Satellite Network Configurations

Capacity Allocation Strategies

• Frequency division multiple access (FDMA)• Time division multiple access (TDMA)• Code division multiple access (CDMA)

FDMA

• a channel access method used in multiple-access protocols as a channelization protocol.

• gives users an individual allocation of one or several frequency bands, or channels.

• Disadvantage: Crosstalk which causes interference on the other frequency and may disrupt the transmission.

Features

• all users share the satellite simultaneously but each user transmits at single frequency.

• can be used with both analog and digital signal. • requires high-performing filters in the radio hardware, in contrast

to TDMA and CDMA. • not vulnerable to the timing problems that TDMA has. Since a predetermined frequency band is available for the entire

period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA.

• Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA.

• Each user transmits and receives at different frequencies as each user gets a unique frequency slot

FDMA & FDD & FDM• FDMA allows multiple users simultaneous access to a certain

system.

• FDD refers to how the radio channel is shared between the uplink and downlink (for instance, the traffic going back and forth between a mobile-phone and a base-station).

• FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel.

• FDMA, on the other hand, is an access method in the data link layer.

Forms of FDMA

• Fixed-assignment multiple access (FAMA)– The assignment of capacity is distributed in a fixed manner among

multiple stations– Demand may fluctuate– Results in the significant underuse of capacity

• Demand-assignment multiple access (DAMA)– Capacity assignment is changed as needed to respond optimally to

demand changes among the multiple stations.– allows all users apparently continuous access of the radio spectrum by

assigning carrier frequencies on a temporary basis using a statistical assignment process.

FAMA-FDMA• FAMA – logical links between stations are preassigned

• FAMA – multiple stations access the satellite by using different frequency bands

• requires manual frequency planning, which is an difficult task in TDMA and FDMA based systems, since such systems are highly sensitive to interference from nearby satellites that are reusing the same channel.

• Another drawback with TDMA and FDMA systems with FAMA is that the number of channels remains constant irrespectively of the number of customers in a footprint. This result in traffic congestion and use of huge bandwidth.

DAMA TYPES

• SCPC• MCPC

SCPC

• refers to using a single signal at a given frequency and bandwidth.

• used on broadcast satellites to indicate that radio stations are not multiplexed as subcarriers onto a single video carrier, but instead independently share a transponder. I

• t may also be used on other communications satellites, or occasionally on non-satellite transmissions.

SCPC contd..• In an SCPC system, satellite bandwidth is dedicated to a single source. • This makes sense if it is being used for something like satellite radio,

which broadcasts continuously.• Another very common application is voice, where a small amount of

fixed bandwidth is required.• However, it does not make sense for burst transmissions like

satellite internet access or telemetry, since a customer would have to pay for the satellite bandwidth even when they were not using it.

• Where multiple access is concerned, SCPC is essentially FDMA. • Some applications use SCPC instead of TDMA, because they require

guaranteed, unrestricted bandwidth. • As satellite TDMA technology improves however, the applications for

SCPC are becoming more limited.

Advantages

• simple and reliable technology • low-cost equipment • any bandwidth (up to a full transponder) – usually 64 kbit/s to 50 Mbit/s

• easy to add additional receive sites (earth stations)

Disadvantages

• inefficient use of satellite bandwidth for burst transmissions, typically encountered with packet data transmission

• usually requires on-site control • When used in remote locations, the transmitting dish

must be protected. • A dish which is moved out of alignment can result in

fines as high as $1,100 per minute (as of 2003) from the satellite operator.

MCPC• several subcarriers are combined into a single bitstream

before being modulated onto a carrier transmitted from a single location to one or more remote sites.

• Uses TDM.• The major disadvantage of MCPC is that all of the signals

must be sent to a single place first, then combined for retransmission.

RESULT: DELAY increases. SOLN: use SCPC.

Frequency-Division Multiple Access

• Factors which limit the number of subchannels provided within a satellite channel via FDMA– Thermal noise– Intermodulation noise– Crosstalk

Reasons for Increasing Use of TDM Techniques

• Cost of digital components continues to drop• Advantages of digital components– Use of error correction

• Increased efficiency of TDM– Lack of intermodulation noise

FAMA-TDMA Operation

• Transmission in the form of repetitive sequence of frames– Each frame is divided into a number of time slots– Each slot is dedicated to a particular transmitter

• Earth stations take turns using uplink channel– Sends data in assigned time slot

• Satellite repeats incoming transmissions– Broadcast to all stations

• Stations must know which slot to use for transmission and which to use for reception

FAMA-TDMA Uplink

FAMA-TDMA

Downlink

Thank you