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Propagation Loss Determination in Cluster Based Gsm Base Stations in Lagos Environs

O. Shoewu*, F.O. Edeko

Department of Electronic and Computer Engineering, Faculty of Engineering, Lagos State University, Epe Campus

Abstract: This work present a propagation loss model developed for cluster GSM base station at 2100 MHz in a suburban environment. The measurement of path loss and receive signal strength at various distance for ten base station in Lagos Nigeria were collected through drive test. COST 231 HATA model was used in analysis with the measured path loss using MATLAB. Results of different comparisons showed that measured and modeled path loss were closest at site 3225-3048 with RMSE of 11.25 and correlation of 0.9931.

Keywords: propagation, receive signal strength, drive test, correlation

Received December 03, 2013; Revised January 19, 2014; Accepted February 07, 2014

Citation: O. Shoewu, F.O. Edeko, Propagation Loss Determination in Cluster Based Gsm Base Stations in Lagos Environs. International Transaction of Electrical and Computer Engineers System, 2 (1). 28-33. January 2014. doi:10.12691/iteces-2-1-5

1. Introduction

Since the mid 1990’s the Telecommunications industry has witnessed rapid growth [1]. Wireless mobile communication networks have become much more pervasive than anyone ever imagined when cellular concept was first developed [2].

High quality and high capacity network are in need today, estimating coverage accurately has become exceedingly important. Therefore for more accurate design coverage of modern cellular networks, measurement of signal strength must be taken into consideration. The cellular concept came into picture which made huge difference in solving the problem of spectral congestion and user’s capacity [4]. With no change in technological concept, it offered high capacity with a limited spectrum allocation. The cellular concept is a system level idea in which a single, high power transmitter is replaced with many low power transmitters. The area serviced by a transmitter is called a cell. Thus each cell has one transmitter. This transmitter is also called base station which provides coverage to only a small portion of the service area. Transmission between the base station and the mobile station do have some power loss this loss is known as path loss and depends particularly on the carrier frequency, antenna height and distance. The range for a given path loss is minimized at higher frequencies. So more cells are required to cover a given area. Neighbor base stations close are assigned different group of channels which reduces interference between the base stations. If the demand increases for the service, the number of base stations may be increased, thus providing additional capacity with no increase in radio spectrum. The advantage of cellular system is that it can serve as many numbers of subscribers with only limited number of channel by efficient channel reuse.

The performance of any wireless communication systems depends on the propagation characteristics of the channel [2]. Channel characteristics have an impact on the design of the transmission strategy. Received signal prediction models play an important role in the RF coverage optimization and efficient use of the available resources. These models can differ in their properties with locations due to different terrain environment. Therefore, extensive study on the effects of radio propagation path-loss had drawn a considerable attention.

Path loss is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space. Path loss is very important in the analysis and design of the link budget of a telecommunication system. It normally includes propagation losses caused by the natural expansion of the radio waves front in free space, absorption losses sometimes called penetration losses, when the signal passes through media not transparent to electromagnetic waves, diffraction losses when part of the radio wave front is obstructed by an opaque obstacle and losses caused by other phenomena.

2. Literature Review

2.1. Empirical Propagation Models

Okumura and Hata are among the two empirical propagation models.

The two basic propagation models are free space loss and plane earth loss would be requiring detailed knowledge of the location and constitutive parameters of building, terrain feature, every tree and terrain feature in the area to be covered. It is too complex to be practical and would be providing an unnecessary amount of detail therefore appropriate way of accounting for these complex effects is by an empirical model [2]. There are many empirical prediction models like, Cost 231-Hata model, Okumura-Hata model, Sakagami-Kuboi model, Cost 231 Walfisch-kegami model.

2.1.1. Okumura Model

Okumura Model–wholly based on measured data-no analytical explanation

•  Among the simplest & best for in terms of path loss accuracy in Cluttered mobile environment.

•  Disadvantage: slow response to rapid terrain changes.

•  Common standard deviations between predicted & measured path loss 10 dB-14 dB.

•  widely used for urban areas.

•  useful for-Frequencies ranging from 150 MHz-1920 MHz.

- Frequencies can be extrapolated to 3 GHz.

- Distances from 1 km to 100 km.

- Base station antenna heights from 30 m-1000 m.

Okumura developed a set of curves in urban areas with quasi-smooth terrain.

•  effective antenna height:

- Base station the = 200 m.

- Mobile: hre = 3 m.

•  Gives median attenuation relative to free space (Amu).

•  Developed from extensive measurements using vertical Omni-directional antennas at base and mobile.

•  Measurements plotted against frequency.

•  Okumura takes urban areas as a reference and applies correction factors.

Urban areas: LdB = A + B log10 R – E

Suburban areas: LdB = A + B log10 – C

Open areas: LdB = A + B log10 R – D

A = 69.55 + 26.16 log10 fC – 13.82 log10 hB

B = 44.9 – 6.55 log10 hB

C = 2 (log₁₀ (fC/28)) 2 + 5.4

D = 4.78 (log₁₀ fC) 2 + 18.33 log10 fC + 40.94

E = 3.2 (log₁₀ (11.7554 hM)) 2 – 4.97 for large cities, fC ≥ 300 MHz

E = 8.29 (log₁₀ (1.54 hM)) 2 – 1.1 for large cities, fC < 300 MHz

E = (1.1 log10 fc – 0.7) hM – (1.56 log10 fC – 0.8) for medium to small cities.

2.1.2. COST 231-Hata model

Okumura-Hata model for medium to small cities has been extended to cover 1500 MHz to 2000 MHz (1999).

LdB = F + B log10 R – E + G

F = 46.3 + 33.9 log10 fc – 13.82 log10 hb

E designed for medium to small cities.

G = 0 dB medium sized cities and suburban areas.

G = 3 dB metropolitan areas.

Accuracy

Extensive measurement in Lithuania [8] at 160, 450, 900 and 1800 MHz:

•  Standard deviation of the error = 5 to 7 dB in urban and suburban environment.

•  Best precision at 900 MHz in urban environment.

•  In rural environment: standard deviation increases up to 15 dB and more.

Measurements in Brazil at 800/900 MHz:

•  mean absolute error = 4.42 dB in urban environment.

•  standard deviation of the error = 2.63 dB path loss prediction could be more accurate, but models are not complex and fast calculations are possible precision greatly depends on the city structure

2.1.3. Ikegami Model

entirely deterministic prediction of field strengths at specified points.

•  Using detail map of building heights, shapes and positions. Trace ray paths.

Restriction has only single reflection from wall accounted for Diffraction calculated using single edge approximation Wall reflection are assumed to be fixed at constant value two ray (reflected, diffracted) are power summed.

•  model tends to underestimate loss at large distance.

•  Variation of frequency is underestimated compared with measurement.

2.1.4. ECC-33 Model

The ECC 33 path loss model, which is developed by Electronic Communication Committee (ECC), is extrapolated from original measurements by Okumura and modified its assumptions so that it more closely represents a fixed wireless access (FWA) system. The path loss model is defined as,

PL(dB) = Afs + Abm - Gt - Gr (13)

where, Afs is free space attenuation, Abm is basic median path loss, Gt is BS height gain factor and Gr is received antenna height gain factor.

These are separately defined as,

Afs = 92.4 + 20 log (d) + 20 log (f) (14)

Abm = 20.41 + 9.83 log (d) + 7.894 log (f) + 9.56 [log (f)]²

Gt = log (hb/200) [13.98 + 5.8 (log (d))²] (16)

For medium city areas,

Gr = [42.57 + 13.7 log (f)] [log (hm) - 0.585] (17)

Where, f is the frequency in GHz, The performance analysis is based on the calculation of received signal strength, path loss between the base station and mobile from the propagation model. The GSM based cellular d is distance between base station and Mobile (km), hB is base station antenna height in meters and mobile antenna height in meters.

3. Methodology for Data Fetch

The concerned methodology deals with drive test tools which contains both software and hardware devices. Here we have deployed TEMS 9.5 for drive test measurements. The complete process of data collection via, TEMS-9.5 or any other advanced version is not a simplistic task. It involves careful setting up of GPS and TEMS enabled hand-set for the purpose of data collection. The first step is to connect the laptop with car’s battery for non-interrupted battery charge up during the drive test. Secondly, a GPS device is connected to the laptop via, usb-2.0 interface and GPS device is placed over the car’s roof. Next, we connect the TEMS help enabled hand-set via, usb-2.0 and following connect the software loaded in the system thereafter. The chosen region is surveyed with the help of this mobile vehicle along with the accessories as mentioned. An HASP (dongle) security key is used for secure access to the TEMS investigation software and readings appear on the map of concerned scenario as the vehicle starts moving, it should be kept in mind that readings will be clear only if regional base-station vectors and regional map vectors are already loaded in to the system. Normally, propagation model measurements are carried out if planning is done for a new network and, if there is an area with changes in the propagation environment such as new buildings, new roads, or else a new frequency band is taken into consideration.

Figure 1. Process of data collection depicted using real-time drive test picture

3.1. The Investigated Environments

The investigation was carried out in Agege which is a sub-urban part of Lagos state southwest Nigeria with a population of 459,939 people. With geographical coordinate of 6 37’19’’ North, 3 19’ 33’’ East.

Figure 2. Nigeria map showing research areas

3.1.1. Data Collection and Setup Features

The TEMS Investigator handset and software versions 9.1 were used to collect samples of the measured signal. The handset was used with precaution as it is known to have a typical accuracy of ± 4 dBm. The calibrated power was -100≤ - < -40 dBm. The device used was TEMS handset given time to settle on a particular range of values. The readouts were made on software in pre-defined format. A geographical positioning system (GPS) receiver was used to collect the location information. The data captured included: logical channel 1 with information on Base station identity code, received signal strength (RxLev), Traffic channel, Timeslot number, path loss, cell identity, RSSI, longitude, latitude, Transmit power-graphic, carrier-to-interference ratio in dB, Timing advance, cell identity and the neighbor list. The GPS had an accuracy of ± 15 m. This is due to the fact that selective variability was switched off in the recent past which improved the accuracy of the GPS dramatically. The network cells within the area being studied were measured for model selection since all relevant parameters such as effective isotropic radiated power (EIRP), antenna type, direction and height were known.

3.1.2. Measurement and Drive Test

The measurement of the received signal strength were collected through drive test with the aid of an Ericson test mobile system (TEMS). This was conducted around twelve base station of a mobile communication network at 10662 MHZ frequency band. The TEMS gives the received signal level at a spherical distance the height of the mobile station is about 1.5 m while the base station consist of heights of 32 m and 34 m. The width of the road is about 20 m. The peak transmitter power is approximately 47 dbm for all base station.

COST 231 hata model will be analyzed and examined with the data collected. The result of this analysis will be used to provide a propagation loss model for the environment.

4. Results and Discusions

TEMS tool was used to measure the signal strength level for uplink and downlink at coverage areas for a cell in the road of Agege. The road of Agege can be considered as sub-urban and also the frequency of the base stations used is 10662 MHz therefore equivalent equations of COST 231 HATA models were used. Path loss was determined by practical measurement for each distance in the cluster, then on that basis a comparison was done between theoretical and experimental values by MATLAB as show below.

Table 1. Data table consisting distance and measured pathloss

Table 2. Data table consisting distance and measured pathloss

Figure 3. Graph for distance and measured path loss

Figure 4. Graph for distance and measured path loss

Figure 5. Graph for distance and measured path loss

Figure 6. Graph for distance and measured path loss

Figure 7. Graph for distance and measured path loss

4.1. Computation

[picture] | (1)

[picture] | (2)

[picture] | (3)

Equation 3 is the equivalent of COST 231 HATA model which is used as a model in MATLAB for comparison between the measured path loss and the propose path loss.

RMSE of each pair of the cluster is also calculated using the formula below =

Also the correlation (r) of the site are calculated using the below formula

[picture] | (4.4)

where

[picture] | (4.5)

and

[picture] | (4.6)

then

[picture] | (4.7)

Table 3. Results for the considered parameters

Figure 8. Showing a snap shot of the drive Test

5. Conclusion

In this work the popular empirical path loss model for mobile communication is studied. Among which is COST 231 HATA model, it is compared with measured path loss obtained from cluster GSM base station in a particular area of Lagos state. The RMSE of each pair of site was also computed and reveals better performance in the said environment. From the MATLAB analysis, measured and model path loss of site 3225-3048 are the closest followed by site 3189-3221. Results show that the characteristic of radio propagation vary considerably from one environment to the other due to terrain factors.

Reference

[1]   | COST Action231, “Digital mobile radio towards future generation system, final report,” tech.rep., European communities, EUR18957,1999.
[2]   | Z.Nadir, “Pathloss Determination Using Okumura-Hata Model and SplineInterpolation for Missing Data for Oman.” WCE 2008, July, 2-4, 2008, London, U.K.
[3]   | Wikipedia.
[4]   | N.T. Surajudeen Bakinde, N. Faruk, A. A. Ayeni, M. Y. Muhammad, M.I” comparative analysis of radio propagation models for wideband code division multiple access (WCDMA) and global system for mobile communications (GSM).
[5]   | Shoewu, O., Edeko, F. O., Analysis of radio wave propagation in Lagos environs, AJSIR, 2011.
[6]   | Purnima K Sharma, R.K.Singh, “Comparative Analysis of Propagation Path Loss Models With Field Measured Data” IJEST, Vol.2(6)in 2010 at 2008-2013. Manju Kumari et al. / International Journal of Engineering Science and Technology (IJEST) ISSN.
[7]   | TEMS Investigation release notes, ASCOM, Document: NT11-21089, www.ascom.com/networktesting, 2011.
[8]   | Mardeni, R. and Kwan, K. F. Optimiza-tion of Hata Propagation Prediction Modelin Suburban Area in Malaysia, Progress in Electromagnetic Research C, Vol. 13, 2010, pp91-106.
[9]   | M. Hata, “Empirical formula for propagation loss in land mobile radio services,” IEEE Trans. Veh. Technol., vol. VT-29, pp. 317-325, Aug. 1980.
[10]   | Yuvraj Singh, “Comparison of Okumura, Hata and COST-231 Models on the Basis of Path Loss and Signal Strength”, International Journal of Computer Applications (0975-8887) Volume 59 No.11, December 2012.