Modulation and Multiplexing ICS 620

Overview Frequency Spectrum Modulation techniques Multiplexing--TDM vs FDM Multiple Access Signal formats

The Bands VLFLFMFHFVHFUHFSHFEHF Submillimeter Range ELF 3MHz30MHz300MHz3GHz30GHz300GHz Far Infra- Red 300KHz30KHz3THz 300mm RadioOptical 3KHz Near Infra- Red 700nm 1PetaHz RedRed OrangeOrange YellowYellow GreenGreen BlueBlue IndigoIndigo VioletViolet 600nm400nm500nm Ultraviolet 1ExaHz X-Ray 1500nm

Frequency Spectrum Limited resource Managed WARC FCC Bands

 Analog Modulation The purpose of a communication system is to transmit information signals (baseband signals) through a communication channel The term baseband is used to designate the band of frequencies representing the original signal as delivered by the input transducer For example, the voice signal from a microphone is a baseband signal, and contains frequencies in the range of Hz The “hello” wave is a baseband signal:

AM radio FM radio/TV Since this baseband signal must be transmitted through a communication channel such as air using electromagnetic waves, an appropriate procedure is needed to shift the range of baseband frequencies to other frequency ranges suitable for transmission, and a corresponding shift back to the original frequency range after reception. This is called the process of modulation and demodulation Remember the radio spectrum: For example, an AM radio system transmits electromagnetic waves with frequencies of around a few hundred kHz (MF band) The FM radio system must operate with frequencies in the range of MHz (VHF band)  Analog Modulation

Since the baseband signal contains frequencies in the audio frequency range (3 kHz), some form of frequency-band shifting must be employed for the radio system to operate satisfactorily This process is accomplished by a device called a modulator The transmitter block in any communications system contains the modulator device The receiver block in any communications system contains the demodulator device The modulator modulates a carrier wave (the electromagnetic wave) which has a frequency that is selected from an appropriate band in the radio spectrum For example, the frequency of a carrier wave for FM can be chosen from the VHF band of the radio spectrum For AM, the frequency of the carrier wave may be chosen to be around a few hundred kHz (from the MF band of the radio spectrum) The demodulator extracts the original baseband signal from the received modulated signal To Summarize: Modulation is the process of impressing a low-frequency information signal (baseband signal )onto a higher frequency carrier signal Modulation is done to bring information signals up to the Radio Frequency (or higher) signal  Analog Modulation

Types of Analog Modulation Amplitude Modulation (AM) Amplitude Modulation (AM) Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant Amplitude modulation is the process of varying the amplitude of a carrier wave in proportion to the amplitude of a baseband signal. The frequency of the carrier remains constant Frequency Modulation (FM) Frequency Modulation (FM) Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant Frequency modulation is the process of varying the frequency of a carrier wave in proportion to the amplitude of a baseband signal. The amplitude of the carrier remains constant Phase Modulation (PM) Phase Modulation (PM) Another form of analog modulation technique which we will not discuss Another form of analog modulation technique which we will not discuss

Amplitude Modulation Carrier wave Baseband signal Modulated wave Amplitude varying- frequency constant

Frequency Modulation Carrier wave Baseband signal Modulated wave Frequency varying- amplitude constant Large amplitude: high frequency Small amplitude: low frequency

AM vs. FM AM requires a simple circuit, and is very easy to generate. It is simple to tune, and is used in almost all short wave broadcasting. The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves?) However, it is quite inefficient, and is susceptible to static and other forms of electrical noise. The main advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals. The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation. The main disadvantage of FM is the larger bandwidth it requires

Amplitude Modulation Uses a higher frequency carrier Most efficient use of frequency Time and Frequency Domain Susceptible to Noise

Most Efficient Use of Frequency Maximum frequency required is: Twice baseband Just baseband (special conditions)

Higher Frequency Carrier Signal Carrier time

Higher Frequency Carrier Power Spectral Density frequency Signal Carrier Baseband watts

Time Domain Signal Carrier time

Time Domain Continued

Detection of Signal time

Frequency Domain Unmodulated frequency Signal Carrier Baseband watts Modulated frequency Signal Carrier Baseband watts Baseband

Spectrum of AM signal

Susceptible to Noise time

Carrier Signal Oscillator Information Signal Modulation Device Low-Power Amplifier Final Amplifier Antenna Simple block diagram of AM modulation

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 can’t be used for synchronization purposes

Frequency Modulation Uses a higher frequency carrier Usually more bandwidth Time and Frequency Domain Resistant to some Noise

Time Domain Signal Carrier time

Time Domain time

Frequency Domain Unmodulated frequency Signal Carrier Baseband watts Modulated frequency Signal Carrier watts

Resistant to Some Noise time

Phase Modulation Uses a higher frequency carrier Fairly efficient use of frequency Time and Frequency Domain Used mainly for data

Time Domain (Instantaneous View) Unmodulated Carrier Modulated Carrier Phase Input

Phase Shift Keying Time Plot 0O0O 90 O 180 O 270 O

Pulse Modulation Uses the sampling rate PAM PDM, PWM PPM PCM

Starts with Sampling Volts time

PAM Volts time Pulse Amplitude Modulation

PDM (a.k.a. PWM) Volts time max = largest Positive min = largest Negative time Pulse Duration Modulation (Pulse Width Modulation)

PPM Volts time max = largest Positive min = largest Negative time Pulse Position Modulation

Pulse Code Modulation n By quantizing the PAM pulse, original signal is only approximated n Leads to quantizing noise n Signal-to-noise ratio for quantizing noise n Thus, each additional bit increases SNR by 6 dB, or a factor of 4

PCM Volts time Pulse Code Modulation

Multiplexing n Frequency Division Multiplexing –Separate each baseband signal into a discrete band –Uses AM-SSB/SC to position each baseband

Frequency Division Multiplexing frequency

Multiplexing Time Division Multiplexing Separate each digital baseband into discrete time slots Cyclical in nature

Time Division Multiplexing Rotation Analogy

FDMA Frequency Division Multiple Access Frequency Time Chan A Chan B Chan C Chan D

TDMA Time Division Multiple Access Frequency Time Chan A Chan B

CDMA Code Division Multiple Access Frequency Time Code

Summary Frequency Spectrum Modulation techniques--AM, FM, Phase, and Pulse Modulation Multiplexing--TDM vs FDM