Wireless Communications

Transcription

1 NETW701 Wireless Communications Dr. Wassim Alexan Winter 2018 Lecture 5

2 NETW705 Mobile Communication Networks Dr. Wassim Alexan Winter 2018 Lecture 5 Wassim Alexan 2

3 Outdoor Propagation Models Radio transmission in mobile communications systems often takes place over irregular terrain The land profile of a particular area needs to be taken into account for estimating the path loss Such a profile may vary from a simple curved earth profile to a highly mountainous profile The presence of trees, buildings and other obstacles must also be taken into account A number of propagation models is available to predict the path loss over irregular terrain Wassim Alexan 3

4 Outdoor Propagation Models Examples of such models are: Longley Rice model Durkin s model Okumura s model Hata model PCS extension to Hata model Walfisch and Bertoni model Wideband PCS Microcell model Wassim Alexan 4

5 Okumura s Model This is one of the most widely used models for signal predictions in urban and suburban mobile communications areas It is applicable for frequencies ranging from 150 MHz to 1920 MHz (but typically extrapolated up to 3000 MHz) It covers distances ranging from 1 km to 100 km and for h t and h r in the range of 30 m to 1000 m The model is based on empirical data collected in detailed propagation tests over various situations of irregular terrain and environmental clutter in Tokyo Wassim Alexan 5

6 Okumura s Model Okumura s model is expressed as L 50 = L f + A mu ( f, d) - G(h te ) - G(h re ) - G AREA (1) L 50 is the 50 th percentile value of the propagation path loss L f is the free space propagation path loss A mu ( f, d) is the median attenuation relative to free space G(h te ) is the base station antenna height gain factor G(h re ) is the mobile antenna height gain factor G AREA is the gain due to the type of environment (correction factor) Once the terrain related parameters are calculated, further necessary correction factors can be added or subtracted Wassim Alexan 6

7 Okumura s Model: A mu Curves Fig. 1. Median attenuation relative to free space (A mu ( f, d)), over a quasi smooth terrain. (Rappaport, Wireless Communications: Principles and Practice, 2 nd edition) A mu is determined from Fig. 1 for different frequencies Wassim Alexan 7

8 Okumura s Model: G AREA Curves Fig. 2. Correction factor, G Area, for different types of terrain. (Rappaport, Wireless Communications: Principles and Practice, 2 nd edition) G AREA is determined from Fig. 2 for different frequencies Wassim Alexan 8

9 Okumura s Model: Height Gain Factors The height gain factors are given by G(h te ) = 20 log h te m < h te < 1000 m (2) G(h re ) = 10 log h re 3 h re 3 m (3) G(h re ) = 20 log h re 3 3 m < h re 10 m (4) Wassim Alexan 9

10 Exercise 1 Find the median path loss using Okumura s model for d = 50 km, h te = 100 m and h re = 10 m in a suburban environment. If the base station transmitter radiates an effective isotropic radiated power (EIRP) of 1 kw at a carrier frequency of 900 MHz, find the power at the receiver. Wassim Alexan 10

11 Exercise 1 Solution The free space path loss can be calculated as L f = 10 log From the Okumura curves λ 2 (4 π) 2 d 2 = 10 log (4 π) 2 ( ) 2 = db and A mu (900 MHz, 50 km) = 43 db G AREA = 9 db Using equations (2) and (4), we have G(h te ) = 20 log h te 100 = 20 log = -6 db Wassim Alexan 11

12 Exercise 1 Solution and G(h re ) = 20 log h re 3 = 20 log 10 3 = db Using equation (1), the total mean path loss is L 50 = L f + A mu ( f, d) - G(h te ) - G(h re ) - G AREA = db + 43 db - (-6) db db - 9 db = db Therefore, the median received power is P r (d) = EIRP(dBm) - L 50 (db) = 60 dbm db = dbm Wassim Alexan 12

13 Hata Model The Hata model is a Japanese empirical formulation of the graphical path loss data provided by Okumura It is valid for frequencies ranging from 150 MHz to 1500 MHz Hata presented the urban area propagation loss as a standard formula and supplied correction equations for the application to other situations Wassim Alexan 13

14 Hata Model The standard formula for the median path loss in urban areas is given by Hata in db as L 50 (urban) = log f c log h te - a(h re ) + ( log h te ) log d (5) L 50 is the median (middle) value or the 50 th median (percentile) value of the propagation path loss f c is the carrier frequency in MHz h te is the effective transmitter (base station) height, ranging from 30 m to 200 m h re is the effective receiver (mobile) height, ranging from 1 m to 10 m d is the T R separation in km a(h re ) is the correction factor for effective mobile antenna height Wassim Alexan 14

15 Hata Model For a small to medium sized city, the mobile antenna correction factor is given in db by a(h re ) = (1.1 log ( f c ) - 0.7) h re - (1.56 log ( f c ) - 0.8) (6) While for a large city, it is given by a(h re ) = 8.29 (log (1.54 h re )) for f c 300 MHz (7) a(h re ) = 3.2 (log (11.75 h re )) for f c > 300 MHz (8) Wassim Alexan 15

16 Hata Model To obtain the path loss in a suburban area, the standard Hata formula in (5) is modified to L 50 (suburban) = L 50 (urban) - 2 log f c (9) While for path loss in open rural areas, the formula in (5) is modified to L 50 (rural) = L 50 (urban) (log f c ) log f c (10) Hata model is well suited for large cell mobile systems but not personal communication systems (PCS) which have cells on the order of 1 km in radius Wassim Alexan 16

17 Exercise 2 Find the median path loss using the Hata model for d = 2.3 km, h te = 180 m and h re = 3 m, for wireless communication taking place in Cairo at a frequency of 870 MHz. How would your answer change if such wireless communication took place in El Ain El Sukhna instead? Wassim Alexan 17

18 Exercise 2 Solution For the wireless communication taking place in Cairo at a frequency of 870 MHz and h re = 3 m, we use the appropriate correction factor a(h re ) = 3.2 (log (11.75 h re )) for f c > 300 MHz a(3) = 3.2 (log ( )) = (11) Then, we use the standard formula for the median path loss in urban areas, with h te = 180 m and d = 2.3 km L 50 (urban) = log f c log h te - a(h re ) + ( log h te ) log d = log log a(3) + ( log 180) log 2.3 = log log ( log 180) log 2.3 = db (12) Wassim Alexan 18

19 Exercise 2 Solution For the wireless communication taking place in El Ain El Sukhna at a frequency of 870 MHz and h re = 3 m, we use the appropriate correction factor a(h re ) = (1.1 log( f c ) - 0.7) h re - (1.56 log f c - 0.8) a(3) = (1.1 log(870) - 0.7) 3 - (1.56 log ) = (13) Then, we use the standard formula for the median path loss in rural areas, with h te = 180 m and d = 2.3 km L 50 (rural) = L 50 (urban) (log f c ) log f c = ( log f c log h te - a(h re ) + ( log h te ) log d) (log f c ) log f c = ( log log a(3) + ( log 180) log 2.3) (log 870) log (14) Wassim Alexan 19

20 = ( log log ( log 180) log 2.3) (log 870) log = db Wassim Alexan 20

21 PCS Extension to the Hata Model This is a European extended version of the Hata model, specifically developed for the 2 GHz range L 50 (urban) = log f c log h te - a(h re ) + ( log h te ) log d + C M (15) Where the mobile antenna correction factor a(h re ) was already defined in equations (9), (10) and (11) f c is the carrier frequency, ranging from 1500 MHz to 2000 MHz h te is the effective transmitter height, ranging from 30 m to 200 m h re is the effective transmitter height, ranging from 1 m to 10 m d is the T R separation, ranging from 1 km m to 20 km Wassim Alexan 21

22 C M = {0, 3} db for {medium sized city and suburban areas, metropolitan centers} Wassim Alexan 22

23 Indoor Propagation Models The proliferation of WiFi, Bluetooth and later introduction of smart home and IoT devices has led to a burst of indoor wireless communications The indoor radio channel differs from the traditional mobile radio channel in a number of aspects: They have much smaller distances They have much greater variability of the environment for a much smaller range of T R separation distances It becomes much more difficult to insure far field radiation A number of propagation models is available to predict the path loss over irregular terrain Wassim Alexan 23

24 Indoor Propagation Models Signal propagation within buildings is strongly affected by specific features such as The building layout The construction materials used in the buildings Building type Open/closed doors Location of antennas Wassim Alexan 24

25 Indoor Propagation Models Examples of such models are: Partition Losses (same floor) Partition Losses (between floors) Log distance Path Loss model Ericsson Multiple Breakpoint Model Attenuation Factor Model Wassim Alexan 25

26 Partition Losses on the Same Floor Buildings have a wide variety of partitions and obstacles which form the internal and external structure Office buildings have large open areas and the offices are separated by moveable partitions. Often, metal reinforced concrete between floors are used The buildings at the GUC also have moveable partitions, but the main structure is made of steel beams Partitions vary widely in their physical and electrical characteristics, making it difficult to apply general models to specific indoor installations Extensive databases of losses for a large number of partitions are performed as shown in Table 1 (next slide) Wassim Alexan 26

27 Partition Losses on the Same Floor Table 1. Average signal loss measurements reported by various researchers for radio paths obstructed by common building materials. (Rappaport, Wireless Communications: Principles and Practice, 2 nd edition) Wassim Alexan 27

28 Partition Losses Between Floors Losses between floors of a building are determined by the external dimensions and materials of the building, as well as the type of constructions used to create the floors and the external surroundings The number of windows in a building and the presence of tinting (which attenuates radio energy) can impact the loss between floors Table 2 (next slide) shows values for floor attenuation factors (FAF) in buildings It is clear that the attenuation through a single floor is greater than the incremental attenuation caused by each additional floor Wassim Alexan 28

29 Partition Losses Between Floors σ Table 2. Average floor attenuation factor in db for 1, 2, 3 and 4 floors in two office buildings. (Rappaport, Wireless Communications: Principles and Practice, 2 nd edition) Wassim Alexan 29

30 Log Distance Path Loss Model This model shows that indoor path loss can be near perfectly modeled using the log normal shadowing model, which we used before, as PL(d) = PL(d) + X σ = PL(d 0 ) + 10 n log d d 0 + X σ (16) Such that the value of n depends on the surroundings and building type X σ is a normal RV in db having a SD of σ db Typical values for various buildings are provided in Table 3 (next slide) Wassim Alexan 30

31 Log Distance Path Loss Model σ Table 3. Path loss exponent and standard deviation measured in different buildings. (Rappaport, Wireless Communications: Principles and Practice, 2 nd edition) Wassim Alexan 31

32 Attenuation Factor Model This is an in building propagation model that includes The effect of building type Variations caused by obstacles This model reduces the SD between measured and predicted path loss to around 4 db compared to 13 db when only the log distance model is used PL(d) = PL(d 0 ) + 10 n SF log d d 0 + FAF + ΣPAF (17) n SF represents the path loss exponent for the same floor measurements FAF represents the floor attenuation factor (selected from Table 2) PAF represents the partition attenuation factor for a specific obstruction encountered by a ray drawn between the Tx and the Rx Wassim Alexan 32

33 Attenuation Factor Model FAF may be replaced by an exponent that accounts for the effects of multiple floor separations PL(d) = PL(d 0 ) + 10 n MF log d d 0 + ΣPAF (18) n MF represents the path loss exponent based on measurements through multiple floors Wassim Alexan 33