13. The speech signal decoding apparatus of claim 6 , wherein the second decoding unit comprises: a gain difference decoding unit decoding a difference between fixed codebook log scale gain values included in the coding information in 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.
6. A speech signal decoding apparatus decoding a speech signal coded by a base layer and at least one speech quality enhancement layer, the speech signal decoding apparatus comprising: a first decoding unit decoding coding information in the base layer from the coded speech signal; a second decoding unit decoding coding information in the speech quality enhancement layer from the coded speech signal according to an operating environment of the speech signal decoding apparatus, wherein the coding information in the speech quality enhancement is obtained by searching a fixed codebook using parameters obtained by the fixed codebook searching unit of the base layer, wherein the parameters comprise: a first correlation d(n) between an impulse response h(i-n) and a target signal x′(n) obtained in the fixed codebook searching unit in the base layer by the following equation, d ⁡ ( n ) = ∑ i = n 39 ⁢ ⁢ x ′ ⁡ ( n ) ⁢ h ⁡ ( i - n ) ⁢ ⁢ n = 0 , … ⁢ , 39 ; a second correlation C detected by using a sign s of each pulse and the first correlation d(n) by the following equation, C = ∑ i = 0 3 ⁢ ⁢ s 1 ⁢ d ⁡ ( m i ) =  d ⁡ ( m 0 )  +  d ⁡ ( m 1 )  +  d ⁡ ( m 2 )  +  d ⁡ ( m 3 )  , wherein m i represents an i th pulse position, and s i represents a sign of an i th pulse; and an amount of energy E of the impulse response h(n) detected by the following equation—, E = ∑ i = 0 3 ⁢ ⁢ ϕ ⁡ ( m i , m j ) + 2 ⁢ ∑ i = 0 2 ⁢ ⁢ ∑ j = i + 1 3 ⁢ ⁢ s i ⁢ s j ⁢ ϕ ⁡ ( m i ⁢ m j ) , wherein Φ(m i , m j ) represents a correlation between the impulse response h(n) with respect to i th and j th pulse positions, s i is the sign of an i th pulse, and s j is a sign of a j th pulse; a calculating unit calculating a signal output from the first decoding unit and a signal output from the second decoding unit, according to the operating environment of the speech signal decoding apparatus; and a speech signal restoring unit synthesizing a signal output from the calculating unit using a linear prediction coding coefficient output from the first decoding unit and restoring the speech signal, wherein the first decoding unit comprises a linear prediction coding coefficient decoding unit decoding linear prediction coding coefficient quantization information included in the coding information in the base layer; a first fixed codebook decoding unit decoding a fixed codebook index included in the coding information in the base layer; an adaptive codebook decoding unit decoding an adaptive codebook index included in the coding information in the base layer; and a gain value decoding unit decoding a fixed codebook gain value and an adaptive codebook gain value included in the coding information in the base layer, and the second decoding unit comprises a gain difference decoding unit decoding quantization information regarding a difference between fixed codebook gain values included in the coding information in the speech quality enhancement layer; and a second fixed codebook decoding unit decoding a fixed codebook index included in the coding information in the speech quality enhancement layer.