Adaptive MLSE-VA receiver for digital cellular radio
First Claim
1. Apparatus for demodulating a time division multiple access (TDMA) signal having a plurality of symbols, said signal comprising at least a synchronizing portion and a data portion within a time slot, said apparatus comprising:
- means for receiving the signal over a channel;
first filter means coupled to an output of the receiving means and comprising a fixed transversal filter having a characteristic selected for matching to a fixed signal pulse characteristic of the received signal for producing a filtered signal, said first filter means characteristic being selected for matching a square root raised cosine pulse characteristic of the received signal; and
second filter means coupled to an output of the first filter means and comprising an adaptive filter having a characteristic selected for adaptively compensating the filtered signal for a time varying impulse response of the channel.
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Abstract
A TDMA RF received signal is demodulated by first being filtered with a fixed transversal filter having a characteristic selected for matching a fixed square root raised cosine pulse characteristic of the received signal. The filtered signal is then adaptively filtered for compensating for a time varying impulse response of the channel. The adaptive filtering is performed initially during a synchronizing portion (preamble) of the filtered signal in accordance with a fast recursive least squares algorithm. Subsequent filter adaptation to a data portion of the filtered signal is accomplished in accordance with a computationally less expensive normalized least mean square procedure. The adaptive filter repetitively applies a modified Viterbi algorithm to blocks of 2D symbols, such that D symbols are released for adapting the adaptive filter means during the data portion of the filtered signal and the signal. The released symbols are also employed for adapting elements required in computing a metric for the modified Viterbi algorithm and the reconstructed signal used to form an error signal that drives the adaptation algorithms.
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Citations
15 Claims
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1. Apparatus for demodulating a time division multiple access (TDMA) signal having a plurality of symbols, said signal comprising at least a synchronizing portion and a data portion within a time slot, said apparatus comprising:
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means for receiving the signal over a channel; first filter means coupled to an output of the receiving means and comprising a fixed transversal filter having a characteristic selected for matching to a fixed signal pulse characteristic of the received signal for producing a filtered signal, said first filter means characteristic being selected for matching a square root raised cosine pulse characteristic of the received signal; and second filter means coupled to an output of the first filter means and comprising an adaptive filter having a characteristic selected for adaptively compensating the filtered signal for a time varying impulse response of the channel. - View Dependent Claims (2, 3, 4, 5)
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6. A method for demodulating a time division multiple access (TDMA) signal having a plurality of symbols, said signal comprising at least a synchronizing portion and a data portion within a time slot, comprising the steps of:
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receiving the signal over a channel; filtering the received signal with fixed transversal filter means having a characteristic selected for matching to a fixed signal pulse characteristic of the received signal, the step of filtering the received signal including matching a filter to a square root raised cosine pulse characteristic of the received signal; and adaptively filtering the filtered signal with adaptive filter means in order to compensate for a time varying impulse response of the channel. - View Dependent Claims (7, 8)
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9. A method for demodulating a time division multiple access (TDMA) RF signal having a plurality of symbols, comprising at least a synchronizing portion and a data portion of a time slot, said method comprising the steps of:
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receiving the RF signal over a channel; filtering the received signal with fixed transversal filter means having a characteristic selected for matching a square root raised cosine pulse characteristic of the received RF signal; and adaptively filtering the filtered signal with adaptive filter means in order to compensate for a time varying impulse response of the channel, the step of adaptively filtering including the steps of, initially adapting the adaptive filter means during the synchronizing portion of the filtered signal in accordance with a recursive least squares algorithm; and subsequently adapting the adaptive filter means during the data portion of the filtered signal in accordance with a normalized least mean square (NLMS) algorithm. - View Dependent Claims (10, 11)
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12. A method for demodulating a time division multiple access (TDMA) RF signal having a plurality of symbols and at least a synchronizing portion and a data portion of a time slot, comprising the steps of:
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(a) receiving the RF signal over a channel; (b) determining a channel impulse response of the received signal over the synchronizing portion; (c) determining signal elements s(-1), S(0), s(1) through autocorrelation, S being defined by ##EQU11## where k represents the index of summation in said autocorrelation operation, N represents the number of samples of the channel impulse response, h(k) represents the estimate of the channel impulse response h(k), and * denotes conjugate; (d) employing a normalized least mean square algorithm for adapting the channel impulse response {h(n)}; (e) operating a maximum likelihood sequence estimator over 2D symbols, said maximum likelihood sequence estimator being implemented by a modified Viterbi algorithm of decision depth D; (f) releasing D symbols; (g) employing the released D symbols to update the channel impulse response; and (h) beginning with a Dth symbol, repeating steps (e), (f) and (g) until a penultimate data block having 2D symbols is received, or, if 2D is not an integral multiple of a number of symbols Nsym, appending {Nsym -(Nsym /2D 2D} symbols to a last block of 2D symbols.
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- 13. In a digital cellular radio receiver having a maximum likelihood sequence estimator demodulator for receiving signal elements, represented by {s(1)}, and determining a value for said signal elements from an estimated channel impulse response, represented by
- space="preserve" listing-type="equation">h(n)=[h.sub.n (1)h.sub.n (2) . . . h.sub.n (N)],
wherein N denotes the number of samples of the channel impulse response, said demodulator including a channel adaption circuit, a matched filter including an adaptive filter matched to the channel impulse response, means coupling the channel adaption circuit to the adaptive filter, a modified Viterbi algorithm generator responsive to said adaptive filter, and a signal reconstruction circuit responsive to said channel adaptation circuit and said modified Viterbi algorithm generator, a method of determining a value for said signal elements comprising the steps of; (a) determining in said channel adaptation circuit a value of a first factor σ
(n) in accordance with σ
(n)=γ
σ
(n-1)+1, with an initial value of σ
(0)=0 and γ
chosen to be a value between 0.9-0.95;(b) determining in said channel adaptation circuit a vector of smoothed channel impulse response coefficients in accordance with the expression ##EQU12## where hs (n) is the vector of smoothed channel impulse response coefficients denoted by [hsn (1), hsn (2) .sub.. . . hsn (N)], the subscript sn denoting a smoothed estimate at time n;
nd(c) determining in said signal reconstruction circuit a value for the signal elements in accordance with the expression;
##EQU13##- View Dependent Claims (14, 15)
Specification