Lab 1 - Radio-Frequency Digital Communication Signals

Lab 1 - Radio-Frequency Digital
Communication Signals
Goal
In this lab, we will survey several radio-frequency communication signal formats. Our goal is to
understand the relationship between the power density spectrum of the transmitted signal, the
rate at which symbol pulses are transmitted, and the rate at which bits are transmitted. These
signaling formats will be generated by the USRP using software that we will be using throughout
the class. We will also measure several “live” communication signals with both the USRP and with
a stand-alone spectrum analyzer.
1 USRP Generated Signals
1.1 Binary-phase-shift-keyed signals
1. We will start with a digital modulation format that transmits one bit per symbol interval
using a single carrier frequency. This format is called binary-phase-shift keying or BPSK.
2. Start USRP TX Digital AM.vi and change the following settings:
a) Carrier Frequency = (999 + n)M where n is the lab station you are working at. So lab
station n = 1 would use carrier frequency fc = 1G
3. Start USRP RX Digital AM.vi and change the following settings:
a) Carrier Frequency = (999 + n)M where n is the lab station you are working at. So lab
station n = 1 would use carrier frequency fc = 1G
4. Using the front panel of the USRP RX Digital AM.vi , view the following:
a) The magnitude of the power spectrum of the signal
p
I 2 + Q2 .
b) The power spectrum of the in-phase component I 2 .
c) The power spectrum of the quadrature component Q2 .
Take a screen shot for each of spectra to include in the lab write up.
5. Now vary the phase of the local oscillator (by using the blue slider bar control below the
constellation graph) on the USRP RX Digital AM.vi and observe what happens to the power
in the in-phase
component as compared to the power in the quadrature component. Is the
p
magnitude I 2 + Q2 of the power density spectrum affected by the phase of the local oscillator? Why or why not?
6. Is it possible to set the phase so that the power density spectrum for the in-phase component
is very close to zero? If the receiver is only sensitive to this component, what would happen?
7. Now move the Tx and Rx antennas relative to one another and see the effect on the power
density spectrum. What is the cause of this effect?
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8. Again vary the phase of local oscillator on the VI and observe what happens to the signal
constellation. Is this behavior consistent with what you observed for the power density
spectrum?
9. Set the phase of the local oscillator so that the constellation is aligned along the I-axis.
Determine the average power of the constellation, which is given by
Pave =
M
1 X
s2
M i=1 i
(1)
where si is the value of the in-phase component and M = 2 for BPSK. (This calculation can
be done off-line using a screen shot of the constellation.)
10. Now move the antennas relative to one another and see the effect of the motion on the
constellation.
11. Reduce the carrier frequency by a factor of two and repeat the same motion. Does the
constellation change at the same rate for the lower carrier frequency? Why or why not?
a) Note: You will need to change the Carrier Frequency for both the Tx and Rx VI’s.
12. Offset the carrier frequency between the transmitter and the receiver by 1 Hz and observe
the constellation. Why does this happen?
a) Note: You only need to apply this offset on the receiver VI using the Carrier Frequency
control.
13. Now move the antennas at a constant rate toward and away from each other. See if you
can “freeze” the rate of rotation by moving the antennas. Why does the rate slow down in
one direction and increase in the other direction? (Hint: Moving the antennas corresponds
producing a small Doppler shift between the transmitter and the receiver.)
1.2 Pulse Amplitude Modulated (PAM) Signals
1. On the transmitter VI, set the Mod Order control value to 4 . The resulting signal constellation is called 4-PAM. Observing this constellation, does every signal point have the same
energy?
a) Note: For the TX VI, if the modulation order is M , then the resulting PAM constellation
will be 2(M /2) = 4.
2. Given that the number of bits per symbol for PAM is equal to M /2, how many bits can be
transmitted per symbol for the 4-PAM format as compared to BPSK?
3. Adjusting the local oscillator phase, align the constellation to the in-phase axis and determine
the average power of this constellation using Eq. (1) with M = 4. Compare this average
power with the average power for the BPSK constellation.
4. Repeat steps (10)-(13) from the previous section to determine if there is any difference between
4-PAM and BPSK with regard to phase and frequency offsets.
5. Compare the power density spectrum of BPSK with the power density spectrum of 4-PAM.
Is there any difference? Why or why not? (Hint: Is the average power the same for each
modulation format?) Note that you do not need to take screen shot for every spectrum.
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1.3 Quadrature-Amplitude Modulated (QAM) Signals
1. On the transmitter VI, reduce the Mod Order control value to 2 and turn on the quadrature
component by disabling the PAM? boolean control. This produces a carrier frequency that is
orthogonal to the single carrier used in the last two parts. This modulation format is called
quadrature phase-shift-keying (QPSK). It is two BPSK signals in phase quadrature meaning
that one BPSK signal has a cosine time dependence while the other BPSK signal has a sine
time dependence.
2. Compare the spectrum of BPSK to the spectrum of QPSK. This can be easily done by toggling
the quadrature component via the
p PAM? control on the transmitter VI. Does the magnitude
of the power density spectrum I 2 + Q2 change for QSPK as compared to BPSK? Does the
power density spectrum for each component change? Why or why not?
3. Repeat parts (7)-(13) from Section 1.1 noting the differences between QPSK and BPSK with
respect to the average signal power and the effect of a phase offset or a frequency offset.
4. On the transmitter VI, set the number of Mod Order control value to 4 and turn on the
quadrature component via the PAM? control. This format is called 16-QAM. It is two 4PAM signals in phase quadrature.
a) Note: For the TX VI, if the modulation order is M , then the resulting QAM constellation will be 2M = 16.
i. The number of bits per symbol for QAM is equal to M .
5. Repeat steps (1)-(3) from this Section (1.3) for the 16-QAM constellation to determine how
this constellation differs from BPSK, 4-PAM and QPSK.
Write-Up for Part 1
1. All questions posed in the narrative.
2. All requested screen shots.
3. Write down the symbol rate and the bit rate for each of the four modulation formats used in
Part 1.
4. Which format can transmit the most number of bits per unit frequency?
5. Which format transmits the largest symbol energy (This is not the average energy per symbol.)
6. Suppose that you are using a carrier of 1 GHz.
a) What is the corresponding wavelength λ ?
b) How far to you have to move the antenna to the phase by 180o ?
c) How many cycles per second of the phase change occur if you are moving at:
i. 1 m/s (Walking)
ii. 30 m/s (In a car)
iii. 250 m/s (In a plane)
(We will discuss estimating the phase of the carrier in Lab 3.)
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2 Live Signals
In this part of the lab, we will use the “USRP Spectrum Analyzer” VI, which is a “soft” spectrum
analyzer that can display a 20 MHz wide section of spectrum anywhere from 50 MHz to 2.2 GHz.
We will also demonstrate the use of a stand-alone power spectrum analyzer.
1. Open “USRP Spectrum Analyzer” VI, which is a “soft” spectrum analyzer.
a) Some Useful Parameters for USRP Spectral Analyzer:
i. Resolution Bandwidth RBW =
duration (in seconds).
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Tacq
(Hz ), where Tacq is the acquisition time or
ii. Frequency Span Fspan = fs , where fs will be the IQ or sampling rate of the USRP.
iii. Cursor Bandwidth ∆F = the distance between the two yellow cursors of the FFT
spectrum graph.
iv. Power in Band P = the average power within the cursor bandwidth ∆F .
2.1 PCS cellphone band (1.85-1.99 GHz)
Several cell phone carriers transmit in the PCS (personal communication services) band. There are
several digital modulation formats that are used within this band including GSM (global system for
mobile communications) and CDMA (code division multiple access) pioneered by San Diego-based
Qualcomm. We will only qualitatively study this band.
1. Display part of the entire band using a resolution bandwidth of 10k. By varying the center
frequency, you should see a modulated signal. If not send someone a text and see if you can
see the signal. Save the spectrum of the part of the band that shows several carriers. (Note
that depending on the time you are logged into the computer, you may not see any cell phone
conversations that are be initiated from the CSE building.)
2. Zoom in on a modulating band near (try 1988 MHz) and determine the bandwidth of the
modulating signal.
3. Capture any screen trace that shows a signal.
4. Depending on the cell phones the other students have in lab, see if you can see the difference
between a 3G signal and a 4G (LTE) signal.
2.2 Commercial FM and digital radio
The commercial FM band lies in the range of 87.5-108 MHz. Several radio channels are now
broadcasting a digital signal in part of their allocated FM frequency band.
1. Scan the FM band using and try and find a channel that is broadcasting digitally based on
the spectrum shown in Figure 1. Take a screen shot of that channel.
2.3 2.4 GHz Wi-Fi band
We will use the stand-alone spectrum analyzer to look at the 2.4 GHz Wi-Fi band because the
USRP cannot demodulate this frequency range. This band is unrestricted and there are many
devices that use it including cordless phones and WiFi.
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-20.000
-25.000
-30.000
Analog
Signal
Power (dBm)
-35.000
-40.000
-45.000
-50.000
-55.000
Digital
sideband
-60.000
-65.000
-70.000
fc
Frequency (Hz)
50 kHz/div
Figure 1: Measured spectrum of commercial FM signal showing both the analog signal as well as
the digital sideband.
1. Using the stand-alone spectrum analyzer, set the center frequency to 2.4 GHz, the span to
250 MHz and the RBW to 100k. You should see many carriers and modulated signals should
appear and disappear depending on the amount of wireless network traffic.
2. Turn your wireless network adaptor on your laptop on and off. You should be able to identify
the carrier and the band that the adaptor is using to connect to the network.
a) Note: Some of the commonly used WiFI channels are channels 1, 6, and 11 with carrier
frequencies at 2.412GHz, 2.437GHz, and 2.462GHz.
3. Try and determine the bandwidth that is used to transmit 802.11. Using your measured value
of bandwidth, calculate how many non-overlapping channels that can be supported assuming
each access point (AP) can hear each other.
a) Note: In the US, only channels 1 through 11 are used.
Write-Up for Part 2
1. All questions posed in the narrative.
2. The write-up for this part of the lab also consists of screen shots for the measured spectrum
for any all of the communication waveforms that you could measure.
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