Technical Field of the Invention
The invention relates to communication networks, and in particular to a method in a
5 first device for estimating the quality of a signal transmitted from a second device to a
third device.
Background to the Invention
Femtocell base stations in a Long Term Evolution (LTE) communication network
10 (otherwise known as Home evolved Node Bs - HeNBs - or Enterprise evolved Node
Bs - EeNBs) are small, low-power, indoor cellular base stations for residential or
business use. They provide better network coverage and capacity than that available
in such environments from the overlying macrocellular LTE network. In addition,
femtocell base stations use a broadband connection to receive data from and send
15 data back to the operator's network (known as "backhaul").
As femtocell base stations can make use of the same frequencies as macrocell base
stations in the macrocellular network, and as they are located within the coverage area
of one or more macrocell base stations in the macrocellular network, it is necessary to
20 ensure that downlink transmissions from the femtocell base station to mobile devices
(otherwise known as User Equipments - UEs) using the femtocell base station do not
interfere substantially with downlink transmissions from macrocell base stations to
mobile devices using the macrocell base stations.
25 Typically, this interference is mitigated by placing a cap on the power that the femtocell
base station can use to transmit signals to mobile devices. The cap on the power can
be set such that, at a specified pathloss from the femtocell base station (for example 80
dB), a signal received by a mobile device from a macrocell base station would meet a
specified quality level (for example a target signal to interference plus noise ratio -
30 SINR). The determination of the cap is subject to a minimum and maximum power
restriction on the transmission power of the femtocell base station, for example 0.001
W and 0.1 W respectively.
However, this approach has limitations in that the transmission power of the femtocell
35 base station will be capped regardless of whether there are any mobilejevtcss negrjo
the femtocell base station that are communicating with a macrocell base station and
?
that need protecting. This cap can lead to the data throughput for mobile devices
communicating with the femtocell base station being unnecessarily restricted.
In providing an approach for setting the maximum permitted transmission power for
5 downlink transmissions from femtocell base stations, it is necessary for the femtocell
base station to determine if there are nearby mobile devices that need protecting.
Therefore, there is a need for a method in which the femtocell base station can
determine the quality of signals being transmitted from a mobile device to another base
10 station.
Summary of the Invention
Therefore, according to a first aspect of the invention, there is provided a method of
estimating a quality of a signal, the method in a first device comprising measuring a
15 signal transmitted from a second device to a third device; determining a value of a
metric from an autocorrelation function of the measured signal; and detemnining an
estimate of the quality of the signal from the determined metric.
Preferably, the step of measuring compnses measuring the signal in the time domain,
20 and the step of detemnining a value of a metric comprises determining the
autocorrelation function of the time domain signal and noise.
Preferably, the step of determining a value of a metric comprises deteimining the
autocorrelation function comprises normalising the measured signal to give a sequence
25 r; taking the fast Fourier transform of this sequence to give f; determining the squared
magnitude of each sample in f; and taking the inverse fast Fourier transform of the
sequence resulting from the step of determining the squared magnitude to give an
autocorrelation sequence a.
30 In a preferred embodiment, the step of determining a value of a metric from an
autocorrelation function of the measured signal comprises calculating the magnitude or
squared magnitude of the autocorrelation function.
Preferably, the step of determining a value of a metric further comprises adjusting or
35 zeroing the central tap in the output of the step of calculating.
3
In a further embodiment, the step of determining a value of a metric further comprises
adjusting or zeroing the tap adjacent the central tap in the output of the step of
calculating.
5 In one embodiment, the step of determining a value of a metric comprises identifying
the tap with the largest magnitude or squared magnitude in the taps remaining in the
output of the step of calculating; and setting the metric to the value of said magnitude
or said squared magnitude of the identified tap.
10 In one embodiment, the step of determining a value of a metric further comprises
adjusting the value of the metric based on the distance of the identified tap from the
central tap.
In another embodiment the step of determining a value of a metric further comprises
15 adjusting the value of the metric based on a function of a peak to average power ratio
of the measured signal.
In this embodiment, the step of adjusting the value of the metric based on a function of
a peak to average power ratio of the measured signal preferably comprises, in the
20 event that the peak to average power ratio of the measured signal is below a threshold
value, adjusting the value of the metnc to a minimum value.
In one embodiment, the step of determining an estimate of the quality of the signal from
the determined metric comprises comparing the determined metric to a look-up table.
25
In an alternative embodiment, the step of determining an estimate of the quality of the
signal from the determined metric comprises using a curve-fitting technique to match
the determined metric to a predetermined relationship between values for the metric
and the quality of the signal.
30
Preferably, the step of measuring comprises measuring a Zadoff-Chu reference signal
transmitted from the second device to the third device, and the quality of the signal is a
signal to noise ratio,
35 Preferably, the step of measuring a Zadoff-Chu reference signal comprises estimating
the position of the Zadoff-Chu reference signal in time.
«
Preferably, the step of measuring comprises measuring a portion of the Zadoff-Chu
reference signal.
5 In one embodiment, the method further comprises the step of using a scheduler to
ensure that no signals will be transmitted to the first device from other devices
associated therewith that might interfere with the execution of the step of measuring.
According to a second aspect of the invention, there is provided a network element for
10 use in a communication network, the network element being configured to perform the
method described above.
Brief Description of the Drawings
The invention will now be described in detail, by way of example only, with reference to
15 the following drawings, in which:
Figure 1 shows an exemplary communication network;
Figure 2 is a flow chart illustrating a method of setting a maximum permitted
20 transmission power for a femtocell base station;
Figure 3 is a flow chart illustrating the method of Figure 2 in more detail;
Figures 4(a) and 4(b) are graphs illustrating the autocorrelation function for time
25 domain reference signals with low and high signal to noise ratios respectively;
Figure 5 is a graph illustrating a plot of autocorrelation function peaks against signal to
noise ratio;
30 Figure 6 is a graph illustrating a plot of peak to average power ratios against signal to
noise ratio;
Figure 7 is a graph illustrating a plot of autocorrelation function peaks against signal to
noise ratio in which the scatter has been reduced;
35
5
Figure 8 is a flow chart illustrating a method of estimating a signal quality of a reference
signal in an uplink in accordance with an exemplary embodiment of the invention;
Figure 9 is a graph illustrating the change in throughput on a macrocell downlink
5 against femtocell base station density in a macrocell sector;
Figure 10 is a graph illustrating the change in throughput on a macrocell downlink
against femtocell base station density for a user equipment at the edge of the
macrocell;
10
Figure 11 is a graph illustrating the change in throughput on a femtocell downlink
against femtocell base station density; and
Figure 12 is a graph illustrating the change in throughput on a femtocell downlink
15 against femtocell base station density for a user equipment at the edge of the
femtocell.
Detailed Description of the Preferred Embodiments
Although the invention will be described below with reference to an LTE communication
20 network and femtocell base stations or HeNBs, it will be appreciated that the invention
is applicable to other types of third or subsequent generation network in which
femtocell base stations (whether for home or business use), or their equivalents in
those networks, can be deployed Moreover, although in the embodiments below the
femtocell base stations and macrocell base stations use the same air interface (LTE), it
25 will be appreciated that the invention can be used in a situation in which the macrocell
and femtocell base stations use the same or corresponding frequencies but different air
interface schemes (for example the macrocell base stations could use WCDMA while
the femtocell base stations use LTE),
30 Figure 1 shows part of an exemplary communication network 2 in which the invention
can be implemented. The communication network 2 includes a plurality of macrocell
base stations 4 (only one of which is shown in Figure 1) that each define a respective
coverage area - indicated by macrocell 6. In an LTE communication network, the
macrocell base stations 4 are referred to as evolved Node Bs (eNBs)
•
6
One or more femtocell base stations 8 (Home eNBs - HeNBs) can be located within the
coverage area 6 of the macrocell base station 4 (although only one femtocell base
station 8 is shown in Figure 1), with each femtocell base station 8 defining a respective
coverage area - indicated by femtocell 10.
5
It will be appreciated that Figure 1 has not been drawn to scale, and that in most realworld
implementations the coverage area 10 of the femtocell base station 8 will be
significantly smaller than the coverage area 6 of the macrocell base station 4.
10 A number of mobile devices (UEs) 12 are also located in the communication network 2
within the coverage area 6 of the macrocell base station 4.
Four mobile devices 12a, 12b, 12c and 12d are each associated with the macrocell
base station 4, meaning that they transmit and/or receive control signalling and/or data
15 using the macrocell base station 4. It will be noted that although the mobile device 12d
is also within the coverage area 10 of the femtocell base station 8, it is associated with
the macrocell base station 4 (this could be due to the signal strength of the macrocell
base station 4 being significantly better for mobile device 12d than the signal strength
of the femtocell base station 8 or the femtocell base station 8 could be restricted to
20 specific subscribers that don't include mobile device 12d, etc.). Mobile devices 12a,
12b, 12c and 12d are referred to collectively herein as "macro-UEs", as they are the
mobile devices/user equipments (UEs) associated with the macrocell base station 4.
Two further mobile devices, 12e and 12f, are located within the coverage area 10 of the
25 femtocell base station 8 and are currently associated with the femtocell base station 8,
meaning that they transmit and/or receive control signalling and/or data using the
femtocell base station 8 Mobile devices 12e and 12f are referred to collectively herein
as "femto-UEs", as they are the mobile devices/user equipments (UEs) associated with
the femtocell base station 8.
30
As described above, it is necessary to ensure that the downlink transmissions from the
femtocell base station 8 to the femto-UEs 12e and 12f do not prevent nearby macro-
UEs (such as macro-UE 12d) from being able to successfully receive downlink
transmissions from the macrocell base station 4 A similar requirement exists for a
35 mobile device that is associated with another femtocell base station, in that the
downlink transmissions from the femtocell base station 8 to the femto-UEs 12e and 12f
7
should not prevent those mobile devices from successfully receiving the downlink
transmissions from their femtocell base station.
As described above, this problem is addressed in conventional networks^ app|yjpg_a
5 cap to the transmission power used by femtocell base stations 8 to transmit signals to
femto-UEs. This cap is set to a value that prevents these downlink signals from
causing an undesirable level of interference to mobile devices that are not associated
with the femtocell base station 8 that are in or near the coverage area 10 of the
femtocell base station 8 (such as mobile device 12d in Figure 1). This cap is applied to
10 the transmission power regardless of whether there are any mobile devices in or near
the coverage area 10 of the femtocell base station 8 (so it would be applied, for
example, even if mobile device 12d was not present).
However, as illustrated in Figure 2, it is determined whether there are any mobile
15 devices that are not associated with the femtocell base station 8 that require protection
from interference caused by downlink transmissions of the femtocell base station 8
(step 101), and the transmission power cap for the femtocell base station 8 is set
accordingly (step 103).
20 A more detailed method of operating a femtocell base station 6 is illustrated in Figure 3.
In Figure 3, steps 111, 113, 117 and 119 correspond to the step of determining (step
101) in Figure 2.
In the following, although the method will be described with reference to protecting
25 mobile device 12d (i.e. a macro-UE) that is associated with macrocetl base station 4
from downlink transmissions from the femtocell base station 8, it will be appreciated
that a similar method can be used to protect a mobile device that is associated with
another femtocell base station.
30 In step 111. the femtocell base station 8 attempts to identify if there are any macro-UEs
12 that are receiving downlink transmissions from a macrocell base station 4.
In LTE, macro-UEs 12 transmit information to the macrocell base station 4 before,
during or after the receipt of a downlink transmission from the macrocell base station 4,
35 for example an acknowledgement (ACK/NACK) signal, a channel quality indicator
(CQI), sounding signals, data signals, etc. Therefore, the femtocell base station 8 can
8
monitor uplink channel(s) used by the macro-UEs for these transmissions to determine
if there are any mobile devices nearby that might need protecting from its downlink
transmissions.
5 In step 113, it is determined whether any signals detected in step 111 originate from
mobile devices that are not being served by (or associated with) the femtocell base
station 8.
If the femtocell base station 8 does not detect any signals from macro-UEs 12, then the
10 femtocell base station 8 can assume that there are no macro-UEs nearby that need
protecting from its downlink transmissions. In this case, in step 115, the maximum
permitted transmission power for the femtocell base station 6 can be set to a high or
relatively high value, for example an upper limit for the transmission power (such as 0.1
W in LTE). The method then returns to step 111 and repeats periodically.
15
If the femtocell base station 8 does detect signals from macro-UEs 12, then the method
moves to step 117 in which the femtocell base station 8 estimates a quality of a
detected signal. This quality can be a signal to noise ratio (SNR), a signal to noise plus
interference ratio (SNIR), a signal strength, or any other measure of the quality of a
20 transmitted signal. In some implementations, depending on the way in which the
femtocell base station 8 detects signals in the uplink, the femtocell base station 8 may
be able to distinguish signals from multiple macro-UEs 12 and can estimate the quality
of each of the signals However, in alternative implementations, the femtocell base
station 8 may not be able to distinguish the signals and therefore performs the
25 estimation on the signal with the highest quality.
In a preferred embodiment of the invention, the femtocell base station 8 identifies
characteristics of the Zadoff-Chu reference signal and estimates the signal to noise
ratio (SNR) of this signal. This embodiment is described in more detail below with
30 reference to Figure 4 It will be noted that in this embodiment the femtocell base
station 8 does not distinguish between signals from multiple macro-UEs 12 and
therefore estimates the SNR for the signal with the highest quality.
In an alternative implementation, the femtocell base station 8 detects and decodes the
35 data in the uplink and determines a quality of the data signals. It will be appreciated by
g
those skilled in the art that alternative techniques can be used by the femtocell base
station 8 to determine a quality of the signals in the uplink.
The femtocell base station 8 then compares the estimated quality (or the highest
5 estimated quality if the femtocell base station 8 can estimate the quality for multiple
signals) with a threshold value (step 119) In a preferred implementation where the
quality is a signal to noise ratio, the threshold can be a value in the range of 10 dB to
30 dB.
10 It will be noted that a macro-UE 12 will need most protection from the downlink
transmissions of the femtocell base station 8 when it is near to the edge of the
coverage area 6 of the macrocell base station 4, as the downlink signals received at
the macro-UE 12 from the macrocell base station 4 will be relatively weak. In this
situation, the macro-UE 12 will need to be transmitting its uplink signals at a relatively
15 high power (due to its distance from the macrocell base station 4). By estimating a
quality of the uplink signal (which will be affected by the transmission power of the
macro-UE 12d and its proximity to the femtocell base station 8), the femtocell base
station 8 can determine whether, and/or the extent to which, the macro-UE 12d needs
protecting from the downlink transmissions of the femtocell base station 8.
20
Therefore, if the estimated quality exceeds the threshold value then the femtocell base
station 8 assumes that the macro-UE 12d that originated the signal needs significant
protection from the downlink transmissions of the femtocell base station 8, and the
maximum permitted transmission power for the femtocell base station 8 should be set
25 at a low or relatively low value (step 121). For example, the maximum permitted
transmission power can be set to a lower limit for the transmission power (such as
0.001 Win LTE).
In one implementation, the femtocell base station 8 sets the maximum permitted
30 transmission power such that, at a specified pathloss from the femtocell base station 8
(for example 80 dB), a signal received by the macro-UE 12d from the macrocell base
station 4 meets or is estimated to meet a specified quality level (for example a target
signal to interference plus noise ratio - SINR), as in a conventional network,
35 The method then returns to step 111 and repeats periodically.
10
If the estimated quality does not exceed the threshold value then the femtocell base
station 8 sets the maximum permitted transmission power to an intermediate value that
lies between an upper and lower limit for the transmission power (step 123). Thus, the
femtocell base station 8 provides some protection for the macro-UE 12d, while allowing
5 downlink transmissions from the femtocell base station 8 to be transmitted at a higher
power than conventional techniques permit. In this way, the data throughput for femto-
UEs 12e and 12f can be improved over the conventional technique.
In a preferred implementation, the intermediate value for the maximum permitted
10 transmission power is selected based on the difference between the estimated quality
of the signal and the threshold value. In particular, the value for the maximum
permitted transmission power can increase in proportion to the difference between the
estimated quality of the signal and the threshold value (up to an upper limit, if
applicable). In a preferred embodiment where the quality is a signal to noise ratio, if
15 the estimated SNR is 5 dB below the threshold value, then the maximum permitted
transmit power can be set to be 5 dB above the low or relatively low value, subject to
the upper limit on the maximum permitted transmit power.
Again, the method returns to step 111 and repeats periodically,
20
In one implementation of the invention, steps 113 and 117 can be combined, in that the
femtocell base station 8 estimates a quality (such as the SNR) of a signal in the uplink
and if the estimated quality is above a particular threshold, then a detection of a macro-
UE 12 is assumed to have been made. This threshold could be the same or different
25 to the threshold used in step 119,
It will be appreciated that a macro-UE 12d may move into the vicinity of the femtocell
base station 8 (i.e. into or near to the coverage area 10 of the femtocell base station 8)
without needing to transmit anything to its associated macroceli base station 4 (for
30 example if the macro-UE 12d is not receiving any downlink transmissions from the
macroceli base station 4), which means that the femtocell base station 8 will not be
able to detect the macro-UE 12d in step 111
However, as the macro-UE 12d may need to monitor downlink control channels from
35 the macroceli base station 4 (for example a broadcast channel - BCH, or a physical
downlink control channel - PDCCH), it is necessary to make sure that the macro-UE
11
12d is able to receive these downlink transmissions. Although these channels are
designed to be relatively robust against interference, the femtocell base station 8 may
still interfere with these channels if the transmission power is sufficiently high.
5 Therefore, in one implementation, the femtocell base station 8 periodically or
intermittently sets the maximum permitted transmission power to the lower limit, in
order to provide the maximum protection for any macro-UEs 12d in its vicinity,
irrespective of whether the femtocell base station 8 detects any signals in steps 111
and 113. For example, the femtocell base station 8 can set the maximum permitted
10 transmission power to the lower limit for 100 milliseconds every 1 second. This will
provide opportunities for any macro-UEs 12d that are not transmitting any uplink
signals to listen for downlink transmissions from the macrocell base station 4.
In an alternative implementation, the femtocell base station 8 can set the maximum
15 permitted transmission power to the lower limit whenever the femtocell base station 8 is
transmitting signals at the same time that the macrocell base station 4 is transmitting
control channel signals. In particular, the femtocell base station 8 will typically be
synchronised with the macrocell base station 4 and the control channel signals will be
sent at predetermined times and on predetermined resource blocks (RBs), so the
20 femtocell base station will know when the macrocell base station 4 will be transmitting
the control channel signals. For example, in LTE, some control channel signals are
transmitted once every 1ms (e.g. PFICH, PDCCH), with the first four of fourteen
symbols transmitted per 1ms carrying control channel signals. Other control channels
(e.g. PBCH, PSCH) are sent less frequently and use approximately seven symbols out
25 of every 140 symbols and a subset of the available resource blocks.
Estimation of the quality of an uplink reference signal
As described above, in a preferred embodiment of the invention, the femtocell base
station 8 identifies characteristics of the Zadoff-Chu reference signal and estimates the
30 signal to noise ratio (SNR) of this signal
Unlike WCDMA networks, in LTE the characteristics of uplink reference signals are
significantly different to the characteristics of both data transmissions and thermal
noise. This method exploits differences in the autocorrelation function between a
35 portion of the time domain reference signal and (filtered) Gaussian noise.
12
For an uplink reference signal occupying a small number of frequency domain resource
blocks, it would be expected that the autocorrelation function with high SNR would
deviate from that due to (filtered) Gaussian noise. However, even with a wideband
spectrally flat reference signal, such as 50 resource blocks (the maximum for a 10 MHz
5 system), the autocorrelation function of a portion of the time domain reference signal
deviates from the filtered Gaussian noise case.
This is true for all the Zadoff-Chu basis sequences, although the nature of the
autocorrelation function does depend on the particular Zadoff-Chu basis sequence. An
10 example of the autocorrelation function for low and high SNR cases with 50 resource
blocks is shown in Figures 4(a) and 4(b) respectively.
It can be seen in Figure 4 that the low SNR case is dominated by the autocorrelation
function of the filtered Gaussian noise, while the high SNR case is dominated by the
15 autocorrelation function of the reference signal.
Figure 5 shows the results of a simulation in which the autocorrelation peaks from a
single reference signal, excluding the central tap, is plotted against the SNR. This plot
was obtained over a range of different reference signal parameters, numbers of
20 resource blocks, numbers of macro-UEs, SNRs from each maao-UE and frequency
resource assignments. The simulation also included fading effects.
Thus, it can be seen from Figure 5 that this metric, based on the autocorrelation
function, can be used to estimate or predict the SNR in many cases. However, there
25 are a number of points in the plot where although the SNR is high, the metric remains
low This scatter to the right hand side of the plot is potentially problematic, since in
these cases nearby macro-UEs might not be protected by the femtocell base station 8.
This scatter can be due to fading as well as differences between the autocorrelation
functions of the different Zadoff-Chu basis sequences.
30
An alternative class of metric for the estimation of the SNR can be based on the
statistics of the time domain waveform. One simple metric is the peak to average
power ratio (PAPR). High SNR reference signals should have low PAPR, whereas
Gaussian noise has a relatively high PAPR.
13
Results for this metric (in linear units) are shown in Figure 6 and it can be seen that
there is an even larger scatter apparent in the PAPR metric than the autocorrelation
metric, and as such the PAPR metric (and other metrics based on statistics of the
power) are less attractive for estimating the SNR of the uplink reference signal.
5
However, it has been observed that the scattering between the autocorrelation and
PAPR metrics is independent, i.e. for the problematic points with high SNR but
abnormally low autocorrelation metric, the PAPR tends to remain low (as expected for
high SNR signals). For such points, the autocorrelation metric can be adjusted
10 (upwards), This approach can be used to reduce the scatter in the autocorrelation
metric, and therefore improve the estimation of the SNR. For example, if the PAPR p
(in linear units) is less than 3, then a minimum value can be applied to the metric, this
minimum value being given by 400+(3-p)*50.
15 Two additional approaches for further reducing the scatter in the autocorrelation metric
have been identified.
Firstly, as the autocorrelation peaks of the reference signals tend to reduce in
magnitude with distance from the main central peak, then some shaping of the
20 autocorrelation function can be applied. To avoid an increase in the "false detection"
rate, it is important that this is only done for samples in the autocorrelation function
which are already significantly above the noise level - and so a threshold is applied
prior to applying this shaping. For example, if the metric is greater than 120 and the
offset from the centre tap is n then the metric can be increased by 0.6n.
25
Secondly, the scatter can be reduced by obtaining results over multiple measurements,
for example by taking the maximum metric obtained from a set of four or eight
measurements,
30 By using all of these techniques, the scatter in the autocorrelation metric is significantly
reduced. Figure 7 illustrates the resulting relationship between the autocorrelation
metric and the SNR.
The femtocell base station 8 can make use of the relationship between the
35 autocorrelation function and the SNR to determine the SNR of an uplink signal. A
14
method of estimating the SNR of the Zadoff-Chu reference signal in accordance with
an embodiment of the invention is shown in more detail in Figure 8.
Firstly, the femtocell base station 8 obtains a "rough" synchronization to the macrocell
5 (via a network monitor mode, or, if the standards allow, via macrocell timing
measurement reports included in mobile device measurements, or via the X2
interface).
This rough synchronization allows the femtocell base station 8 to estimate roughly
10 where in time the uplink reference signals from macro-UEs are likely to be. In nearly all
cases, this is the centre symbol in the 0 5ms uplink sub-frame.
It will be appreciated that this estimation will be subject to some error due to
propagation delay from the macrocell base station 4 and the timing advance used by
15 macro-UEs 12. In the case of over-the-air synchronization, which is assumed
hereafter, the error will be up to one macrocell round-trip propagation delay, which for a
cell of 5km is 33us which is roughly half the duration of an orthogonal frequency
division multiplexing (OFDM) symbol. The error means that signals received from
macro-UEs 12 may arrive earlier than expected at the femtocell base station 8
20
Therefore, in step 201 of Figure 8, the femtocell base station 8 measures or captures a
portion of the uplink reference symbol to give a time domain reference signal. For
example, the femtocell base station 8 obtains the time domain reference signal from
the first 512 samples of the reference symbol (assuming a 10 MHz bandwidth with
25 1024 samples plus a cyclic prefix per OFDM symbol). Despite the timing uncertainty
for over-the-air synchronization, this captured portion of the reference symbol should
only contain reference signal samples from macro-UEs 12 that are near to the
femtocell base station 8 (i.e. there shouldn't be any samples of data symbols).
30 In this step, a scheduler in the femtocell base station 8 may be used to ensure that
there will be no uplink transmissions from femto-UEs 12 to the femtocell base station 8
that might interfere with this measurement.
In step 203, the femtocell base station 8 determines the autocorrelation function for the
35 time domain reference signal and (filtered) Gaussian noise.
15
In one implementation, the femtocell base station 8 does this by normalizing the
captured time domain signal to give unit power, with the resulting sequence being
denoted r, taking the fast Fourier transform (FFT) of this sequence to give f, calculating
the squared magnitude (l2+Q2) for each sample of f and taking the inverse FFT of the
5 resulting sequence to give the autocorrelation sequence a.
As the autocorrelation sequence a determined in step 203 is symmetrical (see Figure
4), only half of the samples in a need to be retained by the femtocell base station 8 for
further processing,
10
In step 205, the femtocell base station 8 takes the magnitude (or, in alternative
implementations, the squared magnitude) of sequence a and then, in step 207, adjusts
or zeros the central tap (corresponding to zero time lag in the autocorrelation function).
15 It may also be necessary to adjust or zero the tap adjacent to the central tap if this tap
is significantly influenced by filtering in the receive path. Such filtering has a fixed
characteristic so the decision as to adjust or zero this tap is a design decision.
Then, in step 209, the femtocell base station 8 finds the tap with the largest magnitude
20 (or squared magnitude) in the remaining taps, and sets the value of a metric m to this
magnitude (or squared magnitude).
The femtocell base station 8 can then determine the signal to noise ratio of the uplink
reference signal using this metric (step 211). The value of the SNR for the determined
25 metric m can be determined from the relationship shown in Figure 5 or Figure 7, for
example using a curve-fitting technique or a look-up table.
As described above, the accuracy of the SNR estimation can be improved by
considering the PAPR of the signal, shaping the autocorrelation function based on the
30 distance of the peak used to determine the metric from the central tap and/or the metric
may be estimated from signals received in multiple time slots,
Therefore, the metric m may be adjusted as a function of distance from the central tap
for example by applying a simple linear function to the metric m determined in step
35 209. This linear function can be as described above.
•
16
Additionally or alternatively, the metric m may be adjusted as a function of the peak to
average power ratio of the captured portion of the uplink reference symbol. Specifically
if the PAPR is below a threshold (for example 3 in linear units) then a minimum value
can be imposed on the metric (again this can be a simple linear function of PAPR).
5 Again, this linear function can be as described above.
Again, additionally or alternatively, the metric m or SNR may be estimated from uplink
reference signals captured in multiple time slots and, for example, the highest value of
the SNR obtained from these measurements can be used by the femtoceli base station
10 8 to adjust its maximum permitted transmission power.
Figures 9 to 12 illustrate the performance benefits of the approach described above.
Figure 9 illustrates how the data throughput on a downlink from a macrocell base
15 station is affected by an increasing number of active femtoceli base stations within the
coverage area of the macrocell base station for both a conventional fixed power cap
and the scheme described above. In particular, it can be seen that there is a negligible
difference in the data throughput between the conventional scheme and the scheme
described above.
20
Figure 10 illustrates how the data throughput on a downlink from a macrocell base
station to cell edge (5 percentile) macro-UEs is affected by an increasing number of
active femtoceli base stations within the coverage area of the macrocell base station
for a conventional scheme and a scheme as described above. Again, there is almost a
25 negligible difference between the two schemes,.
Figure 11 plots the data throughput on a downlink from a femtoceli base station against
the number of active femtoceli base stations within the coverage area of the macrocell
base station for both a conventional fixed power cap and the scheme according to the
30 invention. It can be seen that the scheme described above provides an approximate
increase in data throughput of 5 Mb/s regardless of the number of active femtoceli base
stations, which is roughly equivalent to an improvement of 25% in the data throughput.
Figure 12 plots the data throughput on a downlink from a femtoceli base station to cell
35 edge (5 percentile) femto-UEs against the number of active femtoceli base stations
within the coverage area of the macrocell base station for a conventional scheme and a
17
scheme as described above. It can be seen that for cell edge (5 percentile) femto-UEs
the scheme described above provides an approximate increase in data throughput of
190 kb/s regardless of the number of active femtocell base stations, which translates to
an eight-fold increase in the data throughput.
5
Therefore, these graphs indicate that the adaptation of the maximum permitted
transmission power according to the invention provides performance benefits for femto-
UEs over the conventional fixed maximum permitted transmission power scheme, while
offering the same protection to the macrocell base station downlink.
10
Although the invention has been described in terms of a method of estimating a signal
quality, it will be appreciated that the invention can be embodied in a femtocell base
station that comprises a processor and transceiver circuitry configured to perform the
described method,
15
Furthermore, while the invention has been presented as a method in a femtocell base
station of estimating a quality of a signal transmitted from a macro-UE to a macrocell
base station {or from a femto-UE to another femtocell base station) that allows the
femtocell base station to control its maximum permitted transmission power, it will be
20 appreciated that the signal quality estimated using the method according to the
invention can be used for other purposes, and can be performed by elements in a
communication network other than femtocell base stations, such as macrocell base
stations (eNBs) or mobile devices.
25 While the invention has been illustrated and described in detail in the drawings and
foregoing description, such illustration and description are to be considered illustrative
or exemplary and not restrictive; the invention is not limited to the disclosed
embodiments.
30 Variations to the disclosed embodiments can be understood and effected by those
skilled in the art in practicing the claimed invention, from a study of the drawings, the
disclosure, and the appended claims. In the claims, the word "comprising" does not
exclude other elements or steps, and the indefinite article "a" or "an" does not exclude
a plurality. A single processor or other unit may fulfil the functions of several items
35 recited in the claims. The mere fact that certain measures are recited in mutually
different dependent claims does not indicate that a combination of these measures
18
cannot be used to advantage. A computer program may be storedydistributed on a
suitable medium, such as an optical storage medium or a solid-state medium supplied
together with or as part of other hardware, but may also be distributed in other forms,
such as via the Intemet or other wired or wireless telecommunication systems. Any
5 reference signs in the claims should not be construed as limiting the scope.
-19-

WE CLAIM:
1 A Home evolved Node B HeNB, comprising;
a processor and transceiver circuitry configured to:
monitor at least one uplink signal to detect a reference signal being transmitted from a
user equipment, UE, to a macrocell base station;
calculate depending upon the reference signal being detected, a value of a metric
based on at least one of an autocorrelation function and a peak to average power ratio of
the detected uplink reference signal;
determine an estimate of the quality of the uplink reference signal from the metric;
wherein the HeNB is configured to use the estimated quality of the uplink reference signal
determine if or an extent to which the UE requires protection of its downlink transmissions from
2 The HeNB of claim 1, wherein when the monitoring does not detect a reference signal, the
HeNB determines that there is no user equipment nearby requiring protection from downlink
transmissions from the HeNB and wherein the HeNB is configured to set a maximum permitted
transmission power lo a high or to a relatively high value.
3.The HeNB of claim 1 wherein the processor is configured to calculate the value of the
metric is based on the autocorrelation function and wherein determining the autocorrelation
function comprises.
obtaining a time domain reference signal;
normalising the time domain signal to give unit power;
taking a fast Fourier transform of the time domain signal to give f;
calculating a squared magnitude of each sample of f; and
taking an inverse Fast Fourier Transform of a sequence resulting from the squared
magnitude calculation to give an autocorrelation sequence
44 The HeNB of claim 3, comprising adjusting or zeroing a central lap of the autocorrelation
sequence.
5
a The HeNB of claim 4 compnsrng finding a tap wrth a largest magnitude of the remaining
,aps and setting the value of the menc equal to the largest magnitude tap

6.the henb of any of claims 1to 5 where the quality is one of a singal to nosie snr a signal to noise plus interference ratio, SINR; and a signal stre
7 The HeNB of any one of clarms , to 6, wherein the reference srgna, Is a Zadoff-Chu
reference signal
,8. The HeNB o, any one of claims 1 to 7 wherein t the peaK td average power ratio is below
a threshold a minimum value is imposed on the metric.
10 The HeNB of any one of ctaims 1 to g wheretn when the processor is configured to
esrrmate lhe quality of the signal from a signal to interference plus noise ratio' SlNR and
WherelnwhentheSlNRiSdeterminedbytheprocessortoexceedathresholdvaluetheHeNB
s configured to set a maxlmum permltted transmlsslon powel lowel than a corresponding
sellrng of lhe maximum permilted iransmlsslon power if rhe SINR is determlned noi lo exceed
the threshold value
1 1 A method in in a Home evolved Node B HeNB' of moniloring {or of an uplink reference
srgnal lndrcatlng a nearby presence ot a usel equipment ln communicalion with a macrocell
base slalion, lhe method compnslng
mon[onng at least one uplink signalto detecl a reference signal being transmdted from
a user equipment to a macrocell base stalion;
calcLllating, depending upon the reference sgnal being detected a value of a metric
based on at leasl one of an autocorlelallon funclion and a peak lo average power raiio of the
detected uPlink reference signal'determining an estimate of the quality of t
using the estimated quality of the uplink refere
the user equipment requires protecton of its d
by downlink lransmissions from the HeNB
12 The method of claim 'l l ' comprising when the monitoring does not detect a re{erence
signal, determining thal there is no user equlpment nearby requiring protection from downlink
fansmissions from the HeNB and setting a maximum permitted lransmission power of lhe
HeNB to a high or to a relatively high value

14. A computer program stored on a computeFreadable medium cpmprising program
instructions that when executeO cause a p'rocessor to perform the method of any one of claims
11to 13