Duplexing


Duplexing

Duplexing is a technique employed in communication system for bi-directional or two-way transmission of voice or data.
Down link: data is carried from base station to subscriber terminal. It is also called forward link.
Uplink : data is carried from subscriber terminal to base station. It is also called reverse link.


Two main techniques for dividing forward and reverse communication channels on the same physical transmission medium are

  • Frequency Division Duplex
  • Time Division Duplex

Frequency Division Duplexing(FDD)

  • Allows two-way radio communication by using two distinct radio frequency channels. At any instant of time downlink use channel frequency(f_r) that is different from uplink channel frequency (f_t). It allows simultaneous transmit and receive of data or voice.
  • FDD systems utilize channel plans that are comprised of frequencies with equal bandwidth. This makes FDD ideal for symmetrical communication applications in which the similar traffic flows in both directions, such as voice communications. In applications such as Internet access, which can be very asymmetric in nature, a large percentage of the available Uplink bandwidth remains unused and is, therefore, wasted. Once the channel bandwidth is granted by the regulator, the UL/DL allocation cannot be changed. This leads to unused spectrum for asymmetric operations.
  • The transmitter can desensitize the receiver.
  • Switching of frequencies in FDD require base station. Peer to peer communication not possible.
  • Guard band : A guard band about two times the size of the UL or DL channel is required to separate the UL and DL channels. This amounts to an additional 50% loss in spectrum.

Time Domain Duplexing(TDD)

  • In TDD a single frequency channel is used to transmit signals in both the downstream and upstream directions. Mostly used in fixed wireless point-to-point systems. It allows direct peer-to-peer communications.
  • A duplexer must be designed for operation in the frequency band used by the receiver and transmitter, and must be capable of handling the output power of the transmitter.
  • A duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency, and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver.
  • A duplexer must provide sufficient isolation to prevent receiver desensitization.
  • Guard time is required between Tx and Rx and vice versa. The guard time is equal to a unit’s turn around time plus the round trip delay. A unit’s turn around time is in the order of 50 us. The round trip delay is in the order of 66 us. Thus the round trip delay can absorb the transmitter’s turn around time whenever the direction of traffic switches. The loss in throughput due to guard time for a 5 ms frame is about 2%.

It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results. Thus, bidirectional systems must separate the uplink and downlink channels into orthogonal signaling dimensions, typically using time or frequency dimensions.

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