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Hi
Folks,
There was some confusion regarding the definition of virtual soft handoff and it
would be beneficial if we can all come to the same understanding of these terms.
We can then discuss the implications of these scenarios and backbone messages
required to implement these scenarios. Some of the HO scenarios are listed
below. Please comments on these such that we can come to a common
understanding.
All scenarios refer to a SS which
due to some trigger (load balancing at BS, geographical movement of SS)
disconnects from one BS (primary BS) and moves to the secondary
BS.
1.
Hard Hand-Off:
The SS disconnects from the primary
BS and then reconnects to the secondary BS. At a time the SS is either connected
to the primary BS, not connected to any BS or connected to the secondary
BS.
2.
Make before break (MBB HO)
While the SS is connected to the
primary BS, the SS also connects to the secondary BS and then disconnects from
the primary BS. At a time the SS is either connected to the primary BS,
connected to both the primary and the secondary BS, or connected only to the
secondary BS.
3.
Virtual Soft Hand-Off
Same operation as in
(2).
4.
Soft Hand-Off:
Same operation as in
(2).
As can
be see scenarios 2,3 and 4 describe a very similarly situation. The difference
is in the type of macro diversity gain obtained.
According to my understanding:
1. Hard Hand-Off: provides no diversity
gain.
2. Make before break
(MBB HO) provides no diversity gain.
3. Virtual Soft Hand-Off provides diversity gain by
combining layer 2 information. This is achieved by multicasting the same
information through multiple BSs. On the uplink the diversity gain is obtained
by receiving the Layer 2 PDUs from both the primary and secondary BS and using
an appropriate combining algorithm.
4. Soft Hand-Off:
provides diversity gain by combining PHY information (e.g. MRC of the OFDM/OFDMA
signals). This is again obtained by simultaneous multicast of downlink
information from multiple BS to obtain the downlink diversity and MRC the
received OFDM signals from different BSs at the central RNC to obtain the uplink
diversity.
Virtual Soft Handoff and
soft handoff therefore places different requirements on the backbone messages to
be transferred and also the information which needs to be shared to achieve the
macro diversity gain.
Please comment on this
such that we have to same basic
understanding.
Regards,
-Raja
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