David G. Landon

L-3 Communication Systems-West, Salt Lake City, Utah 

Rainee N. Simons and Edwin G. Wintucky

Glenn Research Center, Cleveland, Ohio

Jun Y. Sun, James S. Winn, Stephen A. Laraway, William K. McIntire, John L. Metz, 

and Francis J. Smith

L-3 Communication Systems-West, Salt Lake City, Utah 

Peak Satellite-to-Earth Data Rates Derived From 

Measurements of a 20 Gbps Bread-Board Modem

NASA/TM—2011-217241

November 2011
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David G. Landon

L-3 Communication Systems-West, Salt Lake City, Utah 

Rainee N. Simons and Edwin G. Wintucky

Glenn Research Center, Cleveland, Ohio

Jun Y. Sun, James S. Winn, Stephen A. Laraway, William K. McIntire, John L. Metz, 

and Francis J. Smith

L-3 Communication Systems-West, Salt Lake City, Utah 

Peak Satellite-to-Earth Data Rates Derived From 

Measurements of a 20 Gbps Bread-Board Modem

NASA/TM—2011-217241

November 2011

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

MILCOM 2011

cosponsored by Armed Forces Communications Electronics Association and

IEEE Communications Society

Baltimore, Maryland, November 7–10, 2011
Available from

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This report contains preliminary fi ndings, 

subject to revision as analysis proceeds.
NASA/TM—2011-217241 1 

Peak Satellite-to-Earth Data Rates Derived From Measurements of a 

20 Gbps Bread-Board Modem 

David G. Landon 

L-3 Communication Systems-West 

Salt Lake City, Utah 84116 

Rainee N. Simons and Edwin G. Wintucky 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Jun Y. Sun, James S. Winn, Stephen A. Laraway, William K. McIntire, John L. Metz, and Francis J. Smith 

L-3 Communication Systems-West 

Salt Lake City, Utah 84116 

Abstract 

A prototype data link using a Ka-band space qualified, high 

efficiency 200 W TWT amplifier and a bread-board modem 

emulator were created to explore the feasibility of very high 

speed communications in satellite-to-earth applications. 

Experiments were conducted using a DVB-S2-like waveform 

with modifications to support up to 20 Gbps through the 

addition of 128 Quadrature Amplitude Modulation (QAM). 

Limited by the bandwidth of the amplifier, a constant peak 

symbol rate of 3.2 Giga-symbols/sec was selected and the 

modulation order was varied to explore what peak data rate 

might be supported by an RF link through this amplifier. 

Using 128-QAM, an implementation loss of 3 dB was 

observed at 20 Gbps, and the loss decreased as data rate or 

bandwidth were reduced. Building on this measured data, 

realistic link budget calculations were completed. Low-Earth 

orbit (LEO) missions based on this TWTA with reasonable 

hardware assumptions and antenna sizing are found to be 

bandwidth-limited, rather than power-limited, making the 

spectral efficiency of 9/10-rate encoded 128-QAM very 

attractive. Assuming a bandwidth allocation of 1 GHz, these 

computations indicate that low-Earth orbit vehicles could 

achieve data rates up to 5 Gbps—an order of magnitude 

beyond the current state-of-practice, yet still within the 

processing power of a current FPGA-based software-defined 

modem. The measured performance results and a description 

of the experimental setup are presented to support these 

conclusions. 

I. Introduction 

Multiple Gigabit per second (Gbps) data rates are of 

considerable interest to both government and commercial 

customers, as evidenced in the roadmap of NASA’s Space 

Communications and Navigation (SCaN) program office 

(Ref. 1). The justification for higher data rates is closely 

linked to the capabilities of spacecraft sensors, which are 

quickly outstripping the ability of current data links to move 

the sensor information back to earth. In response NASA has 

conducted studies and experiments to increase the capacity of 

the data links (Ref. 2). Notable progress has been achieved 

(Ref. 3), and space-to-ground data links are becoming faster, 

but not yet sufficiently so as to satisfy all sensor requirements 

(Refs. 4 and 5). 

Achieving such high data rates is dependant on both the 

spacecraft effective isotropic radiated power (EIRP) and the 

waveform (constellation order and forward error correction). 

Without sufficient power, the data link will always operate in a 

power-limited mode, whereas, without the correct waveform, 

the data link may become bandwidth-limited. By managing both 

the power and bandwidth, the data link performance can 

achieve nearly the modulation-constrained Shannon capacity 

limit (Ref. 6) and remain within the regulatory allocated 

bandwidth. 

NASA Glenn Research Center has developed space 

qualified, high power, high efficiency, Ka-band travelingwave tube amplifiers (TWTAs) for several missions including 

Cassini and ones to Mars, Jupiter, the Moon, and the 

International Space Station (ISS). Amplifier efficiencies have 

progressed from 41 percent for the Cassini design to as high as 

62 percent for the Jupiter mission with RF power levels of 10 

to 200 W. Typical design bandwidths have been 500 MHz or 

less, but as shown later, the amplifiers perform well to 

bandwidths of at least 3 GHz. 

The results reported here focus on the digital video 

broadcasting—satellite—second generation (DVB-S2) 

(Ref. 7) waveform family. Using the DVB-S2 constellations—

quadrature phase shift keying (QPSK) to 32-amplitude phase 

shift keying (APSK)—and the 9/10-rate FEC code as a 

starting point, we have extended the specification to a data rate 

of at least 20 Gbps by adding two higher order constellations, 

64-APSK, and 128-QAM. 

The paper proceeds as follows. In Section II, the waveform 

features and the hardware implementation are described. 

Section III briefly explains the modem emulator technology, a 

method of testing waveforms without a full commitment to 

hardware (i.e., a breadboard modem). Section IV describes the 

equipment used to measure the performance of the data link 
NASA/TM—2011-217241 2 

and Section V presents experimental results. Finally, 

Section VI gives link budget results that rely on the measured 

performance to compute the maximum data rates that are 

feasible for four different types of space missions. 

II. Waveform Design and Implementation 

Details of the DVB-S2 modulation, constellation mapping, 

interleaving, and encoding for QPSK, 8-PSK, 16-APSK, and 

32-APSK modes are available in (Ref. 7). Higher spectral 

efficiencies are available through 64-APSK and 128-QAM. 

In the processes of adding the higher order constellations, we 

observed that bit error rate (BER) performance is surprisingly 

sensitive to the constellation mapping by comparing the BER 

performance of the DVB-S2 constellation for 32-APSK with an 

alternative 32-APSK constellation with the same bit mapping, 

but equally spaced points. We make no claim that our 

extensions of the DVB-S2 modulation suite to higher-order 

modulations have optimized constellations and welcome further 

contributions to the literature on this topic. Rather than 

presenting additional candidates for these modulation types, we 

use the 64-APSK constellation given in (Ref. 8), and the 128-

QAM cross II constellation from (Ref. 9). 

Although results in this paper rely primarily on breadboard 

modem emulation measurements, we are in the process of 

implementing the same waveform at a reduced rate on an 

existing L-3 FPGA-based hardware modem (Ref. 10). Raising 

the hardware rate to 20 Gbps exceeds current funding 

objectives but harmonizes with the current hardware 

architecture. Current firmware loads of the FPGAs support 

QPSK, 8-PSK, 16-APSK, 32-APSK, and 64-APSK and 

(120,128)2

 (Refs. 11 and 12) turbo product code (TPC) with a 

throughput limitation of 4.5 Gbps. Implementation losses can 

be kept reasonable at higher order modulation in this same 

software-defined radio (SDR) to support throughputs similar 

to those studied in this paper. 

The SDR is a two-module (transmit and receive) solution 

(Fig. 1). The analog interface is baseband I/Q-sampled with Xband up-conversion for the transmit module and downconversion for the receive module. The modules use 3.5 Gigasample/sec A/Ds and D/As, and could process a 3.2 GHz RF 

bandwidth with a change in the analog filter. 

We have implemented a DVB-S2 modulator and encoder on 

another platform in a single Virtex-2 Pro 100 FPGA, an older 

member of the Xilinx family. Although that design can encode 

data at over 1 Gbps, in the faster SDR described here the 

extension to 8 Gbps is a relatively straightforward firmware 

change. A boost to 10 Gbps is enabled by spreading 

processing across two modules via I/O devoted to intermodule communication and 20 Gbps could be achieved with 

additional firmware modifications. 

An L-band, IF-sampled version of this modem was used to 

test the limits of the wideband Tracking and Data Relay 

Satellites System (TDRSS) channel. In April 2010, L-3 and 

NASA conducted high data rate demonstrations from the 

Radio Frequency Simulation Operations Center (RFSOC) at 

Goddard Space Flight Center (GSFC) over TDRSS F10 into 

White Sands Complex (Ref. 3). Figure 2 indicates that 

hardware performance was only 1.75 dB from theory in spite 

of the broadband and non-linear distortions of the double-hop 

channel and RF equipment. Modest data rate increases are 

possible, but the TDRSS RF filters significantly attenuate 

higher rates and can lead to considerable losses. 

III. Modem Emulation 

Rather than limiting hardware characterization and 

algorithm validation to modes currently implemented in the 

SDR, we also rely on precise hardware emulation tools. 

The Modem Emulator consists of laboratory test equipment 

and L-3 developed software that provides the capability to test 

waveforms over actual channels, at the same symbol rate and 

bandwidth, while implementing the same digital algorithms as 

used in the SDR. The Modem Emulator was used for 

experimental validation of modulations above 8-PSK in this 

work. 
NASA/TM—2011-217241 3 

Figure 2 illustrates the accuracy of the Modem Emulator in 

predicting hardware performance. Here, measurements were 

performed both with the hardware modem and the Modem 

Emulator over the TDRSS channel as illustrated in (Ref. 3). In 

the emulator, custom C code accurately represents the 

algorithms implemented in the modem transmit and receive 

FPGAs. The emulator generates a wrapping version of the 

same waveform the hardware might generate—only not in real 

time. This wrapping sequence is then played back at full speed 

on an arbitrary waveform generator (AWG). The broadband 

analog waveform is subject to the same RF and channel 

impairments that the hardware modem link experiences. The 

received signal is sampled by a digital oscilloscope and 

processed by the emulator software. Because of its fidelity, the 

Modem Emulator is a very useful tool for experimental 

validation of algorithms, and debugging waveform 

implementations in hardware, which will later run with 

expected performance in the hardware modem. 

IV. Equipment Setup 

The amplifier used for this study was a 200-Watt L-3 ETI 

Model 999HA, Serial #203, Ka-band space TWTA (Ref. 13). 

The TWTA was set up on a test bench at L-3 Communication 

Systems-West (CSW) and its power and phase input/output 

transfer functions were characterized at an operating 

frequency of 33.0 GHz. For this demonstration, the TWTA 

voltages and currents were set so that the saturated output 

power was 125 W. Figure 3 plots the measured normalized 

power and phase responses of the TWTA as a function of its 

input drive. 

Supporting RF hardware was attached to the TWTA to 

match the modem 8 GHz IF to the Ka-band test channel. A 

system level block diagram of the experimental setup is shown 

in Figure 4. The “Modulator” and “Demod” blocks shown 

represent either the hardware modem or the modem emulator. 

The hardware modem was described in Section II previously. 

The Modem Emulator modulator is comprised of a PC hosting 

software that emulates an L-3 software-defined modulator, a 

Tektronix AWG and a Hittite Microwave I/Q Mixer. The 

Modem Emulator demodulator consists of a Tektronix digital 

oscilloscope and a PC hosting software that emulates an L-3 

software-defined demodulator. 

V. Modem Emulator Performance 

Results 

Waveform performance through the 33-GHz test channel was 

assessed by transmitting a waveform from the Modem Emulator 

modulator, through the Ka-band TWTA, and then demodulating 

and measuring the bit error rate of the received signal using the 

Modem Emulator demodulator. In order to induce bit errors for 

the purpose of a BER measurement, the Modem Emulator 

demodulator added digitally generated pseudo noise at precisely 

controlled levels to the received A/D output samples. The 

implementation of the digital noise generator is based on the 

work of Marsaglia and Tsang (Ref. 14) on the Ziggurat method 

for generating random variables. 

Modulation schemes of QPSK, 8-PSK, 16-APSK, 

32-APSK, 64-APSK, and 128-QAM were tested in 

conjunction with the DVB-S2 Low Density Parity Check 

(LDPC) 9/10 FEC codec. For each modulation type, the bit 

rate was selected such that a constant channel symbol rate of 

3.2 Giga-symbols/sec was achieved. This led to an 

information bit rate of 5.7 Gbps for the lowest-order 

modulation tested, QPSK, and up to 20 Gbps for the highestorder modulation tested, 128-QAM. For each waveform under 

test, the input power to the TWTA was set such that its 

average power output back off (OBO) from saturation was 

approximately equal to the peak-to-average power ratio of the 

waveform in an ideal undistorted state. 

Figure 5 plots bit error rate as a function of Eb/N0 for each 

of the modulation types tested with the DVB-S2 LDPC 9/10 

codec. A code block size of 64800 bits and decoder iteration 

count of 16 were used to produce these results. The lowest 

order modulation tested, QPSK, had the lowest 

implementation loss at 0.2 dB, while the highest order 

modulation, 128-QAM, had the largest observed 

implementation loss of 3.0 dB. The large spread in measured 
NASA/TM—2011-217241 4 

implementation loss can be attributed to an underlying system 

noise floor due to quantization noise, thermal noise, and other 

types of distortion-induced noise and intermodulation products 

of the channel. This system noise floor has a much greater 

impact on very densely populated constellations such as 128-

QAM than on sparsely populated ones such as QPSK, because 

the former requires a higher SNR, or conversely, a lower 

relative noise floor, to operate error-free than the latter. There 

is a 10.1 dB spread in required Eb/N0 between QPSK and 128-

QAM (using the DVB-S2 LDPC 9/10 code) and a spread of 

2.8 dB was observed in the implementation loss between the 

two waveforms. 

As discussed in Section VI, for data transmission from a 

spacecraft orbiting near earth, when data throughput is 

bandwidth limited to 300 MHz and received SNR is not a 

limiting factor, information bit rates in the range of 1.8 Gbps 

can be achieved using 128-QAM in conjunction with a 

bandwidth efficient FEC code having a large code rate. The 

observed implementation losses for this test case is much 

lower than that presented in Figure 5. The large improvement 

in performance is expected because the symbol rate of 

1.8 Gbps 128-QAM is only 9 percent of the counterpart 

waveforms shown in Figure 5. Thus, the distortion-induced 

system noise floor is expected to be much lower. A code block 

size of 64800 bits and decoder iteration count of 16 were used 

for this test. In addition, pre-compensation for nonlinear 

effects of the TWTA was applied in order to achieve the 

results shown. The large peak-to-average power ratio of 128-

QAM makes it necessary to pre-compensate when it is 

desirable to operate the TWTA near its maximum output to 

maximize efficiency and EIRP. 

Figure 6 shows experimental results that demonstrate the 

benefit of pre-compensation with the TWTA driven to an 

output back off of 4.5 dB for the 128-QAM (1.8 Gbps) 

waveform tested. The constellation diagram on the left, 

produced with TWTA pre-compensation enabled, is clearly 

superior to the one on the right, which was produced without 

pre-compensation. 
NASA/TM—2011-217241 5 

VI. Supportable Data Rates 

Based on SCaN program goals, we have defined four data 

links that are of significant interest—low-Earth orbit (LEO), 

lunar relay satellites (LRS), second Lagrangian point (L2), and 

deep space (DS). The current state-of-practice data throughput 

for these links are summarized in Table I. In this section, the 

results of link budgets prepared for these four cases based on 

our gigabit per second experiments are considered. In all 

cases, much higher system throughputs are supportable than 

those implemented for the missions given in Table I. 

TABLE I.—CURRENT STATE-OF-PRACTICE THROUGHPUT 

Type 

of 

link 

NASA 

mission 

example 

Operating 

frequency 

band, 

GHz 

TWTA 

saturated 

output power, 

W 

Maximum 

data rate, 

Mbps 

LEO CLARREO X-Band, 8.45 20 150 

LRS LRO K-Band, 25.65 40 100 

L2 JWST K-Band, 25.9 58 28 

DS MRO X- & Ka-Band 

8.45, 32.05 

100 (X-Band) 

35 (Ka-Band) 

6 

TABLE II.—LINK BUDGET ASSUMPTIONS 

Parameter LEO LRS L2 DS 

Frequency, GHz 25.375 25.375 25.375 32.0

Bandwidth, GHz 0.3 to 1 0.3 to 1 0.3 0.3

Tx power, W 40 40 50 to 200 50 to 200

Tx antenna, m 0.1 to 1 0.1 to 1 0.1 to 4 0.1 to 5

Antenna efficiency, 

percent

65 65 65 65

Ground antenna 

efficiency, percent

78 78 78 78

Tx pointing loss, dB 2.0 1.0 1.0 0.2

Orbit height, km 700 3.85e5 1.5e6 1.5e8

Atmospheric loss, dB 7.81 3.70 3.70 1.70 

Rx elevation angle, 

degree 

5.0 10.0 10.0 25.0 

Polarization loss, dB 0.23 0.23 0.23 0.23 

Rx pointing loss, dB 2.0 1.0 1.0 0.2 

Rx antenna, m 2.0 18.3 18.3 34.0 

Clear sky G/T, dBi 28.9 46.5 46.5 60.0 

Note: Tx refers to the spacecraft. Rx refers to the ground terminal.

Following the basic link budget equation and notation in 

Shambayati (Ref. 15) and a 95 percent availability from the 

ITU-R PN-618-7 atmospheric loss model for Madrid, the 

assumptions used to construct the four link budgets are given 

in Table II.

Except for the LEO case, the ground antennas represent 

currently installed terminals in the space network. The G/T 

levels given are for clear sky. The link budget modeling 

equation includes the loss of signal and increase in system 

temperature and consequential degradation of the G/T due to 

the atmospheric losses. 

The Eb/N0 level is determined from the ideal Eb/N0 for a 

BER = 10–9 plus the implementation losses of the entire 

system. When implementing high order constellations, system 

distortions, both linear and non-linear, dominate the 

implementation losses of the data link. To ensure valid link 

budgets, measured performance data were used for the 

modem, frequency converters, and TWTA-induced 

implementation losses. 

Another particularly large impact to the link budget is the 

output back off that is needed for the given constellation order, 

especially those above 8-PSK. Pre-compensation can reduce 

the effects of the non-linear compression of the power 

amplifier but not eliminate it. For this reason we have chosen 

to back off the average output power an amount equal to the 

peak-to-average power ratio of the undistorted waveform (for 

constellation orders higher than 8-PSK.) This approach is 

somewhat conservative and we have shown experimentally 

that it is reasonable to allow some compression in the 

amplifier. The optimum approach would use the OBO that 

minimizes the sum of the OBO and the implementation loss. 

We have not yet determined the optimum back off levels for 

each waveform. Once determined the link budgets would 

perhaps improve by as much as 0.5 to 1 dB for the higher 

order constellations. 

Due to the complexity of the waveform selection, 

determining the OBO and implementation losses, and 

sweeping the antenna diameter, a software program was 

written to calculate the link budget and search the modified 

DVB-S2 waveform set to locate the constellation and FEC 

code rate that maximized the data rate without violating the 

bandwidth constraint. 

A. Low Earth Orbit (LEO) 

Spacecraft orbiting near the earth may pass RF signals 

directly to a ground station or relay them through TDRSS to 

ground (Ref. 2). In this paper we concern ourselves with a 

direct spacecraft-to-ground data link. Given the short 

propagation path, the free space losses are low and we find 

that the LEO link budgets are primarily bandwidth constrained 

rather than power limited. If the proper waveform selection is 

made and the spacecraft antenna diameter is reasonable, very 

high data rates can be achieved as shown in Figure 7. 

The data rates shown can be realized for a 700 km orbit 

with a 5 elevation angle at the ground station. The ground 

station antenna is only 2 m in diameter and relatively low cost. 
NASA/TM—2011-217241 6 

TABLE III.—LEO WAVEFORM SELECTION FOR 

300 MHZ BANDWIDTH 

Diameter, 

m 

Data rate, 

Mbps 

Constellation 

0.1 661.5 8-PSK 

0.2 980.8 16-APSK 

0.3 1044.2 32-APSK 

0.4 1286.9 32-APSK 

0.5 1539.9 64-APSK 

0.6 1544.3 64-APSK 

0.7 1544.3 64-APSK 

0.8 1544.3 64-APSK 

0.9 1750.0 128-QAM 

1.0 1801.7 128-QAM 

Making the right waveform selection is critical to 

maximizing the RF power and bandwidth available. As the 

spacecraft antenna size increases, the spacecraft creates more 

EIRP. Utilizing this increase in power requires changing the 

constellation to increase the data rate without violating the 

bandwidth allocation. Table III lists the corresponding 

waveform selections for ten different spacecraft antenna 

diameters along with the data rate. 

An increase in antenna diameter does not always allow a 

data rate increase since in some cases the power increase is not 

sufficient to support a jump to the next higher order 

constellation. This is seen for antenna diameters of 0.6 through 

0.8 m where the data rate stays constant because there is not 

sufficient power to switch to the 128-QAM constellation. 

B. Lunar Relay Satellite (LRS) 

Path losses to a lunar orbit increase significantly lowering 

the maximum data rate to around 1.6 Gbps. In this case we 

have employed a larger 18.3 m ground antenna with G/T of 

46.5 dB based on the LRO ground station. See Figure 8 for a 

graph of data rate versus spacecraft antenna diameter. 

C. Second Lagrangian Point (L2) 

An example spacecraft to be deployed at this orbital 

location is the James Webb Space Telescope (JWST). Since 

the spacecraft is larger than typical, and the free space loss is 

also larger, the spacecraft antenna was allowed to go to 4 m. 

Additionally, power amplifiers up to 200 W (saturated) 

were investigated. We assumed a fixed bandwidth allocation 

of 300 MHz. In this case data rates of 1 Gbps are possible with 

a 3-m antenna on the spacecraft as shown in Figure 9. 

D. Deep Space (DS) 

A deep space link built around the parameters in Table II 

remains power limited over the range of parameters 

considered in Figure 9. A deeper study is deferred until a 

broader range of system parameters is deemed reasonable for 

exploration. 

VII. Conclusion 

Multi-Gbps modem hardware exists with demonstrated, 

near-ideal performance in satellite links in excess of 1.5 Gbps. 

Hardware-true emulations in the lab of a space-to-ground link 

with a test version of NASA’s space-qualified, high-efficiency 

200 W TWT power amplifier, digital pre-compensation, and 

the industry pervasive DVB-S2 waveforms indicate that RF 

data links offering 20 Gbps are realistic in today’s 

technology—subject to mission-specific link budgets. 

Realistic science-mission scenarios indicate that this 

technology can be applied to increase payload data rates to 

5 Gbps for LEO missions and over 1 Gbps out to the second 

Lagrangian point, which are an order of magnitude higher than 

the current state-of-practice. In applications with greater 

bandwidth and antenna sizes, data rates to 20 Gbps are 

feasible with today’s hardware. These results erase past 

notions of maximum space-to-ground throughput limits.
NASA/TM—2011-217241 7 

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8, pp. 1003-1010, Aug 1998. 

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and Efficiency Space Traveling-Wave Tube Amplifiers 

with Reduced Size and Mass for NASA Missions,” 2008 

IEEE MTT-S Inter. Microwave Symposium Digest, pp. 

319-322, Atlanta, GA, June 15-20, 2008 (Also 

NASA/TM—2008-215220). 

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Generating Random Variables,” Journal of Statistical 

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95, no. 11, pp. 2171-2179, Nov 2007.
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4. TITLE AND SUBTITLE

Peak Satellite-to-Earth Data Rates Derived From Measurements of a 20 Gbps Bread-Board 

Modem 

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6. AUTHOR(S)

Landon, David, G.; Simons, Rainee, N.; Wintucky, Edwin, G.; Sun, Jun, Y.; Winn, James, S.; 

Laraway, Stephen, A.; McIntire, William, K.; Metz, John, L.; Smith, Francis, J. 

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WBS 432938.11.01.03.02.02.15 

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14. ABSTRACT

A prototype data link using a Ka-band space qualified, high efficiency 200 W TWT amplifier and a bread-board modem emulator were 

created to explore the feasibility of very high speed communications in satellite-to-earth applications. Experiments were conducted using a 

DVB-S2-like waveform with modifications to support up to 20 Gbps through the addition of 128-Quadrature Amplitude Modulation 

(QAM). Limited by the bandwidth of the amplifier, a constant peak symbol rate of 3.2 Giga-symbols/sec was selected and the modulation 

order was varied to explore what peak data rate might be supported by an RF link through this amplifier. Using 128-QAM, an 

implementation loss of 3 dB was observed at 20 Gbps, and the loss decreased as data rate or bandwidth were reduced. Building on this 

measured data, realistic link budget calculations were completed. Low-Earth orbit (LEO) missions based on this TWTA with reasonable 

hardware assumptions and antenna sizing are found to be bandwidth-limited, rather than power-limited, making the spectral efficiency of 

9/10-rate encoded 128-QAM very attractive. Assuming a bandwidth allocation of 1 GHz, these computations indicate that low-Earth orbit 

vehicles could achieve data rates up to 5 Gbps-an order of magnitude beyond the current state-of-practice, yet still within the processing 

power of a current FPGA-based software-defined modem. The measured performance results and a description of the experimental setup are 

presented to support these conclusions. 

15. SUBJECT TERMS

Traveling-wave tube amplifier; Software defined modem; Software defined radio; High order modulation; Turbo product code; DVBS2; Low density parity check; PSK; APSK; QAM; Space communications and navigation 

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