Page 342 - From GMS to LTE

328 From GSM to LTE-Advanced Pro and 5G

Slot aggregation: Today, LTE assigns uplink and downlink transmission resources on
a subframe basis. For very high throughput data transmissions in the future, such fine‐
grained scheduling might not be necessary. As a consequence, the new air interface will
likely include methods for slot aggregation, i.e. the network can assign transmission
resources to different devices over several subframes. This reduces signaling overhead.
TDD operation in higher bands: One of the shortcomings of LTE‐TDD today is that
uplink and downlink transmission opportunities have to be synchronized between dif-
ferent base stations as described earlier in this chapter to prevent downlink transmis-
sions from one cell interfering with uplink transmissions of a device in another cell. For
frequencies above 10 GHz this effect is much reduced. This is because such frequencies
will mostly be used for hotspots where high data traffic occurs while larger areas will
continue to be served by macrocells and carriers on lower frequency bands. At higher
frequencies it is thus possible to dynamically change the uplink downlink ratio as
required by traffic conditions, which reduces a major shortcoming of TDD in cellular
networks.
Beamforming: As signal attenuation rises with the frequency used, cellular networks
have so far made use of spectrum below 3 GHz. As there is little spectrum remaining in
this area to further extend capacity, future systems have to use spectrum well above
3 GHz. One method to compensate for signal attenuation that has not been exploited
so far due to its complexity is beamforming. The idea behind beamforming is to direct
the available signal energy in the direction where a mobile device is located, thus extend-
ing the range of the signal. Multi‐User MIMO (MU‐MIMO) takes the concept one step
further and combines multistream simultaneous transmissions to several users by
applying beamforming. MU‐MIMO uses the fact that in a typical cell, users are located
in different directions from the center of the cell and can thus be reached with inde-
pendent signal beams. In other words, MU‐MIMO techniques not only help to increase
the range of a signal but also increase overall throughput in a cell as several transmis-
sions can be made simultaneously.
One problem with beamforming is reaching more distant devices because system
information that is broadcast in a cell cannot be beamformed and sent to a particular
device as the cell is not aware of the location of each subscriber. One solution to this
problem is beam sweeping, i.e. the system information is broadcast periodically but
each time the beam is directed to a different part of the cell. Another solution is to send
the system information in a lower part of the spectrum in which beamforming is not
required to reach all devices located in the cell. A variation of this concept is to combine
a macro cell that operates in the lower frequency ranges with several small cells operat-
ing at higher frequency ranges. These are spread out so that their signals do not inter-
fere with each other and also do not reach all subscribers covered by the macro cell due
to the short range of their high frequency signals. In such a setup, system information
could be broadcast by the macro cell only, which is then informed by devices whether
they can receive one of the small cells as well. The macro cell can then coordinate uplink
and downlink transmissions to the device with that small cell. In effect the device could
send and receive data from the macro cell and the small cells simultaneously. Many of
these concepts have already been specified for LTE (Heterogeneous Networks, HetNet),
but are not used so far due to the fact that macro cells still provide sufficient capacity
and the complexity involved in rolling out small cells, providing fiber‐based connectiv-
ity to them and coordinating transmissions from a macro cell.