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.