Choose Wisely Between QAM and DMT for VDSL

elecommunications operators are responding to the competitive threat of cable and satellite services by offering high-speed applications and converged services to residential and business customers. These applications generally require minimum duplex bit rates in the range of 5 to 6 Mbits/second—both upstream and downstream (symmetric). While there are several digital subscriber line (DSL) technologies available, none meet these high-bandwidth requirements. But help may be at hand. With symmetric data rates up to 26 Mbits/s over distances up to 1,500 meters, and higher asymmetric and short-haul rates, very high-bit-rate DSL (VDSL) might well be the answer. However, before implementing a VDSL-based connection, two decisions have to be made. The first is which transport protocol to use, Ethernet or asynchronous transfer mode (ATM). The second is which of the line-modulation techniques for VDSL should be used—quadrature amplitude modulation (QAM) or discrete multitone (DMT). While the choice of protocol may have more to do with the application requirements, choosing QAM or DMT is not so straightforward. Regional spectral compatibility, noise tolerance, cost, interoperability and relative power consumption must all be factored in. Also, despite the fact that DMT has already been deployed in existing asymmetric DSL (ADSL) schemes, the radically different VDSL model does not make the seamless migration of DMT to VDSL a slam dunk. To make an informed decision between QAM and DMT, it behooves the designer to fully understand the trade-offs and differences between them.