are to be used for 5G:
 
FR1: 410 MHz to 7.125 GHz; FR2: 24.25 to 52.6 GHz

In FR1, 5G adopts 3.3 ~ 3.8, 3.8 ~ 4.2, and 4.4 ~ 4.9 GHz bands on top of the existing sub-3GHz bands in 4G LTE. This posts new requirements for cellular antennas to provide additional frequency coverages in the sub-6GHz frequency range.
 
Table 1: 5G New Radio (NR) operating bands in FR1 1
5G NR operating bands
FR2, or the mmWave frequency range, offers an extremely wide bandwidth up to 2 GHz in some regions. Devices or systems that intend to take advantage of this wide bandwidth require antenna designs to be fundamentally different. Because signal propagation loss is inversely proportional to signal wavelength, mmWave signals suffer severe path losses. To compensate the path loss, increasing antenna Gain through designing phased-array antennas becomes a reliable solution acknowledged by the industry. Phased-array design opens an entirely new realm of antenna design not present in 4G.
 
Table 2: 5G New Radio (NR) operating bands in FR2 1
5G NR uplink and downlink operating bands

Challenging Antenna System Design Due to Co-existence

MIMO functionality requires multiple antennas to co-exist on a device and operate on the same frequency bands. The technology itself has already been used in the 4G LTE network in the form of SU-MIMO and MU-MIMO (Single-user MIMO and Multiple-user MIMO).

In 5G, Massive-MIMO (mMIMO) will be a necessary building block to push the cell capacity and UE downloading data rate to the next level. While most of the mMIMO antenna specifications and technology reviews nowadays focus on the base station side, where 32 or more logical antenna ports are needed, it is expected that the number of antennas on the UE is also going to increase.

Also, because of the enabling of Multiple Access technology in 5G, Bluetooth/WLAN, cellular, etc. are more often to transmit on the UE simultaneously, antenna coexistence issue can only be more complicated to solve. If not properly addressed, antenna coexistence issues can cause either communication range reduction, an unexpected blind spot, or even sporadic connectivity quality drop.

Figure 1 gives an example of antenna efficiency loss due to coexistence. Antennas must be strategically arranged in a 5G UE to achieve the full potency of MIMO.
 
Figure 1: Antenna efficiency reduction when migrating from a SISO to a MIMO system
5G Efficiency and Frequency
 

Design Approaches to New 5G Antenna Design Challenges

Now that we have covered some of the challenges, let's discuss some design considerations that can help to ensure success.
 

Sub-6 GHz Antenna Design Approach

5G antennas can be divided into two categories by their operating frequency: Sub-6GHz and mmWave. Comparing sub-6 GHz 5G with LTE 4G, the system RF front-end and antenna design concepts will be very similar with the only difference being lateral complexity. This means, going from 4G to sub-6 GHz 5G, the same set of componentry will be used in the system side and the antenna will still be an omnidirectional standalone (vs. array) antenna.

In this frequency range, common antenna types such as dipole antenna, monopole antenna, PIFA, IFA, loop antenna, etc. will still play dominating roles as they have been in 2G/3G/4G. Antenna form factors can vary from a simple printed-trace antenna to an intricate Laser Direct Structuring (LDS) antenna.

The conflict between requirements for smaller device size and larger antenna bandwidth will still be the top challenge, only much harder than before. One viable solution to this increasingly intense confrontation is to design an active antenna system.

The most commonly seen active antenna systems can be divided into two categories: active impedance matching and antenna aperture tuning. The active impedance matching technique enables the antenna system to select among different impedance-matching networks based on operating condition changes, while active aperture tuning directly changes the intrinsic characteristics of the antenna.
 
Figure 2: Active matching (left) and active aperture tuning diagram (right)
Active Aperture / Active Matching
Device OEMs can also take advantages of off-the-shelf (OTS) antennas to simplify the antenna design process. However, same with 4G, the same OTS will behave differently when situated in different devices, as different PCBs provides different RF reference even if the antennas themselves are the same. At the minimum, OEMs should expect to have customized antenna matching networks for any selected OTA antennas.
 

mmWave Antenna Design Approach

On mmWave frequencies, several signal propagation path loss greatly constraints the cell size and the bandwidth advantage can be greatly masked by connectivity coverage issues. To compensate signal path loss, phased-array antennas become necessary due to its capability of realizing very high Gain(dBi).

Designing a phased-array antenna for 5G mmWave requires significantly more upfront knowledge on fundamental antenna design concepts, array antenna design practices, mmWave signal propagation behavior, and much more.  At a minimum, a phased-array antenna should be able to steer and optimize the radiation beam to maximize the peak EIRP(dBm) towards a mobile receiving device within its cell sector.  A well-designed phased-array antenna for 5G should also factor in dual polarization, minimizing array size, mitigating side lobe level, improving the beam steering angle range and resolution, suppressing system noise, improve power efficiency, and more.

mmWave antenna testing also presents engineering hurdles. Additional complexity arises with respect to calibration and setup at these high frequencies, where losses in the setup become more pronounced as compared to 4G frequencies. Conservative estimates suggest capital equipment for this testing can require investments of more than $1 million. Choosing a testing partner who understands the specifications and procedures thus becomes critical.
 

About Digi Wireless Design Services

The Digi Wireless Design Services team provides product development engineering services that help you create the right solution for your 5G plans. We have the experience, equipment, infrastructure, and testing tools to help you design the right 5G antennas for your needs. For any inquiry, visit the Digi WDS antenna design page or Contact us.
 
1 3GPP specification series: 38 series
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