The rapid
growth of mobile and internet users has
led towards the development of the third generation of mobile communication
systems. The UMTS is a family of 3G mobile networks designed to offer high
bandwidth radio access.
Enhanced
UMTS is a Umts evolution step, which makes possible an effective high data rate
end to end packet based transmission . IST-SEACORN has proposed a set of
enhancements to UMTS which include among others advanced modulation and radio
transmission techniques improved strategies for IP routing and QOS assurance.
HANDOVER
The handver process is one of the
essential means that guarantees user mobility in a mobile communication network. The concept of mobility is simple.
When a
subscriber moves from the coverage area of one cell to another , a new connection
with the target cell is set up and the connection of the previous cell is
released.
A basic
handover process consists of three main phases:
a) Measurement phase dealing with the
mechanics of measuring important parameters.
b) Decision phase algorithm parameters
and handover criteria
c) Execution dealing with radio resource
allocation and handover signaling.
1.
Operating environment
A handoff algorithm needs to be
designed with the operating radio environment in mind in order to define the
correct parameters needed in the decision phase. Operating environments are
usually separated as:
•
Indoor: low speeds and well-defined mobility paths
•
Outdoor: variable speeds and mobility
paths that depend on each environment separately.
A
further classification of environments can be made according to their cell
size:
•
Pico-cellular and micro-cellular
environments: characterized by small cells and low transmit powers. In
pico-cellular both users and base stations are located indoors, whereas in
micro-cellular only outdoor users are considered. The antenna height in a
microcellular environment is typically at lamppost level (5m above ground).
•
Macro-cellular environments have large
cells (several kilometres) and transmit high output power with the antenna
mounted on a high tower above all surrounding rooftops (15m+ above ground).
POWER CONTROL
Power Control has a
dual operation. Firstly, it keeps interference at minimum levels by controlling
the power transmitted, achieving further to minimize the power consumption at
the mobile user (called User Equipment (UE) in UMTS) and the base stations
(called Node Bs in UMTS). Secondly, it ensures an adequate quality of service
(QoS) level so that the percentage of dropped calls is kept below the
acceptable thresholds.
Power Control is important both in the
uplink and the downlink directions. In the uplink direction control is required
in the situations where UEs are located very close to the Node Bs and are
transmitting with excessive power. This is called the near-far effect and can
result in blocking the whole cell, with UEs that are close to the cell edge
possibly overlooked. If the uplink power is too high interference in
neighbouring cells (inter-cell interference) may also be a direct result of the
near-far effect. In the downlink direction, Power Control directly affects
system capacity. System capacity is determined by the total downlink transmission
power for each cell i.e. when total downlink transmission power is minimized
then the Node B can accept more UEs and the capacity is increased.
No comments:
Post a Comment