FDMA in Satellite Engineering Quiz

Correct Answer: Frequency Division Multiple Access
FDMA stands for Frequency Division Multiple Access, which is a channel access method used in multiple-access protocols. In satellite communications, FDMA divides the available bandwidth into multiple non-overlapping frequency bands, with each user assigned a specific band for the duration of their communication.

Correct Answer: To prevent interference between adjacent channels
Guard bands are unused frequency intervals between adjacent channels in FDMA systems. They help prevent inter-channel interference caused by imperfect filters, spectral spreading, and Doppler shifts in satellite systems. While guard bands reduce overall spectral efficiency, they are essential for maintaining signal integrity.

Correct Answer: No need for precise time synchronization between earth stations
In FDMA, each earth station transmits on its assigned frequency continuously, eliminating the need for precise timing synchronization between stations (unlike TDMA). However, FDMA is susceptible to intermodulation distortion, has lower spectral efficiency than some other techniques, and is less efficient for bursty traffic.

Correct Answer: Intermodulation products are generated due to transponder nonlinearity
Satellite transponders have nonlinear power amplifiers (typically TWTA or SSPA). When multiple carriers pass through these nonlinear devices, intermodulation products are generated at sum and difference frequencies. These products can cause interference, requiring careful power management and guard bands in FDMA systems.

Correct Answer: Preassigned FDMA (PA-FDMA)
In Preassigned FDMA, specific frequency bands are permanently allocated to specific earth stations. This is simple to implement but inefficient when traffic is variable. Demand-Assigned FDMA dynamically allocates frequencies as needed, improving efficiency but requiring more complex control systems.

Correct Answer: Lower power efficiency due to multiple carriers operating simultaneously
In FDMA, multiple carriers operate simultaneously through the satellite transponder, forcing it to operate in a less efficient, linear region to minimize intermodulation. In TDMA, only one carrier uses the transponder at any instant, allowing operation near saturation for higher power efficiency.

Correct Answer: Polarization frequency reuse
Polarization frequency reuse allows the same frequency band to be used twice by transmitting signals with orthogonal polarizations (horizontal and vertical, or left-hand and right-hand circular). This effectively doubles the capacity without requiring additional bandwidth, though it requires more complex antenna systems.

Correct Answer: To separate different frequency bands for individual processing
The input multiplexer (IMUX) in a satellite transponder uses bandpass filters to separate the composite received signal into individual frequency bands. Each band is then processed independently (amplified, frequency converted) before being recombined by the output multiplexer (OMUX) for transmission to earth.

Correct Answer: Each carrier is modulated by a single voice or data channel
SCPC-FDMA assigns one carrier per communication channel. Each earth station's voice or data channel modulates its own carrier at a unique frequency. This is efficient for low-traffic routes and VSAT networks. MCPC (Multiple Channel Per Carrier) combines multiple channels into one carrier for higher traffic routes.

Correct Answer: To minimize intermodulation distortion caused by nonlinear amplification
Power amplifiers in satellite transponders are most efficient near saturation but become nonlinear in this region. When multiple FDMA carriers are amplified simultaneously, nonlinearity creates intermodulation products. Back-off (operating at reduced power levels) linearizes the amplifier but reduces overall power efficiency.

Correct Answer: Available bandwidth and channel spacing
The number of FDMA channels is approximately equal to the total available bandwidth divided by the sum of channel bandwidth and guard band. For example, with 36 MHz total bandwidth, 5 MHz channels, and 1 MHz guard bands, approximately 6 channels could be accommodated (36/(5+1) = 6).

Correct Answer: To shift the frequency from uplink to downlink band
In a bent-pipe (transparent) transponder, the local oscillator provides a stable frequency reference for mixing. The received uplink signals are mixed with the LO frequency to convert them to the downlink frequency band. For example, a C-band transponder might convert 6 GHz uplink to 4 GHz downlink using a 2 GHz LO.