Summary of Open Source SDR Frontend and Measurements for 60-GHz Wireless Experimentation Tesi fast track, laurea triennale Ingegneria Elettronica e Informatica.

1. 1
Relatore:
Prof. Massimiliano
Comisso
Laureando:
Andrea Driutti
Anno accademico 2019-2020
UNIVERSITÀ DEGLI STUDI DI TRIESTE
DIPARTIMENTO DI INGEGNERIA E
ARCHITETTURA
Corso di Laurea Triennale in Ingegneria Elettronica e Informatica
Curriculum Elettronica
Open Source SDR Frontend and
Measurements for 60-GHz Wireless
Experimentation

2. 1 Introduction
In this paper the authors aim at allowing a wider range of research groups to do testbed
experimentations in the 60-GHz band than what is presently possible.
The main goal of this article is to provide an open-source radio front-end design to make 60-
GHz experimentations in wireless communication. Open source because, thanks to this, the
design is completely open to everyone.
Many fonts speak about how, over the years, communication technology demanded an
increasingly exploration of the underutilized millimeter wave frequency spectrum[1]. Therefore
a few studies have been realized on these new carrier frequencies, which have different
properties from conventional lower frequency bands. However, as of now, researchers have
never described anywhere an open-source simple solution for mm-wave experimentation. So
this explains the need of the authors to “fill the gap” and introduce this front-end design.
2 Main notions, concepts, and terms related to the topic
Since the mid-2000s, university groups began to benefit the use of a commodity software
defined radio platform, such as the USRP; the Universal Software Radio Peripheral is a range of
SDR designed and sold by a specific company and intended to be a comparatively inexpensive
hardware platform, commonly used by research labs, universities, and hobbyists.
For mm-wave experimentations, researchers use different platforms (Vubiq development kit and
others) that have the disadvantage of an indirect observation of the communication channel,
e.g. through measurements of throughput or RSSI. Another downside is that not so much
experimentation is allowed with new air interfaces. Instead of using these systems, the authors
describe a new front-end design providing Gerber and drill files, schematics and software;
besides, compared to other boards, it presents Hittite chips with integrated antennas and made of
a simple FR4 substrate.
Measurement results can be found in the paper to characterize the performance of many
algorithms.
The authors also focus on multiple-antenna orthogonal frequency division multiplexing (MIMO
OFDM), which will be applied as basis for the majority of near-future high-rate wireless
systems [2], [3]. The reason is that its main advantage is the ability to cope with severe channel
conditions (attenuation of high frequencies, narrowband interference).
2
Figure 1: Universal Software Radio
Peripheral (USRP N210)

3. 3 The front-end
In this section the target of the authors is to furnish specific details and information on the PCB
used to create the front-end, which is divided in TX, RX and CLK boards.
In recent times new scenarios such as ultradense networks and massive MIMO are foreseen.
Hence, the design showed in the paper simplifies the MIMO setup, because the boards can be
connected together in order to create a system with multiple antennas and multiple transceivers
on a single node.
One of the main problems evinced in this section is running phase coherently: the CLK board,
in the master mode, generates and distributes the clock signal using the on-board crystal, while
in the slave mode it is obtained externally.
One of the main underlined aspects is the connection of the front-end to the USRP. The TX
board requires to be easily usable with other interfaces, such as a signal generator or any other
platform (USRP, spectrum analyzer). A practical solution to this is the insertion of baluns
between the main chip and the I/Q input MCX connectors.
4 Connection to USRP and other platforms
The front-end can be connected to a USRP N210s equipped with BasicRX and BasicTX boards,
which eases both the hardware and software interface to the USRP. The authors provide
information related to this connection because, as it was told a few paragraphs before, the
USRP is a SDR platform ideal for experimentation in wireless communication.
Apart from the hardware features of the connection (ribbons, cables and ports), which are well
described in the paper, the center frequency of the base-band signals, when operating via UHD
(driver), is set to 70MHz.
In order to control the front-end from the USRP, C++ classes and Matlab/Octave functions are
present in the paper.
3
Figure 2: TX board

4. The authors present a system using the front-end connected to other processing platforms that
can interface with analog I and Q signals. For instance a vector signal generator as transmitter
and a spectrum analyzer as the receiver, like it’s shown in figure 3.
One limitation in these interfaced instrument is that they have only a single RF port, while the
TX and RX both require I and Q signals. The solution is displayed using a 90° splitter or, as an
alternative, connecting the unique ports to an input of the TX and, on the other wing, to an
output of the RX.
5 Beamsteering
In this section it’s highlighted how to shape the beam produced by the antenna array. To cover
narrower sectors, indeed, several boards must be combined together. This allows the system to
properly calibrate the phase of each transmitter using an auxiliary RX, because phase is random
at startup and coherent beamforming must be performed using synchronized CLK signals to
several boards.
Another limitation that emerges here is the size of the board, which brings the antennas closer
to each other. This leads to the creation of many sidelobes of comparable strength to the main
lobe. To reduce them, the boards can be re-designed with multiple chips per board (arranged
irregularly) or connected to an external antenna, thanks to new Hittite chipset versions.
4
Figure 3: Vector signal generator + spectrum analyzer setup
Figure 4: Antenna beams

5. 6 Link budget and linearity related measurements
A weak point that emerges here is the indirect access of the TX’s Hittite chip to the power
amplifier output. To perform air measurements then, a reference horn antenna has to be set
aligned with the board. The positions of the two is adjusted to maximize the power received by
the spectrum analyzer.
The authors use a specific Matlab function to generate the input base-band signal (cisoid). Then
the measurements are carried out by the spectrum analyzer (or the RX board in case of higher
amplitudes), which elaborates the power received as a function of the signal amplitude.
A similar arrangement of the system is described to measure the receiver sensitivity.
Carrier leakage is measured as interference caused by cross-talk (noise/electromagnetic
interference that can be generated between two neighboring cables) or a DC offset and is
present as a sine wave within the signal's bandwidth, whose amplitude is independent from it.
This is measured, together with the image signal and the linear gain for various gain settings.
As for intermodulation, the authors aim to develop a polynomial model to characterize 3rd and
5th order distortions. In simple terms, the goal is to allow the assessment of the impact of
distortion by simulation.
All these parameters are tested because researchers want to estimate what can be expected by
the platform in different scenarios.
7 Openly available measurements
In this section, the authors present phase noise and MIMO (2x2) measurements. In the first one
the setup is the same as that described above, using a signal generator, a USRP and an
oscilloscope.
The target here is to estimate the phase noise spectrum, which is obtained from many
consecutive measures and through the open source Matlab code.
Yet a phase noise spectrum does not fully characterize the properties of a signal. Thus MIMO
measurements are depicted using one TX board mounted horizontally and another vertically
(also at the receiver) so that a channel matrix is created.
Then measures are conducted moving the receiver from close to the transmitter to outside the
door of the office room, in a non line-of-sight position.
5
Figure 5: Power received by the
spectrum analyzer

6. 8 Conclusion
The authors provide software, measurements, instructions, hardware design files and all the files
to control the board as open source files.
One of the main thing to remember is that research must be carried on with the use of
measurements and display of results. This can demonstrate the usefulness of the algorithms or
practically verify concept demonstrations. For example, in this approach, imperfections of the
RF front-ends are studied and digital signal processing algorithms are designed to suppress their
impact on system performance.
9 Bibliography
[1] T. S. Rappaport et al., Millimeter wave mobile communications for 5G cellular: It will work!
IEEE Access, vol. 1, pp. 335–349, 2013.
[2] T. Schenk, RF Imperfections in High-Rate Wireless Systems: Impact and Digital
Compensation. Berlin, Germany: Springer-Verlag, 2008.
[3] A. L. Swindlehurst, E. Ayanoglu, P. Heydari, and F. Capolino, Millimeter- wave massive
MIMO: The next wireless revolution? IEEE Commun. Mag., vol. 52, no. 9, pp. 56–62, Sep.
2014.
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