Page 328 - From GMS to LTE

314 From GSM to LTE-Advanced Pro and 5G

the well over 500 pages of 3GPP Technical Report TR 45.820 [36]. In September 2015 a
compromise was reached and subsequently standardized as part of LTE. The details of
this decision were documented in the NB‐IoT work item description contained in
RP‐151621 [37].
Designed for ultra‐low‐cost devices where the modem shall cost less than $5 and
where datarates can be very low in exchange for power efficiency and deep in‐house
coverage, the solution makes a clean break from the mobile broadband approach of
LTE. An NB‐IoT channel is only 180 kHz wide, which is very small compared to mobile
broadband LTE channel bandwidths of 20 MHz or several times that much for devices
supporting carrier aggregation. Nevertheless, NB‐IoT uses Orthogonal Frequency
Division Multiplexing (OFDM) similar to LTE’s physical layer; the same 15 kHz subcar-
rier spacing; OFDM symbol duration, slot format and subframe duration; as well as the
RLC, RRC and MAC protocols standardized for LTE.
From a security point of view, LTE’s authentication and ciphering are also used for
NB‐IoT including the SIM card concept. Small devices might use embedded SIMs
(eSIM) which behave like ordinary SIM cards but are much smaller and are soldered
directly on the circuit board.
A major LTE feature that was left out on purpose in NB‐IoT are handovers in RRC
connected state. If an NB‐IoT device detects that it could be better served by another
cell it has to go to RRC idle state and reselect to the other cell. Also, NB‐IoT does not
support channel measurements and reporting. Both features have been deemed coun-
terproductive as the system has been optimized to transfer only very small chunks of
data. As a consequence there is little need to continue an ongoing data transmission in
another cell or to adapt the channel to changing signal conditions. This will be described
in more detail below. And finally, backwards compatibility to LTE, GSM or UMTS is
also not supported so an NB‐IoT device only needs to support the NB‐IoT part of the
specification.

4.19.5 NB‐IoT – Deployment Options
A channel bandwidth of 180 kHz has been selected as it allows a number of different
deployment scenarios in practice. One option is to deploy one or more NB‐IoT chan-
nels inside a larger LTE channel. The second option is to use the guard band of a full
LTE channel. And finally, a 180 kHz NB‐IoT channel can also replace one GSM channel.
All three deployment scenarios are transparent to non‐NB‐IoT devices, which means
that LTE devices (such as smartphones, tablets, etc.) that do not implement NB‐IoT
functionality simply do not see the NB‐IoT channel inside the main LTE channel or
slightly outside in the guard band. Legacy GSM devices also will not see an NB‐IoT
carrier if used alongside 180 kHz GSM carriers. Such devices will just see noise where
NB‐IoT is active.

4.19.6 NB‐IoT – Air Interface
In the downlink direction, the channel uses Orthogonal Frequency Division Multiplexing
(OFDM) with Quadrature Phase Shift Keying modulation (QPSK, two bits per trans-
mission step) or Binary Phase Shift Keying modulation (BPSK, 1 bit per step). MIMO
(Multiple Input Multiple Output) transmission is not used on an NB‐IoT channel.
Furthermore, there are twelve 15 kHz subcarriers, also referred to as ‘tones’, the same