Comparison between multicode with fixed spreading and single code with variable spreading options in UTRA/TDD

Europe is proposing a system called the Universal Mobile Telecommunication System (UMTS) as a candidate for IMT-2000. It is characterized by a FDD and a TDD mode, both with a CDMA component. Two options are being considered for the spreading for the TDD mode: a multicode option, where each user may have several codes of fixed length, and a single code option, where each user always has one code, but with a variable length. This paper compares the peak-to-average ratio as well as the receiver complexity and performances for both options. It shows that the single code option enables to decrease the complexity by up to 50% and to increase the performances by up to 1 dB, compared to the multicode option. The single code option allows in addition a reduced peak-to-average ratio of the RF signal, resulting in a more efficient power amplifier.