How to have periodical repetition of customized pulse waveform in LT Spice?
PWL sources can be repeated either a set number of times:
PWL REPEAT FOR 7 (file=pwl_data.txt) ENDREPEAT

or forever:
PWL REPEAT FOREVER (file=pwl_data.txt) ENDREPEAT

Source: http://ltwiki.org/index.php?title=Undocumented_LTspice