This document discusses different multiple access techniques used in satellite communication: 1. FDMA assigns each signal a different frequency band to avoid interference. Guard bands are used between signals. It has low interference but wastes capacity and requires narrowband filters. 2. TDMA assigns each channel a time slot within a frame to access the full spectrum. It allows flexible data rates but requires complex synchronization and more bits for framing. 3. CDMA assigns each channel a unique spreading code. It has the highest spectrum efficiency but induces delay and has higher equipment costs.

1. COMMUNICATION TECHNIQUES

2. • A Satellite may have different service stations of its
own located at different places on the earth. They
send carrier signal for the satellite.
• Multiple access techniques enable satellite to take or
give signals from different stations at time without
any interference between them
• FDMA (Frequency Division Multiple Access)
• TDMA (Time Division Multiple Access)
• CDMA (Code Division Multiple Access)

3. Frequency Division Multiple Access
• Assign each signal a different type of
frequency band (range)
• Any two signals should not have same type of
frequency range
• No interference between them
• Guard bands are used between the adjacent
signal spectra to minimize crosstalk between
the channels

5. Advantages
• It reduces the cost and lowers the inter
symbol interference (ISI)
• An FDMA system can be easily implemented.
• Since the transmission is continuous, less
number of bits are required for
synchronization and framing.

6. Disadvantages
• Improving the capacity depends on the signal-
to-interference reduction, or a signal-to-noise
ratio (SNR).
• Guard bands lead to a waste of capacity.
• Hardware implies narrowband filters, which
cannot be realized in VLSI and therefore
increases the cost.

7. Time Division Multiple Access
• Certain time frame to each channel. Within that
time frame, the channel can access the entire
spectrum bandwidth
• Each station got a fixed length or slot
• The slots, which are unused will remain in idle
stage.
• If a channel is transmitting heavier loads, then it
can be assigned a bigger time slot than the
channel which is transmitting lighter loads.
Advantage over FDMA

9. Advantages
• Permits flexible rates (i.e. several slots can be assigned to
a user, for example, each time interval translates 32Kbps,
a user is assigned two 64 Kbps slots per frame).
• Can withstand gusty or variable bit rate traffic. Number
of slots allocated to a user can be changed frame by
frame (for example, two slots in the frame 1, three slots
in the frame 2, one slot in the frame 3, frame 0 of the
notches 4, etc.).
• No guard band required for the wideband system.
• No narrowband filter required for the wideband system

10. Disadvantages
• High data rates of broadband systems require
complex equalization.
• A large number of additional bits are required for
synchronization and supervision.
• Electronics operating at high bit rates increase
energy consumption.
• Complex signal processing is required to synchronize
within short slots.

11. CODE DIVISION MULTIPLE ACCESS
• A unique code has been assigned to each
channel to distinguish from each other . This
code is called spreading code
• A user has access to the whole bandwidth for
the entire duration
• A signal is generated which extends over a
wide bandwidth

12. • Two types
Direct sequence spread spectrum modulation
Frequency hopped spread spectrum modulation

14. Advanatages
• CDMA channel is not easily decodable
• Less interference
• Greatest spectrum efficiency
• CDMA does not require any synchronization

15. Disadavantages
• In while we are using CDMA, code length can
be carefully selected, because it can induce
delay
• Higher cost due to the greater equipment