Higher-level multi-carrier modulation and its implementation

We introduce a CDMA system based on multi-carrier spread spectrum (MC-SS) with one identical code for all users. If the signals are transmitted inside the coherence bandwidth, a pair of surface acoustics wave (SAW) filters can be used for both spreading and despreading the signals at low cost. In general, the employment of a multi-carrier transmission technique increases the dynamic range of the correspondent lowpass time signal, and therefore would require a highly linear power amplifier. By carefully designing the code of a MC-SS signal a significant decrease of several orders of magnitude in dynamic range can be obtained. The spectral properties of the multi-carrier signal are not affected by the code. In some spread spectrum systems, it is desired to support different data rates over the same modem. In the SAW filter approach, neither spreading code nor spreading gain can be altered. The reduced dynamic range of the unmodulated multi-carrier signal, however, can allow for a multi-level modulation of the narrowband signal for higher rate transmission if the signal-to-noise ratio (SNR) is sufficiently high. Different implementations for MC-SS are shown in this paper.