COMS4105/7410代做、代写Communication Systems、代做Java语言、c/c++，Python编程代写

日期：2019-08-25 08:06

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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