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8/10/2019 Coherence in Signal Level Measurement
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Coh erenc e in Signal Level Measurem ents b etw een GSMSOO
and GSM1800 Bands and i ts appl icat ion to Single BCCH
perat on
T.B.
Sgrensen,
P.E.
Mogensen C.
Posch,
N.H.
Moldtl
Center for Personkommunikation (CPK), Aalborg University
Fredrik Bajers Vej 7A-5, DK-9220 Aalborg @st,Denmark
e-mail:
{
tbs, pm} @cpk.auc.dk;
{
cpo, nhm} @dmt.sonofon.dk
bstract
n this paper we investigate the
possibility to operate a dualband GSM network
having co-located GSM900 and GSM1800 cells,
with a single broadcast channel BCCH). Our
investigations are based on simultaneous
dualband signal strength measurements using a
test measurement system, neighbour channel
measurements from a dual band GSM phone,
and Abis trace data. Our results indicate that
base station antennas with dissimilar radiation
patterns for the two bands will effect a change in
mean signal level difference. The characteristics
of the dualband antenna on the mobile station,
especially when interacting with the hand and
head of the user, tend to strongly de-correlate
the mean signal level variations in the two
bands. Both issues, and in particular the latter,
may negatively influence the performance
of
single BCCH operation of co-located GSM900
and GSM1800 cells.
I.
INTRODUCTION
Digital cellular services (GSM 1800) at 1800 MHz
are used extensively to complement existing GSM
(GSM900) cellular networks in city areas, where
the user density is high. The two networks
inherently rely on individual system broadcast
channels (BCCH). The BCCH is broadcast on a
beacon frequency which is transmitted continuously
at maximum power. The BCCH channels require a
much larger frequency reuse distance than the
traffic channel carriers, which can benefit from
power control, discontinuous transmission, and
frequency hopping. The need to transmit a BCCH
beacon for both bands in the case of co-located
GSM900 and GSM
1800
cells implies a significant
reduction in the net network capacity.
In
the following we investigate the possibility
of
single band BCCH operation to increase the
network capacity. The basic idea is described in
Section 11, and a description of the pilot
experimental study that we performed follows in
Section 111. After a presentation of the results of
analysis in Section
V
we end up with a discussion
in Section V and conclusion in VI.
11. SINGLE BAND
BCCH
OPERATION
Mobile station signal strength measurements on the
BCCH beacon frequencies are used to assist initial
cell assignment and hand over between cells. The
GSM specifications allow the MS (Mobile Station)
to report signal strength measurements on the
serving and the six strongest neighbouring cells (the
neighbouring cells BCCH beacon frequency). In
the case of separate BCCH beacon frequencies each
pair of co-located dual band cells will likely
occupy two indexes in the neighbour channel list
(out of only six indexes). For this reason the
efficiency of the hand over mechanism, especially
when having a multi-layer network structure,
reduces significantly. The end result is a reduction
in capacity and quality.
Single BCCH operation of co-located GSM900 and
GSM1800 cells can be implemented by deriving the
1800 MHz signal strength from the
900
MHz
measurements (or vice versa).
In
this case, the
MS
can restrict its measurement reporting to RXLevgm
(signal strength on GSM900 BCCH beacon
frequency), and the radio network should be able to
predict RXLev18m given RXLevgm. This operating
scheme relies heavily on coherence (i.e. strong
correlation) in mean signal level between the two
bands.
The basic operating principle of single BCCH is
illustrated in Figure 1.
A
hand over to GSM1800
can be made for the threshold setting in case B
because, given RXLevgm, we have more than 90
Dansk
Mobil
Telefon (Sonofon), Denmark
0-7803-5435-4/99/$10.00 1999
IEEE
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confidence that RXLev18m is above the required
threshold level.
Signal strength
I
.~
Predicted level
A t.........\:..I...... ......................
Handovermargin
10 probability area
Figure
1
Single band
BCCH
operating principle.
Threshold setting
A:
hand over from GSM900 to
GSM1800 will not be attempted; Threshold setting B:
hand over takes place.
The possibility to predict the 1800 MHz signal
level from 900 MHz measurements has already
been exemplified in the COST231 Hata extensions
[l].
The COST231 extension adds extra
compensation (path loss) terms to the frequency
dependent term of the Hata model. Originally, the
idea was that these additional path loss terms,
together with the observed large scale signal
coherence between GSM900 and GSM 1800
frequency bands, would allow the extensive
knowledge base
of
900
MHz
signal propagation to
be extended to the 1800 MHz band. A summary of
the factors contributing to the path loss difference is
given in [2 ]
According to results input to COST231 [3] the path
loss difference between the two bands was
measured to be within the range of 8.7 11 dB
(urban area) depending on base station height and
position. Further, the standard deviation was found
to be
as
low as 3.3 3.6 dB and the correlation
between slow fading signal variations in the two
bands was high (above 0.9). Slightly different
results were reported in [4]: 6.4
-
7.0 dB mean
difference with a standard deviation of
3.1
dB. Also
in this case the correlation was above 0.9. These
results suggest that prediction is feasible.
However, recent measurement results, obtained
using dual band GSM test mobiles, do not support
the observation of a fixed mean signal strength
difference and a small standard deviation; hence
these GSM network results are in contradiction to
the more ideal propagation measurements that
supported the COST23 1 modelling.
A pilot experiment has been conducted in order to
validate these observations, and further to resolve
the apparent ambiguities. Initially, an urban area
cell was selected to be of particular relevance for
dualband operation,
111.
DUAL
BANDEXPERIMENT
The experiment was conducted in one sector
of
an
existing tri-sectored base station (small urban
macro cell) in Aalborg, Denmark. The urban area is
characterised by 3 to
5
story apartment buildings
with street width varying between 10 and 15 m.
The measurement area is similar to the area used in
[31
The base station uses two single band antennas
placed 4 m apart (horizontal spacing). The
GSM 1800 antenna (18.0 dBi) is aligned vertically,
whereas the GSM900 antenna (17.0 dBi) has a 5.5
down tilt relative to the vertical. The position of the
antennas is 35 m above median ground level.
Halfway in-between the two existing base station
antennas we placed a dualband (wideband
log-
periodic) reference antenna (aligned vertically).
The radiation patterns for this antenna are almost
identical for the two frequency bands with a
horizontal beamwidth of 90 and a (wide) vertical
beamwidth of 65 . The two single band antennas
differ primarily in having a different vertical
beamwidth (GSM1800 6.5 and GSM900
9 )
with
multiple sidelopes (-15 dB). Vertical (E-plane)
radiation patterns can be seen in Figure with tilt
of the GSM900 antenna included.
120
6
240\
1 3 0 0
270
Figure
2
E-plane antenna radiation patterns
for
BTS
single band antennas.
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In the experiment we used the two BCCH beacon
frequencies (1806.4 MHz and 958.2 MHz) plus two
CW signals (1812.4 MHz and 959.0 MHz) which
were transmitted on the reference antenna.
For the data recording we used a four-frequency
measurement system mounted in, a van. The system
has two separate receiving branches for the 900 and
1800MHz bands. Each of the two receiving
branches is sequentially switched in frequency in
order to measure on both the GSM BCCH beacon
frequency and the
CW
frequency. Separate band
(end-fed) dipole antennas were placed on the roof
of the van to receive the four transmitted signals.
All signals were (log) envelope detected in a
bandwidth of
1OOkHz
and recorded at a constant
spatial sample rate of 10 samples per m.
GSMSOO
Dipole Dipole
Power
Combiner
\I
/ .
Power
Measurement
i
Figure 3 The setup used in the van.
Alongside the measurement system, as indicated in
Figure 3, we recorded the measurement reports of a
dual-band mobile station (DB-MS) at 1s intervals.
From the frequency carrier numbers and the BSIC
(base station and network colour code)
identification, we were able to track synchronously
the signal level measurements (RXLev) on the two
BCCH frequencies.
In one set of measurements a passive power-
splitting network provided identical signals for the
DB-MS and the test measurement system (Figure
3), whereas in a second setup, the DB-MS used its
own whip-antenna. The DB-MS was placed in a
fixture at a slight slant angle, just behind the
windscreen. During both measurements a call
connection was established in order to trace the
neighbour channel measurement reports.
The van drove a route of 13km to cover most of
the streets within the half-power beamwidth of the
base station antennas. At the farthest distance the
van was approximately 2 km from the base station.
IV. COHERENCENALYSIS
The signal level measurements provided by the
measurement system were processed to determine
the median level PSO%ver 12.7 m sections as an
estimate of the local mean signal level. The RXLev
measurements, on the other hand, represent
temporal averaging (in dB) over approximately 7
samples (determined from the size of the BCCH
Allocation list) and were used as is.
i.5
2.5
5 7.5
10
12.5 15 17.5 20 22.5 25 27.5
dB
Figure
4
Histogram of the level difference between
900
and 1800MHz signals based on P ~ o )or the
dualband reference antenna.
Figure 4 shows the empirical distribution of the
level difference between CW 900 and 1800 MHz
signals, transmitted from the dualband reference
antenna and received on the dipole antennas. The
distribution is approximate log-normal with 72.5
and 94.5 of the samples having a level difference
within
+o
and +2o
(o
is standard deviation),
respectively. For a normal distribution the
respective values are 68.3 and 95.4 . The mean
level difference is +11.6 dB, and therefore
comparable to the observation in [3]. For the single
band antenna signals the level difference is only
6.9 dB.
All the results for the mean and standard deviation
have been summarised in Table 1. We note that the
standard deviation is comparable to the values
referenced in Section 11. The double entries refer to
the different setups mentioned previously (one is
shown in Figure 3) and have been obtained during
different times of the day. Therefore, we attribute
no significance to the small deviations in the mean
level difference.
The data has been analysed with respect to the
radial distance from the base station, but we found
no significant dependence on distance. Also we
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noted that the standard deviation of the (log-
normal) local mean variations was no different
from one frequency band to the other.
Data
Source
p50
RXLev
(reference)
Single band antennas
I
Network Abis
Table 1 Statistical results of analysis for the level
difference between GSM900 and GSM1800. Grey
shaded italic numbers were obained with the setup in
Figure
3,
whereas the other results were obtained
with separate antennas.
The evaluation and comparison of the DB-MS data
is not as straightforward. Figure 5 shows a sample
plot of the RXLev difference from which it is clear
that the measurements fail occasionally (sample
points below -10 dB).
200 400 6 6 1 1200 1400 16 16
-30
Observation number
Figure
5
Sample plot of the level difference calculated
from RXLev reporting external antenna).
Observations are taken along the measurement route.
From a comparison with the
P S O ~
easurements
(BCCH beacon frequency signal strength
measurements) we concluded that failures are
caused by the RXLevgm measurement reports.
Supposedly, this is due to the fact that the
MS
requires frequency and time synchronisation for a
signal level measurement (it must derive the BSIC)
and therefore is sensitive not only to signal strength
but also channel dispersion and co-channel
interference. CO-channel interference was most
dominant at 900 MHz.
When we exclude the erroneous measurements the
level difference is calculated to be +6.1 dB with the
external antenna signal and +1.7 dB when the DB-
MS uses its own antenna (Table 1). As before, we
observed that the mean difference is constant (no
dependence on distance), but the standard deviation
has increased to approximately 5 dB. This is in part
due to the different, and less accurate, measurement
procedure in the DB-MS evidenced by the increase
from
3.0
dB to 4.8 dB (standard deviation in Table
1) and, with less confidence, the influence from the
mobile antenna (4.8 dB to
5.1
dB).
To further characterise the coherence in signal
strength between the two frequency bands we
investigated the correlation properties for the P50
measurements.
Figure 6 Empirical Distribution Function EDF) of
signal correlation.
Figure
6
shows the correlation between the slow
fading processes at 900 and 1800 MHz. The result
has been obtained by analysing the total
13
km
measurement route in sparse sampled segments of
length 240 m (one sample every 16 m). This allows
us to obtain
a
90 confidence interval estimate
using the bootstrap procedure
[ 5 ]
The three curves
in Figure 6 should be considered individually and
not in comparison; the confidence limit
distributions serve only to illustrate the estimation
accuracy.
If instead we consider the whole data set as a single
sample the correlation turns out to be in the range
0.87 0.89. Clearly, based on Figure 6we may
likely experience a different local mean behaviour
between GSM9OO and GSMl800.
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Finally, Table 1contains a result derived from an
Abis trace on the same cell as we used for the
measurements. We see that when the mobile phone
users are included along with a mixture of different
types
of
DB-MS the situation changes radically.
The mean level difference has actually changed in
favour of GSM1800, and the standard deviation
confirms the trend observed earlier between
PSO
and RXLev derived measurements a significant
increase in standard deviation, and hence less
coherence. It must be emphasised that we cannot
make firm conclusions based on this result, but
increased variability is evident.
v . DISCUSSION
We conjecture that the observed discrepancy in
mean level difference for different base station
antennas is due to differences in the effective
antenna radiation patterns. There will be a small
influence from the antennas themselves (Figure
2)
amplified by the difference in downward tilt and
structures in close proximity to the antenna(s).
This does not prevent signal level prediction for
single BCCH operation. It merely requires that the
radio network obtains a preliminary measurement
of the mean level difference so as to characterise
the cell (environment and BTS antenna
configuration). We suspect that a dualband antenna
tends to equalise propagation conditions and
therefore will be our preferred choice.
At the mobile end of the link the mobile phone user
seems to have a large influence, and most
importantly may possibly be the cause of non-
predictable level differences between the two
bands. This has not been studied in detail in this
experiment, but we infer from other results that it is
a likely cause. In [6] it has been shown that the
local mean variation in received signal strength
caused by different users may vary 8 dB at the
median outage level for the same mobile station.
The impact of these observations is that the hand
over margin shown in Figure I needs to be set high
in order to be confident that a hand over is safe.
This will effectively introduce a gap in the
coverage area of GSM1800 (assuming RXLev9w
reporting only). Eventually, when the margin
becomes very large we may jeopardise the potential
gain that we initially expect from single band
BCCH operation. A simple calculation based on the
DB-MS (own antenna) RXLev statistics in Table
(assuming log-normal distribution) gives a margin
of
6.5
dB at a 90 % confidence level.
VI. CONCLUSION
In this paper, we have reported our investigations
on signal coherence between GSM900 and
GSM1800 frequency bands, which is of major
importance for the operation of a single band
BCCH network.
This investigation shows that despite of quite
favourable propagation conditions for the
prediction of the mean level difference between
GSM900 and 1800 bands, the influence of mobile
station antennas, specifically the interaction with
the user, tends to de-correlate the signal variations.
This necessitates high hand over margins for the
cells in the band without BCCH. We therefore
suggest that further investigations be done to
evaluate the influence on network performance.
Also, we point out that base station antennas have
some influence on the operation of single BCCH
dual band cells.
VII. ACKNOWLEDGEMENTS
The work has been co-sponsored by Nolua
Telecommunications. Their financial support is
very much appreciated.
VIII. REFERENCES
COST telecommunications, Action
23
1, Digital
mobile radio towards future generation systems,
Final report EUR 18957 ISBN 92-828-5416-7,
European Communities, 1999
T.-S. Chu, Larry J. Greenstein, A Quantification
of Link Budget Differences Between the Cellular
and PCS Bands,
IEEE
Transactions
on
Vehicular Technology Vol. 48, No. 1, January
1999,pp. 60-65
P.E.
Mogensen, C. Jensen, J. Bach Andersen,
1800MHz mobile net planning based on
900
MHz measurements,
COST231
TD(91)-08,
Firenze, 22-24 January, 1991
L. Melin, M. Ronnlund, R. Angbratt, Radio
Wave Propagation, A Comparison Between 900
and 1800 MHz,
43rd
Vehicular Technology
Conference
Denver USA, 1993,pp. 250-252
P. Hall, M. A. Martin, Better Nonparametric
Bootstrap Confidence Intervals for the
Correlation Coefficient, Journal of statistical
computation and simulation Vol. 33 No. 16,
G.F. Pedersen, 5 0 ielsen, K. Olesen, I.Z.
Kovacs, Antenna Diversity on a UMTS
Handheld Phone,
To
be published
n
the
proceedings of Personal Indoor and Mobile Radio
Communications, Osaka, Japan, September 12-
15, 1999
1989,pp. 161-172
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