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Preliminary design and user documentation of LTE initial cell selection

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Budiarto Herman
2013-07-02 10:23:16 +03:00
parent 46b47877e5
commit 7f81232309
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85
src/lte/doc/source/lte-design.rst
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@@ -1857,11 +1857,13 @@ The RRC model implemented in the simulator provides the following functionality:
- generation (at the eNB) and interpretation (at the UE) of System
Information (in particular the Master Information Block and, at the
time of this writing, only System Information Block Type 2)
time of this writing, only System Information Block Type 1 and 2)
- initial cell selection
- RRC connection establishment procedure
- RRC reconfiguration procedure, supporting the following use cases:
+ reconfiguration of the SRS configuration index
+ reconfiguration of the PHY TX mode (MIMO)
+ reconfiguration of UE measurements
+ data radio bearer setup
+ handover
- RRC connection re-establishment, supporting the following use
@@ -1943,6 +1945,87 @@ represented in Figure :ref:`fig-lte-enb-rrc-states`.
ENB RRC State Machine for each UE
Broadcast of System Information
+++++++++++++++++++++++++++++++
System information blocks are broadcasted by eNodeB to all attached UEs at
predefined time intervals. The supported system information blocks are:
- Master Information Block (MIB)
Contains parameters related to the PHY layer, generated during cell
configuration and broadcasted every 10 ms (but 40 ms in Section 5.2.1.2 of
[TS36331]_) as a control message.
- System Information Block Type 1 (SIB1)
Contains information regarding network access, broadcasted every 80 ms
via RRC protocol.
- System Information Block Type 2 (SIB2)
Contains UL- and RACH-related settings, scheduled to transmit via RRC
protocol at 16 ms after cell configuration, and then repeats every 80 ms.
Reception of system information is detrimental to the transition of UE RRC
state. SIB1 is necessary for switching from `IDLE_CELL_SELECTION` to
`IDLE_WAIT_SYSTEM_INFO`. After that, MIB and SIB2 are required to switch to
`IDLE_CAMPED_NORMALLY`.
.. _sec-initial-cell-selection:
Initial Cell Selection
++++++++++++++++++++++
UE in `IDLE_CELL_SELECTION` state, e.g. in the beginning of simulation, will
actively seek a suitable cell to attach to.
UE will perform the selection among the surrounding eNodeBs based on several
criteria:
- Rx level criterion;
- quality criterion;
- public land mobile network (PLMN), a.k.a. the network operator; and
- closed subscriber group (CSG).
The first pair of criteria, Rx level and quality, are based on measured cell
RSRP :math:`Q_{rxlevmeas}` and RSRQ :math:`Q_{qualmeas}`, respectively. These
measurements are collected by the PHY layer, as described in
:ref:`phy-ue-measurements`. In order to pass the criteria, both values must be
higher than required minimum RSRP and RSRQ, which can also be expressed as
below:
.. math::
Q_{rxlevmeas} - Q_{rxlevmin} > 0
.. math::
Q_{qualmeas} - Q_{qualmin} > 0
where :math:`Q_{rxlevmin}` and :math:`Q_{qualmin}` are determined by each
eNodeB, and are obtainable by UE from SIB1.
The last pair of criteria, PLMN and CSG, are simple numbers associated with each
eNodeB and UE. When these information are set to values beside their default
values, for example to simulate an environment with multiple network operators,
UE will restrict its cell selection attempts to target only eNodeBs with the
same PLMN ID and CSG ID. The default value of zero disables these criteria.
Section :ref:`sec-network-attachment` of user documentation provides more
details on multi-operator simulation.
When at least one suitable cells are found, the UE will choose the strongest one
(based on RSRP) and connect to it. This is done by issuing a contention-based
random access, followed by `RRCConnectionRequest`, thereby promptly switching
from IDLE mode to CONNECTED mode. Hence, `cell reselection` procedure is *not*
supported in this version of LTE module. (What happen if the eNodeB rejects the
connection request?)
On the other hand, when no suitable cell is found, the UE will stay in
`IDLE_CELL_SELECTION` state, and repeat the cell selection attempt again when
PHY layer provides another set of measurements.
Radio Admission Control
+++++++++++++++++++++++

106
src/lte/doc/source/lte-user.rst
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@@ -39,6 +39,8 @@ of practical examples.
.. _sec-basic-simulation-program:
Basic simulation program
------------------------
@@ -912,6 +914,110 @@ That's all! You can now start your simulation as usual::
.. _sec-network-attachment:
Network Attachment
------------------
As shown in the example in section :ref:`sec-basic-simulation-program`,
attaching a UE to an eNodeB is done by calling ``LteHelper::Attach`` function.
There are other methods of attachment as well, and this section will go through
each of them.
Manual attachment
*****************
This method uses the ``LteHelper::Attach`` function mentioned above, which had
been the only available method in earlier versions of LTE module. It is
typically invoked before the simulation begins::
lteHelper->Attach (ueDevs, enbDev); // attach one or more UEs to a single eNodeB
``LteHelper::InstallEnbDevice`` and ``LteHelper::InstallUeDevice`` functions
must have been called before attaching. In an EPC-enabled simulation, it is also
required to have IPv4 properly pre-installed in the UE.
This method is very simple, but requires you to know exactly which UE belongs to
to which eNodeB before the simulation begins. This can be difficult when the UE
initial position is randomly determined by the simulation script.
In real life, UE will automatically evaluate certain criteria and select the
best cell to attach to, without manual intervention from the user. Obviously
this is not the case in this ``LteHelper::Attach`` function. The other network
attachment methods use more `automatic` approach to network attachment, as will
be described next.
Attachment based on distance
****************************
Normally, UE aims to attach to the cell with the best received signal level
(i.e. the `strongest cell`). In order to determine the strongest cell, the
distance between the UE and the eNodeB may be used as a simple (at least from
simulator point of view) and sometimes practical indicator.
The ``LteHelper::AttachToClosestEnb`` function exists for this purpose. It takes
a ``NetDeviceContainer`` of UEs and another ``NetDeviceContainer`` of eNodeB,
and invokes an ``LteHelper::Attach`` to each UE and its closest eNodeB.
Note that this function does *not* take into account the direction and beamwidth
of the eNodeB receiver antenna. This can be problematic in scenarios such as
three-sectorized cells, where multiple eNodeBs must share the same exact
location.
One trick to mitigate this issue in a three-sectorized cell is to use the
``LteHexGridEnbTopologyHelper`` helper class to setup the eNodeB position and
antenna direction. Each eNodeB within the same site will be positioned a bit
away from the center of the site by a very small offset. This offset is by
default 50 cm, but can be configured by the `SectorOffset` attribute. This small
nudge will slightly vary the distance between the eNodeBs and the UE, making the
eNodeB with the right antenna direction more favorable for
``LteHelper::AttachToClosestEnb`` function.
These functions are invoked before simulation begins, as shown in the example
below (adapted from ``src/lte/examples/lena-dual-stripe.cc`` example)::
Ptr<LteHexGridEnbTopologyHelper> topologyHelper = CreateObject<LteHexGridEnbTopologyHelper> ();
topologyHelper->SetAttribute ("InterSiteDistance", DoubleValue (500));
topologyHelper->SetAttribute ("SectorOffset", DoubleValue (0.5));
topologyHelper->SetAttribute ("SiteHeight", DoubleValue (30));
topologyHelper->SetAttribute ("MinX", DoubleValue (0));
topologyHelper->SetAttribute ("MinY", DoubleValue (0));
topologyHelper->SetAttribute ("GridWidth", UintegerValue (1));
topologyHelper->SetLteHelper (lteHelper);
NetDeviceContainer enbDevs = topologyHelper->SetPositionAndInstallEnbDevice (enbNodes);
lteHelper->SetEnbAntennaModelType ("ns3::ParabolicAntennaModel");
lteHelper->SetEnbAntennaModelAttribute ("Beamwidth", DoubleValue (70));
lteHelper->AttachToClosestEnb (ueDevs, enbDevs);
Attachment based on received signal
***********************************
Sometimes distance does not make a correct criterion for determining the
strongest cell, for instance when the channel condition is fluctuating
because of the addition of fading or shadowing. In these kind of cases,
network attachment should be based on the received signal.
Such method in the current LTE module is implemented in the `initial cell
selection` process. It is automatically triggered by the UE when the previously
described network attachment methods are not invoked.
In more details, it works as follow. When a UE starts the simulation without
being attached to any eNodeB, it will spend some time to measure the signal
strength of the surrounding cells, choose the one with the strongest RSRP, and
then attempt to attach to it.
Even more details can be found in section :ref:`sec-initial-cell-selection` of
design documentation.
Multi-operator environment
**************************
An interesting use case of the initial cell selection process is to setup a
simulation environment with multiple network operators. (To be added later).
.. _sec-configure-ue-measurements:
Configure UE measurements