(a)
A new ICU room is equipped with a central station for simultaneous monitoring of
electrocardiograms (ECG) of 10 patients. The data for 10 patients are brought to a
processing center over wires and are sampled, quantized, binary coded and time division
multiplexed. The multiplexed data are then transmitted to the monitoring station as
shown in Figure 1. As an engineer, you’re asked to design the ECG monitoring system
based on the following specifications:

The ECG signal is 200Hz with voltage range of 0-5V

The maximum tolerable error in sample amplitudes is 0.25% of peak signal
amplitude.

The signal must be sampled at least 25% above the Nyquist sampling rate.

Framing and synchronization (FS) requires additional 1.25% of total data bits. The FS
is added at the beginning of each frame.
Source/patient
Figure 1: ECG Monitoring System
Determine the key parameters of the system design:
(i)
Quantization level.
(Ans: L =200, actual L = 256)
[2 marks]
(ii)
Number of bits used. (Ans: 8)
[2 marks]
(iii)
Step size. (Ans: = 5/256)
[1 mark]
(iv)
Transmission bit rate from each source. (Ans: Rsource = n x fs = 4kbps)
[2 marks]
(v)
Number of channels in one frame. (Ans: 10)
[1 mark]
(vi)
Minimum transmission cable bandwidth required to transmit data from all
sources. (Bmin = R/2 = 40.5/2 = 20.25kHz)
[3 marks]
(vii)
Output SNR from each source.
(Ans: 49.92dB)
[1 mark]
(viii)
Suggest a strategy to improve the output SNR from each source to become at least
66dB. Determine the new transmission bit rate to support this requirement.
Design a TDM system that can accommodate two analog signals and one synchronous digital
signal. The designed system must have the following specifications:

The analog signals are sampled at 1.5 times the Nyquist rate, quantized and then
binary coded.

They are multiplexed using one commutator producing a total output of 24000
bps.

Each channel has equal information bit rate.

The minimum number of bits per sample is 3.
Present your design using a block diagram and label the output of all stages. Indicate the
choice of (i) baseband bandwidth of the analog signals and (ii) the corresponding
quantization level used in your design. Justify your design with calculations.
Ans: 3 inputs, (2 analog and 1 digital)
Output 24kbps so each user transmitting 24k/3 = 8kbps
Input
Analog 1
Analog 2
Digital
BW
A
B
C
Sampling
A x 2 x 1.5
B x 2 x 1.5
-
Based on n= 3 bits, A=B = 888.9Hz
If n is chosen to be 4 bits, then A=B = 666.7Hz
Bitrate
n x A x 2 x 1.5
n x B x 2 x 1.5
8kbps
= 8kbps
= 8kbps
= 8kbps
24kbps
A FDM group multiplexer takes 12 voice channels (4 kHz per channel) and puts them on
subcarriers starting at 60 kHz using SSBSC utilizing upper sideband (USB). The groups are then
multiplexer together to produce a supergroup multiplexer (consists of 5 groups) with a
subcarriers starting at 420 kHz utilizing USB as well.
(i)
Why does the standard voice channel is set at 4 kHz? (ans: including guard band)
[1 mark]
(ii)
Determine the range of the group spectrum. (Ans: 60-108kHz)
[1 mark]
(iii)
Draw the frequency spectrum of the supergroup multiplexer showing all the
pertinent frequency values (Ans: range from 312kHz – 552kHz)
[4 marks]
(iv)
An extra guard band of 1 kHz is now being added between each channel. How
many voice channels can be assigned within the supergroup frequency band?
(Assume the output bandwidth of group/supergroups remain the same)
(Ans : 240/5 = 48 channels)
[2 marks]
(v)
If the original FDM system employed DSBSC instead of SSBSC, what happened
to the output of the groups and supergroups multiplexer?
(ans: number of channel will become halves the original if subcarrier frequency
change accordingly
or the output will be overlapping with one another if subcarrier frequency remain
the same)
[2 marks]
(a)
μ-law compander uses a compressor which relates output to input by the relations and
characteristics as below.
Vout 
Vmax ln( 1  Vin Vmax )
ln( 1   )
Δv
vi
V
The range of allowable voltage is V  10 volts and 256 quantization levels are
employed. The parameter μ determines the degree of compression.
(i)
What is the voltage interval between levels if there is no compression?
(ii)
For μ = 100 what are the minimum and the maximum voltage interval between
levels?
(a)
Figure below shows a phasor diagram and its truth table for a QPSK modulator. The
carrier frequency is 50 kHz with baud rate = 25 kbps. Given the input stream to the
modulator is 101101.
cos ωct
00
01
sin ωct
(0° reference)
-sin ωct
11
10
Binary
Input
Q
I
0
0
0
1
1
0
1
1
QPSK
Output
Phase
135°
45°
-135°
-45°
-cos ωct
(i)
Determine the input bit rate and bandwidth.
[2 marks]
(ii)
Sketch the waveform of the input and modulated signal at the original carrier
frequency.
[3 marks]
(iii)
Determine the degree of phase shift of the carrier modulating the Q bit with
respect to the reference oscillator carrier. Justify your answer. Draw a detail block
diagram of the QPSK modulator that will produce the intended output.