Multiple Access Technology for Mobile System
The transmission bandwidth in different frequency bands differs in range. It is higher than the bandwidth requires for a message to be transmitted. Hence, in order to utilize the frequency spectrum efficiency "Multiple access" methods are introduced.
Cellular systems consist of many users that will share the bandwidth. The ability of the system to carry many signals at the same time is called as "multiple access".
Through "multiple access" it is possible to share the communication capacity of the mobile system among a huge number of base stations.
Types of Multiple Access Techniques:
The three main multiple access methods used to share the bandwidth in a cellular system are:
i) Frequency division multiple access (FDMA)
ii) Time division multiple access (TDMA)
iii) Code division multiple access (CDMA)
In Frequency Division Multiple Access(FDMA). separate channels are assigned to every users and each user is allocated a unique frequency band or channel.
The Time Division Multiple Access (TDMA) systems divide the radio spectrum into time slots and in one slot only one user is allowed to transmit or receive.
In Code Division Multiple Access (CDMA) systems, the narrowband message signal is multiplied by a large bandwidth called a spreading signal. This spreading signal is actually a pseudo-noise code sequence and it has a higher chip rate than the data rate of the message signal.
Types of Multiple Access Depending on the Bandwidth Available:
Depending on the bandwidth that is available to allocate to the users, the multiple access systems are grouped as:
i) Narrowband systems
ii) Wideband systems.
i) Narrowband systems:
In these systems, the available radio spectrum is divided into a large number of narrowband channels. These channels are working with the help of FDD.
The frequency separation is increased to minimize the interference between the forward and reverse links on each channel. However, duplexers and a common transceiver antenna are to be installed in each subscriber unit.
In narrowband FDMA a user is allocated a specific channel that is not shared by other users in observation. If FDD is used then the system is called as FDMA/FDD.
In narrowband TDMA, a user shares the same radio channel but allocates a unique time slot each user on the channel.
A small number of users have different time on a single channel. The radio channels can be allocated using TDD or
FDD while each channel is shared using TDMA Such systems are called TDMA/FDD or TDMA/TDD access systems.
ii) Wideband systems:
In these systems, the transmitting bandwidth of a single channel is larger than the coherence bandwidth of the channel. So, multipath fading will not vary the received signal power. The frequency selective fades occur in a small part of the signal bandwidth at any instant of time.
In these systems there are a large number of transmitters that transmit on the same channel.
If TDMA is used then only one transmitter can access the channel at any instant of time. They can use FDD or TDD,
If CDMA is used it allows all the transmitters to access the channel simultaneously. The systems can use FDD or TDD multiplexing techniques.
Frequency Division Multiple Access (FDMA):
In wireless communication systems, the individual users are allocated individual channels. The channels or the frequency band is unique for each subscriber.
The entire allowed radio spectrum is divided into many parts of the frequency bands and each band or channel is allocated to the user. The channel allocation can be done on a demand basis to the users as a request service.
When a call is processed, no other user can share the same channel. In FDMA/FDD systems the users are assigned a pair of frequencies, one for the forward channel and the other for the reverse channel.
Features of FDMA:
i) If an FDMA channel is not in use, it will be idle and not used by any other user. Hence, there is a chance of resource wastage.
ii) The FDMA channel uses one phone circuit at any instant of time.
iii) If voice channel is assigned in FDMA, then the mobile unit and the base station start transmitting simultaneously.
iv) FDMA needs tight RF filtering to minimize the adjacent channel interference.
v) The FDMA mobile unit uses duplexers as both the transmitter and receiver operate simultaneously. It results in an increase in the cost of subscriber units and base stations.
vi) The complexity of FDMA systems is less.
vii) The FDMA systems have narrow bandwidth as each channel supports only one circuit per carrier.
viii) The symbol time is large in comparison to the delay spread. This indicates that the inter symbol interference is low and no equalization is required in FDMA narrowband systems.
ix) The cost of the cell site is higher in comparison to the TDMA systems.
x) FDMA is a continuous transmission method. So few bits are required for overhead purposes (like synchronization and framing bit).
Time Division Multiple Access (TDMA)
It uses time instead of frequency. Other users share the same time slots of the complete time available.
Each user to allocate a time slot in which the user can access the channel. In each lot, only one user is allowed to transmit or receive.
As shown in the figure each user is assigned a time slot so that channel may be thought of as a time slot that reoccurs every frame where N slots comprise a frame.
The TDMA systems transmit data in burst and buffer methods. The transmission from different users is interfaced into a loop frame structure.
A-frame consists of a number of slots. Every time frame consists of a preamble, an information message, and trail bits. Half of the time slots will be used for the forward link channels and the remaining half for reverse link channels.
The preamble field consists of an address and synchronization data so that both the base stations and subscribers use to identify each other.
The guard bits are used to provide synchronization of different receivers between, different time slots and frames. It is assumed that there are "N" number of slots for N users so that each user can access the channel in their allowed time slot.
The TDMA/FDD systems have identical frame structures that can be used for forward or reverse transmission, but the carrier frequencies will be different for both links.
TDMA Features:
The features of TDMA are:
i) TDMA uses different time slots for transmitting and receiving signals. Hence, duplexers are not required. If FDD used then a switch is used to switch between transmitter and receiver.
ii) As the transmission rates are high adaptive equalization is necessary.
iii) TDMA shares a single carrier frequency with several users, where each user makes use of non-overlapping time slots. The number of time slots depends on parameters like bandwidth, modulation method, etc.
iv) Using TDMA we can allocate different numbers of time slots per frame to different users. Thus, bandwidth can be supplied on demand to different users by reassigning time slots depending on the priority.
v) Data transmission is done in bursts. It results in low battery consumption as the transmitter of the subscriber can be turned off when it is not in use.
vi) Guard time needs to be minimized. If the signal is transmitted at the edges of the time slot, it is suppressed to reduce the guard time. The transmitted spectrum will expand resulting in adjacent channel interference
vii) In TDMA, the handoff process is simple. An enhanced link control like that provide by mobile-assisted handoff (MAHO) can be carried by the subscriber by listening on an idle slot in the TDMA frame.
vii) Because of burst transmissions high synchronization overhead is needed in TDMA systems.
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