coherence in signal level measurement

8/10/2019 Coherence in Signal Level Measurement

1/5

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

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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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coherence in signal level measurement

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