soft handover

1. Why soft handover no longer exist in LTE?
This morning I came across this question in my mind while driving to the office. The reason
seems to be obvious, since there is no longer a RNC in E-UTRAN, data streams from two
active sets can't be combined unless ePC plays a part (or one eNB serves as SRNC),
which is clearly undesirable. However, this topic worth to be looked into a little detail.
The root cause of having soft-handover in CDMA systems is that, in CDMA, each channel is
an interference to other channels, therefore transmit power control is key to maximize
system capacity. Especially at cell edge, a high transmission power could block other UEs
from accessing the network. Here is where soft-handover can play its part. It allows
transmission on different links therefore adds Rx diversity gain. During soft-handover, two
NodeBs transmit/receive the same date stream on different physical channels. The data
from different active set will be combined at UE (downlink) or at RNC (uplink). As such, the
bit stream can be decoded much more reliably than if only one NodeB were
transmitting/receiving. In other words, without soft-handover, to achieve the same SINR,
higher power is required from a NodeB or from UE, which is undesirable.
In LTE, due to the orthogonal on both downlink and uplink within a cell, power control is not
as important as in 3G. Therefore, soft-handover can be dropped from the system design
without having much penalty to the system performance. This reduces network architecture
complexity.
To support soft-handover, Iur is mandatory in case of different NodeBs involved in the soft
handover are not controlled by the same RNC. In this case, the SRNC will be responsible to
combine the data flow and DRNC only controls the NodeB involved in the soft-
handover. The cost of having an Iur interface is significant especially if operators have multi-
vendors RNC. 3GPP removed RNC node from E-UTRAN therefore simplified
the architecture. There is however a X2 interface, which is similar to Iur but not the same.
X2 can be used to forward packets in case of a handover simply for lossless mobility. But
unlike an Iur interface, which have to support much more complex signalling, the X2 is a lot
simpler.
I shall compare X2 with Iur with detail sometime later.