When the wireless industry embarked on the creation of 5G, 2020 used to seem so far away. Now we are quickly closing in on 2020 and this will most certainly be the 5G decade. Every day there are announcements in the press of new field trials and upcoming commercial 5G rollouts. It’s a very exciting time for the wireless industry. Currently, much of the industry 5G focus is on enhanced mobile broad band, driving toward higher and higher network capacity and throughput utilizing beamforming techniques in mid-band and high-band spectrum. We are also beginning to see use cases emerge such as industrial automation that leverage the low latency features of the 5G network architecture.

It was only a few years ago that the industry was debating the feasibility of using mmwave spectrum for mobile communications and framing the challenges that lay ahead for the radio designer [1]. Much has transpired in short time and the industry has progressed rapidly from initial prototypes to successful field trials and now we on the verge of the first commercial 5G mmwave deployments. Many of the initial deployments will be for fixed or nomadic wireless applications, but we will also see truly mobile connectivity at mmwave frequencies in the not so distant future. The first standards are in place, technology is quickly evolving, and much learning has transpired around the deployment of mmwave systems. While we have made much progress, there remain many challenges ahead for the radio designer. Let’s examine some of these challenges for the RF designer in the remainder of this article.

This article is organized into 3 major topics. In the first section, we will discuss some of the leading use cases for mmwave communications and set the stage for the analysis that follows. In the second and third sections we will delve into the architectures and technology for the mmwave basestation systems. In section 2 we will discuss technology for the beamformers and how the required transmit power influences the choice of technology for the system front-end. While the beamformer gets much of the spotlight in the press, there is equally important section of the radio that performs the conversion from bits to mmwave frequency. We will present an example signal chain for this section of the system and suggest some leading-edge components from Analog Devices for radio designer’s consideration.

Deployment Scenarios and Propagation Considerations

When we are developing technology, it’s critical to understand how the technology will ultimately be deployed. In all engineering exercises, there are tradeoffs to be made, and with additional insight creative innovations may emerge. In Figure 1 we highlight two of the common scenarios being explored today in the 28 and 39 GHz spectrum.

Figure 1. 5G mmwave deployment scenarios: (a) fixed wireless access in suburban environment; (b) fixed and mobile deployment in dense urban environment.

Now let’s consider a practical example and derive a simple link budget to illustrate the transmit power requirements for a mmwave basestation as compiled in Table 1. The additional path loss compared to that at cellular frequencies is a major hurdle to overcome at mmwave frequencies but also blockage (buildings, foliage, people, etc.) is a major factor to consider. There is a plethora of work reported in recent years on propagation at mmwave frequencies, with an excellent overview compiled by Rappaport et all in [2]. Several models are discussed and compared, illustrating the dependence of path loss on the environment, and a comparison of line-of-sight (LOS) scenarios versus non-line-of-sight (NLOS). Without going into detail here, we can say that in general an NLOS scenario should be considered for fixed wireless deployment given the desired reach and terrain being considered. In our example, we consider a basestation with 200 m reach in a suburban deployment. Here we have assumed a path loss of 135 dB based on an NLOS outdoor to outdoor link. If we were trying to penetrate from outdoors to indoors then the path loss may be as much as 30 dB higher. Conversely if we assume a LOS model then path loss may be on the order of 110 dB.

In this case we assume 256 elements in the basestation and 64 elements in the CPE. In both cases the output power can be satisfied with silicon implementation. The link is assumed to be asymmetric, which provides some relief in uplink budget. In this case the average link quality should allow for 64 QAM operation in the downlink and 16 QAM in the uplink. The uplink can be improved by increasing the transmit power of the CPE if required, up to the legislated regional limits. If one were to stretch the link reach to 500 m, the path loss will increase to approximately 150 dB. This is doable but makes the radios on both uplink and downlink more complex and power consumption increases dramatically.

mmwave Beamforming

Now let’s consider the various approaches to beamforming; analog, digital and hybrid as shown in Figure 2. I’m sure we are all familiar with the concept of analog beamforming as this topic has been quite popular in the literature in recent years. Here we have data converters translating the digital signals to and from broadband baseband or IF signals, connecting a radio transceiver performing the up and down conversion processes. At RF (example 28 GHz), we split the RF single into several paths where we perform the beamforming, by controlling the phase of each path so that a beam is formed in the far field in the direction of the intended user. This enables a single beam to be steered per data path, so in theory we can serve one user at time with this architecture.

Figure 2. (a) Analog beamforming; (b) Digital beamforming; (c) Hybrid Digital-Analog Beamforming.

The digital beamformer is exactly what it sounds like. The phase shift is purely implemented in the digital circuitry and then fed to the antenna array through an array of transceivers. Simply speaking, each radio transceiver is connected to a single antenna element, but in practice there could be several antenna elements per radio depending on the desired sector shape. The digital approach enables highest capacity and flexibility and enables the roadmap to multi-user MIMO at mmwave frequency, similar to mid-band systems. It is highly complex and given currently available technology will consume an excessive amount of DC power both in RF and digital circuits, however, as technology evolves in the future, digital beamforming will emerge for the mmwave radios.

The most practical and effective beamforming approach in the near term is the hybrid digital-analog beam former which essentially combines digital pre-coding and analog beamforming to create several beams simultaneously in space (spatial multiplexing). By directing power toward the intended users with narrow beams, the basestation can re-use the same spectrum to simultaneously serve more than user in a given timeslot. While there are a few different approaches to the hybrid beamformer reported in the literature, the sub-array approach shown here is most practically implemented and is essentially a step and repeat of analog beamformers. Currently reported systems support from 2 up to 8 digital streams in practice, which can be utilized to simultaneously support individual users, or alternatively provide 2 or more layers of MIMO to a lesser number of users.

Let’s dive a little deeper into the technology choices for the analog beamformer, the building block upon which the hybrid beamformer is constructed as shown in Figure 3. For the treatment here, we have segmented the analog beamforming system into 3 blocks, digital, bits-to-mmwave and beamformer. This is not the way a practical system will be partitioned as one would place all mmwave components in close proximity to mitigate losses, but this division becomes obvious shortly.

The beamformer function is driven by many factors including segment shape and reach, power levels, path loss, thermal constraints etc., and is the section of the mmwave systems that requires some flexibility as the industry learns and matures. Even then, there will continue to be a variety of transmit power levels required to address deployment scenarios ranging from small cell to macro. On the other hand, the bits to mmwave radio for a basestation requires much less flexibility and can largely be derived from the current Release 15 specification [3]. In other words, a designer can re-use the same radio in conjunction with several beamformer configurations. This is not unlike current cellular radio systems where the small signal section is common across platforms and the front-end is more tailored per use case.

We have mapped the progression of possible technologies for the signal chain as we move from digital toward the antenna. Certainly, the digital and mixed signal are produced in fine line bulk CMOS processes. Depending on the requirements of the basestation the entire signal chain may be developed in CMOS or more likely in a mix of technologies to provide the optimum performance for the chain. For example, a common configuration is to use CMOS data converters with a high performance SiGe BiCMOS IF-to-mmwave conversion. The beamformer may be implemented in several technologies as shown, depending on the system requirements as we will discuss next. Depending on the choice of antenna size and transmit power requirements it may be possible to implement a highly integrated silicon approach, or it may be a combination of silicon beamformer with discrete PA and LNA.