CDMA is described in 5.2, “Spread Spectrum and Code Division Multiple Access
(CDMA)” on page 97.
The Qualcomm system uses ten channels of bandwidth 1.25 MHz and allows 118
users on each radio channel at any one time. This is achieved by encoding each
bit of digital data with a unique code known only to the transmitting and
receiving stations. Any other station listening on this channel will only hear
low-level noise. The modulation technique used is Quadrature Phase Shift
Keying (QPSK) and is described in 3.1.6.7, “Quadrature Phase Shift Keying
(QPSK)” on page 66.
One of the significant factors of CDMA is that the mobile station can
communicate with several base stations at the same time, thus avoiding fading
and “blackspots” where the mobile station is shielded from the base station
transmitter by buildings or geographical features. CDMA is most effective when
the cellular phone is actually in a moving vehicle.
Many of the same network techniques are used in the Qualcomm system as in
other digital cellular networks. The main difference is in the use of spread
spectrum in the radio interface.
2.3.4 Japan Digital Cellular (JDC)
Japan′s Digital Cellular system is unique, but based upon IS-54 standards. The
following are the key differences:
•
Channel bandwidth is 25 kHz not 30 kHz
•
Does not coexist with analog channels
•
Three logical channels per frame
•
11.2 Kbps speech bit rate, 42 Kbps transmission rate
Japan was able to allocate dedicated radio channels to digital cellular and
therefore did not have to provide dual analog/digital capability. Three discrete
pairs of bands are available:
•
810 - 826 MHz downlink
•
940 - 956 MHz uplink
•
1429 - 1441 MHz downlink
•
1447 - 1489 MHz uplink
•
1453 - 1465 MHz downlink
•
1501 - 1513 MHz uplink
2.4 Cellular Digital Packet Data (CDPD)
Analog cellular systems throughout the world are becoming congested with
voice traffic, but it is known that the total information carrying capacity of an
analog cellular network is much greater than is currently being utilized. This is
due to a number of factors which all relate with the fact that there are times
when no information is being sent but the radio channel is kept open or idle.
Some of these factors are as follows:
•
“Quiet” time during voice conversations
− Waiting for a response
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