High Efficiency Transmitter Architecture Compatible ...

AIAA 2014-4252 SPACE Conferences & Exposition 4-7 August 2014, San Diego, CA 32nd AIAA International Communications Satellite Systems Conference

High Efficiency Transmitter Architecture Compatible with CCSDS and ECSS Standards for Nano-Satellite Missions. Visweswaran Karunanithi * Master of Science Student, TU Delft, Research Intern at Innovative Solutions In Space.BV, Netherlands

Downloaded by TECHNISCHE UNIVERSITEIT DELFT on September 2, 2014 | http://arc.aiaa.org | DOI: 10.2514/6.2014-4252

Prof. Chris Verhoeven† Associate Professor, Electrical Engineering department, TU Delft, Netherlands. Waldemar Lubbers ‡ Head of RF & E dept. Innovative Solutions In Space.BV, Delft, Netherlands.

Abstract: This paper deals with the need for nano-satellites to start using spectral efficient modulation schemes, provides an overview of the recommendations laid by CCSDS and ECSS for space-to-earth RF links and also the modulation schemes proposed by CCSDS Space link protocol over ETSI DVB-S2 for spectral efficient modulation schemes like QPSK, OQPSK, 16-APSK and 32-APSK. As designing an efficient Power Amplifier is vital for nano-satellites, this paper also discusses some of the efficiency and linearity enhancement techniques to amplify non-constant envelope schemes, finally, concluding with the measurements performed on the designed LINC PA configuration and optimize its performance to get the best EVM for different modulation schemes recommended by CCSDS.

Nomenclature APSK AM/AM AM/PM CCSDS ECSS GSD DVB EER ET EES SR ITU EVM SO LINC OBDH PAE QPSK OQPSK PAPR PA

= = = = = = = = = = = = = = = = = = = = =

Amplitude Phase Shift Keying Amplitude Modulation to Amplitude Modulation Amplitude Modulation to Phase Modulation Consultative Committee for Space Data Systems European Cooperation for Space Standardization Ground Sample distance Digital Video Broadcast Envelope Elimination and Restoration Envelope Tracking Earth Exploration Satellites Space Research. International Telecommunication Union Error Vector Magnitude Space Operations Linear amplification using Non-linear components On-Board Data Handling Power Added Efficiency Quadrature Phase Shift Keying Off-set Quadrature Phase Shift Keying Peak to Average Power Ratio Power Amplifier

*

Master of Science student at TU Delft and Research Intern at Innovative Solutions In Space.BV, AIAA student member. † Associate Professor, Department of Micro-electronics, EEMCS, TU Delft. ‡ Head of RF & Electronics group, Innovative Solutions In Space.BV. 1 American Institute of Aeronautics and Astronautics

Copyright © 2014 by The Aerospace Corporation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

I. Introduction


T

he popularity of nano-satellite missions among universities and space research organizations has grown quite rapidly in the past decade. The focus of the nano-satellite missions has started to shift from educational technology demonstration missions to more complex industrial technology demonstration, science missions and military/government missions. There has been a paradigm shift in the way nano-satellites are being perceived. Space agencies have started to look at nano-satellites as being able to perform serious science missions and capable of replacing bigger satellites in near future. As the trend changes, the amount of data needed to be downlinked by nano-satellites also start to increase. This relation between the downlink data rate and mission objective is elaborated in section IV with the help of case studies. Section II deals with the paradigm shift using an analysis made on a database of nano-satellite transmitters flown between 2003 and 2013 and the need to start moving into spectral efficient modulation schemes. Section III discusses the recommendations by CCSDS/ECSS that need to be followed, in order to design a nano-satellite transmitter that is compatible with ground stations around the world. Section V discusses some of the PA linearity & efficiency enhancement techniques and concludes with implementation of LINC architecture and its performance with 16, 32-APSK and 16, 32, 64-QAM.

II. Nano-Satellite trends In order to understand the need for a more efficient transmitter design, it is important to study the trends in the types of nano-satellite missions, popular down-link data rates, popular modulation schemes and frequency bands that are most commonly used by nano-satellites. In order to carry out this study, a database of the nano-satellite missions and the transmitters flown on them between 2003 and 2013 § was made and analyzed. Based on the analysis, it can be seen that there is a steep increase in the number of nano-satellites launched in 2013. As shown in Figure 1, there were a total of 175 nano-satellite missions launched between this period and approximately 182 transmitters flown all together, of which 83 nano-satellite missions in 2013 alone, which is more than 400% increase in a single year. This upward trend in number of nano-satellite launches is set to continue, as the 2014

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Figure 1. Number of nano-satellites launched between 2003 and 2013. Nano/Microsatellite Market Assessment study done by SpaceWorks projects 2-3 times increase in the number of nano-satellites that will be launched in 2014. The database was completed in January 2014 as a part of the Author’s master thesis based on the information available on the internet and information from B. Klofas’s publication. Launches till 1 st Dec 2013 were considered for the database. An excel sheet of the database can be obtained by an email request to the primary Author. 2 American Institute of Aeronautics and Astronautics §


The nano-satellite missions were categorized based on the payloads they carried and mission objectives ** . The paradigm shift in the mission types is clearly evident from Figure 2; 36% of the missions in 2013 comprise of industrial technology demonstration missions, remote sensing or government/military missions, compared to 13% between 2003 and 2012. As the mission complexity increases, the amount of data generated by the payload and satellite telemetry also increases and as a result higher downlink data rates are needed. In section IV, examples of such missions are discussed in more detail. Figure 3 gives an overview of downlink data rates used by nanosatellite missions between 2003 and 2012, and compared with missions in 2013. 43% of the missions used downlink data rates below 1200 bps between 2003 and 2013 compared to 23% of the missions in 2013. More importantly, an upward trend in using

Figure 2. (Top) Mission types between 2003 and 2012. (Bottom) Missions types in 2013.

Figure 3. (Top) Downlink data rates used in nanosatellite missions between 2003 and 2012 (bottom) downlink data rates used by nano-satellite missions in 2013

**

higher data rates by nano-satellite missions in 2013 was noticed; 24% of the missions used data rates greater than 100 Kbps in 2013 compared to 9% between 2003 and 2012. This need for higher downlink data rates has also forced the nano-satellite developers to start using higher frequency bands such as S-Band, X-band and Ku-band as the available bandwidth in VHF and UHF are limited. Figure 4 gives a comparison between the frequency bands used by nano-satellite missions between 2003 and 2012 to those in 2013. It can be seen that amateur frequency bands in UHF and VHF have been the most popular choices for the downlink. The reason for this trend can be attributed to the wide-spread amateur satellitetracking stations around the world. This approach has benefited both satellite developers and enthusiastic amateur radio operators around the world. A good example is FUNCube†† which was launched in November 2013 and has received 256.5 MB (1,048,644 packets) of telemetry data as of July 2014. It uses 1200 bps BPSK down link and most of this data has been collected by amateur radio operators around the world. This mission has proved that, it is possible to monitor the spacecraft telemetry almost near real-time.

Based on the mission objective and payload, the mission is categorized into: Technology demonstration, Education (university missions having a primary objective of using nano-satellite development to learn about spacecraft design and system engineering, most university missions fall in this category), Amateur, Science, Remote sensing, Education/Technology demonstration, Education/Science, Military and Technology demonstration/Military. †† FUNcube-1 (AO73) http://funcube.org.uk/ 3 American Institute of Aeronautics and Astronautics


Thus, the following can be concluded from the above analysis: - The analysis converges with the 2014 Nano/Microsatellite Market Assessment done by SpaceWorks ‡‡. It can be inferred that the projections for 2014+ shows a number 2-3 times higher than the number of nano/microsatellite missions in 2013. More than half of the missions will be for Earth Observation/Remote Sensing. - The upward trend in the number of nano-satellite missions has led to crowding of free frequency bands and at the same time, as mission complexity increases, there is a need to downlink most of the data harvested by a satellite. This has led to moving towards higher downlink data rates and higher frequency bands that can provide larger bandwidths. - Use of amateur frequency bands (VHF and UHF) with Packet Radio Protocols including APRS that are popular among amateur radio operators will continue, as it has proved to be a win-win situation for both mission operators and amateur radio operators. Redundant radios operating in lower amateur bands were flown alongside a primary high-speed downlink (S-band) in various missions was observed. This trend also shows that flight heritage plays a very important role in choosing the communication architecture. - Nano-satellite cluster launches are getting popular with more than 20 nano-satellites are put into orbits in a single launch§§, as a result bringing down the launch cost. This also means frequency coordination to all the missions becomes that much more difficult as during the initial days of the mission, satellites would be quite close to each other and chances of interference are high. - There is a need to incorporate spectral efficient modulation schemes, in order to utilize the limited frequency bandwidth more responsibly. Figure 4. (Top) Frequency bands used by nano- At the same time, it is wise to start adopting satellite missions between 2003 and 2012. communication standards such as CCSDS and (Bottom) Downlink frequency bands used by ECSS incorporated by the ground stations from nano-satellites in 2013. different space agencies around the world. Section III gives an overview of these standards.

‡‡ http://www.sei.aero/eng/papers/uploads/archive/SpaceWorks_Nano_Microsatellite_Market_Assessment_January_

2014.pdf §§ Minotaur 1 rocket put 29 satellites into orbit in a single launch (19 th Nov 2013) and DNEPR placed 32 satellites in a single launch (21 st Nov 2013). 4 American Institute of Aeronautics and Astronautics


III. CCSDS and ECSS recommendations This section gives an overview of the CCSDS recommendations on RF and Modulation systems stated in CCSDS 401.0-B1 and Bandwidth efficient modulation stated in CCSDS 413.0-G-22 which have to be considered to design a radio transmitter compatible with the existing ground stations. The scope of this paper will be limited to space-to-earth links that belong to Category-A*** missions. The recommendations are categorized into frequency allocation based on the service, occupied Table 1. Recommended frequency allocation and bandwidth, transmitter spurious emissions & corresponding services. harmonic levels and recommended modulation Frequency bands Direction Service schemes. (MHz) (L-Band) 1215 - 1240 Space -> Earth (Down- SR, EES A. Frequency band allocation based on Link) service provided and occupiable bandwidth. (L-Band) 1240 1300 Space -> Earth (Down- SR, EES, The recommendations on frequency band Link) Amateur allocation is done based on the service/operation (L-Band) 1525 -1535 Space -> Earth (DownSO, EES provided by the mission. The different Link) ††† services are: Space Research (SR), Earth (S-Band) 2025 - 2110 Earth -> Space (Uplink) SR, SO, Exploration Satellite Service (EES), Amateur EES Service and Space Operations (SO). Table 1 (S-Band) 2200 2290 Space -> Earth (DownSR, SO, gives a list of frequency bands starting from LLink) EES band to Ka-band and the associated services. (C-Band) 7190 - 7235 Earth -> Space (Uplink) SR These recommendations were obtained from the (X-Band) 8025 - Space -> Earth (Down- EES document ECSS-E-ST-50-05C_Rev2 3 which is 8400 Link) in line with the recommendations stated by (X-Band) 8450 - Space -> Earth (Down- SR CCSDS. Some of the constraints in using these 8500 Link) frequency bands are as follows: (Ka-Band) 25500 - Space -> Earth (Down- SR, EES - L-Band: This band shall not be used for 27000 Link) feeder links of any service. (Ka-Band) 37000 Space -> Earth (Down- SR - S-Band: The maximum occupiable 38000 Link) bandwidth of the downlink shall not (Ka-Band) 40000 - Earth -> Space (Uplink) SR exceed 6MHz, the downlink shall be active only during the period when the 40500 satellite is in the visibility cone of the ground station. - X-Band (8025 to 8400 MHz): The constraints are the same as S-band. - X-band (8400 to 8500 MHz): The maximum occupiable bandwidth shall not exceed mask specified in the Figure 5. - Ka-Band (25.5 GHz – 27 GHz, 37 GHz – 38 GHz and 40 GHz – 40.5 GHz): There are no specific constraints stated in these bands but there shall be an agreement with the frequency coordinator before using these bands.

Figure 5. Maximum allowable bandwidth vs symbol rate3. ***

Altitude below 2 × 10 6 Km (L2 point). Purely science missions such as radio-telescopes fall under the category of SR, engineering missions such as communication satellites fall under the category of SO and earth observation missions such as Remote Sensing fall under the category of EES. 5 American Institute of Aeronautics and Astronautics †††

C. Recommended modulation scheme. The modulation schemes are recommended based on the downlink data rates. The downlink data rates are categorized into high-speed downlink (>2Msps), medium and low-speed downlink ( 2 Msps


Table 2 Maximum occupiable bandwidth corresponding to data rate and frequency band. Frequency Operation Occupied Bandwidth band (MHz) 2200 – 2290 Telemetry (Rs < 300 kHz [Downlink] 10ksps) And Telemetry (10 ksps 1200 kHz or 30 × Rs 8450 – 8500 1.1 x Rs up to 6 MHz at 2Msps) S-band and 10MHz at 8GHz. Rs: 34.2 Symbol – 34.7 rateGHz, 37 – 38 GHz and 40 – 40.5 GHz.

Table 3 Recommended modulation schemes. When using Sub-carrier: - PCM/PM/Bi-phase. - PCM/PM/NRZ. Suppressed carrier modulation (medium rate 70% for a PAPR of 5dB§§§. IF bandwidth; 1.3MHz.


Table 6. MISC-1 mission downlink data rate requirement.

Figure 10. Link margin vs elevation angle for X-band link. §§§

For 16-APSK implemented using a RRC filter with roll-off of 0.4 PAPR is ~5dB. 9 American Institute of Aeronautics and Astronautics


C. VDE-Sat case study8 VDE stands for VHF Data Exchange satellite which helps enabling seamless data exchange for the maritime community. This is an ongoing project at ISISpace BV. A pictorial representation of the concept is shown in Figure 11. The requirements posed on the transmitter by this mission are as follows: 1. Frequency band: VHF (~161.9MHz) 2. Modulation scheme: CCSDS space link protocols over ETSI DVB.S2 3. Data rate: 40 Ksps. 4. Available bandwidth: 50kHz. 5. Maximum PFD: -125 dBW/sqm/4kHz. 6. Transmitter power: 5.98 Watts. Based on these requirements, link budget was calculated with the following assumption: - The satellite antenna had a gain of 5 dBi - Transmit power of 5.9 watts - Modulation scheme: 16-APSK. - Bit-rate of 160 kbps.

Figure 11. VDE-Sat concept 8

With this design, it was possible to limit the occupied bandwidth to 50 kHz. Figure 12 shows a plot of link margin for various elevation angles. D. Conclusion of the case study. The case study show that a lot of upcoming nano-satellite missions need a high data rate downlinks all the harvested data to the ground station and with the present trend, it is very essential to start using spectral efficient Figure 12. Link margin as a function of elevation angle for modulation schemes. The link budget VDE Sat case study. calculations show that it is possible to close the link with spectral efficient schemes like 16-APSK and 32-APSK and as a result occupying lesser bandwidth compared to conventional schemes used by nano-satellites at present.

10 American Institute of Aeronautics and Astronautics

V. Transmitter/Power amplifier architectures In a transmitter, a PA is the final stage of a transmitter chain and is generally the most power hungry component


Microcontroller/DSP generating the baseband signal.

Antenna

I signal PLL

Pre-amplifier

PA

Q signal Modulator

Figure 13. General transmitter chain. in the chain. Figure 13 shows a general block diagram representation of a transmitter chain. PA performs the operation of amplifying the modulated signal to a desired power level to meet the requirements of a link budget. The key design parameters of a PA are efficiency and linearity. With the conventional class of operation such as class-A, AB, B, C, D, E and F, there has to be a trade-off made between these two design parameters. Generally a PA provides highest efficiency when operated in saturation and in the case of class-A, AB and B which are linear, provides the best performance for a non-constant envelope signal but Table 7. PAPR contribution by the constellation alone. the efficiency significantly drops Modulation M-PSK 16-APSK 16-QAM 32-APSK 32-QAM in the power back-off region (power levels lower than the peak envelope power). In the case of PAPR 0dB 1.1dB 2.6 dB 1.4 dB 3.9 dB non-linear amplifier class, such as class-D, E and F, the efficiency is high but at the cost of performance in terms of AM/AM and AM/PM Table 8. PAPR contribution due to RRC filter for different roll-off factors. distortion9 . Thus, with Roll-off 0.1 0.2 0.3 0.4 0.5 conventional class of operation it is not possible to obtain good linearity and at the same time PAPR 7.5dB 5.8dB 4.6dB 3.7dB 3.4dB good efficiency. As stated earlier, the spectrally efficient modulation schemes have a non-constant envelope profile and this is characterized by PAPR, which is the ratio between peak power of the signal envelope to the average power of the envelope. Table 7 gives a comparison of PARP for different modulation schemes and Table 8 gives the additional PARP due to the implementation of a RRC filter. For example, 16-APSK with a RRC filter and having a roll-off factor of 0.5 would give a combined PARP of (1.1dB + 3.4dB) = 4.5 dB. Which means the peak power of the signal envelope is 4.5 dB above the average power level of the signal envelope. Thus, in this case, the amplifier should be able to operate at its peak efficiency up to 4.5dB below its peak output power in order to get the best possible PAE. In order to tackle the challenge of enhancing both linearity and efficiency, the following PA architectures are analyzed and the most suitable architecture is chosen.

A. Doherty Power amplifier 1. Architecture and operating principle The Doherty amplifier was first proposed by W. H. Doherty in 1936 10. A block diagram representation of Doherty PA is shown in Figure 14 (i), A Doherty configuration consists of two non-identical power amplifiers called Carrier PA and Peaking PA. At low input levels the peaking PA is in cut-off and only the carrier PA operates as a 11 American Institute of Aeronautics and Astronautics


Modulated Input Signal S(t)

V1,I1

Zo Ohms, 90 deg.

Carrier PA

Zo Ohms, 90 deg.

linear PA (Class-A, AB). For an input level beyond a certain threshold, the peaking PA turns ON. At this moment, the carrier PA will be operating in saturation and peaking amplifier will provide the additional power required. When the signal envelope reaches peak-power, both the PAs are in saturation. It can be seen in Figure 14 (iii) that the peaking amplifier turns ON at Pmax/4 which is 6dB below the peak power and from this point, the output to the antenna is the combined power from both the PAs. As both the PAs operate in saturation, the configuration operates at its peak efficiency throughout the power back-off region. The working principle of Doherty is by active load pull technique. This is illustrated in the equivalent circuit diagram in Figure 14 (ii). The PAs are replaced by current sources. The current source Carrier PA would see a load RL when Peaking PA is turned off and no currents flows through it. But, when the Peaking PA turns ON, there is current flow (I1 and I2) from both the PAs which causes the peaking PA to experience load-pull effect, resulting in an output power proportional to the cube of the increasing input power. More details about the Doherty can be found in9

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2. Advantages and disadvantages: The advantages are: -

A good efficiency enhancement (PAE) is achieved in the back-off region. - Suitable to amplify signals with large Figure 14. (i) Block-diagram representation of Doherty PA. PAPR of up to 10dB. (ii) Equivalent circuit diagram. (iii) Input vs output power in - Relatively simple to implement when linear scale 11 . compared to EER and Envelope tracking architecture. - Performance in the back-off region can be improved by using multiple stages (N-way Doherty) The disadvantages are: - Gain degradation: Due to the low bias voltage, Peaking PA contributes less output power compared to Carrier PA. - Poor intermodulation distortion: It is caused again due to low biasing of the Peaking PA. - Narrow bandwidth: Caused due to the use of quarter wave impedance transformer. B. Kahn/EER (Envelope Elimination and Restoration) 1. Architecture: This technique was proposed by Kahn11 as a more efficient alternative to linear Class AB RF power amplification for single side band (SSB) transmitters. In the case of non-constant envelope modulation scheme, information is carried by modulating both phase and amplitude with respect to the message signal. In the case of 12 American Institute of Aeronautics and Astronautics


EER, the amplitude information is initially eliminated and the carrier signal containing phase information is amplified by the driver and given as the input to the final PA. The amplitude information (envelope profile) is restored using a class-S modulator and given as the supply voltage to the final stage. Thus, maintaining a high average efficiency regardless of the PARP of the input signal. The block diagram representation of the Kahn/EER is shown in Figure 15. A plot of average efficiency vs back-off power is shown in Figure 15. More details about the architecture can be found in12. 2. Advantages and disadvantages:

Figure 15. Kahn/EER architecture 11

The advantages are as follows: - Ideally operates with a high efficiency over a wide range of signals and power levels (back-off). - Provides excellent linearity as the performance is not dependent on the linearity of the amplifying transistor. The disadvantages of this architecture are: - Complex circuitry: Practical implementation of the circuit shows that the envelope restoration circuitry is complex, as perfect synchronization has to be achieved between the two signal paths. - Needs pre-distortion at higher Figure 16. Average efficiency in the power back-off region11. frequencies. - The switching frequencies of the Class-S modulator should be at least 6 times the RF bandwidth. C. Envelope Tracking (ET) architecture. 1. Architecture and working principle: ET architecture is very similar to EER, the amplitude and phase components are combined at the final PA stage. The Class-S modulator is replaced by a DC-DC converter in the case of ET architecture. The envelope profile is converted to discrete DC levels using the power conditioner and fed as the drain voltage to the final PA. The supply voltage is varied dynamically with sufficient headroom to allow the RF PA to operate in a linear mode. The block diagram representation of ET architecture is shown in Figure 17. 2. Advantages and disadvantages: The advantages of ET architecture are as follows: - Less complex compared to EET. - More efficient compared to Class-A or Class-AB. - Constant efficiency can be maintained in the power back-off region. The main disadvantages of this architecture are:

Figure 17. Block diagram of ET architecture 11

Figure 16. 13 American Institute of Aeronautics and Astronautics

-

Peak efficiency achieved is poor, cannot exceed that of Class-AB. The PAE is poor compared to other efficiency enhancement techniques due to the additional circuitry.


D. Switched Capacitor Digital Power Amplifiers SCDPA. Instead of making use of the linear trans-conductance property of a CMOS transistor, SCDPA makes use of its switching property. The block diagram of SCDPA is shown in Figure 18. Deeply scaled CMOS transistors are poor trans-conductors but very good switches. SCDPAs exploit this property. SCDPAs incorporates the functionality of a Digital-to-analog converter and a PA into the same circuitry. Each of the CMOS transistors are represented by switches (C0, C1, C2…. C7) and they are either at toggle (between Vdd and ground) or only ground potential. This is decided by the control signal given to the gate from the microFigure 18. Conceptual block-diagram of SCDPAs 12 controller/DSP. The output voltage depends on the ratio of the switches that are ON (toggle state) to the total number of switches. For example, C4, C5, C6 and C7 are at ground potential (representing OFF) C0, C1, C2 and C3 are in toggle mode (ON state) then Vout will be 0.5Vdd. Similarly, when all the switches are in toggle mode, maximum output power can be achieved. More details about this architecture can be found in13. Some of the advantages of this architecture are: - Low power consumption. - High signal bandwidth. - Good performance in the power back-off region. (flat efficiency uo-to 13dB PAPR reported12) The main disadvantages of this architecture are: - Not highly linear: AM-PM distortions are high. - Parasitic capacitance in the current technology still limit its performance. - Low PAE (45% reported12) - Low output power. E. LINC architecture (Linear amplification using non-linear components) architecture. As the name suggests, non-constant envelope signals are linearly amplified using non-linear power amplifiers. This is done by first splitting the non-constant envelope signals into two out-of-phase constant envelope signals, amplifying them using a high efficiency non-linear PAs and then combining them using a power combiner to obtain the desired non-constant envelope signal. A block diagram of LINC architecture is shown in Figure 19. A DSP runs a signal component separation algorithm to split the non-constant envelope signal S(t) into S1(t) and S2(t) which are the out-of phase, constant envelope signals. Mathematical expression of these signals are as follows:

Where φ(t) represents the instantaneous phase of the signal and r max represents the peak amplitude of the nonconstant envelope signal. 14 American Institute of Aeronautics and Astronautics

Now, the separated signal components can mathematically be expressed as:

S1(t)

G.S1(t)

Amplifier 1

Antenna

Digital Signal processing unit/signal splitter. Power combiner


And,

Thus, the two constant envelope signals are reduced to a maximum amplitude of r max /2 and have a phase difference of 2.θ(t) between the two signals. θ(t) is defined by:

S2(t)

G.S(t)

G.S2(t)

Amplifier 2

Figure 19. Block-diagram representation of LINC architecture.

Thus, these two signal components are amplified separately using high-efficiency PAs (Class-D, E or F) and the amplified signals G.S1(t) and G.S2(t) are combined to obtain the amplified signal G.S(t). The power combining can be done either using a lossy combiner or a lossless combiner. In the case of a lossy power combining technique such as Wilkinson Power Combiner (WPC) it is possible to obtain high isolation between the outputs of the individual PAs but the drawback is that, power is lost in the isolation resistor, resulting in low PAE. In the case of lossless power combining such as Chireix combining, the isolation between the two PAs is poor resulting in reduced combining efficiency at larger out-phasing angles but with an improved PAE of the system. More details about LINC can be found in Ref 14 Some of the advantages with LINC are as follows: - AM-AM/AM-PM distortions caused by the individual PAs will not affect the performance of the complete system. - Identical PAs are used, resulting in symmetrical signal paths. - Complexity of designing LINC is lower than any of the other efficiency enhancement technique. Some of the disadvantages of LINC are: - While using isolating power combiners, perfect linearity can be assumed but there is a drop in PAE due to the dissipated power in the isolation resistor. - In the case of non-isolated power combining, linearity is compromised for large out-phasing angles.


F. Performance trade-off of different efficiency enhancement techniques: As discussed above, all of the efficiency enhancement techniques discussed so far have both advantages and disadvantages, there is no one particular architecture that is better than the rest8. The best design is chosen based on the application. Thus, a performance trade-off needs to be done between the different architectures to select the most optimal design. In this case, as a transmitter needs to be designed for nano-satellites, a trade-off is done on parameters such as efficiency, linearity, back-off performance, over-head, complexity/cost and area. Each of this parameter is given a weight between 1 and 5 and the total score is calculated to find the suitable design. Table 9 gives the result of a trade-off study performed to come-up with the best candidate.


Table 9. Performance trade-off

As it can be seen from the above table, efficiency and linearity was given a weight of 2 because , most of the architectures discussed are capable of giving a very high efficiency and linearity. The next highest weight is given to performance in the power back-off region followed by over-head and complexity/cost. Thus, a design can score a maximum of 95 in this case. LINC architecture proved to be a better choice for nano-satellite design as it had the simplest design compared to the rest, the over-head required was low and a more a reliable design. A Doherty PA comes close but the over-head and complexity of the design gives LINC an edge. VI. LINC/Chireix amplifier design and measurement results: In order to characterize the performance, LINC PA with Chireix combiner was chosen. The board was designed by CATENA Microelectronics B.V. The individual PAs operate in Class-F mode and the output of the PAs are fed to a Chireix combiner through compensation stubs (the compensation stubs can be used to tune the combiner’s performance for the best compensation angle). The transistors used in the final stage were GaN High Electron Mobility Transistors (HEMT) from CREE. The block diagram in Figure 20 represents the test setup. The performance is evaluated by measuring the EVM (Error Vector Magnitude) which gives an idea of AM/AM and AM/PM distortion caused by the non-linearity in the PA.

MAXIM 2021

S(t)

S: +I,-I,+Q,-Q

MAXIM 2021

S1(t)

MAXIM 2021

S2(t)

Hybrid-coupler [40dB]

S1: +I,-I,+Q,-Q

S2: +I,-I,+Q,-Q PXI National Instruments

LINC PA with mini-circuits driver PAs 150W termination load

VSG

Power Divider/Combiner Power Divider/Combiner

Figure 20. Block diagram of the EVM measurement setup for LINC/Chireix combiner architecture 16 American Institute of Aeronautics and Astronautics


The setup comprises a National Instrument PXIe-1082 with two FlexRIO FPGA (Xilinx Virtex 5 SX50t) addons. Each of the FlexRIO FPGA also has a DAC & ADC with a sampling frequency of 100MHz which are used to produce the baseband differential IQ signals and receive differential IQ signals respectively. The PXI runs LabView in the front end which uses a MATLAB script at the back-end that splits the non-constant envelope signal into two constant envelope signals S1 and S2 (differential baseband IQ) and load it on to the FPGA. The two signal paths are synchronized using a reference and the differential IQ signal is sent to a quadrature mixer (MAXIM 2021). The signals are now mixed with the desired high frequency carrier and the signal is sent to driver PAs (Mini-circuit) to provide a drive of 30dBm at the input of the Class-F PAs. The amplified signals G.S1(t) and G.S2(t) are then combined using a Chireix combiner to obtain the amplified non-constant envelope signal. In this case, the LINC amplifier was designed to deliver an output power of ~41dBm when operated at 900 MHz. This setup was used as a proof of concept as performance of the LINC architecture for APSK modulation schemes has never been characterized in the past. The output of the Table 10. EVM measurement results. LINC PA is coupled back to the receiver Modulation Pout (dBm) Compensation EVM quadrature mixer (MAXIM 2021) and the angles demodulated IQ signal is given to the ADC 16-APSK 40.9 0o 0.21% in FlexRIO0. The Labview runs a matlab o 16-QAM 40.8 20 0.28% script that decodes the received symbol and o 32-APSK 40.8 20 0.32% compares it with expected symbol to o 32-QAM 40.7 20 0.30% calculate the EVM. 64-QAM 39.8 20 o 0.38% The EVM measurements were done for 16-APSK, 32-APSK, 16-QAM, 32-QAM and 64-QAM, for different compensation angles **** and some of the results are present in Table 10. The picture of the measurement setup is shown in Figure 21 and a screen-shot of the LabView interface used to measure the EVM is shown in Figure 22.

Figure 21. Measurement setup

****

Done by varying the length of the compensation stubs, The compensation stub on signal part S1 is shortcircuited hence acts like an inductor, the compensation stub on signal path S2 is open circuited hence acts like a capacitor. 17 American Institute of Aeronautics and Astronautics


Figure 22. Screen-shot of the LabView interface to measure EVM.

VII. Conclusion Based on the study done on the trends in nano-satellite missions and case studies, it is justified that spectral efficient modulation scheme is the need of the hour and it is very important to start adopting communication standards proposed by CCSDS and ECSS. This would reduce the time required to comeup with something entirely new for nano-satellites and more importantly, it would be possible to make use of the existing ground stations around the world. The challenge of efficiently transmitting spectral efficient modulation schemes is addressed in the paper, various methods have been discussed and a tradeoff study is presented. Based on the trade-off, it can be concluded that LINC amplifier architecture is a good candidate for efficiently amplifying non-constant envelope signals recommended by CCSDS DVB.S2 standards. Based on this conclusion, measurements were performed on a LINC amplifier architecture to validate the proof-of-concept and analyze its performance. The EVM measurements gave excellent results, showing very little AM/AM and AM/PM distortions. The EVM results obtained by using two separate signal generators setup for 16-QAM was reported to be 0.67%15 by CATENA, whereas the EVM obtained using the setup discussed above for 16-QAM was 0.28% showing significant improvement. The PAE of the LINC amplifier (final PA transistors and the combiners) was measured to be approximately 60%. The results discussed in this paper confine only to LINC/Chireix architecture, but some of the recent studies have shown that it is possible to use techniques where Wilkinson Power Combiner can be used to combine the outputs of the two signal paths and use a rectifier to recover the power lost in the isolation resistor16 or even use spatial combining techniques17, which would improve the overall efficiency of the transmitter. Thus, among the different efficiency and linearity enhancement techniques, LINC proves to be the most feasible solution for nano-satellite missions. VIII. Acknowledgments The author would like to thank the management of Innovative Solutions In Space.BV (ISISpace BV) and TU Delft for providing this opportunity to work on this topic for his master thesis. The support provided by the RF & Electronics team from ISISpace BV is greatly acknowledged. The author would like to thank Ernst Habekotte and CATENA Microelectronics, Delft for all the support. The author would also like to thank the Prof. P.G.M (Peter) Baltus and A.R. van Dommele from the Electrical engineering department, TU Eindhoven for all the support and guidance. 18 American Institute of Aeronautics and Astronautics

IX. References


1

RF and Modulation systems CCSDS 401.0-B, CCSDS Blue books: Recommended Standards," December 2013. http://public.ccsds.org/publications/BlueBooks.aspx 2 Bandwidth efficient modulation CCSDS 413.0-G-2, CCSDS Green book: Informational report http://public.ccsds.org/publications/archive/413x0g2.pdf 3 ECSS-E-ST-50-05C_Rev2, 04 October 2011. [Online]. Available: ftp://escies.org/ecss.nl/ISO/ARCHIVE/ 4 Space Frequency Coordination Group," 20 June 2012, SFC 21-2R3. 5 CCSDS Space Link Protocol over ETSI DVB-S2 Standards, Blue Book: Recommended standards, March 2013. 6 Triton-1 AIS mission launched on 21 st November 2013. http://www.isispace.nl/cms/index.php/projects/tritonmissions. 7 Andrew Kalman and Adam Raif: “MISCT M- A Novel Approach to Low-Cost Imaging Satellites”, SSC08-X-3 22 nd Annual AIAA/USU Conference on Small Satellites. 8 CPG PTC(13) INFO 16, 3rd Meeting CPG PTC. 9 Frederick H.Raab, Steve Cripps, Peter B. Kenington, “Power Amplifier and Transmitters for RF and Microwave” IEEE Transactions on microwave theory and techniques, VOL. 50, NO. 3, March 2002. 10 Doherty, W. H., “A new high efficiency power amplifier for modulated waves," Proc. IRE, Vol. 24, No. 9, 1163 1182, 1936. 11 Steve Cripps, “RF Power Amplifiers for wireless communications”, second edition, Chapter 11. 12 Frederick H.Raab, “Drive Modulation in Kahn-Technique Teansmitters”, TUF3-14, IEEE MITT-S Digest 1999. 13 Jeffrey S. Walling, “Digital Power Amplifier: A New way to exploit switched capacitor circuit” IEEE communications magazine 2012. 14 Ahmed Birafane, Mohamad El-Asmar, Ammar B.Kouki, “Analyzing LINC Systems”, IEEE microwave magazine, August 2010. 15 Ernst Habekotte and Floris P. van der Wilt, “Experimental out-phasing RF transmitter”, (un-published) 16 Philip A. Godoy, David J. Perreault, Joel L. Dawson, “Outphasing Energy Recovery Amplifier With Resistance Compression for Improved Efficiency”, IEEE Transaction on Microwave Theory and Techniques, Vol. 57, NO. 12, December 2009. 17 Steven Gao, Peter Gardner, “Integrated Antenna/Power Combiner for LINC radio transmitters” IEEE Transaction on Microwave Theory and Techniques, Vol. 53, NO. 03, March 2005.


High Efficiency Transmitter Architecture Compatible ...

High Efficiency Transmitter Architecture Compatible ...

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