DEPARTMENT ELECTRONICS AND COMMUNICATION.
END SEMESTER EXAMINATION REGULAR, DECEMBER 2017
WIRELESS COMMUNICATION
MARKING SCHEME
Semester: 8 Maximum Marks: 60
COURSE CODE: ECE 412 Duration – 3hours
PART A
Question Answer Marks
1 A fixed station in a mobile radio system used for radio communication with 2
mobile stations. Base stations are located at the center or on the edge of a
coverage region and consist of radio channels and transmitter and receiver
antennas mounted on a tower.
2 The actual distance (coverage area) within a cell boundary to the base station terminal 2
is known as footprint.
3 The process of transferring a mobile station from one channel or 2
base station to another is term as handoff or Handover.
4 Techniques such as cell splitting, Sectoring and Coverage Zone approaches are 2
used in practice to expand the capacity (high capacity) of cellular systems.
5 2
Switching center which coordinates the routing of calls in a large
Center service area. In a cellular radio system, the MSC connects the cellular base
stations and the mobiles to the PSTN. An MSC is also called a mobile telephone
switching office.
6 Forward Channel Radio channel used for transmission of information from the 2
base station to the mobile.
Reverse Channel Radio channel used for transmission of information from the
mobile to base station.
7 The public switched telephone network (PSTN) is the world's 2
circuit-switched telephone networks that are operated by
national, regional, or local telephony operators, providing infrastructure
and services for public telecommunication.
8 2
9 ▪ Shadowing 2
▪ Multipath fading Multipath & small scale fading
▪ Time dispersion
▪ Time delay
10 Enhanced Data rates for GSM Evolution (EDGE) also known 2
as Enhanced GPRS (EGPRS), or Enhanced Data rates for Global Evolution) is a
digital mobile phone technology that allows improved data transmission rates as
a backward-compatible extension of GSM. EDGE is considered a pre-3G radio
technology and is part of ITU's 3G definition.
General Packet Radio Services (GPRS) is a packet-based wireless communication
service that promises data rates from 56 up to 114 Kbps and continuous connection
to the Internet for mobile phone and computer users. The higher data rates allow
users to take part in video conferences and interact with multimedia Web sites and
similar applications using mobile handheld devices as well as notebook computers.
GPRS is based on Global System for Mobile (GSM) communication and
complements existing services such circuit-switched cellular phone connections
and the Short Message Service (SMS).
PART B
Question Answer Marks
11.i 5
The GSM system architecture consists of three major interconnected
subsystems that interact between themselves and with the users through certain
network interfaces. The subsystems are the Base Station Subsystem (BSS), Network
and Switching Subsystem (NSS), and the Operation Support Subsystem (OSS). The
Mobile Station (MS) is also a subsystem. but is usually considered to be part of the
BSS for architecture purposes. Equipment and services are designed within GSM to
support one or more of these specific subsystems. The BSS, also known as the radio
subsystem, provides and manages radio transmission paths between the mobile
stations and the Mobile Switching Center (MSC). The BSS also manages the radio
interface between the mobile stations and all other subsystems of GSM. Each BSS
consists of many Base Station Controllers (BSCs) which connect the MS to the
NSS via the MSCs. The NSS manages the switching functions of the system and
allows the MSCs to communicate with other networks such as the PSTN and ISDN.
The OSS supports the operation and maintenance of GSM and allows system
engineers to monitor, diagnose, and troubleshoot all aspects of the GSM system. This
subsystem interacts with the other GSM subsystems, and is provided solely for the
staff of the GSM operating company which provides service facilities for the network.
The block diagram: of the GSM system architecture. The Mobile Stations (MS)
communicate with the Base Station Subsystem (BSS) over the radio air interface. The
BSS consists of many BSCs which connect to a single MSC, and each BSC typically
controls up to several hundred Base Transceiver Stations (BTSs). Some of the BTSs
may be co-located at the BSC, and others may be remotely distributed and physically
connected to the BSC by microwave link or dedicated leased lines. Mobile handoffs
(called handovers, or HO, in the GSM specification) between two BTSs under the
control of the same BSC are handled by the BSC, and not the MSC. This greatly
reduces the switching burden of the MSC.
11.ii. Frequency division duplexing (FDD) provides simultaneous radio transmission 5
channels for the subscriber and the base station, so that they both may constantly
transmit while simultaneously receiving signals from one another. At the base
station, separate transmit and receive antennas are used to accommodate the two
separate channels. At the subscriber unit, however, a single antenna is used for both
transmission to and reception from the base station, and a device called a duplexer is
used inside the subscriber unit to enable the same antenna to be used for
simultaneous transmission and reception. To facilitate FDD, it is necessary to
separate the transmit and receive frequencies by about 5% of the nominal RF
frequency, so that the duplexer can provide sufficient isolation while being
inexpensively manufactured.
12.i.
Paging systems are communication systems that send brief messages to a subscriber.
Depending on the type of service, the message may be either a numeric message, an
alphanumeric message, or a voice message. Paging systems are typically used to
notify a subscriber of the need to call a particular telephone number or travel to a
known location to receive further instructions. In modern paging systems, news
headlines, stock quotations, and faxes may be sent. A message is sent to a paging
subscriber via the paging system access number with a telephone keypad or modern.
The issued message is called a page. The paging system then transmits the page
throughout the service area using base stations which broadcast the page on a radio
carrier.
Paging systems vary widely in their complexity and coverage area. While simple
paging systems may cover a limited range of 2 km to 5 km, or may even be confined
to within individual buildings, wide area paging systems can provide worldwide
coverage. Though paging receivers are -simple and inexpensive, the transmission
system required is quite sophisticated. Wide area paging systems consist of a
network of telephone lines, many base station transmitters, and large radio towers
that simultaneously broadcast a page from each base station. Simulcast transmitters
may be located within the same service area or in different cities or countries.
Paging systems are designed to provide reliable communication to subscribers
wherever they are; whether inside a building, driving on a highway, or flying in an
airplane.
12.ii. Small-scale fading, or simply fading, is used to describe the rapid fluctuation of the 5
amplitude of a radio signal over a short period of time or travel distance, so that
large-scale path loss effects may be ignored. Fading is caused by interference
between two or more versions of the transmitted signal which arrive at the receiver
at slightly different times. These waves, called multipath waves, combine at the
receiver antenna to give a resultant signal which can vary widely in amplitude and
phase, depending on the distribution of the intensity and relative propagation time of
the waves and the bandwidth of the transmitted signal.
Small-Scale Multipath Propagation
Multipath in the radio channel creates small-scale fading effects. The three most
important effects are:
• Rapid changes in signal strength over a small travel distance or time
interval
• Random frequency modulation due to varying Doppler shifts on
different multipath signals
• Time dispersion (echoes) caused by multipath propagation delays.
In built-up urban areas, fading occurs because the height of the mobile antennas are
well below the height of surrounding structures, so there is no single line-of-sight
path to the base station. Even when a line-of-sight exists, multi-path still occurs due
to reflections from the ground and surrounding structures. The incoming radio waves
arrive from different directions with different propagation delays. The signal
received by the mobile at any point in space may consist of a large number of plane
waves having randomly distributed amplitudes,
Doppler Shift
Consider a mobile moving at a constant velocity v, along a path segment having
length d between points X and Y, while it receives signals from a remote source S.
The difference in path lengths traveled by the wave from source S to the mobile at
points X and Y is ∆L = d cosᶱ v∆tcosᶱ, where ∆t is the time required for the mobile
to travel from X to Y, and ᶱ is assumed to be the same at points X and Y since the
source is assumed to be very far away. The phase change in the received signal due
to the difference in path lengths. Relates Doppler shift to the mobile velocity and
the spatial angle between the direction of motion of the mobile and the direction of
arrival of the wave. It can be seen that if the mobile is moving toward the direction
of arrival of the wave, the Doppler shift is positive.
Fast Fading
Depending on how rapidly the transmitted baseband signal changes as compared to
the rate of change of the channel, a channel may be classified either as a fast fading
or slow fading channel. In a fast fading channel, the channel impulse response
changes rapidly within the symbol duration. That is the coherence time of the
channel is smaller than the symbol period of the transmitted signal. This causes
frequency dispersion (also called time selective fading) due to Doppler spreading,
which leads to signal distortion. Viewed in the frequency domain, signal distortion
due to fast fading increases with increasing Doppler
13.i. Cellular Telephone Systems 5
A cellular telephone system provides a wireless connection to the PSTN for any user
location within the radio range of the system. Cellular systems accommodate a large
number of users over a large geographic area, within a limited frequency spectrum.
Cellular radio systems provide high quality service that is often comparable to that
of the landline telephone systems. High capacity is achieved by limiting the
coverage of each base station transmitter to a small geographic area called a cell so
that the same radio channels may be reused by another base station located some
distance away. A sophisticated switching technique called a handoff enables a call to
proceed uninterrupted when the user moves from one cell to another.
Figure 1.5 shows a basic cellular system which consists of mobile stations, base
stations and a mobile switching center (MSC). The Mobile Switching Center is
sometimes called a mobile telephone switching office, since it is responsible for
connecting all mobiles to the PSTN in a cellular system. Each mobile communicates
via radio with one of the base stations and may be handed-off to any number of base
stations throughout the duration of a call. The mobile station contains a transceiver,
an antenna, and control circuitry, and may be mounted in a vehicle or used as a
portable hand-held unit, The base stations consist of several transmitters and
receivers which simultaneously handle full duplex communications and generally
have towers which support several transmitting and receiving antennas. The base
station serves as a bridge between all mobile users in the cell and connects the
simultaneous mobile calls via telephone lines or microwave links to the MSC. The
MSC coordinates the activities of all of the base stations and connects the entire
cellular system to the PSTN. A typical MSC handles 100,000 cellular subscribers
and 5,000 simultaneous conversations at a time, and accommodates all billing and
system maintenance functions, as well. In large cities, several MSCs are used by a
single carrier.
13.ii. Fast Fading 5
Depending on how rapidly the transmitted baseband signal changes as compared to
the rate of change of the channel, a channel may be classified either as a fast fading
or slow fading channel. In a fast fading channel, the channel impulse response
changes rapidly within the symbol duration. That is the coherence time of the
channel is smaller than the symbol period of the transmitted signal. This causes
frequency dispersion (also called time selective fading) due to Doppier spreading,
which leads to signal distortion. Viewed in the frequency domain, signal distortion
due to fast fading increases with increasing Doppler
Slow Fading
In a slow fading channel, the channel impulse response changes at a rate much
slower than the transmitted baseband signal s(t). In this case, the channel may be
assumed to be static over one or several reciprocal bandwidth intervals. In the
frequency domain, this implies that the Doppler spread of the channel is much less
than the bandwidth of the baseband signal. Therefore, a signal undergoes slow
fading if
14.i 5
14.ii. Space Division Multiple Access (SDLIA) 5
•
Space division multiple access (SOMA) controls the radiated energy for each user in
space. It can be seen from Figure 8.8 that SDMA serves different users by using spot
beam antennas. These different areas covered by the antenna beam may be served by
the same frequency (in a TDMA or CDMA system) or different frequencies (in an
FDMA system). Sectorized antennas may be thought of as a primitive application of
SDMA. In the future, adaptive antennas will likely be used to simultaneously steer
energy in the direction of many users at once and appear to be best suited for TDMA
and CDMA base station architectures.
15.i. Speech coders differ widely in their approaches to achieving signal compression. 5
Based on the means by which they achieve compression, speech coders are broadly
classified into two categories: Waveform Coders and Vocoders. Waveform coders
essentially strive to reproduce the time waveform of the speech sign.al as closely as
possible. They are, in principle, designed to be source independent and can hence
code equally well a variety of signals_ They have the advantage of being robust for a
wide range or speech characteristics arid for noisy environments. All these
advantages are preserved with minimal complexity, and in general this class of
coders achieves only moderate economy in transmission bit rate. Examples of
waveform coders include pulse code modulation (PCM), differential pulse code mot -
bd./a/ion (DP CM), adaptive differential pulse cock modulation (ADP M), delta
modulation (DM continuously variable slope delta modulation (CVSDM), and
adaptive predictive coding (APC) Vocoders on the other hand achieve very high
economy in transmission bit rate.
15.ii. Hendon Strategies 5
When a mobile moves into a different cell while a conversation is in progress, the
MSC automatically transfers the call to a new channel belonging to the new base
station. This handoff operation not only involves identifying a new base station, but
also requires that the voice and control signals be allocated to channels associated
with the new base station_
Processing handoffs is an important task in any cellular radio system_ Many handoff
strategies prioritize handoff requests over call initiation requests when allocating
unused channels in a cell site. Hands 's must be performed successfully and as
infrequently as possible, and be acceptable to the users in order to meet these
requirements, system designers must specify an optimum signal
level at which to initiate a handoff once a particular signal level is specified as the
minimum usable
