Page 339 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 325

Scenario 1: Homogeneous Network – Intrasite CoMP: In this scenario, interference
caused by the overlapping coverage of two sectors of a single base station is addressed.
No optical cables are required in this scenario but the interference reduction effect is
limited to the overlapping areas of the sectors of a single cell and is not effective for the
overlap areas between different cell sites.
Scenario 2: Homogeneous Network – RRHs: The first approach to reduce cell‐edge
interference is to transmit a single RF signal to several RRHs. From a mobile device’s
point of view, only a single cell is visible. Owing to the high loss of modulated RF signals
even over coaxial copper cables, the RF signals have to be sent over optical links, which
is a major challenge in practice. On the RRH side, the optical signals are converted to
and from an electromagnetic signal.
Scenarios 3 and 4: Heterogenous Network Scenarios: A high‐power signal from a
macro cell is complemented by low‐power signals of small cells. These are either visible
to the mobile device as individual cells, that is, small cells with their own physical cell‐
IDs, or act as low‐power RRHs in a way that is transparent to the mobile device.
In the downlink direction, the following methods have been defined in the CoMP
work item: firstly, with Joint Transmission (JT), the centralized scheduler can use sev-
eral distributed RRHs to transmit a single signal to the mobile device, thus increasing
the signal quality at the location of the mobile device. Secondly, a somewhat simpler
scheme is Dynamic Point Selection (DPS), that is, the scheduler can quickly assign dif-
ferent antennas in its distribution set for a mobile device depending on its changing
location. And thirdly, a Coordinated Scheduling/Coordinated Beamforming (CS/CB)
method has been defined to shape a transmission via several antennas in such a way as
to concentrate the downlink signal energy in a beam to increase the received signal at
the location of the mobile. This increases the signal strength there while reducing inter-
ference in other coverage locations.
In the uplink direction, two methods have been defined: firstly, in Joint Reception (JR)
mode, a signal can be received at several locations and forwarded to a central process-
ing unit. Again, this requires optical cabling. And secondly a Coordinated Scheduling/
Coordinated Beamforming approach has been defined to tune the signal energy in the
direction of the recipient.
In situations with high uplink interference, it can be beneficial if a small cell receives
the uplink signal while the downlink signal continues to be sent from the macrocell.
Whether CoMP features will be deployed in practice in the future depends on their
effectiveness. The CoMP study in the TR mentioned above comes to the conclusion
that datarates could be improved by between 25% and 50% for mobiles at cell edges with
neighboring cell interference.

4.21 From LTE to 5G

When LTE was initially designed, the main use case was high‐speed Internet access,
often also referred to as Mobile Broadband (MBB). In practice this meant designing a
system that would transport IP packets on all interfaces as fast as possible and designing
an air interface with a low latency when moving between idle and connected state. In
the meantime technology has evolved and many other applications are now also dis-
cussed. These applications often have very diverse requirements for mobile networks