PDC (PERSONAL DIGITAL CELLULAR) TELEPHONE TECHNOLOGY, INTERNATIONAL
PDC (Personal Digital Cellular) is a second-generation technology used in digital cellular telephone communication in Japan. It uses a variation of TDMA (time division multiple access) which divides each cellular channel into individual time slots in order to increase the amount of data that can be carried. Several different mutually incompatible implementations of TDMA technologies are in use worldwide, the most prolific being GSM (Global System for Mobile Communications).
PDC is currently only used in Japan, with the first systems introduced by NTT DoCoMo in 1991 as a replacement for the earlier analog networks. It operates in the 800MHz and 1,500MHz bands, making very efficient use of the available bandwidth. With bandwidth demand so high in Japan, the system can operate in two modes: full rate and half rate. Half-rate channels have reduced speech quality and data transmission rates, but allow more channels to occupy the same bandwidth. Subscriber numbers are so high in Japan that, although PDC is only operational in this one country, it accounted for 12% of global digital subscriptions in December 1999.
Along with the other mobile communication standards, PDC can be developed along a gradual evolutionary path to the global IMT-2000 standard. Indeed, one of the IMT-2000 technologies, WCDMA (Wideband Code Division Multiple Access), is going through initial testing in Japan.
PDC TECHNICAL DETAILS
PDC is the most spectrally efficient of TDMA technologies, with six half-rate (or three full-rate) channels possible in a 25kHz frequency space, compared to three channels in 30kHz in IS-136 and eight channels in 200kHz for GSM. It even compares favorably to CDMA (code division multiple access), using spread-spectrum technology to allow up to 131 channels in a 1,250kHz spectrum band.
Full-rate speech normally requires a digital data transfer rate of 9.6kbps (kilobits per second), as is used in GSM, TDMA IS-136 and CDMA networks. PDC offers two alternative rates; 9.6Kbps in full-rate channels or 5.6kbps in the half-rate channel. The quality of speech along a 5.6kbps connection is significantly lower than the standard 9.6kbps connection, but is a useful trade-off with the number of channels available.
The PDC network supports many advanced features in-line with the other second-generation technologies, such as text messaging and caller identification. Utilizing its Intelligent Network (IN) capabilities, PDC also supports pre-paid calling, personal numbers, Universal Access Numbers, advanced charging schemes and wireless virtual private networks (VPNs). VPNs are closed user groups that allow colleagues working in different locations to communicate through the mobile phone network as though they were using a conventional office phone system.
In Japan indoor coverage is of high importance, providing an important service differentiator for the different networks. PDC has been designed to enable solutions to improve congestion in places such as shopping malls, offices and subway stations. A network of micro and pico base stations can be deployed indoors, along with distributed antenna systems and repeaters, all building upon the planning strengths of the PDC standard.
For the purposes of data transmission, PDC-P (PDC Mobile Packet Data Communication System) has been introduced. This utilizes a packet-based system, letting users use a single channel simultaneously. This is valuable for 'bursty' applications such as internet browsing, where the conventional 'circuit-switched' approach wastes bandwidth by requiring the channel to be permanently dedicated to an individual user. Packet-switched data transmission is also more convenient for the user who is permanently on-line, with them only paying for the volume of data transmitted. By enhancing network efficiency, PDC-P allows a data transfer rate of 28.8Kbps.
THE FUTURE
The move to packet-switched data transfer is one of the important steps towards the common IMT-2000 standard for global mobile communication. PDC therefore offers an upgrade path to networks with a mobile data transfer rate of up to 2Mbps.
