Communication systems by Dr. Fahad Alraddady Assistant professor of CIT
[email protected] Taif University
Communication systems Course Objectives To learn the basic concepts of communication systems Text Book Modern Digital And Analog Communication Systems By B. P Lathi, 4rd Edition Reference books Communication Systems: Analog & Digital – Sanjay Sharma – Kataria & Son Communication Systems by Bruce Carlson Communication Systems 4th ed. By Simon Haykin
INTRODUCTION
Communication: is the process of establishing connection or link between two points for information exchange. Or it is simply the Process of conveying message at a distance or it is the basic process of exchanging information. “A communication system is a process of conveying information from a source to a destination” Information to be transmitted is called message.
INTRODUCTION Communication system is a Components/subsystems act together to accomplish information transfer/exchange. A digital communication system: transfers information from a digital source to the intended receiver (also called the sink). An analog communication system transfers information from an analog source to the sink.
INTRODUCTION
Information is data that has been processed in such a way as to be meaningful to the person who receives it. it is any thing that is communicated. Data: is raw material for data processing. Types of Data Analog: it refers to some thing is continuous. It has continuous value over time. Examples: human voice, video, audio, acoustics or physical parameter: temperature and pressure.
Some examples of Communication Systems
INTRODUCTION Digital: it refers to some thing is discrete. Examples: text, character strings and data stored in memory. To transmit data, it must be transferred to signal. • Signal: it is electric, electronic or optical representation of data, which can be sent over a communication media. • Types of Signals Analog signal: is a continuous wave form that changes smoothly over time as shown in Figure: Digital signal :is discrete , it can only a limited numbers of defined usually 0 and 1.
Types of Data
Types of Signals
9
Requirements of Communication Systems
Rate of information transfer ◦ The rate of information transfer is defined as the amount of information that must be communicated from source to destination. ◦ It will determined the physical form and technique used to transmit and receive information and therefore determines the way system is designed and constructed
Purity of signal received
◦ The received signal must be the same as the transmitted signal
Requirements of Communication Systems
Simplicity of the system ◦ Any communication system must be convenient in order to be effective and efficient and easy to use.
Reliability ◦ Users must be able to depend on a communication system. It must work when needed and transmit and receive information without errors or with an acceptable error.
Elements of a communication system:
Information source: the function of information source is to produce required message which has to be transmitted. Examples speech, television, Facsimile and personal computers. Input Transducer : A transducer is a device which converts one form of energy into another form (electrical, optical or electromagnetic). ◦ To convert the message to a form suitable for the particular type of communication system. ◦ Eg: Speech waves are converted to voltage variation by a microphone.
Elements of a communication system: •
The transmitter couples the input message signal to the channel. It processes and modifies the input for efficient transmission over a channel. • Signal processing operations performed by the transmitter include: Amplification: Filtering: Modulation : it is a process designed to match the properties of the transmitted signal to the channel through a carrier wave. The carrier wave form may be a continuous (analog signal)or a pulse waveform(digital signal). In both cases, the carrier attribute (i.e., the amplitude, frequency or phase) can be changed in continuous or discrete fashion depend on the signal.
Elements of a communication system:
The channel and noise : channel is the medium between the transmitter and the receiver which the signal travel through it. The function of the channel is to provide a physical connection between the transmitter and the receiver.
There are two types of channel, point-to-point channels and broadcasting channels. Examples of point-to-point channels are wirelines, microwave links and optical fibers. Examples of broadcasting channels like satellite which allow several receiving stations to receive simultaneously from a single transmitter. Or Guided media or unguided media.
Elements of a communication system:
The Signal undergoes degradation from noise, interference and distortion. There are three causes of impairment through the channel, which are noise, attenuation and distortion. Attenuation: is a loss of energy and measured in decibel (dB). Distortion: Means that the signal changes its form or shape. Noise : is unwanted signal which tend to interface with the required signal. interface is a Contamination by extraneous signals from human sources.
Elements of a communication system:
The term SNR (signal to noise ratio) is used to measure performance (noise) relative to an information analog signal The term BER (Bit Error Rate) is used in digital system to measure the deterioration of the signal. The receiver extract the input message from the signal and performs this function through the process of demodulation in addition to amplification and filtering. Destination: it converts the electrical message signals to its original form like loudspeakers and PC. Output Transducer :Converts the electrical signal at its input into a form desired by the system used. Eg: Loudspeaker, PC and tape-recorders.
Elements of a communication system: Information source
Input Transducer
Transmitter
noise
Transmission medium
Destination
Output Transducer
Channel
Receiver
Lecture 2 Outline Review Lecture 1 Communication channels Baseband and passband signals. Channel effect, signal-to-noise ratio. Representations of signals. Power Measurments The modulation process. Fundamental limitation of communication Systems.
Communication channels
The channel is the physical path between the transmitter and the receiver. which carries the information. We can said basically there are four types of channels, Telephone channels, Optical fibers, Mobile radio channels and satellite channels. A- Telephone channels: a telephone network use a switching mechanism. The type of the signal is electrical signal. It is linear. The bandwidth is limited band from 300-3100 Hz. B-Optical fiber channel: is a dielectric waveguide which transports light signals from one place to another. It consists of core surrounding by a cladding layer and jacket.
Communication channels
C-Mobile radio channel: the term “Mobile Radio” means the terrestrial situation for the transmitter and the receiver is capable of being moved. The receiver received the signal from multipath. The type of the channel is linear time varying channel. D-Satellite channels : it covers a wide area. There are geostationary satellite and orbit. It could be broadcast and point to pint channel. A channel may linear or non linear. A channel telephone network is linear while satellite channel is non linear. A channel may be time varying like mobile radio channel or time invariant like optical fiber channel.
Communication channels A channel may bandwidth limited or power limited. A telephone channel is bandwidth limited while optical fiber and satellite channel power limited. In summary, we can say the channel properties determine the capacity of the information and the quality of services provided by the system.
Baseband and Passband signals Baseband signal: is that describes signals whose range of frequencies from close to 0 hertz to highest signal frequency. If the signal is transmitted directly without modulation, then it is known as Baseband Transmission. The baseband transmission is used for low frequency and short distance. Passband signal: is a band of frequencies which passes through some filter or set of filters. If the modulating signal is impressed through the carrier signal, the modulated signal is produced. The transmission is called passband transmission and is used for long distance with high frequency.
Baseband signals
Passband signals
CHANNEL EFFECT, Signal-To-Noise Ratio
Signal Bandwidth and Power
In communication System, the bandwidth and the signal power are important factors to control the quality and data rate. The bandwidth can be defined in channel and signal. Bandwidth of channels: is measured in Hertz and can be defined as the range of frequencies that a channel can pass. (In analog communication view) Example: if the channel can transmit with reasonable fidelity a signal whose frequency components from 0 Hz to up to maximum 5kHz, the channel bandwidth B is 5kHz.
CHANNEL EFFECT, Signal-To-Noise Ratio Signal Bandwidth and Power Bandwidth of channel: is measured in bit per second, and can be defined as the speed of bit transmission in a channel or a link. (In Digital communication view) Bandwidth of Signal : is the range of frequencies in a composite signal. The faster a signal changes, the higher its maximum frequency is, and the larger its bandwidth. To have succeful transmit, the channel bandwidth must be larger than the bandwidth of the signal. The signal power: As the power increased, the signal does not effect and the quality of the system is increased as SNR is increased. Power is the rate at which energy is delivered
Representation of Signals To have a knowledge about the communication system, we require some mathematical tools for representation of signal and systems. A- Periodic and nonperiodic signals A periodic signal g(t) is a function of time which satisfies the following condition:
g t g t T0
for all t
Where tdenotes time and T0is the period and define the duration of one complete cycle. Anyg t signal does not satisfy the previous condition is non periodic. In other world T0 does not exist.
Representation of Signals B- Deterministic and Random signals Deterministic signal : it can be modeled as a completely specified function of time or graphically Random or Stochastic : it can not be modeled as a completely specified function of time and must be modeled probabilistically. It can be described as mean value, mean square value, and so on.
Representation of Signals C- Energy and Power signals The electrical signal may be represented by voltage or current and for resistance is equal to on ohm The total energy and the average power are :
2 | g ( t ) | dt
E g lim T
g (t )dt
T
T
1 1 2 2 P lim T | g (t ) | dt lim T g (t ) dt 2T T 2T T
g (t ) is an energy signal if and only if the total energy of the signal is finite and nonzero (i.e 0
Representation of Signals
g (t ) is power signal if and only if the normalized
average power of the signal is finite and nonzero (i.e 0
As a general rule, periodic signals and random signals are classified as power signals
Power Measurement (dB, dBm)
The decibel (dB) is a transmission-measuring unit used to express gain and losses an electronic devices and circuits
for describing relationship between signal and noise ◦ dB 1W ◦ dBm 1mW ◦ example: 100W = 10 log10 100 = 20dB
= 10 log10 100 = 50 dBm 1mW
Power Measurement (dB, dBm)
If two powers are expressed in the same unit (eg: watts or microwatts), their ratio is a dimensionless quantity that can be expressed in decibel form as follows: dB 10 log10 (
P1 ) P2
Where P1 = power level 1 (watts) P2 = power level 2 (watts) The dB value is the difference in dB between P1 and P2
Power Measurement (dB, dBm)
When used in electronic circuits to measure a power gain or loss, that equation can rewritten as Gain
Where
(dB) 10 log10 (
Pout ) Pin
Gain (dB) = power gain (dB) Pout = output power level (watts) Pin = input power level (watts)
Pout absolute Pin
power
• (+) dB - power gain •output power is greater than input power • (-) dB power loss •output power is less than input power
Examples 1. Convert the absolute power ratio of 200 to a power gain in dB Solution: Power gain, Ap (dB) = 10 log10 [200] = 10(2.3) = 23 dB
2. Convert the power gain Ap = 23 dB to an absolute power ratio Solution Power gain, Ap (dB) = 10 log10 [Pout/Pin] 2.3 = log10 [Pout/Pin] [Pout/Pin] = antilog 2.3 = 200
Examples 3. Convert a power level of 200mW to dBm Solutuion: dBm = 10 log10 [200mW/1mW] = 10 log10(200) = 23 dBm
The modulation process •
•
When the information does not match with the channel, the transmitter make some process such as a modulation, which involves varying some parameter of a carrier wave in according with message signal. The process of shifting the baseband signal to passband range is called Modulation. Modulation : is a process designed to match the properties of the transmitted signal to the channel through a carrier wave. The carrier wave form may be a continuous (analog signal)or a pulse waveform(digital signal).
The modulation process •
In both cases, the carrier attribute (i.e., the amplitude, frequency or phase) can be changed in continuous or discrete fashion depend on the signal. The process of shifting the passband signal to baseband frequency range is called Demodulation.
The signal resulting from the process of the modulation is called Modulated signal. The transmitted signal is called Modulating signal. • Benefit of Modulation A-Design practice of an antenna B- remove interference C- reduction of noise
Fundamental limitation of communication Systems •
• • • •
Noise limitation: it is unwanted signal which tend to interface with the transmission and reception of the desired signals in a communication system. Type of noise A- external noise B- internal noise Bandwidth limitation Equipment limitation ◦ Equipment ability
◦ Economy and cost factor ◦ National and international law and agreement as well as standardization (such as ITU etc) ◦ Interaction with existing system
Homework 1 1- Solve Problem 2.1-1a,b and c page 54 2- Solve Problem 2.15 a and b page 54 3- Solve Problem 2.2-1 page 55
