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COMS4105/7410 Communication Systems
Introductory Assignment
Due date: 5pm, 16 August 2019.
Where to submit: the Faculty of EAIT (Hawken Building 50) assignment chute.
Note: All assignments require a cover sheet (available from
https://student.eait.uq.edu.au/coversheets/)
This assignment is 10% of your final mark for COMS4105/7410 and has 20 marks.
Q1 Communication Channel Modelling (7 marks)
The urban environment is an interesting propagation environment, as buildings provide a partially
reflective surface to signals propagating in it. The channel in this system is the wireless
medium between the transmitter and receiver, and we will model it as a half-duplex system.
The transmitter the base station, and the receiver is a vehicle. Fig 1 illustrates this scenario.
Figure 1: Signal Model Diagram – Urban Environment
We assume that the transmitter is fixed at position (0, 0) and the receiver is at a location of
(xr, yr). For simplicity, analysis will be performed in a two dimensional plane. Reflections which
occur, operate like a ‘mirror’, that is, the angle of incidence and departure from the surface must
be equal. Each time the signal reflects it is phase shifted by 180 degrees and attenuated. Due to
the spreading of the signal, there is also a loss associated with distance.
The spreading loss provides a multiplication by L = 1/D2
, where D is the length of the path.
The reflectivity of the buildings is assumed to be 50%. Note that the propagation speed is the
speed of light, c. Each gap between the buildings is 10 m, and the vehicle travels on a line 8 m
away from the transmitter (yr = 8 m).
Question 1.1 Multiple paths between Tx and Rx are superimposed at the receiver. Draw a diagram
including each of these paths and write a formula for each of the path lengths. Allow the
receiver position to be a variable. Do not consider paths with less than 25% signal remaining.
HINT: each path length be in terms of T = (0, 0), R = (xr, yr). ■
Question 1.2 Write the impulse response of the channel assuming an operating frequency of
5.8 GHz. Fix the vehicle location to xr = 50 m.
HINT: h(t) = Aδ(t ? τ ) is an impulse response of a (single path) channel which provides a delay of τ ,
a amplitude change of A. The propagation velocity is the speed of light. ■Question 1.3 We can explore the effect of the channel on a simple modulated system in terms of
the bit error rate (BER). To demonstrate this we will make use of a set of communication system
blocks. Use the provided NRZ encoder and decoder and the top level design to simulate
BER. You will need to implement two types of channels: (a) an additive white gaussian noise
channel (AWGN) and (b) the channel which has the transfer function of the previous question
in addition to AWGN.
Create the code necessary for each of the channels, and complete the top level design.
Plot a BER graph of the level of AWGN (x axis) vs the BER (y axis) for both channels by
using your top level design. SNR should be between 0 dB and 30 dB SNR. BER should
be between 10?6
to 100
. Note that a large number of bits needs to be sent (over 1 million).
Briefly describe the plot itself. What is the maximum/minimum BER, where are they
located, if the SNR was more extreme what BER would you predict (ie SNR = -100 dB
and 100 dB)
NOTE: For your BER graph, a noise power of 1 (the simple randn function), is for a symbol duration
of one sample. If you oversample your signal, you should use the appropriate scaling amplitiude for
your noise power. Assume a bit rate of 100 MBit/sec. ■
Question 1.4 Plot the signal strength of the received signal as the vehicle moves along the path
from +50 m to -50 m. New frequencies are becoming available for communications, which
may provide better features. One of these is the 30 GHz frequency band, which has a much
higher reflectivity of 70%. Repeat this question for the new frequency. Comment on the results.

Q2 Baseband Communication - Line Codes (7 Marks)
In baseband communication systems there are a large number of line codes which are available
to the designer. The best choice depends on the requirements and limitations imposed. Such limitations
may be in terms of the bandwidth available, the need for extensive timing information,
the requirement to reduce the DC component (due to long line lengths).
A line code called ‘Bi-phase Manchester Encoding’ (BMC) combines clock and data into one
signal. In BMC encoding logic 0 is encoded as a transition during the bit period, whereas a logic
1 is encoded as no transition. Additionally, a transition is performed at the beginning of each bit.
The possible representations are shown in Figure 2.
Figure 2: BMC line code, Logic 0 and 1
Question 2.1 Sketch (or plot) BMC representation of the binary bit sequence “0010”. ■
Question 2.2 Using the formulas provided in class, find the power spectral density of the encoding
method assuming a sequence of equally likely 1s and 0s. ■Question 2.3 Create a program which encodes a sequence of bits into BMC encoding. Then find
the simulated power spectral density for a long sequence of bits (>1000). HINT: In order to see
multiple spectral nulls you will need to oversample the original time domain signal ■
Question 2.4 Comment on the bandwidth (with respect to the first spectral null), the available
timing information, and the level of DC component in this signal. Use your results of the
simulation and theoretical result. ■
Q3 RTLSDR – Capturing signals from around us (6 Marks)
We will be using the RTL2832 tuner stick to capture signals in over-the-air experiments. This
small exercise will prepare you for the practicals later in this course. Also, it will give you a
greater understanding and appreciation of the signals being transmitted in the air around us.
Follow the instructions in the course GitHub website (via COMS dashboard) to setup your
RTLSDR device. Also, further instructions are available for the basic usage of the device. One of
the main commands you will be using is rtl_sdr:
rtl_sdr dump.bin -s 2e6 -f 110.9e6 -n 1e6
which will capture one million samples at 2 megasamples per second and a center frequency
of 110.9 MHz to a file called dump.bin.
Question 3.1 The RTLSDR USB software-defined radio device with R820T supports frequencies
from 24 MHz to 1.7 GHz, and sampling rates of 250 kHz to almost 3 MHz. Using the device:
Identify three signals of interest which can be captured (their centre frequency and bandwidth).
They can be:
– an FM radio station
– a DAB radio multiplex
– something else
Capture one second each signal and plot the power spectral density. You should adjust
the sampling rate to as just cover the full bandwidth.
HINT: Know the difference between carrier frequency and bandwidth. ■
Question 3.2 Based on the spectral density plots and your research, describe the
the carrier frequency and bandwidth of the signal,
the signal to noise ratio,
the signal’s purpose,
and comment on any interesting features of the PSD (eg BW efficiency).

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