Aerospace and Electronic Systems Magazine May 2018 - 27

Ivanov et al.
ours, it is easier and faster to write the whole logic of an algorithm
in Python, without using the GUI editor.

EXPERIMENTAL SETUP AND
IMPLEMENTATION CHARACTERISTICS
MEASUREMENT SETUP

Figure 1.

Basic description of the operation of the algorithm within the span of
one frame.

itive operation with the equipment. The USRP incorporates two elements a motherboard and a daughterboard. Our variant is USRP2.
The motherboard consists of two digital-to-analog converters,
which can produce up to 400 MS/s, two analog-to-digital converters with sampling rate up to 100 MS/s, programmable digital upand down-converters, a Gigabit Ethernet Interface, through which
the USRP connects to the host computer. There is also a possibility
for processing bandwidths of up to 100 MHz, and for operating
with two antennas [43]. The daughterboard variant is XCVR2450,
which is a half-duplex transceiver. It supports the 2.4- and 5-GHz
bands and has a typical output power of 100 mW [44].
GNU Radio [45] provides the tools, which allow for many different kinds of transceivers to be easily constructed and then implemented with the USRP. In the recent versions, the usual installation under Linux includes everything that is needed for practical
experiments. The drivers, which operate the equipment directly are
written in C++, but for easier operation, they are connected with
Python scripts via the Simplified Wrapper and Interface Generator.
In this way, the functions can be incorporated into blocks, which
can be used for building the transceivers. The GNU Radio also
provides a graphical user interface (GUI), which simplifies the
process because it resembles Simulink. However, in cases such as

Figure 2.

USRP hardware platform.

MAY - JUNE 2018

Our experiment involves two USRPs (see, Figure 2), which are
several meters apart on two tables close to each other. There are
also obstacles, which form an NLOS scenario (see, Figure 3). One
of them implements the PU transmitter (Tx), while the other is the
SU receiver (Rx), which performs the spectrum sensing function.
For our experiment, we used two types of signals an OFDM
and a direct spread spectrum (DSS). The DSS transmitter is built
using the IEEE 802.15.4 OQPSK PHY block from the IEEE
802.15.4 Transceiver package for GNU Radio [46]. Whereas, the
OFDM signal is created with the help of the OFDM Modulator
block, which is a part of the gr-digital package [45]. Both of the
transmitters are constructed using the GNU Radio editor, which
produces the respective Python files. They are modified in order to
implement the discontinuous transmission.
The PU transmits intermittently for randomly chosen periods of
time. The same holds for the time while it is inactive. The periods
during, which the PU is idle/active are taken preliminary from an
exponential distribution so that its states may also be exponentially
distributed, according to the model we assumed in the previous
System Model section. By varying the upper bound on the two sets
of time periods, we obtain the three different PU configurations as
noted in the System Model section. Each of them runs for 300 seconds and the resulting parameters are calculated by taking the sum
of the active (and the idle, respectively) period and divide it by the
whole runtime of the transmitter (to obtain Pr(ON) and Pr(ON)),
−1
−1
and by finding its mean (for μON
and μOFF
). The corresponding parameters for all three transmitters are presented in Table 1. On the
basis of these parameters, the optimal sensing times for each configuration are obtained in the way described in the System Model
section. In general, the transmitter program implements all three of
the PU configurations, one after the other, so its complete execution time is 900 seconds. The operating frequency is 5 GHz, the
bandwidth is 200 kHz, and the gain is 13 dB.

Figure 3.

Experimental Setup.

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Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine May 2018