1. TLEN 5830-AWL Advanced Wireless Lab
24-Jan-2017
Review Topics:
Analog Modulation (AM)
Frequency Division Multiplexing (FDM)
Double Side-Band, Suppressed Carrier

2. Recommended reference materials
Textbook References: Wireless Communications and Networks, by William Stallings, ISBN , 2002 (1st edition); Wireless Communication Networks and Systems, by Corey Beard & William Stallings (1st edition); all material copyright 2016 Wireless Communications Principles and Practice, by Theodore S. Rappaport, ISBN (2nd edition)

3. Modulation onto an analog signal

4. Amplitude Modulation Amplitude Modulation
cos2πfct = carrier
x(t) = input signal
na = modulation index
Ratio of amplitude of input signal to carrier
a.k.a double sideband transmitted carrier (DSBTC)

5. Amplitude Modulation

6. Spectrum of an AM Signal

7. AM VARIATIONS
Over the years, various modifications to “full” AM have been devised.
The various schemes remove or reduce the carrier output of the modulator and/or one sideband to:
Reduce the energy in the signal
Reduce the spectrum occupancy of the signal

8. VERSIONS OF AM DSB + Carrier
AM Broadcast, very easy to detect, works with very old technology but extremely wasteful of power. Band occupancy is 2B where B is bandwidth of original signal. DSB with Suppressed Carrier is 3 times as efficient but still occupies 2B of the spectrum. Correct detection is a bit difficult as the carrier must be reinserted at the receiver for correct recovery of the signal.
SSB + Carrier
No information is lost by removing one sideband, but efficiency is increased somewhat. Biggest advantage is channel occupancy is B, same as original bandwidth of signal. DSB-SC has some problems detecting properly.
Vestigial Sideband (VSB)
Most of one sideband is attenuated, leaving full (or slightly reduced) carrier but lowering channel occupancy. More complex detection circuitry may not be required for certain signals. Used in US TV broadcasting for both analog and digital transmissions.
SSB – Suppressed Carrier
This is very efficient. Used extensively for voice communication on short wave (including long distance aircraft over water). Companded SSB is extra efficient version coming in use.

9. Single Sideband (SSB) Variant of AM is single sideband (SSB)
Sends only one sideband
Eliminates other sideband and carrier
Advantages
Only half the bandwidth is required
Less power is required
Disadvantages
Suppressed carrier cannot be used for synchronization purposes

10. AMPLITUDE MODULATION IN THE TIME AND FREQUENCY DOMAINS
Above left is time and frequency domain plots (via oscilloscope and spectrum analyzer displays) of an unmodulated carrier.
Top right is modulated carrier and the original signal immediately below.
Bottom right is spectrum of modulated carrier. Note the “sidebands” occupying widening the spectrum to the right and left of the carrier frequency—which is still there and unchanged.

11. AM MODULATION INDEX EXAMPLE (sine wave modulating signal)
m is the modulation index, the depth of modulation compared to the maximum possible: m is a number from 0 to 1, often given as a %.

12. Amplitude Modulation Transmitted power
Pt = total transmitted power in s(t)
Pc = transmitted power in carrier

13. There are various kinds of multiplexing…
Space Division Multiplex (telephone pole lines, separate wire pairs or fibers in cables, crossbar switching offices, footprints in satellite) communications, separate microwave routes)
Time Division Multiplex (circuit switched “party lines”, local area networks, pulse code modulation switches, synchronous and statistical multiplexers, cable DOCSIS modems, TDMA mobile: cell/PCS telephony)
Spread Spectrum (military/government short wave radio, frequency hopping mobile (old) and some cordless telephones, CDMA wireless cordless/mobile phones and IEEE a/g, DCSS IEEE b)
Frequency Division Multiplex (AM, FM, TV broadcasts, cable TV, old telephone “carrier systems”, DSL, wireless LANs, wireless telephones, mobile: cell/PCS telephony “channels”, OFDM: IEEE a/g, “hi-def” radio, HDTV b’cast used outside America, QAM: cable TV, cable modem formats, HDTV over cable, VSB-AM: b’cast analog TV, HDTV b’cast in America)

14. Frequency Division Multiplexing (FDM)
Concept is to convert the energy of signals to occupy different parts of the frequency domain so that many signals can share a “facility” (communications link).
Process to move the energy within the frequency domain is called modulation.
Modulation is employed often so that many signals can be radiated (via wireless transmission), share wires, or for other transmission advantages. More on that later.
The process of sharing facilities by different signals is called multiplexing. We’ll talk about other multiplexing schemes later.
When the sharing method is by means of such frequency conversions, it’s called Frequency Division Multiplexing.
Band pass filters are used to separate out each modulated signal.
Demodulation schemes are used to regenerate each individual signal and restore it to its original spectrum and time domain signals.

15. Modulation
Modulation uses sine waves to establish the general new frequencies that a signal will occupy. These “seed” sine waves are called carriers.
Modulation techniques vary the three parameters of the “carrier” sine wave.
AMPLITUDE MODULATION (AM)
FREQUENCY MODULATION (FM)
PHASE MODULATION (PM)
QUADRATURE MODULATION (QAM)*
*QAM is one of many modulation techniques that combine the two of the basic techniques, amplitude and phase modulation.
Modulation schemes by their nature can produce energy within separate spectral ranges, centered on or near the frequencies of each carrier frequency. They are how FDM systems are constructed.

16. BASIC FDM TELEPHONY SCHEME – USED AS EARLY AS IN THE 1930s

17. FDM Examples…

18. No Channel 1
Little gap 4-5
Big gap 6-7
Huge gap 13-14
Going
Going
Gone 64-68

19. Reasons for Growth of Digital Techniques
Growth in popularity of digital techniques for sending analog data
Repeaters are used instead of amplifiers
No additive noise
TDM is used instead of FDM
No intermodulation noise
Conversion to digital signaling allows use of more efficient digital switching techniques

20. Amplitude Modulation Math in detail for a sine wave modulating signal.
Carrier = A1cos (2πfct)
Modulating Signal = cos (2πfmt)
S(t) = A1 [1 + cos (2πfmt)] cos (2πfct)
Assume m = 1 and remember from your trig that:
cos (2πf1t)] cos (2πf2t) = ½ cos [2π(f1+f2)t] + ½ cos [2π(f1-f2)t]
The AM signal is the unmodified carrier plus two sidebands which occur at exactly the sum and difference of the modulating and carrier frequencies.
S(t) = A1 cos (2πfct) + ½ A1 cos [2π(f1+f2)t] + ½ A1 cos [2π(f1-f2)t]