Satellite multiple accessing implies that more than one user has access to a one or more radio channels within a satellite communications channel. Transponders are typically leased by a company or a common carrier for the purpose of providing voice or data transmission to a multitude of users. The method by which a satellite transponder's bandwidth is used or accessed depends on the multiple accessing method utilized.
The three most commonly used multiple- accessing arrangements: frequency division multiple accessing , time division multiple accessing and code division multiple accessing . With FDMA each earth stations transmission are assigned specific uplink and downlink frequency bands within an allotted satellite bandwidth they may be preassigned or demand assigned . FDMA transmission are separated in the frequency domain and, therefore, must share to total available transponder banswidth as the total transponder power. With TDMA , each earth station transmits a short burst of information during a specific time slot within a TDMA frame. The burst must be synchronized so that each stations burst arrives at the satellite at a different time.
TDMA transmissions are separated in the time domain , and with TDMA the entire transponder bandwidth and power are used for each transmission but for only a prescribed interval of time. With CDMA , all earth stations transmit within the same frequency band and , for all practical purposes, have no limitations on when they may transmit or on which carrier frequency.Thus, with CDMA, the entire satellite transponder bandwidth is used by all stations on a continuous basis. Signal separation is a accomplishes with envelop encryption/decryption techniques.
TDMA, Time division multiple access is the predominant multiple-access method used to-day. It provides the most efficient method of transmitting digitally modulated carriers(PSK) . TDMA is a method of time division multiplexing digitally modulated carriers between participating earth stations within a satellite networks through a common satellite transponder . With TDMA , each earth stations a short burst of a digitally modulated carrier during a precise time slot within a TDMA frame. Each station's burst is synchronized so that it arrives at the satellite transponder at any given time , thus avoiding a collision with another station's carrier. The trans[ponder is an RF=RF repeater that simply receives the earth station transmissions, amplifies them and then retransmits them in a downlink beam that is received by all the participating earth stations.
Each earth station receives the burst from all other earth stations and must select from them the traffic destined only for itself. Transmissions from all earth stations are synchronized to a reference burst. The reference burst as a separate transmission but it may be the preamble that precedes a reference stations transmission of data. also , there may be more than one synchronizing reference burst. The reference burst contain a carrier recovery sequence (CRS) , from which all receiving stations recover a frequency and phase coherent carrier for PSK demodulation.
Also included in the reference burst is a binary sequence for bit timing recovery. At the end of each reference burst, a unique word is transmitted.The unique word is used to establish a precise time reference that each of the stations uses to synchronized the transmission of its burst. The UW is typically a string of 20 successive binary 1s terminated with a binary0. Each earth station receiver demodulate and integrator and threshold detector are designed so that the threshold voltage is reached precisely when the last bit of the UW sequence is integrated. This generates a correlation spike at the output of the threshold detector at the exact time the UW sequence ends.
Each earth station synchronizies the transmission of its carrier to the occurrence of the UW correlation spike . Each station waits a different length of time before it begin transmitting. No two stations will transmit the carrier at the same time. Note the guard time transmissions from successive stations. This is analogous to a guard band in a frequency division multiplexing system. Each stations precedes the transmission of data with a preamble. The preamble is logically equivalent to the reference burst. Because each stations transmission must be received by all other earth stations all stations must recover carrier and clocking information prior to demodulating the data. If demand assignment is used a common signaling channel also must be include in the preamble.
Also included in the reference burst is a binary sequence for bit timing recovery. At the end of each reference burst, a unique word is transmitted.The unique word is used to establish a precise time reference that each of the stations uses to synchronized the transmission of its burst. The UW is typically a string of 20 successive binary 1s terminated with a binary0. Each earth station receiver demodulate and integrator and threshold detector are designed so that the threshold voltage is reached precisely when the last bit of the UW sequence is integrated. This generates a correlation spike at the output of the threshold detector at the exact time the UW sequence ends.
Each earth station synchronizies the transmission of its carrier to the occurrence of the UW correlation spike . Each station waits a different length of time before it begin transmitting. No two stations will transmit the carrier at the same time. Note the guard time transmissions from successive stations. This is analogous to a guard band in a frequency division multiplexing system. Each stations precedes the transmission of data with a preamble. The preamble is logically equivalent to the reference burst. Because each stations transmission must be received by all other earth stations all stations must recover carrier and clocking information prior to demodulating the data. If demand assignment is used a common signaling channel also must be include in the preamble.
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