MOD 8 UNIT 3A

Satellites enable passing to and from geographically separated locations 

Multiple Access = Satellites ability to multiplex signals from numerous terminals in a variety of ways 

Frequency Division Multiple Access (FDMA)

• First access method to be developed (oldest)

• Terminals separated by frequency inside of channels allocated bandwidth

• Between frequencies is guard bands which minimize crosstalk by spacing out users (25MHz spacing)

• Advantageous to support connection oriented applications 

• Inefficient for satellites total capacity due to users not constantly utilizing their frequency
  
  

Time Division Multiple Access (TDMA)

• Assigns terminals dedicated time slots on a channels bandwidth

• Time slot acts as the guard band to prevent interference 

• To employ, stringent timing and network controls must be in place
  
  

Code Division Multiple Access (CDMA)

• Operates as spread spectrum technique

• Uses Pseudorandom noise codes to separate its users 

• All terminals transmit at same time over same frequency 

• Receiving terminal actively looks for specific coded waveform 

• 200 KHz
  
  

Wideband Code Division Multiple Access (WCDMA) 

• Uses wider frequency bands than CDMA for higher throughput for data transmission

• Input signals are digitized and transmitted in coded spread spectrum mode 

• A 5MHz wide carrier is used 

• Asynchronous allowing more flexibility in the allocation resources
  
  

DEMAND ASSIGNED MULTIPLE ACCESS (DAMA)

• Operate on first come first serve basis 

• Allocated to users on as needed basis 

• After communication, frequency is returned to pool where it can be used by next user