Main differences between the third and second generation air
interfaces are described. GSM and IS-95 (the standard for cdmaOne systems) are
the second generation air interfaces considered here. Other second generation
air interfaces are PDC in Japan and US-TDMA mainly in the Americas; these are
based on TDMA (time division multiple access) and have more similarities with
GSM than with IS-95. The second generation systems were built mainly to provide
speech services in macro cells. To understand the background to the differences
between second and third generation systems, we need to look at the new
requirements of the third generation systems which are listed below:
·
Bit
rates up to 2 Mbps;
·
Variable
bit rate to offer bandwidth on demand;
·
Multiplexing
of services with different quality requirements on a single connection, e.g. speech,
video and packet data;
·
Delay
requirements from delay-sensitive real time traffic to flexible best-effort
packet data;
·
Quality
requirements from 10 % frame error rate to 10_6 bit error
rate;
·
Co-existence
of second and third generation systems and inter-system handovers for coverage
enhancements and load balancing;
·
Support
of asymmetric uplink and downlink traffic, e.g. web browsing causes more loading
to downlink than to uplink;
·
High
spectrum efficiency;
·
Co-existence
of FDD and TDD modes.
GSM also covers services and core network aspects, and this GSM
platform will be used together with the WCDMA air interface: see the next
section regarding core networks.
