Comparative Study of Radio Channel Propagation Characteristics for ...

Comparative Study of Radio Channel Propagation Characteristics for 3G/4G Communication Systems Chan-Byoung Chae, Changho Suh, Marcos Katz, DS Park, and Frank H.P. Fitzek† Samsung Electronics Co., Ltd, P.O.BOX 105, Suwon, Korea. E-mail: [email protected], Tel: +82-31-279-4828, Fax: +82-31-279-5130 †Department of Communications Technology, Aalborg University Neils Jernes Vej 12, 9220 Aalborg łst, Denmark

Abstract— A propagation model for 3G/4G communication systems is considered in this paper. The analysis method is based on high frequency ray-tracing and the uniform theory of diffraction. The ray contributions includes diffraction from building edges, reflection from the ground and building and the direct wave. According to the elevation angle of the impinging signal, different propagation components will be presented in the composite received signal. We compared propagation characteristics in 3G/4G communication systems using the above analysis method. The dependency of radio wave propagation with the geometry of the scenario and the frequency band is investigated in detail. The numerical results can be applied in cell planning of new cellular systems.

I. I NTRODUCTION In this paper, we compare the radio propagation characteristics for 3G and 4G radio environments, for the particular and important case where a dominant obstacle is in the path of the signal transmitted from the base station (BS). In a typical cellular system such as 3G, the signal level at a given position can be predicted by analyzing the propagation characteristics of the channel. Results are very much dependant upon location and height of the BS. Thus, the information of the propagation characteristics of the radio channel is useful in cell planning of a cellular system. From the standpoint of services for future 4G systems, and considering backward compatibility with 3G systems, it would be desirable for the former to also have cellular network characteristics. In this paper the Uniform Theory of Diffraction (UTD) [1] will be used to characterize a typical cellular channel. More precisely, the two-dimensional propagation model for building blockage, developed in [2] for satellite communications, will be employed here for characterizing a cellular environment. The rational for using the same model is the fact that the same propagation phenomena takes place in both scenarios, e.g., direct incident wave as well as diffraction and reflection components. The main difference between both cases is the actual dimensions of the problem setting. The radio propagation channel for a Broadband Wireless Local Loop (B-WLL) system operating at 28GHz has been characterized in [3] by using the UTD through according to the model of [1] and [2]. By applying the UTD, the ray contributions are formulated as the sum of direct, reflected and diffracted waves. In this paper, we assume that carrier

frequency is 2GHz for 3G systems and 5 and 60GHz for 4G systems. It is widely accepted and demonstrated that cellular network operation can be achieved in the 2 and 5GHz band but not necessary in the 60 GHz band due to the high attenuation imposed by the channel. Results for the 60GHz band, included here as a reference, will show us this limitation. The propagation characteristics of 2 GHz and 5/60 GHz are analyzed using the computationally efficient UTD method developed in [3]. Two basic scenarios are considered in this paper, building blockage and tree blockage, the difference being that in the former the first- and second-order diffractions are modeled whereas in the latter only the first-order diffraction component is taken into account. The incident Shadow Boundary (ISB) defines the limit angle φISB below which the direct wave will be obstructed by the building while the Reflection Shadow Boundary (RSB) defines the equivalent angle φRSB for reflected wave contributions. The computed results show that in the incident shadow region (0 ≤ θ ≤ φISB ), the signal attenuation is larger for the 5 GHz case and in the reflection shadow region (φRSB ≤ θ ≤ 90o ), the signal level at 5 GHz is more fluctuant than that at 2 GHz according to the incident angle. II. P ROBLEM F ORMULATION The geometry of the propagation model for a cellular-based propagation scenario is illustrated in Figs. 1 and 2 for cases of building and tree blockage, respectively. In the model, a BS is located at the top of a high-rise building. The main purpose of this paper is to provide accurate results for cell planning based on the actual propagation phenomena. This would help to find appropriate coordinates (xBS , yBS , zBS ) for placing the BS when the surrounding environment is known. For this analysis, the received signal strength is independent of the mobile station’s (MS) speed, since the building/tree is assumed of infinite extent in the direction of movement of the MS. Depending on the incident or elevation angle θ, different ray components can reach the MS. The different regions are also illustrated in Figs.1 and 2. This section follows the model and notation of [1] and [2]. Three main contributing components will be present at the receiving end, namely a) incident or direct, b) reflected and c) diffracted waves. In what follows each contributing component will be analyzed in detail.

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Fig. 2.

Tree blockage propagation model and different regions based on the BS elevation angle

The ray contributions to the received signal as a function of the elevation angle can be formulated as follows •

• •

0 ≤ θ ≤ φISB (within the incident shadow region) 1) ground reflections and then second-order diffractions; 2) first and second-order diffractions; 3) first and second-order diffractions followed by reflections from the ground. φISB ≤ θ ≤ φRSB ; 1), 2), and 3) as above: 4) direct wave. φRSB ≤ θ ≤ 90o ; 1), 2), 3), and 4) as above: 5) ground reflections.

A. The Incident Waves The diffracted fields toward the MS can be computed by determining the incident field at the blocking building edges. For the soft (i.e., horizontal) polarization, the incident electric field is in the z direction. The incident plane wave for soft polarization is given by 0

2π

0

Ezi (ref erence) = E0 e−jk0 ρ = E0 e−j λ0 ρ Ezi (mobile) = Ezi (ref erence)· e

2π jλ [(wb +xm ) cos φ+hm sin φ)] 0

(1) .

For hard (vertical) polarization, the magnetic field is in the z direction. The evaluation of the incident plane wave for hard polarization is carried out in the same fashion as for soft polarization. 0

2π

0

Hzi (ref erence) = H0 e−jk0 ρ = H0 e−j λ0 ρ Hzi (mobile) = Hzi (ref erence)·

2π jλ [(wb +xm ) cos φ+hm sin φ)]

e

0

(2) .

B. The Reflected Waves In situations when the BS elevation angles exceed the RSB (θ > φRSB ), a wave reflected from the ground will also reach the MS antenna. At the MS, the reflected waves with respect to the field at the reference point are expressed as [2] Ezr (mobile) =Ezi (ref erence) · Γs (ψ)· 2π

ej λ0 [(wb +xm ) cos φ−hm sin φ)] Hzr (mobile) =Hzi (ref erence) · Γh (ψ)· 2π

ej λ0 [(wb +xm ) cos φ−hm sin φ)] Γs,h (ψ) is the reflection coefficient and given by p cos ψ − a1 ²r − sin2 ψ s,h p Γ (ψ) = cos ψ + a1 ²r − sin2 ψ

(3)

θISB=34.99o θRSB=52.43o

10 hard polarization

10

Normalized Signal Level(dB)


0 0

-10

-20

fc=2GHz fc=5GHz fc=60GHz

-30

-10

-20

-30 fc=2GHz fc=5GHz fc=60GHz

-40

-40

-50 0

20

40

60

80

0

Fig. 3. Normalized signal level versus BS elevation angle for hard polarization in case of building blockage

15

20

soft polarization

0


0


10

Fig. 5. Normalized signal level versus distance between building and receiver for hard polarization

θISB=34.99o θRSB=52.43o

-20

-40

-60

5

Distance between Buliding and Receiver(m)

Base Station Elevation Angle (θ)


-20

-40

-60


-80

-80

-100 0

20

40

60

80

0

Base Station Elevation Angle(θ)

Fig. 4. Normalized signal level versus BS elevation angle for soft polarization in case of building blockage

where ψ is the incidence angle at the ground, measured with respect to the normal at the reflection point, i.e., π π ψ = −θ =φ− . (4) 2 2 For typical mobile communications environments, the following expression for ground relative permittivity was used: 90 ²r = 15 − j . (5) f (M Hz) where a = 1 for soft polarization and a = ²r for hard polarization. C. The Diffracted Waves In a building blockage scenario the diffracted wave contributions to the received signal at the MS are represented by the first and second-order diffractions from the two edges w1 and w2 , respectively (see Fig. 1). The first-order diffraction term from edge w1 is [2] Ezd (ρ1 ) = Ezi (w1 )·Ds (L, φ1 , φ01 , n1 ) ·

5

10

15

20

Distance between Buliding and Receiver(m)

e−jk0 ρ1 √ ρ1

(6)

Fig. 6. Normalized signal level versus distance between building and receiver soft polarization

The UTD diffraction coefficients are given by Ds (L, φ1 , φ01 , n1 ) e−jπ/4 p =− · λ0 · µ½ 2n2π · ¸ · ¸ π + (φ − φ0 ) L cot F 2π g + (φ − φ0 ) 2n λ0 · ¸¾ ¸ · π − (φ − φ0 ) L − + cot F 2π g (φ − φ0 ) 2n λ0 · ¸ · ¸ π + (φ + φ0 ) L + 0 ∓ cot F 2π g (φ + φ ) 2n λ0 · ¸ · ¸ ¶¾ 0 π − (φ + φ ) L − 0 + cot F 2π g (φ + φ ) 2n λ0

(7)

The minus sign between the curly bracket is used for soft polarization and the plus sign for hard polarization. Also L is the distance parameter, n is the wedge index(n1 = n2 = 1.5), and F is the Fresnel transition integral. The present model includes multiple reflection and diffraction terms in different regions. The first-order diffraction term

θISB=21.80

θRSB=57.99

o

10

o

10

hard polarization



hard polarization 0

-10

-20

-30

0

-10

-20


-40


-50

-40 0

20

40

60

0

80

2

Fig. 7. Normalized signal level versus BS elevation angle for hard polarization in case of tree blockage

θISB=21.80

6

8

10

12

Fig. 9. Normalized signal level versus distance between tree and receiver for hard polarization

θRSB=57.99

o

4

Distance between Tree and Receiver(m)


o

soft polarization

10

0


soft polarization


0

-10

-20


-40

-20

-40

-60


-80 0

-50 0

20

40

60

80

2

4

6

8

10

12

Distance between Tree and Receiver(m)


Fig. 8. Normalized signal level versus BS elevation angle for soft polarization in case of tree blockage

w1 for soft polarization are equal to zero, because Ds = 0 at grazing incidence. Similarly, for hard polarization the first-order diffraction term from edge w1 is

from edge w1 is given by wb

hb

Ezd (ρ1 )|w1 =Ezi (ref erence) · ej2π( λ0 cosφ+ λ0 sinφ) ρ1

D

s

(L1 , φ1 , φ01 , n1)

e−j2π λ0 · √ ρ1

(8) wb

ρρ0 ρ + ρ0

hb

Hzd (ρ1 )|w1 =Hzi (ref erence) · ej2π( λ0 cosφ+ λ0 sinφ) ρ1

The distance parameter L is given by L=

Fig. 10. Normalized signal level versus distance between tree and receiver soft polarization

Dh (L1 , φ1 , φ01 , n1) · (9)

where ρ is the distance from the diffraction point to the observation point, and ρ0 is the distance from the source point to the diffraction point. Since the source distance is much greater than the√observation distance(ρ0 >> ρ then L1 ' ρ1 . Note that the λ0 term in the diffraction coefficient of eq. 8 can be incorporated into the amplitude spreading factor. This way all physical dimensions are converted into electrical quantities. The second-order diffraction from edge w2 to edge

e−j2π λ0 √ ρ1

(10)

For the second-order diffraction term from edge w2 to w1 , this term can be expressed as hb

Hzd (ρ1 )|w2 w1 = Hzi (ref erence) · ej2π( λ0 sinφ) ρ2

e−j2π λ0 |ρ2 =wb · √ ρ2

·D

h

(L2 , φ2 , φ02 , n2)|φ2 =0

·D

h

(L21 , φ1 , φ01 , n1)|φ01 =0

ρ1

e−j2π λ0 · √ ρ1 (11)

TABLE I M EAN PARAMETERS FOR S UBURBAN E NVIRONMENT IN CASE OF BUILDING AND TREE BLOCKAGE

System 2GHz/Building 5GHz/Building 60GHz/Building 2GHz/Tree 5GHz/Tree 60GHz/Tree

φISB 34.99o 34.99o 34.99o 21.80o 21.80o 21.80o

φRSB 52.43o 52.43o 52.43o 57.99o 57.99o 57.99o

θ0dB,sof t 38.40o 37.19o 35.58o 26.94o 25.12o 22.76o

where L2 ' ρ2 = wb wb ρ1 L21 = wb + ρ1

(12)

Terms higher than third-order are usually not necessary for accurate computations [4]. Note that in tree blockage, we only consider the first-order diffractions from the top of the tree. III. A NALYTICAL R ESULTS We derive the normalized signal level according to the elevation angle θ between BS and blocking building. The normalized signal level is defined as ¯ total ¯ ¯ Ez (mobile) ¯ s ¯ ¯. S =¯ i (13) Ez (ref erence) ¯ ¯ ¯ total ¯H (mobile) ¯¯ S h = ¯¯ iz . (14) Hz (ref erence) ¯ In typical moderate suburban environment, one to five-story buildings are commonly found. In this study a four-story building was assumed. In addition, the following parameters, defined in Figs.1 and 2., were used: building height hb = 10 m, building width wb = 10 m, MS receiver antenna height hm = 3 m, MS location xm = 10 m from the blocking building, tree height ht = 5 m and tree location xm = 5 m from the MS. A knife-edge approximation was used for the tree, as showed in Fig. 2. For the above defined parameters and according to the geometry of the system we have that (φISB = 34.99o ), (φRSB = 52.43o ) for the building blockage setting and (φISB = 21.80o ), (φRSB = 57.99o ) for the tree blockage setting. The normalized signal level versus BS elevation angle θ is illustrated in Figs. 3, 4, 7 and 8. Figs. 3 and 7 correspond to hard polarization cases while Figs. 4 and 8 correspond to the soft polarization cases. The contributions of different wave components as identified in Section II can be clearly observed in the three well defined regions in the figures. Indeed, in the incident shadow region the diffracted waves are the principal contributing components, in the region limited by φISB and φRSB the direct wave component is also added, and in the reflection shadow region the ground reflected components are further added to the composite received signal. Note that in the incident shadow region the attenuation due to building blockage is more severe than that caused by tree blockage. As can be appreciated, for a building blockage scenario the difference of the signal strength at 20o was approximately 15 dB when low and high frequency bands

θ0dB,hard 38.67o 37.20o 35.64o 27.00o 25.19o 22.77o

θ−20dB,sof t 27.12o 29.98o 33.85o 9.45o 12.25o 19.76o

θ−20dB,hard 3.98o 24.88o 33.57o 28.11o 28.14o 19.88o

are considered, as it could be expected. Figs. 5, 6, 9 and 10 show the results of normalized signal level as a function of the distance between the building or tree and the MS receiver. The shadowing effect of the building or tree is evident when the MS approaches such an obstacle. Note also that as the frequency increases, the signal attenuation becomes larger in the incident shadow region. In other words, line of sight (LOS) becomes the dominant mechanism for propagation. Finally, as it can appreciated from Figs. 3-10, in the 60 GHz band attenuation is severe, rendering such a high frequency band not suitable for cellular operation. In Table 1, the values of φISB , φRSB , θ−20dB,sof t/hard , θ0dB,sof t/hard are the mean values for 2 GHz, 5GHz, and 60 GHz, respectively. We could get an insight on the difference of propagation characteristics between 3G/4G communication systems through the above Table 1. IV. C ONCLUSION A propagation model for building and tree blockage applicable to 3G/4G cellular communication systems has been considered in this paper. High frequency ray-tracing methods and UTD were used to analyze typical propagation scenarios. The ray contributions includes diffraction from building edges, reflection from the ground and building, as well as the direct wave. According to the angle of the impinging wave different components will be present in the composite received signal. We compared the propagation characteristics for 3G/4G communication systems using the above mentioned analysis method. Results strongly depend on the geometry of the considered propagation scenario as well as on the frequency. We can observe that channel attenuation increase with frequency, thus, high frequency bands will impose significant limitations to the link budget. This may justify the use of multihopping approached in some scenarios of future 4G systems R EFERENCES [1] P.A. Tirkas, C.M.Wangsvick, and C.A.Balanis, “Propagation Model for Blockage in Satellite Mobile Communications System,” IEEE Trans. MTT, Vol.46, No.7 pp. July 1998 [2] C.A.Balanis, “Antenna Theory Analysis and Design,” 2nd ed., New York: Wiely, 1996 [3] Chan-Byoung Chae et al, “’Radio Propagation for Building and Tree Blockage at B-WLL,” Telecommunications Review, Vol.9, No.5 pp.727740, Korea, 1999 [4] L.A.Polka, C.A. Balanis, and A.C. Polycarpou, “High-frequency methods for multiple diffraction modeling: Application and comparison,” J. Electromagnetic Waves and Applications, Vol.8, No. 9/10, pp.1223-1246, 1994