Page 252 - From GMS to LTE

238 From GSM to LTE-Advanced Pro and 5G

Synchronizing and requesting initial uplink resources is performed with a random
access procedure on the Physical Random Access Channel (PRACH). In most cases, the
network does not know in advance that a mobile device wants to establish communica-
tion. In these cases, a contention‐based procedure is performed as it is possible that
several devices try to access the network with the same Random Access Channel (RACH)
parameters at the same time. This will result in either only one signal being received or,
in the network, no transmissions being received at all. In both cases, a contention resolu-
tion procedure ensures that the connection establishment attempt is repeated.
Figure 4.12 shows the message exchange during a random access procedure. In the
first step, the mobile sends one of the 64 possible random access preambles on
the RACH. If correctly received, the network replies with a random access response on the
PDSCH which includes:

a timing advance value so that the mobile can synchronize its transmissions;
●
a scheduling grant to send data via the PUSCH;
●
a temporary identifier for the UE that is only valid in the cell to identify further
●
messages between a particular mobile device and the eNode‐B.
The mobile device then returns the received temporary UE identifier to the network.
If properly received, that is, only a single mobile tries to reply with the temporary UE
identifier, the network finalizes the contention resolution procedure with a Random
Access Response message.
During handovers, the new eNode‐B is aware that the mobile is attempting a random
access procedure and can thus reserve dedicated resources for the process. In this case,
there is no risk of several devices using the same resources for the random access pro-
cedure and no contention resolution is required. As this random access procedure is
contention‐free, only the first two messages shown in Figure 4.12 are required.
On the network side, the eNode‐B measures the time of the incoming transmission
relative to its own timing. The farther the mobile device is from the base station, the

Figure 4.12 Random access
Mobile eNodeB procedure.
device

Random access preamble
(PRACH)

Random access response
(PDSCH)

UE identification
RRC signaling, (PUSCH)

Random access response
RRC signaling, (PDSCH)