Signal Processing - March 2016 - 23

Normalized Transmitted
Data in Front−Haul

(BBUs) and the distributed
cell boundaries. Thus, the TR
remote radio heads (RRHs)
system has a simple reconfiguraBBUs
often becomes a bottleneck
tion property, allowing the easy
when there are a large number
addition of new APs to the sysof users/terminal devices (TDs)
tem. The newly added APs in
in the network. Several solutions
the TR system help pick up
have been proposed to tackle
users and reuse the same specFront-Haul
this challenge, such as signal
trum without incurring much inLink
compression and sparse beamterference, while in conventional
forming. However, in these
systems, intercell radio resource
schemes, the data transmitted in
management is needed to coorthe fronthaul is proportional to
dinate resource allocation beTD
T
the aggregated traffic of all TDs,
tween different cells and to limit
and the fronthaul link capacity
the intercell interference. Such a
can still be a bottleneck in a very
self-configuring feature provides
RRH
dense network.
native support for machine-toOn the other hand, if TR
machine (M2M) and D2D comtechnology is used as the air
munications where multiple figure 10. The C-RAN architecture.
interface in C-RAN, due to its
pairs of machines/devices can
unique spatial and temporal
coexist and share the spectrum
separation effects, all TDs are
without complicated transmis15
naturally separated by the
sion coordination strategies.
LTE Uplink and Downlink
location-specific signatures in
Figure 8 shows the perforTR Downlink
both downlink and uplink,
mance comparison in terms of
TR Uplink
11
and the baseband signals for
normalized achievable rate verall TDs can be efficiently
sus the number of APs, where
combined and transmitted.
the normalization is performed
6
This unique feature of TR
over the achievable rate of the
technology can be leveraged
single AP case. It can be seen
to create in essence a tunnelthat the normalized achievable
1
ing effect between the BBUs
rate of conventional systems
and the RRHs to alleviate the
remains unchanged regardless
1
6
11
15
Number of Terminal Devices
traffic load in the fronthaul
of the increase of the number of
link of C-RAN.
APs. This is because neighborThe data rate in the froning APs cannot use the same figure 11. A comparison of the normalized total transmitted data in a
thaul connecting the BBUs
resource in conventional sys- fronthaul between TR-based and LTE-based C-RAN.
and each RRH is only depentems due to the interference. On
seen that the performance degradation of
dent on the system bandwidth and the
the other hand, with the TR system, by
the IEEE 802.11 system is much more
number of bits used for every symbol,
utilizing the spatial focusing effect, all
severe than the TR system. This is
thus serving more TDs will not increase
APs use the same spectrum and thus the
because each link in 802.11 requires an
the traffic on the fronthaul link. Figure
normalized achievable rate increases as
exclusive use of the channel, which is
11 shows the performance comparison
the number of APs increases. Note that
inefficient if there are many users/APs
in terms of normalized total transmitted
although different APs share the same
close to each other. On the contrary, the
data versus the number of TDs, where
spectrum, they are nearly orthogonal with
TR system can tolerate interference
the normalization is performed over the
each other. In traditional systems, such
through the interference mitigation effect
transmitted data of the single TD case.
orthogonality can only be achieved by
of TR so that multiple users/APs can
The normalized total transmitted data in
applying additional techniques, such as
share the same spectrum. Therefore, the
the fronthaul in an LTE-based C-RAN
time, code, or frequency division multiperformance degradation is more graceincreases linearly with the number of
plexing. In the TR system, this nearful, and each user is more robust against
TDs, while that of a TR-based C-RAN
orthogonality is achieved naturally by
the interference from nearby users/APs.
almost keeps constant, showing that the
utilizing the large number of multipath
TR tunneling effect can deliver more
components in the wireless channel.
information in the C-RAN and alleviate
The performance degradation of each
Ideal for cloud-based networking
the burden of the fronthaul caused by
individual user is shown in Figure 9,
In most of the current C-RAN structures as
network densification, a feature essenwhere the normalization is over the
shown in Figure 10, the fronthaul link
tial to next-generation systems.
point-to-point link capacity. It can be
capacity between the baseband units
IEEE Signal Processing Magazine

March 2016

Table of Contents for the Digital Edition of Signal Processing - March 2016