10. The speech signal decoding apparatus of claim 6 , wherein the second decoding unit comprises: a gain difference decoding unit decoding quantization information regarding a difference between log scale gain values of the fixed codebook included in the coding information of the speech quality enhancement layer; and a fixed codebook decoding unit decoding a fixed codebook index included in the coding information of the speech quality enhancement layer.
10. The speech signal decoding apparatus of claim 6 , wherein the second decoding unit comprises: a gain difference decoding unit decoding quantization information regarding a difference between log scale gain values of the fixed codebook included in the coding information of the speech quality enhancement layer; and a fixed codebook decoding unit decoding a fixed codebook index included in the coding information of the speech quality enhancement layer. 11. The speech signal decoding apparatus of claim 10 , wherein the calculating unit comprises: a first adder adding a decoded fixed codebook of the speech quality enhancement layer output from the second fixed codebook decoding unit to a decoded fixed codebook of the base layer output from the first fixed codebook decoding unit; a selector switch selectively transmitting a signal output from the first adder or a decoded fixed codebook of the base layer output from the first fixed codebook decoding unit according to operating conditions of the speech signal decoding apparatus; and a second adder adding a signal output from the selector switch to a decoded adaptive codebook of the base layer output from the adaptive codebook decoding unit.