Figure 2: A direct-sequence CDMA encoding pattern.
Manufacturer Marketed Technology
Figure 3: Shift key modulation.
Two 2. 4 GHz signals
Signal 1 Signal 2
Futaba’s first 2. 4 GHz RC radios
started out with modulated FHSS, and
later models added DSSS, which was
introduced as Futaba Advanced Spread
Spectrum Technology (FASST).
Spektrum’s DSSS-only system (no
hopping) was introduced as DSM,
which was later enhanced with
hopping frequencies called DSM2,
which was followed by DSMX.
Code Division Multiple Access
(CDMA) is the DSSS technology used
by our RC radios. It is designed for
several radios on the same frequency.
Figure 2 illustrates how each data bit is
expanded using several encoding bits.
Note that the encoding pattern for a
‘0’ data bit and a ‘ 1’ data bit are mirror
images of each other.
The higher the ratio of coding bits
for each data bit, the better the DSSS
process can extract data from noise.
The ratio is referred to as process
gain (which will be illustrated and
explained later). RC radios use a
higher process gain than Wi-Fi. Both
DSSS-encoded signals can travel as
far, but our radios can extract clean
data packets at a greater distance
than Wi-Fi. Here are three process
gain examples that help explain a key
benefit of DSSS:
• Most (perhaps all) Wi-Fi uses a
process gain of 11. The range is roughly
850 feet outdoors.
• Transmitters use 64-bit CDMA
codes. Range can be up to 2 to 3 miles
or more, depending on factors such as
• GPS (not 2. 4 GHz) has a process
gain of 1024. Satellite altitude is
12. 6 miles and the distance is farther
near the horizon. GPS satellites use a
common frequency for the 24 to 32
US satellites that employ different
DSSS codes without frequency
It is important to understand that
CDMA communicates the edges of
each coding bit and not the high or
low levels. This section will help you
understand how it works. Those with
an electronics background understand
between rise time
The faster a
from one level
wider the spread
of harmonics and the wider the
The same thing happens if a signal
shifts between two frequencies or
shifts between two phases (e.g. 180°,
90°) of the same frequency. The
faster the shift, the wider the spread
spectrum’s bandwidth. There are
spread spectrum shift key technologies
that accomplish CDMA edge
modulation by shifting between two
frequencies—two or four phases or
Figure 3 illustrates shift keying. Each
rapid shift results in a narrow pulse of
spread spectrum energy that looks like
noise with a frequency response spread
as seen on a spectrum analyzer.
Most of our RC radios use Gaussian
Frequency Shift Keying (GFSK).
Gaussian specifies the type of shaping
filter used to translate the shift keying
into spread spectrum. The Gaussian
filter optimizes performance while
reducing the bandwidth. In this case,
less bandwidth can be a good thing.
The CDMA illustration (Figure 4)
is applicable for the different forms
of shift keying. A multiplier circuit is
fed with a pattern of 0s, 1s, and -1s
timed by the DSSS code series used
by the transmitter. Figure 5 uses an
illustratively short DSS code.
38 Model Aviation FEBRUARY2015 www.ModelAviation.com