Spread-time code-division multiple access

An alternative code-division multiple-access (CDMA) scheme to spread spectrum (SS), called spread time (ST) is proposed for badlimited multiple-access channels. ST-CDMA can be considered the be-frequency dual of SS-CDMA. In ST-GDMA pslendsraodan (PN) sequences are assigned to each user, and the Fourier tcrursf@rm of the transmitted pulse for a given user is d&mined by modulating the phase of the desired transmitted spectnnn by the user’s PN-skquence. The t”Btasd data for a parthdnr mer can be recoverad by sampling t8c output of a Plter matchd tcE tbe w ” s pulse. ImplemenEwtloqs rare dascrilred in which wrface acoustic wave devices are used to perform the matched fislteL.ing or Fourier transformation. Averaged signal-to-intdremoe plus noise ratio (SIR) and spectral afldeacy are campuW fbr bo& asynchronous ST a d dCrect-seqnwe SS-CDMA systuns, assuming an arbitrary c h n d t r M e r fbnetion H(f), whicb is the same between all ppirS sf users. Tlhe resnlts are tlnt same for SS and ST providsd W t the magmnipude of the Fourier transform of the chip h p e in the SS system is the 88me 86 the nragnitwde of the Fader transform OP t4e ST pulse shape. The main advantag@ af tbe ST teehnlqus is the fled whick the transmitted spectrum can be sdwtwl. We dcrjlve the transmitted spectrum tbat maximizes the SIR subject to an average power constraint.