Cabin Propagation Channel Model

A summary of the analysis of the radio propagation measurements in two aircraft bodies at GSM, UMTS and ISM bands is provided below. Results are divided in coverage analysis, large-scale model characterisation, wideband model characterisation in a tap delay form, frequency selectiviness analysis, and attenuation and shadowing study of several cabin objects.

No channel characterization for an aircraft cabin wireless network deployment was available up to now. Hence, results achieved during the WirelessCabin activities are considered of high scientificical value.

COVERAGE RESULTS AND ANALYSIS:

First coverage conclusions emerging from the performance comparison between the use of a conventional antenna and leaky line cable (see D1v1 „Project Review Report Year 1“) pointed out the fact that the leaky line cable distributes the power uniformly over the passenger’s area, without creating well-favoured areas as it happens with conventional antennas.

(a) Transmitter antenna: patch antenna

(b) Transmitter antenna: leaky line

 

These results were confirmed by wideband measurements and an example shows it in the figure on the left. Both plots show the power received by an antenna moving along the right aisle of the A330-200 cabin from the last row until the galley in business class (roughly 29 m). A patch antenna on the transmitter side placed above the front part of the right aisle ceiling was used in the above figure and a leaky line cable over the ceiling of the whole right aisle was used on the below figure. Both measurements were done at 2.45 GHz with a transmitted power of 18 dBm. While the minimum received power in the patch antenna scenario is below -50 dBm, in the leaky line one is kept above ‑35 dBm, more than 15 dB difference. The power in figure (a) would further increase if the receiver moved further on to the first rows, closer to the transmitter position, resulting in an even bigger different coverage situation in the cabin.

Therefore, and without considering capacity restriction, a coaxial cable with leaky sections is the best solution to provide coverage in the whole passenger’s area of an aircraft, whereas a point antenna is a desirable solution to provide coverage in dedicated cabin areas, such as in business class.

 

LARGE-SCALE CHANNEL CHARACTERISATION:

The large-scale channel characterisation for conventional antennas consists in estimating the path loss exponent () and standard deviation (in dB) of a zero-mean Gaussian distributed random variable ( also in dB) based on the best-fit line of the measured data according to the following equation:

                        .

This was done for the two aircraft and all the frequencies were measurements were done; results are summarised in the next table. More than 50 samples were always used. As example, a figure with measured data and a regression curve between normalised received power at 2140 MHz versus distance in the A330-200 is showed.

 

 

f (MHz)

n

s (dB)

A319

800

2.25

4.77

1820

1.94

3.80

1950

1.95

5.09

A330-200

900

4.75

5.29

2140

4.16

6.18

From all results, it can be concluded that a tunnel-like behaviour characterises the propagation  inside  a  small-haul  aircraft,

while in a long-haul aircraft the characterisation is more similar to the one of an indoor office environment.

 

According to the equation describing propagation from a leaky line antenna expressed by:

     .

Large-scale channel characterisation for this antenna provides the estimation of attenuation exponent () and longitudinal loss () of the cable.  depends on the working frequency and is normally given by the manufacturer, but it was also estimated to assess the reliability of the method in calculating . Results on the calculation of  in both aircraft fitted with the theoretical value with a relative error <6% for all frequencies except at 900 MHz, which is a resonant frequency of the cable used. =2.3 was obtained for both aircraft and all frequencies except 900 MHz.

CHANNEL IMPULSE RESPONSE CHARACTERISATION:

A tap delay impulse response characterisastions was carried out for a total of four scenarios: 1- short-range ac with conventional antenna, 2- short-range ac with leaky line cable, 3- long-range ac with conventional antenna, and 4- long-range ac with leaky line cable. In all cases the impulse response can be expressed as the sum of  echoes (or multipath components) having random amplitude , delay , and phase :

                   ,                                  

where  is the Dirac delta function. Characterisation of the in-cabin channel impulse response was developed by determining the statistical properties of the path variables , , , and  in the four scenarios mentioned before.

Due to the big amount of obstacles in the in-cabin channel, the scatterers can be assumed as uncorrelated, and therefore,  were assumed to be uniformly distributed in .

Distribution of number of multipath components

A summary of  distribution in for the four scenarios is presented in the table below. The first phenomena that can be observed is that, in conventional antenna scenarios for both aircrafts, there is a twofold trend for  as the receiver moves away from the transmitter: (i) first  increases because of both the increment of scatters and the decrement of the highest peak of the impulse response which implies a decrease of the threshold used when counting multipath components and hence more echoes are considered; (ii) from a certain point on,  decreases since echoes earlier considered become so weak that are masked by noise and therefore, not counted. The figure with the measurement data and the fitting curves for the A319 is also shown below

1

 

3

 

 

conv. ant.

Nakagami

5 areas, depending on d

initial increase of N

2

 
by higher d, decrease of N

Gaussian

6 areas, depending on d

initial increase of N

4

 
by higher d, decrease of N

leaky line

Gaussian

one single area

 and .

Gaussian

one single area

 and

Tx. antenna

a/c type

short-range a/c

long-range a/c

 

 did not display any trend with respect to neither distance nor position of the receiver for a given row in the leaky line measurements; the same Gaussian characterisation was obtained for both aircraft. It is worth pointing out that this distribution only makes sense at integer values and those lying in the range from 9 to 24 (certainly avoiding negative values). Inspection of the results for all scenarios suggests that the use of the leaky feeder makes the multipath behaviour in terms of number of echoes more uniform over the cabin.

Distribution of path gains

The characterisation of path gains, , was done in two steps: (i) first, the mean amplitude  for the k-bin was studied and determined by an exponential decay (), and then (ii) the variation over this mean value  was modelled in terms of a probability function.

In most cases  and  were found to be function of the Tx-Rx distance, or even lateral dependece of the exponential decrease was observed caused by shadowing of the stowage bins, like the areas identified in the left figure (scenario 3).

Lognormal, Nakagami, lognormal and Weibull distributions characterise  for scenarios 1 to 4 respectively. For further details, refer to deliverable D11.

Distribution of arrival time sequence

Three approaches known in literature for arrival time characterisation were followed (a pure Poisson arrivals process, a modified Poisson process called  developed by Suzuki, and a cluster-type process presented in ’87 by Saleh and Valenzuela), but none of them described satisfactorily the channel impulse response in many cabin positions. Therefore, it was decided to characterise the multipath components arrival time based based on an empirical probability approach.


FREQUENCY SELECTIVNESS ANALYSIS:

A frequency selectiveness study has been also done in terms of mean excess delay (), root mean square delay spread () and the coherence bandwidth () in the same scenarios mentioned in the channel impulse response characterisation.



(ns)


(MHz)

sc. 1

63.7

2.5

sc. 2

45.8

3.47

sc. 3

91

1.75

sc. 4

60.2

2.64

This table summarises the most restrictive  according to the maximum  calculated in each scenario. By comparing these values with the bandwidths of the systems under scope, it is concluded that the use of RAKE receivers receivers or other equalization techniques would improve the performance of WLAN for use in aircraft, while the performance of BluetoothTM will not be highly affected thanks to the frequency hoping scheme.

ATTENUATION, SHADOWING AND FAST FADING EFFECTS CAUSED BY CABIN OBSTACLES:

Wideband meauserement were also investigated in scenarios where either the receiver unit or something around it moved. With these measurements, thanks to the combination of video synchronisation and received signal, it was estimated the attenuation effect that cabin objects might cause. These values should be taken into account in link budget calculations for in-cabin communications. In particular, the following objects/effects were studied: stowage bin attenuation, lavatory’s door attenuation, back-seat table attenuation, lavatory compartment shadowing, seat shadowing, separation walls shadowing, stowage bin shadowing, and fast fading effect caused by a metallic trolley. These moving scenarios can be spitted in three general situations: (i) changing from almost LOS to NLOS (shadowing), (ii) adding an extra obstacle when already in NLOS (attenuation), and (iii) multipath fast fading.

(a) Receiver placed on a seat with back-seat table folded

(b) Receiver placed on a seat with back-seat table unfolded

As an example, this figure summarises the back-seat table attenuation measurement. The figure consists of two video snapshots and two plots of total received power. A vertical blue line in the received power plots marks the instant of time associated to the video snap-shot. A mean difference of 2.5 dB can be observed between both situations.

From the analysis of all the results it can be concluded that when designing an in-cabin radio netowork a fading margin of at least 9 dB (10-12 dB to be on the save side) must be taken into account in the link budget calculation.