Multiple access on narrowband interference channels

We investigate the influence of narrowband interference on different multiple access techniques including classic frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), and orthogonal FDMA (OFDMA). A major finding of this study is that CDMA with pseudonoise spreading sequences (PN-CDMA) does not perform any better than TDMA and that both multiple access schemes give virtually identical performance. We give a supporting interpretation of this result pointing out that TDMA is a special case of CDMA and that the argument of increased robustness to narrowband interference through spectral spreading equally applies to TDMA. Another finding is that orthogonal CDMA (OCDMA) has an entirely different behavior from TDMA and PN-CDMA in that the performance degradation is not the same for all users. Using continuous-wave (CW) interference, it was found that OCDMA can support a significant number of users at interference values which break both TDMA and PN-CDMA. We also point out some similarities between OCDMA and OFDMA, and highlight the fact that OFDMA is the most resistant multiple access technique to narrowband interference.