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Noise cancellation techniques may boost data rates over copper

Using a dsp based technique called vectoring to cancel crosstalk, the performance of very high speed digital subscriber line 2 (VDSL2) technology can be pushed close to its theoretical limit.

Several broadband access equipment vendors have recently detailed vectoring solutions, including Adtran, Alcatel-Lucent and ZTE. Alcatel-Lucent has trialled its technology with more than 24 telecom operators and believes at least six will deploy it.

Vectoring is being welcomed by operators as it enables them to increase broadband rates using their existing copper infrastructure. Even if operators are transitioning to fibre based access networks, these require significant investment and years to roll out. Vectoring provides a parallel approach to achieve very high speed broadband.

Using vectoring, VDSL2 data rates can be boosted to some 100Mbit/s downstream and 40Mbit/s upstream over a 400m link. This compares to 50Mbit/s and 20Mbit/s, respectively, using VDSL2 alone. There is also a large uncertainty in the resulting bit rate for a given loop length but, with vectoring, this uncertainty is removed almost entirely.

Typically, a cable bundle can comprise several hundred telephone copper pairs and crosstalk means the signal in one copper pair leaks into neighbouring ones.

"All my neighbours get a little bit of the signal sent on my pair, and vice versa," said Paul Spruyt, xDSL technology strategist at Alcatel-Lucent. "The signal I receive is not only the useful signal, but also noise contributed from my active VDSL2 neighbours The resulting signal to noise ratio on each pair dictates the overall achievable data rate to the user and, on short loops, crosstalk is the main noise culprit."

To tackle noise, the crosstalk coupling into each VDSL2 line is measured and used to generate an anti noise signal at the digital subscriber line access multiplexer (dslam) equipment to null the crosstalk on each VDSL2 line (see fig 1).

To calculate the crosstalk coupling between the pairs in the cable bundle, use is made of the 'sync' symbol, sent after every 256 data symbols. This equates to a sync symbol being sent every 64ms.

Each sync symbol is modulated with one bit of a pilot sequence. The length of the pilot sequence is dependent on the number of VDSL2 lines in the vectoring group: in a system with 192 VDSL2 lines, 256bit long pilot sequences are used (the next highest power of two).

Moreover, each twisted pair is assigned a unique pilot sequence, with the pilots usually chosen such that they are mutually orthogonal. "If you take two orthogonal pilot sequences, multiply them bit wise and take the average, you always find zero," said Spruyt. "This characteristic speeds and simplifies crosstalk estimation."

The user's DSL modem expects to see the modulated sync symbol but, in reality, sees a modulated sync symbol distorted with crosstalk from the modulated sync symbols transmitted on the neighbouring lines.

The modem measures the error – the crosstalk – and sends it back to the dslam (see fig 2). The dslam correlates the received error values on the 'victim' line with the pilot sequences transmitted on all other 'disturber' lines. By doing this, the dslam gets a measure of the crosstalk coupling for every disturber-victim pair.

The final step is the generation of anti noise within the dslam. This anti noise is injected into the victim line on top of the transmit signal such that it cancels the crosstalk signal picked up over the telephone pair. This process is repeated for each line.
VDSL2 uses discrete multitone (DMT) modulation, where each DMT symbol consists of 4096 tones, split between the upstream and the downstream transmissions. All tones are processed independently in the frequency domain. The resulting frequency domain signal, including the anti noise, is converted back to the time domain using an inverse fast Fourier transform.

It is only recently that advances in silicon have enabled the processing performance needed for vectoring. A fully vectored 200 line VDSL2 system needs a processing ability of some 2.6TMAC/s; one with 400 lines would require four times as much processing power – about 10TMAC/s.

Typically, a VDSL2 line card has 48 ports and performing vectoring on one card – board level vectoring – is relatively straightforward. "It gets more challenging with system level vectoring [across boards and even platforms] and you need significantly more processing horsepower to get to 192 or even 384 ports," said Robert Conger, Adtran's director of product management (see fig 3).

Conger says that what is still missing is the general availability of mass market vectoring asics from merchant VDSL2 chip vendors: companies such as Broadcom, Ikanos Communications and Lantic. Certain chip vendors allow system vendors to use their own vectoring processor while they are also developing their own silicon, said Conger.

System vendors that have already demonstrated vectoring are either using fpgas or their own in house asic. "FPGAs are great for field evaluations, but the difference between fpgas and asics is that the the fpgas consume a lot more power and are not as cost effective," said Conger. "Operators are very conscious of power – if they can wait for the power efficient asic, they prefer that." Most VDSL2 chipset vendors will come to market with their own silicon next year.

Alcatel-Lucent implements its own vector processor, but will not detail its hardware design. What the company has said is that its first generation vectoring system, released in 2011, could process 192 lines. At the Broadband World Forum in October 2012, Alcatel-Lucent unveiled a second-generation system that doubles the capacity to 384 lines.

Adtran can demonstrate system level vectoring now using fpgas, but there will be a further iteration once the asics become available in 2013.

There are other hardware challenges: there is a large amount of data to be transferred within the dslam associated with vectoring. According to Alcatel-Lucent, a 48port VDSL2 card can generate up to 20Gbit/s of vectoring data.
A further practical challenge that operators face when upgrading to vectoring is that not all the users' VDSL2 modems deployed may support vectoring.

"To get maximum benefit, you need to remove all the noise," says Stefaan Vanhastel, marketing director for wireline fixed access at Alcatel-Lucent. "That means you have to be able to measure and cancel the crosstalk from each line in the bundle, even the ones that are on VDSL2."
To tackle this, certain legacy VDSL2 modems can be software upgraded to support vectoring. Those that can't be upgraded to vectoring can be software upgraded to a 'vector friendly' mode. Crosstalk from such a vector friendly line into neighbouring vectored lines can be cancelled, but the 'friendly' line itself does not benefit from the vectoring gain.

But even firmware upgrading is a considerable undertaking for operators, especially when it involves tens or hundreds of thousands of modems. However, not all equipment can be upgraded, even to a friendly mode.

To this aim, Alcatel-Lucent has developed a 'zero touch' approach that allows crosstalk from legacy VDSL2 lines into vectored lines to be cancelled without equipment upgrade. "This significantly facilitates and speeds up the rollout of vectoring," Spruyt concluded.

Roy Rubenstein

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