In multi-carrier CDMA (MC-CDMA) system, timing of the starting point of the desired signal must be estimated to demodulate the desired signal. Especially for asynchronous MC-CDMA systems, rather a complicated timing estimation is needed, due to the random delays of all users. Zong, Wang and Bar-Ness (see VTC2001)proposed a partial sampling MMSE (PS-MMSE) receiver which does not require timing estimation. However, it requires several times more filter's taps than the conventional MC-CDMA receiver. The number of taps is a multiple of, and depends on, the number of subcarriers (i.e., the processing gain). A reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed, which can be adaptively implemented by using the least-mean-square (LMS) algorithm. The proposed structure allows one to choose the number of taps independent of the number of subcarriers, hence having more flexibility in choosing the number of taps than with PS-MMSE receiver. With this structure, we can use fewer taps than with the PS-MMSE receiver and still can obtain comparable performance.

Linear minimum mean square error (LMMSE) receivers for asynchronous multicarrier code division multiple access (MC-CDMA) system under frequency-selective Rayleigh fading channel is studied. The performance of this LMMSE receiver is evaluated and shown to be superior to that of the other two schemes, equal-gain combining (EQC) and maximum-ratio combining (MRC). However, a perfect timing estimation of the desired user is needed for these receivers, as a misaligned sampling interval of these receivers results in severely self intersymbol interference (ISI) and intercarrier interference (ICI) for the desired signal at the output. In order to remove the timing acquisition requirement of a receiver for an asynchronous MC-CDMA system, we proposed a novel partial sampling MMSE (PS-MMSE) receiver. Numerical result shows that the PS-MMSE receiver without timing knowledge provides significantly stronger interference suppression capability than the LMMSE receiver with known timing. Moreover, a so-called reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed to make the number of the receiver's taps independent of the number of subcarriers. Results show that with a proper grouping parameter, a much less complicated RPS-MMSE receiver achieves almost the same performance as the PS-MMSE receiver. Thus, one is able to implement an MMSE receiver without a prior timing requirement to perform multiuser detection for the MC-CDMA system in an asynchronous scenario at the expense of a slight complexity increase.

A new MMSE receiver, which is termed prediction MMSE (PMMSE) receiver, is proposed for joint multiple-access and narrow-band interference suppression in asynchronous DS/CDMA systems. Numerical results indicate that the PMMSE receiver provides considerable advantages over conventional MMSE receiver.