Design an exhaust aftertreatment system for a BS-VI diesel engine.

BS-VI compliance requires 90%+ reduction in NOx and PM compared to BS-IV. A comprehensive system includes: Diesel Oxidation Catalyst (DOC) for HC and CO oxidation plus NO to NO2 conversion; Diesel Particulate Filter (DPF) with active regeneration strategy using fuel injection for soot burning; Selective Catalytic Reduction (SCR) using urea injection (AdBlue) for NOx reduction to N2, sized for 95%+ efficiency; and Ammonia Slip Catalyst (ASC) to prevent NH3 emissions. Control strategy must manage DPF regeneration frequency, urea dosing based on NOx sensors, catalyst temperature windows, and cold start performance. On-Board Diagnostics (OBD) must monitor system functionality and alert to failures.