IEEE - Aerospace and Electronic Systems - March 2020 - 75

Towﬁc et al.
Table 3.

Possible Choices for ðK; bÞ for the X-Band and
Ka -Band Waveforms and the Iris and UST Radios.
Bold Pairs Are Chosen for the Final Implementation
Waveform
Potential
choices for
ðK
K ; bÞ
Iris radio

K a -Band
Waveform
(N
N ¼ 15)

X-band
Waveform
(N
N ¼ 13)
(7, 0.12)

N/A

Figure 6.

(8, 0.28)
UST radio

(21, 0.12)

(5, 0.067)

(24, 0.28)

(6, 0.28)

radio supports only X-band transmission whereas the UST
supports both X-band and Ka -band waveforms.
To see why the DAC frequency is relevant, recall the
equation that relates chip-rate to bandwidth repeated in
the following for convenience:
Rc ¼

BW
FDAC
¼
1þb
K

where K 2 N denotes the oversampling factor (number of
samples per chip) of the particular radio. It is ideal to
select K ¼ FDAC =Rc to be an integer from an FPGA
implementation perspective. In order to choose K, we
must know FDAC , and once K is chosen, we are then able
to select b as (with the restriction that b < 0:3 per our
requirements)
b¼

UST radio is not capable of generating PN DOR signals at
160 MHz on either side of the carrier. This is a typical issue
when it comes to utilizing space-oriented radios as the components used for their implementation must be radiation
hardened and generally do not utilize high-speed components. In order to cope with this, we outline a frequency hopping scheme in the next section that alleviates this limitation
until more capable components are incorporated into spaceflight radios. This scheme also allows for low-bandwidth
capable CubeSats to artificially obtain larger bandwidths,
which improves ΔDOR accuracy.

K a -BAND FREQUENCY HOPPING SCHEME

K Á BW
À 1:
FDAC

For example, the Iris radio DAC frequency is 50 MHz
[13], [14] and the UST radio DAC frequency is 150 MHz
[21]. Based on this, it is possible to generate a table with
possible values of ðK; bÞ that meet the required bandwidth
for the specific radio. For example, Table 3 lists the possible choices for K and b for the Iris and UST radios and
the X-band and Ka -band waveform. No values are listed
for the Ka -band waveform on the Iris radio because the
radio does not support the wide bandwidth required for
the implementation. It is trivial to confirm that the choices
in Table 3 satisfy the requirement that Tc > 10À3 s.
Observe, however, that while the UST radio is capable
of synthesizing the full 32 MHz PN signal, the Ka -band
implementation requirement is that the radio generates two
PN DOR channels (see Table 1). However, due to the location of the second channel (relative to carrier), and due to
limitations of the UST DAC frequency, we observe that the
MARCH 2020

Ka -band frequency hopping scheme. At time t0 þ 2mt to
t0 þ ð2m þ 1Þt for m 2 Z, the configuration transmits the carrier
at frequency fc with PN channels at Æ50 MHz away from fc
(illustrated in the top plot). At time t0 þ ð2m þ 1Þt to
t0 þ ð2m þ 2Þt, the system transmits the waveform illustrated in
the bottom plot where the carrier is 150 MHz away from the original carrier frequency fc with the PN channels still Æ50 MHz
away from fc þ 150 MHz.

Due to the fact that the Ka -band PN channels are wide and
the Ka -band waveform is required to have two subchannels, each 32 MHz wide, it is likely that nascent space
transponders are not capable of synthesizing the entire
192 MHz required signal bandwidth. In order to address
this issue and still meet the Ka -band waveform requirements, we introduce a frequency hopping solution. To
accomplish this, it is assumed that the space transponder
supports dynamic carrier frequency modification. In that
case, the transponder can modify its carrier frequency in
order to push the PN channel further away from the original carrier frequency. For example, consider the case
where the original carrier frequency for a radio is parameterized as fc . Then, it is possible to generate the 32 MHz
PN channels as illustrated in the top plot of Figure 6, each
approximately 50 MHz away from the carrier. However,
and due to the radio bandwidth being limited, it is then
possible to simply modify the carrier frequency to

IEEE A&E SYSTEMS MAGAZINE

IEEE - Aerospace and Electronic Systems - March 2020

Table of Contents for the Digital Edition of IEEE - Aerospace and Electronic Systems - March 2020