paper deals with simulation of Generalized Discrete Fourier Transform spreading codes used in modulation technique in Code Division Multiple Access on real time system using MATLAB environment. Binary sequences can be used as various spreading code in digital modulation technique. The spreading codes are used to increase the spread spectrum and proper utilization of the spread spectrum in cellular communication system. In CDMA for digital modulation technique various spreading codes like Gold codes, M-ary Binary sequences, Walsh codes and Constant modulus function set (GDFT) are used. These codes can be used in various applications like Orthogonal Frequency Division Multiple Access (OFDM), Discrete Multi-tone and Code Division Multiple Access. This paper proposes real time system model in which GDFT spreading code modulated with voice or speech as input analog signal. Performance of Generalized Discrete fourier transform with non linear phase is been determined by some factors. The factors are Bit-Error-Rate, Signal-to-Noise ratio and Mean Square Value of auto-correlation and Mean Square Value of Cross-Correlation responsible to maintain quality of services in digital modulation.