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Advanced Modulation Schemes for LEO Satellite Constellations

Communications via satellites have two unique characteristics: the ability to cover the globe with a flexibility that cannot be duplicated with terrestrial links, and the availability of large bandwidth for intercontinental communications.

 

Modulation is the process of converting data or baseband signals into electrical signals optimized for transmission. Modulation, in general, is achieved by varying some characteristic of a periodic waveform, called the carrier signal,  in accordance with another separate signal called the modulation signal that typically contains information to be transmitted. For example, the modulation signal might be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing a sequence of binary digits, a bitstream from a computer.  The carrier is higher in frequency than the modulation signal. The purpose of modulation is to impress the information on the carrier wave, which is used to carry the information to another location.

 

More recent systems use digital modulation, which impresses a digital signal consisting of a sequence of binary digits (bits), a bitstream, on the carrier. Like analog modulation, in digital modulation systems carrier parameters like phase, frequency or amplitude are varied with information signals.

 

The most fundamental digital modulation schemes are amplitude-shift keying (ASK), phase-shift keying (PSK), frequency-shift keying (FSK), and quadrature amplitude modulation (QAM) as shown below.

 

With strong demand for faster data throughput, satellite communications use high-order modulation schemes to improve their spectral efficiency. However, satellite channel impairments such as large path losses, delays, and Doppler shifts pose severe challenges to the realization of a satellite network. The modulation techniques for satellite communications require not only faster data rates but also minimizing the impacts of the channel impairments.

 

LEO Satellite Constellations Challenges

Low Earth Orbit (LEO) satellites orbit at an altitude of 300-3000 km. As a consequence, they are
characterized by a lower propagation delay, lower propagation losses and a higher Doppler shift than GEO satellites. This fact justifies the need for transmitter-receiver architectures that are robust to Doppler effects when LEO satellite deployments are considered.

 

Low Earth Orbits are in the range between 300km to 3000km of altitude and are characterized for the orbital velocity needed to maintain the satellite in orbit, which is about 7.8km/s in the lowest orbits. As the orbital altitude increases, the velocity is reduced. Due this high velocity, the orbital period is about 100 minutes, consequently the visibility window duration in one point of the Earth is very short.

 

Owing to the low altitude of the orbits, the satellites suffer the atmospheric drag provoked by the gasses of the upper layers of the atmosphere in consequence the velocity of the satellite is reduced and the satellite losses height. This atmospheric drag produces that the angular velocity of the satellite fluctuate over the course of time.

The remote sensing satellites are low earth orbiting satellites that transmit data to ground stations during the visibility time. The visibility times are usually of the orders of 15-20 minutes. The ground stations have to track and acquire the data from remote sensing satellites. There is also a minimum elevation angle for visibility to archive connection between the terminal and the satellite which is the order of 10º.
With the limitation on visibility times on ground station antennas for good RF links, the limitation on the amount of data and data rates arises for LEO satellites in the currently used bands.

As the payloads on the spacecrafts advanced, the demand for reliable communication with higher data rates increased. However, achieving higher data rates with the limited frequency band has some challenges to be considered.

 

When LEO communication is considered, the reliability and robustness of the link should have the highest priority. Therefore, achieving high data rates has to be accompanied by a robust and reliable system.

 

Satellite Channel Model

The channel model for a satellite communications can be characterized in two parts, one part is
due to the scattering and the obstacles around the terminal and the other one is the deterioration of the signal due the Doppler shift.
The satellite propagation model use a line of sight (LoS) component with a Rice distribution and a multipath component (NLoS) with Rayleigh distribution due the reflections of the signal. The delay between these taps are in the order of nanoseconds. Also the LOS component have a power level much larger than the NLoS taps.

 

Doppler Shift

For LEO satellite communication systems, the Doppler frequency at terminals varies with time. This time the varying phenomenon is caused by the line of sight component of the relative velocity vector, evolving from the rapid movement of the satellite in its orbit, in relation to the ground transceiver, which includes satellite velocity and the relative velocity due the Earth’s rotation. This can be characterized by the maximum elevation angle from the terminal to the satellite during the visible time.
More specifically, the Doppler shift is zero when the satellites are in the maximum elevation angle and at its closest position to the terminal; on the other hand, at lower elevation angles this shift is larger.
The LTE standard contemplates that the maximum Doppler shift that a terminal can experiment is that of a high-speed train. In this scenario, the maximum speed is less than 500 km per hour and the carrier’s frequency is 2Ghz. The maximum Doppler shift results in 950Hz. In satellite communication, the Doppler shift will be much larger with the maximum Doppler considered in LTE specifications. Researchers have considered scenario with a carrier frequency of 2GHz, the Doppler shift would be within the range of: -45KHz<=fd<=45kHz. That would make that the communication can’t be established between the mobile terminal and the satellite.

Doppler shift compensation strategies

The high Doppler shift doesn’t allow 4G communication systems such as LTE to be implemented in LEO satellite systems. The Doppler is approximately 50 times bigger than that tolerated in LTE receivers. That’s the reason why a Doppler compensation methods have to be applied.

1rst Method: Doppler Shift compensation at the terminal.

In this method the mobile terminal knows the position of the satellite and its own position. Hence, it has to know the orbit of the satellite to compute the Doppler shift that will be experienced. With these method any terminal can receive the signal in any position of the
Earth.
However, the method has some big disadvantages. One of the biggest problems stems from the limited computational capacity of the terminals. To compensate the Doppler, the terminal needs to calculate the orbit of the satellite and the Doppler shift that the terminal will suffer at each instant. In addition, the way that the terminal has access to the position of the satellite is not a trivial task and seems difficult to obtain.

2nd Method: Doppler Shift compensation at the satellite.

This method consist in defining a static ground cell, where the satellite will provide coverage. In this method the satellite has to know the position of the center of the cell and its own position.
With the GPS system it’s really easy to obtain the coordinates, due the GPS constellation have an attitude orbit of 20180km much higher than the LEO. Based on this information, the satellite only has to compensate the Doppler shift that a user located at center of the cell will experience.
The ground position doesn’t change and these simplify the calculations that the satellite have to do. Applying this method, the terminals in the center of the cell or in the very near positions will receive a negligible Doppler shift. Moreover, terminals that are inside of the cell will suffers a reduced Doppler shift, when compared to the case where no compensation is done. Now the residual Doppler shift depends on the position of the user in the ground cell.
For a cell of 100km radius the maximum difference of Doppler between the center and the worst extreme of the cell is 5KHz. Without any compensation, the Doppler shift will be increased by a factor of 10. The bigger is the cell, the higher is the residual Doppler experienced by edge users. It becomes evident that the residual Doppler increases with the cell size.

Advantages

  • The principal reason is the computational calculations the terminal have to do, in the chosen method the terminal only has to estimate the residual Doppler, which can be done by existing methods exploiting the OFDM modulation format. Nevertheless, in the other method the terminal has to calculate the Doppler shift received during the visibility window duration.
  • The second reason is the occupation of the traffic channels. In the first method the terminal has to receive the information about the parameters of the orbit from other satellite or terrestrial base station. This data will load the traffic channel with control information. But in the chosen method this problem doesn’t exist.

 

Advanced Modulation Schemes for LEO Satellite Constellations

Orthogonal frequency division multiplexing (OFDM)

Orthogonal frequency division multiplexing (OFDM), is a communication technique that divide a transmission bandwidth, into a number of closely evenly-spaced frequency bands. In each one is transmitted one sub-carrier that transports a portion of the information. Each subcarrier is orthogonal to the rest, giving it the name to this multiplexing technique by frequency division.

 

OFDM is a very flexible and efficient modulation technique used in many wireless standard like: LTE, LTE-Advanced, WiMAX, Digital Audio and Video Broadcast, WLAN, and ADSL.
The main advantage of OFDM is its ability to provide robustness against frequency selective
fading, because it divides the overall channel into multiple narrowband signals that can be
modelled as flat fading sub-channels.

 

The most important advantages of OFDM are: Resilient to ISI (inter-symbol interface) due the Cyclic prefix that introduces a guard interval between symbols, resilience to interface due the signal is divided in sub-carriers, if one carrier frequency is interfered only is loss a portion of the transmitted signal and simple channel equalization due the signal is transmitted in many narrowbands signals rather than one fast wideband signal.

 

The fact that each subcarrier is orthogonal to the rest allows that the spectrum of each subcarrier be overlapped and there is no interference, increasing the efficiency from the better use of the spectrum.

 

An OFDM system takes a flow of modulated data (QAM, QPSK,..) and divides it into N parallel flows, that’s the serial to parallel block function. Then each flow is mapped to a subcarrier and combined using the inverse fast Fourier transform (IFFT) obtaining the signal in the time domain to be transmitted. In this moment, the cyclic prefix is added to the signal. This is an important characteristic of the OFDM signal and is based on repeating a small part of the information. This cyclic prefix provides a guard interval between symbols to eliminate intersymbol interference and simplifies the channel estimation and equalization.

 

After adding the cyclic prefix, the signal is converted to serial and it’s transmitted through
the channel. The length of this prefix should be equal or higher than the channel impulse
response.

 

The receiver is basically a reversed version of the transmitter. First of all, the signal is converted serial to parallel. Then, the cyclic prefix is removed. Next, the information conveyed on each subcarrier can be extracted by applying the Fast Fourier Transform (FFT). After that, the signal is converted in a serial data flux.

 

References and Resources also include:

https://upcommons.upc.edu/bitstream/handle/2117/123510/DanielNietoYll_Doppler_compensation_for_LEO.pdf?sequence=2

 

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