On the performance of certain fixed-complexity multiuser detectors in FEXT-limited vectored DSL systems

Multiple-input multiple-output (MIMO) communication has recently attracted considerable interest for transmission over multiple twisted copper pairs in a common binder. The motivation is to overcome the limitations imposed by crosstalk, and thus boost capacity. Far-end crosstalk (FEXT) is the dominant impairment in frequency-division duplex (FDD) digital subscriber line (DSL) systems. While the per-tone MIMO channel matrix is diagonally dominated in this context, noise is colored due to alien FEXT, which is particularly pronounced for short DSL loops - such as those encountered in fiber-to-the-curb (FTTC) or fiber-to-the-basement (FTTB) architectures. This calls for effective multiuser detection strategies at the receiver. We test three candidate detection algorithms (probabilistic data association, sphere decoding, and a recently proposed hybrid thereof) using measured channel data for 300 meter cable, and a fixed upper bound on vector decoding complexity. The results indicate that, when half of the loops in the binder are coordinated, contrary to the case of Rayleigh fading wireless channels, the sphere decoder provides the best performance, even when a pragmatic complexity restriction is employed.