Page 300 - From GMS to LTE

286 From GSM to LTE-Advanced Pro and 5G

4.12.1 Single Frequency Network
Like UMTS and CDMA, the LTE radio access network reuses the same carrier frequencies
for all cells, which can have a bandwidth of up to 20 MHz. In some bands, 20 MHz
channels might not be feasible, however, for a number of reasons:
Not enough spectrum is available because several network operators share the available
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spectrum in a small band. An example is band 20, the European digital dividend band.
As shown at the beginning of this chapter in Table 4.2, only 30 MHz is available for
each direction. If used by more than two operators, the maximum channel bandwidth
per network operator is 10 MHz at best.
Certain bands are not suitable for 20 MHz channels, for example, because of a narrow
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duplex gap between uplink and downlink. This makes it difficult for filters in mobile
devices to properly separate the uplink and the downlink data streams in the
transceiver.

4.12.2 Cell‐Edge Performance
Owing to neighboring cells using the same channel, mobile devices can receive the sig-
nals of several cells. While they are close to one cell, the signals of other cells are much
lower and hence their interference is limited. When a mobile device is at the center of
the coverage areas of several cells, however, two or even more cells might be received
with similar signal strength. If all cells are also heavily loaded in the downlink direction,
the resulting interference at the location of the mobile device can be significant. The
resulting datarate in the downlink direction for this particular user is then very limited
because a robust modulation and coding scheme with good error protection has to be
used. This also impacts the overall capacity of the cell as more time has to be spent
transmitting data to devices at the cell edge at low speed, which cannot then be used to
send data much faster to devices that experience better signal quality.
To improve cell‐edge performance and the overall throughput of an eNode‐B and the
radio network in general, a Load Indication message has been defined in 3GPP TS
36.423 [8] for Inter‐cell Interference Coordination (ICIC). As eNode‐Bs autonomously
decide how they use their air interface, the X2 interface can be used to exchange inter-
ference‐related information between neighboring eNode‐Bs, which can then be used to
configure transmissions in such a way as to reduce the problem.
To reduce interference in the downlink direction, eNode‐Bs can inform their neigh-
bors of the power used for RBs on the frequency axis. This way, an eNode‐B could, for
example, use the highest power only for a limited number of RBs to serve users at the
edge of the cell, while for most other RBs, less power would be used to serve users that
are closer to the center of the cell. As neighboring eNode‐Bs cannot directly measure
the strength of downlink transmissions from neighboring cells, they can use this infor-
mation as an indicator of the RBs in which there is likely to be high interference at the
edge of the cell and hence schedule a different set of RBs for use at the edge of the cell.
This in effect creates two‐tiered cells, as shown for a simplified two‐cell scenario in
Figure 4.27. Such a scheme is also referred to as Fractional Frequency Reuse (FFR) as
only a non‐overlapping fraction of the spectrum is used to serve cell‐edge users.
In the uplink direction, an eNode‐B can measure interference from mobile devices
communicating with another eNode‐B directly and take measures to avoid scheduling