Personal Digital Cellular (PDC)
- the 2G system used in Japan
The Personal Digital Cellular or Pacific Digital Cellular
(PDC) system is a second-generation mobile phone technology introduced in 1991.
Although it is only found in Japan, its use there is very widespread and there
are a considerable number of users. Accordingly it accounts for over 10% of the
world market for second-generation mobile phone users.
PDC was introduced in Japan in 1991 with the move from
analogue to digital technology. It uses TDMA technology and it is very similar
to the US "TDMA" or IS54 / IS136 system but operates in the 800 and 1500 MHz
bands. The modulation scheme, voice frame size, TDMA frame duration, and
interleaving remain the same. The major difference is that it uses a 25 kHz
channel spacing instead of 30 kHz.
Technology
As already mentioned PDC is a TDMA system and it operates by splitting each
channel into several time slots and thereby allowing several users to use the
same frequency channel. For each channel it is possible to support three users
under normal circumstances. However when traffic levels are high it is possible
to use half data rate speech. Although this reduces the speech quality, it
enables six calls to be supported by each channel. This compares very favourably
to GSM that manages eight within each 200 kHz channel.
Speech encoding is an important factor. PDC uses a different
encoder to that used on IS54/IS136. The standard rate is 9.6 kbps along with
similar technologies such as GSM, but when half rate encoding is used this falls
to 5.6 kbps. Although this gives a significant reduction in voice quality, it is
still adequate to maintain intelligibility and enables the network capacity to
be increased to accommodate further calls.
Whilst it may appear that the spectral efficiency of PDC
would result in a lower level of features, it manages to support many of the
expected features including text messaging and caller identification. It also
has Intelligent Network (IN) capabilities and these enable it to support
features such as pre-paid calling, Universal Access Numbers, personal numbers
and virtual private networks (VPN). These VPNs are limited access groups that
allow people working in different locations to use the mobile phone network as
thought it was an internal office phone system.
Data
To facilitate data transmission to a data rate of 28.8 kbps the PDC-P (PDC
mobile Packet Data Communication System) has been introduced. The normal system
uses a circuit switched approach where a complete circuit is switched for a
given user. This is fine for voice communications, but where data applications
are concerned a packet switched approach is far more efficient. Here individual
packets of data are routed to the required user and there is no dedicated
channel. This makes far more efficient use of the channels available because
data transmissions are notoriously "bursty" in nature. By adopting this approach
users can be charged for the data transferred rather than the connection time.
Coverage
One very important feature in Japan is that of coverage, and that of coverage
within buildings such as shopping malls, offices and subway stations. In fact
this is a major sales differentiator between the different operators and as such
it is of great importance. In view of this there is a large number of micro and
pico cells that have been set up. The base stations for these cells use low
power combined with distributed antenna systems and repeaters to ensure the
required coverage.
Migration
One of the important aspects of any system today is the migration path to 3G.
With the move to the greater use of packet data on PDC this provides an
important step towards the IMT2000. This will provide a data transfer rate of up
to 2 Mbps.
|
