The advantages of code division multiple access (CDMA) for cellular voice have become well known. In contrast to orthogonal systems such as time division multiplex access (TDMA) or frequency division multiplex access (FDMA), frequency planning or "orthogonality" coordination (channel allocation) between cells and within the same cell are greatly simplified. The reason is that, unlike TDMA and FDMA where the re-use constraints must account for the worst case (or 95th percentile) interferer, re-use in CDMA is based on the average interference seen from a large number of low power users. Due to this interference averaging property, CDMA simply translates voice activity factor and antenna sectorization into capacity gains. Furthermore, RAKE receivers resolve the multipath components of the spread spectrum signal and translate it into diversity gain.
In spite of the advantages, conventional CDMA systems have very limited per user throughput and are not well suited to "bandwidth on demand" local area network (LAN)-like applications. In fact, current CDMA standards operate in circuit mode, assume a homogeneous user population, and limit each user to a rate which is a small fraction of the system capacity. As mentioned above, CDMA relies on the averaging effect of the interference from a large number of low-rate (voice or circuit-mode data) users. It relies heavily on sophisticated power control to ensure that the average interference from all users from an adjacent cell is a small fraction of the interference from the users within a cell. The imperfect power control in a homogeneous system has a direct impact on system performance.
Moreover, even with perfect power control, users at higher data rates in a system with mixed traffic result in large adjacent cell interference variations which drastically degrade the system capacity. This problem has so far precluded the provision of high data rate services in cellular CDMA.
USPTO Backgrounds
USPTO Backgrounds