Automatic simulcast alignment

Apparatus for simultaneously receiving a plurality of identical RF signals asynchronously from a plurality of transmitter means without mutual interference, each of the RF signals being modulated to convey identical digitally encoded information in the form of symbols, comprising:

• receiving means including a single receiving antenna for simultaneously receiving the plurality of RF signals from said antenna;
• adaptive filter means for adaptively filtering the received RF signals to minimize inter-symbol interference due to a misalignment in time between the received RF signals, the adaptive filter means having an input coupled to an output of the receiving means and comprising a decision feedback equalizer having a plurality (n) of feed-forward taps spaced apart from each other by a fraction of a symbol period and a plurality (m) of feedback taps spaced apart from each other by a symbol period, and modifying means for modifying a signal in accordance with an associated coefficient, each of the feed-forward and feedback taps being coupled to an input of said modifying means;
• and
• recursive generating means having an input coupled to an output of the adaptive filter means for recursively generating the coefficients in accordance with a Complex Fast-Kalman Algorithm.

Apparatus for simultaneously receiving a plurality of identical RF signals asynchronously from a plurality of transmitter means without mutual interference, each of the RF signals being modulated to convey identical digitally encoded information having a preamble portion, a data portion and a midamble portion, comprising:

• receiving means for simultaneously receiving at one location the plurality of RF signals over a channel;
• first filter means having an input coupled to an output of the receiving means and comprising a fixed transversal filter means having a characteristic selected for matching to a fixed signal pulse characteristic of the received signals for producing;
• at the output, a filtered
• second filter means having an input to the output of the first filter means and comprising an adaptive filter means having a characteristic selected for adaptively compensating the filtered signal for a time varying impulse response of the channel, said second filter means including a characteristic selected for adaptively compensating the filtered signal for a misalignment in time between the received RF signals;
• means responsive to the filtered signal for initially adapting the second filter means to a synchronizing portion of the filtered signal in accordance with a Recursive Least Squares procedure, and
• additional means responsive to the filtered signal for subsequently adapting the second filter means to the data portion of the filtered signal in accordance with a Normalized Least Mean Square (NLMS) procedure.

Apparatus for simultaneously receiving a plurality of identical RF signals asynchronously from a plurality of transmitter means without mutual interference, each of the RF signals being modulated to convey identical digitally encoded information having a preamble portion, a data portion and a midamble portion, comprising:

• receiving means for simultaneously receiving at one location the plurality of RF signals over a channel;
• first filter means having an input coupled to an output of the receiving means and comprising a fixed transversal filter means having a characteristic selected for matching to a fixed signal pulse characteristic of the received signals for producing, at the output, a filtered signal:
• and
• second filter means having an input to the output of the first filter means and comprising an adaptive filter means having a characteristic selected for adaptively compensating the filtered signal for a time varying impulse response of the channel, said second filter means including a characteristic selected for adaptively compensating the filtered signal for a misalignment in time between the received RF signals and means for processing the filtered signal in accordance with a Viterbi algorithm repetitively applied on blocks of 2D symbols, where D represents decision depth for the Viterbi algorithm, such that D symbols are released for adapting the second filter means to the data portion of the filtered signal in accordance with a Normalized Least Mean Square (NLMS) procedure.

Apparatus for simultaneously receiving a plurality of identical RF signals asynchronously from a plurality of transmitter means, each of the RF signals being modulated to convey identical digitally encoded information, comprising:

• receiving means for simultaneously receiving the plurality of RF signals;
• adaptive filter means for adaptively filtering the received RF signals to minimize inter-symbol interference due to an effect of a misalignment in time between the received RF signals, the adaptive filter means having a first input coupled to an output of the receiving means and comprising multi-stage lattice decision feedback equalizer means having (M =N1-N2) scalar taps followed by (N2-1) two-dimensional taps and metric forming means for forming a metric in accordance with a predetermined mathematical relationship between the tap output and another tap output, including accumulating means for accumulating a plurality of the formed metrics over a plurality of data packets, each of the taps being coupled to an input of said forming means;
• and
• recursive determining means having an input coupled to outputs of the accumulating means for recursively determining a number of taps for the lattice decision feedback equalizer means for use during a next data packet, the recursive determining means including means for comparing a difference between two accumulating means output signals to a threshold value.

A method for simultaneously receiving a plurality of identical RF signals asynchronously from a plurality of transmitter means without mutual interference, each of the RF signals being modulated to convey identical digitally encoded information, comprising the steps of:

• simultaneously receiving the plurality of RF signals at a single antenna;
• adaptively filtering the received RF signals to minimize inter-symbol interference due to a misalignment in time between the received RF signals, the step of adaptively filtering including operating a decision feedback equalizer means having a plurality (n) of feed-forward taps spaced apart from each other by a fraction of a symbol period and a plurality (m) of feedback taps spaced apart from each other by a symbol period and modifying the output signal from said taps in accordance with an associated coefficient for each of said taps, respectively;
• and
• recursively generating the coefficients in accordance with a Complex Fast-Kalman Algorithm.

In a receiver for receiving a digital simulcast transmission, a method for demodulating a received RF signal having a plurality of symbols comprising at least a synchronizing portion and a data portion of a data packet, comprising the steps of:

• (a) receiving the RF signal over a channel;
• (b) determining a channel impulse response of the received signal over the synchronizing portion;
• (c) employing a normalized least mean square algorithm for adapting the channel impulse response;
• (d) operating a maximum likelihood sequence estimator over 2D symbols, D being decision depth delay;
• (e) releasing D symbols;
• (f) employing the D released symbols to update the channel impulse response;
• and
• (g) beginning with a Dth symbol repeating steps (d), (e) and (f) until a penultimate data block having 2D symbols is received;
• (h) or, when 2D is not an integral multiple of a number of symbols N.sub.sym, appending {N.sub.sym -N.sub.sym / 2D2D} symbols to a last block of 2D symbols.

In a receiver for receiving a digital simulcast transmission, said receiver including an adaptive filter matched to estimated channel characteristics, channel adaptation means coupled to said adaptive filter, a modified Viterbi algorithm generator responsive to said adaptive filter, and signal reconstruction means responsive to said channel adaptation means and said modified Viterbi algorithm generator, a method for determining a value for signal elements, represented by {s(1)}, from an estimated channel impulse response, represented by h(n)=h.sub.n (1)h.sub.n (2) . . . h.sub.n (N), comprising the steps of:

• (a) determining in said channel adaptation means a value of a first factor .sigma.(n)=.gamma..sigma.(n-1)+1, with an initial value of .sigma.(0)=0 and .gamma. chosen to be a value between 0.9-0.95;
• (b) determining in said channel adaptation means a vector of smoothed channel impulse response in coefficients in accordance with the expression ##EQU16## where h.sub.s (n) is the vector of smoothed channel impulse response coefficients denoted by h.sub.sn (1)h.sub.sn (2) . . . h.sub.sn (N), the subscript sn denoting a smoothed estimate at time n;
• and (c) determining in said signal reconstruction means a value for the signal elements in accordance with the expression:
• ##EQU17## where 1=-1,0, 1.