What is the difference between FDD and TDD ?

Frequency Division Duplex

FDD requires two separate communications channels. Wireless systems need two separate frequency bands or channels. A sufficient amount of guard band separates the two bands so the transmitter and receiver don’t interfere with one another. Good filtering or duplexers and possibly shielding are a must to ensure the transmitter does not desensitize the adjacent receiver.

FDD requires two symmetrical segments of spectrum for the uplink and downlink channels.

In a cell phone with a transmitter and receiver operating simultaneously within such close proximity, the receiver must filter out as much of the transmitter signal as possible. The greater the spectrum separation, the more effective the filters.

Time Division Duplex

TDD uses a single frequency band for both transmit and receive. Then it shares that band by assigning alternating time slots to transmit and receive operations (Fig. 3). The information to be transmitted—whether it’s voice, video, or computer data—is in serial binary format. Each time slot may be 1 byte long or could be a frame of multiple bytes.

TDD alternates the transmission and reception of station data over time. Time slots may be variable in length.

Because of the high-speed nature of the data, the communicating parties cannot tell that the transmissions are intermittent. The transmissions are concurrent rather than simultaneous. For digital voice converted back to analog, no one can tell it isn’t full duplex.

In some TDD systems, the alternating time slots are of the same duration or have equal DL and UL times. However, the system doesn’t have to be 50/50 symmetrical. The system can be asymmetrical as required.

For instance, in Internet access, download times are usually much longer than upload times so more or fewer frame time slots are assigned as needed. Some TDD formats offer dynamic bandwidth allocation where time-slot numbers or duration are changed on the fly as required.

The real advantage of TDD is that it only needs a single channel of frequency spectrum. Furthermore, no spectrum-wasteful guard bands or channel separations are needed. The downside is that successful implementation of TDD needs a very precise timing and synchronization system at both the transmitter and receiver to make sure time slots don’t overlap or otherwise interfere with one another.

Applications : 

Most cell-phone systems use FDD. The newer LTE and 4G systems use FDD. Cable TV systems are fully FDD.

Most wireless data transmissions are TDD. WiMAX and Wi-Fi use TDD. So does Bluetooth when piconets are deployed. ZigBee is TDD. Most digital cordless telephones use TDD.

Conclusion

TDD appears to be the better overall choice, but FDD is far more widely implemented because of prior frequency spectrum assignments and earlier technologies. FDD will continue to dominate the cellular business for now. Yet as spectrum becomes more costly and scarce, TDD will become more widely adopted as spectrum is reallocated and repurposed.

Tabular differences : 

What is duplexing ?

Duplexing is the process of achieving two-way communications over a communications channel. It takes two forms: half duplex and full duplex.

In half duplex, the two communicating parties take turns transmitting over a shared channel. Two-way radios work this way. As one party talks, the other listens. Speaking parties often say “Over” to indicate that they’re finished and it’s time for the other party to speak. In networking, a single cable is shared as the two computers communicating take turns sending and receiving data.

Full duplex refers to simultaneous two-way communications. The two communicating stations can send and receive at the same time. Landline telephones and cell phones work this way. Some forms of networking permit simultaneous transmit and receive operations to occur. This is the more desirable form of duplexing, but it is more complex and expensive than half duplexing. 

What is a Sun synchronous orbit?

Sun Synchronous orbit is a special case of the polar orbit where the satellite travels from the north to the south poles. However, in a sun synchronous orbit, the satellite passes over the same part of the Earth at roughly the same local time each day which provides nearly same surface illumination every time. This is a useful for imaging and weather satellites.

How sun-synchronous orbits differ from geo-synchronous?

Geo-synchronous orbits are those orbits in which object revolves in the same time as taken by earth rotation on its axis. Hence, an object in geo-synchronous will always appear stationary at all times from earth. A geo-stationary orbit is a geo-synchronous orbit exactly above equator. With fixed latitude & eccentricity, satellites placed in geo-synchronous orbit will give easy information on weather & communication without monitoring the antenna for signal every single time.

On the other hand, a sun-synchronous orbit faces sun at all times in such a way that an object in its orbit appears ascending or descending from earth at same position at same local time every day. This is because for an object in sun-synchronous orbit, surface illumination angle matches mean solar time also sometimes called as sidereal day of earth. Since it is always lighted, satellite in sun-synchronous orbit is best-suited for Earth observation purposes such as Remote sensing giving a clear & bright view of earth at all times from its imagery.

What is the need and objective of Channel Allocation Scheme. Explain different type of channel allocation strategies.

In radio resource management for wireless and cellular network, channel allocation schemes are required to allocate bandwidth and communication channels to base stations, access points and terminal equipment.

The objective is to achieve maximum system spectral efficiency in bit/s/Hz/site by means of frequency reuse, but still assure a certain grade of service by avoiding co-channel interference and adjacent channel interference among nearby cells or networks that share the bandwidth.

There are two types of strategies that are followed: -

·        Fixed: 

    FCA, fixed channel allocation: Manually assigned by the network

operator

·        Dynamic:

Ø DCA, dynamic channel allocation,

Ø DFS, dynamic frequency selection

Ø Spread spectrum

FCA:

In Fixed Channel Allocation or Fixed Channel Assignment (FCA) each cell is given a predetermined set of frequency channels. FCA requires manual frequency planning, which is an arduous task in TDMA and FDMA based systems, since such systems are highly sensitive to cochannel interference from nearby cells that are reusing the same channel.

This results in traffic congestion and some calls being lost when traffic gets heavy in some cells, and idle capacity in other cells.

DCA and DFS:

Dynamic Frequency Selection (DFS) may be applied in wireless networks with several adjacent non-centrally controlled access points.

A more efficient way of channel allocation would be Dynamic Channel Allocation or Dynamic Channel Assignment (DCA) in which voice channel are not allocated to cell permanently, instead for every call request base station

request channel from MSC.

Spread spectrum:

Spread spectrum can be considered as an alternative to complex DCA algorithms. Spread spectrum avoids cochannel interference between adjacent cells, since the probability that users in nearby cells use the same spreading code is insignificant.

What is Avalanche Photo diode. Mention its applications.

An avalanche photodiode is a semiconductor-based photodetector (photodiode) which is operated with a relatively high reverse voltage (typically tens or even hundreds of volts), sometimes just below breakdown. In this regime, carriers (electrons and holes) excited by absorbed photons are strongly accelerated in the strong internal electric field, so that they can generate secondary carriers, as it also occurs in photomultipliers. The avalanche process, which may take place over a distance of only a few micrometers, for example, effectively amplifies the photocurrent by a significant factor. Therefore, avalanche photodiodes can be used for very sensitive detectors, which need less electronic signal amplification and are thus less susceptible to electronic noise.

Typical applications of avalanche photodiodes include 

• receivers in optical fiber communications, 
• range finding, imaging, 
• high-speed laser scanners, 
• laser microscopy, 
• and optical time domain reflectometry (OTDR).

Geiger Mode for Single Photon Counting :

When operated in the so-called Geiger mode with carefully designed electronics, avalanche photodiodes can be used even for single photon counting with dark count rates well below 1 kHz and with a quantum efficiency of several tens of percent, sometimes even well above 50%. The Geiger mode means that the diode is operated slightly above the breakdown threshold voltage, where a single electron–hole pair (generated by absorption of a photon or by a thermal fluctuation) can trigger a strong avalanche. In the case of such an event, an electronic quenching circuit reduces the voltage at the diode below the threshold voltage for a short time, so that the avalanche is stopped and the detector is ready for detection of further photons after some recovery time of e.g. 100 ns. That dead time constitutes a substantial limitation of this technology. It limits the count rate to the order of 10 MHz, whereas an avalanche diode in linear mode (i.e., operated with lower reverse voltage) may be operated with a bandwidth of many gigahertzes.

Photon-counting APDs are also called SPADs = single-photon avalanche diodes. They can be used in quantum optics experiments (for example, for quantum cryptography) and in some of the applications mentioned above if an extremely high sensitivity is required.

What is Ideality Factor in p-n junction diode ?

The ideality factor of a diode is a measure of how closely the diode follows the ideal diode equation. The derivation of the simple diode equation uses certain assumption about the cell. In practice, there are second order effects so that the diode does not follow the simple diode equation and the ideality factor provides a way of describing them.

Recombination mechanisms

The ideal diode equation assumes that all the recombination occurs via band to band or recombination via traps in the bulk areas from the device (i.e. not in the junction). Using that assumption the derivation produces the ideal diode equation below and the ideality factor, n, is equal to one.

However recombination does occur in other ways and in other areas of the device. These recombinations produce ideality factors that deviate from the ideal. Deriving the ideal diode equation by considering the number of carriers the need to come together during the process produces the results in the table below.

Recombination Type	Ideality factor	Description
SRH, band to band (low level injection)	1	Recombination limited by minority carrier.
SRH, band to band (high level injection)	2	Recombination limited by both carrier types.
Auger	2/3	Two majority and one minority carriers required for recombination.
Depletion region (junction)	2	two carriers limit recombination.

Explain TDM , FDM , Statistical Multiplexing briefly.

Frequency division multiplexing (FDM): Divide the frequency spectrum into smaller subchannels, giving each user exclusive use of a sub channel (e.g., radio and TV). One problem with FDM is that a user is given all of the frequency to use, and if the user has no data to send, bandwidth is

wasted — it cannot be used by another user.

Time division multiplexing (TDM): Use time slicing to give each user the full bandwidth, but for only a fraction of a second at a time (analogous to time sharing in operating systems). Again, if the user doesn’t have data to sent during his timeslice, the bandwidth is not used (e.g., wasted).

Statistical multiplexing: Allocate bandwidth to arriving packets on demand. This leads to the most efficient use of channel bandwidth because it only carries useful data.That is, channel bandwidth is allocated to packets that are waiting for transmission, and a user generating no packets doesn’t use any of the channel resources.

What is an Electromagnetic Wave?

A wave produced by the acceleration of an electric charge and propagated by the  periodic variation of intensities of, usually,perpendicular electric and magnetic fields.

Explain Satellite Graveyard and its purpose.

Due to the non-spherical shape of Earth, one more effect called as the “Satellite Graveyard” is seen. The non-spherical shape leads to the small value of eccentricity (10^-5) at the equatorial plane. This causes a gravity gradient on GEO satellite and makes them drift to one of the two stable points which coincide with minor axis of the equatorial ellipse. 

Working satellites are made to drift back to their position but out-of-service satellites are eventually drifted to these points, and making that point a Satellite Graveyard.

 (Note: A graveyard orbit, also called a supersynchronous orbit, junk orbit or disposal orbit, is an orbit significantly above GEO where satellites are intentionally placed at the end of their operational life. It is a measure performed in order to lower the probability of collisions with operational spacecraft and of the generation of additional space debris. The points where the graveyard is made are separated by 1800 on the equator and are set approximately on 750 E longitude and 1050 W longitude.)

What are disadvantages of satellite in Geostationary Earth Orbit ?

Disadvantages of GEO:

Northern or southern regions of the Earth (poles) have more problems receiving these satellites due to the low elevation above a latitude of 60°, i.e., larger antennas are needed in this case. Shading of the signals is seen in cities due to high buildings and the low elevation further away from the equator limit transmission quality. The transmit power needed is relatively high which causes problems for battery powered devices. These satellites cannot be used for small mobile phones. The biggest problem for voice and also data communication is the high latency as without having any handovers, the signal has to at least travel 72,000 kms. Due to the large footprint, either frequencies cannot be reused or the GEO satellite needs special antennas focusing on a smaller footprint. Transferring a GEO into orbit is very expensive.

State 3 conditions for a geostationary satellite.

There are three conditions which lead to geostationary satellites. Lifetime expectancy of these satellites is 15 years. 

1) The satellite should be placed 37,786 kms (approximated to 36,000 kms) above the surface of the earth. 
2) These satellites must travel in the rotational speed of earth, and in the direction of motion of earth, that is eastward. 
3) The inclination of satellite with respect to earth must be 0 degree .

Why Uplink Frequency In Mobile Phones Is Less Than Downlink Frequency?

The reason is power supply of mobile phone is a battery bank, and it is very limited,if uplink frequency is more,i.e. frequency of wave from mobile phone to telecom tower is more, then large amount of power is needed to transfer this frequency,so more power should be consumed in mobile phone so user have to charge his mobile phone after a very less interval which must be annoying for users,after noticing this fact designers made a system in which uplink frequency is less than downlink frequency.

Why is the uplink frequency greater than download frequency in satellite communication?

There are two main reasons for the up-link frequency being grater than down-link frequency. Attenuation and Power limits.

• On the earth, the transmitting station has a large bank of power and also may need to transmit to multiple satellites. Higher frequency means the signal has higher energy (E=h\v$E=h\text{v}$), though this has the effect of being attenuated easily by the atmosphere. Hence, the signal needs to be of higher intensity to compensate. Example, blue light is absorbed easily by the atmosphere than red, hence the evenings have red skies. So why do we need higher frequencies?
• The atmosphere has the Ionosphere
  
  
   at an altitude of about 60–1000 Km. This layer contains ionized particles of gas that block out and reflect these lower frequency waves. Hence, we need to use signals with higher frequency.
• Since the ground stations have rather larger sources of power as mentioned above, they can afford the higher power consumption to get the signal to the satellite. Also, satellites only have so much power source to amplify the received signal.

The satellite faces technical problems of its own.

• Satellites are powered by solar panels and this calls for low power transmission. Low frequency signals can only be transmitted efficiently by the satellite.
• It also does not face the problem of signals getting reflected by the ionosphere. So lower frequency signals are used by satellites.
• This is not a problem to receiving stations on earth as they use sensitive circuits and can afford to have large antennas that usually operate on LOS (Line Of Sight) which ensures the signal is captured reliably.

What is faraday rotation in satellite Comm. ?

Faraday rotation

Faraday rotation is an effect that affects satellite propagation. Faraday rotation results from the fact that the ionosphere is a magneto-ionic region. The Faraday rotation of a signal causes different elements of a signal to travel in different ways, particularly rotating the plane of polarization. This can create some problems with reception. A linearly polarized signal can be considered as two contra-rotating circularly polarized signals. The phase velocities of these two signals vary in a magnetic medium such as the ionosphere and as a result the polarization of the signal changes. The degree of change is dependent upon the state of the ionosphere and it follows the same pattern as that experienced for HF ionospheric communications changing over the course of the day, with the seasons and over the sunspot cycle.

Explain Geostationary and Geosynchronous Orbits.

A geostationary orbit is one in which a satellite orbits the earth at exactly the same speed as the earth turns and at the same latitude, specifically zero, the latitude of the equator. A satellite orbiting in a geostationary orbit appears to be hovering in the same spot in the sky, and is directly over the same patch of ground at all times.

A geosynchronous orbit is one in which the satellite is synchronized with the earth's rotation, but the orbit is tilted with respect to the plane of the equator. A satellite in a geosynchronous orbit will wander up and down in latitude, although it will stay over the same line of longitude. Although the
terms 'geostationary' and 'geosynchronous' are sometimes used interchangeably, they are not the same technically; geostationary orbit is a subset of all possible geosynchronous orbits.

Geostationary objects in orbit must be at a certain distance above the earth; any closer and the orbit would decay, and farther out they would escape the earth's gravity altogether. This distance is 35,786 kilometers (22,236 miles) from the surface.

What is effect of Non spherical shape of earth on GEO satellite? What do you mean by station keeping ?

As the shape of Earth is not a perfect sphere, it causes some variations in the path followed by the satellites around the primary. As the Earth is bulging from the equatorial belt, and keeping in mind that an orbit is not a physical entity, and it is the forces resulting from an oblate Earth which act on
the satellite produce a change in the orbital parameters.

Due to the non-spherical shape of Earth, one more effect called as the “Satellite Graveyard” is seen. The non-spherical shape leads to the small value of eccentricity (10^-5 ) at the equatorial plane. This causes a gravity gradient on GEO satellite and makes them drift to one of the two stable points which
coincide with minor axis of the equatorial ellipse.

In addition to having its attitude controlled, it is important that a geostationary satellite be kept in its correct orbital slot. The equatorial ellipticity of the earth causes geostationary satellites to drift slowly along the orbit, to one of two stable points, at 75°E and 105°W.

To counter this drift, an oppositely directed velocity component is imparted to the satellite by means of jets, which are pulsed once every 2 or 3 weeks. These maneuvers are termed east-west station-keeping maneuvers.

What is the effect of atmospheric drag on LEO satellites ?

For Low Earth orbiting satellites, the effect of atmospheric drag is more pronounces. The impact of this drag is maximum at the point of perigee. Drag (pull towards the Earth) has an effect on velocity of Satellite (velocity reduces).

This causes the satellite to not reach the apogee height successive revolutions. This leads to a change in value of semi-major axis and eccentricity. Satellites in service are maneuvered by the earth station back to their original orbital position.

What is Fourier Transform ? Why do we do fourier transform ?

Fourier Transform :

            Fourier transform gives a frequency domain representation of time domain signals. It is applicable for both periodic and aperiodic signals.

            An aperiodic signal is one which is periodic with an infinite period. As the period increases the fundamental frequency decreases which makes aperiodic signal infinitesimally close in frequency and the representation in terms of linear combination takes the form of an integral rather than sum. The resulting spectrum is called Fourier Transform.

            It is an extension of Fourier series for aperiodic signals.

By fourier transform we can represent the signal from time domain to frequency domain , thus we can find the various frequency components contained in given signal. It helps us to find the total Bandwidth required for transmission of given signal.

Explain few limitations of fourier series and how to overcome those limitations.

Limitations of Fourier series:

• ·         It can be used only for periodic inputs and thus not applicable for aperiodic one.
• ·         It cannot be used for unstable or even marginally stable systems.

Solution to first limitation is Fourier Transform, that is applicable for aperiodic also.

Solution to second limitation is Laplace transform by using e^st  .

What is the need of FFT?

FFT and DFT yields same result and are actually same transforms of converting time or spatial domain signal into frequency domain signal. FFT is actually an optimized way of getting DFT of a time domain signal. Computers use FFT method for faster processing to convey time domain signal to frequency domain signal. The no. of multiplications and additions in case of FFT are way less than using the actual DFT formula.

This picture has dimensions 240×320240×320, which is 76,800 pixels.

It takes roughly 7.67 milliseconds to perform a two-dimensional FFT of this image. How long does it take to evaluate the DFT without using the FFT algorithm? As far as I know, it’s over one minute. I stopped the program when one minute had passed.

Another comparison is below : 

N-point DFT	N-point FFT
Multiplications: N2	Multiplications: (N/2) log2(N)
Addition: N(N-1)	Addition: Nlog2(N)
Example: N =1000 Additions: 1000x999 Multiplications: 106	Example: N =1000 Additions: 1000log2(1000) = 104(approx.) Multiplications: 5120 (approx..)

Why do spacecraft launch from near the equator if possible?

It is better to launch rockets closer to the equator because the Earth rotates at a greater speed here than that at either pole. This extra speed at the equator means a rocket needs less thrust (and therefore less fuel) to launch into orbit.

In addition, launching at the equator provides an additional 1,036 mph (1,667 km/h) of speed once the vehicle reaches orbit. This speed bonus means the vehicle needs less fuel, and that freed space can be used to carry more pay load.

Also read this :

https://www.quora.com/Why-do-spacecraft-launch-from-near-the-equator-if-possible/answer/Robert-Frost-1?srid=X0dT

Differenece between FIR and IIR Filter.

FIR vs IIR filter:

FIR Filter	IIR Filter
A finite impulse response (FIR) filter produces an output that will go to zero when the input goes to zero and stays zero	An infinite impulse response (IIR) filter produces an output that may continue indefinitely even after its input goes to zero and stays zero.
Its response to an impulse function input is finite	Its response to an impulse function input is infinite
FIR filter uses only current and past input digital samples to obtain a current output sample value. It does not utilize past output samples. Simple FIR equation is mention below. y(n)= h(0)x(n) + h(1)x(n-1) + h(2)x(n-2) + h(3)x(n-3) + h(4)x(n-4)	IIR filter uses current input sample value, past input and output samples to obtain current output sample value. Simple IIR equation is mention below.  y(n)= b(0)x(n) + b(1)x(n-1) + b(2)x(n-2) + b(3)x(n-3) + a(1)y(n-1) + a(2)y(n-2) + a(3)y(n-3)
FIR filters are preferred due to its linear phase response and also they are non-recursive. Feedback is not involved in FIR; hence they are stable	IIR filters are not stable as they are recursive in nature and feedback is also involved in the process of calculating output sample values.
FIR have no analog equivalent	IIR filters have analog equivalent
FIR filters are used as anti-aliasing, low pass and baseband filters	IIR filters are used as notch (band stop), band pass functions
Transfer function of FIR filter will have only zeros, need more memory	Transfer function of IIR filter will have both zeros and poles and will require less memory than FIR counterpart.
An FIR filter’s z-Transform has zeroes and poles only at 0 + j0, so it is always stable	An IIR filter’s z-Transform has pole(s) and may also have zeroes, so it may be unstable if any of the poles are outside the unit circle
FIR Filters have linear phase.	IIR filters don’t have linear phase so they are used at places where phase distortion is tolerable.

FIR filter need higher order than IIR filter to achieve same performance. Delay is more than IIR filter. It has lower sensitivity than IIR filter. These are disadvantages of FIR filters.

Why are FIR filters preferred over IIR filters?

Choice of the filter depends on the application.

Merits of FIR filters -

1) FIR filters have an amazing property called Linear phase. Linear phase property implies that the phase is a linear function of the frequency. It ensures that signals of all frequencies are delayed by the same amount of time, thereby eliminating the possibility of phase distortion. Pretty darn useful in audio applications.

2) FIR filters are always stable i.e. for a finite input, the output is always finite. This property is always useful.

Demerits of FIR filters -

1) For a desired frequency response, with tight constraints on the passband, transition band and the stopband, a FIR filter may have large number of coefficients. An IIR filter can satisfy the constraints with lesser number of coefficients. For the same reason, IIR filters are used for filtering out sensor noise.


If the application demands linear phase, then we go for FIR filters. If there is no linear phase constraint, and the desired frequency response can be approximated by lesser number of coefficients using a stable IIR filter, then we choose IIR filters.