Multiple Access -4.CDMA
In the CDMA system, different spectrum-spread codes are set for each user, and multiple users share the same broadband radio channel. Previously, the usage of a wide bandwidth by a single communications channel had been regarded as disadvantageous in terms of effective frequency utilization. However, high efficiency in frequency usage has been demonstrated since the introduction of transmission power control technology. This has been possible because transmission power control has resolved the near/far problem in which reception signals from a user distant from a base station were masked by reception from a nearer user in the uplink. As a consequence, CDMA is the multiple access system that is now attracting the most attention as a core technology for the next-generation mobile communications system.
CDMA is a system based on spectrum-spread technology, which makes it less susceptible to the impact of noise and interference by substantially spreading the bandwidth range of signals after modulation. In addition, because of its broadband characteristics, fading resistance can be achieved by RAKE multi-path synthesis.
It also offers major advantages in that spectrum-spread modulation signals provide excellent privacy functions against third-party interception. Moreover, because the use of spectrum-spread modulation signals allows for the accurate calculation of relative values for signal arrival timing, this characteristic can be used to accurately measure the present location of the mobile unit Q a function that is applied in GPS and car-mounted navigation systems.
There are two basic types of CDMA implementation methodologies : FH (frequency hopping) and DS (direct spread) systems. As it is difficult to use the FH system on a practical basis unless a super-fast synthesizer is employed, the DS system is attracting attention as the most feasible method.
Concept of CDMA