V.P. Ipatov, A.B. Khachaturian

Many modern telecommunication and navigation-timing services (cdmaOne, cdma2000, WCDMA, GPS, GLONASS, etc.) are CDMA based. The merits of CDMA technology are widely recognized and stem from its spread spectrum nature. As an air congestion grows the matter of keeping out-of-band emission within the prescribed limits, CDMA networks included, becomes more and more acute. Just for one, the GLONASS L1 signal currently penetrates into the radio astronomy band 1610.31613.8 MHz badly hampering normal radio telescope operation. To solve the problem radically spectral efficient modulation is a good option. In particular, continuous phase modulation (CPM) provides a significant narrowing of the bandwidth spanning 99 % of the total emitted power versus the traditional BPSK. One of the key criteria of a CDMA signature ensemble quality is the multiple access interference (MAI) mean power. When passing on from BPSK to CPM it is of great importance to select CPM format which optimizes MAI performance subject to the limited time-frequency resource. In the paper the way is presented to calculate the autocorrelation function of an arbitrary CPM signal be the latter of either full or partial response type. After that the 99%-bandwidth is readily found and the MAI power is computed in terms of its product with the assigned processing time. Further, the main outcome of the study is the rating of CPM modes according to the MAI suppression capability. Among the number of modulation formats considered the classic MSK appears to be the best in this regard. This dismisses the previously stated by some experts conjecture that the Gaussian MSK can be the preferable choice of modulation mode for a spectral efficient CDMA.