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Radio-propagation measurements and modeling in indoor stairwells at millimeter-wave bands
Physical Communication 38 (2020) 100955
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Radio-propagation measurements and modeling in indoor stairwells at millimeter-wave bands ∗
A.O. Aldhaibani a , , Tharek A. Rahman b , Abdulmalik Alwarafy c a b c
College of Engineering & Petroleum, Hadhramout University, Yemen Wireless Communication Centre, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Malaysia Department of Electrical Engineering, King Saud University, Saudi Arabia
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info
Article history: Received 18 June 2019 Received in revised form 4 October 2019 Accepted 28 November 2019 Available online 30 November 2019 Keywords: Indoor propagation Stairwell Millimeter-wave 5G network Path loss models Polarization
a b s t r a c t A good understanding of signal wave attenuation along a stairwell is necessary for successful wireless network planning. In this study, the propagation characteristics in indoor multi-floor stairwell environments are examined under the millimeter frequency band. An extensive signal measurement is conducted in two stairwells. These measurements provide information important for ensuring consistent radio transmission of data in such environment, which then enables immediate response during emergency operations. In these measurements, directional horn antennas for co-polarization and cross-polarization in transmitters and receivers are used for different millimeter frequencies (26, 28, 32, and 38 GHz). The feasibility of using the millimeter wave, which is suitable for indoor 5G wireless networks, in two different indoor stairwell environments is investigated. Four different models are employed for path loss investigation. Under single frequencies, the close-in free-space reference distance and floating-intercept path loss models are adopted. For multiple frequencies, the close-in free space reference distance with frequency dependent path loss exponent and alpha-beta-gamma models are applied. The measurements provide data on the received power at multi-floor stairwells through the stair steps, which are useful for path loss study. Accordingly, the path loss exponent values, standard deviations, and other parameters are obtained. © 2019 Published by Elsevier B.V.
1. Introduction Currently, users demand access to high-quality services at any time and location and hence create substantial traffic in wireless networks [1]. Thus, the present cellular networks that cover large cell areas cannot be expected to provide sufficient capacity and satisfactory data rates. 3G cellular system services have been developed, and large data rates are realized in the current LTE/LTE-A systems. However, technical challenges and data rates continue to increase because of pressure from data traffic, which remains as a major issue in the future of mobile networks [2]. Increasing the frequency bandwidth is necessary because the frequency spectrum at low-frequency bands (below 6 GHz) is insufficient. To satisfy the rising demands, especially for indoor applications, dense small-cell deployment has become a main direction for the traditional and next-generation communication systems [3]. Researchers are presently examining the millimeter-wave (mm-wave) frequencies in 5G systems, which can increase data ∗ Corresponding author. E-mail address: [email protected] (A.O. Aldhaibani). https://doi.org/10.1016/j.phycom.2019.100955 1874-4907/© 2019 Published by Elsevier B.V.
speed and bandwidth [4]. However, at high frequencies, high path loss with distance and deep shadowing occurs because of weak diffraction reflection [5]. Thus, studies on radio propagation properties at high-frequency bands are important. Different types (deterministic, empirical, and stochastic) of large-scale path loss models exist, but measurement-based path loss models provide realistic insight into the propagation characteristics of a wireless channel [2,6]. In [7], single-frequency and multi-frequency path loss models are defined. The distance d in the models are 3D T–R separation distances based on measurements. Additionally, both co- and cross-polarized path loss models as well as combinedpolarization path loss models are established for directional and omnidirectional cases. Site-specific propagation models for different indoor environments with the features of low power and short distance are needed to describe the channel characteristics more accurately. Propagation models are formulas for calculating large-scale path loss (or gain) and are usually established empirically on the basis of measurements. Path loss is dependent mainly on the environment, frequency, and height of antennas; hence, a propagation model can only be applied to sites similar to where the model is developed [8]. In 5G systems, a large path loss can be minimized by designing small cells or hot spot systems within a confined coverage area,
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where high-speed data transmission can be realized [9]. Recently, 5G channel propagation at 20 and 30 GHz bands for indoor and outdoor environments, but not in stair environments, has been investigated. In [9], extensive propagation measurement campaigns are conducted at 28 and 38 GHz, which measured path loss and delay spread. Obtaining this information is vital for the design and operation of future 5G cellular networks that use the mm-wave spectrum. The comparison of three candidates’ large-scale propagation path loss models for use over the entire microwave and mm-wave radio spectrum is presented in [10]. A comparison was also provided for the 2–73 GHz band for indoor office and shopping mall environments. Further measurement campaign conducted the propagation measurements, and channel modeling was carried out at 28, 38, 60, and 73 GHz in urban microcell, urban macrocell, and indoor hotspot scenarios [11]. Alvarez et al. constructed an indoor radio channel in the range 1–9 GHz using omnidirectional antennas and arranged four environments (line-of-sight [LOS], Soft-non-LOS [NLOS], Hard-NLOS, and corridor). The estimated path loss exponents (PLEs) with verity reference distances were high in the Hard-NLOS senior compared with those in other environments [12]. The development of wireless networks to provide communication services at emergency sites is required [13]. As a crucial part of a building structure, stairwells play an important role in daily functioning and emergency situations. Wireless coverage along stairwells and understanding radio propagation in such environments is crucial in allowing public safety personnel communication and information sharing for a swift and effective response [14]. Propagation measurements were reported in [8] for the multi-floor stairwell using measurements at 2.4 GHz. Fundamental propagation mechanisms, such as reflection from stairwell walls and transmission through the stair steps, were investigated. In [3], the path loss model with receiving antenna height dependence in an indoor stair environment was proposed on the basis of intensive measurements. The parameters of the model, such as path loss, antenna heights, and shadowing, were extracted. Radio propagation in four typical stairwells was studied through measurement at two frequencies (2.4 and 5.8 GHz) in [15]. The path loss exponent (PLE) values were derived for vertical and horizontal polarizations. In [16], a path loss (PL) model for a stairwell environment considers the effect of building floor height as well as unique PL patterns on several stair flights, which incorporate floor attenuation factors that allow easy comparison with established indoor PL models at 0.9 and 1.8 GHz. Essential insights into the relation of PL to distance for several frequency bands were presented in [17]. These points are useful in facilitating wireless network planning under a stairwell setting. The results in this study can be exploited by the designers of small-cell wireless communications system. This paper provides the propagation characteristics of the mm wave at 26, 28, 32, and 38 GHz bands in a tropical city (Kuala Lumpur). An extensive measurement is conducted in two different indoor stairwell environments with four different mm-wave candidate frequencies for 5G wireless networks. The measurement uses a highly directive horn antenna with co-polarization and cross-polarization at the transmitter (Tx) and receiver (Rx). These measurements are used to collect all reflected signals with gains above the noise floor. Four popular propagation modeling approaches are applied and include single-frequency models, such as close-in free space reference distance (CIM) and floating-intercept (FIM) path loss models, and multi-frequency models, such as alpha-beta-gamma (ABG) and close-in free space reference distance with frequency-dependent PLE (CIF) models. The propagation characteristics, such as path loss, PLE, β , and standard deviation (STD), in multi-floor stairwells obtained by
Table 1 Number of stairs steps for each section of stairwells. Stairwell
Section S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
Stair 1 Steps
12
12
12
12
12
12
12
12
12
12
Stair 2 Steps
13
13
13
13
13
13
13
13
13
13
measurement are investigated. The remainder of this paper is organized as follows. The stair structures are described in Section 2. The measurement system is discussed in Section 3. The results and discussion are presented in Section 4. The conclusions are provided in Section 5. 2. Stair structures Two dog-leg stairwells for biggest building at Universiti Teknologi Malaysia in Kuala Lumpur campus have been chosen for this study. At stair 1, the stairwell resides in the corner of the building and has width 2.9 m whereas the width for each half landing is 1.6 m as in Fig. 1, while the stair 2 located in the middle of the building beside the building entrance, it has 3.9 m width whereas the width for each half landing is 1.9 m as in Fig. 1. Two sections of stair steps are found between each two floors (the ten stair sections are denoted by S1, S2, S3, . . . , S10, in Fig. 1). A door from each stairwell leads to the floor inside the building, but it is kept closed during the measurement of the stairwells. In Fig. 2 a typical stair environment composed of the stair steps, railings, platforms, and so on presented. In stair 1, each stair section has around 12 steps, and each step has a (tread) depth of 0.29 m, and a height (rise) of 0.18 m. While in stair 2, each stair section has around 13 steps, and each step has a (tread) depth of 0.32 m, and a height (rise) of 0.13 m. The staircases are of a dog-leg type configuration, which is the most common type built inside modern buildings [18,19]. Notably, the laws in most countries forbid the use of combustible materials as components or finishing of staircases to provide safe passage in case of fire [16,17]. At stair 1, The Tx is positioned on the 15th floor, and the Rx is moved along the stairwell starting from the 15th floor to the 10th floor, while in stair 2 the Tx is located in 6th floor and the Rx move down until first floor, as shown in Fig. 2. From our observations, most stair steps are made of reinforced concrete, and the stairwell walls are made of concrete as well. Ceilings/floors in modern offices and buildings are mostly made of concrete. This similarity of the structures and materials of stairwells in modern office building ensures that the proposed path loss models can be applied to many, if not all, of the indoor stairwell propagation environments in modern buildings. The numbers of steps for each section of stairwells are listed in Table 1. 3. Measurement system During the measurement, 10 stair flights denoted by S1 until S10 and 5 half-landings are within the transmitter-to-receiver separation distance. The measurement setup consists of a transmitter, Tx, and a receiver, Rx, operating at 26, 28, 32, and 38 GHz. Fig. 3 illustrates the block diagram of the measurement system, and Fig. 2 shows a photograph of the equipment in the measurement site. Specifically, the transmitter consists of a signal generator (Anritsu MG3694C Signal Generator 40 GHz) with a power transmit level (Pt ) of 10 dBm. The receiver includes a spectrum analyzer (Anritsu MS2720T), which provides accurate RF power measurements over a wide frequency range. With an
A.O. Aldhaibani, T.A. Rahman and A. Alwarafy / Physical Communication 38 (2020) 100955
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Fig. 1. Cross-sectional view of investigated dog-leg stairwell.
Fig. 2. Measurement hardware setup and stair section view.
antenna attached, RF power measurement becomes the received signal strength (RSSI). RSSI measurements can be combined with on-screen map displays to become a versatile solution for mapping the coverage of RF transmitters. Horn antennas are used at both transmitting and receiving ends. The antennas have a gain and beam width depending on the frequency used, the 3 dB beam width for vertical plane in the range 19.7–14 deg. while for horizontal plane, the 3 dB beam width in the range 15.2–20.2 deg. The antenna gain in the range 18.80 dbi–21.34 dbi., At the transmitter, transmission power is used to extend the dynamic measurement range. The signal generator is placed on top of an elevated stand; the transmitting antenna’s height is 1.25 m above
the ground. Three different transmit/receive antenna polarizations are considered in the measurements: vertical/vertical (v–v), horizontal/horizontal (h–h), and vertical/horizontal (v–h). The receiving antenna is installed on a wooden tripod with a height of approximately 1.2 m. The Rx antenna moves along the adjacent sections of the stairs steps by three steps. Thus, according to the measurement scheme, the effective range of the Tx–Rx separation is about 1–22.5 m. It is envisioned to be the typical applicable range of the indoor small cell deployment. A low noise amplifier (LNA) is added to the front end of the receiver to extend the measurement dynamic range in both stairwells.
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Fig. 3. System block diagram.
4. Results and discussion The measurements are conducted in two stairwells. In the measurement scheme, the Tx antenna is fixed at one location, and the Rx antenna is moved on a wooden arm. On every three stair steps, the Rx antenna records 10 values for the signal and these values are averaged offline to obtain the mean power. Eight typical measured points are obtained at a stairwell between two floors. The material of the stair rails can influence the measurement. However, the material’s effects may be less important than the wide surfaces of the surrounding walls, stair steps, and stairwell ceilings, given that the main reflection and transmission occur therein. Antenna polarization (co-polarization and cross polarization) is considered in this work. Fig. 4 shows the path loss with the total number of stair steps (120) for five floors under vertical polarization in stairwells 1 and 2. The mean path loss increases as the receiving antenna moves down along the stairs. Notably, a sudden rise in path loss is observed at the first six junctions (each section comprises 12 steps between consecutive stair sections) (Fig. 4). We call this sudden rise in path loss as the junction between each section. When the Rx antenna is located beyond 100 stair steps in stairwell 1 at 28 GHz (Fig. 4a), the path loss appears to settle down because of weak signal reception caused by the reflection. Notably, in the first stair section, the LOS path contributes to the majority of the received signal. The reflected path also contributes to the signal reception because of the reflectivity of the stairwell walls. In the last three sections, the front wall blocks the signal of the Tx antenna from reaching the Rx antenna directly and causes the sudden signal drop. Meanwhile, at the 28 GHz band in stairwell 2 (Fig. 4a), the signal stabilizes in the last five sections but produces a path loss more effectively than in stairwell 1 in the same band. The main propagation mechanisms in the stairwell include reflections from the stairwell walls, transmissions through the stairs, and their combinations. In Fig. 4b, a clear increase in path loss at the 38 GHz frequency band, (38 GHz) is observed at the junctions between the consecutive stair sections with vertical polarizations. The large path loss increase is more apparent at the first junction (around stair step 12 for stairwells) than at other sites because the LOS scenario exists for the entire first section of the stairwells in each measurement. When Rx turns at the second junction (second 12 steps) to proceed to the next section of the stairwell, the LOS path is lost and a significant increase in path loss transpires for the first four sections because of multiple reflections. In the last remaining junctions, the path loss does not change and the junctions appear flat, which indicates that the signal is weak and
would soon disappear from the stairwell floors. The path loss value for stairwell 2 is less than the path loss for stairwell 1 at 38 GHz (Fig. 4b). Generally, the 28 and 38 GHz frequency bands in stairwell 2 are superior to those in stairwell 1. Therefore, the results are dependent on frequency and distance (Fig. 4a, b). The effect of the LOS path is substantial in the first section at all frequencies. However, for the high-frequency band of 38 GHz, the received power weakens in the farthest section, thus demonstrating the path loss stability at 38 GHz after 60 stair steps for both stairs. In summary, the path loss in stairwell 2 is superior to that in stairwell 1 because the larger width of the former stairwell. Stairwell 2 may not reflect the path as much as stairwell 1 does, which then makes the signal weak, and excessive reflection also transforms the signal as noise. This result indicates that the path loss is not only dependent on frequency but also affected by environmental objects, which include the stairwell wall structure. Path Loss Models: A—Single frequency path loss models To model how different operating frequencies may affect path loss along the stairwells. Measured path loss values that were obtained were analyzed pertaining to their relation to distance in m. In this work, different path loss models are investigated. Path loss is an indication of power loss in the channel [15] and can be expressed as follows: P (d) = 10 Log
(
Pt
) (1)
Pr
where (Pt ) is the transmitted power; (Pr ) is the received power that is proportional to 1/dn ; d is the separation distance between Tx and Rx; and, n is the path loss exponent (PLE). The mean power predicted by (1) is a random variable, which can be characterized by adding an extra term W, a log-normal distribution for both outdoor and indoor propagation environments. Thus, for single frequency to calculate PLE, the close-in free space reference distance (CIM) model is used. The CIM path loss model under the stairwell environment could be described [11] and expressed as follows: PL (f , d) [dB] = PL (f , d0 ) + 10 n log10
(
d d0
)
+ Wσ ,
(2)
where PL (f , d0 ) is the free-space path loss at a reference distance (d0 = 1 m), n is the PLE, and Wσ is the random variable that represents the shadowing effect with zero mean Gaussian distribution and STD of σ in dB. In Fig. 5 the measured path loss and its linear fit as a function of the separation distance between Tx and Rx antennas at all frequencies for both stairwells are shown. The path loss is measured in each section of the stairwells with certain distances. The total distance for the five floors is 22.5 m. The path loss increases with distance. Compared with the path loss presented in Fig. 4, the path loss in Fig. 5 shows a different appearance when plotted as a function of stair steps and separation distances. Evidently, when plotted as a function of separation distance, path loss prediction varies more significantly than when plotted as a function of stair steps. In Fig. 5, the path loss variation with Tx–Rx separation distances at 26, 28, 32, and 38 GHz frequencies when the transmitted horn antenna is directed toward the stairs. Many reflected paths are created on the basis of the stair structure. Moreover, the reflected paths hold a strong power because of the reflection from the surrounding environment, particularly, from the concrete stairs, walls, and ceiling. The PLE at all frequencies for stairwell 2 is better than that for stairwell 1 (Fig. 5a, b). This result is observed because of the wider stairwell 2 and its resultant lower signal reflection (when the walls are closer to one another, the signal is reflected more rapidly and within a shorter distance, and thus weakens as noise). The PLEs in stairwell
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Fig. 4. Path losses for two stairwells at 28 and 38 GHz.
1 are between 7.52 and 7.36; the PLE is high at the 26 or 32 GHz frequency band and lowest at the 38 GHz band. However, these values are acceptable for emergency situations in the stairwell environment because the received power can be acquired within five floors. The PLEs in stairwell 2 are between 6.99 and 7.5 (Fig. 5b). The highest PLE is achieved at 32 GHz, whereas the lowest value is at 38 GHz. Therefore, 38 GHz attains the best result among the frequencies tested in both stairwells in such a specific environment. Fig. 6 demonstrates the relation of path loss bearing a Tx–Rx separation distance with co-polarization and cross-polarization under the CIM model at the 28 and 38 GHz frequency bands. Notably, from the 20 and 30 bands, 28 and 38 GHz, respectively, were chosen because of the popularity of the last two frequencies. In the last few years, the 28 and 38 GHz bands have been considered potential bands for future 5G broadband cellular communication networks [9]. Clearly, the path loss increases with increasing separation distance in all cases. In stairwell 1, the horizontal polarizations achieve a superior performance to the vertical and horizontal–vertical polarizations at both 28 and 38 GHz bands (Fig. 6a, b). This finding may be explained in terms of the orientation of the electric field with respect to the stairwell structure. For the polarization, the electric field is parallel to the stair structure; hence, strong reflection and diffraction effects are expected. In this study, the 38 GHz band performs better than the 28 GHz band in all the polarization cases. The worst results are found in the cross-polarization at both bands. All the parameters in stairwell 1 for all polarization cases at all frequencies are shown in Table 2. In Fig. 6b, the co- and crosspolarizations at 28 and 38 GHz in stairwell 2 are displayed. The horizontal–horizontal polarization in most of the cases is better than other polarizations at both frequencies because of the same reason mentioned above (parallel orientation of electric field to the stair structure). Hence, strong reflection and diffraction effects are also expected. Such performance is followed by that of vertical–vertical polarizations, whereas the cross-polarization attains the worst results in both frequencies. Fig. 6 shows that the 38 GHz band is more effective than the 28 GHz band. The parameters for stairwell 2 at all polarizations are listed in Table 3. Furthermore, the distance reaches 22.5 m (five floors), but the path loss stabilizes at the end of the stairwell. Therefore, the signal is weak and disappears at the end of the stairwell. Another model which has been used in 3GPP and WINNER II models is the FIM model. This model has two parameters and
Table 2 Path loss model parameters for the CIM model at all frequencies for stairwell 1. Single-frequency CIM path loss model for d0 = 1 m Freq. (GHz)
Pol.
PLE
σ (dB)
26
V–V H–H V–H
7.52 7.31 7.79
9.95 9 6.93
28
V–V H–H V–H
7.6 7.33 7.86
8.86 8.99 5.57
V–V H–H V–H
7.48 7.6 7.96
8.49 8.65 7.89
V–V H–H V–H
7.36 7.26 7.57
8.98 8.95 5.55
32
38
does not consider a physically based anchor to the transmitted power, and has a similar form to CIM model as [20]: PLFI (d) [dB] = α + 10.β log10 (d) + WσFI
(3)
where α [dB] and β (e.g. the average path loss exponent) are the floating intercept and the slop line, respectively. The showing effects are represented by random variable WσFI which has Gaussian distribution with zero mean and standard deviation σ FI [dB]. Table 4 indicates the parameter values for the indoor channels of the FIM model in both stairwells. Compared with Table 2, the difference in STD between the CIM and FIM models is small. The CIM model provides the benefit of simple comparisons of measurements across many frequency bands by using merely one parameter because the frequency-dependent effects of the model are primarily contained in the 1 m free space path loss. The environment provides additional and substantial frequencydependent loss beyond the first meter of free-space propagation. The parameter values are identical to those in previous research [15]. Fig. 7 shows a plot of the path loss versus the distance under the FIM model at all frequencies in both stairwells. The STD of the shadow fading is slightly smaller under the FIM model than in the CIM model in Fig. 5. With a separation distance of 22.5 m, the difference in STD between the FIM and CIM models is less than 3 dB. The line slopes differ between the FIM and CIM models. The FIM model in stairwell 1 attains a β range of 9.09–10.92 and STD range of 6.28–8.07 dB for the identical measurement data
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Fig. 5. CIM model for (a) stairwells 1 and (b) stairwells 2, under all the frequencies tested.
Fig. 6. a and b, Measured path loss polarization for stairwells 1 and 2 at 28 and 38 GHz with CIM.
sets. By contrast, the β range is 6.73–9.81 and STD range is 8.01–
for stairwell 1. Meanwhile, the lowest STD is at 26 GHz in both
9.98 dB for stairwell 2. The STD for stairwell 2 is greater than that
stairwells and the highest STDs are at 38 and 32 GHz in stairwells
A.O. Aldhaibani, T.A. Rahman and A. Alwarafy / Physical Communication 38 (2020) 100955
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Fig. 7. a, b Measured path losses for stairwells 1 and 2 at 26, 28, 32 and 38 GHz under the FIM model. Table 3 Path loss model parameters for CIM model at all frequencies for stairwell 2..
Table 5 Parameter values for both ABG and CIF models in stairwells 1 and 2. Multi-frequency (ABG and CIF) path loss model parameters
Single-frequency CIM path loss model for d0 = 1 m Freq. (GHz)
Pol.
PLE
σ (dB)
ABG
α
β
γ
σ (dB)
26
V–V H–H V–H
7.06 7.36 7.52
10.16 12.10 8.6
Stairwell 1 Stairwell 2
9.75 8.22
12.9 8.81
1.57 2.81
7.38 10.06
CIF
n
b
f0
σ (dB)
28
V–V H–H V–H
7.15 6.99 7.27
10.46 13.55 6.48
Stairwell 1 Stairwell 2
7.44 7.18
−0.04 −0.013
31 GHz 31 GHz
9.04 10.34
32
V–V H–H V–H
7.55 7.35 7.57
9.98 11.51 7.56
38
V–V H–H V–H
6.99 6.99 7.29
9.69 10.45 8.52
(CIF) path loss model is a multi-frequency model that employs the same physically motivated FSPL anchor at 1 m as the CIM model. The CIF model equation is presented in [7]: PL
CIF
(f , d) [dB] = FSPL(f , d0 ) + 10 n 1 + b
Table 4 Path loss model parameters for FIM models at all frequencies for both stairwells.
( × log10
Single-frequency FIM path loss model Freq.
26 28 32 38
Stairwell 1
Stairwell 2
β
STD (σ ) (dB)
β
STD (σ ) (dB)
10.92 9.86 9.17 9.09
6.28 6.67 7.58 8.07
9.81 8.97 7.37 6.73
8.01 9.6 9.98 9.67
1 and 2, respectively. Notably, the slope of the FIM model often holds no physical basis. By contrast, the CIM model is physically based and adequately estimates path loss data points. B—Multi-frequency path loss models A multi-frequency model known as the alpha-beta-gamma (ABG) model which includes a frequency dependent γ , distancedependent α terms and optimization factor β to describe path loss at various frequencies [7,12]. The ABG model equation is given by [7]: PLABG (f , d) [dB] = 10 α log10 d d0 + β
( / )
+ 10γ log 10(f /1 GHz) + WσABG
(
(4)
This ABG model is used to investigate multi-frequency based on physical distance (d0 ) of 1 m. Another two-parameter multi-frequency model can be considered to be an extension of the CIM model. The close-in free space reference distance with frequency dependent path loss exponent
d d0
)
+ XσCIF
(
f − f0
))
f0 (5)
Where d0 = 1 m, n denotes the distance dependency of path loss (e.g. the path loss exponent, or PLE), b is an intuitive model-fitting parameter that represent the slop of linear frequency dependency of path loss. Noted that, all parameters for CI, FI, CIF and ABG path loss models are estimated based on minimum mean square error (MMSE) approach [7]. Fig. 8a and b display the ABG and CIF path loss models. The CIF standard division lies within the difference of 2 dB from ABG in stairwell 1 and around 0.3 dB in stairwell 2. An extremely small difference is achieved using one less model parameter (at the CIF model). For stairwell 1, the ABG model reduces the STD by around 1.66 dB relative to the CIF model, whereas in stairwell 2, the ABG model is 0.28 dB smaller than the CIF model. Table 5 shows the parameter values for both ABG and CIF models in both stairwells. Thus, the difference in STD between the CIF and ABG models is less than 2 dB. As shown subsequently, the CIF and ABG models are even closer in performance (a fraction of a dB difference in STD, which is well within the typical measurement error from cable flexing, pointing errors, or temperature variations) for these environments. 5. Conlusion The propagation characteristics of an mm-wave 5G network in two stairwell environments are investigated through experimental measurements. The measurements are obtained at 20
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Fig. 8. Multi-frequency CIF and ABG path loss models in (a) stairwell 1 and (b) stairwell 2.
and 30 GHz bands (26, 28, 32, and 38 GHz) for every three stair steps for five levels. The transmitter is placed at the top floor, and the receiver is moved along the stairwells. The propagation mechanisms for each of the 10th stair sections are analyzed. The proposed research can be applied to predict the signal level of different stair step structures and provide a set of criteria for cell planning in an indoor environment. The particular stairwells investigated involve reflective side walls. Thus, the reflected signal from the walls weakens in the lowest floors. The PLE values and the associated STDs are extracted. When the conventional separation distance is used in a path loss model, the n values are significantly higher than the values for urban and other indoor propagation environments. Single-frequency (CIM and FIM) and multi-frequency (ABG and CIF) path loss models are investigated. The directional horn antennas for co-polarization and cross-polarization at the Tx and Rx are used for all frequencies. The path loss model offers virtually identical results whether under the single- or multi-frequency path loss models. This finding indicates the faster signal drop in stairwells than those in other environments. The results of this study are beneficial for understanding radio propagation in an indoor stairwell environment, which is crucial in emergency situations (i.e., law enforcement and firefighting purposes). The results can also help in developing effective indoor communication systems and may be useful for designing small-cell wireless communication systems.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The author would like to acknowledge and express sincere appreciation to Universiti Teknologi Malaysia and Ministry of Higher Education, Malaysia for financing this research project under grant No. Q.J130000.21A2.02E95 and No. A.J090601.5357.07085TABUNG PUSAT KECEMERLANGAN PENGAJIAN TINGGI (HICoE) PUSAT KOMUNIKASI WAYARLES (WCC). References [1] A.I. Sulyman, A. Alwarafy, G. Maccartney, T. Rappaport, A. Al-Sanie, Directional radio propagation path loss models for millimeter-wave wireless networks in the 28, 60, and 73 GHz bands, IEEE Trans. Wireless Commun. PP (99) 1. [2] M. Kim, Y. Konishi, Y. Chang, J.i. Takada, Large scale parameters and double-directional characterization of indoor wideband radio multipath channels at 11 GHz, IEEE Trans. Antennas and Propagation 62 (1) (2014) 430–441. [3] Yu Yu, Yang Liu, Wen-Jun Lu, Hong-Bo Zhu, Path loss model with antenna height dependency under indoor stair environment, Int. J. Antennas Propag. (2014) 1–6.
A.O. Aldhaibani, T.A. Rahman and A. Alwarafy / Physical Communication 38 (2020) 100955 [4] A.F. Abouraddy, S.M. Elnoubi, Statistical modeling of the indoor radio channel at 10 GHz through propagation measurements-partI: Narrowband measurements and modeling, IEEE Trans. Veh. Technol. 49 (5) (2000) 1491–1507. [5] W.C.Y. Lee, Mobile Communications Engineering, second ed., McGraw-Hill, New York, NY, USA, 1982. [6] A.I. Sulyman, A. Alwarafy, H.E. Seleem, K. Humadi, A. Alsanie, Path loss channel models for 5G cellular communications in Riyadh city at 60 GHz, in: 2016 IEEE International Conference on Communications, ICC0, Kuala Lumpur, 2016, pp. 1–6. [7] G.R. Maccartney, T.S. Rappaport, S. Sun, S. Deng, Indoor office wideband millimeter-wave propagation measurements and channel models at 28 and 73 GHz for ultra-dense 5G wireless networks, IEEE Access 3 (2015) 2388–2424. [8] S.Y. Lim, Z. Yun, J.M. Baker, N. Celik, H.s. Youn, M.F. Iskander, Propagation modeling and measurement for a multifloor stairwell, IEEE Antennas Wirel. Propag. Lett. 8 (2009) 583–586. [9] T.S. Rappaport, et al., Millimeter wave mobile communications for 5G cellular: It will work!, IEEE Access 1 (2013) 335–349. [10] S. Sun, et al., Investigation of prediction accuracy, sensitivity, and parameter stability of large-scale propagation path loss models for 5G wireless communications, IEEE Trans. Veh. Technol. 65 (5) (2016) 2843–2860. [11] T.S. Rappaport, G.R. MacCartney, M.K. Samimi, S. Sun, Wideband millimeter-wave propagation measurements and channel models for future wireless communication system design, IEEE Trans. Commun. 63 (2015) 3029–3056. [12] A. Alvarez, G. Valera, M. Lobeira, et al., Ultra wideband channel model for indoor environments, J. Commun. Netw. 5 (2003) 309–318. [13] Omar Abdul Aziz, Tharek Abdul Rahman, Modelling the impact of operating frequencies on path loss and shadowing along multi-floor stairwell for 0.7 GHz–2.5 GHz range, Prog. Electromagn. Res. M 40 (2014) 69–78. [14] Omar Abdul Aziz, Tharek Abdul Rahman, Investigation of path loss prediction in different multi-floor stairwells at 900 MHz and 1800 MHz, Prog. Electromagn. Res. M 39 (2014) 27–39. [15] S.Y. Lim, Z. Yun, M.F. Iskander, Propagation measurement and modeling for indoor stairwells at 2.4 and 5.8 GHz, IEEE Trans. Antennas and Propagation 62 (9) (2014) 4754–4761. [16] A. Hoffmann, R. Muehlnikel, Experimental and numerical investigation of fire development in a real fire in a fivestorey apartment building, Fire Mater. 35 (2010) 453–462. [17] Building Department, The government of Hong Kong special administrative unit, code of practice for fire safety in buildings, 2011. [18] S. Emmitt, C.A. Gorse, Barry’S Introduction to Construction of Buildings, second ed., Wiley–Blackwell, 2010. [19] C. Hartwell, N. Pevsner, The Buildings of England, Tale University Press, Lancashire: North, 2009. [20] M.K. Samimi, T.S. Rappaport, G.R. MacCartney, Probabilistic omnidirectional path loss models for millimeter-wave outdoor communications, IEEE Wirel. Commun. Lett. 4 (4) (2015) 357–360.
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A.O. Aldhaibani obtained his Ph.D. degree in communication Eng. from The University Malaysia Perlis, Malaysia, 2015, and M.Sc. from University of Technology Malaysia (UTM) 2012. In 2016 he a pointed as a postdoc fellow in University of Technology Malaysia(UTM) to work in the experimental propagation channel of 5G networks at the real field (Kuala Lumpur Area). Currently, he working as a lecturer at Hadhramout University. He has been working in the field of optical wireless technologies solution for 4G networks, computer networking, passive optical networks (PON), wireless sensors network and fiber wireless (FiWi), both in experimental and academically research valued work. Currently he is working at Hadhramout University. Tharek Abd Rahman Abdul Rahman Tharek (M’14) received the B.Sc. degree (Hons.) in electrical engineering from the University of Strathclyde, Glasgow, U.K., the M.Sc. degree in communication engineering from University of Manchester Institute of Science and Technology (UMIST), Manchester, U.K., and the Ph.D. degree in mobile communication from the University of Bristol, Bristol, U.K. He is currently a Professor of Wireless Communication with the Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia. He is the Director of Wireless Communication Centre (WCC), Faculty of Electrical Engineering, Universiti Teknologi Malaysia, and currently conducting research related to mobile communications, antenna and propagation. He has also conducted various short courses related to mobile and satellite communication to the telecommunication industry and government agencies, since 1988. He has authored more than 300 scientific papers in journals and conferences and obtained many national and international awards. He is also a Consultant for many communication companies and an active member in several research academic entities. Abdulmalik Alwarafy received the B.S. degree in electrical engineering with a minor in communication from Ibb University, Ibb, Yemen, in 2009, and the M.Sc. degree in electrical engineering with a major in communications from King Saud University, Riyadh, Saudi Arabia, in 2015, where he is currently pursuing the Ph.D. degree in electrical engineering.,His current research interests include millimeter-wave measurements and propagation channel modeling and analysis for the fifth-generation cellular mobile communications.
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Full length article
Radio-propagation measurements and modeling in indoor stairwells at millimeter-wave bands ∗
A.O. Aldhaibani a , , Tharek A. Rahman b , Abdulmalik Alwarafy c a b c
College of Engineering & Petroleum, Hadhramout University, Yemen Wireless Communication Centre, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Malaysia Department of Electrical Engineering, King Saud University, Saudi Arabia
article
info
Article history: Received 18 June 2019 Received in revised form 4 October 2019 Accepted 28 November 2019 Available online 30 November 2019 Keywords: Indoor propagation Stairwell Millimeter-wave 5G network Path loss models Polarization
a b s t r a c t A good understanding of signal wave attenuation along a stairwell is necessary for successful wireless network planning. In this study, the propagation characteristics in indoor multi-floor stairwell environments are examined under the millimeter frequency band. An extensive signal measurement is conducted in two stairwells. These measurements provide information important for ensuring consistent radio transmission of data in such environment, which then enables immediate response during emergency operations. In these measurements, directional horn antennas for co-polarization and cross-polarization in transmitters and receivers are used for different millimeter frequencies (26, 28, 32, and 38 GHz). The feasibility of using the millimeter wave, which is suitable for indoor 5G wireless networks, in two different indoor stairwell environments is investigated. Four different models are employed for path loss investigation. Under single frequencies, the close-in free-space reference distance and floating-intercept path loss models are adopted. For multiple frequencies, the close-in free space reference distance with frequency dependent path loss exponent and alpha-beta-gamma models are applied. The measurements provide data on the received power at multi-floor stairwells through the stair steps, which are useful for path loss study. Accordingly, the path loss exponent values, standard deviations, and other parameters are obtained. © 2019 Published by Elsevier B.V.
1. Introduction Currently, users demand access to high-quality services at any time and location and hence create substantial traffic in wireless networks [1]. Thus, the present cellular networks that cover large cell areas cannot be expected to provide sufficient capacity and satisfactory data rates. 3G cellular system services have been developed, and large data rates are realized in the current LTE/LTE-A systems. However, technical challenges and data rates continue to increase because of pressure from data traffic, which remains as a major issue in the future of mobile networks [2]. Increasing the frequency bandwidth is necessary because the frequency spectrum at low-frequency bands (below 6 GHz) is insufficient. To satisfy the rising demands, especially for indoor applications, dense small-cell deployment has become a main direction for the traditional and next-generation communication systems [3]. Researchers are presently examining the millimeter-wave (mm-wave) frequencies in 5G systems, which can increase data ∗ Corresponding author. E-mail address: [email protected] (A.O. Aldhaibani). https://doi.org/10.1016/j.phycom.2019.100955 1874-4907/© 2019 Published by Elsevier B.V.
speed and bandwidth [4]. However, at high frequencies, high path loss with distance and deep shadowing occurs because of weak diffraction reflection [5]. Thus, studies on radio propagation properties at high-frequency bands are important. Different types (deterministic, empirical, and stochastic) of large-scale path loss models exist, but measurement-based path loss models provide realistic insight into the propagation characteristics of a wireless channel [2,6]. In [7], single-frequency and multi-frequency path loss models are defined. The distance d in the models are 3D T–R separation distances based on measurements. Additionally, both co- and cross-polarized path loss models as well as combinedpolarization path loss models are established for directional and omnidirectional cases. Site-specific propagation models for different indoor environments with the features of low power and short distance are needed to describe the channel characteristics more accurately. Propagation models are formulas for calculating large-scale path loss (or gain) and are usually established empirically on the basis of measurements. Path loss is dependent mainly on the environment, frequency, and height of antennas; hence, a propagation model can only be applied to sites similar to where the model is developed [8]. In 5G systems, a large path loss can be minimized by designing small cells or hot spot systems within a confined coverage area,
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where high-speed data transmission can be realized [9]. Recently, 5G channel propagation at 20 and 30 GHz bands for indoor and outdoor environments, but not in stair environments, has been investigated. In [9], extensive propagation measurement campaigns are conducted at 28 and 38 GHz, which measured path loss and delay spread. Obtaining this information is vital for the design and operation of future 5G cellular networks that use the mm-wave spectrum. The comparison of three candidates’ large-scale propagation path loss models for use over the entire microwave and mm-wave radio spectrum is presented in [10]. A comparison was also provided for the 2–73 GHz band for indoor office and shopping mall environments. Further measurement campaign conducted the propagation measurements, and channel modeling was carried out at 28, 38, 60, and 73 GHz in urban microcell, urban macrocell, and indoor hotspot scenarios [11]. Alvarez et al. constructed an indoor radio channel in the range 1–9 GHz using omnidirectional antennas and arranged four environments (line-of-sight [LOS], Soft-non-LOS [NLOS], Hard-NLOS, and corridor). The estimated path loss exponents (PLEs) with verity reference distances were high in the Hard-NLOS senior compared with those in other environments [12]. The development of wireless networks to provide communication services at emergency sites is required [13]. As a crucial part of a building structure, stairwells play an important role in daily functioning and emergency situations. Wireless coverage along stairwells and understanding radio propagation in such environments is crucial in allowing public safety personnel communication and information sharing for a swift and effective response [14]. Propagation measurements were reported in [8] for the multi-floor stairwell using measurements at 2.4 GHz. Fundamental propagation mechanisms, such as reflection from stairwell walls and transmission through the stair steps, were investigated. In [3], the path loss model with receiving antenna height dependence in an indoor stair environment was proposed on the basis of intensive measurements. The parameters of the model, such as path loss, antenna heights, and shadowing, were extracted. Radio propagation in four typical stairwells was studied through measurement at two frequencies (2.4 and 5.8 GHz) in [15]. The path loss exponent (PLE) values were derived for vertical and horizontal polarizations. In [16], a path loss (PL) model for a stairwell environment considers the effect of building floor height as well as unique PL patterns on several stair flights, which incorporate floor attenuation factors that allow easy comparison with established indoor PL models at 0.9 and 1.8 GHz. Essential insights into the relation of PL to distance for several frequency bands were presented in [17]. These points are useful in facilitating wireless network planning under a stairwell setting. The results in this study can be exploited by the designers of small-cell wireless communications system. This paper provides the propagation characteristics of the mm wave at 26, 28, 32, and 38 GHz bands in a tropical city (Kuala Lumpur). An extensive measurement is conducted in two different indoor stairwell environments with four different mm-wave candidate frequencies for 5G wireless networks. The measurement uses a highly directive horn antenna with co-polarization and cross-polarization at the transmitter (Tx) and receiver (Rx). These measurements are used to collect all reflected signals with gains above the noise floor. Four popular propagation modeling approaches are applied and include single-frequency models, such as close-in free space reference distance (CIM) and floating-intercept (FIM) path loss models, and multi-frequency models, such as alpha-beta-gamma (ABG) and close-in free space reference distance with frequency-dependent PLE (CIF) models. The propagation characteristics, such as path loss, PLE, β , and standard deviation (STD), in multi-floor stairwells obtained by
Table 1 Number of stairs steps for each section of stairwells. Stairwell
Section S1
S2
S3
S4
S5
S6
S7
S8
S9
S10
Stair 1 Steps
12
12
12
12
12
12
12
12
12
12
Stair 2 Steps
13
13
13
13
13
13
13
13
13
13
measurement are investigated. The remainder of this paper is organized as follows. The stair structures are described in Section 2. The measurement system is discussed in Section 3. The results and discussion are presented in Section 4. The conclusions are provided in Section 5. 2. Stair structures Two dog-leg stairwells for biggest building at Universiti Teknologi Malaysia in Kuala Lumpur campus have been chosen for this study. At stair 1, the stairwell resides in the corner of the building and has width 2.9 m whereas the width for each half landing is 1.6 m as in Fig. 1, while the stair 2 located in the middle of the building beside the building entrance, it has 3.9 m width whereas the width for each half landing is 1.9 m as in Fig. 1. Two sections of stair steps are found between each two floors (the ten stair sections are denoted by S1, S2, S3, . . . , S10, in Fig. 1). A door from each stairwell leads to the floor inside the building, but it is kept closed during the measurement of the stairwells. In Fig. 2 a typical stair environment composed of the stair steps, railings, platforms, and so on presented. In stair 1, each stair section has around 12 steps, and each step has a (tread) depth of 0.29 m, and a height (rise) of 0.18 m. While in stair 2, each stair section has around 13 steps, and each step has a (tread) depth of 0.32 m, and a height (rise) of 0.13 m. The staircases are of a dog-leg type configuration, which is the most common type built inside modern buildings [18,19]. Notably, the laws in most countries forbid the use of combustible materials as components or finishing of staircases to provide safe passage in case of fire [16,17]. At stair 1, The Tx is positioned on the 15th floor, and the Rx is moved along the stairwell starting from the 15th floor to the 10th floor, while in stair 2 the Tx is located in 6th floor and the Rx move down until first floor, as shown in Fig. 2. From our observations, most stair steps are made of reinforced concrete, and the stairwell walls are made of concrete as well. Ceilings/floors in modern offices and buildings are mostly made of concrete. This similarity of the structures and materials of stairwells in modern office building ensures that the proposed path loss models can be applied to many, if not all, of the indoor stairwell propagation environments in modern buildings. The numbers of steps for each section of stairwells are listed in Table 1. 3. Measurement system During the measurement, 10 stair flights denoted by S1 until S10 and 5 half-landings are within the transmitter-to-receiver separation distance. The measurement setup consists of a transmitter, Tx, and a receiver, Rx, operating at 26, 28, 32, and 38 GHz. Fig. 3 illustrates the block diagram of the measurement system, and Fig. 2 shows a photograph of the equipment in the measurement site. Specifically, the transmitter consists of a signal generator (Anritsu MG3694C Signal Generator 40 GHz) with a power transmit level (Pt ) of 10 dBm. The receiver includes a spectrum analyzer (Anritsu MS2720T), which provides accurate RF power measurements over a wide frequency range. With an
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Fig. 1. Cross-sectional view of investigated dog-leg stairwell.
Fig. 2. Measurement hardware setup and stair section view.
antenna attached, RF power measurement becomes the received signal strength (RSSI). RSSI measurements can be combined with on-screen map displays to become a versatile solution for mapping the coverage of RF transmitters. Horn antennas are used at both transmitting and receiving ends. The antennas have a gain and beam width depending on the frequency used, the 3 dB beam width for vertical plane in the range 19.7–14 deg. while for horizontal plane, the 3 dB beam width in the range 15.2–20.2 deg. The antenna gain in the range 18.80 dbi–21.34 dbi., At the transmitter, transmission power is used to extend the dynamic measurement range. The signal generator is placed on top of an elevated stand; the transmitting antenna’s height is 1.25 m above
the ground. Three different transmit/receive antenna polarizations are considered in the measurements: vertical/vertical (v–v), horizontal/horizontal (h–h), and vertical/horizontal (v–h). The receiving antenna is installed on a wooden tripod with a height of approximately 1.2 m. The Rx antenna moves along the adjacent sections of the stairs steps by three steps. Thus, according to the measurement scheme, the effective range of the Tx–Rx separation is about 1–22.5 m. It is envisioned to be the typical applicable range of the indoor small cell deployment. A low noise amplifier (LNA) is added to the front end of the receiver to extend the measurement dynamic range in both stairwells.
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Fig. 3. System block diagram.
4. Results and discussion The measurements are conducted in two stairwells. In the measurement scheme, the Tx antenna is fixed at one location, and the Rx antenna is moved on a wooden arm. On every three stair steps, the Rx antenna records 10 values for the signal and these values are averaged offline to obtain the mean power. Eight typical measured points are obtained at a stairwell between two floors. The material of the stair rails can influence the measurement. However, the material’s effects may be less important than the wide surfaces of the surrounding walls, stair steps, and stairwell ceilings, given that the main reflection and transmission occur therein. Antenna polarization (co-polarization and cross polarization) is considered in this work. Fig. 4 shows the path loss with the total number of stair steps (120) for five floors under vertical polarization in stairwells 1 and 2. The mean path loss increases as the receiving antenna moves down along the stairs. Notably, a sudden rise in path loss is observed at the first six junctions (each section comprises 12 steps between consecutive stair sections) (Fig. 4). We call this sudden rise in path loss as the junction between each section. When the Rx antenna is located beyond 100 stair steps in stairwell 1 at 28 GHz (Fig. 4a), the path loss appears to settle down because of weak signal reception caused by the reflection. Notably, in the first stair section, the LOS path contributes to the majority of the received signal. The reflected path also contributes to the signal reception because of the reflectivity of the stairwell walls. In the last three sections, the front wall blocks the signal of the Tx antenna from reaching the Rx antenna directly and causes the sudden signal drop. Meanwhile, at the 28 GHz band in stairwell 2 (Fig. 4a), the signal stabilizes in the last five sections but produces a path loss more effectively than in stairwell 1 in the same band. The main propagation mechanisms in the stairwell include reflections from the stairwell walls, transmissions through the stairs, and their combinations. In Fig. 4b, a clear increase in path loss at the 38 GHz frequency band, (38 GHz) is observed at the junctions between the consecutive stair sections with vertical polarizations. The large path loss increase is more apparent at the first junction (around stair step 12 for stairwells) than at other sites because the LOS scenario exists for the entire first section of the stairwells in each measurement. When Rx turns at the second junction (second 12 steps) to proceed to the next section of the stairwell, the LOS path is lost and a significant increase in path loss transpires for the first four sections because of multiple reflections. In the last remaining junctions, the path loss does not change and the junctions appear flat, which indicates that the signal is weak and
would soon disappear from the stairwell floors. The path loss value for stairwell 2 is less than the path loss for stairwell 1 at 38 GHz (Fig. 4b). Generally, the 28 and 38 GHz frequency bands in stairwell 2 are superior to those in stairwell 1. Therefore, the results are dependent on frequency and distance (Fig. 4a, b). The effect of the LOS path is substantial in the first section at all frequencies. However, for the high-frequency band of 38 GHz, the received power weakens in the farthest section, thus demonstrating the path loss stability at 38 GHz after 60 stair steps for both stairs. In summary, the path loss in stairwell 2 is superior to that in stairwell 1 because the larger width of the former stairwell. Stairwell 2 may not reflect the path as much as stairwell 1 does, which then makes the signal weak, and excessive reflection also transforms the signal as noise. This result indicates that the path loss is not only dependent on frequency but also affected by environmental objects, which include the stairwell wall structure. Path Loss Models: A—Single frequency path loss models To model how different operating frequencies may affect path loss along the stairwells. Measured path loss values that were obtained were analyzed pertaining to their relation to distance in m. In this work, different path loss models are investigated. Path loss is an indication of power loss in the channel [15] and can be expressed as follows: P (d) = 10 Log
(
Pt
) (1)
Pr
where (Pt ) is the transmitted power; (Pr ) is the received power that is proportional to 1/dn ; d is the separation distance between Tx and Rx; and, n is the path loss exponent (PLE). The mean power predicted by (1) is a random variable, which can be characterized by adding an extra term W, a log-normal distribution for both outdoor and indoor propagation environments. Thus, for single frequency to calculate PLE, the close-in free space reference distance (CIM) model is used. The CIM path loss model under the stairwell environment could be described [11] and expressed as follows: PL (f , d) [dB] = PL (f , d0 ) + 10 n log10
(
d d0
)
+ Wσ ,
(2)
where PL (f , d0 ) is the free-space path loss at a reference distance (d0 = 1 m), n is the PLE, and Wσ is the random variable that represents the shadowing effect with zero mean Gaussian distribution and STD of σ in dB. In Fig. 5 the measured path loss and its linear fit as a function of the separation distance between Tx and Rx antennas at all frequencies for both stairwells are shown. The path loss is measured in each section of the stairwells with certain distances. The total distance for the five floors is 22.5 m. The path loss increases with distance. Compared with the path loss presented in Fig. 4, the path loss in Fig. 5 shows a different appearance when plotted as a function of stair steps and separation distances. Evidently, when plotted as a function of separation distance, path loss prediction varies more significantly than when plotted as a function of stair steps. In Fig. 5, the path loss variation with Tx–Rx separation distances at 26, 28, 32, and 38 GHz frequencies when the transmitted horn antenna is directed toward the stairs. Many reflected paths are created on the basis of the stair structure. Moreover, the reflected paths hold a strong power because of the reflection from the surrounding environment, particularly, from the concrete stairs, walls, and ceiling. The PLE at all frequencies for stairwell 2 is better than that for stairwell 1 (Fig. 5a, b). This result is observed because of the wider stairwell 2 and its resultant lower signal reflection (when the walls are closer to one another, the signal is reflected more rapidly and within a shorter distance, and thus weakens as noise). The PLEs in stairwell
A.O. Aldhaibani, T.A. Rahman and A. Alwarafy / Physical Communication 38 (2020) 100955
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Fig. 4. Path losses for two stairwells at 28 and 38 GHz.
1 are between 7.52 and 7.36; the PLE is high at the 26 or 32 GHz frequency band and lowest at the 38 GHz band. However, these values are acceptable for emergency situations in the stairwell environment because the received power can be acquired within five floors. The PLEs in stairwell 2 are between 6.99 and 7.5 (Fig. 5b). The highest PLE is achieved at 32 GHz, whereas the lowest value is at 38 GHz. Therefore, 38 GHz attains the best result among the frequencies tested in both stairwells in such a specific environment. Fig. 6 demonstrates the relation of path loss bearing a Tx–Rx separation distance with co-polarization and cross-polarization under the CIM model at the 28 and 38 GHz frequency bands. Notably, from the 20 and 30 bands, 28 and 38 GHz, respectively, were chosen because of the popularity of the last two frequencies. In the last few years, the 28 and 38 GHz bands have been considered potential bands for future 5G broadband cellular communication networks [9]. Clearly, the path loss increases with increasing separation distance in all cases. In stairwell 1, the horizontal polarizations achieve a superior performance to the vertical and horizontal–vertical polarizations at both 28 and 38 GHz bands (Fig. 6a, b). This finding may be explained in terms of the orientation of the electric field with respect to the stairwell structure. For the polarization, the electric field is parallel to the stair structure; hence, strong reflection and diffraction effects are expected. In this study, the 38 GHz band performs better than the 28 GHz band in all the polarization cases. The worst results are found in the cross-polarization at both bands. All the parameters in stairwell 1 for all polarization cases at all frequencies are shown in Table 2. In Fig. 6b, the co- and crosspolarizations at 28 and 38 GHz in stairwell 2 are displayed. The horizontal–horizontal polarization in most of the cases is better than other polarizations at both frequencies because of the same reason mentioned above (parallel orientation of electric field to the stair structure). Hence, strong reflection and diffraction effects are also expected. Such performance is followed by that of vertical–vertical polarizations, whereas the cross-polarization attains the worst results in both frequencies. Fig. 6 shows that the 38 GHz band is more effective than the 28 GHz band. The parameters for stairwell 2 at all polarizations are listed in Table 3. Furthermore, the distance reaches 22.5 m (five floors), but the path loss stabilizes at the end of the stairwell. Therefore, the signal is weak and disappears at the end of the stairwell. Another model which has been used in 3GPP and WINNER II models is the FIM model. This model has two parameters and
Table 2 Path loss model parameters for the CIM model at all frequencies for stairwell 1. Single-frequency CIM path loss model for d0 = 1 m Freq. (GHz)
Pol.
PLE
σ (dB)
26
V–V H–H V–H
7.52 7.31 7.79
9.95 9 6.93
28
V–V H–H V–H
7.6 7.33 7.86
8.86 8.99 5.57
V–V H–H V–H
7.48 7.6 7.96
8.49 8.65 7.89
V–V H–H V–H
7.36 7.26 7.57
8.98 8.95 5.55
32
38
does not consider a physically based anchor to the transmitted power, and has a similar form to CIM model as [20]: PLFI (d) [dB] = α + 10.β log10 (d) + WσFI
(3)
where α [dB] and β (e.g. the average path loss exponent) are the floating intercept and the slop line, respectively. The showing effects are represented by random variable WσFI which has Gaussian distribution with zero mean and standard deviation σ FI [dB]. Table 4 indicates the parameter values for the indoor channels of the FIM model in both stairwells. Compared with Table 2, the difference in STD between the CIM and FIM models is small. The CIM model provides the benefit of simple comparisons of measurements across many frequency bands by using merely one parameter because the frequency-dependent effects of the model are primarily contained in the 1 m free space path loss. The environment provides additional and substantial frequencydependent loss beyond the first meter of free-space propagation. The parameter values are identical to those in previous research [15]. Fig. 7 shows a plot of the path loss versus the distance under the FIM model at all frequencies in both stairwells. The STD of the shadow fading is slightly smaller under the FIM model than in the CIM model in Fig. 5. With a separation distance of 22.5 m, the difference in STD between the FIM and CIM models is less than 3 dB. The line slopes differ between the FIM and CIM models. The FIM model in stairwell 1 attains a β range of 9.09–10.92 and STD range of 6.28–8.07 dB for the identical measurement data
6
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Fig. 5. CIM model for (a) stairwells 1 and (b) stairwells 2, under all the frequencies tested.
Fig. 6. a and b, Measured path loss polarization for stairwells 1 and 2 at 28 and 38 GHz with CIM.
sets. By contrast, the β range is 6.73–9.81 and STD range is 8.01–
for stairwell 1. Meanwhile, the lowest STD is at 26 GHz in both
9.98 dB for stairwell 2. The STD for stairwell 2 is greater than that
stairwells and the highest STDs are at 38 and 32 GHz in stairwells
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Fig. 7. a, b Measured path losses for stairwells 1 and 2 at 26, 28, 32 and 38 GHz under the FIM model. Table 3 Path loss model parameters for CIM model at all frequencies for stairwell 2..
Table 5 Parameter values for both ABG and CIF models in stairwells 1 and 2. Multi-frequency (ABG and CIF) path loss model parameters
Single-frequency CIM path loss model for d0 = 1 m Freq. (GHz)
Pol.
PLE
σ (dB)
ABG
α
β
γ
σ (dB)
26
V–V H–H V–H
7.06 7.36 7.52
10.16 12.10 8.6
Stairwell 1 Stairwell 2
9.75 8.22
12.9 8.81
1.57 2.81
7.38 10.06
CIF
n
b
f0
σ (dB)
28
V–V H–H V–H
7.15 6.99 7.27
10.46 13.55 6.48
Stairwell 1 Stairwell 2
7.44 7.18
−0.04 −0.013
31 GHz 31 GHz
9.04 10.34
32
V–V H–H V–H
7.55 7.35 7.57
9.98 11.51 7.56
38
V–V H–H V–H
6.99 6.99 7.29
9.69 10.45 8.52
(CIF) path loss model is a multi-frequency model that employs the same physically motivated FSPL anchor at 1 m as the CIM model. The CIF model equation is presented in [7]: PL
CIF
(f , d) [dB] = FSPL(f , d0 ) + 10 n 1 + b
Table 4 Path loss model parameters for FIM models at all frequencies for both stairwells.
( × log10
Single-frequency FIM path loss model Freq.
26 28 32 38
Stairwell 1
Stairwell 2
β
STD (σ ) (dB)
β
STD (σ ) (dB)
10.92 9.86 9.17 9.09
6.28 6.67 7.58 8.07
9.81 8.97 7.37 6.73
8.01 9.6 9.98 9.67
1 and 2, respectively. Notably, the slope of the FIM model often holds no physical basis. By contrast, the CIM model is physically based and adequately estimates path loss data points. B—Multi-frequency path loss models A multi-frequency model known as the alpha-beta-gamma (ABG) model which includes a frequency dependent γ , distancedependent α terms and optimization factor β to describe path loss at various frequencies [7,12]. The ABG model equation is given by [7]: PLABG (f , d) [dB] = 10 α log10 d d0 + β
( / )
+ 10γ log 10(f /1 GHz) + WσABG
(
(4)
This ABG model is used to investigate multi-frequency based on physical distance (d0 ) of 1 m. Another two-parameter multi-frequency model can be considered to be an extension of the CIM model. The close-in free space reference distance with frequency dependent path loss exponent
d d0
)
+ XσCIF
(
f − f0
))
f0 (5)
Where d0 = 1 m, n denotes the distance dependency of path loss (e.g. the path loss exponent, or PLE), b is an intuitive model-fitting parameter that represent the slop of linear frequency dependency of path loss. Noted that, all parameters for CI, FI, CIF and ABG path loss models are estimated based on minimum mean square error (MMSE) approach [7]. Fig. 8a and b display the ABG and CIF path loss models. The CIF standard division lies within the difference of 2 dB from ABG in stairwell 1 and around 0.3 dB in stairwell 2. An extremely small difference is achieved using one less model parameter (at the CIF model). For stairwell 1, the ABG model reduces the STD by around 1.66 dB relative to the CIF model, whereas in stairwell 2, the ABG model is 0.28 dB smaller than the CIF model. Table 5 shows the parameter values for both ABG and CIF models in both stairwells. Thus, the difference in STD between the CIF and ABG models is less than 2 dB. As shown subsequently, the CIF and ABG models are even closer in performance (a fraction of a dB difference in STD, which is well within the typical measurement error from cable flexing, pointing errors, or temperature variations) for these environments. 5. Conlusion The propagation characteristics of an mm-wave 5G network in two stairwell environments are investigated through experimental measurements. The measurements are obtained at 20
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A.O. Aldhaibani, T.A. Rahman and A. Alwarafy / Physical Communication 38 (2020) 100955
Fig. 8. Multi-frequency CIF and ABG path loss models in (a) stairwell 1 and (b) stairwell 2.
and 30 GHz bands (26, 28, 32, and 38 GHz) for every three stair steps for five levels. The transmitter is placed at the top floor, and the receiver is moved along the stairwells. The propagation mechanisms for each of the 10th stair sections are analyzed. The proposed research can be applied to predict the signal level of different stair step structures and provide a set of criteria for cell planning in an indoor environment. The particular stairwells investigated involve reflective side walls. Thus, the reflected signal from the walls weakens in the lowest floors. The PLE values and the associated STDs are extracted. When the conventional separation distance is used in a path loss model, the n values are significantly higher than the values for urban and other indoor propagation environments. Single-frequency (CIM and FIM) and multi-frequency (ABG and CIF) path loss models are investigated. The directional horn antennas for co-polarization and cross-polarization at the Tx and Rx are used for all frequencies. The path loss model offers virtually identical results whether under the single- or multi-frequency path loss models. This finding indicates the faster signal drop in stairwells than those in other environments. The results of this study are beneficial for understanding radio propagation in an indoor stairwell environment, which is crucial in emergency situations (i.e., law enforcement and firefighting purposes). The results can also help in developing effective indoor communication systems and may be useful for designing small-cell wireless communication systems.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment The author would like to acknowledge and express sincere appreciation to Universiti Teknologi Malaysia and Ministry of Higher Education, Malaysia for financing this research project under grant No. Q.J130000.21A2.02E95 and No. A.J090601.5357.07085TABUNG PUSAT KECEMERLANGAN PENGAJIAN TINGGI (HICoE) PUSAT KOMUNIKASI WAYARLES (WCC). References [1] A.I. Sulyman, A. Alwarafy, G. Maccartney, T. Rappaport, A. Al-Sanie, Directional radio propagation path loss models for millimeter-wave wireless networks in the 28, 60, and 73 GHz bands, IEEE Trans. Wireless Commun. PP (99) 1. [2] M. Kim, Y. Konishi, Y. Chang, J.i. Takada, Large scale parameters and double-directional characterization of indoor wideband radio multipath channels at 11 GHz, IEEE Trans. Antennas and Propagation 62 (1) (2014) 430–441. [3] Yu Yu, Yang Liu, Wen-Jun Lu, Hong-Bo Zhu, Path loss model with antenna height dependency under indoor stair environment, Int. J. Antennas Propag. (2014) 1–6.
A.O. Aldhaibani, T.A. Rahman and A. Alwarafy / Physical Communication 38 (2020) 100955 [4] A.F. Abouraddy, S.M. Elnoubi, Statistical modeling of the indoor radio channel at 10 GHz through propagation measurements-partI: Narrowband measurements and modeling, IEEE Trans. Veh. Technol. 49 (5) (2000) 1491–1507. [5] W.C.Y. Lee, Mobile Communications Engineering, second ed., McGraw-Hill, New York, NY, USA, 1982. [6] A.I. Sulyman, A. Alwarafy, H.E. Seleem, K. Humadi, A. Alsanie, Path loss channel models for 5G cellular communications in Riyadh city at 60 GHz, in: 2016 IEEE International Conference on Communications, ICC0, Kuala Lumpur, 2016, pp. 1–6. [7] G.R. Maccartney, T.S. Rappaport, S. Sun, S. Deng, Indoor office wideband millimeter-wave propagation measurements and channel models at 28 and 73 GHz for ultra-dense 5G wireless networks, IEEE Access 3 (2015) 2388–2424. [8] S.Y. Lim, Z. Yun, J.M. Baker, N. Celik, H.s. Youn, M.F. Iskander, Propagation modeling and measurement for a multifloor stairwell, IEEE Antennas Wirel. Propag. Lett. 8 (2009) 583–586. [9] T.S. Rappaport, et al., Millimeter wave mobile communications for 5G cellular: It will work!, IEEE Access 1 (2013) 335–349. [10] S. Sun, et al., Investigation of prediction accuracy, sensitivity, and parameter stability of large-scale propagation path loss models for 5G wireless communications, IEEE Trans. Veh. Technol. 65 (5) (2016) 2843–2860. [11] T.S. Rappaport, G.R. MacCartney, M.K. Samimi, S. Sun, Wideband millimeter-wave propagation measurements and channel models for future wireless communication system design, IEEE Trans. Commun. 63 (2015) 3029–3056. [12] A. Alvarez, G. Valera, M. Lobeira, et al., Ultra wideband channel model for indoor environments, J. Commun. Netw. 5 (2003) 309–318. [13] Omar Abdul Aziz, Tharek Abdul Rahman, Modelling the impact of operating frequencies on path loss and shadowing along multi-floor stairwell for 0.7 GHz–2.5 GHz range, Prog. Electromagn. Res. M 40 (2014) 69–78. [14] Omar Abdul Aziz, Tharek Abdul Rahman, Investigation of path loss prediction in different multi-floor stairwells at 900 MHz and 1800 MHz, Prog. Electromagn. Res. M 39 (2014) 27–39. [15] S.Y. Lim, Z. Yun, M.F. Iskander, Propagation measurement and modeling for indoor stairwells at 2.4 and 5.8 GHz, IEEE Trans. Antennas and Propagation 62 (9) (2014) 4754–4761. [16] A. Hoffmann, R. Muehlnikel, Experimental and numerical investigation of fire development in a real fire in a fivestorey apartment building, Fire Mater. 35 (2010) 453–462. [17] Building Department, The government of Hong Kong special administrative unit, code of practice for fire safety in buildings, 2011. [18] S. Emmitt, C.A. Gorse, Barry’S Introduction to Construction of Buildings, second ed., Wiley–Blackwell, 2010. [19] C. Hartwell, N. Pevsner, The Buildings of England, Tale University Press, Lancashire: North, 2009. [20] M.K. Samimi, T.S. Rappaport, G.R. MacCartney, Probabilistic omnidirectional path loss models for millimeter-wave outdoor communications, IEEE Wirel. Commun. Lett. 4 (4) (2015) 357–360.
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A.O. Aldhaibani obtained his Ph.D. degree in communication Eng. from The University Malaysia Perlis, Malaysia, 2015, and M.Sc. from University of Technology Malaysia (UTM) 2012. In 2016 he a pointed as a postdoc fellow in University of Technology Malaysia(UTM) to work in the experimental propagation channel of 5G networks at the real field (Kuala Lumpur Area). Currently, he working as a lecturer at Hadhramout University. He has been working in the field of optical wireless technologies solution for 4G networks, computer networking, passive optical networks (PON), wireless sensors network and fiber wireless (FiWi), both in experimental and academically research valued work. Currently he is working at Hadhramout University. Tharek Abd Rahman Abdul Rahman Tharek (M’14) received the B.Sc. degree (Hons.) in electrical engineering from the University of Strathclyde, Glasgow, U.K., the M.Sc. degree in communication engineering from University of Manchester Institute of Science and Technology (UMIST), Manchester, U.K., and the Ph.D. degree in mobile communication from the University of Bristol, Bristol, U.K. He is currently a Professor of Wireless Communication with the Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia. He is the Director of Wireless Communication Centre (WCC), Faculty of Electrical Engineering, Universiti Teknologi Malaysia, and currently conducting research related to mobile communications, antenna and propagation. He has also conducted various short courses related to mobile and satellite communication to the telecommunication industry and government agencies, since 1988. He has authored more than 300 scientific papers in journals and conferences and obtained many national and international awards. He is also a Consultant for many communication companies and an active member in several research academic entities. Abdulmalik Alwarafy received the B.S. degree in electrical engineering with a minor in communication from Ibb University, Ibb, Yemen, in 2009, and the M.Sc. degree in electrical engineering with a major in communications from King Saud University, Riyadh, Saudi Arabia, in 2015, where he is currently pursuing the Ph.D. degree in electrical engineering.,His current research interests include millimeter-wave measurements and propagation channel modeling and analysis for the fifth-generation cellular mobile communications.