This paper provides a performance comparison of mixed modulation codewords and shortened codewords defined in DOCSIS 3.1 specifications. In mixed modulation codewords, the same modulation constellation is not used for all sub-carriers of the OFDM symbol. The performance comparison is needed to decide efficient transmission parameters at the view of signal-to-noise ratio and spectral efficiency. In this paper, we present the more efficient codeword scheme from the comparison.