Cost 231 Walfisch-Ikegami Model

COST 231 Walfisch- Ikegami Model

The parameters, excess path loss from Walfisch-Bertoni model [1] and final building path loss from Ikegami Model [2] are combined in this model with a few empirical correction parameters. This model is statistical and not deterministic because you can only insert a characteristic value, with no considerations of topographical database of buildings. The model is restricted to flat urban terrain [3].

The parameters used in Cost 231 Walfisch- Ikegami [4] are denoted in figure 1.

Figure 1[3]. Geometry of Cost 231 Walfisch- Ikegami

The formulation of the model is given as follow:

If a free LOS exists in a street canyon then, path loss defined as

L_los=42.6+26logR+20logf for R ³20m

If a non-LOS exists, path loss defined as follow:

If L_rts+L_msd<0

L_FS represents free space loss, L_rts is rooftop to street diffraction and scatter loss, L_rts is the multiscreen loss.

h_roof>h_mobile

if L_rts<0

The rooftop to street diffraction and scatter loss L_rts represents the coupling of wave propagating along the multi –screen path into the street mobile located.

for 0=<j<35

for 35=<j<55

for 55=<j<=90

where L_ori defined as_,

where j is the angle between incidences coming from base station and road , in degrees shown in following figure.

Figure 2.2.3.2 Definition of Street Orientation angle j.

Dh_mobile=h_roof-h_mobile

Dh_Base= h_base-h_roof

The multiscreen diffraction loss L_msd is an integral for which Walfisch-Bertoni model approximate a solution to this for the cases base station antenna height is greater than the average rooftop. COST 231 extended this solution to the cases base station antenna height is lower than the average rooftop by including empirical functions.

for h_base>h_roof

                                        for h_base<=h_roof

for h_base>h_roof

   for h_base<=h_roof

h_base>h_roof

for R>=0.5 km and h_base<=h_roof

for R<0.5 km and h_base<=h_roof

for medium sized cities and suburban centers with moderate tree density

for metropolitan centers

The term k_a denotes the increase of the path loss for base station antennas below the rooftops of adjacent buildings. The terms k_d and k_f control the dependence of the multi screen diffraction loss versus distance and radio frequency.

Restrictions of the model is given as follow:

Frequency (MHz)	800-2000 MHz
Base Station Height (h_base)	4-50 m
Mobile Height (h_mobile)	1-3 m
Distance R,km	0.02-5 km

Table 1 Restrictions of the Cost 231 WI Model

In case of that data on the structure of buildings and roads are not available, following values could be taken as default.

b=20...........50 m

w=b/2

h_roof= 3m(number of floors)+roof

roof=3 m for pitched

0 m for flat

j=90⁰

The COST Walfisch-Ikegami Model is included in Report 567-4 by ITU-R. The estimation of the models agrees with measurements well for the antenna heights above roof- top [5,6]. By using Wireless Simulator Program, this model is compared with Okumura’s results in the following figure 3.

Figure 3 Comparison of Cost WI Model with Okumura’s results.

Terrain Parameters:

Average Width: 73.8 m

Average Building Height: 11.81m

Percentage of Buildings: 37%

Study Parameters:

Frequency: 900 MHz,

TX Height (hb)=51m

Mobile Height (hm)=1.5m

TX Gain: 13 dBi

City Size: Small/Medium

Area Type :

Okumura-Hata Path Loss :Open Area

Okumura-Hata Path Loss (1) : Suburban Area

Okumura-Hata Path Loss (2) : Urban Area

COST WI Model has mean difference 9.52 dB for Open area, 9.05 dB for Suburban Area, 18.99 dB for Urban area. Actually, average height of buildings and average spacing value implies terrain is more suitable for Suburban area. Comparison should be based on Suburban area result. The same model is compared also with Modified Hata Model for the same terrain and following figure 4 is obtained by using Wireless Simulator Program.

Figure 4 Comparison of Cost WI model with Modified Hata Results.

Terrain Parameters:

Average Width: 73.8 m

Average Building Height: 10.93m

Percentage of Buildings: 37%

Study Parameters:

Frequency: 900 MHz,

TX Height (hb)=51m

Mobile Height (hm)=1.5m

TX Gain: 13 dBi

City Size: Small/Medium

Area Type :

Modified Hata Path Loss :Open Area

Modified Hata Path Loss (1) : Suburban Area

Modified Hata Path Loss (2) : Urban Area

As seen from the figure, COST WI Model correlates with Modified Hata Model for Open Area case, for the other cases, the mean differences are 19.08 dB for Suburban and 29.02 dB for urban area. Actually, these differences could be caused by terrain irregularity of Tokyo where Okumura took his measurements.

This model work for both cases of base station antenna height above and below rooftop levels. However, prediction error becomes larger for base station height is near or below to rooftop level because model works well for the cases propagation is over rooftops. As mentioned in calculations, the model uses rooftop diffraction loss of Walfisch- Bertoni. For base station antenna heights below roof top level, other propagation models, diffraction around building edges and multiple reflections from building walls could be dominant. Therefore, use of model for micro cell prediction could yields large errors.

Since multiple diffraction loss is approximated for settled field strength, model agrees with measurements for 1>d_s called settled-distance. The settled-distance is given by following formula.

In literature, it is claimed that Cost 231 Walfisch –Ikegami model applies diffraction loss from the last roof-top to the street erroneously [7]. In [7], it is mentioned that COST 231 Walfisch-Ikegami model predicts path loss 8.7 dB more optimistic than it is supposed to be. More detail on this comment could be found in [7].

[1] J.Walfisch and H.L. Bertoni, “ A Theoretical model of UHF propagation in urban environments,” IEEE Trans. Antennas Propagat., vol.36, 1988, pp.1788-1796

[2] F.Ikegami, T.Takeuchi, and S.Yoshida, “Theoretical prediction of mean field strength for Urban Mobile Radio”, IEEE Trans. Antennas Propagat., Vol.39, No.3, 1991

[3] Doble John., “Introduction to Radio Propagation for Fixed and Mobile Communications”, Artech House, Boston-London, 1996

[4] Cost Final Report, http://www.lx.it.pt/cost231/

[5] K.Low, “Comparison of CW-measurements performed in Darmstadt with the flat edge model”,COST 231 TD(92) 8,Vienna, January,1992

[6] K.Low, “Comparison of urban propagation models with CW-measurements”,Proc. Vehicular Techno. Conf., UTC’92, pp 936-942,1992

[7] Har D., Watson, A.M.,and Chadney, A.G., “Comment on Diffraction Loss of Rooftop –to- Street in COST 231-Walfisch-Ikegami Model”, IEEE Trans.Veh. Technol. Vol.48,No.5,1999, pp. 1451-1452