What is multiple access technique? Do a comparative study among FDMA,TDMA and CDMA.
Multiple Access Techniques are techniques to access a single channel by multiple users. They provide multiple access to the channel. A “channel” refers to a system resource allocated to a given mobile user enabling the user to establish communication with the network (other users). Multiple access techniques are used to allow a large number of mobile users to share the allocated spectrum in the most efficient manner. As the spectrum is limited, so the sharing is required to increase the capacity of cell or over a geographical area by allowing the available bandwidth to be used at the same time by different users.
A cellular system divides any given area into cells where a mobile unit in each cell communicates with a base station. The main aim in the cellular system design is to be able to increase the capacity of the channel i.e. to handle as many calls as possible in a given bandwidth with a sufficient level of quality of service. There are several different ways to allow access to the channel. These includes mainly the
1) Frequency division multiple-access (FDMA)
2) Time division multiple-access (TDMA)
3) Code division multiple-access (CDMA)
Frequency division multiple access (FDMA):
It is a technology by which the total bandwidth available to the system is divided into frequencies. This division is done between non overlapping frequencies that are then assigned to each communicating pair.FDMA is used mainly for analog transmission. In FDMA all users share the satellite simultaneously but each user transmits at single frequency.
For eg: For example in GSM entire frequency band of 25 MHz is divided into 124 RF carriers of bandwidth 200 KHz each. In Satellite applications entire transponder band of 500 MHz is divided into 24 channels each of bandwidth 40MHz (36 MHz useful and 4MHz guard band).
Code division multiple access (CDMA):
Unlike FDMA, CDMA separates calls by code. Every bit of a conversation is been tagged with a specific and unique code. The system gets a call, it allocates a unique code to that particular conversation, now the data is split into small parts and is tagged with the unique code given to the conversation of which they are part of. Now, this data in small pieces is sent over a number of the discrete frequencies available for use at any time in the specified range. The system then at the end reassembles the conversation from the coded bits and deliver it. For example in CDMA IS-95 standard entire bandwidth of 1.225 MHz is shared by various channels/users using unique 64 Walsh Codes.
Time division multiple access (TDMA):
What do you mean by co-channel interference and system capacity?
Briefly describe the congestion control, slow start and fast retransmit mechanism.
Write short note on Call setup of GSM network for mobile-to-mobile call.