- a) The constellation diagram below represents a multi-level digital modulation system.
How many bits are encoded in each symbol? [1]
What is the name of this type of modulation? [1]
What does each point in the diagram represent? [2]
b) What type of network is defined below? [4]
▪ Prior to sending data a communication path is set up.
▪ The connection is known as a virtual circuit which defines a fixed path, but NOT a
dedicated path.
▪ Virtual Circuit Identifiers (VCI) are used to route the packets, Full network
addresses are not needed.
c) If a digital line code is defined as below, sketch example waveforms for the cases
of non-return-zero (NRZ) and return-to-zero (RZ) pulses. [6]
Three voltage levels (+V, 0, -V) are used to represent two binary symbols. Zeros are
represented by the absence of a pulse and ones are represented by alternating pulses
of amplitudes +V and –V.
2
- A passive optical network (PON) has the following specification:
Parameter Value
Link budget 30 dB
Fibre loss 0.2dB/km (at 1550nm)
Total connector loss 6 dB
Operating wavelength 1550 nm
Passive splitter insertion loss 0 dB (Ideal splitter)
Downstream aggregate bit rate 10Gb/s
Fixed downstream bit rate per user 156.25 Mb/s
To ensure the required downstream bit rate can be maintained for all users
simultaneously, determine:
• The maximum number of users, and therefore, [2]
• The maximum fibre length of the PON. [6] - A wireless communication system uses OFDM modulation with the following
parameters:
Parameter Value
Data carrying subcarriers 54
Data symbol duration 3.2μs
Guard interval between symbols 0.4μs
Modulation formats QPSK, 16QAM, 32QAM or 64QAM
Error correction coding rate 2/3
Determine:
• The symbol rate. [2]
• The maximum and minimum gross signal bit rates (all subcarriers used). [4]
• The gross bit rate if the modulation formats are allocated as follows. [3]
Modulation Number of subcarriers
QPSK 18
16QAM 15
32QAM 11
64QAM 10
What determines the modulation format that can be used on a specific subcarrier?
[4] - If a mobile system requires a signal to noise ratio (SNR) of 10dB in order to operate,
and the propagation coefficient = 3, what is the smallest cluster size that should be
used? (Cluster sizes can be 3, 4, 7 and 12) [6]
(Assume that interference only originates from the 6 closest cells using the same frequencies)
3
If a cluster size of 4 is to be used what is the minimum SNR the mobile system can
use? [3] - The diagram below shows an SDH-based network which connects 2.048Mb/s
tributaries between 4 different locations according to the table shown.
a) For all 5 nodes in the network, indicate what type of network node is used. [4]
b) For all 4 STM links in the network, indicate what type the link is in the SDH
hierarchy (lowest required capacity). [5]
c) What is the key aspect of SDH which avoids the “multiplexer mountain” associated
with PDH networks? [3]
d) STM signals support high bit rates and long transmission distances, state what type
of transmission links are employed and explain why