Coherence in Signal Level Measurements between GSMQOO and GSM1800 Bands and its application to Single BCCH

Operation

T.B. S@rensen, P.E. Mogensen, C. Posch’, N.H. Moldt+

Center for Personkommunikation (CPK), Aalborg University Fredrik Bajers Vej 7A-5, DK-9220 Aalborg Ost, Denmark

e-mail: { tbs, pm} @cpk.auc.dk; { cpo, nhm} @dmt.sonofon.dk+

Abstract - In 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 (GSM1800) 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

’ Dansk Mobil Telefon (Sonofon), Denmark

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network capacity. The basic idea is described in Section II, and a description of the pilot experimental study that we performed follows in Section III. After a presentation of the results of analysis in Section IV 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 cell’s 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 GSM 1800 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 RXLev is00 given RXLevgoo 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 %

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confidence that RXLevlsW is above the required threshold level.

: Signal strength

A

B

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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 [ 11. The COST23 1 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 GSM1800 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.

III. DUALBANDEXPERIMENT

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 GSM1800 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 2 with tilt of the GSM900 antenna included.

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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 1800 MHz 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 100 kHz and recorded at a constant spatial sample rate of 10 samples per m.

I Power I

Measurement System

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 13 km 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. COHERENCE ANALYSIS

The signal level measurements provided by the measurement system were processed to determine the median level Psow over 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.

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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 fo and f2o ((3 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 II. 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.

Table 1 Statistical results of analysis for the level difference between GSM900 and GSMlSOO. 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).

1000 1200 14% 1600 1800

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 PSO% measurements (BCCH beacon frequency signal strength measurements) we concluded that failures are caused by the RXLevgOo 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 +I.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 Pso% measurements.

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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 6 we may likelv exnerience a different local mean behaviour between GSM900 and GSM 1800.

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Finally, Table 1 contains 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 GSMl800, 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. ln [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 1 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 GSMl800 (assuming RXLevgm 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 1 (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 GSM9OO 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 Nokia Telecommunications. Their financial support is very much appreciated.

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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, “1800 MHz mobile net planning based on 900 MHz measurements”, COST231 TD(91)-08, Firenze, 22-24 January, 199 1 L. Melin, M. RBnnlund, 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, 1989, pp. 161-172 G.F. Pedersen, J.O. Nielsen, K. Olesen, I.Z. Kovacs, “Antenna Diversity on a UMTS Handheld Phone”, To be published in the proceedings of Personal Indoor and Mobile Radio Communications, Osaka, Japan, September 12- 15, 1999

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