Principles of Communication
Course Outline 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Introduction to Communications Systems Noise Amplitude Modulation Single-Sideband Techniques Frequency Modulation Radio Receivers Radiation and Propagation of Waves Pulse Modulation Digital Modulation Broadband Communication System
Course Outline 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Introduction to Communications Systems Noise Amplitude Modulation Single-Sideband Techniques Frequency Modulation Radio Receivers Radiation and Propagation of Waves Pulse Modulation Digital Modulation Broadband Communication System
Principles of Communicat Communication ion LAB 1. 2. 3. 4. 5. 6. 7. 8.
Passive, Active Filters, Tuned Circuits AM Transmitter Frequency Modulation Pulse Amplitude Modulation Diode Detection Time Division Multiplexing Frequency Division Multiplexing Suggested Project : superheterodyne receiver
Introduction: Communication System
Communication systems are designed to transmit information from one place to another. Communication systems Design concerns: 1. Selection of the information – bearing waveform; 2. Bandwidth and power of the waveform; 3. Effect of system noise on the received information; 4. Cost of the system.
RA 9292 Definition: Communications – refers to process of sending and/or receiving information, data, signals and/or messages between two or more points by radio, cable, optical waveguide or other devices and wired or wireless medium.
Evolution of Communication System:
Smoke Signal
Tribal Drum
Morse Code
Evolution of Communication System: 1837 – Samuel Morse invented the telegraph system, the first to be commercially successful communication system which uses electricity in sending messages. 1866 – the use of telegraph cables that runs under water. 1898 – twelve transatlantic cables in operation. 1876 – Alexander Graham Bell invented the telephone, the first voice communication by electrical means.
Importance of a radio in communication as a medium; 1865 – James Clerk Maxwell constructed the theoretical framework in radio communication. 1887 – Heinrich Rudolph Hertz verified the theories of Maxwell. 1901 – Guglielmo Marconi accomplished the first transatlantic communication via radio. 1906 – transmitters began to use specially designed high frequency alternators to transmit voice. 1920 – began the regular radio broadcasting.
The use of electronic system; 1904 – Sir John Ambrose Fleming invented the diode tube. 1906 – Lee De Forest invented the triode for amplification. 1947 – Brattain, Bardeen, and Shockley invented the transistor. 1948 – transistor began to use for amplification.
Two Broad Categories of Communication System 1.
Analog Communication System
2.
Digital Communication System
Analog Communication System An analog communication system transfers information from an analog source to the sink.
Digital Communication System A digital communication system transfers information from a digital source to the sink.
Analog Information Source An analog information source produces messages that are defined on a continuous form. (e.g. microphone)
Digital Information Source A digital information source produces a finite set of possible messages. (e.g. typewriter)
Digital and Analog Communication Systems
A digital waveform is defined as a function of time that can have a discrete set of amplitude values.
An Analog waveform is a function that has a continuous range of values. ( )
x t
( )
x t
t
t
Analog
Digital
Digital Communication
Advantages • • • • • •
Relatively inexpensive digital circuits may be used; Privacy is preserved by using data encryption; Data from voice, video, and data sources may be merged and transmitted over a common digital transmission system; In long-distance systems, noise dose not accumulate from repeater to repeater. Data regeneration is possible Errors in detected data may be small, even when there is a large amount of noise on the received signal; Errors may often be corrected by the use of coding.
Disadvantages • •
Generally, more bandwidth is required than that for analog systems; Synchronization is required.
Deterministic and Random Waveform
A Deterministic waveform can be modeled as a completely specified function of time.
A Random Waveform (or stochastic waveform) cannot be modeled as a completely specified function of time and must be modeled probabilistically.
Basic Properties of the em signal;
• Wavelength is the distance
A
B
required to complete one cycle at a particular frequency. (i.e the distance from point A to B represents v one wavelength)
f where:
= =
and k is dielectric constant of a certain medium
• The frequency is the number of cycles
(i.e. from A to B) in a one-second period. It may be the number of voltage polarity alternations or em field oscillations that takes place in a span of time. cycle 1 Hz
1
second
• Amplitude is the maximum displacement of a continuous wave. Amplitude
A
B
A m p l i t u d e
Time
• Phase is the location of the travelling wave at a fixed point in time. 90o
here, when phase is at 900
= sin + 90
180o 0o
360o
270o
Sample problems: 1. Calculate the wavelength in (a) free space, (b) transmission line, with dielectric medium constant of 1.5, corresponding to a frequency of 27 MHz. 2. Express the positive cosine function representation of the following signals ;
= 50 sin + (b) = 10 1 − (a)
Bandwidth
•
The span of frequencies within the spectrum occupied by a signal for conveying information (e.g. music uses 0 to 20 KHz --> BW = 20KHz)
Water (information)
•
B a n d w i d t h
Sending information in a short amount of time requires more bandwidth
Standard voice channel bandwidth; f1 = 300Hz , f2 = 3400Hz BW = 3400 Hz – 300 Hz BW= 3.4 kHz
TV broadcasting channel bandwidth; e.g. ABS-CBN operates at 54 MHz to 60 MHz BW = 60 MHz – 54 MHz BW = 6 MHz
Decibels - used in almost every part of electronic communication system system to express the ratio of two power levels or voltage v oltage levels. Definition: - one tenth of a Bel
= log( ) = = log( ) 10 Example Problem; 1. Find the ratio between P2 and P1, in dB, if (a) P1 = 2W and P2 = 3W; (b) P1 = 3W and P2 = 2W
dB Gain vs dB Loss If Po is the output power power of a device and Pi is the input power then the Gain in dB is;
= 10 lo log If Po is less than Pi then the negative gain results to a Loss in the system. Example: 1. An attenuator has a loss of 26 dB. If the power of 2 W is applied to the attenuator, find the output power.
Other dB notation dBm – based dBm – based on the reference 1mW of power at the input.
= 10 lo log 1
dBW – use dBW – use 1W as reference input i nput power dBk – use – use 1kW as reference Note : dB+dBk = dBK dB+dBm = dBm dB+dBW = dBW
Example Problems: 1. Convert a power level of 5W to (a)dBm and (b)dBk 2. If a signal with a power level of -12dBm were applied to the system as shown, What is the output level of the system? in
Ap Ap 20dB 15dB 3. Convert 10 dBW to dBm and dBk.
out
dB in Current and Voltage Gains: From power formula;
= =
It follows that, voltage gain in dB is;
= 10 log = 20log( )
Similarly the current gain in dB is given by;
= 10 log = 20 log Ex. A signal in cable tv system has an amplitude of 3mV in 75 ohms. Calculate its level in dBmV and dBm.
Parts of a Communication System
All communication systems contain three main sub systems:
Transmitter Channel Receiver
Other parts of communication system: Information Input Source Output Destination/Sink Noise
Transmitter
Receiver
Block Diagram of A Communication System TRANSMITTER: The signal-processing block is used for more efficient transmission. Examples: In an analog system, the signal processor may be an analog low-pass filter to restrict the bandwidth of m(t). In a hybrid system, the signal processor may be an analog-to-digital converter (ADC) to produce digital signals that represent samples of the analog input signal.
Block Diagram of A Communication System TRANSMITTER:
The transmitter carrier circuit converts the processed base band signal into a frequency band that is appropriate for the transmission medium of the channel. Example: An amplitude – modulated (AM) broadcasting station with an assigned frequency of 850 kHz has a carrier frequency f c=850kHz. The mapping of the base band input information waveform m(t) into the band pass signal s(t) is called modulation. It will be shown that any band pass signal has the form
= ω +
ω = 2
If R(t)=1 and θ(t) = 0, s(t) would be a pure sinusoid of frequency f =f c with zero bandwidth.
Block Diagram of A Communication System Channel: Channels represents the path in which signals travel from transmitter to receiver. Very general classification of channels are:
Wire: Twisted-pair telephone line, coaxial cable, waveguide, and fiber-optic cables. Wireless: Air vacuum, and seawater.
In general, the channel medium attenuates the signal so that the delivered information deteriorated from that of the source. The channel noise may arise from natural electrical disturbances or from artificial sources.
Block Diagram of A Communication System Receiver: The receiver takes the corrupted signal at the channel output and converts it to be a base band signal that can be handled by the receiver’s base band processor.
The base band processor cleans up this signal and delivers an estimate of the source information to the communication system output.
In digital systems, the measure of signal deterioration is usually taken to be the probability of bit error P(e) – also called Bit Error Rate (BER) of the delivered data m(t).
In analog systems, the performance measure is usually taken to be the Signal-to-noise Ratio (SNR) at the receiver output.
How to measure the effectiveness of a communication system? We can measure the “GOODNESS” of a communication system in many ways: How close is the estimate to the original signal m(t)? Better estimate = higher quality transmission Signal to Noise Ratio (SNR) for analog m(t) Bit Error Rate (BER) for digital m(t) How much power is required to transmit s(t)? Lower power = longer battery life, less interference How much bandwidth B is required to transmit s(t)? Less B means more users can share the channel Exception: Spread Spectrum -- users use same B. How much information is transmitted? In analog systems information is related to B of m(t). In digital systems information is expressed in bits/sec.
Frequency Bands
Regulations specify, modulation type, bandwidth, power, type of information and etc. that a user can transmit over designed frequency bands.
Frequency assignments and technical standards are set internationally by International Telecommunication Union (ITU). Locally, these are set by NTC.
Each nation of ITU retains sovereignty over spectral usage and standards adopted in its territory.
Each nation is expected to abide by the overall frequency plan adopted by ITU.
Assignment: 1.
2.
Write the complete list of the VHF and UHF television channel and its corresponding operating frequencies. Name the two basic type of spectrum analyzer, and briefly describe how each works.
3.
What is the difference between the white and pink noise?
4.
Why is the noise power bandwidth greater than the half power bandwidth of a system?
Noise
•
Noise is any unwanted signal (random) that corrupts and distort the desired signal V
Noise Added
t
•
Effects of Noise:
• Noise can cause the user to misunderstand the original signal • Noise can cause the receiving system to malfunction • Noise can result in a less efficient system
General Types of Noise 1. External Noise – noise created and defined within the property of the channel or medium. a.
Atmospheric Noise – also called “static” because lightning is the principal source of this noise.
b.
Industrial noise – a man-made noise thru the intervention of man and/or man’s machine and equipment. Space Noise – or extra-terrestrial noise comes from the sun, stars, and other space bodies which radiates energy.
c.
2. Internal Noise – originates within the communication equipment. a. Thermal Noise – produced by random motion of electrons in a conductor due to heat.
∝ ∝ = Where: Pn - noise power, in Watts T – temperature in Kelvin k – Boltzmann’s constant, 1.38x10-23 J/K
Noise calculation Noise Voltage;
= 4 Due to several sources;
= 4 + + ⋯ + = √( + + ⋯ + )
b.
Shot Noise – due to random arrival of electrons or random variations in current flow in an active device.
= 2 Where: In – rms noise current, in Ampere Io – dc bias current , in Ampere q – charge of an electron, 1.6x10-19 Coulomb
Exercises: 1. A diode noise generator is required to produce 10 uV of noise in a receiver with an input impedance of 75 ohms, and a noise power bandwidth of 200kHz. What is the current through the diode? 2. The circuit shows two resistors in series at two different temperatures. Find the total noise voltage and noise power produced at the load, over a bandwidth of 100 kHz.
Signal-to-Noise Ratio
•
The SNR is important in determining how well the system will operate or how successfully the system can recover a weak signal dB -10 dBm
Noise floor -50 dBm
f (MHz)
SNRdB
S 10 log S dB N dB N
Where: S = signal power (in watts) N = noise power (in watts)
Variations of SNR Ratio of signal plus noise power to noise power alone.
+
S/N for FM receivers that included distortion (SINAD).
++ +
Sample Problem: 1.
A receiver produces a noise power of 100 mW with no signal. The output level increases to 10 W when a signal is applied. Calculate the (S+N)/N in dB.
2.
Determine the signal power of a receiver with SNR = 28 dB. This receives an equivalent noise power of 500 mW.
Noise Figure Definition: -a figure of merit which indicates how much a component or series of stages degrades the signal to noise ratio of a system.
=
Where: S/Nin - input signal to noise ratio S/Nout – output signal to noise ratio S/N in dB; NF(dB) = S/Nin(dB) - S/Nout(dB)
Example: 1. The signal power at the input of the degenerating circuit in a receiving section is 1500 uW and the noise power received is 5 uW. At the output these values are 100 mW and 3700 uW, respectively. What is the NF of the sytem, in dB?
Equivalent Noise Temperature:
= +1 290 Where: NF - noise figure Teq – equivalent noise temperature Ex. An amplifier has an equivalent noise temp. of (a)45K, (b)89oF. Calculate the dB noise figure.
Information Capacity
•
Information capacity is the rate at which data can be transferred by a communications system
•
Shannon’s Theory
S C B log 2 1 N where C = Channel or info capacity (in bits per sec) B = channel bandwidth (in Hertz) S = signal power (in watts) N = noise power (in watts)
Information Measurement Definition: Information Measure (I j) The information sent from a digital source (I j) when the jth massage is transmitted is given by:
where P j is the probability of transmitting the jth message. • •
•
Messages that are less likely to occur (smaller value for Pj) provide more information (large value of Ij). The information measure depends on only the likelihood of sending the message and does not depend on possible interpretation of the content. For units of bits, the base 2 logarithm is used; if natural logarithm is used, the units are “nats”; if the base 10 logarithm is used, the units are “hartley”.
Information Measurement Definition: Average Information (H ) The average information measure of a digital source is,
where m is the number of possible different source messages. The average information is also called Entropy.
Definition: Source Rate (R) The source rate is defined as,
where H is the average information T is the time required to send a message.
Information Measurement Ex. Find the information content of message that consists of a digital word 12 digits long in which each digit may take on one of four possible levels. The probability of sending any of the four levels is assumed to be equal, and the level in any digit does not depend on the values taken on by pervious digits.
Answer: Possible combinations of 12 digits ( # of possible messages) = 412 Because each level is equally likely, all different words are equally likely.
Channel Capacity & Ideal Comm. Systems For digital communication systems, the “Optimum System” may defined as the system that minimize the probability of bit error at the system output subject to constraints on the energy and channel bandwidth. Q: Is it possible to invent a system with no error at the output even when we have noise introduced into the channel? Yes under certain assumptions ! According Shannon the probability of error would approach zero, if R
Where R - Rate of information (bits/s) ; C - Channel capacity (bits/s) B - Channel bandwidth in Hz ; S/N - the signal-to-noise power ratio Note: Capacity is the maximum amount of information that a particular channel can transmit. It is a theoretical upper limit. The limit can be approached by using Error Correction
Channel Capacity & Ideal Comm. Systems In analog systems, the OPTIMUM SYSTEM might be defined as the one that achieves the Largest signal-to-noise ratio at the receiver output, subject to design constraints such as channel bandwidth and transmitted power. Question: Is it possible to design a system with infinite signal-to-noise ratio at the output when noise is introduced by the channel? Answer: No! DIMENSIONALITY THEOREM for Digital Signalling: Nyquist showed that if a pulse represents one bit of data, noninterfering pulses can be sent over a channel no faster than 2B pulses/s, where B is the channel bandwidth.
Modes of Transmission:
Simplex
sending of information in one direction only e.g. broadcast radio/TV
Half-duplex
sending of information in either direction, but in only one direction at a time e.g. Two-way radio
Full-duplex
sending of information in both directions, simultaneously
e.g. Public-Switched Telephone Network (PSTN)
•
Modulation is the process by which the communications signal that contains the information (e.g. voice, data) is combined with another signal (I.e. carrier)
•
The result is a signal at frequencies more compatible with the application and in a desired part of the spectrum.
FM Basics (Analog Modulation) Carrier = A sin 2f c t
Info Signal = sin 2 f m t
A
1
t
t
Note: f m << f c
A
FM signal = A sin (2f c t + m sin2 f m t ) t
freq> f C
freq< f C freq= f C
Note: m = modulation index = maximum carrier frequency shift divided by the frequency of the modulating signal, f m
AM Basics (Analog Modulation) Carrier = V c sin 2f c t Vc
Info Signal = V msin 2 f m t Vm
t
t Note: V m < V c Note: f m << f c
Vc + Vm envelope
Vc
AM Signal = ( Vc + Vmsin 2 f mt ) sin 2 f c t
Vc - Vm t
carrier freq, f c
Digital Modulation
• FSK – Frequency Shift Keying Binary States: Logic 0 (high), Logic 1(low)
• ASK – Amplitude Shift Keying Binary States: Logic 0 (absence of the carrier)
• PSK – Phase Shift Keying Binary States: Logic 0 (0º, -90º), Logic 1(180º, +90º)
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Multiplexing and Transmission Media
In this section:
Multiplexing:
combining several sessions on to one medium
Properties of transmission media:
Characteristics of various transmission media that impact their suitability for applicaitons.
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Multiplexing
Multiplexing: combining several signals onto one line.
Frequency division multiplexing (FDM): using different frequency ranges for different signals
Demultiplexing: taking a multiplexed signal and recovering its original components
Wave division multiplexing (WDM): same as FDM, but with optical signals.
Time division multiplexing (TDM): each signal is allocated to a periodic time slot.
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Frequency Division Multiplexing
Suppose that we have three phone signals that we want to combine onto one line with higher bandwidth. Allocate 4 KHz of bandwidth to each signal, which includes a “guard band” of unused frequency range to ensure signals don’t overlap. Each signal originally uses the range 0.3 – 3.3 KHz. Transform each signal to a different frequency range: Signal 1: 20 – 24 KHz channel Use 20.5 KHz to 23.5 KHz, with 0.5 KHz of guard band on each end. Signal 2: 24 – 28 KHz Signal 3: 28 – 32 KHz At receiver, filters are used to isolate each channel, and then the frequency is transformed back to its original range.
67
FDM
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FDM applications
High capacity phone lines
AM radio: 530 KHz to 1700 KHz, 10 KHz bandwidth per station
FM radio: 88 MHz to 108 MHz, 200 KHz bandwidth per station
TV broadcasts: 6 MHz bandwidth per TV channel
First generation cell phones: each user gets two 30 KHz channels (sending, receiving).
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Wave Division Multiplexing Essentially the same as FDM, except the signals are optical and prisms are used to combine/split signals instead of electrical components. Used to combine signals of different frequencies (i.e. colours) onto one fibreoptic cable.
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Time Division Multiplexing (TDM)
TDM is a digital method, as opposed to FDM which is analog. Each signal is split into fixed-size units of time, and units from each signal are sent alternately. If signal represents bits, use a fixed size block of bits as the unit. Suppose that we have 3 signals to combine, and the time unit is 1 ms. Cycle through the signals as follows: Send signal 1 for 1 ms. Send signal 2 for 1 ms. Send signal 3 for 1 ms. Each signal has its own time slot; if it has nothing to send, the slot is left empty to preserve synchronization.
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TDM
