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Mobile and Broadband Access Networks
Lab session OPNET: UMTS - Part 2
Background information
Abram Schoutteet, Bart Slock
1 UMTS Practicum CASE 2: Soft Handover Gain
1.1 Background
The macro diversity gain is responsible for the increase in system performance caused
by soft handovers. The soft handover gain can be easily illustrated using the following
example. As a radio connection is supported by two or more radio links during the soft
handover, it is possible to use links with lower quality as long as the combined effect is
sufficient to provide the requested quality of the radio connection. Imagine a desired BLER
of 1/10000. When using two bearers with a BLER of only 1/100 this can be achieved,
as a block error will only occur if both links fail. This will happen with a probability of
1/100 * 1/100 = 1/10000. Lowering the link quality enables the transmission power
to be kept at lower levels res ulting in less interference in the system.
On the other hand every soft handover results in one or more extra connections in
the radio network layer, consuming additional resources and causing interference.
In the downlink direction an additional base station transmitting at relatively high power
levels causes the total amount of power present in the system to rise significantly. In the
uplink direction a mobile station in soft handover also transmits two or more signals -
which are received by two or more different Node B’s. Hence the interference present in
the system increases although not as significantly as in the downlink direction. This is
due to the higher power levels used by the transmitter in the base station. Typically in
the downlink data is transmitted at values up to some Watts whereas mobile terminals
transmit at around 125mW. Soft handovers do not only consume e xtra radio resources but
multiple-link connections require also more orthogonal codes and consume extra resources
in the Node B and the RNC. Also the signalling capacity over the Iub interface between the
Node B and the RNC in the UTRA network part is more extensively used when multiple
links are allowed.
For efficient resource usage it is therefore necessary to allow the appropriate amount of
soft handovers in the system to maximise the s oft handover gain while keeping the increase
in interference due to multiple link connections below ac ceptable levels.
Another advantage of soft handover is its soft capacity. CDMA networks are not hard
capacity limited. This means that additional users cannot be hard blocked due to a
1
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- wohu0072012-11-20全英文的看的不是很懂啊
- victor7bo2012-12-06全英文的更容易让你明白嘛
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