Channel Characteristics

Channel Characteristics

Mobile Satellite Communication Networks Ray E Sheriff and Y Fun Hu Copyright q 2001 John Wiley & Sons Ltd ISBNs: 0-471-72047-X (Hardback); 0-470-845562 (Electronic) Channel Characteristics 4.1 Introduction This chapter considers the propagation environment in which a mobile-satellite system operates The space between the transmitter and receiver is termed the channel In a mobilesatellite network, there are two types of channel to be considered: the mobile channel, between the mobile terminal and the satellite; and the fixed channel, between the fixed Earth station or gateway and the satellite These two channels have very different characteristics, which need to be taken into account during the system design phase The more critical of the two links is the mobile channel, since transmitter power, receiver gain and satellite visibility are restricted in comparison to the fixed-link The basic transmission chain is shown in Figure 4.1 By definition, the mobile terminal operates in a dynamic, often hostile environment in which propagation conditions are constantly changing In a mobile’s case, the local operational environment has a significant impact on the achievable quality of service (QoS) The different categories of mobile terminal, be it land, aeronautical or maritime, also each have their own distinctive channel characteristics that need to be considered On the contrary, the fixed Earth station or gateway can be optimally located to guarantee visibility to the satellite at all times, reducing the effect of the local environment to a minimum In this case, for frequencies above 10 GHz, natural phenomena, in particular rain, govern propagation impairments Here, it is the local climatic variations that need to be taken into account These very different environments translate into how the respective target link availabilities are specified for each channel In the mobile-link, a service availability of 80–99% is usually targeted, whereas for the fixed-link, availabilities of 99.9–99.99% for the worst-month case can be specified The following reviews the current status of channel modelling from a mobile and a fixed perspective 4.2 Land Mobile Channel Characteristics 4.2.1 Local Environment Spurred on by the needs of the mobile-satellite industry, the 10 years spanning the mid-1980s Mobile Satellite Communication Networks 116 Figure 4.1 Mobile network propagation environment to the mid-1990s witnessed significant effort around the world in characterising the land mobile-satellite channel The vast majority of these measurement campaigns were focused on the UHF and L-/S-bands, however, by the mid-1990s, with a number of mobile-satellite systems in operation, focus had switched to characterising the next phase in mobile-satellite development, that of broadband technology at the Ka-band and above The received land mobile-satellite signal consists of the combination of three components: the direct line-of-sight (LOS) wave, the diffuse wave and the specular ground reflection The direct LOS wave arrives at the receiver without reflection from the surrounding environment The only L-/S-band propagation impairments that significantly affect the direct component are free space loss (FSL) and shadowing FSL is related to operating frequency and transmission distance This will be discussed further in the following chapter Tropospheric effects can be considered negligible at frequencies below 10 GHz, while impairments introduced by the ionosphere, in particular, Faraday rotation can be effectively counteracted by the selective use of transmission polarisation Systems operating at above 10 GHz need to take into account tropospheric impairments and these will be considered further when discussing the fixed-link channel characteristics Shadowing occurs when an obstacle, such as a tree or a building, impedes visibility to the satellite This results in the attenuation of the received signal to such an extent that transmissions meeting a certain QoS may not be possible The diffuse component comprises multipath reflected signals from the surrounding environment, such as buildings, trees and telegraph poles Unlike terrestrial mobile networks, Channel Characteristics 117 which rely on multipath propagation, multipath has only a minor effect on mobile-satellite links in most practical operating environments [VUC-92] The specular ground component is a result of the reception of the reflected signal from the ground near to the mobile Antennas of low gain, wide beamwidth operating via satellites with low elevation angle are particularly susceptible to this form of impairment Such a scenario could include hand-held cellular like terminals operating via a non-geostationary satellite, for example The first step towards modelling the mobile-satellite channel is to identify and categorise typical transmission environments [VUC-92] This is usually achieved by dividing the environment into three broad categories: † Urban areas, characterised by almost complete obstruction of the direct wave † Open and rural areas, with no obstruction of the direct wave † Suburban and tree shadowed environments, where intermittent partial obstruction of the direct wave occurs As far as land mobile-satellite systems are concerned, it is the last two of the above environments that are of particular interest In urban areas, visibility to the satellite is difficult to guarantee, resulting in the multipath component dominating reception Thus, at the mobile, a signal of random amplitude and phase is received This would be the case unless multisatellite constellations are used with a high guaranteed minimum elevation angle Here, satellite diversity techniques allowing optimum reception of one or more satellite signals could be used to counteract the effect of shadowing The fade margin specifies the additional transmit power that is needed in order to compensate for the effects of fading, such that the receiver is able to operate above the threshold or the minimum signal level that is required to satisfy the performance criteria of the link The threshold value is determined from the link budget, which is discussed in the following chapter The urban propagation environment places severe constraints on the mobile-satellite network For example, in order to achieve a fade margin in the region of 6–10 dB in urban and rural environments, a continuous guaranteed minimum user-to-satellite elevation angle of at least 508 is required [JAH-00] The compensation for such a fade margin should not be beyond the technical capabilities of a system and could be incorporated into the link design However, to achieve such a high minimum elevation angle using a low Earth orbit constellation would require a constellation of upwards of 100 satellites On the other hand, for a guaranteed minimum elevation angle of 208, a fade margin in the region of 25–35 dB, would be required for the same grade of service, which is clearly unpractical While these figures demonstrate the impracticalities of providing coverage in urban areas, in reality, for an integrated space/terrestrial environment, in an urban environment, terrestrial cellular coverage would take priority and this is indeed how systems like GLOBALSTAR operate In open and rural areas, where direct LOS to the satellite can be achieved with a fairly high degree of certainty, the multipath phenomenon is the most dominant link impairment The multipath component can either add constructively (resulting in signal enhancement) or destructively (causing a fade) to the direct wave component This results in the received mobile-satellite transmissions being subject to significant fluctuations in signal power In tree shadowed environments, in addition to the multipath effect, the presence of trees will result in the random attenuation of the strength of the direct path signal The depth of the fade is dependent on a number of parameters including tree type, height, as well as season due 118 Mobile Satellite Communication Networks to the leaf density on the trees Whether a mobile is transmitting on the left or right hand side of the road could also have a bearing on the depth of the fade [GOL-89, PIN-95], due to the LOS path length variation through the tree canopy being different for each side of the road Fades of up to 20 dB at the L-band have been reported due to shadowing caused by roadside trees in a suburban environment [GOL-92] In suburban areas, the major contribution to signal degradation is caused by buildings and other man-made obstacles These obstacles manifest as shadowing of the direct LOS signal, resulting in attenuation of the received signal The motion of the mobile through suburban areas results in the continuous variation in the received signal strength and variation in the received phase The effect of moving up in frequency to the K-/Ka-bands imposes further constraints on the design of the link Experimental measurement campaigns performed in Southern California by the jet propulsion laboratory as part of the Advanced Communications Technologies Satellite (ACTS) (not to be confused with the European ACTS R&D Programme) Mobile Programme reported results for three typical transmission environments [PIN-95] For an environment in which infrequent, partial blocking of the LOS component occurred, fade depths of 8, and dB were measured at the K-band, corresponding to fade levels of 1, and 5%, respectively This implies that for 1% of the time, the faded received signal is greater than dB below the reference pilot level and so on In an environment in which occasional complete shadowing of the LOS component occurred, corresponding fade depths of 27, 17.5 and 12.5 dB were measured Lastly, for an environment in which frequent complete blockage of the LOS occurred, fade depths of greater than 30 dB were obtained for as low as a 5% fade level In all environments, fade depths at the K- and Ka-bands were found to be essentially the same Similar degrees of fading were found during European measurement campaigns, such as under the European Union’s ACTS programme SECOMS project The results demonstrate the difficulty in providing reliable mobile communications to any environment in which the LOS to the satellite may be restricted Channel modelling is classified into two categories: narrowband and wideband In the narrowband scenario, the influence of the propagation environment can be considered to be the same or similar for all frequencies within the band of interest Consequently, the influence of the propagation medium can be characterised by a single, carrier frequency In the wideband scenario, on the other hand, the influence of the propagation medium does not affect all components occupying the band in a similar way, thus causing distortion to selective spectral components 4.2.2 Narrowband Channel Models 4.2.2.1 Overview Narrowband channel characterisation is primarily aimed at establishing the amplitude variation of the signal transmitted through the channel The vast majority of measurement campaigns have used the narrowband approach and, consequently, a number of narrowband models have been proposed These models can be classified as being either (a) empirical with regression line fits to measured data, (b) statistical or (c) geometric-analytical Empirical models can be used to characterise the sensitivity of the results to critical parameters, e.g elevation angle, frequency Statistical models such as the Rayleigh, Rician and log-normal distributions or their combinations for use in different transmission environ- Channel Characteristics 119 ments, are especially useful for software simulation analysis; whilst geometric-analytical models provide an understanding of the transmission environment, through the modelling of the topography of the environment 4.2.2.2 Empirical Regression Models A number of measurement campaigns performed throughout the world have aimed to categorise the mobile-satellite channel These measurement campaigns have attempted to emulate the satellite by using experimental air borne platforms, that is helicopters, aircraft, air balloons, or in some cases existing geostationary satellite systems have been used The following briefly describes some of the most widely cited models Empirical Roadside Shadowing (ERS) Model This model is used to characterise the effect of fading predominantly due to roadside trees The model is based upon measurements performed in rural and suburban environments in central Maryland, US, using helicopter-mobile and satellite-mobile links at the L-band [VOG-92] Measurements were performed for elevation angles in the range 20–608; the 208 measurements utilised a mobile-satellite link, and the remainder a helicopter The subsequent empirical expression derived from the measurement campaign for a frequency of 1.5 GHz is given by AL P; u; fL ị ẳ 2MuịlnP Nuị dB for fL ẳ 1:5 GHz 4:1ị Muị ẳ a bu cu2 4:2ị Nuị ẳ d u e 4:3ị a ẳ 3:44; b ẳ 0:0975; c ¼ 20:002; d ¼ 20:443 and e ¼ 34:76 ð4:4Þ where AL(P,u ,fL ) denotes the value of the fade exceeded in decibels, L denotes the L-band, fL is the frequency at L-band in GHz and is equal to 1.5 GHz in the equation; P is the percentage of the distance travelled over which the fade is exceeded (in the range 1–20%) or the outage Table 4.1 ERS model characteristics Elevation angle (8) M(u ) N(u ) 20 25 30 35 40 45 50 55 60 4.59 4.63 4.57 4.40 4.14 3.78 3.32 2.75 2.09 25.90 23.69 21.47 19.26 17.04 14.83 12.61 10.40 8.18 120 Mobile Satellite Communication Networks Figure 4.2 Fading at 1.5 GHz due to roadside shadowing versus path elevation angle probability in the range of 1–20% for a given fade margin u is the elevation angle From the above, the following model characteristics can be derived (Table 4.1) The following relationship between UHF and L-band has been derived for fade depth in tree shadowed areas for P in the range of 1–30% [VOG-88] sffiffiffiffiffiffi fL dB 4:5ị AL P; u; fL ị ẳ Auhf P; u; fuhf Þ f uhf Similarly, it was found that when scaling from L-band (1.3 GHz) to S-band (2.6 GHz) the following relationship applies: As ðP; u; fs Þ < 1:41AL ðP; u; fL Þ dB ð4:6Þ The ERS model can be found in Ref [ITU-99a], which provides formulae that can be used to extend the operational range of the model The conversion from L-band to K-band and vice versa in the frequency range 850 MHz to 20 GHz can be obtained from the formula, for an outage probability within the range 20% $ P $ 1% [ITU-99a]: & !' 1 ffi pffiffi pffiffiffi AK ðP; u; fK Þ ¼ AL ðP; u; fL Þexp 1:5 dB ð4:7Þ fL fK Channel Characteristics 121 Elevation angles below 208 down to 78 are assumed to have the same value of fade as that at 208 Figure 4.2 shows the results of application of the ERS model at 1.5 GHz for a range of elevation angles To increase the percentage of distance travelled (or outage probability) to the limits of 80% $ P $ 20%, the ITU recommends the following formula:   80 ln AL2K ðP; u; fL2K Þ ¼ AK ð20%; u; fK Þ ð4:8Þ ln4 P where AL2K() denotes the attenuation for frequencies between 0.85 and 20 GHz An extension to the ERS model is also provided for elevation angles greater than 608 at frequencies of 1.6 and 2.6 GHz, respectively [ITU-99a] This is achieved by applying the ERS model for an elevation angle of 608 and then linearly interpolating between the calculated 608 value and the fade values for an 808 elevation angle given in Table 4.2 Linear interpolation should also be performed between the figures given in Table 4.2 and 908 elevation, for which the fade exceeded is assumed to be dB Table 4.2 Fades exceed (dB) at 808 elevation [ITU-99a] P (%) Tree-shadowed 1.6 GHz 10 15 20 30 2.6 GHz 4.1 2.0 1.5 1.4 1.3 1.2 9.0 5.2 3.8 3.2 2.8 2.5 As noted earlier, the season of operation effects the degree of attenuation experienced by the transmission The following expression is used to take into account the effect of foliage on trees, at UHF, for P in the range 1–30%, indicating a 24% increase in attenuation due to the presence of leaves Afull foliageị ẳ 1:24Ano foliageị dB 4:9ị The Modified Empirical Roadside Shadowing (MERS) Model The European Space Agency modified the ERS model in order to increase the elevation angle range up to 808 and the percentage of optical shadowing up to 80% One form of the MERS is given by AðP; uÞlnðPÞ BðuÞ dB where P and u are as in the ERS model A(u ) and B(u ) are dened by: Auị ẳ a1 u2 a2 u a3 ð4:10Þ Mobile Satellite Communication Networks 122 a1 ¼ 1:117 £ 1024 ; a2 ¼ 20:0701; a3 ẳ 6:1304 Buị ẳ b1 u2 b2 u b3 b1 ¼ 0:0032; b2 ¼ 20:6612; b3 ¼ 37:8581 Empirical Fading Model This model is based upon the measurements performed by the University of Surrey, UK, simultaneously using three bands (L, S and Ku) [BUT-92] Elevation angles were within the range 60–808 Its basic form is similar to the ERS model with the addition of a frequency-scaling factor It is given by MP; u; f ị ẳ au; f ÞlnðPÞ cðu; f Þ ð4:11Þ where aðu; f Þ ¼ 0:029u 0:182f 6:315 cðu; f Þ ¼ 20:129u 1:483f 21:374 The model is valid for the following ranges: P, link outage probability, 1–20%; f, frequency, 1.5–10.5 GHz; u , elevation angle, 60–808 The model has been extended by combining it with the ERS, resulting in the combined EFM (CEFM) model This is valid for elevation angles in the range 20–808 with regression coefficients: aðu; f ị ẳ 0:002u2 0:15u 0:2f 0:7 cu; f ị ẳ 20:33u 1:5f 27:2 4.2.2.3 Probability Distribution Models Probability distribution models can be used to describe and characterise, with some degree of accuracy, the multipath and shadowing phenomena This form of modelling allows the dynamic nature of the channel to be modelled In turn, this enables the performance of the system to be evaluated for different environments Essentially, a combination of three probability density functions (PDF) are used to characterise the channel: Rician (when a direct wave is present and is dominant over multipath reception), Rayleigh (when no direct wave is present and multipath reception is dominant) and log-normal (for shadowing of the direct wave when no significant multipath reception is present) Complete Obstruction of the Direct Wave In an urban environment, the received signal is characterised by virtually a complete obstruction of the direct wave In this case, the received signal will be dominated by multipath reception The received signal comprises, therefore, of the summation of all diffuse components This can be represented by two orthogonal independent voltage phasors, X and Y, which arrive with random phase and amplitude The phase of the diffuse component can be characterised by a uniform probability density function within the range 0–2p , while the amplitude can be categorised by a Rayleigh distribution of the form Channel Characteristics 123 r r2 PRayleigh rị ẳ exp 2 sm sm ! ð4:12Þ where r is the signal envelope given by: q r ẳ x2 y2 4:13ị and s m is the mean received scattered power of the diffuse component due to multipath propagation ă As noted in Chapter 1, for an unmodulated carrier, fc, the Doppler shift, fd, of a diffuse component arriving at an incident angle u i is given by: fd ¼ vfc cosui Hz c 4:14ị ă where u i is in the range 0–2p This results in a maximum Doppler shift fm of ^ vfc/c, where c is the speed of light (