Department of  

CSE

Mobile And Wireless

Communication
Subject code: ESC-CSE-308G
Semester: VI
 UNIT 1 - Content
• Introduction: Application, History, Market Scenario, Reference Model and
Overview, Wireless Local Loop and Cellular system.
• Wireless Transmission: Frequencies, Signals, Antennae, Signal Propagation,
Multiplexing, Modulation, Spread Spectrum.
• MAC Layer: Specialized MAC, SDMA, FDMA, TDMA – Fixed TDM, Classical
ALOHA, Slotted,
• ALOHA, CSMA, DAMA, PKMA, Reservation TDMA. Collision Avoidance, Polling,
Inhibit Sense, Multiple Access, CDMA.
• Broadcasting: Unidirectional Distribution Systems, Digital Audio Broadcasting,
Digital Video Broadcasting, Convergence of Mobile and Broadcasting Techniques.

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Why Wireless?

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Limitations

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Range of Wireless Systems

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 History of Mobile Radio Communications

• Major Mobile Radio Systems

• 1934 - Police Radio uses conventional AM mobile communication system.
• 1935 - Edwin Armstrong demonstrate FM
• 1946 - First public mobile telephone service - push-to-talk
• 1960 - Improved Mobile Telephone Service, IMTS - full duplex
• 1960 - Bell Lab introduce the concept of Cellular mobile system
• 1968 - AT&T propose the concept of Cellular mobile system to FCC.
• 1976 - Bell Mobile Phone service, poor service due to call blocking
• 1983 - Advanced Mobile Phone System (AMPS), FDMA, FM
• 1991 - Global System for Mobile (GSM), TDMA, GMSK
• 1991 - U.S. Digital Cellular (USDC) IS-54, TDMA, DQPSK
• 1993 - IS-95, CDMA, QPSK, BPSK
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 Wireless Local Loop
WLL is a system that connects subscriber to the local telephone
station wirelessly.
• Systems WLL is based on: U WLL

• Cellular WANU
WASU
Transceiver
• Satellite
• Microcellular Air TWLL
Trunk Switch WLL AM Interface
PSTN
• Other names function Controller HLR
• Radio In The Loop (RITL)
• Fixed-Radio Access (FRA).

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A general WLL setup

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 WLL services
• Business related, eg: Conference calling
• Calling cards, coin phones
• MODEM
• ISDN (64kbps)(Integrated Services Digital Network)
• Toll-quality service
• Expand from a central office to about 8 kms
• Low license cost
• Subscriber costs equivalent or better than copper

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 Cellular system
• Provide connection to the PSTN
for any user location within the
radio range of the system.
• Characteristic:
– Large number of users
– Large Geographic area
– Limited frequency spectrum
– Reuse of the radio frequency by
the concept of “cell’’.

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 Hexagonal Clusters
• Honey Comb Structure is used.
• Concept of Frequency Reuse can be implemented
• Why Hexagon:
• hexagon structure provide no overlap and no gap
structure with maximum coverage.

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 Speed, Wavelength, Frequency
• Frequency and wave length: λ = c/f wave length λ,
• speed of light c ≅ 3x108m/s, frequency f

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Radio Frequency Bands

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Frequencies for communication

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Frequencies for mobile communication

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Signals

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Signals

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 Antennas
• Radiation and reception of electromagnetic waves,
coupling of wires to space for radio transmission
• Isotropic radiator: equal radiation in all directions
(three dimensional) - only a theoretical reference
antenna
• Real antennas are not isotropic radiators but, e.g.,
dipoles with different lengths. Eg: λ/4 on car roofs
• directed and sectorized antennas are Often used for
microwave connections or base stations for mobile
phones. E.g., radio coverage of a valley.

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Signal propagation ranges

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 Signal propagation
• Receiving power of signal is proportional to 1/d² in vacuum, It depends
much more in real environments (d = distance between sender and receiver.)
• It is additionally influenced by:
Fading (Frequency Dependent)
Shadowing
Reflection At Large Obstacles
Refraction Depending On The Density Of A Medium
Scattering At Small Obstacles
Diffraction At Edges

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Free-space Propagation

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Multipath propagation

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 Multiplexing Techniques

• Multiplexing techniques are used to allow many users to share a common

transmission resource. In our case the users are mobile and the transmission
resource is the radio spectrum. Sharing a common resource requires an access
mechanism that will control the multiplexing mechanism.
• As in wireline systems, it is desirable to allow the simultaneous transmission of
information between two users engaged in a connection. This is called
duplexing.
• Two types of duplexing exist:
• Frequency division duplexing (FDD), whereby two frequency channels are assigned to a
connection, one channel for each direction of transmission.
• Time division duplexing (TDD), whereby two time slots (closely placed in time for duplex
effect) are assigned to a connection, one slot for each direction of transmission.
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 Multiplexing
channels k i

k1 k2 k3 k4 k5 k6

• Multiplexing in 3 dimensions c

• time (t) (TDM) t c

• frequency (f) (FDM) t

s1
• Space (SDM) f
s2
• code (c) (CDM) f
c
t
• Goal: multiple use
of a shared medium s3
f

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 Narrowband versus Wideband

• These multiple access schemes can be grouped into two categories:

• Narrowband systems - the total spectrum is divided into a large number of
narrow radio bands that are shared.
• Wideband systems - the total spectrum is used by each mobile unit for both
directions of transmission.

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 Frequency Division Multiplexing (FDM)
• Separation of the whole spectrum into smaller frequency bands
• A channel gets a certain band of the spectrum for the whole time – orthogonal system
• Advantages:
• no dynamic coordination
necessary, i.e., sync. and
k1 k2 k3 k4 k5 k6
framing c
• works also for analog signals
f
• Disadvantages:
• waste of bandwidth
if the traffic is
distributed unevenly
• inflexible
• guard bands
• narrow filters
t

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 Example 6.2

Five channels, each with a 100-kHz bandwidth, are to be multiplexed together.

What is the minimum bandwidth of the link if there is a need for a guard band
of 10 kHz between the channels to prevent interference?

6.27
Solution
For five channels, we need at least four guard bands. This means that the required bandwidth is at least
5 × 100 + 4 × 10 = 540 kHz,
as shown in Figure.

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 Time Division Multiplexing (TDM)
• A channel gets the whole spectrum for a certain amount of time – orthogonal system
• Advantages:
• only one carrier in the
medium at any time
• throughput high - supports bursts
• flexible – multiple slots k1 k2 k3 k4 k5 k6
• no guard bands ?!
• Disadvantages: c
• Framing and precise f
synchronization
necessary
• high bit rates
at each
Tx/Rx is required. t

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 Space Division Multiple Access (SMDA)
• more than one user can be
allocated to the same physical
communications channel
simultaneously in the same
cell

• separated by angle only

• in a SDMA system, two users

will be allocated to the same
time slot and carrier
frequency at the same time
and in the same cell
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 Spread spectrum technology: CDM
• Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for
duration of the interference
• Solution: spread the narrow band signal into a broad band signal using a special code
power• protection against narrowspread
interference band signal
interference power signal

spread
detection at interference
receiver

f f

protection against narrowband interference

• Side effects:
• coexistence of several signals without dynamic coordination
• tap-proof
• Alternatives: Direct Sequence, Frequency Hopping

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Figure 6.27 Spread spectrum

6.33
 Code Division Multiplexing (CDM)
• Each channel has a unique code k 1 k2 k3 k4 k5 k6

(not necessarily orthogonal)

c
• All channels use the same spectrum at the
same time
• Advantages:
• bandwidth efficient
• no coordination and synchronization necessary f
• good protection against interference and tapping
• Disadvantages:
• lower user data rates due to high gains required
to reduce interference t

• more complex signal regeneration

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 Orthogonal Codes
• A pair of codes is said to be orthogonal if the cross correlation is zero: R (0) =
xy
0.

• For two m-bit codes: x1,x2,x3,...,xm and y1,y2,y3,...,ym:

For example: x = 0011 and y = 0110 : Chip Sequence

Replace 0 with -1, 1 stays as is. Then:

x = -1 -1 1 1
y = -1 1 1 -1
-----------------
Rxy(0) = 1 -1 +1 -1 = 0

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 Types of CDM I

• Two types exist:

• Direct Sequence CDM (DS-CDM)
• spreads the narrowband user signal (Rbps) over the full spectrum by multiplying it by a
very wide bandwidth signal (W). This is done by taking every bit in the user stream and
replacing it with a pseudonoise (PN) code (a long bit sequence called the chip sequence).
The codes are orthogonal (or approx.. orthogonal).
• This results in a processing gain G = W/R (chips/bit). The higher G the better the system
performance
Frequency as the lower the interference. G indicates the number of possible codes.
2

Not all of the codes are orthogonal.

Code
CDMA

Time

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 Types of CDM II

• Frequency hopping CDM (FH-CDM)

• FH-CDM is based on a narrowband FDM system in which an individual user’s
transmission is spread out over a number of channels over time (the channel choice is
varied in a pseudorandom fashion). If the carrier is changed every symbol then it is
referred to as a fast FH system, if it is changed every few symbols it is a slow FH system.
A A B B A
B A B A

B A B A
A B

B A B

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Figure 6.28 Frequency hopping spread spectrum (FHSS)

6.38
Figure 6.29 Frequency selection in FHSS

6.39
Figure 6.30 FHSS cycles

6.40
Figure 6.31 Bandwidth sharing

6.41
 FHSS (Frequency Hopping Spread
Spectrum)
• Discrete changes of carrier frequency
• sequence of frequency changes determined via pseudo random number sequence
• Two versions
• Fast Hopping:
several frequencies per user bit
• Slow Hopping:
several user bits per frequency
• Advantages
• frequency selective fading and interference limited to short period
• simple implementation
• uses only small portion of spectrum at any time
• Disadvantages
• not as robust as DSSS
• simpler to detect MWC (ESC-CSE-308G)
2.32.1
42
Figure 6.32 DSSS

6.45
DS-SS (Direct Sequence Spread Spectrum)
• XOR of the signal with pseudo-random number (chipping sequence)
• many chips per bit (e.g., 128) result in higher bandwidth of the signal
tb
• Advantages
user data
• reduces frequency selective
0 1 XOR
fading tc
• in cellular networks chipping
• base stations can use the sequence
0 1 1 0 1 0 1 0 1 1 0 1 0 1
same frequency range =

• several base stations can resulting

detect and recover the signal signal

• soft handover 0 1 1 0 1 0 1 1 0 0 1 0 1 0

• Disadvantages tb: bit period

tc: chip period
• precise power control necessary
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 Modulation

• Digital modulation
• digital data is translated into an analog signal (baseband)
• ASK, FSK, PSK - main focus in this chapter
• differences in spectral efficiency, power efficiency, robustness
• Analog modulation
• shifts center frequency of baseband signal up to the radio carrier
• Basic schemes
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)
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Modulation and Demodulation
analog
baseband
digital
signal
data digital analog
101101001 modulation modulation radio transmitter

radio
carrier

analog
baseband
digital
signal
analog synchronization data
demodulation decision 101101001 radio receiver

radio
carrier

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 Digital Modulation
• Modulation of digital signals known as Shift Keying 1 0 1

• Amplitude Shift Keying (ASK):

• very simple t
• low bandwidth requirements
• very susceptible to interference 1 0 1

• Frequency Shift Keying (FSK):

t
• needs larger bandwidth

1 0 1

• Phase Shift Keying (PSK):

t
• more complex
• robust against interference

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 Advanced Frequency Shift Keying
• bandwidth needed for FSK depends on the distance between the
carrier frequencies
• special pre-computation avoids sudden phase shifts
 MSK (Minimum Shift Keying)

• bit separated into even and odd bits, the duration of each bit is
doubled
• depending on the bit values (even, odd) the higher or lower
frequency, original or inverted is chosen
• the frequency of one carrier is twice the frequency of the other
• even higher bandwidth efficiency using a Gaussian low-pass filter 
GMSK (Gaussian MSK), used in GSM
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 Advanced Phase Shift Keying
• BPSK (Binary Phase Shift Keying): Q

• bit value 0: sine wave

• bit value 1: inverted sine wave I
1 0
• very simple PSK
• low spectral efficiency 10 Q 11
• robust, used e.g. in satellite systems
• QPSK (Quadrature Phase Shift Keying): I
• 2 bits coded as one symbol
• symbol determines shift of sine wave 00 01
• needs less bandwidth compared to BPSK A
• more complex
• Often also transmission of relative, not absolute
phase shift: DQPSK - Differential QPSK (IS-136,
PACS, PHS)
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MWC (ESC-CSE-308G) 54
 Issues with CDM

• CDM has a soft capacity. The more users the more codes that are used. However as more codes
are used the signal to interference (S/I) ratio will drop and the bit error rate (BER) will go up for all
users.
• CDM requires tight power control as it suffers from far-near effect. In other words, a user close to
the base station transmitting with the same power as a user farther away will drown the latter’s
signal. All signals must have more or less equal power at the receiver.
• Rake receivers can be used to improve signal reception. Time delayed versions (a chip or more
delayed) of the signal (multipath signals) can be collected and used to make bit level decisions.
• Soft handoffs can be used. Mobiles can switch base stations without switching carriers. Two base
stations receive the mobile signal and the mobile is receiving from two base stations (one of the
rake receivers is used to listen to other signals).
• Burst transmission - reduces interference

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