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5G/Massive MIMO Channel

Toward 5G: What’s Changing and How to Address Design and Test Challenges?

May 14, 2014
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Even as LTE and LTE Advanced (4G cellular systems) are being deployed, work is already starting on their successor, 5G. This article describes the needs that demand continued development of mobile communications systems, and explains some of what is currently happening to bring 5G from theory to reality.

According to ABI Research, more than 30 billion devices will be wirelessly connected to the Internet of Things (now becoming the Internet of Everything) by 2020, and the most active area of research is in connecting these devices to the internet via low power radio. So, the question that must be answered is, “How do tomorrow’s communications networks support a change to our lives that is bigger than the arrival of the Internet itself?”

What’s Changing

The need for high speed connectivity is the common denominator as we look ahead to next-generation networks. Achieving 24/7 access to high bandwidth services requires that the industry continues on its current growth path; going far beyond simple voice and data services and moving to a world with billions of Internet-connected devices, ranging from simple sensors to high definition streaming video terminals, with “everything everywhere and always connected.”

For individual subscribers, the main wireless delivery mechanisms have become public and private Wi-Fi hotspots routed from fixed-line networks, and third and fourth generation cellular networks (aka “mobile broadband”). In cellular networks, successive advances in mobile network technology and system specifications have provided higher cell capacity and consequent improvements in single-user data rate. Along with the latest mobile network specifications, there is a concurrent move to the Evolved Packet Core (EPC) – the simplified all-packet network architecture designed specifically to improve data throughput and latency, and to better match the air interface part of the mobile network to the architecture of the network’s backhaul and of fixed-line networks.

These improvements have produced new services and a new set of end-user expectations, witnessed by the rise of more complex and data-hungry applications for smartphones and tablets. The latest of these is “seamless connectivity” – the ability to move an application amongst devices: for instance, tablet to smartphone to home entertainment center – without interruption of the content. To provide this capability requires access to, and control of, the content over multiple networks: WiFi hotspot, cellular and landline. (It’s not just a technical challenge – associated billing requires a plethora of roaming agreements as well.)

The vision for 2020 that is presented in the studies for fifth generation mobile networks “5G” is one of “everything everywhere and always connected.” It assumes devices can operate on frequencies from a few hundred megahertz to (in some cases) 80 GHz. Indoor cell sizes may be as small as a single room. Since wireless spectrum is limited, it employs pico- and femto-cells to maximize frequency reuse at RF. ITU’s definition of 4G has an expectation of 1 Gbps single-user data rate. The goal for 5G is not necessarily to increase this, but to have a high capacity network capable of delivering this rate to a much bigger user community; in other words, to provide higher aggregate capacity for more simultaneous users. None of the studies have yielded specific details of the core network that joins everything together, but they assume that seamless connectivity, mentioned earlier, will be a given.

To support vastly increased numbers of devices and performance requirements, the latest studies postulate the key network attributes that will be required: an integrated wireline/wireless network, where the wireless part comprises a dense network of small cells with capacity enhanced through high-order spatial multiplexing (MIMO), cell data rates of the order of 10 GB/s and round-trip latency of 1 ms. Most studies now assume multiple air interfaces, which will include operation at microwave or millimeter wave frequencies. With these attributes, the combined network will support everything from simple M2M devices to immersive virtual reality streaming, with monitoring and control of literally billions of sensors and multiple simultaneous streaming services, and will support the massive data collection and distribution needs of the “Internet of Things.” With the massive infrastructure costs involved, it’s difficult to see individual operators affording the investment separately; shared, jointly-managed resources have been predicted as being much more likely.

In the mobile world, capacity gains come essentially from three areas: more spectrum, better efficiency and better frequency reuse through progressively smaller cell size. The fourth generation networks currently being built use more frequency bands than previous generations, and can use both broader channel bandwidths and aggregated carriers (the combination of two or more channels at different frequencies to further increase capacity). The work on EPC does recognize, and seek to limit, the packet delivery overhead in wireless networks, since signaling absorbs (finite) network capacity. However, with mobile data consumption currently forecast to almost double year-on-year for the next five years, network operators maintain that they will struggle to meet long-term demand without even more spectrum. Freeing up frequency bands currently used for other systems will become a major priority.

The official process of 5G standardization should be launched in the 2015-2016 time frame. The International Telecommunication Union (ITU) holds an international conference every three to four years, known as the World Radiocommunication Conference (WRC), to agree on international radio frequency issues, including frequency allocation standards for mobile networks. The next WRC is scheduled to be held in Geneva in 2015 (ITU-R WRC-15). The 5G standard is expected to be one of the topics of discussion for international delegates.

Design and Test Challenges

Compared to previous generations of mobile network, 5G presents a number of new design and test challenges. The first of these is the extension to higher frequencies. Experimental use of frequencies from 28 to 80 GHz and multiple-antenna configurations for either enhanced capacity (spatial multiplexing or MIMO) or signal steering are featured in a number of studies. New techniques for self-organizing network topologies, software defined radios capable of multiple air interface standards and software defined networks based on cloud computing are already being proposed for future 4G standards releases, and these will be extended to 5G. The integration of wireline and wireless networks may provide both anti-trust and regulatory issues beyond the obvious technical challenges.

Component and system design and test at microwave and millimeter wave frequencies is a well-understood and established science, but its application to high volume, low cost devices for the consumer market is new. Two major differences are the much wider modulation bandwidth than other wireless communications systems and the physical construction of the devices which may not allow direct RF connection. There’s already an unlicensed frequency band being used at 60 GHz for the wireless LAN standard 802.11ad, which features a 2 GHz channel bandwidth. Similar use may be made of licensed spectrum in the 28 GHz range, where Samsung and others have already reported experimental results, and in the 70 GHz range, where a number of university studies are underway. In “real” user devices, these frequencies would likely use antenna components bonded directly to the transmitter and receiver chips, making connection to test equipment difficult. Base station radios will typically feature antenna matrices for beam steering (directing RF attributes toward a specific device), and/or multiple transmit/receive streams (MIMO) for capacity enhancement. Testing user devices will mean duplicating these real-world conditions, and test equipment suppliers will need to provide new channel measurements and simulation models for initial development, and complex baseband and microwave sources for performance verification. Figure 1 shows a system designed for testing 802.11ad components at 60 GHz, and gives an idea of what might be needed for millimeter wave 5G design and development.

Figure 1

Figure 1 WLAN 802.11ad test system working at 60 GHz.

Potential new physical layer technology (PHY) attributes are still to be decided, but it’s likely that any 5G devices will need to operate in a number of different radio access networks (RAN). The new PHY will include new, more efficient modulation and coding schemes that are efficient at very high symbol and bit rates (e.g., the use of Golay sequences in 802.11ad). Challenges here include everything from battery power consumption (meeting user expectations) to demodulating and decoding data from multiple carriers using different PHY characteristics simultaneously.

Today, much of the research into next-generation systems is being carried out in universities, either on their own or with the help and direction of commercial partners; or in the R&D departments of network operators, chip manufacturers and network equipment manufacturers. While the standardization process for next-generation cellular systems has not yet officially started, it’s likely that some of the work currently underway will be incorporated into the 5G systems we’ll begin to see rolling out around 2020.

Vector network analyzers enable in-depth design and test of millimeter wave components such as the antenna array elements needed for beam-steering and MIMO. Software can enable a system-level design automation environment that accelerates design and verification of communications systems at the physical layer, where advanced digital signal processing meets RF. It combines with measurement products to create an expandable environment for modeling, implementing and validating next-generation communications systems. It enables a virtual system to be verified from the first day of a project, beginning with simulation models, and gradually incorporating more measurements as the design is translated into working hardware. It can be used in conjunction with signal sources to create complex arbitrary waveforms to test theoretical channel models in the real world. Systems can give a comprehensive set of tools that work with a range of signal analysis products for demodulation and vector signal analysis. Together these measurement, simulation and signal generation and analysis tools enable the exploration of virtually every facet of the components and signals that will become part of the advanced designs needed for next-generation communications systems.

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