In a cellular code division multiple access (CDMA) system, uplink capacity is limited by interference. For a ground-based CDMA system or a high altitude platform station (HAPS) CDMA system, the interference is contributed by users in the same cell and in other cells. In this paper, a novel scheme is proposed to reduce both the same-cell and other-cell interferences in a cellular CDMA system. The proposed scheme attempts to lower the required transmitting power for each user by integrating terrestrial base stations (BSs) with a HAPS to provide two separate paths for signal reception. Numerical results show that the total interference is significantly reduced and the uplink capacity is improved under the proposed scheme, compared to either a HAPS CDMA system or a ground-based CDMA system.