Digital Signal Processing Reference
In-Depth Information
Ta b l e 2 . 3
Transmission bandwidths in LTE
Channel bandwidth [MHz]
1
.
4
3
5
10
15
20
Number of resource blocks [NRB]
6
15
25
50
75
100
Fig. 2.4
Frequency planning
for inter-cell interference
(
white
denotes the central
region and the
shaded regions
have a less efficient frequency
reuse)
consists of two, in the time domain, consecutive resource blocks within a subframe.
Additionally, QPSK is used for all control information and data can be transmitted
with different modulations like QPSK, 16-QAM, and 64-QAM.
Thanks to the flexible resource allocation in time, frequency and space, and the
scalable bandwidths supported by LTE, it is expected that at least 200 users per
cell should be supported in the active state for a bandwidth of up to 5 MHz and at
least 400 users for higher bandwidths. More users are expected to be supported in a
passive (sleeping) state [26].
In a 20 MHz bandwidth, which is the maximum allowed bandwidth, up to 100
Resource Blocks (RB) could be allocated to different users (Table
2.3
). A RB is
the minimum frequency-time unit allocated to a single user and consists of 12 sub-
carriers. Assuming 400 active users per cell should be supported, each user should
be allocated 1 RB every 4 consecutive sub-frames (1 ms TTI). The throughput per
user in this worst-case scenario will be around 250 kbps. This throughput is for the
downlink (DL) direction and will slightly decrease when considering the overhead
of the control information.
The cell-edge performance is a more challenging task since inter-cell interference
can degrade the system capacity. A way to limit this interference is through inter-
cell interference coordination, which can be achieved by restricting the transmission
power in different parts of the spectrum in combination with a flexible frequency
planning. A possible flexible frequency planning is a soft frequency reuse scheme
which is characterized by a frequency reuse factor 1 in the central region of a cell,
and a less efficient frequency reuse near the cell edge as in Fig.
2.4
.
When the mobile station (or UE) is near the base station (or eNB), the received
power of the user signal is strong, and the interference from other cells is weak. So
at the inner part of the cell, all the sub-carriers can be used to achieve high data rate
communication (Fig.
2.5
).
