Page 281 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 267

another tunnel is established between the Serving‐GW and the PDN‐GW. Data arriv-
ing for the mobile device can be immediately forwarded to the device. Data waiting to
be transmitted in the uplink direction can also be sent immediately, either over con-
tinuously allocated RBs on the uplink shared channel or, during times of lower activ-
ity, after a quick scheduling request via the uplink control channel. Furthermore, the
mobile device actively monitors the signal quality of the serving cell and the signal
quality of neighboring cells and reports the measurements to the network. The net-
work can then perform a handover procedure when another cell is better suited to
serve the mobile device.

Measurements for Handover
A handover is controlled autonomously by each eNode‐B and the eNode‐B also decides
if and when mobile devices should send measurement reports, either periodically or
event triggered. The standard is flexible in this regard so that different eNode‐B vendors
can use different strategies for measurement reporting. Measurement configuration
parameters are sent to the mobile device after an RRC connection has been established
as shown in Figure 4.19 with an RRC Connection Reconfiguration message.
While mobile devices can easily measure the signal quality of neighboring cells on
the same channel, transmission gaps are required to measure the signal quality of
LTE, UMTS and GSM neighboring cells on other channels. Such measurements
are thus only configured if the eNode‐B detects that the signal quality of the current
cell is decreasing and no other intrafrequency cell is available to take over the
connection.
Unlike in GSM, where only the Received Signal Strength Indication (RSSI) is used for
the decision, LTE uses two criteria. This is necessary as neighboring base stations trans-
mit on the same channel. A mobile device thus receives not only the signal of the cur-
rent serving cell but also the signals of neighboring cells, which, from its point of view,
are noise for the ongoing data transfer. In LTE, the following criteria are used to describe
the current reception conditions:
RSRP: The Reference Signal Received Power, expressed in dBm (the power relative to
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1 mW on a logarithmic scale). With this parameter, different cells using the same
carrier frequency can be compared and handover or cell reselection decisions can be
taken. For example, a strong and hence very good RSRP value equals −50 dBm on a
−9
logarithmic scale or 0.000001 mW (10 W) on a linear scale. A weak RSRP value,
which still allows reception in practice but at lower speeds, is −90 dBm, which equals
0.000000001 mW (10 −12 W) on a linear scale.
RSSI: The Received Signal Strength Indication. This value includes the total power
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received, including the interference from neighboring cells and other sources.
RSRQ: The Reference Signal Received Quality. It equals the RSRP divided by the
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RSSI. The better this value the better can the signal of the cell be received compared
to the interference generated by other cells. The RSRQ is usually expressed on a loga-
rithmic scale in decibel (dB) and is negative as the reference signal power is smaller
than the overall power received. The closer the negative value is to 0, the better the
RSRQ. In practice, an RSRQ of −10 results in very low transmission speeds. An RSRQ
of −3 or higher results in very good transmission speeds if the overall signal strength
(RSRP) of the cell is also high.