In this paper, we present a half rate speech coder using trellis excitation. The coder combines code-excited linear prediction (CELP) system and trellis quantization method using the codebook expansion, and it produces higher speech quality than the typical CELP coder for the same transmission rate. A subjective comparison with 3~8 bit .$mu$-law PCM indicates that the half rate coder provides speech quality between 5-bit and 6-bit $mu$-law PCM .