Abstract:
The primary problem addressed in this thesis is to develop a Multi-carrier Code Division Multiple Access (MC-CDMA) Digital Subscriber Line (DSL) modem, capable of delivering 1 Gbps aggregated throughput over a 200 m single twisted-pair of 0.5 mm copper wire. A secondary problem is too much self-interference within Digital Subscriber Line (DSL) systems. Self-interference is defined as the disturbance or interference that is created by similar DSL technology (i.e. one VDSL service on another VDSL service) operating within the same frequency band. The interference, referred to as Crosstalk is due to magnetic cross-coupling of signals between adjacent twisted pairs. Far end crosstalk (FEXT) for short loops is still a huge problem. Current techniques like Vectoring makes use of pre-coding in downstream transmission and makes use of Multi-user Detection and interference Cancellation (MUD-IC) in upstream transmission. Overall these techniques (pre-coding and MUD-IC) have a high computational overhead. Using Super Orthogonal Complete Complementary (SOCC) spreading codes the effect of crosstalk is mitigated, since the SOCC spreading/despreading process inherently provide IC and does not require overhead for pre-coding and MUD-IC.
The designed GDSL modem provides Multi-user interference (MUI)-free communication in a highly crosstalk dominated environment (Near end crosstalk and Far-end crosstalk) by incorporating Super Orthogonal Complete Complementary (SOCC) spreading into the existing xDSL modem architecture.
A novel code allocation algorithm was also developed to provide all users with equal transfer ratios (i.e. transferred bits to offered bits), even with bad line profiles and a high amount of offered bits. Implementation and simulation of existing network characteristics, including attenuation, Near-end Crosstalk (NEXT) and Far-end Crosstalk (FEXT) used ITU and NIPPC standards as guidelines.
1 Gbps aggregate throughput was obtained for the GDSL system over a single pair of 0.5 mm diameter copper wire over a distance of 190 m with only GDSL modems present, or 185 m to 148 m, depending on whether ADSL2+ or VDSL2 Profile 30a bypass profiles are implemented. Performance analysis in a practical MIMO crosstalk environment showed that the GDSL system have acceptable BER performance for a fully loaded system of 64 users, even for very short loops severe FEXT).
