Standards for the first and second phases of 5G are moving towards completion, and early deployments can piggy-back on 4G LTE infrastructure. But there's plenty of work to do before the technical advances in 5G's radio spec can support new use cases.
Although 5G is being heavily marketed as a new technology, it's neither particularly new nor a single technology. If mobile technology were a long-running TV series, 5G is a mid-season reboot, with new characters introduced alongside the old, new plot arcs complementing existing storylines, and a publicity drive that rather overstates the case. However, the possibilities for future development are much enhanced.
There have been three major new generations of mobile technology: 2G replaced analogue with digital; 3G began the switch to data-centric networking; and 4G completed that move. 5G has three main focuses -- mobile networking, IoT, and very high-performance industrial control -- of which mobile networking will be the most important for most people over the next few years, and which is best thought of as a continuation of 4G's Long Term Evolution (LTE) under a new flag. Indeed, this stage of 5G is known as NSA (Non Stand Alone) as it will run alongside and interoperate with existing LTE networks. SA (Stand Alone) comes later.
Which is not to say that there aren't significant innovations in 5G. While the 5G standardisation process covers core network and base station topology as well as other aspects of running high-performance networks, most of the factors that will affect our first experiences of 5G are affected by the subset of standards called New Radio, or 5G NR. Although work on NR was only started in the spring of 2016, it quickly rolled up the until-then very disparate research area and has already produced a number of nearly-there pre-standard references (see boxout below).
15, that difficult stage…
5G NR is developed by a group called 3GPP, the 3G Partnership Project, and the first version of the standard is called Release 15. 3GPP is so called because it was first formed to standardise 3G; it has considerable authority as an international group that brings together standards committees, regulators and industry bodies, and the legal issues over renaming it were too onerous when 4G came along. Release 15 is the 18th major standard, which fact is an excellent indicator of how organisations at this level actually work.
Release 15 has been produced at some speed. Starting in early 2016, a preliminary release in March 2018 was declared complete enough for manufacturers to start preliminary production, By the third quarter of 2018, both Ericsson and Huawei said they'd deployed more than 10,000 base stations on that release. A further standard update appeared in September, with a 'feature freeze' final pre-standard version of Release 15 promised for December. However, chips developed by Qualcomm to the September release were reported by industry site Light Reading to have proved incompatible with the March-release-based base stations, potentially requiring a hardware swap.
As a result the December freeze has been postponed to March 2019 with knock-on delays for Release 16, which is expected to bring the low-latency and high-speed aspects of 5G to prominence. The difficulties, according to 3GPP, were caused by a lack of communication between the technical subgroups working on the Radio Access Network side, those defining the overall system configuration, and those in charge of the core network configuration. Citing overwhelming workloads, the 3GPP said that there had been no time for a coordination meeting of all the subgroups prior to the September release.
The industry is sympathetic, with players like Samsung saying that they're not changing their roll-out plans. Samsung is expected to show a 28GHz-enabled 5G handset at Mobile World Congress in February 2019.
5G NR includes major advances over LTE, each with specific benefits.
Most importantly, there's masses of new airspace. 5G NR includes millimetre-wave (mmWave) spectrum (>24GHz) for the first time, with the first release of 5G including frequencies from below 1GHz up to 52.6GHz. The high-frequency spectrum (> 6GHz) comes in many different bands that vary by region, as well as many that are not yet fully available due to existing services that must be closed or moved.
The high-band allocations can support very high data rates and intensive frequency reuse, providing very dense, high-performance networking. They have very limited range for a given transmission power compared to lower bands and more stringent health and safety limits, and they are more susceptible to environmental issues like heavy rainfall and seasonal leaf growth. Conversely, the very small wavelength makes it much easier to build very high-performance antennas of small physical size.
The high bands will be used to overlay existing LTE networks, providing much higher bandwidth on demand to reduce LTE (and eventually, 5G) mid- and low-band congestion, as well as fibre-speed home and office fixed wireless access (FWA) broadband. The 28GHz bands have seen the most attention, with the UK breakdown by region and operator being typical of how a territory already well-serviced with LTE will allocate resources:
Ultra-lean design is a key 5G NR design principle, reducing energy consumption and interference. LTE relies on a number of always-on signals transmitted by base stations -- beacons that show which cells are available, reference channels that terminals and base stations use to configure data links, command channels for tracking mobility and so on. In LTE, these signals don't take up a significant percentage of the overall channel usage, but 5G will have a much denser network with more cells, which will on average have quite a low actual usage rate. The always-on signals will thus take a greater percentage of power, and will interfere more with adjacent cells, leading to lower throughput.
Wherever possible 5G reduces or switches off such signals until they're actually needed. The reference signal, for example, is only transmitted once data transfer is under way. This means the handset and base station have to optimise the signal on the fly, but the overall benefit to throughput for the network is notable.
Ultra-lean design is also a key component of forward compatibility, a specific requirement in 5G NR for curiously unspecific ends. The basic rule is to leave as much room as possible in implementations to allow future developments. In practice, this means minimising non-data carrying transmissions (reducing overall interference and spectrum use), having a high degree of frequency and time-domain flexibility in 5G designs, and providing paths for reconfiguration in the future both in the hardware and in the specification itself.
This latter decision came about through experience with LTE, which encodes a number of design decisions in the specification such as when and where error-correction happens: if a new service finds these decisions inefficient or even disabling, then there's nothing that can be done. A reconfigurable standard can improve on old decisions. Also, new basic technologies such as software-defined radio (SDR) have moved much radio engineering from hardware into software, meaning that changing operating characteristics in ways that once took a complete hardware revision can now be pushed out as a software update. 5G is the first generation to fully embrace this.
Modulation and framing
5G modulation and framing is also an increment from existing ideas, but a significant one. Like LTE (and recent wi-fi standards, and just about every modern digital wireless system), 5G NR uses ODFM as its underlying modulation scheme. ODFM (orthogonal frequency division multiplexing) combines multiple subchannels within a channel, and is known to be both robust against interference and efficient in its use of frequencies. It's also highly flexible, as different numbers of subcarriers can be added to increase a channel capacity, or numbers reduced to provide much lower-power, lower-bandwidth options.
5G NR can choose subcarrier spacing from 15kHz to 240kHz, with a maximum 3300 subcarriers in simultaneous use on one channel. However, channels can be no more than 400MHz wide. The standard is frequency agnostic, meaning any subcarrier configuration can be used on any band. In practice, the mid- and low-band frequencies below 6GHz have markedly different channel and noise characteristics, as well as different maximum bandwidths, to the high-band allocations, so will use 15 to 60kHz channel spacing, while high-band will use 60 to 120kHz. There are currently no 5G band allocations between 6GHz and 24.25GHz, but the standard allows for optimal ODFM configuration to match any future expansion into this spectrum.
Not all devices on 5G NR have to support all bandwidths, which is a change from LTE. Furthermore, 5G NR supports adaptive bandwidth, letting devices move to a low-bandwidth, low-power configuration when appropriate, and gearing up to higher bandwidths only when necessary. This creates the opportunity for very low average power devices that can still deliver high performance -- IoT networks, for example, which normally only need small amounts of data for telemetry, but nevertheless need to be able to update their firmware for security and feature patches. The 5G NR specification refers to these different configurations as 'bandwidth parts', and in theory a device can support multiple bandwidth parts simultaneously on the same channel, although the first 5G NR release limits devices to one bandwidth part at a time.
Within a subchannel, data is divided up into frames of ten milliseconds each, further subdivided into ten 1ms subframes. Those subframes are themselves divided into slots of 14 OFDM symbols apiece. Thus, wider bandwidth subchannels have more OFDM symbols per second and each slot thus gets shorter, but the basic frame structure stays the same. At the lowest subcarrier spacing, 15kHz, the frames are identical to LTE, simplifying compatibility.
LTE and similar systems allocate bandwidth to different devices by slot, but 5G NR has a mechanism for a transmission to start within a slot, effectively creating what are called 'mini-slots'. This is especially useful for the high bands, which can have very large OFDM symbols and thus the ability to use just a few to carry a relatively short message improves both channel reuse and latency. Another potential advantage is if, or when, 5G expands to unlicensed spectrum, which normally comes with a 'listen before use' rule to prevent interference. If a channel appears quiet, the ability to start a transmission without having to wait for a slot boundary reduces the chance of another device grabbing the channel.
Other low-latency adaptations in 5G NR are tight requirements for data transmissions to start after a channel is granted, and restrictions on processing delay for data streams. This is achieved in the higher network layers by changing header structures so that processing can begin without the full packet information being known, and at the physical layer by having the radio receive essential information from reference and downlink control signals instead of deriving it from the symbol stream.
5G NR has a much more advanced concept of beamforming than LTE. Beamforming is the manipulation of the signals fed to and received from complex antennas to create beams in space that focus power in a particular direction. LTE could do this for data; 5G NR extends this to control channels too, while increasing the precision and adaptability overall for operation under different conditions. At the high bands, beamforming will mostly be used to increase range by energy focus, while at the mid and low bands below 6GHz, where attenuation is less of a problem, beamforming will be a key part of MIMO, the multiple-in multiple-out spatial channel technique that increases bandwidth for multiple devices in the same area. Although not part of the first release, 5G NR will support distributed-MIMO, where a user can receive different parts of the same data stream from multiple sites.
This touches on the other major areas of 5G beyond the radio: how base stations communicate with each other and with the core network, how the operators manage the whole system for reliability and profit, and what shapes the new network uses built on the back of these technologies will take. Don't expect the full picture to become clear for three to five years: 5G in 2019 will be as much about groundwork as immediate results.
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