1. A CELP speech decoder, comprising: an adaptive codebook configured to generate an adaptive code vector; a random codebook configured to generate a random code vector, the random codebook comprising: an input vector provider configured to provide an input vector including at least one pulse, each pulse having a position and a polarity, a fixed waveform storage configured to store at least one fixed waveform, a selector configured to select at least one of a first process and a second process based on a value of an adaptive codebook gain, and a convolution section configured to generate the random code vector by convoluting the at least one fixed waveform with the input vector when the first process is selected; and a synthesis filter configured to output synthesized speech by performing linear prediction coefficient synthesis on a signal based on the adaptive code vector and the random code vector.