Page 282 - From GMS to LTE

268 From GSM to LTE-Advanced Pro and 5G

Network optimizations try to improve both the RSRP and RSRQ values. This means
that in as many places as possible there should always be only one dominant cell with a
strong signal. This means that the RSRP is high (e.g. −50 dBm) and the RSRQ is also
high (e.g. −3). If two cells are received with an equal signal power, the RSRPs of the two
cells might be strong while the resulting RSRQ for each cell is very low (e.g. −8), as the
signals interfere with each other.
In practice, both the RSRP and the RSRQ are used for handover decisions. On the one
hand, the observed neighbor cell should have a strong signal, that is, the received
reference signals should be strong. Hence, the RSRP should be high. On the other hand,
the interference should be as low as possible, which means that the quality expressed
with the RSRQ should be as high as possible. This is not always the case. At the edge of
the LTE coverage area, for example, the signal quality (RSRQ) might be high as there are
no or only weak neighboring LTE cells that interfere with the signal of an observed cell.
The RSRP of the cell, however, is very low owing to the high attenuation caused by the
distance between the mobile device and the cell. In such a scenario, it does not make
sense to hand over the connection to a cell that only has a better RSRQ if there are other
alternatives. Instead, the eNode‐B could decide to redirect the connection to a UMTS
access network if the reception conditions of such a cell are better. This is discussed in
more detail in Section 4.9.
The following list shows a number of LTE measurement events that are configured
by the network during connection setup or later on, for example, due to deteriorating
signal conditions:
For LTE to LTE handovers:
Event A1: The serving cell becomes better than a threshold value;
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Event A2: The serving cell becomes worse than a threshold value;
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Event A3: The neighbor cell becomes better than the serving cell;
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Event A4: The neighbor cell becomes better than a threshold value;
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Event A5: The serving becomes worse than a threshold value, neighbor becomes bet-
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ter than another threshold value.
For LTE to UMTS or GSM handovers:
Event B1: The inter‐RAT neighbor cell becomes better than a threshold value;
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Event B2: The serving cell becomes worse than threshold 1 and inter‐RAT neighbor
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becomes better than threshold 2.
Typically, B1 or B2 events are configured by the network once the LTE signal strength
reported by an A2 event falls to a very low level and no neighboring LTE cells are
reported with a stronger signal.
Measurements are configured by the eNode‐B with RRCConnectionReconfiguration
messages. The configuration consists of three parts:
Part 1 – Measurement Objects: When a radio connection is established, signal con-
ditions are good and the device is in the frequency band preferred by the network, only
the current LTE carrier frequency is typically configured as a measurement object. If
signal conditions are not ideal or if the device is on a lower‐priority LTE frequency
band, LTE carriers in several frequency bands are configured as measurement objects.
A transmission gap configuration is then required to enable the device to periodically
re‐tune to other frequencies to perform measurements. If no neighboring LTE cells can