The aim of this project was to characterize the narrowband and wideband land mobile satellite and railroad radio channel at Ku/Ka band for mobile multimedia services including multipath and rain fading effects.
An experiment simulating satellite links with the transmitter installed in an aircraft has been carried out in greater Linz area (Austria). Various mobile environments have been considered such as rural, urban, suburban, and railroad.
Characteristic model parameters have been determined for narrowband and wideband and these parameters have been included in a simulator software.
Land mobile and railroad environments have become of interest to satellite multimedia system designers. There is a significant potential of users travelling daily in various vehicles. The aim for higher bit rates and the congestion of lower frequencies have driven the designers to seek answers at Ku/Ka bands. The satellite channel in these bands is rather challenging with a multipath and rain fading and low link margin. Ku/Ka band propagation in mobile environments has not been measured extensively and the wideband behaviour at Ka-band has hardly ever been measured before.
The main objective of this activity is the modelling of the narrowband and wideband propagation effects on Ku and Ka bands, based mainly on a measurement campaign:
- Considering various land mobile environments (rural, railroad, urban, suburban, etc.),
- Considering available data and literature,
- Extract narrow and wideband model parameters for the different environments and elevations,
- Integrate these parameters in a simulation software.
Propagation mechanisms: (1) free space attenuation, (2) ground reflection, (3) reflection and scattering from multipath sources, (4) attenuation due rain cells, (5) attenuation due foliage, (6) shadowing due obstacles, (7) diffraction around objects, (8) Doppler shift due movement of the satellite and mobile receiver
The main challenges arise from the experiment setup. The key issues were:
- Integration of RF equipment into the aircraft and the measurement van,
- Guaranteeing an exact flight pattern,
- Documentation of flight pattern,
- Data quality,
- Availability of periods of rain/no-rain.
The later issue was the major challenge and risk to the activity, since due to the complexity of the experiment, it was not possible to postpone indefinitely the campaign in the absence of rain, the campaign had to be plan based on statistics and rain predictions. Eventually, the first campaign only recorded very light rain over a few minutes. This unexpected fact was partially mitigated through additional measurements with a beacon receiver and tracking antenna on Hotbird-6 during periods of rain and no rain over the same routes.
A unique experiment was carried out for an enhanced understanding of propagation mobile channel characteristics at Ku- and Ka-band frequencies. Wideband- and Narrowband results do show excellent plausibility. As expected the usage of such high frequencies requires careful planning. Even minor path shadowing elements like electricity poles cause strong signal degradation. Comparison of narrowband and wideband relative power level variations are in very good agreement.
In this study, Ku- and Ka-band scattering and multipath effects could be statistically described for four different environments and documented for a variety of individual obstacles. The extracted narrowband - especially Average Fade Duration and Level Crossing Rate parameters - can be useful for efficient coding, while the extracted wideband parameters have been used for the modeling of this channel in various environments. With the available auxiliary data (Rx and Tx position as well as video) it was possible to describe several striking scattering effects. Within certain limits the particular results are certainly transferable to environments of similar type in other locations.
An operational channel model simulator for various environments have been delivered for time-series simulation of channel characteristics.
For the experiment a variety of requirements have been identified. It should:
- Consider various mobile environments (rural, railroad, urban, suburban),
- Simulate satellite links with various azimuth- and elevation angles,
- Use sufficient dynamic range to characterize multipath also at atmospheric attenuation conditions,
- Give full documentation of the environment, so that ideally, fadings and reflections in the signal can be physically explained.
Since the experiments had to simulate land mobile satellite reception realistically, an airborne transmit platform was used. This allowed to investigate the channel in azimuth and elevation dependence.
The principal experiment setup uses an Elektrobit Propsound Channel Sounder (PS-CS) at 17.6 GHz. The sounding signal with 200 MHz bandwidth was utilized in the experiment, allowing measurements of link attenuation and delay spread introduced by multipath. The PS-CS has been used in SISO configuration. For comparison purposes a narrowband pure sine wave was also transmitted and measured.
The basic concept for the experimental configuration is to let the mobile receiver move on a spot-like position so that the movement of the car would not change the measurement geometry. This requirement is fulfilled by choosing measurement routes within an area of less than 1 square kilometre, while the transmitter was carried at a link distance of 10 km. A Hercules C130 was chosen, as such an aircraft is able to move on a hemisphere of 10 km radius around the spotlike position of the moving receiver. The measurement geometry thus includes full azimuth circles by the transmitter on the aircraft, and elevation steps at 20, 40 and 60 degrees seen from the receiver, as indicated in Fig. 1.
As an additional independent reference measurement an actual Ku-band satellite beacon signal at 11.7 GHz from Hellasat has been used. Along with the mobile receivers, a stationary beacon receiver at 19.701 GHz has been set up in the area of Linz, Austria, to measure the HotBird 6 Ka-band beacon. Fig. 2 shows an overview of these channel measurement instruments.
To support the analysis, various auxiliary data have been simultaneously recorded: the position of transmitter and receiver using GPS readings, with the aircraft attitude documented by IMU recordings. To study the environmental conditions in detail, up-looking cameras with fish-eye lenses have been mounted on the roof of the measurement vans, with GPS position and time inserted into the video stream.
Four scenarios were investigated: rural, urban, suburban and overhead power lines (measured below a trolley bus route and comparable to railway power lines). In both measurements (17.6 GHz and 11.7 GHz) the very same routes have been considered, they reside in and around the city of Linz. The measurements were carried out at a nominal mobile RX speed of 40 km/h (17.6 GHz) and 20 km/h (11.7 GHz). The maximum Doppler at 17.6 GHz was 1.75 kHz mainly due to an elliptical flight track. At 11.7 GHz the maximum Doppler was 180 Hz only (due to the movement of the RX in the van).
click for larger image
First and second-order statistics of the narrowband channel such as CDF, level crossing rate (LCR [1/s]), and average fade duration (AFD [s]) were extracted. These parameters give important information for coding especially when it comes to error mitigation. They depend on the speed of the mobile receiver.
During wideband analysis, the following parameters were extracted: delay spread, mean excess delay, maximum delay, total received power, and peak power.
Effects of scatterers with broadside illumination
The project main tasks were:
- Review of channel models,
- Identification of requirement for the experiment,
- Planning and design of the experiment and integration of measurement equipment,
- Execution of the Experiment,
- Narrow-and wideband data analysis,
- Model development and testing,
- Simulator software development,
- Additional satellite measurements execution and analysis.