Hello Friends,

This is regarding the terms from Computer Networks TDMA and FDMA. These two pre-3G technologies that have been utilized to improve the performance of the 2G network.

FDMA (Frequency Division Multiplexing Access):

FDMA is a method of dividing a single band into 30 discrete channels. Each channel would then be capable of handling separate traffic, whether it’s a call or a data transfer. This is a higher level of multiplexing and should not be confused with FDM (Frequency Division Multiplexing) which is a low level multiplexing process and occurs on the physical layer. FDMA hardware includes high performance filters that also help it in avoiding near-far problems which can deteriorate the call quality.

Some Features of FDMA:

  • In FDMA all users share the satellite simultaneously but each user transmits at single frequency.
  • FDMA can be used with both analog and digital signal.
  • FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.
  • FDMA is not vulnerable to the timing problems that TDMA has. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.
  • Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA.
  • Each user transmits and receives at different frequencies as each user gets a unique frequency slot.

Advantages of FDMA:

  • Does not need any base-control station
  • Data that is transferred between the stations will never be lost.
  • After the data transmission, the effect of DELAY DISTORTION is very small and can be ignored.
  • No need of Network Timing
  • Channel Operations in FDMA are simple.
  • FDMA algorithms are easy to understand and implement.

Disadvantages of FDMA:

  • Impossible for receiver to receive the data from more than one station at a single point of time
  • Maximum Data Rate for every channel is small and fixed.
  • Requires special filters to avoid interference between any narrow channel.

TDMA (Time Division Multiplexing Access):

TDMA is the second form of high level multiplexing that also allows more subscribers to use the same frequency band. TDMA divides a single channel into 3 discrete time partitions. Each time partition is only for a few milliseconds at a time, therefore each channel is sending data in a round robin fashion. In order to ensure that the data is properly synchronized, each time packet contains data in between 2 guard intervals (Explained Below). The processes of TDMA are a little bit more difficult to implement due to the timing required so that the data packets are distinguishable. It also requires a little bit more overhead in synchronization due to the multiple packets that need to be put back together in order to form a single signal.

Guard Intervals: Guard intervals are used to ensure that distinct transmissions do not interfere with one another. These transmissions may belong to different users or to the same user. The purpose of the guard interval is to introduce immunity to propagation delays, echoes and reflections, to which digital data is normally very sensitive.

Some Features of TDMA:

  • TDMA shares single carrier frequency with multiple users.
  • Non-continuous transmission makes handoff simpler.
  • Slots can be assigned on demand in dynamic TDMA.
  • Advanced equalization may be necessary for high data rates if the channel is “frequency selective” and creates Inter-symbol interference.
  • Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA
  • Frequency/slot allocation complexity.
  • Pulsating power envelope: Interference with other devices.

Advantages of TDMA:

  • TDMA offers the ability to carry the data at the rate of 64kbps to 120mbps.
  • Can be adapted to any kind of transmission.
  • It ensures that, there is no interference in the transmission system.
  • It is cost-effective.

Disadvantages of TDMA:

  • It suffers from attenuation impairment when signal strength falls off.
  • The dead time between time slots can limit the potential bandwidth of TDMA channel.
  • TDMA can be affected from the noise impairment , which is an additional signal inserted between the sender and the receiver .

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