The main objectives of this project was to investigate the applicability and enumerate the potential gains of using Carrier Aggregation (CA) in satellite systems with taking into considerations the various nuances therein:
1. Define relevant scenarios for the possible implementation of CA schemes. The candidate scenarios can be characterized based on services, operational frequency bands, nature of CA (beam level, transponder level etc.).
2. Assessing the impact of CA schemes on the design and operation of gateway, payload and user terminal. The axes for impact analyses include hardware and processing complexity, modifications to existing standards and end-to-end system performance analysis.
3. Demonstrate and examine the performance of CA under different scenarios highlighting the implementation consequences on RF, PHY, MAC and link layers, where an end-to-end laboratory software-based testbed called “Carrier Aggregation Demonstrator (CADSAT)” is developed for this evaluation.
4. Based on the designing insights and the outcomes of the demonstration and testing, this project defines the technological roadmap to fill the gaps between the existing technologies and those of future that are needed for CA implementation.
Figure (1) Carrier Aggregation in GEO FSS forward link.
Figure (2) Multi‐Satellite CA (GEO‐GEO / MEO‐MEO / GEO‐MEO)
Carrier Aggregation (CA) can be seen as an extension or improved-version of Channel Bonding (CB), which is included in the DVB-S2X standard. CA considers Adaptive Coding and Modulation (ACM), and can be applied to intra-band contiguous, intra-band non-contiguous and inter-band carriers, allowing an arbitrary number of aggregated carriers.
The success of the deployment of CA in satellite communications systems entail the development of three key components:
|Packet scheduler and Load Balancing at GW side||Implement the user-carrier association and schedule the PDUs across the carriers based on the link capacities and congestion, so that at the receiver side the packet disordering is minimized.|
|Packet merging unit (FIFO-based) at the UT side||Convert the multiple streams of the aggregated carriers into a single stream of received packets|
|Multiuser Aggregation and Access Control||Designing the carrier allocation strategy for all the user terminals of the system and determining the multiplexing of each carrier.|
|UT chip able to simultaneously lock-in to multiple carriers||This component already exists in the market.|
In general, CA has been shown to provide an extra degree of flexibility, which can be exploited in certain circumstances in order to provide a better satellite resource exploitation rendering minimal unmet and unused capacity. It shall be noted that, when the demand is significantly high compared to the system capacity, there is nothing that CA can do to improve the situation, as CA does not add additional capacity to the system. Therefore, CA should be seen as an additional degree of flexibility, which can help in particular situations where unbalanced congested-underused carriers exists. In this context, significant gains in terms of peak data rate can be achieved with CA when compared to the single carrier case.
Carrier aggregation provides a high flexibility in spectrum aggregation and brings extraordinary system-level performance benefits in terms of demand satisfaction and radio resource optimization. Some of the new additional features that arise with carrier aggregation deployment in satellite systems are summarized as follows:
- Unlike channel bonding, carrier aggregation technique operates with Adaptive Coding and Modulation (ACM) functionalities that are fundamental for broadband traffic
- Aggregated channels may not be located in the same frequency band, i.e. a multibeam multicarrier scenario is feasible in carrier aggregation, whereas channel bonding requires same frequency channel band.
- CA allows to exploit the extra/unused resources of the system to enhance the offered capacity at the congested carriers.
- CA shall be seen as an additional degree of flexibility that can achieve higher demand satisfaction without the need to reshuffle the whole user-carrier assignment by simply exploiting underutilized resources of other carriers of the system.
Carrier Aggregation (CA) is implemented by means of three main blocks. First, at the Gateway side, a Multiuser Aggregation and Access Control (MAAC) block represents the main intelligence of the system that is in charge of designing the carrier allocation strategy for all the user terminals in the system as well as the multiplexing of each carrier. Also in the gateway side, a key module is the load balancing and PDU scheduler, that is responsible for applying the decisions of the MAAC by distributing the incoming PDUs across the available carriers. The load balancing and PDU scheduler block has to be carefully designed such that the PDUs are distributed across the selected carriers based on the link capacities so that the PDU disordering is minimized at the receiver side.
Finally, at the receiver side, the most important block is the traffic-merging block that takes the PDU streams of the aggregated carriers as input and converts them into a single stream of received PDUs.
CADSAT started in January 2018 and concluded in July 2020.
The project was composed of four main tasks:
1- Carrier aggregation scenario definition and selection.
2- CADSAT design and implementation
3- CADSAT testing
4- CADSAT development roadmap.