Page 330 - From GMS to LTE

316 From GSM to LTE-Advanced Pro and 5G

Like in LTE, narrowband variants of the reference signals as well as primary and sec-
ondary synchronization signals are used so mobile devices can detect the presence of an
NB‐IoT channel and synchronize to it.
User data and system information is transmitted over a shared channel, the
Narrowband Physical Downlink Shared Channel (NPDSCH). In the uplink direction its
equivalent is used for uplink user data and acknowledgment of data received in the
downlink direction.
The Narrowband Physical Downlink Control Channel (NPDCCH) is used for the fol-
lowing purposes:
Downlink Assignments: When data arrives for an NB‐IoT device at the eNode‐B the
●
downlink control channel is used to schedule downlink transmission assignments.
Each assignment includes the position and number of subframes that are dedicated on
the downlink shared channel for the subscriber, how often the data is repeated blindly
to improve the link budget and whether an acknowledgement is required from the
mobile device after receipt of the data. Unlike in LTE where a downlink assignment
references the current subframe, an NB‐IoT assignment applies with a delay of between
5 and 10 subframes between assignment and use of the downlink channel. Also, an
assignment can include more than just one subframe due to the very narrow channel
bandwidth. Another reason for this inter‐subframe scheduling is that two downlink
assignments can be broadcast per subframe. As only one data transmission can be
included in a subsequent subframe the two assignments must have different delays.
Uplink Grants: When the eNode‐B becomes aware that a device has data waiting in
●
its output buffer, e.g. via a buffer status information or random access request, it
schedules uplink transmission opportunities. Uplink data is always acknowledged by
the eNode‐B.
Paging: When downlink data arrives for a mobile device that is currently in RRC idle
●
state, the device must be paged. A typical paging interval is between one and two seconds.
Due to the narrow channel bandwidth, the NPDCCH is not transmitted in every sub-
frame in the first symbols but only once over many subframes and then takes the com-
plete subframe. The periodicity of this and other channels (e.g. the random access
channel periodicity in the uplink, which can be from 40 ms to 2.56s) is announced in the
system information so all mobile devices know when to observe the downlink for
potential assignments, uplink grants and Paging messages.
Like in LTE, System Information messages, referred to as SIB NB in NB‐IoT, are trans-
mitted over the downlink shared channel. The Master Information Block (MIB), however,
is sent separately. The MIB contains only 34 bits and is sent over a period of 640 ms with
many repetitions for robustness. It contains among other information the four most sig-
nificant bits of the System Frame Number, four bits that describe the size and location of
the SIB1‐NB, five bits containing a system information value tag and 1 bit to indicate if
access class barring is applied to limit system access to a subset of devices in case of over-
load. All other information is then taken from the System Information messages.

4.19.8 NB‐IoT Multicarrier Operation
To increase capacity, several NB‐IoT channels (carriers) can be configured per sector of
an eNode‐B. One of these carriers is then declared the anchor carrier and broadcasts all