Design Con 2015

G.Vector boosts VDSL2

-May 14, 2013

As the world’s demand for more bandwidth continues to grow, DSL (digital subscriber line) technologies stay competitive by constantly improving utilization of the existing telephone copper wires. One of the latest major improvements in DSL was named VDSL (very-high-bit-rate DSL) and it opened up the possibility of download speeds close to 100Mb/s. Furthermore the low cost of the existing infrastructure makes DSL a very attractive choice for many businesses and still remains the most popular broadband technology in the world.

To stay on the course of improving the rates of today’s DSL deployment, a new standard called G.Vector was recently developed. The purpose of the standard is to battle one of the biggest enemies of any DSL line: crosstalk. A full description of G.Vector can be found in the ITU-T G.993.5 document. The primary focus of the standard is to minimize the effects of far-end crosstalk (FEXT), therefore improving the bitrate on a DSL line. The effects of FEXT carry negative impact on the DSL performance. This occurs when a signal from one line gets coupled onto the neighboring line, causing interference.  Figure 1 shows the FEXT path between the DSLAM (DSL access multiplexer) and two CPEs (customer premise equipment).


Figure 1, Effects of FEXT

To improve the performance of a DSL line, G.Vector analyzes the signals in the xDSL wire bundle transmitted by every transceiver and the interferences they cause to the neighboring lines. This is followed by mathematically compensating/pre-distorting each signal as to minimize the effect of FEXT. By scanning for FEXT before transmitting a signal, a G.Vector line is able to account for the interference before it negatively impacts the performance. By nature, FEXT is an additive electric noise and therefore an example of its effects can be shown with simple sine waves.

Figure 2 shows the effects that FEXT can have on a DSL line. In this situation, a signal is transmitted without analyzing FEXT from other lines. The system treats the existing FEXT as part of the overall electric noise on the line, and, as a result, the signal to be transmitted maybe received incorrectly due to higher noise floor or negatively impacted during transmission when additional lines become active.

Figure 3 shows an example of how G.Vector accounts for FEXT by pre-distorting/compensating the signal before its transmission, therefore insuring that the received signal can carry as much information as possible. By the time the signal is received, pre-distortion is eliminated during its travel though the medium.  In order to effectively remove FEXT, every DSL line in a bundle should support G.Vector and be a member of the same G.Vector group. This is important because every G.Vector enabled line performs a network measurement of the medium and compensates the signal based on knowing what the neighboring lines are transmitting. The result of G.Vector can be a significant performance increase.


Figure 2, Example of received signal with interference  



Figure 3, Example of received signal without interference
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