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Aeronautical Channel Model for Broadband L-Band Satellite Communication

Madhavendra Richharia, Naveen Kaluvala Inmarsat, Advanced Systems Division, 99 City Road, London EC1Y 1AX, , Great Britain; E Mail:

Panos Fines Wireless Intelligent Systems Ltd., 64 Hatfield House, Baltic Street West, London Ec1Y 0SU, Great Britain

Marcos Alvarez-Diaz Dept. of Signal Theory and Communications, University of Vigo, Campus Universitario, 36310 Vigo, Spain,

Axel Jahn TriaGnoSys GmbH, Argelsrieder Feld 22, D-82234 Wessling, Germany

Abstract This paper describes an aeronautical propagation model suitable for evaluating the performance of wideband mobile satellite systems (MSS), such as Inmarsat’s Broadband Global Area Network (BGAN) system. Geometry of propagation is combined with estimates of scattered power from sea surface, weighted by antenna directivity, to provide a statistical representation of signals received on an aircraft. Due to the variability in scattered signal levels, propagation delays and angles of arrival, the multi-path statistics are multi-dimensional exhibiting an amplitude-delay-Doppler spread. The amplitude and delay contours of the multi-path are dependent on the propagation geometry. The concentric amplitude iso-contours demonstrate that the highest scattered power arrives from a relatively narrow central region, which differs from the specular point. Since the location of maximum scattering region with respect to the specular point has an elevation angle and sea state dependence, the minimum delay component may not represent the highest magnitude. The total multi-path power is evaluated by spatial integration of the scattered power. At a given delay, the dominant components of scattered signals arrive from the region along the aircraft-satellite bearing. The statistical characteristics are used to create a model suitable for computer simulation to facilitate investigation of the proposed BGAN aeronautical system. The model comprises of a direct signal combined with outputs of an n-tap delay line, each of which is statistically independent Rayleigh distributed with a Gaussian spectral shape. It is observed that for a typical flight path the channel affects the performance of un-equalized BGAN high rate bearers adversely. Concept and results of an equalizer are included to demonstrate performance improvements.

Introduction Inmarsat’s next generation mobile satellite communication system, known as Broadband Global Area Network system (BGAN), is an L-band broadband system offering multimedia services to land portable user terminals (UT) initially and subsequently to airborne, land vehicular and maritime terminals. BGAN air interface employs coded 16-QAM and QPSK modulation schemes at bandwidths up to 200 KHz. The air interface has been well-characterized for the land portable channels. Inmarsat has undertaken a study to assess the performance of the air interface in the mobile environment. One of the fundamental requirements for undertaking the study is a parametric propagation model in each environment and UT class. To the best of authors’ knowledge, the propagation characteristics of wide-band MSS channels have not been modeled in sufficient detail for the task in hand. Hence, propagation models, tailored to each category of BGAN environment were developed, based on established theory.

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Investigations revealed that coherence bandwidth of maritime and land mobile channels were 3-5 MHz wide and thus can be treated as flat fading channel; however, in a typical aeronautical environment, the multi-path delays were observed to be sufficiently large to possibly affect the performance of BGAN high rate bearers adversely. This paper therefore addresses the aeronautical wide-band model. The model is based on delay-Doppler analysis of Earth surface scattering, especially sea scattering, which is generally more pronounced than scattering from land. The received satellite signal at an aeronautical terminal consists of a direct component and multipath echoes. The multipath component is a combination of the specular reflection (coherent multipath component) and the scattered or diffuse reflections (incoherent component). From both the theory and measurements it is known that, as long as the grazing angle is higher than a few degrees and the sea is slightly rough, the specular component is much lower than the scattered component. This means that the Rice law that describes the received signal level is driven only by the carrier to multipath component, C/M. The multipath component of the satellite channel was calculated by integration of the spatial scattering distribution on the surface and subsequently of the Power Delay Spectrum. The Doppler model was developed from propagation geometry for the flight scenario. Different antenna patterns and environments were used to provide results for representative flight phases. The propagation characteristics with three classes of antennas, expected to be used in Aeronautical BGAN, were modeled at a typical cruising altitude for calm and rough sea conditions at low (5 deg) and high (20 deg) elevation angles. The propagation model was used to characterize the performance of BGAN bearers for each type of antenna in a typical aeronautical environment. The results demonstrated that the performance of the high rate bearers was indeed susceptible to the power-delay profiles. Hence work was undertaken to develop a suitable channel equalization scheme at the receiver. Several studies were undertaken, each using a different approach. This paper presents the results of one of the approaches to demonstrate the advantage offered through equalization. The technique relies on iterative estimation and cancellation of multi-path echoes.

Wide-band Model A wideband model is considered a complex superposition consisting of the direct signal between satellite and

aircraft and the multipath components [1]. In the aeronautical environments, the multipath component is a combination of the specular reflection (coherent multipath component) and the scattered or diffuse reflections (incoherent component), cf. Figure 1. The figure shows equal contours of differential delays with respect to the specular reflection of a satellite-to-aircraft propagation path. The differential delay between the direct signal and the specular reflection signal can be computed directly from the geometry and is depicted in Figure 2 for varying satellite elevation and aircraft altitude levels.

Figure 1. Propagation geometry of specular and scattered reflections.

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Figure 2. Differential delay, as a function of elevation angle and aircraft altitude; Differential delay is given as the delay between the time of arrival of a direct ray and a ray reflected from the specular point.

The scattered component intensity heavily depends on the sea state (calm/rough sea). From both the theory and measurements ([2], [3], [4]), it is clear that, as long as the grazing angle is higher than a few degrees and the sea is slightly rough, the scattered component dominates. The characteristic parameter of the channel is the carrier to multipath component, C/M.

In the case of not very small grazing angles, the C/M typically increases with the satellite elevation. Only for small grazing angle this tendency is inverted. The worst value is found for elevations between 3° and 7°, for moderate sea states. The main reason for this trend is that the Fresnel reflection coefficient decreases for high incident angles on the sea surface. There exists a standardized prediction method [5] (following [6]) for calculating the fading depth due to the multipath influence, however the method can only predict C/M (and the fading depth), which might be enough for a narrowband analysis, but not wideband components as delay and phase information is missing. Thus we have used a computational model for the aeronautical wideband channel. The inputs to the model are the satellite and aircraft antenna positions, the sea state and the sea surface model state. The evaluation of the wideband channel consists of the calculation of the spatial distribution of the scattering intensity and the subsequent calculation of the Power Delay Spectrum. Different sea state scenarios have been taken into account:

• A “calm-moderate” sea state, corresponding to a significant wave height of H = 1.5 m, and an rms sea slope of β0 = 0.07 (higher value of the recommended interval in [7]).

• A “moderately rough” sea state, corresponding to a significant wave height of H = 3 m, and an rms sea slope of β0 = 0.07.

The procedure is explained in Figures 3 & 4. Figure 4 represents the distribution of the scattering intensity as solid contour lines as seen from an aircraft. The scattering distribution is obtained following [7] (Equation 6), which uses theory and results from [8,9,10]. The total power received from the multipath is calculated as follows:

sss GSMP θφθσπ

ddtan4

1 220 Γ= ∫∫

where: – φs is the azimuth offset angle with respect to aircraft antenna boresight. – θs is the elevation offset angle with respect to boresight. – σ0 is the scattering cross section of a sea surface element calculated as indicated in [7] (equations 4, 5a, 5b)

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– S is the shadowing factor accounting for the signal shadowing caused by the waves themselves. – Γ is the Fresnel reflection coefficient of the sea surface, calculated for the incident angle on a tilted surface. – G is the antenna gain for the incoming angle of arrival defined by φs and θs.

Each point of the Power Delay Spectrum is obtained by integrating all the scattered contributions presenting the same delay with respect to the specular component. Geometrically, the specular component represents the shortest reflected path. [The locus of points with the same excess delay with respect to the specular component approximates an ellipse whose centre drifts from the specular point from the horizon away as the delay increases:. Examples of equal delay reflection points are shown in Figure 3.

Typical Results and discussion Using the wide-band model described in the preceding section, we have been able to characterize the signal

coming from the sea surface. In this section, we show the results obtained for typical conditions as an example. The conditions are given by an aeroplane at velocity of 224 m/s and altitude of 8543 meters, a satellite with elevation of 20 degrees and an omni-directional antenna working at a carrier frequency of 1542 MHz; a rough sea surface was considered.

Figure 3 shows the contours of equal delay of the signals reflected at the sea surface; the reflection point is determined by the elevation and azimuth angles from which the scattered or reflected signal is received. The delay contours are distributed around the specular reflection point, which is assigned zero delay (it is related to the shortest reflected path).

Figure 3. Delay contours measured in µs with respect to the specular reflection point, for a satellite elevation of 20 degrees and aircraft altitude 8543 meters. Figure 4 shows the geometrical distribution of the scattered power coming from the sea surface in the case under study. In this case, the maximum of the scattered power is approximately at the specular point. The main observed feature is the shape of the contours of equal scattered power. They do not present the same shape of the delay contours of Figure 3, but are wider in azimuth for elevations below the maximum of scattered power. By assigning for each point its scattered power to its related delay, we can build the power delay profile of the studied system by integrating the scattered power contribution on equal delay paths. As an example, Figure 5 shows the resulting power-delay-profile of an aero channel at 20° elevation at rougher sea state [11] normalised with respect to the total

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of the multipath power. Therefore, this plot shows the shape and time extension of the delay spectrum but not its actual level relative to the LOS component which should be weighted with the C/M factor.

Figure 4. Power contour at sea surface referenced to the maximum scattering point and directed towards the aircraft; Incoming angle for maximum scattering point = -19.98 deg; altitude = 8534 m; Omni-directional antenna; Frequency = 1542 MHz; Satellite elevation = 20 deg; Rough sea.

0 2 4 6 8 10 12 14-40

-35

-30

-25

-20

-15

-10

Delay (µs)

No

rmal

ised

Pow

er D

elay

Spe

ctru

m (

dB)

ε = 20.0°, Ha = 8534.4 m. Sea state: H = 3 m, β0

= 0.07

Figure 5. Power delay profile for an aeronautical channel; altitude = 8534 m; Omni-directional antenna; Frequency = 1542 MHz; Satellite elevation = 20 deg; Rough sea.

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Figure 6 shows the probability density of the received power for the delay interval [19.5664 us, 20.0664 us]. This

range of delays is around the differential delay of the specular point for altitude 8534 m and elevation 20º. Figure was developed to obtain a probability distribution for a given delay for the n-tap wide-band simulator; which was used later for the design of a channel equalizer with Rayleigh approximation for the received scattered multipath power. Figure 7 shows the probability density of the differential Doppler at the receiver for the delay range 19.5664 - 20.0664 us. It has been obtained by sampling the Doppler at each point of the N point space within the given delay range and estimating probability in the same way as in figure 6. Note that the dip at 0 occurs because the differential Doppler is ~0 around the specular point. Its purpose is to derive a Doppler model at each delay tap of the n-tap wide-band model using a Gaussian approximation. The distribution presents a maximum around 10 Hz monotonically decreasing for higher and lower differential Doppler values.

Figure 6. Probability distribution of power as received at aircraft from the region of maximum scattering for delays in the range 19.5664 us - 20.0664 us; Aircraft altitude = 8543 m; Satellite elevation = 20 deg; Omni-directional antenna; Frequency = 1542 MHz; Rough sea.

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Figure 7. Differential Doppler probability for the lowest differential-delay region (19.5664 - 20.0664 us) ; Aircraft altitude = 8543 m; Satellite elevation = 20 deg; Omni-directional antenna; Frequency = 1542 MHz; Rough sea; Aircraft velocity=224 m/s. The dip at 0 Hz corresponds to the rays emanating from the specular point region. Differential Doppler = Doppler (direct path) – Doppler (scattered path)

For elevations lower than 20 degrees, we have found that the maximum scattered power ceases to be centered

about the specular point; instead, it is located at even more negative elevations. This displacement is greater the lower the elevation is.

The results illustrate the behavior for a system with an omni-directional antenna. The use of directive antennas would help in rejecting the scattered component. For example, and roughly speaking, if the satellite elevation is 20 degrees the main scattered component arrives from a point located 40 degrees below in elevation. A directive antenna would attenuate signals received from that location, so the effect of the scattered signal would be clearly lowered.

Simulation Model The aeronautical channel can be modeled as a n-tap delay line (see figure 8). The received signal exhibits a delay spread caused by a multi-path echoes. Each tap of the n-tap model represents an echo component at a given delay with Rayleigh distributed envelope, a Gaussian spectral shape and a Doppler offset. The outputs of the taps are combined with the direct signal to simulate the received signal level at aircraft. The magnitude of the nth tap is adjusted by the gain control, Kn and the overall magnitude of the multi-path by K.

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W G N

W G N

LP F 1

LP F 1

D elay 1 & Freq O ffset

π /2 p h ase sh if t

K 1

Unfaded S ignal

Faded S ignal Line - of - S ite

W G N

W G N

LP F 2

LP F 2

π /2 ph as e sh if t

K 2

W G N

W G N

LP F 3

LP F 3

π /2 ph as e sh if t

K 3

W G N

W G N

LP F 4

LP F 4

π /2 ph as e sh if t

K 4

W G N

W G N

LP F 5

LP F 5

π /2 ph as e sh if t

K 5

K

D elay 2 & Freq O ffset

D elay 3 & Freq O ffset

D elay 4 & Freq O ffset

D elay 5 & Freq O ffset

D el a y : M u lt i -p at h d e la y F re q o ffs e t: As s o c ia te d D o p p ler W GN : U n c o rre la te d n o is e s o ur ces LP F : L o w p as s B u tt e rw o rt h fi lter s K : A d j u s ted t o se t C /M

Figure 8. Receiver equalizer model

IV. Aeronautical channel equaliser

A. Equalizer Model . Depending on the signal bandwidth, the received signal is subject to selective fading. An example of a typical delay profile and its effects on the received signal spectrum is shown in Figure 9. Clearly, the severity and selectivity increases as the signal bandwidth increases.

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4 .10 6 6 .10 6 8 .10 6 1 .10 525

20

15

10

5

0Delay Profile

(s)

Dif

fuse

d po

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ativ

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LO

S (d

B)

1 0.5 0 0.5 130

20

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10

LOSBdw=42kHzBdw=84kHzBdw=190kHz

Received Spectrum

Fs

dB

Figure 9. Typical aeronautical power delay profile and signal spectrum We have implemented a number of channel estimators for the multi-tap channel. Instead of trying to identify the complete delay profile characteristics once, the successive intersymbol interference (ISI) cancellation, performed in a sequential orderly manner, was proven quite effective. The equalizer is shown in Figure 10. The equalizer consists of an iterative equalizer. In each iteration the channel estimator assumes a 1-tap channel model and performs channel estimation, regeneration of the multipath and interference cancellation. Each equalizer iteration identifies and ‘cleans’ part of the ISI improving the signal quality for the following iteration. The channel parameters must be estimated accurately, before any ISI removal is possible. For this a number of reference symbols is needed within a short observation time. In practice, the observation time is chosen so that the channel state remains approximately unchanged within that time. By doing so, the parameters can be regarded as time independent variables. The number of reference symbols is chosen so that the estimates have small variance due to the thermal noise. PSAMS and UW symbols are a convenient choice but also hard decisions before decoding or tentative decisions while decoding may be also necessary in many cases. A modulator uses all the available reference symbols to regenerate a ‘clean’ reference waveform without ISI and thermal noise. The regenerated waveform is then subtracted from the received waveform to isolate the ISI. The ISI is then correlated with the regenerated waveform over time, phase and gain and the best match produces the best estimates of the reflection delay, phase and amplitude. In general, channel estimation is a not a trivial process since the framing information is minimum with very significant channel noise at low coding rates. One could keep searching for all the significant reflections that are present and identify all the multipath reflection parameters. However, this may be a very unreliable process if the number and quality of the reference symbols are not sufficient. Once the channel parameters are known with some accuracy, significant levels of ISI can be removed from the received signal assisting the decoder to compensate for the effects of the thermal noise and any remaining ISI. By doing so the BGAN receiver can supply more and better quality reference symbols to the channel estimator which, in turn, can supply better channel estimates to the ISI data decision quality improves as the ISI elements are removed and this, in turn, produces more and better quality reference symbols which further improves the channel estimation and the ISI removal.

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Figure 10. BGAN aeronautical channel equalizer

B. Results The number of equalizer iterations and the exact design details of each section is a compromise between performance and complexity and the overall design may be quite different depending on the data rate, modulation, coding rate and burst/frame format. For each transmission mode the equalizer can be optimized for performance and speed. In this study we have investigated the equalizer for a broadband short burst pi/4-QPSK transmission with six equalizer iterations. This equalizer was proven quite effective for all aeronautical operational scenarios. The operation is demonstrated in the following figures. Figure 11 is a ‘snapshot’ of the matched filter samples as they are applied to the input of the turbo decoder. The channel assumed is at low elevation over calm see and no AWGN was added for clarity. Clearly, there is considerable ISI added on the signal constellation. Figure 11 also demonstrates the signal constellation after six iterations at the input of the decoder. It is evident that nearly all the ISI has been removed.

Received Constellation with ISI

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0.5 0 0.5 1 1.5

R5T45QRE

Received Constel lati on wi th Cancel led ISI

-1,5

-1

-0,5

0

0,5

1

1,5

-1,5 -1 -0,5 0 0,5 1 1,5

R5T45QRE

Figure 11. Typical input (l) and output (r) signal constellation of the equalizer

BGAN Equalizer

BGAN Receiver

Demodulator

ChannelEstimator

ISIEqualization

ReceivedSignal

RecoveredData

BGANModulator

Decoder

Iterations: 0..5

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Figure 12 shows the obtained performance in terms of packet error rate for a fast changing channel (fd=40Hz). In this figure, the ideal receiver is one with zero implementation loss measured over an unfaded AWGN channel. The Rx curve indicates the performance of a receiver without equalization and Rx+Eq1, Rx+Eq2,…,Rx+Eq6 indicates the performance after 1 to 6 equalization iterations. Clearly, the performance can be improved with one or more equalization iterations, however, the improvement drops significantly after the second iteration.

Figure 12. PER performance of the BGAN aeronautical equalizer

Conclusion The paper presents a wide-band aeronautical model which has been used to characterize the performance of a

wide-band MSS channel. It is observed that the specular and the maximum scattering points are not co-incident. The delay and Doppler contours form concentric rings due to the geometry. The delay profile is a function of satellite elevation, sea state and antenna beam width. For practical purposes, the behavior can be modeled by a tapped delay line, with each arm exhibiting a Rayleigh distribution and a Gaussian shaped spectral profile. It is demonstrated that a MSS channels such as used in BGAN are susceptible to the delay spread and that equalization improves the performance quite significantly.

Acknowledgments The authors acknowledge ESA for funding a part of the work under its ARTES-3 initiative.

R5T4 5QR E, at 12 .5 degrees and fd=40 Hz

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

57 58 59 60 61 62 63

C/No ( dBHz)

Pa

cke

t E

rro

r R

ate

Rx Rx+Eq1 Rx+Eq2 Rx+Eq3 Rx+Eq4 Rx+Eq5 Rx+Eq6 Ideal Rx+AWGN

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