TDMA BURSTS ACQUISITION SYSTEM

• US 3,813,496 A
• Filed: 10/02/1972
• Issued: 05/28/1974
• Est. Priority Date: 10/02/1972
• Status: Expired due to Term

• Current Assignee: International Telecommunications Satellite Organization
• Original Assignee: Wilfrid G. Maillet
• Inventors: Maillet, Wilfrid G.
• Primary Examiner(s): Blakeslee, Ralph D.
• Application Number:
  US05/293,905
• Time in Patent Office: 603 Days
• Field of Search: 179/15BS 325/4 178/69.5R
• US Class Current: 370/324
• CPC Class Codes: H04B 7/2126 using a reference station
• CPC Class Codes: Y02D 30/70 in wireless communication networks

Abstract

In a time division multiple access communication system, burst time acquisition is accomplished automatically and rapidly taking advantage of the known range to the satellite. When the transmitter is turned on or when burst synchronization is lost, a low power acquisition signal, occurring at the frame rate and controlled in time by the burst synchronizer start pulse, is transmitted. At the receiver, an aperture is generated providing a window occupying all or a portion of the time slot assigned to the burst of the station. The acquisition signal is caused to precess across the entire frame until the acquisition signal is received during the generation of said window. When the latter occurs the start time is set back a number of bit positions corresponding to the amount of precession of the acquisition signal during a round trip delay time from the station to the repeater and back to the station. The normal burst transmission resumes thereafter.

Claims

1. In a TDMA system of the type in which multiple stations access a repeater on a time divided basis by periodically transmitting bursts of communication which form a TDMA frame, wherein at least one burst in said frame includes a frame reference indicia and wherein all other bursts include a normal reference indicia, and wherein all bursts have assigned time slots within said TDMA frame, the improvement comprising, at a local station, a. means for generating a periodic start signal occurring at said frame rate, b. acquisition signal generating means for generating and transmitting an acquisition signal at a substantially lower power than normal burst transmission in response to each said start signal, c. scanning means, responsive to said initiating means for shifting the start signals m bits per frame in one direction, whereby said acquisition signals are effectively scanned across said frame at the rate of m bits per frame, d. receiver means for receiving bursts and acquisition signals relayed via said repeater, e. means responsive to the reception of said frame reference indicia for generating an aperture during the time slot assigned to said local station, and f. means responsive to the reception of said acquisition signals during said aperture for disabling said scanning means and for shifting said start pulses K bits in a direction opposite to said one direction where K equal m times the approximate number of frames generated during a round trip delay from the station to the repeater and back to the station.

2. Apparatus as claimed in claim 1, wherein said means responsive to said acquisition signals comprises, a. means for detecting each received acquisition signal which arrives in coincidence with said aperture, b. first counter means responsive to said detected acquisition signals for providing an output signal when x detected acquisition signals are applied thereto prior to said first counter means being reset, c. second means responsive to said detected acquisition signals and to said apertures for providing a reset output y frames after detection of said first detected acquisition signal, said reset output being connected to reset said first counter means and said second means, and where y >

or = x, d. first logic means responsive to the output pulse from said first counter means for causing said scanning means to be disabled, and e. said second logic means responsive to the output pulse from said first counter means for causing said start signal generating means to shift its start signals K bits in said opposite direction.

3. Apparatus as claimed in claim 1, wherein said start pulse generating means comprises, a. a source of clock pulses, b. a frame counter for counting said clock pulses from said source, c. first recycle means for causing, when actuated, said frame counter to recycle when it reaches a count of N, where N clock pulses is the number of clock pulses in a frame, d. second recycle means for causing, when actuated, said frame counter to recycle at a count of N+m, where m <

<

N and is either a positive or negative integer, e. third recycle means for causing, when actuated, said frame counter to recycle at a count of N-K, where K equals m times the approximate number of frames generated during said round trip delay time, and f. decoder means connected to said frame counter for providing a start pulse output when said frame counter reaches a fixed count, said fixed count being no greater than the lesser of N or N+m.

4. AppAratus as claimed in claim 3, wherein said scanning means comprises, a. means for disabling said first and third recycle means, and b. means for actuating said second recycle means.

5. Apparatus as claimed in claim 4, wherein said means for shifting said start pulses K bits comprises, a. means for disabling said first and second recycle means and actuating said third recycle means, and b. means responsive to the first recycle caused by said third recycle means for disabling said third recycle means and for actuating said first recycle means.

6. Apparatus as claimed in claim 5, wherein said acquisition signal generating means includes an acquisition signal modulator having a power output which is at least 20 dB down from the power output of the normal burst transmission.

7. A local station for a TDMA communications network of the type wherein each station is assigned a time slot within a TDMA frame for accessing a repeater, said local station being of the type which includes, a. source of clock pulses, b. frame counter means for counting said clock pulses, c. first recycle means for causing said frame counter to recycle when it reaches a count of N, where N is equal to the number of clock pulses per TDMA frame, d. decoder means responsive to a fixed count of said frame counter for providing output start pulses, e. burst transmission means, including preamble generator means and data generator means, responsive to each said start pulse for transmitting a burst of communication, said burst including a preamble portion and a data portion and said preamble portion including a unique code word, f. receiver means for receiving communications repeated by said repeater, said receiver means including means for providing a frame reference signal upon receipt of a particular predetermined reference code, and means for providing a local station signal upon receipt of said unique code word in the preamble of said local station, g. means responsive to said frame reference signal and said local station signal for accumulating an error amount representing the position error of said local station burst in said TDMA frame, h. second recycle means responsive to said error amount for causing said frame counter to recycle at a count of N+p until said error amount is eliminated, where p is an integer much less than N and is either positive or negative dependent upon the direction of said position error, i. means responsive to the operation of said second recycle means for disabling said first recycle means while said second recycle measn is operating to cause recycling at N+p, and j. means responsive to said frame reference signal and said local station signal for providing a sync loss output signal when the local station burst is not within a predetermined range of its assigned position in the TDMA frame, the improvement comprising:

aa. means, when enabled, responsive to said start pulses for generating and transmitting an acquisition signal at a power substantially lower than the power of said local station burst, bb. third recycle means for causing said frame counter to recycle at a count of N+m where m is a positive or negative integer much less than N, cc. fourth recycle means for causing said frame counter to recycle at a count of N-K, where K equals m times the approximate number of TDMA frames per round trip time to the repeater, dd. first logic means for disabling said burst transmission means and said first and second recycle means, and for enabling said acquisition signal generating means and said third recycle means, said first logic means being actuated by said sync loss signal or by turn on of said station, ee. means responsive to said frame reference signal for generating an aperture pulse which covers a portion of the time slot assigned to said local station, ff. means responsive to said apErture pulses and connected to said receiver means for detecting acquisition signals received during the time covered by said aperture pulses, gg. second logic means responsive to the detection of a predetermined number of acquisition signals during a predetermined number of frames for disabling said third recycle means and enabling said fourth recycle means, and hh. third logic means for disabling said third recycle means and for enabling said burst transmission means and said first recycle means after said third recycle means has caused one recycling of said frame counter.

8. The improvement as claimed in claim 7 wherein said first logic means comprises separate means for disabling the transmission of the preamble portion and the data portion of said burst, and wherein said third logic means comprises, a. means following the first recycle of said frame counter at N-K for enabling said first recycle means to cause said frame counter to recycle at counts of N, and for enabling the transmission of the preamble portion only of said burst, b. means operative a fixed delay time after operation of said last-mentioned means for enabling said second recycle means, whereby said first and second recycle means and said error accumulating means become operative to alter the start pulses and place the now generated preamble in the proper assigned position, and c. means operative a short delay after the operation of said last-mentioned means for testing said error amount and enabling the transmission of said data portion of said burst when said error amount is reduced to a predetermined value.

9. The improvement as claimed in claim 7 wherein said means for detecting acquisition signals received during the time covered by said aperture pulses comprises, a. gating means responsive to the coincidence of a received acquisition signal and an aperature pulse for providing an output pulse, b. counting means for counting said output pulses and providing a detection indicating signal upon the accumulation of a predetermined number of said output pulses, c. means responsive to the first said output pulse for resetting said counting means a predetermined number of frames thereafter, whereby a detection signal is generated by said counting means only if said predetermined number of output pulses occur within said predetermined number of frames.