Several system vendors including Adtran, Alcatel-Lucent and ZTE have announced vectoring technology that boosts the performance of very-high-bit-rate digital subscriber line (VDSL2) broadband access technology. Vectoring is used to counter crosstalk - signal leakage between the telephony twisted wire pairs that curtails VDSL2's bit rate performance – as is now explained.
There is a large uncertainty in the resulting VDSL2 bit rate for a given loop length. With vectoring this uncertainty is almost removed
Paul Spruyt, Alcatel-Lucent
Two key characteristics of the local loop limit the performance of digital subscriber line (DSL) technology: signal attenuation and crosstalk.
Attenuation is due to the limited spectrum of the telephone twisted pair, designed for low frequency voice calls not high-speed data transmission. Analogue telephony uses only 4kHz of spectrum, whereas ADSL uses 1.1MHz and ADSL2+ 2.2MHz. The even higher speed VDSL2 has several flavours: 8b is 8.5MHz, 17a is 17.6MHz while 30a spans 30MHz.
The higher frequencies induce greater attenuation and hence the wider the spectrum, the shorter the copper loop length over which data can be sent. This is why higher speed VDSL2 technology requires the central office or, more commonly, the cabinet to be closer to the user, up to 2.5km away - although in most cases VDSL2 is deployed on loops shorter than 1.5km.
The second effect, crosstalk, describes the leakage of the signal in a copper pair into neighbouring pairs. “All my neighbours get a little bit of the signal sent on my pair, and vice versa: the signal I receive is not only the useful signal transmitted on my pair but also noise, the contributed components from all my active VDSL2 neighbours,” says Paul Spruyt, xDSL technology strategist at Alcatel-Lucent.
Typical a cable bundle comprises several tens to several hundred copper pairs. The signal-to-noise ratio on each pair dictates the overall achievable data rate to the user and on short loops it is the crosstalk that is the main noise culprit.
Vectoring boosts VDSL2 data rates to some 100 megabits-per-second (Mbps) downstream and 40Mbps upstream over 400m. This compares to 50Mbps and 20Mbps, respectively, without vectoring. There is a large uncertainty in the resulting VDSL2 bit rate for a given loop length. "With vectoring this uncertainty is almost removed," says Spruyt.
Vectoring
The term vectoring refers to the digital signal processing (DSP) computations involved to cancel the crosstalk. The computation involves multiplying pre-coder matrices with Nx1 data sets – or vectors – representing the transmit signals.
The crosstalk coupling into each VDSL2 line is measured and used to generate an anti-noise signal in the DSLAM to null the crosstalk on each line.
To calculate the crosstalk coupling between the pairs in the cable bundle, use is made of the ‘sync’ symbol that is sent after every 256 data symbols, equating to a sync symbol every 64ms or about 16 a second.
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, 256-bit-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 pilots sequences and multiply them bit-wise, and you take the average, you always find zero,” says Spruyt. "This characteristic speeds up and simplifies the crosstalk estimation.”
A 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. 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 multi-tone (DMT) modulation where each DMT symbol consists of 4096 tones, split between the upstream (from the DSL modem to the DSLAM) and the downstream (to the user) 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.
The above describes the crosstalk pre-compensation or pre-coding in the downstream direction: anti-noise signals are generated and injected in the DSLAM prior to transmission of the signal on the line.
For the upstream, the inverse occurs: the DSLAM generates and adds the anti-noise after reception of the signal distorted with crosstalk. This technique is known as post-compensation or post-coding. In this case the DSL modem sends the pilot modulated sync symbols and the DSLAM measures the error signal and performs the correlations and anti-noise calculations.
Challenges
One key challenge is the amount of computations to be performed in real-time. For a fully-vectored 200-line VDSL2 system, some 2,600 billion multiply-accumulates per second - 2.6TMAC/s - need to be calculated. A system of 400 lines would require four times as much processing power, about 10TMAC/s.
Alcatel-Lucent’s first-generation vectoring system that was released end 2011 could process 192 lines. At the recent Broadband World Forum show in October, Alcatel-Lucent unveiled its second-generation system that doubles the capacity to 384 lines.
For larger cable bundles, the crosstalk contributions from certain more distant disturbers to a victim line are negligible. Also, for large vectoring systems, pairs typically do not stay together in the same cable but get split over multiple smaller cables that do not interfere with each other. “There is a possibility to reduce complexity by sparse matrix computations rather than a full matrix,” says Spruyt, but for smaller systems full matrix computation is preferred as the disturbers can’t be ignored.
There are other challenges.
There is a large amount of data to be transferred within the DSLAM associated with the vectoring. According to Alcatel-Lucent, a 48-port VDSL2 card can generate up to 20 Gigabit-per-second (Gbps) of vectoring data.
There is also the need for strict synchronization – for vectoring to work the DMT symbols of all lines need to be aligned within about 1 microsecond. As such, the clock needs to be distributed with great care across the DSLAM.
Adding or removing a VDSL2 line also must not affect active lines which requires that crosstalk is estimated and cancelled before any damage is done. The same applies when switching off a VDSL2 modem which may affect the terminating impedance of a twisted pair and modify the crosstalk coupling. Hence the crosstalk needs to be monitored in real-time.
Zero touch
A further challenge that operators face when upgrading to vectoring is that not all the users' VDSL2 modems may support vectoring. This means that crosstalk from such lines can’t be cancelled which significantly reduces the vectoring benefits for the users with vectoring DSL modems on the same cable.
To tackle this, certain legacy VDSL2 modems can be software upgraded to support vectoring. Others, 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.
Upgrading the modem firmware is also a considerable undertaking for the telecom operators especially when it involves tens or hundreds of thousands of modems.
Moreover, not all the CPEs can be upgraded to friendly mode. To this aim, Alcatel Lucent has developed a 'zero-touch' approach that allows cancelling the crosstalk from legacy VDSL2 lines into a vectored lines without CPE upgrade. “This significantly facilitates and speeds up the roll-out of vectoring” says Spruyt.
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