Turbo multiuser detection for coded DMT VDSL systems

H. Dai, V. Poor
2002 IEEE Journal on Selected Areas in Communications  
In recent years, iterative processing techniques with soft-in/soft-out (SISO) components have received considerable attention. Such techniques, based on the so-called turbo principle, are exemplified through turbo decoding, turbo equalization and turbo multiuser detection. In this paper, turbo multiuser detection is applied to a discrete multitone (DMT) very-high-rate digital subscriber line (VDSL) system to combat crosstalk signals and to obtain substantial coding gain. The proposed iterative
more » ... MT receiver is shown to achieve an overall 7.0 dB gain over the uncoded optimum receiver at a bit error rate of 7 10 − for a channel with severe intersymbol interference and additive white Gaussian noise and with one dominant crosstalk signal. Impulse noise is detrimental to the proposed scheme but can be overcome through erasure decoding techniques, as is shown by example. Digital Subscriber Line (DSL) technology provides transport of high-bit-rate digital information over telephone subscriber lines. Various DSL techniques (Basic Rate ISDN, HDSL, ADSL, and VDSL) involving sophisticated digital transmission schemes and extensive signal processing have recently become practical due to advances in microelectronics. The latest in DSL technology is very-high-rate DSL (VDSL), which provides tens of megabits per second to those customers who desire broadband entertainment or data services. At such high rates, signals on twisted pairs can be reliably transmitted at most to a few thousand feet. Thus, VDSL will primarily be used for loops fed from an optical network unit (ONU) or a central office (CO) to a customer premises, i.e., it addresses the socalled "last mile" problem. The modulation scheme for VDSL can either be multicarrier-based or single carrierbased, typically discrete multitone (DMT) and carrierless amplitude/phase modulation (CAP)/quadrature amplitude modulation (QAM). The duplexing methods can be either time-division duplexing (TDD) or frequency-division duplexing (FDD) [9], [20]. Intersymbol interference (ISI) is one of the major obstacles to high-data-rate, bandwidth-efficient communications. Multicarrier modulation (MCM), following Shannon's optimum transmission suggestion, achieves the highest performance in channels with ISI. DMT is a particular form of MCM that has been found to be well suited for DSL application and is adopted in ANSI T1.413 ADSL standards. With this approach, a channel is divided into many independent ISI-free subchannels in the frequency domain, and power and bits are allocated adaptively according to the channel characteristics [8] , [20] . The advantages of using DMT for VDSL include optimality for data transmission, adaptivity to changing environments and flexibility in bandwidth management. Normally VDSL signals occupy the band 300 KHz to 30 MHz within the twisted-pair bandwidth, and are separated from POTS/ISDN signals by splitter devices. Noise on a phone line usually occurs because of imperfect balance of the twisted pair. There are many types of noises that couple through imperfect balance into the phone line, the most common of which are crosstalk noise, radio noise and impulse noise. While the radio noise problem can be solved or at least alleviated by restricting VDSL transmission within radio bands, crosstalk and impulse noise are two 1 © = X are reconstructed based on these detected bits. X
doi:10.1109/49.983354 fatcat:hrfraclnxbcwxlo2g2jm3hewuu