5 years to 5G: Enabling rapid 5G system development
24 June 2015
The goals for the 5G Radio Access Network (RAN) are lofty indeed and have been discussed at length by industry experts. What has received far less airtime is, “What exactly is the best path to 5G?”
5G RAN has many challenges but also a range of ideas which can be implemented in hardware, both for prototyping, which needs to happen over the next 3 years, and ultimately for production deployment, which is to commence in 2020.
The goal of 5G is to provide a 1000x increase in capacity, supporting 100+ billion connections with data rates to 10Gbps and less than 1msec latency. However, these new networks will not just support the fastest links and fattest data pipes; they also aim to improve upon the capabilities of current networks.
Developing 5G networks that meet these goals will require a combination of existing systems such as LTE-Advanced and WiFi, combined with revolutionary technologies designed to support new uses such as IoT, augmented reality, immersive gaming, and UHD (ultra-high-definition) streaming video.
5G standards are far from final, but many of the elements that will form part of the 5G architecture are already crystallising. These new networks will employ many new frequency bands to augment existing wireless frequencies, enabling delivery of high capacity. Such tremendous connectivity and capacity will demand many more basestation nodes. These will be small cells and high-capacity cloud or virtual RAN architectures serving many hundreds of radio heads. Content caching and processing at the edge will all have a place in the new 5G networks, enabling high speed streaming video and reality augmentation for coverage of high-demand events.
Major innovation is also needed at the lowest levels to accommodate broad requirements for both video and augmented reality. The needs of M2M networks will drive innovation in the physical layer, air interface definition, and control plane structures.
New frequency bands
5G will see some of the spectrum below 6GHz being re-purposed for use with newer technologies, particularly for non-line-of-sight (NLOS) requirements. Existing cellular bands will be augmented with new spectrum allocations above 6GHz, able to supply much wider contiguous spectrum. Additionally, carrier aggregation techniques will be used to combine chunks of spectrum that are not co-located within the same band to further improve peak data rates. The core bands will provide up to 100MHz of instantaneous bandwidth and the new extended bands will provide contiguous chunks of spectrum with as much as 500Mhz in bandwidth.
Release 12 of the 3GPP standard, stated for freeze in 2015, provides for early massive MIMO systems. These systems take active antennas to a new level. Large arrays of radiating elements require horizontal and vertical beam forming to significantly increase capacity and coverage. Massive MIMO in turn requires significantly more processing power.
Advanced physical layer
Current 4G OFDM air interfaces deliver high-speed data with limited support for low-power M2M communications. As a result, air interface technology and the 5G physical layer will be augmented using new bands of spectrum. Many new candidate air interfaces are being considered to provide support for sub-1msec latency with 10Gbps throughput. Other interfaces that can cater to the needs of simple sensor data transmission will not require such low latency or high data throughput so it’s likely that 5G will not employ a single air interface technology.
In addition, the physical layer will require new coding and modulation schemes, protocols, and framing structures brought about by disparate end-user requirements. The 5G infrastructure must automatically determine the type of channel needed and adapt based on conditions such as precipitation or moving objects affecting line of sight. Cognitive radio techniques and advanced adaptive coding and modulation schemes will allow equipment to provide the best possible connections.
New architectures including Cloud RAN and Virtual RAN take a more centralised approach for greater CapEx and OpEx savings. Centralising baseband processing and backhaul functions to serve many hundreds or thousands of remote radios enable the use of GPU-centric server farms with localised data-centre processing at the edge. This change places challenges on the fronthauling aspects of the networks, where the data from many hundreds of radios must be transmitted to data centres over various media. 5G infrastructure will also push in other directions including core virtualisation components such as Software Defined Networks (SDN) and Network Functions Virtualisation (NFV); resulting in a more software-centric, server-based architecture that allows use of commodity servers and distributed processing.
Implementing massive MIMO
The benefits of massive MIMO are undisputed but the cost is enormous due to the computational burden involving large matrices and linear algebra for beam forming calculations for each antenna. As a result, massive MIMO will hugely increase both connectivity and signal-processing requirements. High-speed connectivity is required between the digital front end (DFE) processing and the analogue domain and between the baseband processing and the radio processing, which require some form of serial transceiver. DSP for DFE and beamforming algorithms demands wide bandwidths and high sample rates, necessitating agile, high-performance signal-processing.
Massive MIMO antenna algorithms can be realised with current technology, as shown in Figure 2, but as massive MIMO systems scale to larger arrays of antenna elements, greater levels of integration will be required and made possible by future device generations.
FPGAs and SoCs enable massive connectivity and capacity
The capacity and latency goals that 5G demands will have a knock-on effect to the requirements of the infrastructure equipment. 5G systems must support massive connectivity and capacity that can only be served through the use of high-throughput communications including 10Ge, 40Ge, PCIe, and future evolutions of CPRI. Capacity increases will come from new modem architectures, advanced radio technology, and new modulation schemes.
FPGAs have long been used in wireless infrastructure equipment due to their high performance, which permits rapid implementation of complex signal-processing algorithms. The latest Xilinx 20nm UltraScale FPGA devices can support over 8 TMACS and more than 5Tbps of serial transceiver bandwidth. Xilinx All Programmable SoC devices couple a high-performance FPGA fabric with a fully integrated processing subsystem based on dual ARM Cortex A9 MPCore processor’s, which can be used to efficiently implement higher layers of the complex 5G protocol stacks.
High end fronthauling
Fronthauling is an evolving market driven heavily by the centralisation of baseband processing, which in turn drives the need for IQ data fronthauling by wireless, copper, or fibre media. Current connectivity standards exist in the form of CPRI and OBSAI. Figure 3 shows a state-of-the-art CPRI aggregator implemented in FPGA. 5G is likely to have a different implementation for some processing elements.
Advanced physical layer
Development of the physical layer for 5G is underway with many candidate technologies. Evaluating the relative merits of new candidate air-interface technologies needs is best done with FPGAs, which enable rapid implementation of required algorithms and interfaces. The inherent re-programmability of FPGAs permits rapid design changes to demonstrate improvements or to add features with very little schedule impact.
High-level synthesis tools ease development of advanced 5G algorithms. For example, Xilinx’s Vivado HLS enables algorithm developers and system architects to design in C/C++ and then synthesise to RTL as shown in Figure 4. Popular 3rd party tools including MATLAB and Simulink can also be used for front-end design.
With the advent of All Programamble SoCs, such as the Xilinx Zynq SoC family, ARM processors are readily available for implementing scheduling and other higher-level protocols.
5G prototyping platforms
5G standards do not yet exist but companies are still keen to start prototyping. Creating custom hardware and software to implement 5G functions is both time consuming and costly. BEEcube has developed a range of 5G prototyping hardware that takes the pain out of developing such systems.
BEEcube provides several platforms (see Figure 5) that can easily be tiled together to create huge amounts of DSP processing power and large amounts of optical connectivity for CPRI aggregator fronthauling designs. Each platform supports the VITA-57 FMC analogue cards to be fitted easily for direct RF sampling or for interfacing 1GHz of bandwidth to a 60GHz transceiver. BEEcube also delivers all the required 5G interfaces, enabling the system designer to focus on developing the algorithms without getting stuck on the interface standards.
5G production technology
FPGAs will be used for prototyping 5G wireless infrastructure over the next few years. However, when it comes to deployment technology, cases can be made for either ASICs or FPGAs.
The decision of whether to keep a design in an FPGA or migrate to an ASIC for production is a question of economics. With more serial transceivers, DSP slices, block RAMs, DLLs, PLLs, processor sub-systems, memory interfaces, PCIe interfaces, and other blocks, the FPGA’s hardware penalty for re-programmability continues to diminish. In parallel, the risk of severe ASIC design bugs increases exponentially as overall 5G system complexity increases.
We’re still five years away from commercial deployment but many companies need to prototype these emerging algorithms and applications now as standards begin to firm up.
Xilinx FPGAs and Zynq SoC devices coupled with commercially available 5G prototyping platforms can save significant development time versus the development of custom prototyping platforms. These tools allow system architects and designers to find the best architectures and algorithms, rather than spend their time architecting the platform on which to prototype.
As we look to 2020 for widespread 5G deployment, it is likely that most OEMs will sell production equipment based on FPGAs and All Programmable SoCs. The hardware complexity of 5G’s physical layer is too challenging to guarantee that ASIC implementations will be free of severe hardware bugs. Keeping the hardware soft will be the wise path chosen by the smartest OEMs.
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