**COMMUNICATION SYSTEMSBTB15103 & BTB15104 / S1`13

Topic 1Introduction to Communication System

Communication Technology Section

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**IntroductionCommunicationsTransfer of information from one place to another.Should be efficient, reliable and secured.A communication system is a process of conveying information from a source to a destination.

Communication systemComponents/subsystems act together to accomplish information transfer/exchange.

Electronic Communication systemTransmission, reception, and processing of information between two or more locations using electronic circuits. An electronic communication system is transferring information using an electrical field as a mean of signal.

**Electromagnetic Signal

**Significance of Human CommunicationCommunication is the process of exchanging information.Main barriers are language and distance.

Methods of communication:Face to faceSignalsWritten word (letters)Electrical innovations:TelegraphTelephoneRadioTelevisionInternet (computer)

**RequirementsRate of information transferThe rate of information transfer is defined as the amount of information that must be communicated from source to destination.It will determine the physical form and technique used to transmit and receive information and therefore determines the way system is designed and constructed . Purity of signal receivedThe received signal must be the same as the transmitted signal.

**Simplicity of the systemAny 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. Requirements

**Elements of Communication system

Block diagram of Communication system

**Input TransducerA device that converts energy from one form to another.Convert an input signal into an electrical waveform.

Example: microphone converts human voice into electrical signal referred to as the baseband signal or message signal.Elements of Communication systemBaseband/message signal

**TransmitterModifies or converts the baseband signal into format appropriate for efficient channel of transmission. [Example: If the channel is fiber optic cable, the transmitter converts the baseband signal into light frequency and the transmitted signal is light].Transmitter also use to reformat/reshape the signal so that the channel will not distort is as much. Modulation takes place in the transmitter. It involves static variation of amplitude, phase or frequency of the carrier in accordance to a message signal.Elements of Communication system

**Channel (Transmission medium)Physical medium through which the transmitter output is sent.Divided into 2 basic groups:Guided Electromagnetic Wave Channel (wire, coaxial cable, optical fiber)Electromagnetic Wave Propagation Channel (wireless broadcast channel, mobile radio channel, satellite)Introduces distortion, noise and interference in the channel, transmitted signal is attenuated and distorted. Signal attenuation increase along with the length of channel.Elements of Communication system

**ReceiverTo extract the desired signal from the output channel and to convert it to a form suitable for the output transducer.Demodulation takes place in the receiver.

Output TransducerConverts electrical signals to its original waveform.

Elements of Communication system

**Communication System Examples

**Communication System Examples

**Analog vs. DigitalThe signal can be analog or digital message:AnalogAn analog signal is a smoothly and continuously varying voltage or current. Examples are:Sine waveVoiceVideo (TV)

DigitalDigital signals change in steps or in discrete increments. Most digital signals use binary or two-state codes. Examples are:Telegraph (Morse code)Continuous wave (CW) codeSerial binary code (used in computers)

**Analog Vs Digital (Advantages and Disadvantages)

Digital Analog Advantages: Inexpensive Privacy preserved (Data encrypt.) Can merge different data Error correctionDisadvantages: Expensive No privacy preserved Cannot merge different data No error correction capability

Disadvantages: Larger bandwidth Synchronization problem is relatively difficult.Advantages: Smaller bandwidth Synchronization problem is relatively easier.

**Mode of CommunicationBroadcastingInvolves the use of a single powerful transmitter transmit to many receivers. Demodulation takes place in the receiver.Information-bearing signals flow in one direction.Example : TV and radio (Simplex).

Point-to-point CommunicationWhere a communication process takes place over a link between a single transmitter and a receiver.Information-bearing signals flow in bidirectional, which requires the use of a transmitter and receiver at each end of the link.Example: Telephone (Full Duplex) and walkie talkie (Half Duplex).

**Baseband vs Modulated SignalBaseband SignalBase band signal is the modulating signal/original information signal either in a digital or analog form (intelligent/message) in communication system. Example: voice signal (300Hz 3400Hz)

Transmission of original information whether analog or digital, directly into transmission medium is called baseband transmission.

Modulated SignalModulated signal is baseband signal which its original frequency is shifted to higher frequency to facilitate transmission purposes.

**Carrier signalwaveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying informationusually a much higher frequency than the input signal. Its purpose is either to - transmit the information through space as an electromagnetic wave (e.g in radio communication) - allow several carriers at different frequencies to share a common physical transmission medium by frequency division multiplexing (e.g tv system).

**Block Diagram of Modulation Process Modulation is the process of putting information onto a high frequency carrier in a transmitter.

**Modulation

Modulation

Process of changing baseband signals to facilitate the transmission medium

Process of modulation

Frequency translation such as AM, FM, PM etcSampling and coding such as PAM, PCM etcKeying such as ASK, FSK etc

**Types of modulation :

Analogue modulations are frequency translation method caused by changing the appropriate quantity in a carrier signal

Digital modulation is the result of changing analogue signal into binary ones by sampling and coding

Keying modulations are digital signals subsequently modulated by the frequency modulation by using one or other analogue method

Modulation

**Why Modulate?Reduce noise and interferenceBy using proper frequency where noise and interference are at minimumIncreasing power is costly and may damage equipment

Frequency AssignmentFor TV and radio broadcasting, each station has a different assigned carrier

MultiplexingCombining several signals for simultaneous transmission on one channel by placing each signal on different carrier frequency

Why Modulate?Modulation is also important because:

Ease of radiation - related to antenna design & smaller size. Low loss and low dispersion.Simultaneous transmission of several signals enables the multiplexing i.e combining multiple signals for tx at the same time over the same carrier.

**Modulation ex:

**SNR, Bandwidth and Rate of Communication1) Signal to Noise Ratio (SNR)SNR is defined as the ratio of signal power to noise power. Noise distorts the signal and accumulated along the path.It is normally measured in Decibel (dB), defined as 10 times the algorithm (to base 10) of the power ratio.dBm is a dB level using a 1mW reference.dBW is a dB level using a 1W reference.

SNR, Bandwidth and Rate of CommunicationExample 1: Convert 100W to dB and dBm.

Answer : dB = 10 log10 [100W/1W] = 10(2) = 20 dB

dBm = 10 log10 [100W/1mW] = 10 log10 [100W/(1x10-3W] = 10(5) = 50 dBm

SNR, Bandwidth and Rate of CommunicationExample 2: A measured value of 10mW will result in what dBm power level?

Answer : dBm = 10 log10 [10mW/1mW] = 10(1) = 10 dBm

SNR, Bandwidth and Rate of CommunicationExample 3: A laser diode outputs +10dBm. Convert this value to WattsAnswer : dBm = 10 log 10 (x W/1mW) 10 = 10 log 10 (x W/1mW) x W/1mW = 10(10/10) x W = 10 (1mW) x = 10 (1x10-3) x = 0.01 Watts

**SNR, Bandwidth and Rate of Communication2) BandwidthBandwidth is the portion of the electromagnetic spectrum occupied by a signal.Specifically, bandwidth is the difference between the upper and lower frequency limits of the signal or the equipment operation range.Figure below shows the bandwidth of the voice frequency range from 300 to 3000Hz. The upper frequency is f2 and the lower frequency is f1. The bandwidth, then is BW = f2 f1

SNR, Bandwidth and Rate of Communication

Bandwidth of an information signal is the difference between the highest and lowest frequencies contained in the information.

Bandwidth of a communication channel is the difference between the highest and lowest frequencies that the channel will allow signals to pass through it (ie: its pass band).

Data rate proportional to bandwidth

**SNR, Bandwidth and Rate of Communication3) Rate of CommunicationRate of information transfer is directly proportional with its bandwidth. Shannon limit for information capacity, C

C = B log2 (1 + SNR) Shannon Hartley Theorem = 3.32 B log10 (1 + SNR)

where C = information capacity (bps) B = bandwidth (Hz) SNR = signal to noise ratio (no unit)

SNR, Bandwidth and Rate of CommunicationExample 4: For a standard telephone circuit with a SNR of 30dB and a bandwidth of 2.7 kHz, determine the Shannon limit for information capacity.

Answer : C = 3.32 B log10 (1+SNR) = 3.32 (2.7x103) log10 (1+30) = 13368.57 bps = 13.368 kbps

SNR, Bandwidth and Rate of CommunicationExample 5: The telephone channel has a bandwidth of about 3kHz. Calculate the capacity of a telephone channel that has an SNR of 1023.

Answer : C = 3.32B log10 (1+SNR) = 3.32 (3x103) log10 (1+1023) = 29982.59 bps = 29.983 kbps

**Electromagnetic Frequency SpectrumThe electromagnetic frequency spectrum is divided into subsections, or bands, with each band having a different name and boundary.

The International Telecommunications Union (ITU) is an international agency in control of allocating frequencies and services within the overall frequency spectrum.

Electromagnetic Frequency SpectrumElectromagnetic waves are signals that oscillate; i.e the amplitudes of the electric and magnetic fields vary at a specific rate.These oscillation may occur at a very low frequency or at an extremely high frequency.The range of electromagnetic signals encompassing all frequencies is referred to as the electromagnetic spectrum.

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Frequency and WavelengthA signal is located on EM spectrum according to its frequency & wavelength

1) Frequency, f (Hz)Frequency is the number of cycles of a repetitive wave that occur in a given period of time.Frequency is measured in cycles per second (cps).The unit of frequency is the hertz (Hz).

2) Wavelength, (m) Wavelength is the distance occupied by one cycle of a wave and is usually expressed in meters.Wavelength is also the distance traveled by an electromagnetic wave during the time of one cycle.

= c/f

Wavelength = speed of light frequency

As speed of light = 3 108 meters/second

Therefore = 3 108 / fExample:Determine the wavelength if the frequency is 4MHz?

= 3 x 108/ (4 x 106) = 75 mFrequency and Wavelength

**Frequency Bands3 30 kHz VLF (very low freq)Ground wave 30 300 kHz LF(low freq)Ground wave 300 3000 kHz MF(medium freq)Ground wave/sky wave 3 30 MHz HF (high freq)Sky wave (Ionospheric) 30 300 MHz VHF (very high freq) Space wave (LOS) 300 3000 MHz UHF (ultrahigh freq)Space wave (LOS) 3 30 GHz SHF (superhigh freq)LOS/Satellite 30 300 GHz EHF (Extremely high freq)LOS/SatelliteInternational Telecommunications Union (ITU) Band Designation

Electromagnetic Frequency SpectrumExample 6A signal with a wavelength of 1.5m , what is its frequency?

Answer : = 3x108 / f f = 3x108 / = 200 x 106 = 200 MHz

Electromagnetic Frequency SpectrumExample 7The maximum peaks of an electromagnetic wave are separated by a distance of 0.203m. What is the frequency in MHz and GHz?

Answer : f = 3x108 / = 3x108 / (75/3.28) = 3x108 / (75/3.28) = 6.86 x 109 = 6.86 GHz

**Radio Communication SystemIt is wireless communication system

The information is being carried by the electromagnetic waves, which is propagated in free space

Electromagnetic waves are waves that travel at the speed of light and made up of an electrical field and magnetic field at right angles to one another and to the direction of propagation

**Block diagram of a radio communication systemRadio Communication System

**Propagation WavesThe three waves propagation methods: Ground wave propagation Sky wave propagation Space wave propagationFig: Radio wave propagation methods

**Propagation WavesThere are three main type of propagations:

Ground (space) wave propagationDominants mode for frequencies below 2 MHzThe movement tend to follow the contour of the earth with large antenna size

Sky-wave propagationDominants mode for frequencies between 2 30 MHz rangeSky waves are those waves that radiated towards ionosphere. By a process of refraction and reflection, the receiver on the earth will receive the signal. The various layers of the ionosphere have specific effects on the propagation of radio waves.

**Propagation WavesThere are areas of no coverage along the earth surface between transmitting and receiving antennaThe angle of reflection and the loss of signal depend on the frequency, time, season, activities of the sun etc

Space wave propagation (Line Of Sight) Dominants mode for frequencies above 30 MHz where in propagates in straight lineNo refraction and can almost propagates through ionosphere

**Propagation WavesBut the signal path has to be above horizon to avoid blocking leading antenna to be placed on tall towersThe distance to radio horizon is

d = 2h miles

**Satellite CommunicationSatellite employs LOS radio transmission over very long distanceIt offers broad coverage even across the ocean and can handle bulk of very long distance telecommunication

**Historical Development

YearEvents18441876190419231936196219661972TelegraphTelephoneAM RadioTelevisionFM RadioSatelliteOptical links using laser and fiber opticsCellular Telephone

**Historical Development

YearEvents197519751980198119821985198919901990-2000First digital telephone switchWideband communication system (cable TV etc)Compact disc is developed by Philip & SonyFCC adopts rules for commercial cellular telephoneInternet is used to replace ARPANETFax machines widely available in officesFirst SONET standard optical fiber products releasedWWW becomes part of the internetDigital communication system (ISDN, BISDN, HDTV, handheld computers, digital cellular etc Global telecom system

**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 noisedB 1WdBm 1mWexample: 100W = 10 log10 100 = 2dB = 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: 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 Where Gain (dB) = power gain (dB)Pout = output power level (watts) Pin = input power level (watts)absolute power (+) dB - power gain output power is greater than input power (-) dB power lossoutput power is less than input power

**Examples1. 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 dB2. Convert the power gain Ap = 23 dB to an absolute power ratioSolutionPower gain, Ap (dB) = 10 log10 [Pout/Pin] 2.3 = log10 [Pout/Pin] [Pout/Pin] = antilog 2.3 = 200

NoiseNoise is any interference that disturbs the legible transmission of a signal (any unwanted signal in a CS)It is a random, undesirable electric energy that enters the communication system via the communication medium and interferes with the transmitted message.Noise can occur in various ways : External noise (generated outside the device or circuit) Internal noise (produced by circuit component)

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Noise Power**

Noise power is interfering and unwanted power in an electrical device or system. is defined as :

N (Watt) = KTB or N(dBm) = 10log(KTB/1mW)

K = Boltzmanns Constant (1.38 x10-23 J/K)T = Temperature in Kelvin (273+ ???Celcius)B = Signal Bandwidth

**Limitation in a Communication SystemThere are two categories of limitations:Technological constraintEquipment abilityEconomy and cost factorNational and international law and agreement as well as standardization (such as ITU etc)Interaction with existing systemPhysical constraint BandwidthThe difference between the upper frequency and lower frequency of the signal or the equipment operation range NoiseAny unwanted electrical energy present in the usable passband of a communication circuit

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