This encoding scheme may be known by other names, but I know it as "pulse-width encoding" and I do not know of an 8051 serial peripheral resource capable of generating it. I think you will need to use an I/O port and either bit-bang it if you are using a "generic" 8051 derivative, or see if an 8051 derivative with advanced timer facilities (PCA?) can provide some hardware assist. I've never used the PCA, so can't say one way or the other, but I have had to generate and decode this type of encoding back in the old days when there were no 8051's with advanced timer facilities. Have fun.