1. Introduction
at regular intervals in a pseudo-random way through a
pre-established code. Several modern applications make
Phase-Shift Keying (PSK) modulations are the best known use of Frequency Hopping to avoid interference, among
and most used numerical modulations in the field of com- them, the most known is the Bluetooth technology [
8
]. If
munications. The information is encoded in the carrier several terminals transmit on the same carrier, collisions
phase which assumes discrete values as a function of the are created. By applying Frequency Hopping according
bits or sequence of bits to be transmitted. The signal to a certain law, this problem is avoided. Obviously only
amplitude and frequency remain constant. Since the in- those who transmit and receive know the sequence of
formation is coded in the phase, this type of modulation frequency jumps and where there should be interference
has an excellent robustness towards amplitude errors in a given instant, it will be of short duration because
of the received symbols [
1, 2
]. In modern applications at the next instant, the frequencies will most likely be
there was the necessity to strengthen the communication diferent. With the difusion of the IoT, secure
commuprotocols mostly for avoiding interference both volun- nications will be always more required. Sensible data,
tary (such as the case of jammers, very used in electronic locally processed by smart nodes equipped with machine
warfare [
3
]) and involuntary (such as the transmission learning algorithms[
9
] (that nowadays find application in
of several devices on the same frequency [
4
]). One of the always more fields [ 10],[
11
],[
12
],[
13
],[
14
],[
15
],[
16
] must
most used technique useful to avoid these issues is the be transmitted over the internet in the safest way
posSpread Spectrum [
5
]. The Spread Spectrum was initially sible In such a context, FPGAs can be a powerful and
developed in the military field to avoid radio intercep- valuable ally for the de-centralization of computing as
tions. Originally, any type of transmission that guaran- for example in [
17
].
teed a non-stationary spectrum was considered a spread The paper is structured as follows: Section 2 gives a
spectrum. With the introduction of the Code Division brief introduction on PSK modulation, Section 2.1 gives
Multiple Access (CDMA) [
6
], the spread spectrum is gen- some info about frequency hopping and its
implemenerated by multiplying the information by a code. Spread tation by using Digital Direct Synthesis, in Section 3
Spectrum can be also generated by using the frequency simulation and implementation results are shown, end,
hopping technique, the so called Frequency-Hopping ifnally, in Section 4 some conclusion are discussed.
Spread Spectrum (FHSS) [
7
]. As for CDMA, FHSS is a
transmission technique used to increase the bandwidth of
a signal. It consists in varying the transmission frequency 2. PSK Modulation
There are several types of PSK modulation. Among the
most used are:
• Binary phase-shift keying (BPSK): it associates
the two binary digits 1 and 0 with two diferent
carrier phase values, such as 0° and 180°.
• Quadrature phase-shift keying (QPSK): in this
case we have 4 equally spaced phases to represent
An easy way to generate the carrier is the use of a
Digital Direct Synthesizer [
18
] based on a 2 locations
memory where the samples for the wave generation are
stored. The binary information, i.e. the shift phase, is
given by adding an ofset to the read-address. Ofset 0
means 0° phase (bit 0), ofset 2 − 1 means 180° phase
(bit 1). A simplified scheme of our system is depicted in
Fig.2.1.
In our experiments the number of bit N for the address
has been set to 10, as consequence the number of memory
locations is 1024. Also the number of bit of each memory
word is N=10 bit, but in general these two parameter can
be diferent. The tuning word is the input used to set the
carrier frequency following the law:
=
·
2
(1)
The input SEL is used to generate the binary informa
tion: SEL=0 → bit 0, SEL=1 → bit 1.
Implementation
In [
7
] the authors present a frequency hopping system
based on Frequency Shift Key (FSK), in this work, we
focused our attention on PSK modulation. In detail, we
implemented a BPSK modulatior but the developed
system is suitable for any PSK modulation. Fig.2 depicts the
time behavior of a BPSK modulation. The carrier phase
(b) changes at varying of binary signal (a).
In Fig.2, is instead shown the constellation of a BPSK.
Before the implementation, the system has been simu
lated first in floating point and then in fixed point by
using Simulink. After the system simulation the system
has been coded in VHDL and implemented in a Xilinx
Zynq 7020 FPGA. Fig.3 shows the exact correspondence
between the simulink simulation and the Xilinx Vivado.
The Fig.3 shows the positive timing closure of the
system with a clock period of 3 ns corresponding to 333
2.1. Frequency Hopping with Digital MHz.
About the power consumption, we did first an
estima
Direct Synthesis (DDS) tion without considering the switching activity of the
The aim of a frequency-hopping-based system is to jump circuit nodes (see Fig.3), and then we repeated the same
to diferent frequencies. An example is depicted in Fig.2.1, estimation by using the value of switching activity
prowhere are shown all 79 frequencies where a device Blue- vided by the simulation (see Fig.3). In the latter case, the
tooth may work. value depends on the input test vectors.
In both cases the estimated power is similar and is 147 mW for vectorless estimation and 120 mW for vectorbased estimation. The last aspect of the system implementation is the
resources utilization on FPGA. The synthesis results are
shown in Fig.3
As can be seen, the resource utilization is very low. By
increasing the value of N, the resource utilization will
increase linearly with the exception of the memory used to
store the wave samples that will increase exponentially.