EXPLAIN THE EFFECTS OF THE EARTH’S EARTH ’S ATMOSPHERE ON SPACE WAVE RADIO
SIGNAL Effects: absorption, refraction -radio propagation is the behavior of radio waves when they are transmitted or propagated from one point on the earth to another or into various parts of the atmosphere -radio waves are affected by reflection, re flection, refraction, diffraction, absorption, polarization and scattering
Absorption -when a radio waves reaches an obstacle some of its energy is absorbed and converted into another kind of energy while another part is attenuated and continues to propagate and another part may be reflected Refraction -the direction of an electromagnetic wave changes as it moves from an area of one refractive index to another -the angle of incidence and the angle of refraction are linked by Snell’s law -refraction is caused by an abrupt change in velocity of the upper part of a radio wave as it strikes or enters a new medium
EXPLAIN WHAT IS MEANT BY DUCTING AND ITS CAUSES -atmospheric ducting is a mode of propagation of elect romagnetic radiation, usually in the lower layers of the earth’s atmosphere where the waves are bent by atmospheric refraction
DESCRIBE THE THREE BASIC CAUSES OF FADING OF RADIO SIGNALS ON THE VHF BANDS AND ABOVE Causes: absorption, free space loss, multipath (Rayleigh) Absorption -in the atmosphere there are some atoms and molecules so when the signal get s against those atoms and molecules it will lose energy due to atoms and molecules absorption Free Space Loss (Fig 1) -free space loss is often defines de fines as the loss caused by an electromagnetic wave as it propagates on a straight line through a vacuum with no absorption or reflec tion of energy from nearby objects -the major loss of energy is because of the spreading out of the wave front as it travels from the transmitter -as the distance increases the area ar ea of the wave front spreads out, o ut, much like the beam of flash light -this means that the amount of energy contained within any unit area on the wave front decreases as distance increases Multi-path (Rayleigh) (Fig 2) -multi-path are usually overhead, although ground reflections can occasionally be a factor
-multi-path fading has a tendency to build up during night time hours w ith a peak in the early morning and then disappear as convection currents caused by heat produced during the early daylight hours breakup the layers -using frequency or space diversity can generally m inimize the effects of multipath fading -in Rayleigh environment there is no dominant path -all of the received e nergy is low power reflected signals, this leads to fast fading which can lead to severe attenuation of the received rece ived signal up to 40dB when compared to non-fading channel
EXPLAIN WHAT IS MEANT BY THE TERM DIFFRACTION D IFFRACTION OF SPACE WAVE RADIO SIGNALS AND ITS CAUSES (Fig 3) Diffraction -a phenomenon whereby a wave travelling in a straight path bends around an obstacle Causes -obstruction causes diffraction as electromagnetic waves are diffracted over the top and around the sides of an obstruction -the direct wave form that just get by the obstruction become new sources of wave fronts that start filling in the void, making the shadow zone a finite e ntity -the lower the frequency of the wave the quicker is the process of diffraction i.e. smaller size
DESCRIBE THE FACTORS AFFECTING THE PROPAGATION OF RADIO SIGNALS INSIDE BUILDINGS Frequency -high frequencies reflect of buildings and result in multiple reflections c ausing multipath fading -high frequencies penetrate through building walls and result in losses -loss increases with frequency and polarization Building structures -penetration losses depend on wall thickness and building materials used -penetration loss is the difference betwee n the average signal level outside building and inside e.g. steel framed, office block, factories -height of building floor elevation average first floor penetration loss is 10,4 dB and decrease by 1,9dB per floor Location of building -average penetration loss in a building in urban areas is 5 dB greater than in rural area (same floor) Climate/ Weather -rain, fog, etc. affect internal inte rnal building signal propagation
EXPLAIN THE IMPORTANCE OF CARRIER TO INTERFERENCE (C/I) IN RADIO COMMUNICATION SYSTEMS
-a measure of how much our signal is going to be affected after passing through the devices in radio communication -the ratio of power in an RF carrier to the interference power in the channel -C/R resembles the carrier to noise ratio which is the signal to noise ratio of a modulated signal before demodulation -the C/R is very important in cellular systems in order to determine the maximum allowed interference level for which the system will still work -It defines the level of interference
EXPLAIN THE CAUSES OF INTERFERENCE TO RADIO SIGNALS AT VHF AND ABOVE Causes: Co-channel, Adjacent channel, Intermodulation Co-channel interference -co-channel interference is caused by an unwanted signal at the same frequency as the wanted signal -in a VHF/UHF frequency modulation receiver co-c hannel interference is not important since it is eliminated by the capture effect Adjacent channel interference -occurs when transmission from adjacent channels (channels next to one another in the frequency domain) interfere with each other -adjacent channel interference result from important filters in receivers that allow nearby fre quencies that allow nearby frequencies to enter the receiver Intermodulation interference -the undesired combining of several signals in a non-linear device, producing new unwanted frequencies which can cause interference in adjacent receivers located at repeater sites -when two or more signals at frequencies f1 and f2 are applied to a non-linear characteristic they will generate intermodulation products
EXPLAIN THE NEED FOR EMPLOYING DIVERSITY RECEPTION TECHNIQUES IN RADIO SYSTEMS -the need of using diversity is to increase the reliability of the system by increasing its availability -e.g. a reliability percentage of 99.99% co rresponds to about 53 minutes of outage time per year while a reliability percentage of 99.9999% amounts to only about 32 seconds of outage time per year
DESCRIBE THE FOLLOWING METHODS OF RECEPTION 1. Frequency Diversity 2. Space Diversity 3. Polarization Diversity Frequency Diversity -it is simply modulating two different RF carrier frequencies with the same IF intelligence the n transmitting both RF signal to a given destination -at the destination both carriers are demodulated and the one that yields the better quality IF signal is selected
-frequency diversity arrangements provide complete and simplex equipment redundancy, and have the additional advantage of providing two complete transmitter to receiver e lectrical paths -its advantage is that it doubles the amount of frequency spectrum and equipment necessary Space diversity -with space diversity the output of a transmitter is fed to two or more antennae that are physically separated by an appreciable number of wavelength -similarly at the receiving end, there may be more than one antennae providing the input signal to the receiver -if multiple receiving antennae are used, they must also be separated by an appreciable number of wavelength Polarization diversity -with polarization diversity a single RF carrier is propagated with two different electromagnetic polarizations (vertical and horizontal) -electromagnetic waves of different polarization do not nece ssarily experience the same transmission impairments -polarization diversity is generally used in conjunction with space diversity -one transmit/receive antenna pair is vertically polarized and the other is horizontally -it is also possible to use frequency, space and polarization diversity simultaneously
EXPLAIN WHAT IS MEANT BY ANTENNA GAIN -a measure of how much more power in dB an antenna will radiate in a ce rtain direction with respect to that which would be radiated by a reference antenna that is an isotropic point source or dipole -antenna gain describes how much power is t ransmitted in the direction of peak radiation to that of an isotropic source -an antenna with a gain of 3db means that the power received far from the antenna will be 3db higher than what would be received from a lossless isotropic antenna with the same input power -it is a measure of its directional properties and indicates the extent to which radiation is c oncentrated in particular direction or the exte nt to which the antenna receives signals bette r from one direction than from all others -antenna gain is defined relative to a reference antenna and is the same whet her the aerial is used for transmission or reception, but may be defined in terms of either
EXPLAIN WHAT IS MEANT BY THE BEAMWIDTH OF AN ANTENNA (Fig 4) -the beam-width of an antenna is the angle subtended by the 3db points on the major lobe of the radiation pattern -the angular separation between the half power (3dB) points on an antennas radiation pattern -the beam-width of an antenna is the angular w idth of a beam within which the radiation exceeds some specified fraction of the maximum value
EXPLAIN WHAT IS MEANT BY THE EFFECTIVE LENGTH OF AN ANTENNA
-the effective length leff of an antenna is that length which if it is carried a uniform current at the same amplitude as the input current to the antenna would produce some field strength at a given point in the equatorial plane of the antenna -this means that the product of the physical length of the antenna and the mean current flowing in the antenna must be equal to the product of the effective length and the assumed uniform current
EXPLAIN WHAT IS MEANT BY THE RADIATION RESISTANCE OF AN ANTENNA -the portion of an antenna’s input impedance that is the result of power radiated into space is called the radiation resistance Rr -it is the ratio of power radiated by the antenna to the square of the current at the feed point Rr=P/^2 -it should be noted that Rr is not the resistance of the conductors that form the antenna. It is simply an effective resistance that is related to the power radiated by the antenna -the power radiated from an antenna is dissipated in fictitious resistance known as the radiation resistance -power radiated = I^2Rr where I is the current fed into the antenna and Rr is the radiation resistance -power radiated from an antenna is always less t han the power fed into it because some power is lost in the antenna
EXPLAIN WHAT IS MEANT BY EFFECTIVE ISOTROPIC RADIATED POWER (EIRP) -it is a product of radio frequency transmit power and its antenna gain -EIRP is referred to as an isotropic point of source -the difference between ERP and EIRP is 2dB: ERP=EIRP – 2dB -EIRP is equivalent to transmit power that an isotropic antenna would have to radiate to achieve the same power density in the chosen direction at a given point as another antenna
EXPLAIN THE OPERATION OF PARABOLIC DISH ANTENNA WHEN USED ON ULTRA HIGH FREQUENCY (UHF) BANDS AND ABOVE -the parabolic dish antenna is essentially a large metal dish that is illuminated by a source of spherical wave front and which converts the incident radiation into a wave having a plane wave front -the feed point is placed at t he focal point of the dish, energy is radiated from the feed point with a spherical wave front and is directed on to the dish -the geometry of the dish is such that the distance from the focal point to the dish and then to an arbitrary plane X on the other side of the focal point is a constant, regardless of which point on the surface of the dish is considered. Thus the distance RAX=RBX=RCX=RDX=RFX -this ensures that the spherical wave front produced by the feed point is converted into a plane wave front at the plane X -the reflected waves are all parallel with one another and form a highly directive radio wave
DETERMINE THE GAIN OF A DISH ANTENNA GIVEN THE DISH DIAMETER AND FREQUENCY (OR WAVELENGTH OF OPERATION) (FIG 5)
EXPLAIN WHAT IS MEANT BY THE EFFECTIVE APARTURE OF A PARABOLIC DISH ANTENNA -it is the effective radio frequency plane wave front collecting area that is effective signal aperture are Ac = G lambda squared/ 4pi where G = power gain contained and lambda is wavelength
EXPLAIN WHAT IS MEANT BY FREE SPACE PATH LOSS AND CALCULATE THE LOSS GIVEN THE DISTANCE AND THE FREQUENCY OR WAVELENGTH OF OPERATION (FIG 6) -free space path loss is often defined as the loss incurred by an electromagnetic wav e as it propagates in a straight line through a vacuum with no absorption or reflection of energy from nearby objec ts
EXPLAIN THE NEED FOR CARRIER FREQUENCY STABILITY IN RADIO TRANSMITTERS -there is need for carrier frequency stability in radio transmitters to avoid inter ference with adjacent (in frequency) channels -in the case of SSB and ISB systems the suppressed carriers must be reinserted at the receiver with correct frequency -this requirement will clearly be made harder if the carrier frequency at the transmitter is not constant -that any frequency drift should be very small and t he maximum allowance change in frequency is normally specified as so many parts per million
DESCRIBE FREQUENCY SYNTHESISERS AND THEIR APPLICATION IN RADIO TRANSMITTERS AND RECEIVERS (FIG 7) -a frequency synthesizer is a circuit that drives a large number of discrete frequencies singly or simultaneously from an accurate high stability crystal oscillator source -each of the derived frequencies has the accuracy and the stability of the reference source -the two basic methods are direct and indirect -a frequency synthesizer must be able to cover a wide frequency band so that the receiver can work over the whole of the turning range -direct output frequency is obtained by mixing, multiplication and filtering -indirect output frequency is obtained from a VCO w hose accuracy is maintained by a phase locking oscillator to a standard frequency
EXPLAIN THE FUNCTIONS OF A FINAL STAGE AMPLIFIER IN A RADIO TRANSMITTER -it must develop the r.f. power output that the transmitter is to produce and must do so with the maximum possible efficiency -the selectivity of the stage must be such that the side frequencies of the signal are transmitted but unwanted harmonics are not
-the output stage should be operated linearly and should not be proof to unwanted oscillators known as parasitic oscillators -the stage must be easy to tune to the operating frequencies and to couple with optimum efficiency to the antenna feeder
DESCRIBE THE DIFFERENCE BETWEEN LOW LEVEL AND HIGH LEVEL MODULATION (FIG 8) -the modulation process is carried out at t he final amplifier stage where the carrier as at its final power level -high level modulation offers an advantage in that high efficiency class C amplifier stage can be used throughout the high frequency part of the transmitter -disadvantage associated with high level modulation is in the high audio frequency power which the class B stage must supply to adequately modulate the output stage which demand an expensive and physically large circuit -modulation process can be carried out at a low level stage and then the required power level
SKETCH THE BLOCK DIAGRAM OF A SIMPLE AM TRANSMITTER AND DESCRIBE THE FUNCTION OF EACH STAGE (FIG 9) -RF oscillator generates radio frequency carrier wave -Buffer amplifier – low gain, high input impedance linear amplifier used to isolate the oscillator from high power amplifier because they provide constant load to t he oscillator from high power amplifier because they provide constant load to the oscillator and hence reduce short term variations -Carrier drive – amplifies the carrier to a level that is adequate to the modulator -Modulator – used to modulate the carrier wave through enabling beating together of audio frequency and radio frequency -Linear power amplifier – raises radio frequency to the adequate and specifies the r.f. for transmission -Coupling network – feeder cable which converts the transmitter to the antenna, for max imum transfer must be impedance matched
SKETCH THE BLOCK DIAGRAM OF A SIMPLE FM TRANSMITTER AND DESCRIBE THE FUNTION OF EACH STAGE (FIG 10) -Amplitude limiter – to remove any amplitude modulation introduced by the modulator -Frequency multiplier – is a stage that increases or multiplies the frequency of the already g enerated signals -Class B or C (tuned power amplifier) – it develops the power output that the transmitter must produce (amplifies the signal to the specified output power) -Automatic Frequency Control – is applied to increase the stability of the signal since the inherent frequency stability of an LC oscillator is inadequate -Frequency Modulator – converts an input signal voltage into an output frequency change
EXPLAIN THE METHODS OF ELIMINATING HARMONICS IN RADIO TRANSMITTERS
-to eliminate harmonics in radio transmitters the anode or collect output circuit must be tuned to resonant at the required frequency of operation -the circuit must possess sufficient selectivity so to be able to discriminate against the side frequencies of the signal -these requirements are satisfied by using an inductor of high quality factor and a low pass capacitor -in addition the tuned circuit must provide a suitable load impedance to the transmitter amplifier and must also transfer maximum power to the load i.e. feeder cable for the antenna -to further suppress harmonics in radio transmitter, filter circuit must be employed calculated and designed to offer maximum attenuation or impedance to all frequencies outside the modulated radio signal bandwidth
DESCRIBE THE NEED FOR PRE-EMPHASIS IN FREQUENCY MODULATED TRANSMITTERS (FIG 11) -to provide a more satisfactory approach to the efficient use of the allowed frequency band is based on pre-emphasis in the transmitter -it allows the low frequency and high fre quency portions of the power spectral density of the m essage are equalized in such a way that the message fully occupies the frequency bond allotted to it, before the noise is introduced in the receiver -an improvement in the output SNR of the FM system can be achieved if the modulating signal is preemphasized -signal is passed through the pre-emphasis network which amplifies the high frequencies components of the signal more than the frequency components
EXPLAIN WHY THE CLASS OF BIAS USED IS AN IMPORTANT FACTOR IN A RADIO TRANSMITTER -it is the one that determines the efficiency load, matching network driving buffering or modulating function of the transmitter components -the function of the amplifier stage in a radio transmitter and what the function calls for helps determine the type of class bias that is whether it is class A, B, C buffer amplifiers -the class of bias is such an important factor the bias determines whether the amplifier can be used as a final stage power amplifier e.g. class A o r C use pre-amplifier (class A) -it determines the efficiency level of t he amplifier e.g. class B maximum efficiency is about 78.5% class 80-90% class A 25% -in addition the class of bias in a radio tr ansmitter is important, it gives a guideline as to whether the amplifier can be used for high power output or low power output -biasing of amplifiers is also important as it is used to produce amplifiers relevant or desired input or output impedance for example emitter followers which have very high input impedance and low output impedance are used as buffer amplifier with low gain or unity gain provides buffering between oscillator and carrier drive amplifier
JUSTIFY THE USE OF CLASS C BIAS FOR MOBILE RADIO TRANSMITTER -save battery since amplifier is on for robust duration and hence battery charge last longer -high efficiency 78% and 90%
-responds rapidly to frequency change, since in mobile communication systems frequency change rapidly and readily -low output power for example 20watts, 100watts
EVALUATE THE REQUIREMENT FOR POWER LEVEL CONTAINED IN RADIO TRANSMITTERS WHICH ARE: FIXED POSITION, MOBILE AND HAND HELD Fixed Position -power level control is required in fixed position transmitter to ensure signals get to rece ivers at a convenient level -if received too high by the receiver it will result in distortion such as clipping of signals, if too low it might strain the AGC -power level control also helps to limit co -channel interference as well as adjacent channel inte rference -efficient to transmit at appropriate level of power depending on the size of service ar ea, atmospheric conditions -too high signals may be clipped in receiver -improve signal to noise ratio Mobile transmitter -to counter path loss as distance to receiver changes -to maintain minimum acceptable SNR -minimize effect on the battery AGC -environmental and ground conditions will continuously change this means that factors affecting radio wave propagation will be continuously changing, this translates into the need to c ontinually control transmitter power levels Handheld transmitter -helps to control or reduce power drawn from the battery -maintain minimum acceptable SNR -radio signals from handheld transmitter are going to be affecte d by number of propagation characteristics which need to be solved by implementing power level control
EXPLAIN WHY THE DUAL CONVERSION SUPERHETERODYNE PROCESS IS NORMALLY USED FOR COMMUNICATION RECEIVERS DESIGNED FOR USE ON THE HIGH FREQUENCY BANDS AND ABOVE -for good image frequency rejection a relatively high IF is selected -however for high gain selective IF amplifiers that are easily neutralized, a low IF is more desirable -the solution is to use two intermediate frequencies the first IF is relatively high for good image frequency rejection and the second IF is relatively low for easy amplification -the first image frequency could be 1 0,625MHz for instance which pushes image frequency 21,25MHz away from the desired RF -the first IF is immediately down converted to 455 KHz and fed to a series of high gain IF amplifier -the dual conversion super-heterodyne process is normally used because the first IF can give a wide separation between the wanted signal frequency and the usage channel signal frequency -the second IF can be low making good adjacent channel selectivity easier to obtain
SKETCH THE BLOCK SYSTEM DIAGRAM OF A DOUBLE SUPERHETERODYNE RECEIVER DESIGNED TO RECEIVE AMPLITUDE MODULATED SIGNALS AND EXPLAIN THE FUNCTION OF EACH STAGE (FIG 12) Dual conversion AM super-heterodyne receiver -RF amplifier – determines sensitivity of the receiver, however bec ause the RF amplifier is the first active device encountered by a received signal it is the primary contributor of noise -First detector stage – includes an RF oscillator and a mixer converter as the first detector stage to provide the first IF which is r elatively high in MHz for good image frequency rej ection IF = Fo – Fs -Second detector stage – down converts the first IF to the second IF which is fed to IF amplifiers through IF filters -IF amplifier stage – provides gain and selectivity is achieved in it -Audio detector – converts the IF enveloped back to original source information, the audio detector can be as simple as a diode or as complex as a phase locked loop or balanced demodulator -Audio amplifier stage – amplifies the detected output to provide sufficient power to operate the loudspeaker
SKETCH THE BLOCK SYSTEM DIAGRAM OF A DOUBLE SUPERHETERODYNE RADIO RECEIVER DESIGNED TO RECEIVE FREQUENCY MODULATED SIGNALS AND EXPLAIN THE FUNCTION OF EACH STAGE (FIG 13) -Audio detector stage – in the FM receiver system is made up of the limiter, frequency discrimination and de-emphasis stage -Limiter – clips off or removes the amplitude peak variation due to noise or unwanted signal amplitude modulation due to non-linearity of the proceeding stage -Discriminator or frequency discriminator – a frequency dependent circuit used to produce an output voltage that is directly proportional to the instantaneous frequency at its input
EVALUATE THE MAJOR DIFFERENCES IN THE DESIGN OF THE RECEIVERS ABOVE -the receiver used to receive AM signals are essentially identical to those used for FM, except for the method used to extract the audio information from the composite IF waveform -the major differences are in the inclusion of frequency dependent circuits in the audio detector stage and these include limiter, discriminator and de-emphasis stages -amplification, the r.f. stage in an AM receiver may not include amplification whereas for FM receiver it always provides gain
EXPLAIN WHAT IS MEANT BY NOISE FIGURE IN COMMUNICATION RECEIVERS -noise figure is essentially a measure of t he gain of the amplifier to the noise added by the amplifier -therefore the noise figure is improved e ither by reducing the internal noise or by incre asing amplifier gain -the noise figure F is the ratio of the S/N ratio at the input to a device, to S/N ratio at the output
F = input S/N ratio Output S/N ratio
CALCULATE THE NOISE FIGURE FOR A CASCADED SERIES OF STAGES GIVEN THE NOISE FIGURE AND POWER GAINS FOR THE INDIVIDUAL STAGES (FIG 14) -when two or more amplifiers or devices are cascaded together as shown on fig 14 the total noise figure is an accumulation of the individual noise figure
EXPLAIN WHY IT IS IMPORTANT THAT THE FIRST STAGE OF A HIGHLY SENSITIVE RADIO RECEIVER IS DESIGNED FOR LOW INTERNAL NOISE PROPERTIES -it can be seen that the noise figure of the first amplifier (F1) contributes the most toward the o verall noise figure -this is because the noise introduced in the first stage is amplified by each of the succe eding amplifier -therefore it is important that the first stage of sensitive radio receivers be designed to be low internal noise properties that is very low noise generation and very high gain (A1) -when compared to the noise introduced in the first stage the noise added by e ach succeeding amplifier is effectively reduced by a factor equal to the product of the gains of the proceeding stages
EXPLAIN WHY IT IS IMPORTANT THAT THE NOISE FIGURE SHOULD BE AS LOW AS POSSIBLE -the lowest figure is 1 or 0 dB produced by a noiseless device -the higher the noise figure, the worse the signal to noise ratio at the output -so it is important to keep the noise factor as low as possible as close to 0dB as possible that is theoretically it should be 0dB -this is because it means signal degradation by noise is not going to be any worse due to passing through other devices such as amplifiers so noise figure is a measure of how much noise other devices through which the signal passes will contribute to degradation of t he quality of the signal with respect to noise -basically noise figure indicates the relative increase of t he noise power to increase in signal power -because at higher frequencies the antenna noise is much smaller than the internally generated noise thus the signal is less degraded if the noise figure is small
EVALUATE THE NEED FOR THE FOLLOWING SUBSYSTEMS IN COMMUNICATION RECEIVERS Squelch -when the carrier is not present at the input of the receiver the AGC disappears, the receiver sensitively becomes maximum and it amplifies any noise present at t he input -squelch is a device which cuts off the receiver output in the absence of car rier signal Noise limiters -these circuits are needed to remove any amplitude variations (produced by noise or interference) from the FM wave
-basically it acts as a slicer or clipper limiter to cut off both the positive and negative peaks of the input wave at some predetermined level keeping the amplitude of the output constant -it is a circuit where output is a constant amplitude for all inputs above a critical value Automatic Gain Control (AGC) (Fig 15) -AGC is applied to a receiver to maintain the carrier level at the detector input at a more or less constant value -as the amplitude of the wanted car rier that appears at the input of a radio receiver may fluctuate widely by perhaps 100dB or more De-emphasis -to perform the inverse operation of t he pre-emphasis by de-emphasizing the high frequency components so as to restore the original signal-power distribution of the message -in such a process the high frequency component of the noise at the discriminator output are also reduced, thereby effectively increasing the o utput signal to noise ratio of the system -de-emphasis is needed to distort the signal and it will be necessary at the receive to restore the various components of the signal to their o riginal amplitude relationships (Fig 16)
EXPLAIN THE NEED FOR DIPLEXERS IN RADIO TRANSCEIVERS AND HOW DUPLEX OPERATION MAY BE ACHIEVED WITH THE AID OF DIPLEXERS -reduces the number of antennas -eliminates the need for extra wiring
EXPLAIN WHAT IS MEANT BY SPREAD SPECTRUM ALSO KNOWN AS CODE DIVISION MULTIPLE ACCESS TECHNIQUES IN RADIO COMMUNICATION -spread spectrum (CDMA) system is a system that does not follow the channelization principles of traditional cellular radio communication systems -rather than dividing the allocated frequency spectrum into narrow bandwidth channels, one of each user information is transmitted (spread) over a very wide fre quency spectrum with as many as 20 mobile subscriber units simultaneously using the same carrier frequency within the same frequency band -spread spectrum is a technique whereby a modulated wave form is modulated spread a second time in such a way as to generate an expanded bandwidth wideband signal that does not significantly interfere with other signals -bandwidth expansion is achieved by a second modulation means that is independent of the information message
DESCRIBE THE FOLLOWING FORMS OF SPREAD SPECTRUM TECHNIQUES FREQUENCY, TIME HOPPINGS AND DIRECT SEQUENCE Frequency hopping -frequency hopping spread spectrum was first used by the military to ensure reliable anti-jam and to secure communications in a battlefield environment
-the fundamental concept of frequency hopping is to break a message into fixed size blocks of data which each block transmitted in sequence except on a different carrier frequency -with frequency hopping pseudorandom code is used to generate a unique frequency hopping sequence -the sequence in which the frequencies are selected must be known by both the transmitter and the receiver prior to the beginning of the transmission -the transmitter sends a block on a radio frequency carrier and then switches (hops) to the next frequency in the sequence and so on -after reception of a block of data on one frequency the receiver switches to the next frequency in the sequence -each transmitter in the system has a different hoping sequence to prevent a subscriber from interfering with transmissions from other subscribers using the same radio channel fre quency Time hopping (Fig 17) -it is whereby some frequency allocated for the service are divided into a number o f timeslots and channel is jumped from one timeslot to another -if the codes act as an on/off gate to the transmitted signal it is called time hopping spread spectrum frequency Direct sequence spread spectrum (Fig 18) -in direct sequence system, a high bit rate pseudorandom code is added to a low bit rate information signal to generate a high bit rate pseudorandom signal closely resembling noise that contains both the original data signal and pseudorandom code -again before successful transmission the pseudorandom code must be known to both the transmitter and the internal receiver -when a receiver detects a direct sequence transmission it simply subtracts the pseudorandom signal from the composite receive signal to extra information data
EXPLAIN THAT INTERNATIONAL MOBILE TELECOMMUNICATION – 2000 (IMT – 2000) WAS THE ITU VISION OF A GLOBAL FEDERATION OF COMPATIBLE FIXED AND MOBILE TELECOMMUNICATION SYSTEMS -the number 2000 stood for tree things 1. The year it was supposed to go into service 2. The frequency it was supposed to o perate at (MHz) 3. The bandwidth the service should have (KHz) -the basic services that the IMT 2000 network is supposed to provide to its users are 1. High quality voice transmission 2. Messaging (replacing e-mail, fax, sms, chat, e tc.) 3. Multimedia (playing music, viewing video film television) 4. Internet access (web surfing, including pages with video)
EXPLAIN WHY THE INTRODUCTION OF PACKET SWITCHING TECHNIQUES IN THE FORM OF THE GENERAL PACKET RADIO SERVICE (GPRS) INTO EXISTING GSM NETWORKS CAN BE REGARDED AS THE FIRST STEP IN THE EVOLUTION TOWARDS THIRD GENERATION 3G NETWORK
-because GPRS is packet switched which supports point to point (PTP) service (IP and X25 networks) -multiple users share the same transmission channel theoretical limit is 160Kbps (using 8 time slots) -GPRS traffic is second priority tr affic -GPRS traffic uses its own channel -GPRS provides quality of service -asymmetric channels are possible -user is charged for the total packets he sent/ received -latency is very high (a round trip ping being t ypically above 600-700ms and often reading one second round trip time)
DESCRIBE THE IMPLEMENTATION OF ENHANCED DATA RATES FOR GSM EVOLUTION (EDGE) AS A LEGITIMATE ROUTE TO THE PROVISION OF 3G SERVICES FOR THOSE EXISTING TDMA NETWORK OPERATORS WHO DID NOT SECURE ADDITIONAL 3G SPECTRUM LICENCES -EDGE is a technology at the air interface to GSM, which provides an improved modulation scheme and improves spectrum efficiency -EDGE uses 8 phase shift keying rather than standard GSM Gaussian Minimum Shift Keying -this has the effect of increasing data rates with rates as high as 384Kbps in a 200 KHz GSM carrier (4 time slot GPRS operation) -EDGE can deliver 3G mobile multimedia services using existing frequencies, bandwidth and carrier structure (no core network challenges)
EXPLAIN WHY CODE DIVISION MULTIPLE ACCESS (CDMA) SYSTEMS WERE THE DOMINANT CHOICE OF THE PROPOSALS SUBMITTED TO THE ITU -in this scheme each user is assigned a unique pseudonoise (PN) code -CDMA systems provide much larger capacity -more efficient utilization of the spectrum -better speech quality using low bit rate linear prediction codes -larger bandwidth per channel -handoff strategies used in a CDMA system provide for a better coverage and lead to an improvement in the system performance -more efficient and dynamic power controls which limit interferenc e
STATE THAT WCDMA WAS THE EUROPEAN PROPOSAL FOR THE IMT-2000 AIR INTERFACE -WCDMA was proposed by Ericson -this system uses direct sequence spread spec trum -it runs in a 5 MHz bandwidth and has been designed to interwork with GSM networks although it is not backward compatible with GSM
DESCRIBE HOW IN CDMA DIRECT SEQUENCE (CDMA-DS) NARROW BAND INFORMATION SIGNALS ARE MULTIPLIED BY UNIQUE HIGH SPEED CODES EFFECTIVELY SPREADING THE SIGNAL BANDWIDTH -this is one where user information bits are spread over a wide bandwidth by multiplying this information signal (user data) directly with CDMA spreading codes comprised with chips -the rate of change of these chips must be higher than the user data rate and this impose their rate on the resulting signal -this resulting signal is therefore more complex and will use up more bandwidth
DEFINE THE CONCEPT OF ORTHOGONALITY AS APPLIED TO THE ORTHOGONAL VARIABLE SPREADING FACTOR (OVSF) CODES USED IN THE UMTS -OVSF is an implementation of CDMA where before each signal is transmitted the spectrum is spread through the user by user code -these codes are derived from an OVSF code tree and each user is given a different unique code -an OVSF code tree is a complete binary tree that reflects the construction hard wire metal -are channelization codes which spread signal and user data -these codes are inherently more tolerant of interference caused by multiple users -various physical channels may exist at any time on a radio interface of 3g systems e.g. at a mobile station there may be one or more physical channels, a dedicated physical common control channel, a physical random access channel and a physical common packet channel -codes are mutually orthogonal and may spread each physical channel by variable spreading factor
DESCRIBE HOW A CODE TREE CAN BE USED TO GENERATE MULTIPLE OVSF CODES AND THE LIMITATION IMPOSED BY THE USE OF HIGHER ORDER CODES (FIG 19) EXPLAIN THE PRINCIPLE BEHIND MULTIPLE CODE TRANSMISSION AND RECEPTION (FIG 20) Scrambling Codes -to differentiate signals from different sources at t he transmission side and at the receiver, the reverse process occurs, with the signals first de -spread using scrambling code and then combined with the channelization codes Channelization code -different transmissions from a single source (mobile station in the uplink or Node B on the downlink) are separated by channelization codes
EXPLAIN THE REASONS FOR SCRAMBLING IN THE UMTS AIR INTERFACE -on the uplink, scrambling codes are used to isolate the different user accessing the same cell -on the downlink, scrambling codes are used to prevent co-channel interference from adjacent cells
DEFINE AND DIFFERENTIATE BETWEEN THE CONCEPT OF SPREADING FACTOR AND PROCESSING GAIN (FIG 21) Spreading Factor -the ratio of the chip rate to the data rate (chip rate is the clock rate of the spreading code) Processing gain -it is defined as the gain in signal to noise ratio SNR obtained by the use of spread spect rum, specifically we write PG =Tb/ Tc which represents the gain archived by processing a spread spectrum signal over an unspread signal -in a WCDMA system the data rate is 30 Kbps and the chip rate 3 .84 Mbps
EXPLAIN HOW MULTIPLE PATH PROPAGATION CAN BE USED TO ADVANTAGE IN CDMA SYSTEMS -multipath propagation reduced fading with w ider bandwidth -multipath suggest that this is more than one tr ansmission available between transmitter and the receiver which enables the system to close the transmission which yield better quality -multipath diversity increases reliability of system by increasing its availability
DESCRIBE THE OPERATION OF RADIO ACTIVATED KEY ENTRY (RAKE) RECEIVER (FIG 22) -the RAKE receiver utilizes multiple correlation recovers (rake fingers) which are dynamically allocated to de-spread the signal at those delay positions where significant energy arrives -in each finger the received signal is de-spread using a spreading code which is time aligned w ith the delay of the required multipath signal -at each finger other multipath which arrive more than the chip duration delayed either side will simply appear as noise and so will be suppressed from the decoded signal -the resulting fading compensated and de-spread signals derived from each finger are then combined to obtain a single output from the RAKE receiver thus obtaining multipath diversity
EXPLAIN WHY THERE IS A NEED FOR RIGID POWER CONTROL IN CDMA AND HOW IT IS IMPLEMENTED IN THE UMTS AIR INTERFACE (FIG 23) (FIG 24) -power control is required in the uplink to counteract the near-far effect -this involves reducing multiple access interference through the e qualization of all the various receive powers -power control is needed because without this capability the system will not work efficiently and attain its full capacity Open loop power control -it simply measures the interference conditions within a channel and adjusts t ransmission power accordingly based on parameters broadcast by the cell
Outer loop power control -outer loop control is usually implemented within the Radio network Controller(RNC) by means of performing checks on some form of frame quality indicator which is tagged along with the user data to indicate when error rates become unacceptable
EXPLAIN WHY ERROR PROTECTION USED IN THE UMTS AIR INTERFACE -like in all radio systems the link reliability is subjected to various disturbances that exist in the atmosphere with fading radio frequency interference -these can cause serious duko losses -so to improve the system performance against the random disturbances and the inherent error s due to other short coming of the air interface protection schemes are required
DESCRIBE THE DIFFERENT TYPES OF ERROR PROTECTION USED IN THE UMTS AIR INTERFACE Automatic Repeat Request (ARQ) -an error control method for data t ransmission that uses acknowledgements (messages sent by the receiver indicating that it has correc tly received a data frame or packet) and timeouts to achieve reliable data transmission over an unreliable service -types of ARQ protocol include 1. Stop and wait ARQ 2. Go-back-N ARQ 3. Selective repeat ARQ Forward Error Correction (FEC) -a technique used for controlling errors in data transmission over unreliable or noisy communication channels -types of FEC are block codes and convolutional codes Cyclic Redundancy Checks (CRC) -an error detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data -CRCs are so called because the check value is a redundancy and the algorithm is based on cyclic codes
DISCUSS UMTS NETWORK PLANNING MAKING REFERENCE TO THE FOLLOWING FACTORS 1. Simple Frequency re-use -the process in which the same set of frequencies (channel) can be allocated to more than one cell, provided the cells are separated by sufficient distance (efficient utilization of spectral) 2. OVSF code availability -as the codes can be used up so there must be the availability of the OVSF code all the time for efficiency of the system
3. Service availability/ QOS -is the one that makes sure that call duration, call success, busy hour rates, etc. are calculated to ensure service availability 4. Mutual Interference -channels using the same frequency change code in adjacent channel -mutual interference are selected because many cells disappear as source of interference -UMTS is an interference limited network in w hich mutual interference hinders the network capacity 5. Cell Breathing -refers to the effective expansion or contraction of a given cell due to sudden changes in the number of mobile users within the cell
DESCRIBE THE TYPES OF CHANNELS WHICH EXIST IN UMTS SYSTEMS Logical Channels -service access point between the Radio Link Control (RLC) and Media Access Control (MAC) define the logical channel -the set of logical channels is defined in or der to transmit each specific type of information that may be required for communication with the higher layer -logical channel therefore determines the band of information that will be used within it Transport Channel -service access point (SAP) between the medium access control (MAC) and the physical layer define the transport channel -there are two types of transport channel: dedicated transport channel (DCH) that are unique for each user and common transport channel that are share d by all users in a cell Physical Channel -service access point (SAP) between the physical layer and the actual transmission medium define the physical channels -each physical will have a specific transmission purpose and characteristic and it is these physical channels which are differential using channelization codes in the WCDMA spreading process
EXPLAIN HOW LOGICAL TRANSPORT AND PHYSICAL CHANNELS ARE MAPPED TOGETHER (FIG 25) DESCRIBE THE COMPOSITION OF THE FRAME STRUCTURES OF THE MAJOR PHYSICAL CAHNNELS IN UMTS (FIG 26) DESCRIBE THE AIR INTERFACE PROTOCOL STACKS FOR RELEASE 99 (R99) IN THE CIRCUIT SWITCHED (CS) DOMAIN AND THE PACKET SWITCHED (PS) DOMAIN FOR BOTH THE USER AND CONTROL PLANE (FIG 27)
EXPLAIN THE FUNCTIONALITY OF THE FOLLOWING PROTOCOLS Radio link Control -provides a reliable transmission of data blocks over the air interface using selective ARQ type procedure, where data blocks received in error are retransmitted by source Medium Access Control -determines how different types of information should be transmitted over a physical channel (medium) and controls the timing of those transmission Packet Data Convergence Protocol (PDCP) -helps to achieve the goal of inter working by adapting different network layer protocols to the RLC so that the user data can be transferred across the UTRAN transparently -improves the transmission for delay sensitive information such as voice or video. Does so by reducing the size of header fields within different compression/ decompression algorithms Broadcast/ Multicast Control (BMC) -responsible for transforming messages from the network that can be broadcast or multicast to all mobile stations in a cell -this entity on the network side per iodically estimate the volume of the cell broadcast traffic and forwards the information to the RRC layer using an indication primitive
WITH THE AID OF A DIAGRAM DESCRIBE THE COMPOSITION AND FUNCTIONALITY OF THE UMTS TERRESTRIAL RADIO ACCESS NETWORK (UTRAN) INCLUDING THE ASSOCIATED INTERFACES (FIG 28) EXPLAIN THE CONCEPT OF MAERO DIVERSITY AND THE ADVANTAGES IT PROVIDES (FIG 29) -maero diversity means that transmission power can be controlled according to the cell from which the mobile receives the greatest signal strength The advantages it provides -so the mobile will not rather wait until signal strength in an adjacent cell exce eds that of the current cell, the maero diversity state can be read when the signal strength of a neighbouring cell is still below that of the current cell yet exceeds a certain threshold -signal quality and handoff robustness is improved -maero diversity is used to move a call from one base station that is approaching congestion to another with better capacity
WITH THE AID OF A DIAGRAM DESCRIBE THE COMPOSITION AND FUNCTIONALITY OF R99 UMTS CORE NETWORK INCLUDING THE MAJOR INTERFACES (FIG 30)
DESCRIBE THE RESPONSIBILITIES OF THE CN Subscriber Connection -monitors the session, billing, disconnect user, connectivity, allocate resources Subscriber Management -managing subscribers, creating the subscribers identification authentication carrying out location updates, tracking subscriber, allocate services Network Management -observes and maintains quality of entire network -network management for base station subsystems BSS responsible for radio path control and that every call is connected through BSS -also include the management of network subsystems (NSS) which include MSC/ VLR/ GMSC within it
DESCRIBE THE RESPONSIBILITIES OF ALL R99 CN COMPONENTS Serving GPRS Support Node (SGSN) -provides GPRS services to a mobile station in the serving area of its associated mobile switching centr e (MSC) -locates MSCs subscribing to GPRS and add information to the HLR control access to the network by performing authentication Gateway GPRS Support Node (GGSN) -provides an interface between a GPRS network and an external network such as Packet Switched Public data Network (PSPDN) Mobile Switching Centre (MSC) -are high performance digital ISDN switches -they set up connections to other MSC and to the BSC via the A interface Authentication Centre (AuC) -as the radio interfaces and mobile station MS are particularly vulnerable, a separate AuC has been defined to protect user identity and data transmission Equipment Identity Register (EIR) -the EIR is the database for all NEs that is it stores all device identification registered for this network, as MS are mobile they can be easily stolen Home Location Register (HLR) -is the most important database in a UMTS system as it stores all user relevant information -this comprises static information such as the mobile subscriber ISDN (MSISDN) subscribed services and the authentication key Visitor location Register (VLR) -the VLR associated to each MSC is very dynamic database which stores all important information needed for the MS users currently in the LA that is associated to the MSC
-maintains base station subsystem (BSS) connections -mobility management
EXPLAIN THE ADVANTAGES AND DISADVANTAGES OF SEPARATE PACKET SWITCHED AND CIRCUIT SWITCHED DOMAINS IN THE R99 CN Advantages -voice is more prioritized than data hence combining -easy to migrate from GPRS to UMTS -decentralizing point of failure if one fails e.g. voice, data c an still work Disadvantages -it is costly because more resources are needed in establishing links -complex interfaces and more complicated network design
DESCRIBE THE GPRS TUNNELING PROTOCOL (GTP) AND THE INTERFACES WHERE IT IS IMPLEMENTED -this is the protocol that defines the process of adding addresses and protocol information to protocol data units so that they can be routed with PLMN or between PLMNs -tunneling is used to mean encapsulating an original packet with view header -GTP is used to transfer IP datagram between 2GTP-U end points in t he user plane in interface UMTS -interfaces where it is implemented 2 IUPs, Gn, Gp -MSC server is a soft switch variant of MSC which provides circuit switched calling, mobility management and GSM services to the mobile phone roaming within the area that it serves, MSC server functionality enables split between control control and user plane
DESCRIBE THE DIFFERENCES BETWEEN THE R99 PHASE 4 AND PHASE 5 UMTS NETWORK ARCHITECTURE (FIG 31) DESCRIBE THE SECURITY THREATS THAT EXIST IN UMTS AND HOW NETWORK OPERATORS CAN GUARD AGAINST THEM Unauthorized access to data -introducing secured pin numbers, complex encryption codes, authentication processes, frequency hopping use complicated large numbers Threats by integrity -by protecting the frame structure by implementing error checks, error detection, parity check, cyclic redundancy check (CRC), cell sequence Denial of service -it is whereby by the network will reject someone who fails to pin his number from the network Unauthorized access to resources -by implementing authentication methods
-network administrator checking for the resources being requested
STATE THE FACTORS TO BE TAKEN INTO ACCOUNT WHEN IMPLEMENTING A 3G NETWORK -financial restrictions -environmental/ health restrictions -licensing conditions -applications (varied and wanted) -learning from the experience of other operators -technical limitations
SKETCH THE BLOCK SYSTEM DIAGRAM OF A MICROWAVE RADIO (LINE OF SIGHT) REPEATER SYSTEM AND EXPLAIN THE FUNCTION OF EACH STAGE (FIG 32) -the diagrams show a typical layout for a m icrowave radio link -information originates and terminates at the terminal stations, whereas the r epeater simply relay the information to the next downlink microwave station -a microwave radio link comprises of two terminal stations where as the repeater simply relay the information to the next downlink microwave station -a microwave radio link comprises of two terminal stations on each end that are interconnected by three repeater stations -the microwave stations must be geographically placed in such a way that the terrain does not interfere with transmissions between stations
EXPLAIN THE IMPORTANCE OF THE CARRIER TO NOISE FREQUENCY -it is probably the most important parameter considered when evaluating the performance of a microwave communication system -it is a measure of the system’s performance in the presence of noise -this can be used to determine how much the signal is degraded by noise across t he wide carrier bandwidth
EXPLAIN THE IMPORATNCE OF THE ENERGY OF BIT TO NOISE RATIO (Eb/No) AND ITS RELEVANCE TO THE BIT ERROR RATE (BER) -Eb/No is one of the most important and most often used parameters when evaluating a digital radio system -Eb/No ratio is a convenient way to compare digital systems that use different transmission rates, modulation schemes or encoding t echniques. Mathematically Eb/No is Eb = C/Fb = CB No N/B NFb Its relevance to the Bit-error-rate (BER)
-the bit error rate (BER) is a function of the carrier-to-noise ratios the high the ratio the less likely we are to have noise induced bit errors -the lower it is the high the probability of noise induced errors -while the Eb/No ratio is used to estimate the bit error rate cannot be directly measured
CARRY OUT CALCULATIONS FOR THE ABOVE (FIG 33) EXPLAIN WHAT IS MEANT BY SYSTEM THRESHOLD IN RADIO SYSTEMS -the system threshold is the minimum signal or carrier power that is available at radio receiver input (e.g. LNA) if the signal is less than t his value then the system cannot perform satisfactorily in such a w ay as to produce usable baseband signal -the receiver threshold is dependant on the wideband noise power present at the input of the receiver
CARRY OUT THE CALCULATIONS FOR THE ABOVE (FIG 34) CALCULATE LINK BUDGET EQUATIONS FOR MICROWAVE RADIO RELAY SYSTEMS (FIG 35) -link equations are used to analyze a r adio link performance that is carrier power to noise density (No) ratio performance or a radio link
EXPLAIN WHAT IS MEANT BY THE TERM FADE MARGIN AND DESCRIBE ITS IMPORTANCE WHEN CARRYING OUT LINK BUDGETS -fade margin is essentially a “fade-factor” included in system gain equations that considers the non ideal and less predictable characteristics of radio wave propagation such as multipath propagation (multipath loss) and terrain sensitivity -fading margin is the margin by which fading changes from normal predictable fading -it is used as a safety value to protect signal against falling below threshold value. It is the difference between link budget and threshold value -fade margin could be due to building penetration as signal penetrates through building materials
EVALUATE THE ADVANTAGES AND DISADVANTAGES OF COMMUNICATING BY MEANS OF GEOSYNCHRONOUS SATELLITES RATHER THAN SATELLITES IN LOW EARTH ORBITS Advantages -Geosynchronous satellites remain almost stationary in respect to a given ear th station. Consequently expensive trading equipment is not required at t he earth stations -Geosynchronous satellites are available to all earth stations within their shadow 100% of the time. The shadow of a satellite includes all the earth stations that have a line of sight path to it and lie within the radiation pattern of the satellite antenna
-There is no need to switch from a geosynchronous satellite to another as they orbit ove rhead consequently there are no transmission breakers due to switching times -the effects of Doppler shifts are negligible Disadvantages -Geosynchronous satellites require sophisticated and heavy propulsion devices on board to keep them in a fixed orbit -High altitude geosynchronous satellite introduce much longer propagation delays. The round trip propagation delay between two earth stations through a geosynchronous satellite is between 500ms and 600ms -Geosynchronous satellites require higher transmit powers and more sensitive receivers because of the longer distances and greater path losses -High precision spacemanship is required to place a geosynchronous satellite into orbit and to ke ep it there
EXPLAIN WHY THE ANGLE OF ELEVATION IS AN IMPORTANT FACTOR IN SATELLITE COMMUNICATION -because the angle of elevation allows the e lectromagnetic wave to be radiated from the earth station antenna pointing directly toward a satellite and the horizontal plane -it also determines the distance in w hich the electromagnetic wave has to be propagated such as when the angle of elevation is smaller the greater the distance a propagated wave must pass through the earth’s atmosphere, if the angle of elevation is too small and the distance the electromagnetic w ave travels through the earth’s atmosphere is too long
EVALUATE THE REASONS FOR THE POPULARITY OF THE 6/4 GHz BANDS FOR SATELLITE COMMUNICATION PURPOSES -for the corresponding angles of elevation the 6/4 GHz bands has least attenuation due to atmospheric absorption that is in normal atmospheric conditions (deviation angles at 5 degrees and above 0.2dB of signal power is lost -the 14/12 GHz band is comparatively severely affected that is for 14/12 GHz with angle of elevation 5 degrees and above in normal atmospheric conditions attenuation less than 20% of signal power lost -6/4 GHz band are least attenuation in normal, rainy, fog atmospheric conditions -sky noise is low at 4GHz and hence it is possible to design and construct rec eive station system with lower noise temperature at 4GHz -6/4 GHz band offer minimum propagation problems
EXPLAIN WHAT IS MEANT BY EQUIVALENT NOISE TEMPERATURE IN SATELLITE COMMUNICATION SYSTEMS -equivalent noise temperature (ENT) is a hypothetical value that can be calculated but cannot be measured -ENT is often used rather than noise figure because it is a more accurate method of expressing the noise contributed by a device or a receiver when evaluating its performance
-essentially ENT represents the noise power present at the input to a device plus the noise added internally by that device
EXPLAIN THE IMPORTANCE OF THE G/Te RATION TO SATELLITE COMMUNICATION SYSTEMS -G/Te ratio in satellite communication systems is a figure of measurement used to represent the quality of a satellite or earth station receiver -the G/Te is a gain-to-equivalent noise temperature ratio of the receive antenna -the communication performance of the system is determined by the G/Te ratio that is the receive antenna gain G and the noise t emperature (Tr) expressed in dB G/Te =Ar + A(LNA) Te
DESCRIBE THE MULTIPLEXING AND MULTIPLE ACCESS ARRANGEMENTS FOR COMMUNICATION SATELLITES (FIG 36) -satellites multiple accessing implies that more than one user has access to more radio channels (transponders) within a satellite communication channel -the three most commonly used multiple accessing arrangements are FDMA, TDMA, CDMA Frequency Division Multiple Accessing (FDMA) -is a method of multiple accessing where a given RF bandwidth is divided into smaller frequency bands called subdivisions. Its subdivisions has its own IF carrier frequency -a control mechanism is used to ensure t hat two or more earth stations do not transmit in the same subdivision at the same time Time Division Multiple Accessing (TDMA) -TDMA is a method of time division multiplexing digitally modulated carriers between participating earth stations within a satellite network through a common satellite t ransponder -with TDMA each earth station transmit a short burst of a digitally modulated carrier during a precise time slot (epoch) within a TDMA frame Code Division Multiple Accessing (CDMA) -CDMA there are no restrictions on time and bandwidth each earth station transmitter may tr ansmit whenever it wishes and can use any or all the bandwidth allocated a particular satellite system or channel because there is no limitation on the bandwidth -CDMA is sometimes referred as a spread spectrum multiple access transmission can spread throughout the entire allocated bandwidth
DESCRIBE SATELLITE SUBSYSTEMS Attitude and orbit control -attitude stabilization and orbit determination -the satellite has to stay in one place in its geosynchronous orbit so that it appears stationary to its users, the spacecraft will need to be kept on station within an accuracy of +/-0.4 -this ensures that the users will never need to realign their antenna on the ground
-the satellite will always be within a square of 40miles sites (64km) -its position within that square is monitored continuously by COMSAT or owner who uses a large tracking and ranging station for that purpose -when the satellite is drifting towards one side o f the square under the influence of o rbital disturbances that it suffer the owner takes action to bring it back Telemetry tracking and command -telemetry involves measuring and sensing remote device sending the result or information over telecommunication networks -this action consists of sending command signals to electrically driven valves in the satellite which control the movement of the satellite -telemetry trading and command also involves exchanging housekeeping data with c ontrol centre, this information on number of channels and security of communication Power systems -supply of power (electric energy) at various voltage levels, this involves power and voltage regulation in the spacecraft -solar cells are used to generate the electrical power requirements Communication subsystem -consists of BPF, LNA, frequency down converter and LPF and power amplifier -BPF selects UL rf channel from the transponder bandwidth -LNA parametric amplifier offer s signal pre-amplification that is achieves amplification -down converter is a mixer which with the aid of MIN shift oscillator down converts the uplink (UL) frequency to the down link (DL) frequency (i.e. 6GHz to 4GHz) -by non-linear reactance elements excited with high fr equency power called pumping power applied to them from outside Spacecraft antennas -used for the capturing and rec eption of r.f. signal from earth station and for transmission of downlink signals to earth station -it provides antenna gain and directivity to both rece ived and transmitted signals -size of the antenna determines the magnitude or size of the diameter of illumination or foot print or coverage area on earth in other words the smaller the diameter the larger the coverage beam or footprint or earth. The bigger the antenna the smaller the foot print EXPLAIN HOW A SATELLITE MAINTAINS ITS ATTITUDE RELATIVE TO THE EART H’S
SURFACE -the satellite maintains its attitude relative to the earth’s surface control commands send from the ground command station as well as its orbited speed -upon being launched and placed into orbit 35880km above the e arth’s surface of an orbited speed of 11200km/hr it is expected to go on circulating the earth in 24hr synchronism forever -however this is not the case as the satellite will be subjected to a range of disturbances if left unchecked would relentlessly take the satellite out of or bit -the disturbances firstly arise from shape of the earth, which is not a sphere but slightly squashed at the northern hemisphere, with southern hemisphere larger than the north hence this asymmetric causes
the centre of gravity not to be in line with spin of t he earth and also that the equator fall to southern hemisphere -in addition the earth is not even c ircular as seen from polies it has budgets at the equator -as this is not enough the earth does not stay constant in its spin it vibrates like gigantic bay of jelly as its soft insides expand and contract
SKETCH THE BLOCK (SYSTEM) DIAGRAM OF A SATELLITE TRANSPONDER AND EXPLAIN THE FUNCTION OF EACH STAGE (FIG 37) -a typical satellite transponder is made up of t he receiver, frequency converter section and transmitter section -it is WRF to RF repeater -receiver section is made up of receive antenna BPF, LNA, down converter stages -transmitter section consists of high power amplifier (HPA) (10watts to 500watts) and BPF and transmit antenna BPF stage -it is the input band limiting device tuned to filter and accept the frequency band allocated to the service it is handling e.g. designed to accept fre quencies around 6GHz from the earth’s station Low Noise Amplifier (LNA) -acts as the preamplifier a common device used as an LNA in a tuned code. P rovides wideband low noise amplification of very low level signals received from the earth’s station -frequency translator (a shift oscillator and BPF) which converts t he high band uplink frequency to the low band downlink frequency -the low level power amplifier which is commonly a t ravelling wave tube, amplifies the RF signal for transmission through the downlink to earth’s station receives
DESCRIBE TYPICAL EARTH STATION EQUIPMENT (FIG 38) Antenna -captures signals arriving from satellite (DL) signals and focus signals at the satellite (UL) signals -the antennas used for satellite ground equipment are the parabolic circular dish types which reflect t he captured down link signals to the focus of t he parabolic a collecting is maintained for transmission purposes the horn emits the signals which are reflected off the parabola to form the uplink beam Low noise receiver -the low noise receiver equipment is made up of diplexer LPF (to receive 4GHz rf signals on the DL) low noise amplifier LNA, down converter, IF amplifier and demodulator -the LNA is generally mounted as close to the antenna feed as possible so that t ransmission line losses to the LNA are kept at absolute minimum High power transmitter -earth station transmitter are normally called upon to handle high powe r levels and so they are equipped with high power amplifier (HPA) -the power levels for earth station can range from a few hundred watts to several kilowatts
-the basic function of a power amplifier (PA) in an earth station is to amplify the low level RF carrier provided by ground communication equipment Tracking -the satellite does not stay in exactly the same position relative to the earth during its orbit -the earth station antenna consequently has an automatic tracking system. The tr acking system results in AZ/EL angle tracking movements it stays locked to the satellite Terrestrial network Interface -the multiplexers and de-multiplexers are terrestrial interface equipment -on the transmit side the terrestrial interface accepts incoming digital streams in the form of terrestrial line codes -similarly on the receive side, the interface accepts the received data or analogue RF signal which put onto the de-multiplexer and then onto terrestr ial links
EVALUATE THE OPHANS IN SATELLITE LAUNCH VEHICLES AND THE ORGANISERS RESPONSIBLE -today two different satellite launching systems are used to place satellites into geosynchronous orbit The Ariane rocket built in France with subsytems supplied several European countries -when Ariane is used for launching it climbs on a curving path to about 320km -at the required speed of 36480km/ hr parallel to the earth’s surface -at 320km, it releases the satellite on its elliphical transfer orbit up to g eosynchronous altitude and back again The space shuttle, built in USA -the US space shuttle launch process is different -the shuttle operates only in a circ ular orbit at about 240 to 320km altitude -its speed at that height is constant at the balancing speed of 26200kph -with shuttle therefore the satellite has first to be removed from the cargo body, by means of either mechanical locks and springs -it is then left for a short time in low circular orbit while the shuttle pilot removes himself to a safe distance of several miles before any motor ignition occurs in the satellite
EVALUATE THE ROLE OF VERY SMALL APARTURE SATELLITES (VSAT) IN NATIONAL/ INTERNATIONAL COMMUNICATION -VSATs are digital and designed primarily to support data communication on point to multipoint basis for large private network found in I /N communication -in role such as retail, inventory management, c redit verification and authorization and general transaction processing -VSATs provide user with services comparable to large gateway and terrestrial networks at a fraction of the cost -VSAT networks are designed to provide role of international and domestic communication services for data distribution and collection requirements -VSAT technology is by far the easiest way to connect far off places to the information highway in North
