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University of Tehran, Alborz Campus
EE 00: Mobile Communications
Mark: 6 %
Solution to Assignment # 1: Evolution of Mobile Networks and
Standardization and Fundamentals of Mobile Communications part 1
Coverage: Topic 1 and partially Topic 2
1. LTE stands for Long Term Evolution which it was initiated as a project by
telecommunication standard body known as the Third Generation Partnership
Project (3GPP). LTE is the evolution path for carriers with both GSM/UMTS
networks and CDMA2000 networks.
a. What are the key benefits that LTE bring to cellular systems?
b. What are the main components of LTE network architecture? Describe each
component along with its block diagram?
a) Solution
Great throughput: both downlink as well as uplink can achieve high data rates, so as a
consequence it can support high throughput.
Low latency: Time required for users to connect to the network is in rang e of a few hundred
milliseconds, so it can support many delay-sensitive applications/services, e.g. VoIP, and online
gaming.
Flexibility enhancement: Both Frequency Division Duplex (FDD) and Time Division Duplex
(TDD) can be used on same platform which provides mobile operators with supporting multiuser
environments efficiently.
Superior end-user experience: Optimized signaling for connection establishment and other airinterface and mobility management procedures have further improved the user experience. Also,
it is reduced latency (to 10 ms) for better user experience.
All-in-one Connection: LT E will also back seamless connection to existing networks such as
GSM, CDMA and WCDMA.
Plug and play: most drivers for user’s devices are automatically identified, and new drivers are
loaded for the hardware if needed, and activated to work with the newly connected device.
b) Solution (for further reading, see [1])
As we discussed in class, similar to a typical mobile network, LTE has three major components
radio access network, core network, and user terminal. The high-level network architecture of
LT E is included ensuing three key components:
1. The User Equipment (UE).
2. The Evolved UMT S Terrestrial Radio Access Network (E-UT RAN).
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3. The Evolved Packet Core (EPC).
The interfaces between the different parts of the system are denoted Uu, S1 and SGi as shown in
Figure:
The evolved packet core communicates with packet data networks in the outside world such as
the internet, private corporate networks or the IP multimedia subsystem.
1. The User Equipment (UE)
The mobile equipment comprised of the following important modules:
-Mobile Termination (MT): This handles all the communication functions.
-Terminal Equipment (TE): This terminates the data streams
- Universal Integrated Circuit Card (UICC): his is also known as the SIM card for LTE
equipment. It runs an application known as the Universal Subscriber Identity Module (USIM).
USIM stores user-specific data such as phone number, home network identity and security keys
etc.
2. The E-UTRAN (The access network)
As shown in figure below and similar to other radio access networks (discussed in class), the EUT RAN provide connectivity between the mobile and the evolved packet core. It has one
component, the evolved base stations, called eNodeB or eNB. Their major functions are similar
to other base station defined in the legacy networks as discussed in class: handling data
transmission/reception over air interface and also controlling radio signals over air interface.
Each eBN connects to nearby base stations by the X2 interface, which is mainly used for
signalling and packet forwarding during handover/handoff.
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3. The Evolved Packet Core (EPC) (The core network)
Again similar to other core networks which were discussed in class such as a GSM network, the
EPC’s architecture has following major components as shown below:
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 The Home Subscriber Server (HSS) component has is a central database that contains
information about all the network operator's subscribers.
 The Packet Data Network (PDN) Gateway (P-GW) communicates with the outside
networks such as packet data networks PDN, using SGi interface.
 The serving gateway (S-GW) acts as a router, and forwards data between the base station/
eNodeB and the PDN gateway.
 The mobility management entity (MME) controls the high-level operation of the mobile
by means of signalling messages and Home Subscriber Server (HSS).
2. Consider signal-to-interference ratio of 15 dB in the worst case that can be tolerated
within a cellular network. Find the optimal value of N for (a) omnidirectional
antennas, (b) 120° sectoring, and (c) 60° sectoring. Can it be used sectoring? If so,
which condition (120° or 60°) would be used? (Assume a path loss exponent of n=4)
a) Solution
Optimal N for omnidirectional antennas
=6, n=4, =
(√
)
>
(√
)
→
15 dB = 31.62
(
)
→ N>4.59 → N=7 since N=5,6 are not
permitted
b) Solution
120° sectoring →
(
=2
)
→ N>2.65 → N=3
b) Solution
60° sectoring →
=1
(
)
→ N>1.87 → N=3 since N=2 is not permitted
Given the capacity losses associated with interference, 60° sectoring is the good choice (higher
S/I with N=3). But giving capacity losses associated trunking (the probably of blocking is
increase when we use 60° over 120°) so It is likely that 120° sectoring is good option.
3. A receiver in an urban cellular radio system detects a 1 mW signal at d 0 = d=1m
from the transmitter. In order to mitigate co-channel interference effects, it is
required that the signal received at any base station receiver from another base
station transmitter which operates with the same channel must be below –100dBm.
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A measurement team has determined that the average path loss exponent in the
system is n = 3. (a) Determine the major radius of each cell if a 7-cell reuse pattern is
used. (b) What is the major radius if a 4-cell reuse pattern is used?
a) Solution
=1 m
=1 mW n=3 N=7
Interference from another BS < -100 dBm
√
( )
√
(
(
) →
)
(√
(√
from
)
)
→ R>470 m
b) Solution
for N=4
(
√
)
(√
)
R>622 m
4. Consider your company won a license to build a U.S. cellular system (the
application cost for the license was only $500!). Your license is to cover 140 square
km. Assume a base station costs $500,000 and a Mobile Telephone Switching Office
(MTSO) costs $1,500,000. An extra $500,000 is needed to advertise and start the
business. You have convinced the bank to loan you $6 million, with the idea that in
four years you will have earned $10 million in gross billing revenues, and will have
paid off the loan.
(a) How many base stations (i.e., cell sites) will you be able to install for $6 million?
(b) Assuming the earth is flat and subscribers are uniformly distributed on the
ground, what assumption can you make about the coverage area of each of your cell
sites? What is the major radius of each of your cells, assuming a hexagonal mosaic?
(c) Assume that the average customer will pay $50 per month over a four year
period. Assume that on the first day you turn your system on, you have a certain
number of customers which remains fixed throughout the year. On the first day of
each New Year, the number of customers using your system doubles and then
remains fixed for the rest of that year. What is the minimum number of customers
you must have on the first day of service in order to have earned $10 million in gross
billing revenues by the end of the 4th year of operation?
(d) For your answer in (c), how many users per square km are needed on the first day of
service in order to reach the $10 million mark after the 4th year?
a) Solution
Number of BSs (6-1.5-0.5)/ 0.5 = 8
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b) Solution
Area= 140 km2, so per cell 140/8= 17.5 km2
Area of hexagonal cell= (3√
)
(
)
, so
√
= 2.6 km
c) Solution
a customer pays 50 *12= $ 600 per year
let M be the number of customer on the first day, so revenues over 4 years
(M+2M+4M+8M)*600=9000 M
, so M=1112
d) Solution
Users per square km = M / area= 1112/140= 8 users/km2
5. Mobile IP, as proposed by the Internet Engineering Task Force (IETF) provides
an efficient, scalable mechanism for node mobility within the Internet. Describe
briefly mobile IP protocol based on IPV4 and IPV6?
Solution (detailed reading in [2])
Mobile IP facilitates node movement and its change for the point of attachment to the
Internet without changing an IP address. This allows them to maintain transport and
higher-layer connections while moving. Node mobility is realized without the need to
propagate host specific routes throughout the Internet routing fabric. Mobile IP is
intended to solve node mobility issues over the IP layer. It is just as suitable for mobility
across homogeneous media as it is for mobility across heterogeneous media.
The mobile node uses two IP addresses: a fixed home address and a care-of address that
changes at each new point of attachment. IP Mobility based on IPv4 protocol
enhancements that allow transparent routing of IP datagrams to mobile nodes in the
Internet. Each mobile node is always identified by its home address, regardless of its
current point of attachment to the Internet. While situated away from its home, a mobile
node is also associated with a care-of address, which provides information about its
current point of attachment to the Internet. The protocol provides for registering the
care-of address with a home agent. The home agent sends datagrams destined for the
mobile node through a tunnel to the care- of address. After arriving at the end of the
tunnel, each datagram is then delivered to the mobile node. Mobile IPv6 is the IP
mobility implementation for the next generation of the Internet Protocol, IPv6.