A high-speed downlink telecommunication system is required to meet various applications for small satellites such as earth observation. The purpose of this research is to develop a high-data-rate (typically over 300Mbps) communication system. Generally, the operation at nonlinear region provides high efficiency for a RF power amplifier. However the amplitude-phase modulated signal, which is an efficient scheme in term of frequency band, requires high linearity. In order to amplify amplitude-phase modulated signal for high data rate, a 2W X Band GaN-HEMT power amplifier and digital pre-distortion technique were developed. In this paper measurements and simulations of the system are presented

Atsushi Tomiki

Hirobumi Saito

Hiromi Watanabe

Naohiko Iwakiri

Osman Ceylan

Tomoya Fukami

CiteSeerX

 
 
 
1 
 
Development of high-speed and high-efficiency downlink transmitter with 
GaN-HEMT amplifier and pre-distortion technique for nano/small satellite 
 
 Hiromi WATANABE
1)
, Tomoya FUKAMI
1)
, Atsushi TOMIKI
2)  
Hirobumi SAITO
2)
, Naohiko IWAKIRI
1)
, Osman CEYLAN
3) 
 
1)Department of Aeronautics and Astronautics, The University of Tokyo, Tokyo, Japan  
2)The Institute of Space and Astronautical Science, JAXA, Sagamihara, Japan 
3) Faculty of Electrical and Electronic Engineering, Istanbul Technical University, Turkey 
 
 
  A high-speed downlink telecommunication system is required to meet various applications for small satellites 
such as earth observation. The purpose of this research is to develop a high-data-rate (typically over 300Mbps) 
communication system. Generally, the operation at nonlinear region provides high efficiency for a RF power 
amplifier. However the amplitude-phase modulated signal, which is an efficient scheme in term of frequency band, 
requires high linearity. In order to amplify amplitude-phase modulated signal for high data rate, a 2W X Band 
GaN-HEMT power amplifier and digital pre-distortion technique were developed. In this paper measurements and 
simulations of the system are presented. 
 
Key Words: Small satellite, Downlink, X band, GaN-HEMT power amplifier, Pre-distortion 
 
 
Nomenclature 
 
Downlink :  telecommunication from satellites 
to ground 
X Band :  8GHz – 12GHz 
SAR :  synthetic aperture radar 
 
1.  Introduction 
 
  In a satellite communication system, main limitations are 
down link capabilities as well as sensing capabilities. Recent 
earth observations such as optical imaging or SAR imaging by 
satellites require higher spatial resolutions and more image 
data. This means that the down link data volume should be 
increased. Recently technologies of small satellites have been 
so matured that many earth observation missions are proposed 
[1]. However, it is true that nano/small satellite missions still 
have many limitations of satellite functions compared to large 
satellites. 
The purpose of this research is to develop a high-data-rate 
(typically over 300Mbps) communication system which can 
be applicable to small satellites of 50 kg class in LEO (Low 
Earth Orbit). System will be demonstrated on orbit by 
“Hodoyoshi” small Satellite project, scheduled to be launched 
in within a few years [2]. 
A typical small satellite with 50kg can generate only as small 
as power of around 100W as total and can distribute roughly 
20W. This is a power constraint for a high-data-rate 
communication system for small satellites. Several hundred 
Mbps down link system on conventional large satellite 
consume one or more hundreds watt. 
In this project, it has been developing the communication 
subsystem both for the flight hardware as well as the ground 
system, paying attention to reduce the DC power consumption 
and the mass of onboard instruments. 
Section II describes outline of our novel high-data-rate down 
link system for nano/small satellites and other conventional 
downlink systems. In section III, it is presented that developed 
novel GaN HEMT X-band power amplifier. In section IV, the 
information is given about pre-distortion technique and 
measurement results. Section V shows the result of total dose 
testing for space qualification. 
 
2.  Transmitter system 
 
  One conventional way to increase the down link data 
volume is to use data relay satellites. In this expensive scheme, 
a long visible time compensates its relatively low bit rate for 
long communication range between geo-synchronous orbit 
and LEO. Therefore, a cost-effective approach to increase the 
down link data volume seems to increase a data rate of 
downlink to an earth station which has a short visible time 
around 10 minutes. Fig.1 shows down link data rates of LEO 
satellites and their mass [3]. The figure indicates that down 
link data rates are proportional roughly to linear or square of 
satellite mass. This is because in general a downlink with a 
high data rate requires high DC power and large mass. 
Conventional downlink systems of large satellites have 
capability of hundreds Mbps and in most cases they utilize X 
band (8025-8400MHz) for earth observation. The maximum 
bandwidth is 375MHz and in most cases a convolution coding 
with r=1/2 is applied. Therefore multi phase-shift-keying and 
amplitude-phase modulation are necessary to achieve higher 
bit rates than 300 Mbps.  
 
 
 
 
 2 
Table 1.  System outline. 
Center frequency 8.16 GHz 
Band width 150 MHz 
Modulation scheme QPSK,16QAM 
Mod. symbol rate 10 Msps,100 Msps 
Avg. RF power 33 dBm (2W) 
DC power consumption < 20 W 
 
Fig.1. Down link bit rate vs. satellite mass for low earth orbit. ★denotes 
presented novel technology. 
 
Fig.2 DC power consumption vs. data bit rate for onboard high-data-rate 
transmitter. ◆and● correspond to conventional transmitters with 8PSK 
and 16QAM, respectively. ★ denotes novel transmitter with 16QAM. 
RF power levels are also indicated. 
 
These modulations, however, are sensitive to nonlinear 
distortion of RF power amplifiers (PA). RF PAs have to be 
operated in linear region, which causes reduction of power 
efficiency. Also they require digital processing circuits with 
several hundred MHz clock. Fig.2 shows power consumptions 
as a function of data bit rate for onboard X-band transmitters 
with high data rates. Their power consumptions increase as 
the data bit rates increase with bandwidth-effective 
modulations.  
 
3.  SIC GaN HEMT X-band Power Amplifier 
 
The GaN transistor has some advantages for space 
applications such as better heat dissipation, smaller size etc. 
Therefore, in PA design, die package bare GaN HEMT 
transistor was used. Triquint TFG2023-01 GaN transistor was 
preferred for this project. The nonlinear EEHEMT based 
model (GaN on SiC Process) of the transistor which was 
supported by company, used [4] at AWR Microwave Office 
for computer aided design of the amplifier. All transmission 
lines were modeled on electromagnetic solver of the software 
tool with the frequency that including third harmonics 
frequency to increase accuracy. Design contains two different 
circuit boards: Transistor mounted substrate (LTCC) and 
input/output circuit substrate (Figure 3). Two substrates are 
connected to each other by wire bonds. Additionally, designed 
novel power amplifier was optimized for nano/small satellites: 
Smaller package design, high PAE (Power Added efficiency) 
and optimization for pre-distortion technique.  
A GaN-HEMT class AB power amplifier (Fig.4) designed 
to meet requirements is shown in Table 2. The symbol rate is 
100Msps and the effective frequency bandwidth is about 
150MHz (roll off coefficient is 0.5), which is just 1.8% of the 
carrier frequency 8.16 GHz. The frequency dependency of 
Gain characteristics shown in Fig.5 are by less than 1dB while 
the nonlinearities are by 2-3 dB in terms of compression 
power level.  
But we found a problem in heat dissipation path. As thermal 
behavior of GaN-HEMT is not well understood[5,6] and the 
bare chip has as small as 0.6mm*0.8mm foot print, we 
concluded that our design was not good and have to improve. 
Some materials such as LTCC substrate and glue under 
substrate may be changed. 
 
Table 2.  Specifications of GaN-HEMT class AB power amplifier. 
Drain voltage 28 V 
Quiescent current  125 mA 
Class AB 
Operating frequency 8.15 - 8.35 GHz 
Avg. output power 34 dBm 
Peak. output power >36 dBm 
Size 33.6×28.5×9 mm3 
 
Fig.3 Structure of the Circuit Layout 
 
Fig.4 Photograph of GaN-HEMT class-ABpower amplifier engineering 
model (EM) 
Downlink Bit Rate (Mbps) 
D
C
 p
o
w
er
 c
o
n
su
m
p
ti
o
n
 (
w
) 
Satellite Mass (kg) 
D
o
w
n
li
n
k
 B
it
 R
at
e 
(M
b
p
s)
 
28.5mm 
 
 
 
3 
PAE(Power Added Efficiency)
10
20
30
40
50
60
15 17 19 21 23 25 27 29
Drive Power [dBm]
P
A
E
 [
%
]
Frequency dependency (Gain and Phase shift)
2
4
6
8
10
12
15 17 19 21 23 25 27 29
Drive power [dBm]
G
ai
n
 [
d
B
]
-10
-5
0
5
10
15
P
h
as
e 
sh
if
t 
[d
eg
]
8085MHz
8160MHz
8235MHz
  
Fig.5. Frequency characteristics of  power amplifier (measurement) 
  
Fig.6. Comparison of simulation and measurement(Gain and Phase shift) 
 
Fig.7. Comparison of simulation and measurement (PAE) 
 
Table 3.  X-TX Engineering model specifications. 
Weight 1.33kg 
Size 120mm×120mm×73mm 
DC power consumption 20W 
Avg. RF power 33dBm 
 
A Phase shift characteristic of Fig.5 shows that phase shift is less 
than 7 degrees over the entire frequency range. This value is 
about 1/3 to 1/2 for the value of a conventional GaAs FET power 
amplifier [7]. As a result, this power amplifier can be used in 
smaller OBO (Output Back Off) point. 
 
4.  X-TX Transmitter 
 
4.1.  Transmitter Engineering Model 
The X-TX engineering model (EM) is made for the purpose 
of evaluation of our novel GaN-HEMT amplifier, high-speed 
digital circuits and pre-distortion algorithm (Fig. 8., Fig. 9. 
And Table. 3.). The X-TX EM has become very compact as a 
transmitter with over 300Mbps. We have obtained some 
preliminary results. 
 
4.2.  Measurement Result 
The typical characteristics we have already obtained are 
shown figures below. Fig.10 shows AM-AM characteristics of 
each frequency, 150MHz bandwidth centered at 8.16GHz. 
The non-linearity is 3-4dB in the saturated region, whereas 
variation of the AM-AM characteristic is about 1.5dB. This 
result that the frequency dependence is much smaller than 
nonlinearity indicates that it may be enough to apply 
pre-distortion without memory effect. 
Figure 11 shows AM-PM characteristics of each frequency. 
The phase shift depends on the amplitude is smaller than 8 
degrees, and this value is very similar to the results of a single 
power amplifier. This result also indicates that recursive 
calculation for compensating frequency characteristics are not 
important in pre-distortion. 
 
Fig.9 Photograph of the new transmitter (engineering model including 
power supply) 1st floor: Analog RF block and 2nd floor: Digital block  
   measured 
simulated 
   measured 
simulated 
Fig.8. Block diagram of our X-band transmitter 
 
 4 
 
Fig.10 AM-AM characteristics,Baseband input level is relative value. 
 
Fig.11 AM-PM characteristics, Baseband input level is relative value. 
 
Fig.12 Constellation plot, 100Msps, w/o noise,199992 symbols,  
OBO 2dB, avg. output power 33dB 
 
 
 
5.  16QAM Demodulation And Pre-Distortion Technique 
   
Pre-distortion is a technique that improves EVM using 
distorted baseband signals [8]. This technique is new 
technology in small satellite communication, because it 
requires large amount of computation. An optimized 
pre-distortion technique is being developed for small satellites 
(Figure 8).  In order to decrease amount of computation, 
gradual saturation characteristics and less frequency 
dependency are required. Considering its wide dynamic range 
of 16QAM signal, the new PA has enough power between 
P1dB and saturation power, and in spite of 150MHz wide BW, 
its frequency dependency is measured to be as low as 1dB at 
maximum as shown in Fig.5. As a result, Pre-distortion will 
improve EVM easily. 
 
5.1.  16-QAM Decode Test 
We conducted a simple communication experiment without 
pre-distortion and constellation plot is shown in Fig.12. 
Observed constellation deviation is simple, and is considered 
as a simple effect of AM-AM nonlinearity. EVM (Error 
Vector Magnitude) is measured to be 10.8% rms at OBO 2 
dB. 
 
6.  Space Qualification 
 
6.1.  Ionization doze 
As one of space environment tests, we performed total 
ionization test for the GaN amplifiers. This GaN amplifier 
will be applied to small earth observation satellites for high 
data rate down links. The orbits are sun-synchronized orbit 
with altitude of 500-800km. Total ionization dozes are 
expected to be 20k rad for five years mission assuming 
shield of aluminum 2mm thickness. The GaN devices will 
be powered on only when data are down linked to the 
ground station for less than an hour per day. We irradiated 
20k rad to a GaN amplifier module in one hour at a Co 60 
radiation facility. The GaN amplifier is not powered on 
during the test to simulate our operation condition in space. 
Characteristics of input/output power relation and 
power-additive efficiency do not change with power 
accuracy of 0.1dB between before/after the radiation tests. 
This test shows our GaN is tolerant for the total ionization 
doze. 
 
6.2.  Heat Cycle 
In low earth orbit, the satellite travels about 15 times 
around the earth, then the GaN-HEMT amplifier will be 
exposed to heat cycle. But we have heat conduction 
problem as stated above, some materials such as substrate 
and glue under substrate may be changed. We have to pay 
attention to heat cycle. 
 
7.  Conclusions 
 
This research has focused on a power efficient X-band 
downlink system with higher than 300Mbps data rate. 
Space conditions and satellite restrictions were considered 
0 5 10 15 20
20
22
24
26
28
30
32
34
36
EM Input-Output characteristics
Baseband Input Level [dB]
O
u
tp
u
t 
L
e
ve
l 
[d
B
m
]
 
 
8085MHz
8110MHz
8135MHz
8160MHz
8185MHz
8210MHz
8235MHz
0 5 10 15 20
-2
-1
0
1
2
3
4
5
6
7
EM Input-Phase shift characteristics
Baseband Input Level [dB]
P
h
as
e
 s
h
if
t 
[d
e
g]
 
 
8085MHz
8110MHz
8135MHz
8160MHz
8185MHz
8210MHz
8235MHz
 
 
 
5 
during the design. In order to achieve both the signal 
quality and power saving, Class AB, X Band, GaN-HEMT 
power amplifier was carefully designed for space 
conditions and digital pre-distortion technique was 
developed. Engineering model of the X-TX system still has 
small problems, but our new downlink system is almost 
ready to use in a small satellite. 
 
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Development of high-speed and high-efficiency downlink transmitter with GaN-HEMT amplifier and pre-distortion technique for nano/small satellite

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Development of high-speed and high-efficiency downlink transmitter with GaN-HEMT amplifier and pre-distortion technique for nano/small satellite

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