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Advantages and Disadvantages of TDMA and FDMA

This page covers advantages and disadvantages of TDMA and FDMA. It mentions TDMA advantages, TDMA disadvantages, FDMA advantages and FDMA disadvantages. TDMA stands for Time Division Multiple Access and FDMA stands for Frequency Division Multiple Access.

Introduction:
To understand concept of TDMA and FDMA, we will take example of GSM system where in mobile user-1 utilizes resources of base station (i.e. frequency and time) to communicate with other mobile user-2. The transmissions from mobile user to base station is known as uplink and transmissions from base station to mobile user is known as downlink.

What is FDMA?

FDMA in GSM900

In GSM, large frequency band (25 MHz) is divided into smaller frequency bands (200 KHz) known as channels. Moreover separate frequency bands are allocated for uplink (890 to 915 MHz) and downlink (935 to 960 MHz) as shown in the figure-1. Total of 124 channels are available with each having 200KHz bandwidth in each direction (uplink and downlink). For communication between users and Base station, one dedicated frequency is used for uplink and one for downlink. Hence simultaneous transmissions are possible in GSM. This process is known as FDMA (Frequency Division Multiple Access).
Example: FDMA is used to share satellite bandwidth among multiple ground stations (e.g. VSATs).

Benefits or advantages of FDMA

Following are the benefits or advantages of FDMA:
➨It allocates dedicated frequencies to different stations. Moreover there are separate bands for both uplink and downlink. Hence stations transmit and receive continuosly at their allocated frequencies.
➨It is very simple to implement with respect to hardware resources.
➨FDMA is efficient when constant traffic is required to be managed with less number of user population.

Drawbacks or disadvantages of FDMA

Following are the disadvantages of FDMA:
➨In FDMA, frequencies are allocated permanently and hence spectrum will be wasted when stations are not transmitting or receiving.
➨Network and spectrum planning is cumbersome and time consuming.
➨It uses guard bands to prevent interference. This wastes very useful and scarce frequency resources.
➨It requires RF filters to meet stringent adjacent channel rejection specifications. This increases cost of the system.
➨Maximum bit rate per channel is constant and hence can not be used for varying data rate requirements as per QoS and priorities.

What is TDMA?

TDMA-Time Division Multiple Access

We already know that frequency is very critical resource to utilize it for communication purpose. Due to this and in order to support more number of users with a single base station, eight time slots are allocated to each channels in GSM as shown in figure-2. Hence eight users can communicate with single frequency channel. This process is known as TDMA (Time Division Multiple Access). GSM system utilizes both TDMA and FDMA to use available time and frequency resources efficiently.

Benefits or advantages of TDMA

Following are the benefits or advantages of TDMA:
➨Here stations transmit on single frequency (say 'f1') but at different time instants. Likewise they receive on another frequency (say 'f2') at different time slots. As TDMA makes efficient use of the spectrum, more number of users can use same spectrum than FDMA system.
➨The operational costs of TDMA networks are lower compare to traditional FDMA network.
➨The different types of traffic e.g. voice, data and video are transmitted using TDMA technique as this require different data rates which can be easily achieved using allocation of multiple time slots.
➨The resources are utilized effectively as per need basis and hence battery life can be enhanced.
➨It can be used along with DAMA to utilize frequency/time resources efficiently on need basis.

Drawbacks or disadvantages of TDMA

Following are the disadvantages of TDMA:
➨Network and spectrum planning require more efforts.
➨Multipath interference affects call quality.
➨Switching from one base station cell to the other cell results into dropped calls.



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