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G.fast: Moving Copper Access into the Gigabit Era

The History of Copper Access Technologies

Traditional copper networks, which primarily carry voice services, provided data services using dial-up modems and ISDNs in early days. With their limited access rates, traditional copper networks struggled to keep up with the rapidly growing demand for bandwidth driven by data services.

ADSL access technology was the first technology to bring people into the broadband access era, with downstream access rates of up to 8 Mbit/s.

Later on, ADSL2+ technology extended the signal frequency band from 1.1 MHz to 2.2 MHz and improved the maximum downstream rate to 24 Mbit/s. It replaced ADSL technology and was adopted throughout the industry.

Later on, VDSL technology was able to improve on both upstream and downstream rates and make symmetric access possible, which overcame the disadvantage of ADSL2+'s asymmetric access.

However, there arose a dispute between VDSL's two modulation techniques, QAM and DMT, and VDSL was incompatible with ADSL2+. In response, VDSL evolved to VDSL2, which was fully compatible with ADSL2+. VDSL2's access rates of up to 100 Mbit/s transitioned copper access technology into the "Fast Broadband" era. VDSL2 works on both the 17 MHz and 30 MHz frequency band, which can be further divided up into multiple upstream and downstream subchannels, effectively providing even higher bandwidth over short distances. As such, VDSL2 is ideal for short-distance applications such as FTTC (fiber to the cabinet) and FTTB (fiber to the building), whereas ADSL2+ is more suited for DSLAM equipment in central offices.

Although VDSL2 can ideally provide speeds up to 100 Mbit/s, it is challenging for VDSL2 to reach 100 Mbit/s access speeds due to crosstalk between lines. To address this issue, vectoring technology, which can eliminate crosstalk, was developed, raising copper access speeds to de facto rates of 100 Mbit/s. However, VDSL2 technology is a bottleneck to increasing transmission rates because vectoring technology is both a crosstalk cancelation technology and a VDSL2 technology. The maximum rate vectoring technology can reach is the maximum rate that a noiseless, single copper pair applying VDSL2 can reach.

To help enable copper access to reach 1000 Mbit/s rates, G.fast technology has emerged and will transition copper access into the gigabit era.

Key Technologies in G.fast

Back in 2010, Huawei began to research access technologies that enable ultra-high-speed transmission over short-distance twisted pairs to address issues associated with FTTH (fiber to the home) applications. For Huawei's customers in Europe such as BT (British Telecom) and FT (France Telecom), FTTH is expensive to implement in brown fields because of expensive labor, scattered population, problems with home fiber wiring, and slow roll-outs. As an FTTH alternative, European operators chose to lay out fiber to the distribution point (FTTDp) and reuse existing access media, such as telephone lines and coaxial cables, to provide ultra-high-speed broadband access. Because the last-part transmission was over such a short distance, the operators anticipated they could achieve gigabit speeds using existing access media.

To address the last-part transmission issue, G.fast technology was developed. In order to work successfully, G.fast must be capable of ultra-high speeds, which entails extending the frequency spectrum. A wider frequency band results in a higher access speed. VDSL2 currently works on 17 MHz or 30 MHz, while G.fast will work on 106 MHz or even 212 MHz. Of course, the frequency spectrum cannot be extended infinitely. Like spectrum resources in the wireless communication sector, spectrum resources in the fixed communication sector must be properly planned, to prevent conflicts with spectra already in use and to reserve space for future technologies. For example, the Office of Communications (Ofcom) in the UK has defined a strict ANFP (access network frequency plan) for spectrum application. The ADSL2+ spectrum is allowed only in exchanges, and the VDSL2 spectrum can be deployed only at FTTC street cabinet sites. The G.fast spectrum may need to avoid the frequency bands that are already in use.

G.fast technology will use the 106 MHz frequency band in the initial stage and 212 MHz in the future. The wider the frequency band, the higher bandwidth G.fast can achieve. However, higher frequencies also mean shorter transmission distances, higher costs, and greater power consumption. The frequency band that is ultimately used is a compromise between performance, costs, and implementation.

Similar to VDSL2, G.fast performance is affected by crosstalk between lines. Without the vectoring noise cancelation process, G.fast rates are severely degraded. Figure 1 illustrates simulation results of G.fast rates over 100-meter lines. Some lines are capable of up to 1.3 Gbit/s. If crosstalk is present and vectoring processing is absent, the G.fast rate drops sharply to about 200 Mbit/s. This occurs because G.fast operates at a very high frequency and the impact of crosstalk on G.fast is much more severe than on VDSL2. Therefore, G.fast must use a more advanced vectoring technology to cancel crosstalk between lines.

Figure 1 G.fast simulation results over 100-meter lines

Two vectoring options are currently available for G.fast: the improved linear precoding algorithm and the non-linear precoding algorithm. The latter obtains more gain than the former on high frequencies but obtains almost the same gain as the former on low frequencies. We can see in Figure 1, that on the majority of lines, the rate achieved using the non-linear algorithm is higher than that achieved using the linear algorithm. However, due to its complexity, the non-linear algorithm requires a more powerful processor than the linear algorithm, which results in implementation difficulties, high power consumption, and more costs. As such, the first release of the G.fast standard uses the improved linear precoding algorithm.

G.fast technology uses the same DMT modulation technology as VDSL2. To address scenarios in which the network is upgraded from VDSL2 to G.fast, but terminals are not, the standard requires that G.fast be backward compatible with VDSL2 CPEs. Operators generally prefer to upgrade their equipment first and then allow end users to use VDSL2 CPEs until they upgrade to G.fast terminals.

Unlike VDSL2, G.fast technology does not use FDD (frequency division duplex). Instead, G.fast uses TDD (time division duplex). In FDD mode, different frequency bands are used for upstream and downstream transmission, whereas in TDD mode, different timeslots are used for upstream and downstream transmission. TDD facilitates hardware implementation and flexible downstream/upstream ratio definition (to establish symmetric access). Reports of G.fast bandwidth exceeding 1 Gbit/s generally refer to the sum of the upstream and downstream bandwidths.

Running on a very high frequency band, G.fast technology applies only to short-distance transmission. Therefore, G.fast devices reside close to end users and may encounter the power supply issue. To resolve this issue, G.fast devices rely on reverse power feeding. That is, end users' terminals supply power to G.fast devices through access media. In addition, G.fast makes use of a range of energy-saving technologies, effectively reducing G.fast's power consumption per line less than that of VDSL2 while helping facilitate the implementation of reverse power feeding.

Progress of G.fast

Huawei began development of G.fast in 2010. By the end of 2011, Huawei had debuted the first G.fast prototype in the industry, which reached access speeds of up to 1 Gbit/s over a single, 100-meter copper pair. The prototype was considered groundbreaking in the telecommunications industry. The G.fast group, founded by the ITU-T in 2011, has drawn participation from a large number of operators, chipset manufacturers, and equipment suppliers. Huawei has actively participated in the development of the G.fast standard and submitted a significant number of proposals contributing to key G.fast technologies, including TDD full-duplex mode, TDD basic frame structure, peer-to-peer architecture, forward error correction transmission mechanisms, power-saving modes, and online re-configuration, etc. Currently, core technical solutions are determined, and the implementation process and parameter definition are under study. The G.fast standard document will be ready to consent  by the end of 2013.

While contributing to the development of the G.fast standard, Huawei has invested in the promotion of G.fast commercialization, and has already started development of G.fast products. Based on its wide range of contributions and technical strengths, Huawei is well positioned for success in providing the industry with the first standard specification-compliant G.fast product, which can accommodate standard variations through software programmability. Currently, European operators show great interests in G.fast ultra-high-speed broadband access, and some have recently revealed plans for G.fast trials. As a leading equipment provider, Huawei will actively coordinate with these operators to ensure the success of these trials.

G.fast technology is capable of gigabit access speeds over existing copper lines. This has undoubtedly strengthened operators' confidence to continue investing in their existing copper infrastructure to build ultra-fast broadband networks. Copper infrastructure is a very important resource for fixed line operators. In addition, building ultra-fast broadband networks by reusing existing copper infrastructure with innovative technologies results in a relatively quick network roll-out and a rapid return on investment. Therefore, G.fast is an increasingly appealing option in more and more operators' eyes.

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