Urgent Recruitment for Elect Engineers 70 vacancies

Recruitment for OIl & Gas Projects - Qatar.
Mechanical Technicians (Maint) - 5 Years Exp - 50 Nos
Electrical Technicians (Projects) - 20 Nos
Excellent Salary + Food + Accommodation
Final Interview at Chennai on 10th July 2014
Rush your resume to hr@crescentcareers.com | Ph 044 4500 6070 / 8090
For detailed jd visit www.crescentcareers.com

30 Interview qns for EEE

Why star delta starter is preferred with induction motor?

Star delta starter is preferred with induction motor due to following reasons:
• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

State the difference between generator and alternator

Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Why AC systems are preferred over DC systems?

Due to following reasons, AC systems are preferred over DC systems:
a. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
c. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

How can you relate power engineering with electrical engineering?

Power engineering is a sub division of electrical engineering. It deals with generation, transmission and distribution of energy in electrical form. Design of all power equipments also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on grid systems and they might work on off grid systems that are not connected to the system.

What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:
• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.

Why back emf used for a dc motor? highlight its significance.

The induced emf developed when the rotating conductors of the armature between the poles of magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.

What is slip in an induction motor?

Slip can be defined as the difference between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

Explain the application of storage batteries.

Storage batteries are used for various purposes, some of the applications are mentioned below:

• For the operation of protective devices and for emergency lighting at generating stations and substations.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange, laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

9. Explain advantages of storage batteries

Few advantages of storage batteries are mentioned below:
• Most efficient form of storing energy portably.
• Stored energy is available immediately because there is no lag of time for delivering the stored energy.
• Reliable source for supply of energy.
• The energy can be drawn at a fairly constant rate.

10. What are the different methods for the starting of a synchronous motor.

Starting methods: Synchronous motor can be started by the following two methods:
• By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.
• By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.

11. Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.

Following motors are used: -
• Vacuum cleaners- Universal motor.
• Phonographic appliances – Hysteresis motor.
• Vending machines – Shaded pole motor.
• Refrigerators – Capacitor split phase motors.
• Rolling mills – Cumulative motors.
• Lathes – DC shunt motors.
• Power factor improvement – Synchronous motors.

12. State Thevenin’s Theorem:

According to thevenin’s theorem, the current flowing through a load resistance
Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).

13. State Norton’s Theorem

The Norton’s theorem explains the fact that there are two terminals and they are as follows:
• One is terminal active network containing voltage sources
• Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance.
The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.

14. State Maximum power transfer theorem

The Maximum power transfer theorem explains about the load that a resistance will extract from the network. This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.

15. Explain different losses in a transformer.

There are two types of losses occurring in transformer:
• Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:
o eddy current loss
o Hysteresis loss.
These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.
• Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.

16. Explain different types of D.C motors? Give their applications

Different type of DC motors and their applications are as follows:-
• Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.
• Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coalmines etc.
• Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.

17. Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.

Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of compoles or interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.

18. Comment on the working principle of operation of a single-phase transformer.

Working principle of operation of a single-phase transformer can be explained as
An AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self inductance and known as self induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.

19. Define the following terms:-

• Reliability,
• Maximum demand,
• Reserve-generating capacity,
• Availability (operational).

Reliability: It is the capacity of the power system to serve all power demands without failure over long periods.
Maximum Demand: It is maximum load demand required in a power station during a given period.
Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.
Availability: As the percentage of the time a unit is available to produce power whether needed by the system or not.

20. Mention the disadvantages of low power factor? How can it be improved?

Disadvantages of low power factor:
• Line losses are 1.57 times unity power factor.
• Larger generators and transformers are required.
• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.
• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size

21. State the methods of improving power factor?

Methods of improving power factor:
• By connecting static capacitors in parallel with the load operating at lagging power factor.
• A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.

22. State the factors, for the choice of electrical system for an aero turbine.

The choice of electrical system for an aero turbine is guided by three factors:
• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.
• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.
• Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.

23. What are the advantages of VSCF wind electrical system?

Advantages of VSCF wind electrical system are:
• No complex pitch changing mechanism is needed.
• Aero turbine always operates at maximum efficiency point.
• Extra energy in the high wind speed region of the speed – duration curve can be extracted
• Significant reduction in aerodynamic stresses, which are associated with constant – speed operation.

24. Explain the terms real power, apparent power and reactive power for ac circuits and also the units used.

• Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit of real power is watt. i.e. Expressed as W or kW.
• Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.
• Reactive Power: It is the product of voltage, current and sine of angle between the voltage and current i.e. Reactive power = voltage X current X sinj or Reactive power = V I sin j and has no other unit but expressed in VAR or KVAR.

25. Define the following: Average demand, Maximum demand, Demand factor, Load factor.

• Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.
• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.
• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.
• Load Factor: It is defined as the ratio of the average power to the maximum demand.

26. Explain forward resistance, static resistance and dynamic resistance of a pn junction diode.

• Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward-resistance.
• DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

27. How does Zener phenomenon differ from Avalanche breakdown?

The phenomenon when the depletion region expands and the potential barrier increases leading to a very high electric field across the junction, due to which suddenly the reverse current increases under a very high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur independently or both of these may occur simultaneously. Diode junctions that breakdown below 5v are caused by Zener Effect. Junctions that experience breakdown above 5v are caused by avalanche-effect. The Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

28. Compare JFET’s and MOSFET’s.

Comparison of JFET’s and MOSFET’s:
• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial.
• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.
• JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.
• When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.

30. Explain thin film resistors and wire-wound resistors

a. Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.
b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

31. What is a differential amplifier? Also, explain CMRR.

Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal.
CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.

Electical Interview videos helpful to yours

Top 50 Interview Questions and their answers for Freshers

1. Tell me about yourself

The most often asked question in interviews. You need to have a short statement prepared in your mind. Be careful that it does not sound rehearsed. Limit it to work-related items unless instructed otherwise. Talk about things you have done and jobs you have held that relate to the position you are interviewing for. Start with the item farthest back and work up to the present.

2. Why did you leave your last job?

Stay positive regardless of the circumstances. Never refer to a major problem with management and never speak ill of supervisors, co-workers or the organization. I f you do, you will be the one looking bad. Keep smiling and talk about leaving for a positive reason such as an opportunity, a chance to do something special or other forward-looking reasons.

3. What experience do you have in this field?

Speak about specifics that relate to the position you are applying for. If you do not have specific experience, get as close as you can.

4. Do you consider yourself successful?

You should always answer yes and briefly explain why. A good explanation is that you have set goals, and you have met some and are on track to achieve the others.

5. What do co-workers say about you?

Be prepared with a quote or two from co-workers. Either a specific statement or a paraphrase will work. Jill Clark, a co-worker at Smith Company, always said I was the hardest workers she had ever known. It is as powerful as Jill having said it at the interview herself.

6. What do you know about this organization?

This question is one reason to do some research on the organization before the interview. Find out where they have been and where they are going. What are the current issues and who are the major players?

7. What have you done to improve your knowledge in the last year?

Try to include improvement activities that relate to the job. A wide variety of activities can be mentioned as positive self-improvement. Have some good ones handy to mention.

8. Are you applying for other jobs?

Be honest but do not spend a lot of time in this area. Keep the focus on this job and what you can do for this organization. Anything else is a distraction.

9. Why do you want to work for this organization?

This may take some thought and certainly, should be based on the research you have done on the organization. Sincerity is extremely important here and will easily be used. Relate it to your long-term career goals.

10. Do you know anyone who works for us?

Be aware of the policy on relatives working for the organization. This can affect your answer even though they asked about friends not relatives. Be careful to mention a friend only if they are well thought of.

11. What kind of salary do you need?

A loaded question. A nasty little game that you will probably lose if you answer first. So, do not answer it. Instead, say something like, that,s a tough question. Can you tell me the range for this position? In most cases, the interviewer, taken off guard, will tell you. If not, say that it can depend on the details of the job. Then give a wide range.

12. Are you a team player?

You are, of course, a team player. Be sure to have examples ready. Specifics that show you often perform for the good of the team rather than for yourself is good evidence of your team attitude. Do not brag; just say it in a matter-of-fact tone? This is a key point.

13. How long would you expect to work for us if hired?

Specifics here are not good. Something like this should work: I,d like it to be a long time. Or As long as we both feel I,m doing a good job.

14. Have you ever had to fire anyone? How did you feel about that?

This is serious. Do not make light of it or in any way seem like you like to fire people. At the same time, you will do it when it is the right thing to do. When it comes to the organization versus the individual who has created a harmful situation, you will protect the organization. Remember firing is not the same as layoff or reduction in force.

15. What is your philosophy towards work?

The interviewer is not looking for a long or flowery dissertation here. Do you have strong feelings that the job gets done? Yes. That,s the type of answer were that works best here. Short and positive, showing a benefit to the organization.

16. If you had enough money to retire right now, would you?

Answer yes if you would. But since you need to work, this is the type of work you prefer. Do not say yes if you do not mean it.

17. Have you ever been asked to leave a position?

If you have not, say no. If you have, be honest, brief and avoid saying negative things about the people or organization involved.

18. Explain how you would be an asset to this organization?

You should be anxious for this question. It gives you a chance to highlight your best points as they relate to the position being discussed. Give a little advance thought to this relationship.


19. Why should we hire you?

Point out how your assets meet what the organization needs. Do not mention any other candidates to make a comparison.

20. Tell me about a suggestion you have made?

Have a good one ready. Be sure and use a suggestion that was accepted and was then considered successful. One related to the type of work applied for is a real plus.

21. What irritates you about co-workers?

This is a trap question. Think real hard but fail to come up with anything that irritates you. A short statement that you seem to get along with folks is great.

22. What is your greatest strength?

Numerous answers are good, just stay positive. A few good examples: Your ability to prioritize, Your problem-solving skills, Your ability to work under pressure, Your ability to focus on projects, Your professional expertise, Your leadership skills, Your positive attitude

23. Tell me about your dream job.

Stay away from a specific job. You cannot win. If you say the job you are contending for is it, you strain credibility. If you say another job is it, you plant the suspicion that you will be dissatisfied with this position if hired. The best is to stay genetic and say something like: A job where I love the work, like the people, can contribute and can,t wait to get to work.

24. Why do you think you would do well at this job?

Give several reasons and include skills, experience and interest.

25. What are you looking for in a job?

Stay away from a specific job. You cannot win. If you say the job you are contending for is it, you strain credibility. If you say another job is it, you plant the suspicion that you will be dissatisfied with this position if hired. The best is to stay genetic and say something like: A job where I love the work, like the people, can contribute and can,t wait to get to work.

26. What kind of person would you refuse to work with?

Do not be trivial. It would take disloyalty to the organization, violence or lawbreaking to get you to object. Minor objections will label you as a whiner.

27. What is more important to you: the money or the work?

Money is always important, but the work is the most important. There is no better answer.

28. What would your previous supervisor say your strongest point is?

There are numerous good possibilities: Loyalty, Energy, Positive attitude, Leadership, Team player, Expertise, Initiative, Patience, Hard work, Creativity, Problem solver

29. Tell me about a problem you had with a supervisor?

Biggest trap of all. This is a test to see if you will speak ill of your boss. If you fall for it and tell about a problem with a former boss, you may well below the interview right there. Stay positive and develop a poor memory about any trouble with a supervisor.

30. What has disappointed you about a job?

Don,t get trivial or negative. Safe areas are few but can include: Not enough of a challenge. You were laid off in a reduction Company did not win a contract, which would have given you more responsibility.

31. Tell me about your ability to work under pressure?

You may say that you thrive under certain types of pressure. Give an example that relates to the type of position applied for.

32. Do your skills match this job or another job more closely?

Probably this one. Do not give fuel to the suspicion that you may want another job more than this one.

33. What motivates you to do your best on the job?

This is a personal trait that only you can say, but good examples are: Challenge, Achievement and Recognition.

34. Are you willing to work overtime? Nights? Weekends?

This is up to you. Be totally honest.

35. How would you know you were successful on this job?

Several ways are good measures: You set high standards for yourself and meet them. Your outcomes are a success. Your boss tells you that you are successful.

36. Would you be willing to relocate if required?

You should be clear on this with your family prior to the interview if you think there is a chance it may come up. Do not say yes just to get the job if the real answer is no. This can create a lot of problems later on in your career. Be honest at this point and save yourself future grief.

37. Are you willing to put the interests of the organization ahead of your own?

This is a straight loyalty and dedication question. Do not worry about the deep ethical and philosophical implications. Just say yes.

38. Describe your management style.

Try to avoid labels. Some of the more common labels, like progressive, salesman or consensus, can have several meanings or descriptions depending on which management expert you listen to. The situational style is safe, because it says you will manage according to the situation, instead of one size fits all.

39. What have you learned from mistakes on the job?

Here you have to come up with something or you strain credibility. Make it small, well-intentioned mistake with a positive lesson learned. An example would be working too far ahead of colleagues on a project and thus throwing coordination off.

40. Do you have any blind spots?

Trick question. If you know about blind spots, they are no longer blind spots. Do not reveal any personal areas of concern here. Let them do their own discovery on your bad points. Do not hand it to them.

41. If you were hiring a person for this job, what would you look for?

Be careful to mention traits that are needed and that you have.

42. Do you think you are overqualified for this position?

Regardless of your qualifications, state that you are very well qualified for the position.

43. How do you propose to compensate for your lack of experience?

First, if you have experience that the interviewer does not know about, bring that up: Then, point out (if true) that you are a hard working quick learner.

44. What qualities do you look for in a boss?

Be generic and positive. Safe qualities are knowledgeable, a sense of humor, fair, loyal to subordinates and holder of high standards. All bosses think they have these traits.

45. Tell me about a time when you helped resolve a dispute between others?

Pick a specific incident. Concentrate on your problem solving technique and not the dispute you settled.

46. What position do you prefer on a team working on a project?

Be honest. If you are comfortable in different roles, point that out.


47. Describe your work ethic.

Emphasize benefits to the organization. Things like, determination to get the job done and work hard but enjoy your work are good.

48. What has been your biggest professional disappointment?

Be sure that you refer to something that was beyond your control. Show acceptance and no negative feelings.

49. Tell me about the most fun you have had on the job.

Talk about having fun by accomplishing something for the organization.

50. Do you have any questions for me?

Always have some questions prepared. Questions prepared where you will be an asset to the organization are good. How soon will I be able to be productive? And what type of projects will I be able to assist on? are examples.

Top 20 Interview Qns forr ELECTRONICS

What are different categories of antenna and give an example of each?

Different categories of antenna are as follows :

1. Wire Antennas - Short Dipole Antenna
2. Microstrip Antennas - Rectangular Microstrip (Patch) Antennas
3. Reflector Antennas - Corner Reflector
4. Travelling Wave Antennas - Helical Antennas
5. Aperture Antennas - Slot Antenna
6. Other Antennas - NFC Antennas

What is handover and what are its types?

Handover in mobile communication refers to the process of transferring a call from one network cell to another without breaking the call.
There are two types of handover which are as follows :

Hard Handoff : hard handoff is the process in which the cell connection is disconnected from the previous cell before it is made with the new one.

Soft Handoff : It is the process in which a new connection is established first before disconnecting the old one. It is thus more efficient and smart.

What is ionospheric bending?

When a radio wave travels into the ionospheric layer it experiences refraction due to difference in density. The density of ionospheric layer is rarer than the layer below which causes the radio wave to be bent away from the normal. Also the radio wave experiences a force from the ions in the ionospheric layer. If incident at the correct angle the radio wave is completely reflected back to the inner atmosphere due to total internal reflection. This phenomenon is called ionospheric reflection and is used in mobile communication for radio wave propagation also known as ionospheric bending of radio waves.

What is CDMA?

CDMA stands for Code Division Multiple Access which uses digital format. In CDMA systems several transmissions via the radio interface take place simultaneously on the same frequency bandwidth. User data is combined at the transmitter’s side with a code, then transmitted. On air, all transmission get mixed. At the receiver's side the same code is used as in the transmitter’s side. The code helps the receiver to filter the user information of the transmitter from incoming mixture of all transmissions on the same frequency band and same time.

Explain the concept of frequency re-use.

The whole of the geographical area is divided into hexagonal shape geometrical area called cell and each cell having its own transceiver. Each BTS (cell site) allocated different band of frequency or different channel. Each BTS antenna is designed in such a way that i cover cell area in which it is placed with frequency allotted without interfering other cell signals. The design process of selecting and allocating channel groups for all of the cellular base station within system is called frequency reuse.

Explain Bluetooth.

Bluetooth is designed to be a personal area network, where participating entities are mobile and require sporadic communication with others. It is omni directional i.e. it does not have line of sight limitation like infra red does. Ericsson started the work on Bluetooth and named it after the Danish king Harold Biuetooth. Bluetooth operates in the 2.4 GHz area of spectrum and provides a range of 10 metres. It offers transfer speeds of around 720 Kbps

7. What are GPRS services?

GPRS services are defined to fall in one of the two categories :

- PTP ( Point to point)
- PTM ( Point to Multipoint)
Some of the GPRS services are not likely to be provided by network operators during early deployment of GPRS due in part to the phased development of standard. Market demand is another factor affecting the decision of operators regarding which services to offer first.

8. What are the advantages of CDMA?

Advantages of CDMa are as follows :

1. Frequency diversity : Transmission is spread out over a large bandwidth due to that less affected by noise. If bandwidth is increased S/N ratio increases, which means noise will be reduced.
2. Multiplication Resistance : Chipping codes used for CDMA not only exhibit low correlation but also low autocorrelation. Hence a version of the signal that is delayed by more than one chip interval does not interfere with dominant signal as in other multipath environments.
3. Privacy : Due to spread spectrum is obtained by the use of noise like signals, where each user has a unique code, so privacy is inherent.
4. Graceful Degradation. In CDMA, more users access the system simultaneously as compared to FDMa, TDMA.

9. What are the advantages of spread spectrum?

SPread spectrum has the following advantages :

1. No crosstalk interference.
2. Better voice quality/data integrity and less static noise.
3. Lowered susceptibility to multipath fading.
4. Inherent security.
5. Co-existence.
6. Longer operating distances.
7. Hard to detect.
8. Hard to intercept or demodulate.
9. Harder to jam than narrow bands.
10. Use of ranging and radar.

10. Explain the steps involved in demodulating a signal.

Once the signal is coded, modulated and then sent, the receiver must demodulate the signal. This is usually done in two steps :
1. Spectrum spreading (e.g., direct sequence or frequency hopping) modulation is removed.
2. The remaining information bearing signal is demodulated by multiplying with a local reference identical in structure and synchronised with received signal.

11. How can a Pseudo Random Noise COde be usable?

To be usable for direct sequence spreading, a PN code must meet the following conditions :

1. Sequence must be built from 2 levelled numbers.
2. The codes must have sharp autocorrelation peak to enable code synchronization.
3. Codes must have a low cross-correlation value, the lower it is, more are the number of users which can be allowed in the system.
4. The codes should be “balanced” i.e. the difference between ones and zeros in code may only be 1.

12. What are the drawbacks of walsh codes?

Walsh codes have the following drawbacks :

1. The codes do not have a single, narrow autocorrelation peak.
2. The spreading is not over the whole bandwidth; instead the energy is spread over a number of discrete frequency components.
3. Although the full sequence cross correlation is identically zero, this does not hold for a partial sequence cross correlation function. Thus advantage of using orthogonal codes is lost.
4. Orthogonality is also affected by channel properties like multi path.

13. Explain radio environment in building.

Building penetration : Building penetration depends on the material used for construction and architecture used. This varies building to building and is based on building construction.
Building Height Effect : The signal strength is always higher at top floor and generally floor gain height is about 2.7dB/floor which is not dependent on building construction.
Building Floor Reception : The signal isolation between floors in a multi floor building is on the average about 20dB. Within a floor of 150 * 150 feet, the propagation loss due to interior walls, depending on the wall materials is about 20 dB between the strong and the weak areas.

14. List some advantages of GSM.

Here are some advantages of GSM :
1. GSM is mature, this maturity means a more stable network with robust features.
2. Less signal deterioration inside buildings.
3. Ability to use repeaters.
4. Talk time is generally higher in GSM phones due to pulse nature of transmission.
5. The availability of Subscriber Identity Modules allows users to switch networks and handset at will.
6. GSM covers virtually all parts of world so international roaming is not a problem.

15. What are the various types of numbers for network identity?

Various types of number for network identity are as follows :
1. MSISDN ( Mobile station ISDN) Number : It is international mobile subscriber number which is normally called mobile number. It is unique worldwide.
2. MSRN ( Mobile Subscriber Routing Number) : MSRN is used during mobile terminate trunk call to provide location of mobile subscriber.
3. HON ( Hand Over Number ) : HON is used for providing information required to transfer call from one B?SC to another BSC or to another MSC.
4. ISMI ( International Mobile Subscriber Identity Number) : Purpose of ISMI is for location update and authentication.
5. TMSI ( Temporary Mobile Subscriber Identity ) : TMSI is used instead of IMSI to improve security efficiency of network.
6. IMEI : International Mobile Equipment Identity.

16. What are the functions of Base Station System ( BSS)?

Functions of BSS are as follows :
1. Radio path control.
2. BTS and TC control.
3. Connection establishment with MS-NSS.
4. Mobility management, speech transcoding.
5. Connection of statistical data.

17. What are the parts of Network Management System ( NMS )?

Following are the parts of network management system :
1. OMC : Operation and maintenance center - Computerized monitoring center.
2. NMC : Network Management Center - Centralized control of a network is done here.
3. OSS : Operation and support system - Used for supporting activities performed in an OMC and/or NMC.

18. What are applications of DSP?

Some selected applications or digital signal processing that are often encountered in daily life are listed as follows:
1. Telecommunication: Echo cancellation in telephone networks.
2. Military Radar signal processing
3. Consumer electronics Digital Audio/TV
4. Instrumentation and control
5. Image processing image representation, image compression
6. Speech processing speech analysis methods are used in automatic speech recognition
7. Medicine Medical diagnostic instrumentation such as computerised tomography (CT)
8. Seismology DSP techniques are employed in geophysical exploration for oil and gas.
9. Signal Filtering Removal of unwanted background noise.

19. What is analog-to-digital conversion of signals?

A discrete-time signal is defined by specifying its value only at discrete times, called sampling instants. When the sampled values are quantised and encoded, a digital signal is obtained. A digital signal is obtained from the analog signal by using an analog-to-digital converter. This entire process is referred to as the conversion of signals from analog to digital form.

20. What are the properties of ROC for z-Transform?

Properties of the ROC for the z-Transform :
1. X(z) converges uniformly if and only if the ROC of the z-transform X(z) of the sequence includes the unit circle. The ROC of X(z) consists of a ring in the z-plane centered about the origin. That is, the ROC of the z-transform of x(n) has values of z for which x(n) r-n is absolutely summable.
2. The ROC does not contain any poles.
3. When x(n) is of finite duration then the ROC is the entire z-plane, except possibly z=0 and/or z=infinity.
4. If x(n) is a right sided sequence, the ROC will not include infinity.
5. If x(n) is a left sided sequence, the ROC will not include z=0. However if x(n)=0 for all n>0, the ROC will include z=0.
6. If x(n) is two sided and if the circle |z| = r0 is in the ROC, then the ROC will consist of a ring in the z-plane that includes the circle |z|=r0.
7. If X(z) is rational, then the ROC extends to infinity, i.e. the ROC is bounded by poles.
8. If x(n) is causal, then the ROC includes z=infinity.
9. If x(n) is anti- causal, trhen the ROC includes z=0.