We propose a novel 20 Gb/s WDM-PON upstream transmission scheme using 4-PAM modulated free-running 1550nm VCSEL and adaptive Single-Carrier Frequency-Domain Equalization (SC-FDE). A transmission distance of 20km is achieved in direct detection system. OCIS codes: (140.7260) Vertical surface cavity emitting diodes; (060.4510) Optical communications