2. Radio Subsystem Architecture

Written By: Tristan Voon

Subsystem Requirements and Specifications

Key subsystem requirements are cascaded down from the overall mission concept and mission level requirements. The full list of radio subsystem requirements are shown in Appendix C.

In general, the main subsystem requirements for the radio can be summarised into a few main themes:

1. Transmitting the data over the correct frequency band
2. Transmitting the amount of data as required by the mission CONOPS
3. The necessity for the communication subsystem to be error tolerant
4. Transmitting the data directly to a size, weight and power constrained ground terminal - a very small aperture terminal (VSAT)

Specifications are then determined that could fit these requirements:

A mechanical envelope was first defined in which the space based radio segment would fit. This allows the mechanical team some certainty to plan the layout of the main bus structure. The modulation scheme was primarily chosen for its versatility and its proven Forward Error Correction Schemes (Low Density Parity Check, Bose–Chaudhuri–Hocquenghem). Target Data rates were then selected based on the mission requirements, available modulation scheme of the DVBS2 protocol, and the supported bandwidths on the radio. A link budget was then calculated, and a design EIRP selected based on the link requirement and availability of components, which will be elaborated on in the subsequent sections

Subsystem Architecture and Philosophy

The radio subsystem is designed with the Gomspace Link S/X as the core of the space segment. The Link S/X is a flight proven software defined radio (SDR) from Gomspace, which is flexible and offers a high degree of configurability. As an SDR, the radio does not produce a signal that is ready to be transmitted, but instead an intermediate frequency (IF) on which the desired data is modulated. This IF has to be conditioned by passing it to an upconverter followed by a power amplifier, which converts the IF to the correct frequency and at the correct power level. The antenna then radiates the signal through free space, where it is picked up by the Very Small Aperture Terminal (VSAT) on the ground. The Low Noise Blocks (LNB) then amplifies the signal and downconverts it back to an IF frequency, from which a dedicated modem or appropriately configured SDR can recover the data. As the satellite is constantly in motion, the ground segment will need to track the satellite as it passes over the sky, and both the VSAT and LNB are mounted on a gimbal.

Beyond the simplified diagram above which shows the Ku band signal chain, the Link S/X is also connected to an S and X band antenna. A bi-directional S - Band Link is thus available, which can be used to update model weights from the ground. A high speed, X band downlink is also provided, which can be used for full image transfers and also as a backup to the Ku band link. A full overview of the radio system (including the S and X band links) is provided in Appendix C

The radio subsystem is designed with the goal of maximising reliability in the most cost effective manner possible. To do this, a combination of in house and flight proven parts are used. In general, components which are readily available or are mission critical (like the SDR) will be procured commercially, allowing cost savings for the former and some measure of reliability for the latter. Components designed and made in house are typically those that are difficult to obtain commercially, and for which there may be some redundancy available. This is the case for the upconverter and patch antenna. In house development also has the benefit of being able to achieve tighter integration with the rest of the main satellite bus, as more design flexibility is afforded

Link Budget Analysis

The link budget validates whether or not sufficient power is received on the ground side to enable data recovery from the transmitted Signal. The various losses in the system are evaluated, such as Free Space Loss and rain attenuation. This informs certain factors like the Design EIRP required, and therefore can assist in helping us assess if variables like transmission power and antenna gain were selected correctly. The link budget was calculated for the following parameters:

In particular, rain attenuation, being a higher cause for concern due to the high frequencies involved, required more attention. The ITU618 model was used, based on Singapore’s geographical latitude, and a 99% link availability. This allows us to estimate the rain fade at
different slant ranges (directly related to the elevation angles). At higher confidence intervals (eg. 99.99%) the estimated rain fade increases, representing the greater losses that have to be overcome to achieve a similar link quality. More details of the link calculation can be found in Appendix D.

The first portion of the link budget analysis was to determine the Energy per symbol relative to the noise floor - the Es/No term.

A value of 0 indicates that the Signal power is equivalent to the noise power. The Es/No was calculated for two bandwidths that are supported by the Link S/X radio. The bandwidth of the transmitted signal represents the symbol rate at which the radio is transmitting. The tradeoff lies between the speed of the data transfer and the link reliability - a higher bandwidth (and hence higher symbol rate) results in the signal power being distributed over a wider bandwidth, therefore the Es/No drops for the same elevation angle.

The minimum Es/No required is determined by the modulation scheme and the receiver sensitivity. In DVB-S2, the modulation scheme can be changed to accommodate different link conditions, and each different MODCOD (modulation code) corresponds to a different Es/No required at the Ground Terminal. Therefore, the data rate can be estimated with the following graph:

For DVB-S2, the minimum Es/No required is -2.35 dB. This corresponds to a MODCOD of QPSK1/4, where each symbol encodes two bits of data and 3 bits of error correction is used for every 1 bit of usable data sent. Any Es/No below -2.35 dB is deemed unusable. It is observed that the 2 MHz signal achieves a lower peak data rate (8 Mbps), but is able to maintain a much more stable link relative to the 30 MHz signal.

For the intended mission CONOPS of transferring ship images and location extracted by the AI model, the volume of data to be transmitted relative to the data rates achieved is negligible - even at 2 a 2 MHz symbol rate. The detailed calculation may be found in Appendix E