1. Mobile Communications
Instructor
M. Naman Chaudhary
MS(Multimedia and Communication)
Muhammad Ali Jinnah University Islamabad Campus

2. Lecture # 6+7

3. Wireless Transmission
CH = 2
Mobile Communications
JOCHEN SCHILLER
2nd Edition

4. Modulation Digital modulation Analog modulation Motivation
digital data (0 and 1) is translated into an analog signal (base band)
Required if digital data has to be transmitted over a media that only allows for analog transmission - old analog telephone system and wireless networks
differences in spectral efficiency, power efficiency, robustness
Analog modulation
shifts center frequency of baseband signal up to the radio carrier
Motivation
smaller antennas (e.g., /4, 1 MHz over 1.8 GHz)
Frequency Division Multiplexing
medium characteristics: long waves for submarines, short waves for handheld devices, very short waves for directed microwave transmission
Basic schemes
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)

5. Modulation and demodulation
analog
baseband
signal
digital
data
digital
modulation
analog
modulation
radio transmitter
radio
carrier
analog
baseband
signal
digital
data
analog
demodulation
synchronization
decision
radio receiver
radio
carrier

6. Digital modulation Modulation of digital signals known as Shift Keying1 1
Modulation of digital signals known
as Shift Keying
Amplitude Shift Keying (ASK):
very simple
low bandwidth requirements
very susceptible to interference
Frequency Shift Keying (FSK):
needs larger bandwidth
Phase Shift Keying (PSK):
more complex
robust against interference t 1 1 t 1 1 t

7. 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
Equivalent to offset QPSK
even higher bandwidth efficiency using a Gaussian low-pass filter  GMSK (Gaussian MSK), used in GSM

8. Example of MSK 1 1 1 1 data bit even 0 1 0 1 even bits odd 0 0 1 11 1 1
data
bit
even
even bits
odd
signal h n n h value
odd bits
low frequency
h: high frequency
n: low frequency
+: original signal
-: inverted signal
high frequency
MSK
signal t
No phase shifts!

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

10. Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation
it is possible to code n bits using one symbol
2n discrete levels, n=2 identical to QPSK
bit error rate increases with n, but less errors compared to comparable PSK schemes
Example: 16-QAM (4 bits = 1 symbol)
Symbols 0011 and 0001 have the same phase φ, but different amplitude a and 1000 have different phase, but same amplitude.
 used in standard 9600 bit/s modems Q
0010
0001
0011
0000 φ a I
1000

11. Hierarchical Modulation
DVB-T modulates two separate data streams onto a single DVB-T stream
High Priority (HP) embedded within a Low Priority (LP) stream
Multi-carrier modulation
splitting data into several components, and sending each of these components over separate carrier signals
(about 2000 or 8000 carriers)
QPSK, 16 QAM, 64QAM
Example: 64QAM
good reception: resolve the entire 64QAM constellation
poor reception, mobile reception: resolve only QPSK portion
6 bit per QAM symbol, 2 most significant determine QPSK
HP service coded in QPSK (2 bit), LP uses remaining 4 bit
HP for the standard resolution
LP for the high resolution Q 10 I 00
000010
010101

12. Antennas: isotropic radiator
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 always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of radiation around an antenna z y z
ideal
isotropic
radiator y x x

13. Antennas: simple dipoles
Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole
Example: Radiation pattern of a simple Hertzian dipole
Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)
/4
/2 y y z
simple
dipole x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane)

14. Antennas: directed and sectorized
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z
directed
antenna x z x
side view (xy-plane)
side view (yz-plane)
top view (xz-plane) z z
sectorized
antenna x x
top view, 3 sector
top view, 6 sector

15. Antennas: diversity Grouping of 2 or more antennas Antenna diversity
multi-element antenna arrays
Antenna diversity
switched diversity, selection diversity
receiver chooses antenna with largest output
diversity combining
combine output power to produce gain
Co-phasing needed to avoid cancellation
/2
/2
/4
/2
/4
/2 + +
ground plane

16. Signal propagation ranges
Transmission range
communication possible
low error rate
Detection range
detection of the signal possible
no communication possible
Interference range
signal may not be detected
signal adds to the background noise
sender
transmission
distance
detection
interference

17. Signal propagation Wave Propagation Ground Waves(<2MHz)
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d² (d = distance between sender and receiver)
Receiving power 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
Wave Propagation
Ground Waves(<2MHz)
Sky Waves (<30 MHz)
Line of Sight(>30MHz)
shadowing
reflection
refraction
scattering
diffraction

18. Spread Spectrum

19. Spread Spectrum

20. Spread Spectrum Transmitter Input is fed into a channel encoder
Produces analog signal with narrow bandwidth
Signal is further modulated using sequence of digits
Spreading code or spreading sequence
Generated by pseudonoise, or pseudo-random number generator
Effect of modulation is to increase bandwidth of signal to be transmitted
On receiving end, digit sequence is used to demodulate the spread spectrum signal
Signal is fed into a channel decoder to recover data

21. Spread Spectrum What can be gained from apparent waste of spectrum?
Immunity from various kinds of noise and multipath distortion
Can be used for hiding and encrypting signals
Several users can independently use the same higher bandwidth with very little interference

22. Frequency Hoping Spread Spectrum (FHSS)
Signal is broadcast over seemingly random series of radio frequencies
A number of channels allocated for the FH signal
Width of each channel corresponds to bandwidth of input signal
Signal hops from frequency to frequency at fixed intervals
Transmitter operates in one channel at a time
Bits are transmitted using some encoding scheme
At each successive interval, a new carrier frequency is selected

23. Frequency Hoping Spread Spectrum
Channel sequence dictated by spreading code
Receiver, hopping between frequencies in synchronization with transmitter, picks up message
Advantages
Eavesdroppers hear only unintelligible blips
Attempts to jam signal on one frequency succeed only at knocking out a few bits

24. Frequency Hoping Spread Spectrum Example

25. DSSS (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

26. DSSS (Direct Sequence Spread Spectrum)

27. DSSS (Direct Sequence Spread Spectrum)

28. Cell Structure

29. Frequency planning

30. Frequency planning

31. Cell breathing

32. Next Week Lecture Plan Telecommunication Systems CH = 4
Mobile Communications by
JOCHEN SCHILLER

33. Uploaded by Ahmad Mushtaq www.AhmadMushtaq.com Student at
Department of Information Technology,
Bahauddin Zakariya University,
Multan, Pakistan