Minimization of excess bandwidth in pulse amplitude modulated data transmission
AssigneeBell Telephone Laboratories, Incorporated
InventorAaron D. Wyner
Life of this patent
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Abstract
Pulse amplitude modulated data systems typically utilize linear combinations of Nyquist pulses to transmit data signals. Ideally, required bandwidth for the transmitted signal is 1/(2T)=.rho./2L where .rho. is the data rate, T the signaling interval, and L the number of bits per signal. In practice, however, an "excess bandwidth" of at least 10-20 percent is required. According to the invention, data signals are encoded to produce an encoded signal which can be transmitted over a communications channel having excess bandwidth of merely 2-4 percent. The encoding scheme utilizes linear combinations of characteristic sequences which are known as discrete prolate spheroidal sequences. A transmitter and a corresponding receiver are disclosed.
Claims16
Method for transmitting data signals, here designated as first, comprising the steps of
- (1) encoding a first block of .nu. of said first signals into a second block of N encoded signals, N being greater than .nu., said second block being a linear combination of .nu. characteristic sequences, said linear combination being defined by said first data signals of said first block, and said characteristic sequences being approximations of discrete prolate spheroidal sequences or of a nonzero multiple of discrete prolate spheroidal sequences, and
- (2) modulating the amplitude of pulses by said encoded signals.
A data transmitter comprising
- (1) connection means to a source of data signals, here designated as first,
- (2) an encoder comprising transformation means for transforming a first block of .nu. of said signals into a second block of N encoded signals, N being greater than .nu., said second block being a linear combination of .nu. characteristic sequences, said linear combination being defined by said first signals of said first block, and said characteristic sequences being approximations of discrete prolate spheroidal sequences or of a nonzero multiple of discrete prolate spheroidal sequences,
- (3) a pulse amplitude modulator connected to said encoder, and
- (4) connection means to a communications channel.
A receiver for receiving pulse amplitude modulated signals, here designated as first, comprising
- (1) connection means to a band-limited communications channel,
- (2) a decoder comprising transformation means for transforming a first block of N of said first signals into a second block of .nu. decoded signals, N being greater than .nu., said first signals of said first block being numbered from n=1 to n=N and said decoded signals of said second block being numbered from j=1 to j=.nu., said second block being a linear combination of N characteristic sequences, said linear combination being defined by said first signals of said first block, and the j-th element of the n-th of said characteristic sequences being the n-th component of the j-th member of a set of approximations of discrete prolate spheroidal sequences or of a nonzero multiple of discrete prolate spheroidal sequences, and
- (3) connection means to a data sink.
Method for transmitting amplitude modulated pulses over a communications channel, amplitudes of at least one block, here designated second, of N of said pulses being modulated by N encoded signals, said encoded signals having been obtained by encoding a block, here designated first, of .nu. data signals, N being greater than .nu., said second block being a linear combination of .nu. characteristic sequences, said linear combination being defined by said data signals of said first block, and said characteristic sequences being approximations of discrete prolate spheroidal sequences or of a nonzero multiple of discrete prolate spheroidal sequences, said method comprising allocation of bandwidth to said channel so that allocated bandwidth exceeds Nyquist bandwidth by less than 10 percent.
Dependencies are inferred from the claim text.