SPECTRUM SENSING ON LTE FEMTOCELLS FOR
GSM SPECTRUM RE-FARMING USING XILINX FPGAs
Jörg Lotze (CTVR, Trinity College Dublin, Ireland. jlotze@tcd.ie);
Suhaib A. Fahmy (CTVR, Trinity College Dublin, Ireland. suhaib.fahmy@tcd.ie);
Barıs¸ Özgül (CTVR, Trinity College Dublin, Ireland. ozgulb@tcd.ie);
Juanjo Noguera (Xilinx Research Labs, Ireland. juanjo.noguera@xilinx.com);
Linda E. Doyle (CTVR, Trinity College Dublin, Ireland. linda.doyle@tcd.ie).
ABSTRACT
Femtocells are a promising solution to provide high coverage
and high data rates inside consumer’s homes while cutting op-
erator costs significantly. Next generation Long Term Evolu-
tion (LTE) femtocells are likely to be deployed in GSM spec-
trum, increasing frequency utilisation and allowing a smooth
transition to LTE. This paper proposes to use a spectrum sens-
ing technique specialised for LTE signals to avoid interfer-
ence between neighbour femtocells without operator inter-
vention. Simulation results of the detection characteristics
are given. A demonstrator has been implemented on a Xilinx
ML507 Virtex 5 prototyping board, using our flexible FPGA-
based cognitive radio framework, which allows to use run-
time reconfiguration of the FPGA to switch between sensing
and normal operation. It demonstrates the sensing algorithm
on a real platform.
1. INTRODUCTION
Femtocells are consumer-deployed small base stations that
use the Internet as backhaul to provide fast and exclusive cel-
lular services to consumers’ homes. They offer great poten-
tial to increase system capacity and coverage, and at the same
time minimise operational costs [1]. Autonomous deploy-
ment is essential since operators have no control over where
exactly a femtocell will be installed. It is required that fem-
tocells avoid interference with macrocells and neighbouring
femtocells, through careful power control and frequency al-
location.
It is likely that next generation Long Term Evolution
(LTE) systems will be deployed in vacant GSM spectrum to
maximise frequency reuse [2]. This is a perfect opportunity
for femtocells, enabling a smooth transition to LTE. Figure 1
illustrates a deployment scenario, where the frequency chan-
nels allocated to GSM macrocells B and C comprise the spec-
trum available for the operation of femtocells inside the area
covered by GSM macrocell A.
Unlike UMTS, where all base stations use the same
frequency bands and can be distinguished via code, LTE
requires exclusive frequency access. Therefore femtocells
within close proximity need to coordinate the use of spectrum
Fig. 1: GSM frequency re-use and femtocell deployment.
to avoid interference. However, due to autonomous setup,
femtocells do not know about the properties of other nearby
femtocells in advance and cannot coordinate with them.
An attractive solution to this problem is to avoid interfer-
ence by carefully controlling transmission power so as to only
just cover the user’s home. Yet, this method cannot guarantee
interference-free operation since the femtocell must also pro-
vide complete coverage in the user’s home. If the user places
the femtocell too close to an outside wall or window, it may
not be able to give full coverage while avoiding leakage to a
neighbour at the same time. Thus, an LTE femtocell needs to
detect if the frequency band it intends to use is already occu-
pied by another nearby femtocell before starting to operate.
This can be achieved by acting like a mobile handset and try-
ing to decode the neighbour’s control channel, but the signal
can be too weak for a reliable detection, while still likely to
cause interference.
A promising solution to this problem is spectrum sens-
ing. It allows a femtocell to detect the presence of neighbour-
ing femtocells without the need to decode their signals. It is
even possible to detect weak signals with this technique, to
guarantee interference-free operation.
Proceedings of the SDR ’09 Technical Conference and Product Exposition, Copyright c© 2009 SDR Forum, Inc. All Rights Reserved.

Sensing is required during initial system startup, and
very rarely during operation since the femtocells are not ex-
pected to change their operating frequency. Hence it is bene-
ficial to use the same hardware resources for both sensing and
normal operation modes, and reconfigure to one mode or the
other, as needed. Xilinx FPGAs allow the reconfiguration of
parts of the device while other parts continue to operate. This
capability can be leveraged to allow switching between sens-
ing and normal operation without wasting hardware resources
and power, and thus using a smaller device.
In this paper we describe a sensing technique suitable for
LTE femtocells as well as an implementation and demonstra-
tion of it on a run-time reconfigurable FPGA-based cognitive
radio platform. The sensing algorithm, along with detection
characteristics, is described in Section 2, and the hardware
implementation is discussed in Section 3. Section 4 draws
the conclusions.
2. SENSING LTE SIGNALS
The sensing algorithm in this paper is based on estimat-
ing time-averaged power spectral density (PSD), performing
moving average filtering and decision thresholding, followed
by an appropriate peak detection technique. The proposed al-
gorithm is tailored for sensing LTE signals and also takes into
account femtocell network frequency planning, as adopted by
the mobile operator, to improve detection performance and
reduce false alarms. In order to retrieve the frequency plan-
ning information, the femtocell basestation can decode the
macrocell basestation’s identity over the air after locking onto
the GSM broadcast control channel (BCCH) carrier available
in the GSM downlink. It can then use the decoded informa-
tion to interrogate an operator-specific database through its
Internet backhaul and obtain the frequencies and transmis-
sion bandwidth allowed for use by LTE femtocells. This is
why the algorithm discussed here is only designed to sense
LTE signal activity in these predefined frequencies; this im-
proves robustness and computational efficiency significantly.
In the remainder of this section, after giving brief information
on LTE signals, the proposed sensing algorithm is described
in detail and simulation results are presented to illustrate de-
tection characteristics.
2.1. LTE Physical Layer Properties
The LTE [3] downlink and uplink transmission schemes
are based on orthogonal frequency division multiple access
(OFDMA) and single carrier frequency division multiple ac-
cess (SC-FDMA), respectively. LTE supports several channel
bandwidth modes ranging from 1.4 MHz to 20 MHz. Scal-
able bandwidth is a property of LTE that makes it attrac-
tive for deployment in existing GSM uplink and/or downlink
bands which can reach 35 MHz and 75 MHz for GSM-900
and GSM-1800, respectively [4]. The basic LTE scheduling
unit in both downlink and uplink is called a resource block
(RB) and consists of 12 subcarriers with a spacing of 15 kHz
(corresponding to 180 kHz overall) in the frequency domain
and 6 or 7 consecutive OFDM symbols (SC-FDMA symbols
for the uplink) in the time domain. The number of available
RBs in the frequency domain varies depending on the chan-
nel bandwidth, which increases from 6 to 100 when the band-
width changes from 1.4 MHz to 20 MHz, respectively. In the
time domain, each RB spans a slot, with a duration equivalent
to 6 or 7 symbols (0.5 ms). Two slots correspond to a sub-
frame and 10 subframes typically form a frame (10 ms). LTE
supports both time division duplexing (TDD) and frequency
division duplexing (FDD). For TDD, a subframe within a
frame can be allocated to downlink or uplink transmissions.
In the case of FDD, because the downlink and uplink trans-
missions are separated in the frequency domain, there is no
allocation of subframes in time.
2.2. Sensing Algorithm
As explained in the beginning of Section 2, the spectrum
available to the LTE femtocell is divided into channels. Since
a femtocell basestation serves only a few home-based users
within a very short transmission range, the channel bandwidth
is likely to be the smallest possible LTE bandwidth, 1.4 MHz.
In the simulations and implementation we assume this band-
width, although the same algorithm can be applied to higher
LTE bandwidths. Using a small bandwidth is also preferable,
since it allows the allocation of more frequency channels in
the unoccupied GSM spectrum and, therefore, increases the
number of neighbouring LTE femtocells which can coexist
without mutual interference. In order to avoid such interfer-
ence, a femtocell basestation needs to detect the signal activ-
ity in the frequency channels used by the neighbouring fem-
tocells.
The sensing algorithm in this paper is based on signal de-
tection using a time-averaged PSD estimate. The key feature
that is making this sensing method reliable is the LTE-specific
control and synchronisation signalling which typically oc-
cupies 72 subcarriers (all available subcarriers in 1.4 MHz
mode, see Section 2.1) around the DC carrier. For instance,
regardless of TDD or FDD mode, corresponding subcarriers
of some OFDM symbols in particular downlink subframes
are always allocated, even with no user activity. These carry
data for certain LTE physical channels, such as data for the
physical broadcast channel or for primary and secondary syn-
chronisation signalling [3]. If we also consider the fact that
LTE or GSM signals are the only sources of activity in the re-
farmed GSM spectrum, we see that it is possible to obtain a
distinctive spectral shape in the presence of an LTE signal af-
ter sufficient time-averaging. It is also important to note that
a long time average is reasonable since sensing takes place
during initial system startup and very rarely during runtime.
The sensing algorithm for LTE femtocells has to discover
whether there is an LTE signal present in a particular chan-
nel. A flow-chart of the complete sensing algorithm is shown
Proceedings of the SDR ’09 Technical Conference and Product Exposition, Copyright c© 2009 SDR Forum, Inc. All Rights Reserved.