SPREAD-TIME CODE-DIVISION MULTIPLE-ACCESS

An alternative code-division multiple-access (CDMA) scheme to spread s pectrum (SS), called spread time (ST) is proposed for bandlimited mult iple-access channels, ST-CDMA can be considered the time-frequency dua l of SS-CDMA. In ST-CDMA pseudorandom (PN) sequences are assigned to e ach user, and the Fourier transform of the transmitted pulse for a giv en user is determined by modulating the phase of the desired transmitt ed spectrum by the user's PN-sequence. The transmitted data for a part icular user can be recovered by sampling the output of a filter matche d to the user's pulse, Implementations are described in which surface acoustic wave devices are used to perform the matched filtering or Fou rier transformation. Averaged signal-to-interference plus noise ratio (SIR) and spectral efficiency are computed for both asynchronous ST an d direct-sequence SS-CDMA systems, assuming an arbitrary channel trans fer function H(f), which is the same between ah pairs of users, The re sults are the same for SS and ST provided that the magnitude of the Fo urier transform of the chip shape in the SS system is the same as the magnitude of the Fourier transform of the ST pulse shape, The main adv antage of the ST technique is the flexibility with which the transmitt ed spectrum can be selected, We derive the transmitted spectrum that m aximizes the SIR subject to an average power constraint.