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2 System Aspects 2.1 Fundamentals of Radio Transmission In mobile radio systems, unlike wired networks, electromagnetic signals are transmitted in free space (see Figure 2.1). Therefore a total familiarity with the propagation characteristics of radio waves is a prerequisite in the develop- ment of mobile radio systems. In principle, the Maxwell equations explain all the phenomena of wave propagation. However, when used in the mobile radio area, this method can result in some complicated calculations or may not be applicable at all if the geometry or material constants are not known exactly. Therefore special methods were developed to determine the characteristics of radio channels, and these consider the key physical effects in different models. The choice of model depends on the frequency and range of the radio waves, the characteristics of the propagation medium and the antenna arrangement. The propagation of electromagnetic waves in free space is extremely com- plex. Depending on the frequency and the corresponding wavelength, elec- tromagnetic waves propagate as ground waves, surface waves, space waves or direct waves. The type of propagation is correlated with the range, or dis- tance, at which a signal can be received (see Figure 2.2). The general rule is that the higher the frequency of the wave to be transmitted, the shorter the range. Based on the curvature of the earth, waves of a lower frequency, i.e., larger wavelength, propagate as ground or surface waves. These waves can still be received from a great distance and even in tunnels. T R mitter Trans- FilterFilter Receiver Feeder lines W W Z 0 Z Z Free space antenna Receive antenna Transmit Figure 2.1: Radio transmission path: transmitter–receiver. Z 0 and Z W are the radio wave resistances in free space and on the antenna feeder link Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright © 1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 28 2 System Aspects 30 kHz 300 kHz 3 MHz 30 MHz 300 MHz 3 GHz 30 GHz frequency EHFSHFUHFVHFHFMFLF Submarine surface waves Ground / 1000 km Radio horizon Radar Space waves Radio Navigation Data, Radio and Television Broadcasting Line-of-Sight Radio Satellite Radio Space waves Geom. horizon Direct waves 100-150 km Figure 2.2: Propagation and range of electromagnetic waves in free space In the higher frequencies it is usually space waves that form. Along with direct radiation, which, depending on the roughness and the conductivity of the earth’s surface, is quickly attenuated, these waves are diffracted and reflected based on their frequency in the troposphere or in the ionosphere. The range for lower frequencies lies between 100 and 150 km, whereas it decreases with higher frequencies because of the increasing transparency of the ionosphere, referred to as the radio horizon. When solar activity is intense, space waves can cover a distance of several thousand kilometres owing to multiple reflection on the conductive layers of the ionosphere and the earth’s surface. Waves with a frequency above 3 GHz propagate as direct waves, and con- sequently can only be received within the geometric (optical) horizon. Another factor that determines the range of electromagnetic waves is their power. The field strength of an electromagnetic wave in free space decreases in inverse proportion to the distance to the transmitter, and the receiver input power therefore fades with the square of the distance. The received power for omnidirectional antennas can be described on the basis of the law of free-space propagation. An ideal point-shaped source, a so-called isotropic radiator of signal en- ergy, transmits its power P T uniformly distributed into all directions. Such a transmitter cannot be realized physically. The power density flow F through the surface of a sphere at a distance d from an ideal radiator (see Figure 2.3) can be expressed as F = P T 4πd 2 [W/m 2 ] (2.1) In most cases antennas are used that focus the radiated power into one direction. The resultant antenna gain g(Θ) into the direction Θ is expressed by the radiated power normalized to the mean power, where P 0 represents the total transmit power emitted from the antenna. g(Θ) = P (Θ)4π P 0 (2.2) 2.1 Fundamentals of Radio Transmission 29 Distance d Area Isotropic source Figure 2.3: Power density flow F The maximum signal energy radiated from the antenna is transmitted into the direction of the main lobe. The maximum antenna gain g max at Θ = 0 gives the amplification measure in comparison with an isotropic radiator using the same signal energy. According to Equation (2.1), the power density flow of an ideal loss-less antenna with gain g T is F = P T g T 4πd 2 [W/m 2 ] (2.3) The product P T g T is called EIRP (Effective Isotropically Radiated Power). This is the transmit power necessary with an omnidirectional isotropic ra- diator to reach the same power density flow as with a directional antenna diagramme. The energy arriving at the receiver is P R = P T g T g R  λ 4πd  2 (2.4) In Equation (2.4) P T represents the power radiated by the transmitter and P R the input power of the receiver. g T and g R stand for the corresponding absolute antenna gains. λ is the wavelength and d the distance between sender and receiver. The free-space path loss L =  λ 4πd  2 (2.5) describes the spatial diffusion of the transmitted energy over a path of length d, and g R is the receive antenna gain. In a logarithmic representation this produces the path loss (P T − P R ) L F = −10 log g T − 10 log g R + 20 log f + 20 log d − 20 log c 4π with c representing the wave propagation speed. In a simple case scenario with isotropic antennas the free-space attenuation L 0 is produced without antenna gain as the difference between received power and radiated power: 30 2 System Aspects L 0 [dB] = P R [dBm] − P T [dBm] = −10 log  P R [mW] P T [mW]  = −20 log  λ 4πd  (2.6) 2.1.1 Attenuation Weather conditions cause changes to the atmosphere, which in turn affect the propagation conditions of waves. Attenuation is frequency-dependent and has a considerable affect on some frequencies, and a lesser one on others. For example, in the higher-frequency ranges above about 12 GHz attenuation is strong when it is foggy or raining because of the scattering and absorption of electromagnetic waves on drops of water. Figure 2.4 shows the frequency-dependent attenuation of radio waves with horizontal free-space propagation in which, as applicable, the appropriate at- tenuation values for fog (B) or rain of different intensity (A) still need to be added to the gaseous attenuation (curve C). What is remarkable are the resonant local attenuation maxima caused by water vapour (at 23, 150, etc., GHz) or oxygen (at 60 and 110 GHz). Based on 60 GHz as an example, Figure 2.5 shows the propagation atten- uation and the energy per symbol E s related to N 0 (noise power), referred to as the signal-to-noise ratio, for antenna gain of g T = g R = 18 dB. These gains are achieved with directional antennas with approximately 20 ◦ · 20 ◦ beam angles. The electric transmit power in the example is 25 mW, thereby pro- ducing the value 2 dBW = 1.6 W for the radiated microwave power (EIRP). The ranges which can be achieved are 800 m in good weather conditions and 500 m in rainy conditions (50 mm/h). 2.1.2 Propagation over Flat Terrain Free-space propagation is of little practical importance in mobile communica- tions, because in reality obstacles and reflective surfaces will always appear in the propagation path. Along with attenuation caused by distance, a radiated wave also loses energy through reflection, transmission and diffraction due to obstacles. A simple calculation [27] can be carried out for a relatively simple case scenario: two-path propagation over a reflecting surface (see Figure 2.6). In this case P R P T = g T g R  h 1 h 2 d 2  2 d  h 1 , h 2 is a frequency-independent term. The corresponding path loss L P is L P = −10 log g T − 10 log g R − 20 log h 1 − 20 log h 2 + 40 log d 2.1 Fundamentals of Radio Transmission 31 Temperature: Water vapour: Sea level: 1 atm (1013.16 mbar) 20 C: Gaseous 2 10 3 0.1 g/m 3 A: Rain 2 10 100 O 2 H O 2 H O 2 Pressure: O C 2 7.5 g / m B: Fog O m H 3 µ Wavelength: Centimetre Millimetre 10 cm 1 cm 1 mmλ Submillimetre 0.01 0.1 1 10 100 0.02 0.05 0.2 0.5 2 5 20 50 200 500 525 2 5 2 5 Frequency (GHz) Specific attenuation (dB / km) for a horizontal path 10 A A A A A B B B C C 150 mm/h 50 mm/h 25 mm/h 0.25 mm/h Figure 2.4: Attenuation of radio propagation depending on the frequency due to gaseous constituents and precipitation for transmission through the atmosphere, (from CCIR Rep. 719, 721) 32 2 System Aspects E N / = 18 dB = 18 dB 2 ) / [dB] 0s Signal-to-noise ratio ( 10 Path loss / dB Path loss at 60 GHz 90 100 110 120 130 140 150 160 170 60 50 40 30 20 10 0 -10 Gain Free-space propagation Distance / m 800 m (good weather)500 m (rain) 3 4 5 6 7 8 9 98765432 Rain plus oxygen +32 dB Oxygen attenuation +15 dB 2 3 10 3 Noise figure Data rate = 10 dB = 1 Mbit/s g R T F R g = 25 mWTransmit power P T Figure 2.5: Attenuation due to weather conditions h h 1 h 2 + h 1 h 1 h 2 d - 2 h 2 Reflected wave φ θ θ Figure 2.6: Model for two-path propagation due to reflection and with isotropic antennas L F dB = 120 − 20 log h 1 m − 20 log h 2 m + 40 log d km (2.7) In this model the receive power decreases much faster (∼ 1/d 4 ) than with free-space propagation (∼ 1/d 2 ). This also depicts the reality of a mobile radio environment more closely but does not take into account the fact that actual ground surfaces are rough, therefore causing wave scattering in addition to reflection. Furthermore, obstacles in the propagation path and the type of buildings that exist have an impact on attenuation. 2.1 Fundamentals of Radio Transmission 33 E N / ) / [dB] 0s Signal-to-noise ratio ( 2 3 4 6 25 -10 60 50 40 = 18 dB 20 10 7 = 1 GHz f = 60 GHz f 0 998 = 18 dB T P = 25 mW g R F T R g = 1 Mbit/s = 10 dB 3 10 876543 Distance / m 2 10 30 3 Path loss / dB 60 80 100 120 140 160 Direct plus reflected wave 1 GHz Direct plus reflected wave 60 GHz mean Figure 2.7: Propagation attenuation in two-path model taking into account O 2 absorption With the introduction of the propagation coefficent γ, the following applies to isotropic antennas: P R = P T g T g R  λ 4π  2 1 d γ (2.8) Realistic values for γ are between 2 (free-space propagation) and 5 (strong attenuation, e.g., because of city buildings). Different models can be used for calculating the path loss based on these parameters, and are presented in Section 2.2. Figure 2.7 compares the resulting propagation attenuation at 1 GHz and at 60 GHz, taking into account O 2 absorption and interference caused by two- path propagation. This interference leads to signal fading in sharply defined geographical areas, and this is also relevant within the transmission range. 2.1.3 Fading in Propagation with a Large Number of Reflectors (Multipath Propagation) Fading refers to fluctuations in the amplitude of a received signal that oc- cur owing to propagation-related interference. Multipath propagation caused by reflection and the scattering of radio waves lead to a situation in which transmitted signals arrive phase-shifted over paths of different lengths at the receiver and are superimposed there. This interference can strengthen, distort or even eliminate the received signal. There are many conditions that cause fading, and these will be covered below. 34 2 System Aspects Transmitter Receiver Figure 2.8: Multipath propagation In a realistic radio environment waves reach a receiver not only over a direct path but also on several other paths from different directions (see Figure 2.8). A typical feature of multipath propagation (frequency-selective with broad- band signals) is the existence of drops and boosts in level within the channel bandwidth that sometimes fall below the sensitivity threshold of the receiver or modulate it beyond its linear range. The individual component waves can thereby superimpose themselves con- structively or destructively and produce a stationary signal profile, referred to as multipath fading, which produces a typical signal profile on a path when the receiver is moving, referred to as short-term fading (see Figure 2.9). The different time delays of component waves result in the widening of a channel’s impulse response. This dispersion (or delay spread) can cause interference between transmitted symbols (intersymbol interference). Furthermore, depending on the direction of incidence of a component wave, the moving receiver experiences either a positive or a negative Doppler shift, which results in a widening of the frequency spectrum. In general the time characteristics of a signal envelope pattern can be de- scribed as follows: r(t) = m(t)r 0 (t) (2.9) Here m(t) signifies the current mean value of the signal level and r 0 (t) refers to the part caused by short-term fading. The local mean value m(t) can be deduced from the overall signal level r(t) by averaging r(t) over a range of 40–200 λ [21]. The receive level can sometimes be improved considerably through the use of a diversity receiver with two antennas positioned in close proximity to each other (n · λ/2; n = 1, 2, . . .). Because of the different propagation paths of the radio waves, the receiving minima and maxima affected by fading of both antennas occur at different locations in the radio field, thereby always enabling 2.1 Fundamentals of Radio Transmission 35 i +1i a i V ( ) r t i b i a Signal envelope of the tr ( ) tr ( ) b µ 100 0.1 10 1 0.01 0 0.2 0.4 s Mean Fade margin 0 t t t i t R T receive voltage Fade duration Connection duration ThresholdR for threshold R Figure 2.9: Receive signal voltage at a moving terminal under multipath fading (overall and in detail) Selection diversityScanning diversity A A 2 r t ( ) 2 r t ( ) 1 r t ( ) 1 r t ( ) tr ( ) tr ( ) Figure 2.10: Diversity reception the receiver to pick up the strongest available receive signal. See Figure 2.10, which shows the signal profile r i (t) of two antennas and the receive signal r(t). With scanning diversity an antenna is replaced by a prevalent antenna when its signal level drops below a threshold A. With selection diversity it is always the antenna with the highest signal level that is used. 2.1.4 A Statistical Description of the Transmission Channel It is only possible to provide a generic description of a transmission channel on the basis of a real-life scenario. In the frequency range of mobile radio being considered, changes such as the movement of reflectors alter propagation conditions. Signal statistics is another way of developing a mathematical understanding of the propagation channel. 36 2 System Aspects 2.1.4.1 Gaussian Distribution The distribution function resulting from the superposition of an infinite num- ber of statistically independent random variables is, based on the central limit theorem, a Gaussian function: p(x) = 1 √ 2πσ e − (x−m) 2 2σ 2 (2.10) No particular distribution function is required for the individual overlaid ran- dom variables, and they can even be uniformly distributed. The only prereq- uisite is that the variances of the individual random variables should be small in comparison with the overall variance. A complete description of the Gaussian distribution is provided through its mean value m and the variance σ 2 . 2.1.4.2 Rayleigh Distribution On the assumption that all component waves are approximately incident at a plane and approximately have the same amplitude, a Rayleigh distribution occurs for the envelope of the signal. This assumption applies in particular when the receiver has no line-of-sight connection with the transmitter because of the lack of dominance of any particular component wave (see Figure 2.8). The distribution density function of the envelope r(t) is p(r) = r σ 2 e − r 2 2σ 2 (2.11) with the mean value, quadratic mean value and variance E{r} = σ  π 2 , E{r 2 } = 2σ 2 , σ 2 r = σ 2  4 − π 2  For the representation with r(t) = m(t) · r 0 (t) a normalization of E{r 2 0 } = 1 is common and useful. The logarithmic representation with y = 20 log r 0 therefore produces p(y) = 10 y/10 20 log e e −10 y/10 with the mean value, variance and standard deviation (C = 0.5772 . . . is Euler’s constant) E{y} = − C · 10 log e = −2.51 dB σ 2 y = (10 log e) 2 π 2 /6 = 31.03 dB, σ y = 5.57 dB Figure 2.11 illustrates the distribution in half-logarithmic scaling. [...]... 2 System Aspects Diraction Diraction describes the modication of propagating waves when obstructed A wave is diracted into the shadow space of an obstruction, thereby enabling it to reach an area that it could ordinarily only reach along a direct path through transmission The eect of diraction becomes greater as the ratio of the wavelength to the dimension of the obstacle increases Diraction is negligible... lognormal distribution, i.e., Lm = log m(t) is normally distributed with a standard deviation of approximately 4 dB [21, 27] This 2.2 Models to Calculate the Radio Field 43 is also called lognormal fading This approximation applies to statistics for large built-up areas 2.1.9 Interference Caused by Other Systems In addition to the interference caused by radio wave propagation, which has already been discussed,... through dierent methods that involve transmitting several connections simultaneously in multiplex mode Multiplexing is a technique permitting multiple use of the transmission capacity of a medium The following techniques are used in radio systems: Frequency-division multiplexing (FDM) Time-division multiplexing (TDM) Code-division multiplexing (CDM) Space-division multiplexing (SDM) In addition... cellular network supplying an extended area 2.3 Cellular Systems Traditional radio networks (1st-generation) that try to provide coverage to large areas by increasing the transmitting power of the individual base stations are only able to serve a limited number of subscribers because of the bandwidth used In these radio networks an allocated radio channel is retained as long as possible, even if a receiver... neighbouring base station Cellular systems must therefore be capable of carrying out a change in radio channel as well as in base station during a connection This process is called handover (see Section 3.6) A mobile radio network must be aware of which mobile radio users are currently roaming in its radio coverage area to enable it to page them if necessary Mobile stations are therefore always assigned... applications process needing communications support It contains standard services for supporting data transmission between user processes (e.g le transfer), providing distributed database access, allowing a process to be run on dierent computers, and controlling and managing distributed systems 62 2 2.6 System Aspects Allocation of Radio Channels Utilization of the capacity of a transmission medium can be improved... dB 2.2.5 Radio Propagation in Microcells Todays radio networks use hierarchical cell structures with small microcells below the level of conventional macrocells This increases network capacity in areas with high trac volumes (city centres, trade fairs, etc.) Microcells cover areas extending up to several 100 m, with radio illumination greatly aected by the geometry of the base station surroundings The... dened, neighbouring supply areas must use dierent radio channels in order to avoid interference Where user density is high, this results in a heavy demand for frequencies, which, however, are restricted because of the scarcity of available spectrum The poor utilization of frequency spectrum in these radio networks and the increase in the number of mobile radio users, which these systems could no longer... value of a signal level can be calculated using empirical models and diraction models Some applications, such as the calculation of radio propagation in networks with microcells ( . Ground / 1000 km Radio horizon Radar Space waves Radio Navigation Data, Radio and Television Broadcasting Line-of-Sight Radio Satellite Radio Space waves. 2.1: Radio transmission path: transmitter–receiver. Z 0 and Z W are the radio wave resistances in free space and on the antenna feeder link Mobile Radio