Page 338 - From GMS to LTE

324 From GSM to LTE-Advanced Pro and 5G

With this scheme, the macro cell coordinates with the small cells as to which of its
subframes it leaves empty in the time domain. The small cells will then use the empty
macro cell subframes for their own data transmission, thus avoiding interference from
the macrocell in those subframes and reciprocally not causing interference for devices
being served by the macro cell. Again, a balance has to be found between the gain of
reduced interference and the loss of transmission capacity in the macro cell and the
small cells operating in its coverage area because of the split of subframe resources. This
can be done, for example, by adapting the number of blank subframes to the amount of
data traffic being handled by the small cells, which requires interaction between the
macro cell and the small cells in its coverage area.

4.20.4 Coordinated Multipoint Operation
Release 8 ICIC and Release 10 eICIC are methods to enable neighboring cells to avoid
interference by coordinating when and where each base station transmits in the down-
link direction. In 3GPP Release 11, additional methods were defined to further improve
cooperation between different radio network elements. These methods are referred to
as Coordinated Multipoint Operation (CoMP).
For a better understanding of these methods, it is perhaps worthwhile to take a quick
look back at how UMTS reduces interference at the cell edge. Right from the start of the
first UMTS networks, the soft handover mechanism has been used to transmit a voice
data stream sent from several base stations simultaneously to a single user to improve
reception conditions. In the uplink direction, several base stations can pick up the signal.
This is possible because there is a central controlling element in UMTS, the RNC, which
forwards a downlink data packet to several NodeBs for transmission and receives the
same uplink data packet from several NodeBs in the uplink direction. Based on error
correction information, it can then select which of the copies to forward to the core
network. It is important to note, however, that the selection is based on packet level
rather than on individual bits or layer 1 RF signals. With the introduction of HSDPA for
data transmission, the concept had to be partly abandoned because the RNC no longer
controlled when and how data in the downlink direction is sent, as this responsibility was
moved to the NodeBs to improve the scheduling performance and to reduce the load on
the RNC. However, in the uplink direction, the soft handover approach continues to be
used for HSUPA (E‐DCH). For circuit‐switched voice calls, however, soft handover
mechanisms in both downlink and uplink directions are still used today.
In LTE, soft handover mechanisms have never been defined as all scheduling deci-
sions are based in the eNode‐Bs and because there is no central radio control instance
as in UMTS. As a consequence, only hard handovers are possible up to 3GPP Release
10. With CoMP in 3GPP Release 11, a number of mechanisms with an effect similar to
UMTS soft handover are introduced in LTE. They are not called soft handover, how-
ever, because CoMP multipoint transmission methods are based on the distribution of
ready‐to‐transmit RF signals rather than data packets that are modulated into an RF
signal at several base stations.
3GPP TR 36.819 [45] describes four network deployment scenarios for different
CoMP mechanisms. All mechanisms have in common that a central element is required
for controlling RF transmissions in the downlink and uplink directions. This is a new
concept in LTE where so far, the eNode‐Bs are largely autonomous: