Unit 1: The wireless channel

Transcription

1 Unit 1: The wireless channel Wireless communications course Ronal D. Montoya M. August 23, /26

2 Outline I 1. Empirical path loss models Overview 2. The Okumura model 3. Hata model 4. COST 231 model 5. Standford University Interim (SUI) model 6. Indoor attenuation factors 2/26

3 Empirical path loss models I Mobile communication systems operate in complex propagation environments that cannot be accurately modeled by free-space path loss or ray tracing. Several path loss models have been developed over the years to predict path loss in typical wireless environments such as large urban macrocells, urban microcells and inside buildings. These models are mainly based on empirical measurements over a given distance in a given frequency range and a particular geographical area or building. 1. Empirical path loss models 3/26

4 Empirical path loss models II Applications of these models are not always restricted to environments in which the empirical measurements were made (Consider an adjusted model). Many wireless systems use these models as a basis for performance analysis. They were initially developed for urban macrocells, today are adjusted for outdoor microcells and indoor propagation. Empirical measurements of the P r P t as a f (d) include the effects of path loss, shadowing, and multipath. 1. Empirical path loss models 4/26

5 Empirical path loss models III In order to remove multipath effects, empirical measurements for path loss typically average their P r measurements and the corresponding path loss at a given d over several λ c. This average path loss is called the local mean attenuation (LMA), and is measured in several places with similar propagation characteristics. 1. Empirical path loss models 5/26

6 The Okumura model I This model is applicable over 1 d 100 Km and frequency ranges of 150 f c 1500 MHz. Okumura used extensive measurements of base station-to-mobile signal attenuation throughout Tokyo to develop a set of curves giving median attenuation relative to free space of signal propagation in irregular terrain. The base station heights for these measurements were m. 2. The Okumura model 6/26

7 The Okumura model II The path loss formula of Okumura is given by: P L (d) [db] = L (f c, d) + A mu (f c, d) G (h t ) G (h r ) G AREA (1) Where: L (f c, d) : Free space path loss at distance d and carrier frequency f c. A mu (f c, d) : The median attenuation in addition to free space path loss across all environments. G (h t ) : The base station antenna height gain factor. 2. The Okumura model 7/26

8 The Okumura model III G (h r ) : The mobile antenna height gain factor. G AREA : The gain due to the type of environment. Okumura derived empirical formulas for the base station and the mobile antenna height gain factors: { G (h t ) = 20 log 10 (h t /200) 30m < h t < 1000m (2) G (h r ) = { 10 log 10 (h r /3) h t 3m 20 log 10 (h r /3) 3m < h t < 10m (3) 2. The Okumura model 8/26

9 The Okumura model IV Correction factors related to terrain are also developed to improve the model accuracy. Okumura s model has a db empirical standard deviation between the P L predicted by the model and the measured P L used to develop the model. 2. The Okumura model 9/26

10 Hata model I The Hata model is an empirical formulation of the graphical path loss data provided by Okumura and is valid over roughly the same range of frequencies, MHz. This empirical model simplifies calculation of path loss since it is a closed-form formula and is not based on empirical curves for the different parameters. 3. Hata model 10/26

11 Hata model II The empirical path loss in urban areas under the Hata model is given by: P L,urban (d) [db] = 69, , 16 log 10 (f c ) 13, 82 log 10 (h t ) + a (h r ) + [44, 9 6, 55 log 10 (h t )] log 10 (d) (4) 3. Hata model 11/26

12 Hata model III a (h r ) is a correction factor for the mobile antenna height h r based on the size of the coverage area. For small to medium sized cities, this factor is given by: a (h r ) [db] = (1, 1 log 10 (f c ) 0, 7) h r (1, 56 log 10 (f c ) 0, 8) [db] (5) For larger cities and f c > 300 MHz: a (h r ) [db] = 3, 2 [log 10 (11, 75h r )] 2 h r 4, 97 [db] (6) 3. Hata model 12/26

13 Hata model IV Corrections to the urban model are made for suburban and rural propagation environments, respectively: [ ( )] 2 fc P L,suburban (d) [db] = P L,urban (d) 2 log 10 5, 4 [db] (7) 28 P L,rural (d) [db] = P L,urban (d) 4, 78 [log 10 (f c )] , 33 log 10 (f c ) K [db] (8) 3. Hata model 13/26

14 Hata model V Where K: K = 35, 94 : for the countryside. K = 40, 94 : for the desert. Anotations: The Hata model well-approximates the Okumura model for distances d > 1 Km. f c > 1500 MHz is not covered by this model (PCS, Wi-Fi, ZigBee, Bluetooth, WiMAX, WiBRO?). Indoor environments are also not captured with the Hata model. 3. Hata model 14/26

15 COST 231 model I The Hata model was extended by the European cooperative for scientific and technical research (EURO-COST) up to f c = 2 GHz, and is given by: P L,urban (d) [db] = 46, , 9 log 10 (f c ) 13, 82 log 10 (h t ) (9) a (h r ) + [44, 9 6, 55 log 10 (h t )] log 10 (d) + C M Where: a (h r ) is the same as Hata model. 4. COST 231 model 15/26

16 COST 231 model II C M = 0 [db] for medium sized cities. C M = 3 [db] for metropolitan areas. The restrictions for the COST 231 (Hata extension) model are: 500 MHz < f c < 2 GHz. 30 m < h t < 200 m. 1 m < h r < 10 m. 1 Km < d < 20 Km. 4. COST 231 model 16/26

17 Standford University Interim (SUI) model I It has an extension of the Hata model MHz < f c < 3, 5 GHz. 2 m < h r < 10 m. 0, 1 Km < d < 8 Km. The SUI model describes three types of terrain T t : terrain A, B and C. Terrain A: used to describe hilly areas with moderate or very dense vegetation, or dense populated urban area. Maximum path loss. 5. Standford University Interim (SUI) model 17/26

18 Standford University Interim (SUI) model II Terrain B: used to describe the hilly terrains with rare vegetation, flat terrains with moderate or heavy tree densities, or suburban environment. Medium path loss. Terrain C: suitable for flat terrains with light vegetation or rural areas. Minimum path loss. The basic path loss expression of The SUI model with correction factors is (for d > d 0 ): P L,Tt (d) [db] = A (λ c ) + 10γ log 10 ( d d 0 ) + X (f c ) + X (h r ) + S (10) 5. Standford University Interim (SUI) model 18/26

19 Standford University Interim (SUI) model III Where: d 0 = 100 m. (4πd A (λ c ) = 20 log 0 10 λ c ). Path loss exponent γ = a bh t + c h t. Coefficients a, b, and c are given in the next table. γ = 2 for LOS in an urban area, 3 < γ < 5 for urban NLOS environment, and γ > 5 for indoor propagation. The frequency correction factor: X f = 6 log 10 ( fc 2000 ) 5. Standford University Interim (SUI) model 19/26

20 Standford University Interim (SUI) model IV The receiver antenna correction factor: { ( 10, 8 log hr ) X (h r ) = ( log hr ) T t = A, B T t = C The log normally distributed factor S, for shadow fading because of trees and other clutter on a propagation paths. 8, 2 S 10, 6. f c is given in MHz and λ c in m. (11) 5. Standford University Interim (SUI) model 20/26

21 Standford University Interim (SUI) model V Model parameter T t = A T t = B T t = C a 4,6 4 3,6 b 0,0075 0,0065 0,005 c 12,6 17,1 20 Table: SUI model parameters for different terrain types. Homework: Read about the ECC-33 and Ericsson models. 5. Standford University Interim (SUI) model 21/26

22 Indoor attenuation Figure: Indoor signal strength coverage heat plot. 6. Indoor attenuation factors 22/26

23 Indoor attenuation factors I It s difficult to find a generic model that can be accurately determine the empirical path loss in a specific indoor setting (partition materials and dielectric properties vary widely). Indoor path loss models must accurately capture the effects of attenuation across floors due to partitions, as well as between floors. The attenuation per floor is greatest for the first floor that is passed through and decreases with each subsequent floor. 6. Indoor attenuation factors 23/26

24 Indoor attenuation factors II Partition type Partition Loss [db] Cloth 1,4 Double plasterboard wall 3,4 Foil insulation 3,9 Concrete wall 13 Aluminium siding 20,4 All metal 26 Table: Typical partition losses measured at MHz. 6. Indoor attenuation factors 24/26

25 Indoor attenuation factors III The experimental data for floor and partition loss can be added to an analytical or empirical db path loss model P L (d) as: Where: P L [dbm] = P t [dbm] P L (d) F AF i P AF i (12) F AF i : Floor attenuation factor for the ith floor traversed by the signal. P AF i : Partition attenuation factor for the ith floor traversed by the signal. N f i=1 N p i=1 6. Indoor attenuation factors 25/26

26 Indoor attenuation factors IV N f : Number of floors traversed by the signal. P AF i : Number of partitions traversed by the signal. 6. Indoor attenuation factors 26/26