## 1 Introduction  
In the last decade, the amount of data acquired by Earth observation satellites has been steadily increasing due to improvements in the resolution of on-board instruments, which translates into a greater capacity request in downlink data from a wireless communication link [1]. As the number of instruments on board have been increasing, maximization over downlink data rate and the application of bandwidth-efficient modulation schemes stand out as active research lines in satellite communications. In 2012, the Consultative Committee for Space Data Systems (CCSDS) published a recommendation of standard CCSDS 131.2-B-1 [2], which describes a spectrally efficient and high data-rate telemetry transmission scheme based on Serial Concatenated Convolution Codes (SCCC) [3]. Specifically, CCSDS 131.2-B-1 presents a variable and adaptive coding and modulation (VCM/ACM) plan composed of 27 transmission schemes with the goal of adapting communications according to the downlink channel [4]. Until today, CCSDS 131.2-B-1 has been regarded as a relatively new standard, generating little research attention. However, it is strongly endorsed by the European Space Agency (ESA) for next-generation applications in space communications [5].  
On the other hand, new space technologies are pushing for a more efficient and faster methodology to design and develop novel communication systems embedded on satellites. An interesting proposal is the implementation of the whole system on chip (SoC).  
This strategy is quite risky due to the hazardous space environment that can impair the reliability-sensitive communication systems. The Soc paradigm imposes that part of the system (or the whole one) is implemented in software on a micro-processor, while the other part (or even the full system) is described in hardware on a field-programmable gate array (FPGA) [6]. In the case of satellite systems, this is a delicate task because the hostile space environment leaves the embedded software unreliable. In order to create a flexible system which is at the same time reprogrammable, compact and low weight, as well as high-speed, the project Madrid Flight on Chip [7] proposes the implementation of such a system on FPGA hardware.  
Adaptive communication methods have been studied for low-Earth-orbit satellites in the literature [8; 9]. In [4], an adaptive coding and modulation (ACM) scheme was shown with 27 modes, and later standardized by the Consultative Committee for Space Data Systems (CCSDS) in [2].  
This paper proposes the synthesis and verification of the traditional 27 modes system specified in [2] where interconnection takes place using the AXI-stream protocol [10]. To the best of the authors' knowledge, this is the first demonstration of an adaptive transmitter in the literature based on the CCDS standard. Modern adaptive communication systems usually require high computation power to meet speed requirements and enhance throughput. FPGAs are the most popular platforms for systems that require great performance and flexibility [11; 12].  
This paper is structured in the following way: firstly, the transmitter structure is detailed. Secondly, the algorithms are written and verified by MATLAB simulations. Once the MATLAB code was developed, the VHDL code was written to achieve a hardware design. Lastly, some behavioral simulations in VIVADO verify the correct RTL implementation of the transmitter.  
The main contributions of this paper are:  
* A flexible FPGA design and implementation for an adaptive transmitter for high-speed satellite telemetry following standard CCSDS 131.2-B.1;
* The optimization of the system design;
* Validation of the design with a reference model.  
The remaining parts of this paper are organized as follows. Section 2 presents the transmitter structure with ACM. Section 3 discusses the flexible and adaptive embedded prototype of the transmitter structure. Section 4 shows the simulation results from VIVADO. Finally, Section 5 states the conclusions drawn from the research.