Synchronizer for MSK burst communications
AssigneeCommunications Satellite Corporation
InventorSmith A. Rhodes
Life of this patent
Estimating the term…
Abstract
A synchronization preamble for each TDMA burst consists of an alternating sequence of the two MSK tones on every other MSK bit interval. Such a preamble does not exhibit random polarity reversals at the two MSK frequencies as occurs during the message burst. The receiving circuit therefore directly processes the preamble symbols to extract carrier phase and symbol synchronization without conventionally squaring the IF to remove polarity modulation. A 6 dB increase in synchronization SNR is thereby provided since the requisite synchronizer filtering need only be effected at the fundamental frequencies F.sub.L and F.sub.H rather than at the their second harmonics 2F.sub.L and 2F.sub.H as is required when squaring is employed.
Claims12
An apparatus for demodulating a burst signal having a preamble portion and a message portion comprising
- first (70) and second (65) mixers having said burst signal applied directly thereto;
- a third mixer having said burst signal applied thereto by way of delay means;
- first and second band pass filters receiving an output of said third mixer, said first and second band pass filters having upper and lower center frequencies, respectively;
- first signal combining means (90, 95) receiving outputs from said first and second band pass filters for providing first and second reference signals in phase quadrature, said first and second reference signals being applied to said first and second mixers, respectively, to thereby demodulate said burst signal with (i) said first reference signal in said first mixer and (ii) said second reference signal in said second mixer;
- second signal combining means receiving outputs from said first and second band pass filters for providing first and second timing signals;
- first and second data generating means receiving outputs from said first and second mixers, respectively, for generating first and second binary signals under the control of said first and second timing signals;
- means (135) for applying said first binary signal to said third mixer when said message portion of said burst signal is being received to thereby demodulate the polarity of said message portion in accordance with said first binary signal;
- and
- means for applying a constant signal level to said third mixer when said preamble portion of said burst signal is being received to thereby avoid polarity demodulation of said preamble portion of said burst signal.
An apparatus for providing carrier and symbol synchronization for a burst transmission signal having short synchronization preamble and message portions, wherein carrier phase and symbol timing are acquired for each burst from said short synchronization preamble, said apparatus comprising:
- a first mixer (75) having a delayed burst transmission applied thereto for providing a first mixed signal;
- first and second band pass filter means receiving said first mixed signal to provide first and second filtered signals;
- first signal combining means receiving said first and second filtered signals for providing first and second carrier phase reference signals in phase quadrature;
- second signal combining means receiving said first and second filtered signals for providing first and second symbol timing signals, said first and second symbol timing signals being offset in time from one another;
- means for applying a constant signal level to said first mixer when said preamble portion is being received to thereby avoid polarity demodulation of said preamble portion;
- and
- means for applying a variable signal level to said first mixer when said message portion is being received to thereby demodulate the polarity of said message portion in accordance with said variable signal level.
A method of transmitting a group of MSK symbols at a predetermined rate in a burst of data having a preamble portion and a message portion, individual symbols being transmitted in individual symbol intervals, said method comprising:
- (a) generating a first sinusoidal carrier signal;
- (b) generating a second sinusoidal carrier signal in phase quadrature with respect to said first sinusoidal signal;
- (c) modulating said first sinusoidal carrier signal with a first modulation signal to produce a first modulated signal;
- (d) modulating said second sinusoidal carrier signal with a second modulation signal in phase quadrature with respect to said first modulation signal to produce a second modulated signal;
- (e) selectively modulating the polarity of said first and second modulated signals with first and second data signals, respectively, to produce first and second final signals, each said first and second data signals having possible transition times at every other MSK symbol interval, said first and second possible transition times being staggered in time by one symbol interval;
- (f) adding said first and second final signals to produce a transmission signal;
- and
- (g) maintaining said first data signal at a constant level during said preamble portion whereby no polarity reversals in said first sinusoidal carrier occur during said preamble.
Dependencies are inferred from the claim text.