# Access Techniques The problem of multiple access arises when several carriers are handled simultaneously by a satellite repeater that is a nodal point of the network. The satellite repeater consists of several adjacent channels (called transponders), whose bandwidth is a fraction of the total repeater bandwidth. Any specific carrier falls within a given repeater channel. In terms of multiple access, there are two aspects to be considered: - Multiple access to a particular repeater channel (i.e. a transponder) - Multiple access to a 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. This rectangle represents the bandwidth of the channel and its duration of operation (figure below). 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. This discrimination can be achieved: - As a function of the carrier energies in frequency domain. If the spectra of the carriers each occupy a different sub-band, the receiver can discriminate between carriers by filtering. This is the principle of frequency division multiple access (FDMA, part (a) of figure below). - As a function of the carrier energies in frequency domain. Several carriers received sequentially by the receiver can be discriminated by temporal gating even if they occupy the same frequency band. This is the principle of time division multiple access (TDMA, part b of figure below). - By the addition of a signature that is known to the receiver and is specific to each carrier. This ensures identification of the carrier even when all the carriers occupy the same frequency band simultaneously. The signature is most often realized by means of pseudorandom codes (pseudonoise codes); hence the name code division multiple access (CDMA, part c of fig below). The use of such codes has the effect of broadening the carrier spectrum in comparison with that which it would have if modulated only by the useful information. This is why CDMA is also sometimes called spread spectrum multiple access (SSMA). ![[Pasted image 20250101133059.png]] > [!Figure] > _The principles of multiple access: (a) frequency division multiple access (FDMA); (b) time division multiple access (TDMA); (c) code division multiple access (CDMA). (B = channel (transponder) bandwidth.)_ (source: #ref/Maral ) # Link Parameters ## Equivalent Isotropically Radiated Power (EIRP) EIRP, equivalent isotropically radiated power, is the most important parameter of the majority of payloads, since all payloads have at least a downlink. It comes from a combination of the power into the transmitting antenna’s terminal and the antenna gain. EIRP is defined on all directions. EIRP in a given direction is the total power that the antenna would radiate if it could have the gain in all directions that it has in the one direction. ## G/T $G/T_s$ , the figure of merit of the payload as a receiver, is the most important parameter for satellite uplinks (or for ground stations). For a satellite with only a few uplinking stations, $G/T_s$ need only be defined in their directions (allowing for antenna pointing error), but otherwise, it is defined in every direction, like EIRP. $G/T_s$ comes from a combination of the receive antenna gain and the system noise temperature $T_s$. The parameter sets the uplink signal‐to‐noise ratio (SNR). # The Transponder A transponder is a chain of elements that provide a processing path through the payload for a channel. Most satellites have many transponders. A particular transponder exists only as long as the payload switches are turned so as to allow the channel to be processed wholly; when the switches are turned other ways, the transponder as an entity is gone and some of its units may be incorporated into different transponders; the action may be later reversed or followed by a different such action. Typically most of the units in a transponder are shared by other transponders; other channels merge into this channel for a while, are separated out, and so on. The basic structure of a bent‐pipe transponder is always pretty much the same. The transponder elements shown below are fewer than in any actual transponder but they are enough to represent the major ones in any transponder. ![[Pasted image 20250101132236.png]] > [!Figure] > _Simplified block diagram of a transponder of a bent‐pipe payload_ (source: #ref/Braun ) Note that the transponder is taken to include the receive and transmit antennas (which is not always the case in the literature). The signal is shown as traveling from left to right. The radio frequency (RF) lines coming out of the receive antenna and going into the transmit antenna are shown as waveguide and the rest as coaxial cable, but other mixes are possible. The bent‐pipe transponder units are the following, in order of signal flow: - Receive [[Antennas|antenna]], which receives the signal from the uplink coverage area to the exclusion of other areas - Preselect [[RF Building Blocks#Filter|filter]], which suppresses uplink interference - Low‐noise [[RF Building Blocks#Amplifier|amplifier]] (LNA), which boosts the received signal to a level where the noise added by the rest of the payload units will not cause serious degradation, and itself adds little noise - Frequency converter, usually a lot [[RF Building Blocks#Mixer|downconverter]], to convert from the receive uplink frequency to the transmit downlink frequency. The LNA and the frequency converter are together sometimes known as the receiver - Input multiplexer (IMUX), a bank of filters for separating out and/or combining channels - Preamplifier (preamp), which boosts the signal level to what the high‐power amplifier (HPA) needs to output the desired RF power - HPA, which boosts the signal level to what the transmit antenna needs to close the link - Electronic power conditioner (EPC), that provides direct current (DC) power to rest of the HPA subsystem, namely the preamp and HPA - Output multiplexer (OMUX), a bank of filters for combining and/or separating out channels - Transmit antenna, which transmits the signal to the downlink coverage area to the exclusion of other areas. Each unit has a large number of possible variations. For example, the HPA subsystem comes in two main varieties, one with a traveling‐wave tube amplifier (TWTA) and the other with a solid‐state amplifier (SSPA), as shown in the figure below. The TWTA‐type subsystem has three or four units as shown, including the channel amplifier (CAMP), which is the preamp. A CAMP with TWTA linearizer is a linearizer‐channel amplifier (LCAMP). The SSPA usually incorporates all functions into just one unit. Other meanings of “transponder”, occurring in a statement of the number of transponders in payload, but not used in #ref/Braun , are number of TWTAs and, for TV broadcast, equivalent number of 36 MHz-wide channels. Active devices in a transponder are those that amplify the signal. These are the LNA, the frequency converter, the preamp, and the HPA. The rest of the units are passive. ![[Pasted image 20250101140149.png]] > [!Figure] > _Two kinds of HPA subsystem: (a) TWTA subsystem, (b) SSPA unit._ (source: #ref/Braun ) # Payload Architecture In SATCOM, the payload consists of the antennas and the repeater. Therefore the payload architecture is the way in which the transponders are linked together for the purposes of reliability, selection and reuse of signal paths, best usage of the spacecraft bus capabilities, mass and cost savings, and so on. The payload architecture also includes the selection and  arrangement of redundant active units. The payload units are linked together by means of integration elements, which include waveguide, coaxial cable, and switches. ==WIP== ![[Pasted image 20240623172859.png]] ![[Pasted image 20240623174112.png]] - Spot-beam: The whole frequency resource allocated to a beam is switched on board. This can correspond to a channel or several channels (typically 125 or 250 MHz in Ka-band). - Channel: This is equivalent to the frequency resource that is classically transmitted through a transponder (typically 36 or 72 MHz). - Carrier: This can be an FDMA carrier transmitted by a satellite terminal or earth station (typically from a few kHz up to tens of MHz depending on the earth station's radio capability). ![[Pasted image 20240623174408.png]] > [!warning] > This section is under #development # SATCOM Antenna Technologies ## Reflector, Single-Beam Antennas A reflector antenna has at least a main reflector and a feed. By far the most common form of main reflector is a paraboloid or section of a paraboloid. The general idea of a reflector antenna is that the feed either is or appears to be at the focus of the paraboloid, so that upon reflection the rays (see note below) emerge parallel and in phase in planes perpendicular to the paraboloid’s axis, as shown in the figure below. If the antenna has only a main reflector and a feed, it is a single‐reflector antenna, for which the feed is at the focal point. If additionally it has a subreflector, it is a dual‐reflector antenna, and the feed is no longer actually at the focus but virtually there. In either case, the phase center of the feed must be known so the antenna geometry can be set up correctly. A reflector antenna is inherently wideband if the phase center of the feed or virtual feed is exactly at the focus for all intended frequencies. ![[Pasted image 20250113093052.png]] > [!Figure] > _Geometry of paraboloid._ (source: #ref/Braun ) > [!info] > In reflector antennas, the concept of "rays" is used because reflector antennas are often analyzed and designed using **geometrical optics** principles. Rays provide a simplified representation of how electromagnetic waves propagate and interact with the reflector's surface, allowing engineers to predict how the antenna focuses and directs energy. > By modeling the incident waves as rays, the path of each ray can be traced as it reflects off the parabolic or other curved surfaces. This approach helps visualize the transformation of incoming waves into a collimated beam or vice versa, making it easier to understand and optimize the antenna's focusing properties. > The radiation pattern, while ultimately the goal of interest, emerges as a result of the constructive and destructive interference of the reflected rays. The "ray-based" perspective offers a practical way to analyze how the antenna's shape and geometry influence wave propagation and energy directionality before fully transitioning to a field-based analysis of the resulting radiation pattern. When an entire paraboloid forms the reflector, the reflector is said to be center‐ fed because the feed or virtual feed points down the paraboloid’s central axis. If the paraboloid is partial, the antenna is offset‐fed because the feed or virtual feed is not set to point down the paraboloid’s axis, but it is still at the focus. The primary radiation pattern is that of the feed. The main reflector’s pattern is the secondary radiation pattern. The tangential electric field on the feed’s aperture and the antenna geometry determine the tangential electric field on the reflector’s aperture and thus its antenna pattern. The sense of circular [[Antennas#Polarization|polarization]] is reversed every time the signal is reflected, so a RHCP single‐reflector antenna requires a LHCP feed. The only type of feed in use today for single‐beam reflector antennas is the single horn. An exception is a phased array forming the feed pattern for an antenna that creates a contoured beam. > [!warning] > This section is under #development ## Multi-Beam Antennas > [!warning] > This section is under #development