EN 253 205 (2/2562)Mobile Communication

Modulation Techniques

Asst. Prof. Nararat RuangchaijatuponElectrical EngineeringKhon Kaen University

Office: EN04325A, Email: nararat@kku.ac.th

2

Wired Transmission Techniques• Baseband transmission

– LANs: TP cable, coaxial cable, optical fiber– Line coding: e.g. Manchester

• Voice-band transmission– Analog: carrier modulation (e.g. AM, FM, PM)– Digital: digital modulation (e.g. ASK, FSK,

PSK, QAM)– To eliminate DC component, more bandwidth

efficiency over telephone channel

3

Consideration in the Design of Wireless Modem Techniques

• Fading, system type, service requirement, users’ equipment

1. Bandwidth efficiency2. Power efficiency3. Out-of-band radiation4. Resistance to multipath5. Constant envelope modulation

4

Bandwidth Efficiency

• Analog -> Digital, TDMA - > CDMA• Increase system capacity -> increase

revenues• Need modulation schemes that provide

efficient utilization of available bandwidth• Cellular industry vs. WLANs industry

(circuit-switched service vs. burst-mode traffic)

5

Power Efficiency

• Battery-oriented application: battery size, size and weight of portable terminals, recharging interval

• Power need to operate electronics in the terminal vs. receiver design complexity

• Power to radiate the signal at the antenna• Higher data rates require higher operating

levels of SNR

6

Out-of-band Radiation

• Signal energy lying outside the main lobe

• Adjacent-channel interference (ACI)

• Serious interference for mobile terminals in a farther distance

• Need to be kept under a specified level

7

Constant Envelope Modulation

• Signal to be amplified should have a constant envelope vs. nonlinearity of amplifier

• Frequency modulation (FM)– Large side lobes

• Gaussian minimum shift keying (GMSK)• π/4 - Quadrature Phase Shift Keying

8

3 Categories of Modulation Techniques

• Pulse transmission techniques -> Infrared-based applications

• Basic modulation techniques for TDMA cellular

• Spread spectrum techniques in CDMA, WLANs– Variation of spread spectrum techniques ->

OFDM in WLANs

9

Carrier Modulated Transmission

• Mixed with a carrier signal at higher frequency– For FDM– Applications: TV, AM/FM radio, cellular, etc.– Size of antenna is on the order of

transmission wavelength• Higher frequency, better coverage, reduced

antenna size, higher data rate, complex RF circuit, reduced penetration

10

Digital Cellular Transmission

• AM, FM, PM• Fading causes amplitude fluctuation• GMSK -> GSM, CDPD• π/4 - QPSK -> North American TDMA

cellular

11

Digital Frequency Modulation and GMSK

• Digital frequency modulation = FSK

• For multi-level input (PAM), the output still has constant envelope with multiple frequency levels

12

Frequency Spacing• For optimal detection -> ensure

orthogonality of transmitted symbols• In FSK, minimum distance = 1/T

– Non-coherent detection (receiver is not locked to the phase of the transmitted carrier)

– T = duration of the transmitted data symbols• In MSK (minimum shift keying), minimum

distance = 1/2T– For coherent detection

13

Gaussian MSK• Filtered to reduce side

lobes• Gaussian filter

smoothes transition from one frequency tone to another, and hence, reduces side lobes

• Constant-envelope continuous phase modulation

• GMSK Modulator

14

GMSK (cont.)• Time-bandwidth

produce BbT– Bb: 3-dB bandwidth of the

Gaussian filter– T: symbol duration

• BW of filter is narrower small side lobes power in side lobes, ACI– In GSM (voice), BbT is

0.3– In CDPD, BbT is 0.5

15

Digital Phase Modulation andπ/4 -QPSK

• BPSK• Signal

constellation• Error rate

– Distance between 2 points in the constellation

– Level of noise

16

BPSK – Non-Coherent Detection

• Use carrier in the current bit as the reference carrier of the following bit

• Advantage - The receiver doesn’t need to recover phase of the transmitted carrier

17

BPSK – Non-Coherent Detection (cont.)

• The value of the XOR of the consequent bits are transmitted instead of sending the actual bits

• The two phases that are transmitted represent the change in the polarity of the current bit with respect to the previous bit

• The value of the delay is equal to the duration of a transmitted bit

18

Quadrature Phase Shift Keying

• Four-phase PSK• Represents a block of 2 information bits• 0°, 90°, 180°, 270°• 00, 01, 10, 11• Implementation: using the same bandwidth

and orthogonal carriers

19

QPSK (cont.)

20

Comparison: BPSK, MSK, QPSK

21

Comparison: BPSK, MSK, QPSK (cont.)

• Width of main lobe– Occupied bandwidth– Data rate

• Peak of side lobes– Adjacent-channel interference

• Filtering technique can be used to further reduce the side lobes– If the baseband data stream is kept in rectangular

form, the side lobes are very high

22

Basic Implementation of PSFs for BPSK

• Pulse-Shape filtering to control side lobes• Pulses with smoother transition

23

Further Improvements to QPSK

• Offset or staggered QPSK (OQPSK or SQPSK)– The two branches are offset by T/2 sec– Envelopes of the in-phase and quadrature-

phase signals overlap one another– The peaks of one branch occurs in between

the peaks of the other branch– Fewer fluctuation in amplitude– Increase constancy in amplitude

24

Further Improvements to QPSK (cont.)

• OQPSK or SQPSK– Changes in phase of carrier occurs every T/2

sec (rather than T sec)– Phase shift by +- 90 degrees– Avoid +-180 degrees phase shift– Increase constancy in phase

• Improve power requirement and reduce side lobe

• But difficult to develop non-coherent receiver, sensitive to multipath fading

25

π/4-QPSK

• QPSK signal constellation is shifted by 45°each symbol interval T– Phase transition from one symbol to the next

is +- 45 degrees and +- 135 degrees– Reduce amplitude variation

26

π/4-QPSK (cont.)Signal constellation and phase transitions

27

π/4-QPSK (cont.)

• Can be implemented with– Coherent detection– Differential coherent detection– Discriminator detection

• Used in– North American Digital Cellular TDMA– IS-136– Japanese Digital Cellular standard

28

Broadband Modems for Higher Speed

• Voice-oriented networks– Low-speed modulation technique– Concern: Coverage & mobility

• Data-oriented networks– WLANs, Point-to-point fixed wireless networks– Concern: Achievable data rate– In order to increase speed

• OFDM IEEE 802.11a & HIPERLAN-2

29

Orthogonal Frequency Division Multiplexing

• Multirate, multisymbol, multicarrier modulation (MCM)

• Using orthogonality of the adjacent carriers

• OFDM = an FDM system a single user uses all FDM channels together

30

Multicarrier Modulation

• Instead of using 1 carrier, Rs symbols/sec• OFDM uses N carriers, Rs/N symbols/sec• Carriers are spaced by about Rs/N Hz• Each of carriers is modulated with the rate

of Rs/N symbols/sec• Multipath is reduced by about 1/N• Less distortion in each demodulated

symbol

31

Multicarrier Modulation (cont.)• The symbols are made sufficient long

relative to the multipath spread– No need for anti-multipath signal processing

• Frequency selective fading– Error control coding across symbols in

different subchannels• Increase data rate by increasing the

number of subcarriers– Complexity and limited transmitted power

32

Multicarrier Modulation (cont.)

33

Multisymbol Modulation

• Multiamplitude and multiphase• To increase data rate• Recall QPSK vs. BPBK

– Same symbol transmission rate (occupied bandwidth), double bit transmission rate

• Increasing the number of symbols in the constellation allowing more encoding of the number of bits per symbol

34

Multisymbol Modulation (cont.)• 16-QAM (4 bits/symbol) vs. 64-QAM (6 bits/symbol)• The number of bits per symbol of a signal

constellation represents the increase of the data rate

35

Multirate Transmission

• To increase reliability under degraded channel condition

• “Fallback” mode of operation• Users move away from BS

– SNR reduces– Modems fall to lower rate– Reasonable error rates at lower SNR

36

Spread Spectrum Transmissions

• FHSS, DSSS• Advantages

– Overlay onto other systems’ bands with minimal performance impact

– Wide band vs. frequency selective fading multipath channel

– Urban & indoor have heavy multipath– Anti-interference and interception– Unlicensed spread spectrum

37

Frequency Hopping Spread Spectrum

• Invented by a movie star Hedy Lamarr• To protect guided torpedoes from jamming• Shift the center frequency of the

transmitted signal• Frequency hops occur in a random pattern

– The pattern known only to transmitter and receiver• The required transmission bandwidth spans

equal to the number of hopped frequencies

38

FHSS (cont.)

39

FHSS (cont.)

40

FHSS (cont.)• Random number generator• The occurrence of frequencies is statistically

independent of one another• Pseudorandom pattern• Slow frequency hopping long packet• Fast frequency hopping very short packet• In military, fast FHSS, sometimes the same bits

are transmitted using several frequencies

41

FHSS (cont.)• GSM supports an optional frequency-hopping

pattern of 217.6 hops/sec– Frequency selective fading & Co-channel interference

• CDMA with FHSS– Assigning a different hopping pattern to each terminal– The code are random and independent from one

another– Patterns are selected so that 2 users never hop to the

same frequency– The number of frequency slots limits the number of

users

42

IEEE 802.11 using FHSS

• 78 hopping channels separated by 1 MHz• Divided into 3 patterns of 26 hops• Allows 3 different systems to coexist

without any hop collision• Allows 3 APs in the same area

– 3 folds increase in capacity

43

Direct Sequence Spread Spectrum

• Each transmitted information bit is spread (mapped) into N smaller pulses (chips)

• N - Processing gain/Bandwidth expansion factor• Despreader correlates the received signal with

the duplicated transmitted spreading signal• The autocorrelation function of a good random

code has a very high peak-to-sidelobe ratio– Multiply the pulse with the delayed version of itself

and then integrate over the duration of the pulse

44

DSSS (cont.)

45

DSSS (cont.)

46

DSSS (cont.)

• Compared with FHSS, the bandwidth of DSSS is always wide

• DSSS provides a robust signal with better coverage area than FHSS

• FHSS can be implemented with much slower sampling rates– Saving implementation costs– Saving power consumption of the mobile units

47

High-speed Modem for Spread Spectrum Technology

• From the receiver’s point of view, it is very similar to a pulse transmission technique that sends a narrow pulse every N slots

• It is used to measure the multipath characteristic and impulse response of the channel

• PPM-DSSS can be used in case of no serious multipath

Question & Discussion

Assignment