Access Networks- Mobile

The Access Network is defined as the setting up the communication between end user terminal and the transmission Network. 

The bandwidth allocated for a mobile telecom operator is a very expensive resource. Therefore the company must occupy the frequencies very efficiently.

The widely used moble technology is GSM(Global System for Mobile Communications).GSM networks operate in a number of different carrier frequency ranges.GSM 900MHZ Band is used, when increasing the coverage. Typically GSM 900MHz can cover wide area compared with 1800MHz band.When the frequency getting higher and higher air lose increases. Sometimes, 900 Band will not enough to deliver high customer demand ,in that case we have to use 1800 band to increase the capacity.

We are using different access technologies to address each user  privately with the limited resource frequencies.TDMA,FDMA and CDMA are the access technologies which allow to share the limited bandwidth with subscribers.

•        FDMA (Frequency Division Multiple Access)          In FDMA, signals from various users are assigned to various frequencies. (Just as in an analog system) Cross talk between adjacent channel users is avoided by maintaining a frequency guard band between them.  Advantages of FDMA are that the capacity increases can be done by reducing the information rate and efficient digital codes. Also the technological advances required for implementing the system are simple
•     TDMA (Time Division Multiple Access)         This is the access method used in GSM. In a TDMA system data from each user is conveyed in time intervals called slots. Several slots make up a frame. Each slot is made up of a preamble plus information bits addressed to various (mobile) stations. The functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver.     However it is a disadvantage that TDMA requires a large amount of signal processing for matched filtering and correlation detection for synchronization with a time slot.

•  CDMA (Code Division Multiple Access)   The primary advantage of CDMA is its ability to tolerate a fair amount of interfering signals compared to FDMA and TDMA. So the problems of frequency band assignment and adjacent channel interference is greatly reduced. With CDMA, adjacent micro cells share the same frequencies whereas with FDMA/TDMA it is not feasible for adjacent micro cells to share the same frequencies because of interference. 

I will discuss about these access technologies in later posts. 

   

A New Set of Articles

I am extremely sorry that i couldn't write any thing for a long period.But I thought to start bloging again about technical stuffs.So I thought to write a complete set of articles about telecommunication.Wait for articles

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Importance of the parallel operation of transformers

 

 

Transformers are placed in parallel by electric utilities when they want to provide a 'stronger' voltage source and will result in higher available fault current that can be delivered downstream. Usually the utility transformer can serve its own load but two are put together to achieve with a ties primary and secondary bus to 'stiffen' the voltage to ride through system load changes. Because here we have more transformers connected in parallel. So even if, one of the transformers gets into a fault or is taken out for maintenance or repair, the load can continued to be serviced. Therefore by this we can reduce the spare capacity also.

Before two or more transformers connected in parallel and to share a common load satisfactorily, there are some conditions to be satisfied. Those are the voltage ratio (turns ratio) and the per unit impedance of each machine on its own base must be the same. When two transformers are connected in parallel, the impedances of the transformers must match (within 10%) to divide the load approximately equally between the two transformers or to divide the load according to the rating of each transformer. If the transformers to be connected in parallel are equipped with load tap changing windings, then the impedances for each of the tap changer positions must match. If these conditions are not met, then one of the transformers could conceivably carry a continuous overload, resulting in overheating.

Also the polarity must be the same, so that there is no circulating current between the transformers.
The polarity of connection in the case of single phase transformers can be either same or opposite. Inside the loop formed by the two secondary the resulting voltage must be zero. If wrong polarity is chosen the two voltages get added and short circuit results.     The phase sequence must be the same and no phase difference must exist between the voltages of the two transformers. When these requirements are satisfying we can have a good parallel operation of transformers which is very useful and favourable in practice.

 

Generally, application of parallel transformers allows achieving the following benefits:

 

1. Reducing the total capacity of electrical transformers (as compared to separate their work). The decrease of total installed capacity is reached:
   

• by lowering the overall  demand load to the diversity of loads connected to different transformers
  
• by using a higher load rate of parallel transformers
  
• less required backup in case of electrical transformer failure
  

2) Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded transformers

3) Improving the power quality due to the stable level of short circuit current throughout the network

4) Increasing the reliability of operation of protective devices in the case of phase-to-earth short circuits in the network.

5) Possibility of placing electrical transformers in operation phase-by-phase

Importance of the parallel operation of transformers

Transformers are placed in parallel by electric utilities when they want to provide a ‘stronger’ voltage source and will result in higher available fault current that can be delivered downstream.  Usually the utility transformer can serve its own load but two are put together to achieve with a ties primary and secondary bus to ‘stiffen’ the voltage to ride through system load changes. Because here we have more transformers connected in parallel. So even if, one of the transformers gets into a fault or is taken out for maintenance or repair, the load can continued to be serviced. Therefore by this we can reduce the spare capacity also. 

Before two or more transformers connected in parallel and to share a common load satisfactorily, there are some conditions to be satisfied. Those are the voltage ratio (turns ratio) and the per unit impedance of each machine on its own base must be the same. When two transformers are connected in parallel, the impedances of the transformers must match (within 10%) to divide the load approximately equally between the two transformers or to divide the load according to the rating of each transformer. If the transformers to be connected in parallel are equipped with load tap changing windings, then the impedances for each of the tap changer positions must match. If these conditions are not met, then one of the transformers could conceivably carry a continuous overload, resulting in overheating.

Also the polarity must be the same, so that there is no circulating current between the transformers. The polarity of connection in the case of single phase transformers can be either same or opposite. Inside the loop formed by the two secondary the resulting voltage must be zero. If wrong polarity is chosen the two voltages get added and short circuit results.     The phase sequence must be the same and no phase difference must exist between the voltages of the two transformers. When these requirements are satisfying we can have a good parallel operation of transformers which is very useful and favourable in practice.

Generally, application of parallel transformers allows achieving the following benefits:

1)      Reducing the total capacity of electrical transformers (as compared to separate their work). The decrease of total installed capacity is reached:

·         by lowering the overall  demand load to the diversity of loads connected to different transformers

·         by using a higher load rate of parallel transformers

·         less required backup in case of electrical transformer failure

      2)   Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded transformers

      3)    Improving the power quality due to the stable level of short circuit current throughout the network

      4)  Increasing the reliability of operation of protective devices in the case of phase-to-earth short circuits in the network.

     5)   Possibility of placing electrical transformers in operation phase-by-phase