Page 341 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 327

different zones on the time and frequency axis which are configured in a certain way to
meet different demands. This can be achieved with the following measures:
Ultra‐lean always‐on transmissions: For energy efficiency it has been decided to
limit always‐on transmissions such as the system information in the broadcast channel
as much as possible and to transfer them in such a way as to make reception as power
efficient as possible for mobile devices.
Self‐contained transmission: In LTE, control channels that transmit information
such as uplink and downlink assignments are spread over the complete bandwidth of a
channel. This has made it impossible, for example, to fully integrate the NB‐IoT air
interface into the overall LTE channel. Instead, NB‐IoT has its own separate control
channels and if deployed inside an overall LTE carrier as described earlier in this chap-
ter, it has to ignore the overlapping resource elements used for LTE. This is clearly not
ideal. As a consequence the new air interface localizes different types of transmissions
in the overall channel. This way, IoT devices that have to be very power efficient do not
have to observe the overall channel but only the parts in which data is transferred for
them. High‐speed devices also transfer their data in other parts of the channel and are
not required to understand how data is transferred in other parts of the channel. This
requires a redesign of reference signal transmissions which can no longer be evenly
distributed in the channel.
Flexible timing relations: In LTE there are fixed timing relations for many proce-
dures. For example, the LTE specification requires that a HARQ ACK/NACK is sent
after a specific number of subframes. While this delay was required due to the amount
of processing power available in mobile devices when LTE was designed, today’s devices
could send ACK/NACKs much sooner. They are prevented from doing so as the system
has to be backwards compatible. The new radio interface will address this limitation
from today’s perspective by avoiding such stringent timing relationships.
Scalable OFDM spacing: LTE was specifically designed for frequencies below
6 GHz. In this frequency range it was ideal to use Orthogonal Frequency Division
Multiplexing (OFDM) with a subcarrier spacing of 15 kHz. This subcarrier spacing
offered a good balance between limiting the delay spread occurring when data is trans-
mitted over a few kilometers, delay in channels with a maximum bandwidth of 20 MHz
and signal processing overhead. As 5G shall also tap into spectrum up to 100 GHz with
much shorter distances over which data is transmitted and channel bandwidths of hun-
dreds of MHz, a much larger subcarrier spacing of several hundred kHz is required. As
transmission distances are much shorter there is much less delay spread, which allows
such significantly larger subcarrier bandwidths. Larger subcarrier bandwidths will also
allow decrease of the individual symbol transmission times, which in turn further
reduces air interface latency, as will be further discussed below. Instead of 1 millisecond
(1000 microseconds) subframes, subframe lengths of 0.2 milliseconds (200 microsec-
onds) together with faster HARQ ACK/NACK responses can further reduce latency of
the air interface.
Better windowing of the OFDM signal: At each side of an LTE channel a rather large
guard band is required today. This is because of signal leakage at the cell edge which
produces interference for neighboring channels. To encapsulate individual transmis-
sions as described above and even to use different subcarrier spacings in a single chan-
nel, guard band requirements have to be significantly reduced. This can be achieved by
better windowing or filtering of the OFDM signal.