Copper Reborn

JAN 2013 . ISSUE 6821 22

Thanks to new technologies such as Vectoring

and G.fast, copper is being revitalized as an integral

part of ultra-broadband networking.

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JAN 2013 . ISSUE 68

Huawei Communicate

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Gigabit access over copperbsolescence looms large over DSL copper access, thanks to bandwidth-heavy applications such as HDTV, whi le compet ing media , such as

coaxial cable, are evolving to deliver higher speeds. Huawei meets this challenge with Giga DSL (a new-generation single twisted pair DSL access technology), for which the prototype arrived in December 2011. Delivering 1Gbps uplink & downlink speeds within 100 meters, this technology has the viability to keep copper resources fully utilized over the next decade.

Service and competition challenges

People are shifting their focus from mere web browsing to online video, with HD/3D video either in place or on the way; all are posing much greater demands on fixed bandwidth, for which 20 to 50Mbps has become basic. Access of 100Mbps or higher will soon become the mainstream, and yet ADSL2+ and VDSL2 can deliver peak throughputs of only 25 and 100Mbps, respectively (generally 50Mbps from VDSL2 when inter-line crosstalk is

accounted for). Multi-service operators (MSOs) are using

DOCSIS 3.0 technology to provide up to 200Mbps coaxial access to residential homes, far beyond the capabilities of legacy DSL. Although FTTH can provide over 100Mbps access, its large-scale deployment has been hindered by distribution costs and onsite obstacles.

Gigabit access over copper

Legacy copper utilization through new DSL technologies is now a very efficient route to high-speed broadband. Huawei released its industry-first Gigabit DSL prototype in December 2011. Tests with several major operators show that when using 0.5mm copper, this prototype delivers uplink/downlink speeds of 550Mbps within 200 meters and up to 1Gbps within 100 meters. Huawei Giga DSL delivers the following key features:

Expanded frequency range

Wider frequency ranges can deliver enhanced bandwidth over a given length of copper. VDSL2 uses a 30MHz frequency range, del iver ing

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Giga DSL: Gigabit access over copper22

FTTB/C sites: Where it all comes together

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COMBO: Broadband ability, narrowband reliability

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By Huang Lei

Giga DSL

Copper Reborn

JAN 2013 . ISSUE 6823 24

Editor: Michael huangzhuojian@huawei.com

100Mbps downlink speed, while Giga DSL expands the frequency range to 100MHz, and can be further extended to 200MHz, enabling far superior downlink.

TDD/OFDM duplexing & modulation

VDSL2 utilizes frequency division duplex (FDD) technology and orthogonal frequency-division multiplexing (OFDM) modulation, as well as different frequency bands for uplink and downlink, though there may be signal echo at the receivers, which becomes significant if the frequency band is wide, leading to poor system performance.

Compared with VDSL2, Giga DSL uses a higher frequency band, as well as time division duplexing (TDD) and OFDM, both of which allocate different timeslots for uplink and downlink data transmission, with reduced echo interference, simplified circuit design, and reduced complexity in digital-analog conversion. TDD and OFDM are also easier to implement as the transmitters and receivers use the same t ime/frequency conversion module; they can also help adjust the ratio of uplink and downlink timeslots and rates, facilitating smooth bandwidth allocation and management.

Fewer subcarriers, less complexity

With h i ghe r f r equency band s , OFDM subcarriers require increased intervals, reducing the number of subcarriers and ensuring a simple physical layer. VDSL2 adopts 4,096 subcarriers, with 4.3125KHz spacing for each, while Giga DSL reduces this number to 2,048, making for an

interval approximately twelve times larger than that for the former. As an added bonus, Giga DSL is compatible with legacy DSL technology, including ADSL2+/VDSL2, so operation can continue during the upgrade process.

Discontinuous modality

Giga DSL has relatively lower power consumption in terms of signal transmission and line drivers, but higher consumption in digital signal processors and digital-analog converters, thanks to the higher frequency bands employed. When the transmission rate is low, Giga DSL uses discontinuous modes and disables some t imes lots , requir ing no modulation on either the transmission or reception side, while digital signal processors, digital-analog converters, and line drivers can be disabled or kept in sleep mode to conserve power. Compared with ADSL2+/VDSL2, which require 100mW signal transmission power, Giga DSL requires a hundred times less.

Giga DSL benefits

Huawei Giga DSL delivers 1Gbps of data access over a single twisted pair, enabling operators to meet user demand, enhance profits, and better compete with the coax-enabled services offered by MSOs. Giga DSL also features reduced inter ference, low power consumption, and backwards compatibility with legacy technologies, ensuring smooth evolution and an extended product lifecycle.

However, Giga DSL deployment requires certain conditions. First, of course, is that twisted pairs must already be in place for home access. Second, the transmission distance should be less than 250 meters. And finally, when Giga DSL is used in a fiber to the distribution point (FTTDp) scenario, it should serve less than 50 subscribers, as costs need be kept in check that relate to the various small-capacity equipment involved, while keeping the setup ecofriendly and easy to maintain.

In the future, other technologies may coexist with Giga DSL, and their inter-line crosstalk, counteraction, and other relevant issues demand our continuous attention on the research front. Giga DSL also has higher requirements in terms of power supply and maintenance, and the relevant solutions are being developed.

Giga DSL: Gigabit access over copper

Figure 1 Giga DSL prototype & performance

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Total rate (DS+US) in Mbps

Giga DSL

VDSL2 30a (no crosstalk)

VDSL2 17a (no crosstalk)

Loop length: 0.5mm