SATELLITE MULTIPLE ACCESS TECHNIQUES
Mobile users
share the wireless spectrum at the same time. The sharing might be based on
frequency, time, or code. Wireless telephony employs duplexing technology,
which permits talking and listening at the same time, similar to FDD frequency.
Division Duplexing provides each user with two bands of frequencies. Because a
simplex channel is made up of a forward and a reverse channel, each duplex
channel is made up of one simplex channel.
The forward
and reverse channels are separated by a frequency range across the system. TDD,
or time division duplexing, uses time instead of frequency to have identical
forward and reverse channels.
Both FDD and
TDD have advantages and drawbacks. In wireless communication systems, these
multiple access methods utilizing any of these approaches are utilized.
DIFFERENT
MULTIPLE ACCESS TECHNIQUES:
THE THREE
important multiple access techniques are:
1 TIME
DIVISION MULTIPLE ACCESS
2 FREQUENCY
DIVISION MULTIPLE ACCESS
3 CODE
DIVISION MULTIPLE ACCESS
Time
Division Multiple Access (TDMA)
The time slots of the whole available time are shared between several people. To achieve bidirectional communication, each duplex channel (TDD) includes distinct forward and reverse time slots. The figure represents a TDD time slot with the same frequency:
This TDD mode
allows any transceiver to function as both a transmitter and a receiver. TDD is
commonly seen in cordless phones. The TDMA concept is shown. In a TDMA system,
the entire spectrum is split into time slots, with each time slot allowing just
one user to utilize the radio channel.
Digital data transmission and digital modulation are permitted in TDMA. Many customers can tune in to their favorite station within their designated time intervals. As demonstrated, several users' transmissions are interlaced into a single frame.
The synchronization
information and address information are included in the preamble field in this
way. The guard bits are used to allow separate receivers to be synchronized
across different frames and time periods.
It is
believed that there are N time slots for N users, with each user accessing the
channel during their allotted time period.
The following
are some of the characteristics of TDMA: 1)Each user of the TDMA multiple
access systems has the same carrier frequency, but no time periods overlap.
Frequency Division Multiple Access in Mobile Communication
All users share the satellite at the same time using FDMA,
but each broadcasts in its frequency band. When using analog modulation, when
signals are present all of the time, this is the most typical technique used.
As shown in the diagram, the available transponder bandwidth is shared among
the users, allowing them to all broadcast at the same time.
The most basic type of multiple access is FDMA. It assigns a
carrier frequency (or many carrier frequencies for busy stations) and a
bandwidth around that carrier frequency to an earth station permanently. All of
the station's outgoing traffic is frequency modulated on that carrier,
regardless of destination.
Each carrier is allocated a specific frequency band for the
uplink in fixed-frequency operation, and other carriers share that band.
Several carriers share frequency bands for demand multiple access (DMA), with a
specific band assigned based on availability at the time of need.
In an FDMA system, each carrier spectra must be sufficiently
separated from one another to avoid carrier crosstalk. The nearby low-power
carrier will be affected by a high-power carrier. Excessive separation, on the
other hand, causes satellite bandwidth to be squandered. For accessing, FDMA
uses extremely easy frequency adjustment and provides virtually independent
channel off operation.
A satellite's frequency plane is made up of the carrier
frequencies and bandwidth allotted to all ground stations. Every FDMA station
must be able to receive at least one carrier from all of the other stations in
the network.
Bidirectional communication is feasible in conversational
telephonic systems at the same time, and it is also necessary for cellular
communication. A duplexer is a device that allows you to talk and listen at the
same time. Frequency multiplexing/duplexing is referred to as frequency
division duplexing (FDD) since it is done with frequency.
The duplex channel in FDD has two simplex channels, forward
and reverse, as shown in the diagram. The forward frequency band transmits
radio traffic from the base station BTS to the mobile unit, whereas the reverse
frequency band transmits radio traffic from the mobile unit to the base station
BTS. To enable simultaneous conversion, a duplex device is kept in both the
mobile unit and the base station.
Some
features of the FDMA scheme:
1) To
eliminate adjacent channel interferences ACI, FDMA requires appropriate
filtering on the receiver side.
2) In the
FDMA scheme, if a channel is not in use, it is considered idle and is not used
by other users. As a result, there is a risk of resource waste.
3)A single
phone circuit can be handled by an FDMA channel at a time.
Types:
1) fixed
assignment multiple access FAMA
2) Demand
assignment multiple access DAMA
Non-linear
effects in FDMA SCHEME OF MULTIPLEXING:
Multiple radio channels share the same antenna at the base
station in this system. For optimum power and efficiency, power amplifiers are
run at saturation, and they are non-linear. Intermodulation frequency
production is caused by the dispersion of signals throughout the whole
frequency domain. It will amplify interferences in the actual transmission,
therefore IM should be kept to a minimum.
Code Division Multiple Access (CDMA)
Many earth stations broadcast orthogonally coded spread-spectrum signals in the same frequency range at the same time via CDMA. Decoding systems receive several stations' mixed broadcasts and retrieve one. Users can broadcast at the same time and share the frequency allotment under the CDMA system. The diagram represents the block diagram of the CDMA System.
Several users utilize the whole transponder bandwidth in this
system at all times. Signals from various users are encoded so that information
from a single transmitter can only be recognized and retrieved by a correctly
synchronized receiving station that understands the code. That is, each
receiving station has its code, referred to as its address, and anytime a
transmitting station intends to send a message to that receiver, it simply
modulates its broadcast with the desired receiver's address.
At an earth station, carrier separation is accomplished by
identifying the carrier with the correct address. These addresses are often in
the form of a periodic binary sequence that modulates the carrier or changes
the carrier's frequency state. The carrier correlation procedure is used to
identify addresses. For code generators, a digital address is obtained.
A station address generator repeats its address sequence,
which is overlaid on the carrier alongside the data.
CDMA is better suited to military tactical communication environments where numerous small groups of mobile stations are only connected for a short period at irregular intervals.
There may be an issue of near-far impact since the same
channel is used by multiple users. When compared to other multiple access
methods, the major advantage of CDMA is the lower degree of interference. At
the receiving end, the receiver selectively adjusts to hearing the indented
signal of the users since each user/subscriber is assigned a distinct pseudo-random
codeword that is orthogonal to all other pseudo-random codewords of remaining
users. To avoid near-far issues, CDMA uses proper power control methods.
Some features of CDMA multiple access schemes:
1) In CDMA, if the spreading sequences are not perfectly
orthogonal from one user to another, there is a risk of self-jamming. As a
result, this spreading sequence or pseudo-random noise code must be meticulously
prepared before being multiplied by the message signal.
2) Compared to TDMA and FDMA, CDMA has a higher soft capacity
limit.
3) The RSSI (radio signal strength indicator) is used in CDMA
to improve power control.
Types
1) Fixed assignment multiple access
2) Demand assignment multiple access