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Multiple access and routing for LEO satellite constellations

Multiple access to a particular repeater channel (transponder) implies prior multiple access to the satellite repeater.

Each satellite repeater channel (transponder) amplifies every carrier whose spectrum falls within its passband at a time when the channel is in an operational state. The resource offered by each channel can thus be represented in the form of a rectangle in the time–frequency plane. In the absence of special precautions, carriers would occupy this rectangle simultaneously and mutually interfere. To avoid this interference, it is necessary for receivers (an earth station receiver for a transparent satellite and an on-board satellite receiver for a regenerative satellite) to be able to discriminate between the received carriers.


When this  discrimination is achieved in frequency domain its called FDMA, in the temporal domain called TDMA and code domain it’s called CDMA.


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 mobility and routing issues

Due to their low altitude (typically 700 to 1400 km), LEO satellites move at rapid speeds relative
to the ground terminals. Speeds at over 25,000 km/hour, with satellite visibility times of only a
few minutes before handover occurs to another satellite, are the norm.

This high mobility leads to a rapidly and regularly-changing network topology, and raises
numerous issues for the networking layer with respect to routing. Terrestrial Internet routing protocols, such as Open Shortest Path First (OSPF) and Routing Information Protocol (RIP), rely on exchanging topology information when network connections are established or changed. In LEO constellations, this topology information quickly becomes obsolete and must constantly be
refreshed with new information. The overhead of regularly providing this information is an
obstacle to considering satellites as conventional Internet routers.


The virtual node concept

The virtual node concept aims to exploit the regularity of the constellation’s topology. Again,
the goal is to hide the mobility of satellites from routing protocols running over the constellation.
In this scheme, information concerning terrestrial constellation users, and how to communicate with them, is state that relates to a region of the Earth and is maintained in a fixed position relative to the surface of the Earth.

Constellation users communicate with the virtual entity containing state pertaining to them: this is the virtual node. This virtual node is embodied at any given time by a satellite, and a virtual
network of these nodes is embodied at any time by the satellite constellation.

As the satellites move and as users perform handovers, state, such as routing table entries or
channel allocation information, is transferred continuously from one satellite to another.
Routing is performed in the fixed virtual network, by using a common routing protocol.

Strategies dependent on topology

These strategies use proprietary routing protocols that have explicit knowledge of the constellation topology and the satellite mobility. Such protocols require that there is always a path
between two communicating ground hosts, and that routing is loop-free.

Each proprietary protocol will be very specific to the design of a certain type of constellation.
The Footprint Handover Routing Protocol is a simple example of such a protocol for polar
Walker star constellations


MF-TDMA (Multi-Frequency, Time Division Multiple Access) is the leading technology for dynamically sharing bandwidth resources in an over-the-air, two-way communications network. The access method is a combination of FDMA and TDMA, that is, the frequency band of a transponder contains several different carriers, and each carrier works in the way of narrow-band TDMA.


Thus, each frequency channel is divided into several timeslots that can be assigned to multiple connections. The timeslots are the resource that needs to be allocated to each connection. Each connection is assigned a fixed portion of the resource-based on its quality-of-service (QoS) requirements.


Each terminal initiates communication (with some other terminal) by making a connection request. The connection is supported through the allocation of the commonly shared resources
(i.e., set of timeslots) managed by a satellite. In an MF-TDMA satellite system, timeslots are allocated in groups, called bursts. Each burst is composed of a single string of contiguous timeslots over which a terminal transmits its data. A terminal transmits, to the satellite, its bursts in the assigned position of the frame according to a transmit burst time plan (BTP) and receives bursts in the assigned position of the frame, returned by the transponder, according to a receive BTP.


Clock synchronization is the key technology of the system satellite communication system which based on MF-TDMA.


For satellite networks MF-TDMA is the dominant technology because it provides the most bandwidth and the greatest overall efficiency and service quality, while also allowing the dynamic sharing of that bandwidth among many (tens of thousands) of transmitters in a two-way communication mode. MF-TDMA networks can have either a star-topology, a fully meshed or partially meshed topologies.


ST Engineering iDirect Achieves World’s First Live MF-TDMA Demo on Telesat LEO Satellite in Oct 2020

ST Engineering iDirect, a company of ST Engineering North America, announced the successful completion of the first Over-the-Air (OTA) testing of iDirect’s Multi-Frequency Time Division Multiple Access (MF-TDMA) return link on the Telesat Phase-1 Low Earth Orbit (LEO) satellite. The milestone achievement demonstrated dynamic sharing of bandwidth among multiple terminals within a LEO constellation, a capability that extends the capacity and flexibility of Telesat’s multi-beam beam hopping architecture, and opens up a wide range of use cases for Telesat’s LEO customers in the commercial, government, and defense markets for land, land-mobile, aeronautical, maritime, and other applications.


The testing was conducted at Telesat’s Allan Park facility and featured ST Engineering iDirect’s VSAT platform networked across multiple satellite modems. The iDirect platform was able to compensate fully for the LEO satellite link dynamics, including time, frequency, signal variation and Doppler effects. Short guard times (the time intervals required between radio bursts to prevent self-interference) were achieved, comparable in length to guard times used on GEO satellite links, without compromising capacity or spectral efficiency.


The ability to leverage MF-TDMA to efficiently share bandwidth on satellite ground-to-space links improves the capacity, performance and affordability of broadband services delivered over LEO satellite constellations.

To put the link to the test, the team conducted a video conference with engineers at Allan Park and achieved seamless connectivity, low jitter and low packet loss, resulting in a high Quality of Experience (QoE) that exceeded the level typically achieved over GEO satellite networks.


“This is a significant success for the ST Engineering iDirect and Telesat teams,” said Bart Van Poucke, Vice President of Product Management at ST Engineering iDirect. “We have achieved the benefits of MF-TDMA efficiency whilst unlocking the low latency offered by LEO satellites. This demonstration confirms the wide addressable market for LEO and particularly for applications that require mission-critical communications. We are proud to have been part of these tests and thank Telesat for the opportunity.”


“Satellite service providers are eager to take advantage of Telesat LEO’s affordable, low latency, high-speed connectivity to deliver secure Internet, VPN, video conferencing and cloud applications to their customers,” stated Erwin Hudson, Telesat’s Vice President of LEO. “I congratulate the ST Engineering iDirect engineering team on their successful testing campaign. They demonstrated the powerful advantages that MF-TDMA brings to LEO networks, a capability that can provide increased flexibility and higher capacity for our customers while allowing us to support a greater number of end users on each LEO satellite.”


MRC Mx-DMA Multi-Resolution coding (MRC) and Mx-DMA MRC (Multi-Resolution Coding)

In a Satellite Return Link (RTN), a single up to thousands of terminals (e.g. cruise ships, fishing boats, broadband users, towers or airplanes) access a single gateway through a single satellite transponder, thereby sharing a common bandwidth. The specifications per terminal may vary greatly, from low (fishing boat) to high throughput (cruise ship), from low (IoT or downloading) to high (audio call) jitter sensitivity, from low-cost (broadband users) to high-end (cruise ships) terminals.


Thousands of terminals should potentially be able to log on in a few seconds, e.g. due to mobility or when the network recovers from an outage. For a profitable business, the required number of Multi-Carrier Demodulators (MCDs) needed at the gateway to demodulate up to thousands of terminals should be limited and as low as possible, without compromising the efficiency of transmission (number of bits per Hz that can be transmitted). In order to address this, we have designed and productized a single return technology that captures all the above requirements and which represents a significant improvement on our previous 3 return (RTN) link technologies SCPC, Mx-DMA and CPM, write ST Engineering iDirect reserachers.


This new RTN link technology is referred to as MRC Mx-DMA. Multi-Resolution coding (MRC) can demodulate, on a single MCD, any combination of terminal transmissions (from high demanding (high throughput, no jitter) to low cost (low throughput, no jitter sensitive transmissions) terminals. Keep-alive traffic does not cost any noticeable bandwidth. Guard bands are minimized. Jitter is as low as a single FEC word duration, even though a burst from a terminal spans up to 100 FEC words. Automatic in-band regrowth detection without any calibration prevents terminals from saturating BUCs while maximizing power transmission. This results in a single RTN link transmission that is as efficient as SCPC for high throughput terminals while achieving CPM like scalabilities, overbooking ratios and efficiencies for services with high overbooking. There is no need for mode switching between CPM and SCPC, as it is all done within a single time-frequency frame using a single technology on a single MCD.


All terminals share a single bandwidth pool and can log on in an unsolicited way (thousands at the same time in a few seconds). The proposed multi-resolution coding (MRC) satellite return channel waveform solves the problem of finding a balance between TDMA flexibility and SCPC efficiency in a unified waveform. The waveform is highly parametrizable allowing to cope with very different user scenarios. The MRC waveform optimizes spectral efficiency in all traffic conditions and minimizes system jitter. The waveform needs to allow nearly continuous carriers minimizing time guards, frequency guard bands and synchronisation overheads. Next, the waveform keeps the possibility to allocate very small time/frequency chunks to serve terminals with low traffic demand.


Mx-DMA MRC adjusts the frequency plan, symbol rate modulation, transmission length, code block size and power in real-time for every terminal in the network, based on return traffic demand, QoS settings and link conditions. But, designing an Mx- DMA MRC link does not require precise knowledge of the traffic and terminal mix, as the link self optimizes in real-time, providing a far simpler way to manage complex traffic demands. This also eliminates the need for the cumbersome trade-offs needed when predefining return carrier plans for a mix of terminal and service types as required.


The efficiency of the proposed waveform varies a lot with the terminal traffic demand. For low bit rates, efficiency is lower because of the smaller FEC codewords, but also, and mainly because the proportionally higher guard times, frequency guard bands and synchronization overheads.


The presented MRC waveform supports transmit modes from 100kBaud to 100MBaud, allowing terminal bit rate ranges from 1kbps to 100Mbps. The MCD is such that the whole aggregated bandwidth can be allocated at will by the MRC controller in order to optimize the spectral usage. There is no limitation in MRC on the allocation complexity. The MCD design is such that high and low bandwidth carriers can co-exist, and the allocation is bounded only by the aggregated bandwidth and not by the allocation complexity.


A new MF-TDMA waveform is proposed. The new MRC waveform complies with HRC and CPM performances and with a large variety of end user needs (high and low traffic demands). A time multiplexed hardware architecture has been presented based on a time multiplexed demodulator, allowing cost effective implementation and low processing jitter.


Innovations over HRC 

Scalable Demodulator Technology. Unlike HRC where each terminal is assigned its own carrier, with MRC, terminals not transmitting traffic, will seamlessly log-off and automatically restart transmission when needed. This means that there is no idle capacity consumption, enabling the technology to support a wide mix of traffic profiles in a shared return capacity.  Mx-DMA MRC supports a minimum transmit length of 5ms, allowing up to 5,000 active terminals with a single multi-carrier demodulator; meaning that less hardware is needed at the hub, resulting in significant capex savings.

High Resolution Bandwidth Allocation.  Mx-DMA MRC redistributes the available spectral resources 25 times per second, allowing it to seamlessly adapt to changing traffic demand and link conditions. Industry leading granularity in bandwidth assignment, lowest latency and jitter and highest efficiency for any traffic profile are made possible by a minimum transmission length of 5ms tied to a symbol rate of 100ksps and a 5% roll-off.

Adaptive Payload Length.  Mx-DMA MRC adapts the payload length in real time, versus using the industry norm of pre-coded static payload length.  By using adaptive code lengths, MRC optimizes efficiencies based on transmission length, resulting in reduced jitter.  This also provides important efficiency gains for bursty traffic patterns, such as those associated with voice, Internet of Things (IoT) and Supervisory Control and Data Acquisition (SCADA)applications.

Automatic Regrowth Control.  Ensures that the BUC always operates at its most efficient operations point, so reducing BUC cost and allowing GAN.


Building on the well-established, award winning Mx-DMA® HRC (High Resolution Coding) technology, Mx-DMA MRC (Multi-Resolution Coding) offers unprecedented service agility, extending the availability of Mx-DMA to very large networks and expanding the applicability and use of the technology to include a full spectrum of use cases.


Mx-DMA MRC brings forth the full scalability of TDMA return link technologies to the original Mx-DMA HRC return at the same SCPC-like efficiency levels. Service providers can cover a myriad of use cases in a single return link, from cruise ships and large enterprise customers to SCADA and broadband access, sharing satellite capacity more efficiently over a group of satellite terminals and applications achieving the lowest Total Cost of Ownership (TCO).


Mx-DMA MRC is a return link technology that combines the benefits of MF-TDMA (ideal for bursty traffic and higher contention services) and the spectrum efficiency of Mx-DMA HRC into a single return technology suited to a greatly expanded set of applications. Mx-DMA MRC scales in MHz independent of the number of terminals so customers may be served with a single return link for the majority of their use cases, minimizing operational complexity and maximizing statistic multiplexing. Designing an Mx-DMA MRC link does not require precise knowledge of the traffic and terminal mix as the link self-optimizes in real time. Moreover, the high efficiency enables bandwidth savings, higher throughput, better network availability and substantial terminal cost savings.


References and Resources also include:

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