Long-Term Objectives in System Planning: Delivering Consistency

Long-Term Objectives in System Planning:
Delivering Consistency
Radio bandwidth in a 3G network is more adaptive than radio bandwidth in a 2G
network, which in turn is more adaptive than radio bandwidth in a 1G network. The
OVSF code structure provides an elegant mechanism for balancing variable user data
rates and allocating (code domain distributed) power. Pilot symbol based measurement
reports provide a fast (1500 Hz) mechanism for measuring radio channel quality,
which in turn provides the basis for accurate measurement reports, which in turn provides
the basis for effective load balancing and admission control. Load balancing and
admission control allow resources to be shared across the IP RAN network and radio
layer in a more dynamically adaptable way than would be possible in existing legacy
access networks (optimized for predominantly constant-rate offered traffic).
At the radio system level, load balancing between Node B transceivers helps to distribute
offered noise. This helps improve average Node B sensitivity, which in turn
helps improve coverage. Radio system capacity constraints are determined by the
amount of RF power available both at the Node B and in the handset. The downlink
can relatively quickly become code-limited because of the limited number of OVSF
codes available. In larger cells, orthogonality can be a problem because of delay spread
on the channel, and this can cause downlink performance degradation.
Coded channel streams (OVSF and long code streams) represent a phase argument
that needs to be coherently demodulated, decorrelated, and combined by the receiver.
Time, phase, or amplitude ambiguity on the radio channel will potentially impair performance
(increase bit error rates).
Network performance is improved over time by optimizing radio layer and network
layer performance parameters. There is a need for consistent quality, which in turn
requires careful implementation of soft handover algorithms to avoid session discontinuity
(or what used to be called high dropped call rates).
In the longer term (3 to 5 years), there is substantial gain potential in IMT2000,
which will result in a more consistent radio channel. The challenge will be to deliver
equivalent consistency into and through the IP RAN and IP core network to provide
measurable and manageable end-to-end performance.