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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering – Award
(Bachelor of Engineering in Electronic Engineering – Award)
(NFQ – Level 8)
Autumn 2005
Mobile Communication Systems Engineering
(Time: 2 Hours)
Answer any three questions for full marks
Maximum available marks is 100.
Examiners: Dr. Dirk Pesch Prof. Cyril Burkley Mr. John Ryan
Q1. (a) Calculate the average speed of a mobile station given the following:
- The average time is t = 15msec that the mobile receives a radio signal below the signal amplitude level of R = 20μV.
- The mobile station experiences an average received power level of P (^) R = -60dBm (normalised to 1Ω resistance). The signal is subject to Rayleigh fading.
- The carrier frequency of the signal is fc = 900MHz and the speed of light is 300,000 km/sec. You may find some of the information given in the Appendix below useful for the required calculations. [8 marks]
(b) List two requirements for modulation schemes suitable for mobile systems. Briefly describe the advantage of using QPSK over BPSK. [9 marks]
(c) A cellular system with two cell sizes is depicted in Figure 1. Calculate the signal to interference ratio (in dB) that mobile station M, located at the cell fringe of the centre cell, experiences caused by interfering base stations Ai , i = 1 … 6. Assume that the interfering base stations are located in the centre of their respective cells, that all cells use omni-directional antennae, constant
transmitter power P TX , and a propagation path loss exponent of γ = 3.5. [16.33 marks]
A 1
R
2R
M
1
2
3
4
2
3
4
A 4
A 2
A 3
A 6
A 5
B
Figure 1
Q2. (a)^ Consider a TDMA based multiple access mobile radio system with bandwidth B = 1.25MHz. i) Calculate the maximum theoretically possible error free data rate per user when there are K = 10 users in the system, the average received power per user is P = -45dBmW, and the total noise power density is N 0 = 5.75 ⋅ 10 - (^16) W/s.
ii) Show that the total data rate of the TDMA system increases without bound when the number of users increases. State why there is a limit to this increase in real systems. [11 marks] (b) Consider the convolutional encoder with constraint length K = 4 shown in Figure 2. Write down the channel encoded binary sequence generated for the source message M = 11001011. Assume that all shift registers have an initial value of zero. [11 marks]
ii) Determine if a DS-CDMA system with a stability threshold of η = 0.1 and
a signal to interference ratio requirement is S/I = -11dB can accommodate the same amount of capacity. [12 marks] (c) Compare the two cellular radio systems listed in Table 1 in terms of the blocking probability that subscribers experience when the load is 40 Erlangs (Table ). Each system has a total bandwidth of 4 MHz. [12.33 marks] Table 1. Mobile Radio System Parameters System A (^) System B Carrier spacing 40 kHz 200 kHz Slots per TDMA frame 4 8 Cluster size K with omni- directional antennae
Appendix:
Some formulae you may find useful in answering questions 1 to 4.
Level crossing rate and average fade duration for Rayleigh fading channel:
2
L R = 2 π fm ρ e −^ ρ ,
ρ π
ρ
2
f m
e
t
= , with
P R
R
ρ = and
c
m
v
f
Shannon channel capacity theorem (^)
= ^ +
N
C B log 1 P 2
Stable Erlang capacity of a DS-CDMA multiple access system (^ )(^ )
0
E I
n BR b
≤ + −^ η
Cosine formula: c^2 = a^2 + b^2 − 2 ab cos γ
Relationship between frequency resue distance, cell size and cluster size in hexagonal shaped
cellular environments K R
D = 3
Pollazcek-Kinchine formula for the average time in a M/G/1 queuing system
X^2
W ; X is service time random variable, μ is the mean service rate; μ
ρ=λ
∞
−∞
X = xf ( x ) dx ; ∫
∞
−∞
X^2 = x^2 f ( x ) dx ; f(x) is pdf of the service time for continuous random variables
∑^ (^ )
∞
k 0
X Xk PXk and ( k )
k
X ∑ Xk PX
∞
0
(^2 2) , for discrete random variables
Table 2. Erlang-B call attempt blocking formula
