  Kernel adaptive filtering (KAF) is proposed for nonlinearity-tolerant optical
direct detection. For 7x128Gbit/s PAM4 transmission over 33.6km 7-core-fiber,
KAF only needs 10 equalizer taps to reach KP4-FEC limit (BER@2.2e-4), whereas
decision-feedback-equalizer needs 43 equalizer taps to reach HD-FEC limit
(BER@3.8e-3).
