NS-3 GSoC project (LTE CA handover) update

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NS-3 GSoC project (LTE CA handover) update

Alexander Krotov-2
I have just pushed a test suite to my GSoC repo with a test for handover
between component carriers other than first one:
https://bitbucket.org/labdsf/ns-3-dev/commits/3584f971905367e8b471e6f4872b2769be764bf8

It is inter-eNB (aka X2) handover only for now.

What I did so far:
1. Aggregated information about UE in eNB component carrier manager into
one structure. This makes it easier to change what information about UE is
stored. I expected that I will need to store information about which
component carriers are enabled on UE, but apparently it is not needed yet.
Primary goal is to make HO work with CA, so component carrier manager is ok
with just enabling all other cells served by the same eNB as SCC.

2. Added API to detach PHY from the channel. It is required to avoid PHY
previously attached to SCC from receiving signals which are transmitted on
the same frequency when UE is attached to PCC on the same frequency.

3. Fixed a bug which resulted in PHY dividing signals with different
SpectrumModels:
https://bitbucket.org/labdsf/ns-3-dev/commits/610318f10ae8fbafbd40035a0789953d028dd562
The problem arises sometimes when PHY is changing frequency on which it
operates, that is any inter-frequency handover.

4. Make eNBs identifiable by any of its cell id within EPC:
https://bitbucket.org/labdsf/ns-3-dev/commits/db74ffdafa56cda6fdf96e5f3cc841da4d36d0ee
It is required to send HO messages over X2, for example. Each eNB still has
one MAC address, all interfaces are multiplexed and it does not really
matter which cell ID is used. That is why I use .at (0) to get any cell ID
when I need to fill "sourceCellId" field.

According to X2 specification (
http://www.etsi.org/deliver/etsi_ts/136400_136499/136423/11.02.00_60/ts_136423v110200p.pdf
) there is no such thing as source cell. There is only a "target cell"
field, but it is put into RRC message and not used for addressing. Messages
are exchanged between eNBs, which are identified by their addresses.

In theory each cell can serve up to 255 cells. In NS-3 each eNB had exactly
one cell, that is why, for example, LteHexGridEnbTopologyHelper creates 3
eNBs to serve 3 co-located cells and handover between them happens over X2
interface.

5. Added a test suite which tests all 1CC and 2CC cases of inter-eNB
handover. Now going to add intra-eNB testcases and make them work correctly.
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