Near Ground Path Loss Prediction for UMTS 2100 MHz Frequency Band Over Propagating Over a Smooth-Earth Terrain

In this paper, path loss prediction for near ground propagation of third generation (3G)-based Universal Mobile Telecommunications System (UMTS) network signal in the 2100 MHz frequency band over a smooth-earth terrain is presented. Particularly, the attenuation due to diffraction is estimated based on ITU-R Recommendation P.526-13 for diffraction over smooth earth. Furthermore, the total pathloss is determined using the Blomquist empirical model which combined free-space loss, plane-earth loss and the diffraction loss over smooth earth. In the study, two drive tests are conducted for the UMTS 2100 MHz frequency band in suburban area of Uyo. The Blomquist empirical model was tuned with respect to the first drive test pathloss dataset. The results show that with the training data (first drive test data), the untuned Blomquist empirical model has RMSE=10.21344 dB, Prediction Accuracy = 89.92%, minimum Prediction error = 9.02 dB and maximum Prediction error = -34.05 dB. Also, with the training data, the optimized Blomquist empirical model has RMSE=1.625388dB, Prediction Accuracy = 98.48%, minimum Prediction error = 5.34 dB and maximum Prediction error = -5.40 dB. Furthermore, with the cross validation data (second drive test data), the optimized Blomquist empirical model has RMSE=1.831368 dB, Prediction Accuracy = 98.24%, minimum Prediction error = 5.25 dB and maximum Prediction error = -6.15 dB. The results show that for the given terrain under study, the tuned or optimized Blomquist empirical model can effectively predict the pathloss for the UMTS network signal in the 2100 MHz frequency band.


Introduction
Power density of an electromagnetic wave or signal as it propagates through the environment in which it is travelling. Reliable path loss prediction methods are required for efficient planning of wireless communication links. An efficient prediction method minimizes interference levels and also helps to optimize the link parameters [1][2][3]. Pathloss prediction is the act of estimating the expected pathloss that may be experienced by radio wave as it propagates in a given environment. Pathloss prediction utilizes analytical expressions called pathloss models to estimate the estimate the expected pathloss for any given sign.
The rudimentary concept used in estimating expected path loss in wireless communication links is the free space, which is defined as a region free of all objects that might absorb or reflect radio energy [4]. However, in practice for line-of-sight (LOS) or near LOS communication system, in which the wave is propagated in the atmosphere and near the ground, the free space transmission equivalent is modified through various causes such as atmospheric refraction, reflection, etc [4]. Particularly, when radio wave propagates near the ground with a line of sight (LOS) condition, the path loss can be better described by the plane earth (PE) path loss model rather than the free space model. In addition, when the receiver is obstructed by object like tree, then knife edge diffraction loss need to be considered along with the plane earth (PE) path loss. In practice, due to the effect of obstructions, such receiver close to the ground and below obstruction height are considered non-line-of-sight paths. In that case, Blomquist empirical formula may be used to find the total pathloss over such non-line-of-sight paths with ground reflections [5], [6].
In this paper, path loss prediction for near ground propagation of third generation (3G)-based Universal Mobile Telecommunications System (UMTS) network signal in the 2100 MHz frequency band over a smooth-earth terrain is presented [7][8][9]. Particularly, the attenuation due to diffraction is estimated based on ITU-R Recommendation P.526-13 for diffraction over smooth earth. Furthermore, the total pathloss is determined using the Blomquist empirical model which combined free-space loss, plane-earth loss and the diffraction loss over smooth earth. In the study, two drive tests are conducted for the UMTS 2100 MHz frequency band in suburban area of Uyo metropolis of Akwa Ibom state. The total loss model was tuned with respect to the first drive test pathloss dataset. The prediction performance of the tuned and the untuned model are compared. The tuned model is then cross validated using the second drive test data.

Calculation of the Basic Transmission Loss
In most cases, the basic free-space attenuation model fails to effectively predict the total transmission loss. In such cases, along with the free space pathloss ( L ), some other attenuation factors are imposed on the radiowave due to medium effects. Consequently, the sum of free-space attenuation (Lfsp) and medium loss (Lm) is defined as basic transmission loss (L ) given by: where: where: f is the frequency (MHz) and d is the distance (km). Among others, medium losses include: Atmospheric absorption loss due to gases, vapor, and aerosols; reflection loss, including focusing or defocusing due to curvature of reflecting layer; scattering of radiowave due to irregularities in the atmospheric refractive index or by hydrometeors; diffraction loss due to obstructions; radio precipitation due to rain and snow; temporal climatic effects such as fog and cloud; antenna to medium coupling loss; polarization coupling loss and multipath adverse effects. In most analysis, the value of many of the medium loss components is negligible when compared to others and so they are ignored. In this paper, the medium losses considered include only the ground reflection loss (L ) and attenuation due to diffraction (L ). In such case, Blomquist empirical method is used to determine the total path loss as follows [5,6]; where all the losses are in dB.
In view of the nature of the terrain considered in the case study area, the ITU-R Recommendation P.526-13 method for estimating attenuation due to diffraction over smooth-earth propagation path is use. According to ITU-R Recommendation P.526-13, the standard method for calculating the transmission loss due to diffraction over a smooth-earth is defined as follows [10], [11]: where: X is the normalised length of the path; Y and Y are the normalised length antenna heights; a . is the equivalent Earth radius a is the actual Earth radius (6370km); k is the applicable effective earth radius k-factor. The following k-factors are normally applied: k = 4/3 under the median link planning "standard atmosphere" criteria (50%); k = 3 under the long term "annual" interference criteria (20%); and k = 20 under the short term "worst month3" interference criteria (0.01%) h *+ is the transmitter antenna height (m); h ,+ is the receiver antenna height (m); d is the path length (km); f is the frequency (MHz).
F(X) = 11 + 10 log(X) -17.6X (8) with the height gain term: When radio wave propagates over ground, direct ray in addition to ground reflected ray are received. The ground reflection loss in dB is given by the plane earth model as; Where d is the distance in meters between the transmitter and receiver, h *+ is the transmitter antenna height in meters, and h ,+ is the receiver antenna height in meters.

Drive Test Measurement Campaign
A handheld Samsung I9500 Galaxy S4 mobile phone was used to take measurement of received signal strength (RSS) from the UMTS 2100 GHz network. The RSS measurements were taken two times along dual lane tarred road in a suburban part of Uyo metropolis. The Samsung I9500 Galaxy S4 has CellMapper Android application installed. The CellMapper captures and displays advanced GSM/CDMA/UMTS/LTE current and neighbouring cells' low level data and can also record and export the data as comma-separated values (CSV) file. Data captured by the CellMapper comprises the current and neighbouring cells RSS in decibels (dB), the current cells cell ID (CID), local area code (LAC). The RSS along with the respective longitudes and latitudes were recorded at each measurement (receiver) point. In addition, the UMTS base station (transmitter) was located, and its longitude and latitude were recorded.

Calculation of the Measured Pathloss from the Measured RSS
After the measurements, Haversine formula was used to determine the distance between the mast (transmitter) and of the receiver locations.
The RSS value recorded at each of the receiving point is converted to measured pathloss (PL ) in dB by using the formula:  Table 1. The receiver locations, distance, RSS, measured path loss and Okumura-Hata model predicted Pathloss are also given in Table 1.

Prediction Perf ormance Analysis of the Model
In order to evaluate the prediction performance of the model, the root mean square error (RMSE), prediction accuracy (PA), the absolute minimum prediction error (AMNPE) and the absolute maximum prediction error (AMXPE) are calculated for the models.
Let PL ( . PQ,. )(R) be the measured path loss (dB), let PL (S,. RT*. )(R) be the predicted path loss (dB) and let PL ( . PQ,. ) UUUUUUUUUUUUUUUU be the mean of measured path loss and let n be the number of measured data points. The RMSE is estimated as: ^_ (12) Then, the prediction Accuracy (PA) based on mean absolute percentage error (MAPE) is calculated as: #m × 100% (13) The absolute minimum prediction error (AMNPE) is given for all i as;  Table 1 and Table 2 as well as figure 1 show the first and the second drive tests datasets of measured received signal strength (RSSI), measured pathloss and the distance (d) of the measurement points from the transmitter base station. In the optimization process, the total pathloss in the original Blomquist empirical model is considered as consisting of free space pathloss ( L ) and excess pathloss due to ground reflection and diffraction ' L + L (. Table 3 and figure 2 show the field measured pathloss (dBm) and the pathloss predicted by the untuned Blomquist model versus distance. The Correlation Coefficient (r) between the prediction residual (error) of the untuned Blomquist model and the excess pathloss due to ground reflection and diffraction is -0.97283. In view of the very strong correction, the optimization process is performed by generating a correction factor which is a function of the excess pathloss that minimizes the root mean square error. The correction factor obtained is given as; The pathloss predicted by the tuned or optimized Blomquist model versus distance is given in table 3. The data and pathloss predicted in table 3 are with respect to the training data which is the first drive test field measured data. The second drive test field measured data is used to cross validate the prediction performance of the models. Table 4 and figure 3 show the result of the cross validation process.  The prediction performance results in Table 5 show that with the training data (first drive test data), the untuned Blomquist empirical model has RMSE=10.67 dB, Prediction Accuracy = 89.51%, minimum Prediction error = 9.02 dB and maximum Prediction error = -34.29 dB. With the training data, the optimized Blomquist empirical model has RMSE=1.7497dB, Prediction Accuracy = 98.306%, minimum Prediction error = -4.395 dB and maximum Prediction error = 6.338 dB. Furthermore, with the cross validation data (second drive test data), the optimized Blomquist empirical model has RMSE=1.833 dB, Prediction Accuracy = 98.24%, minimum Prediction error = 5.43 dB and maximum Prediction error = -9.278 dB. The results show that for the given terrain under study, the tuned or optimized Blomquist empirical model can effectively predict the pathloss for the UMTS network signal in the 2100 MHz frequency band.

Conclusion
Pathloss prediction for near ground propagation of third generation (3G)-based Universal Mobile Telecommunications System (UMTS) network signal in the 2100 MHz frequency band over a smooth-earth terrain is presented. Attenuation due to diffraction is estimated based on ITU-R Recommendation P.526-13 for diffraction over smooth earth. Also, the total pathloss is determined using the Blomquist empirical model which combined free-space loss, plane-earth loss and the diffraction loss over smooth earth. Two drive tests are conducted for the UMTS 2100 MHz frequency band in suburban area. The results show that for the given terrain under study, the tuned or optimized Blomquist empirical model can effectively predict the pathloss for the UMTS network signal in the 2100 MHz frequency band.