Pulse or digital communications – Transmitters – Angle modulation
Reexamination Certificate
1996-12-18
2001-11-20
Vo, Don N. (Department: 2631)
Pulse or digital communications
Transmitters
Angle modulation
C375S305000
Reexamination Certificate
active
06320914
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus for transmitting digital information streams in a bandwidth-efficient manner. Such a need arises for example in digital wireless telephone systems that transmit digitally coded speech and wish to obtain the highest amount of traffic in an allocated radio band. The invention may also be applied to transmitting digital data along telephone lines using MODEMs.
BACKGROUND OF THE INVENTION
Minimum Shift Keying (MSK) is a known binary modulation that impresses binary information bits onto a radio frequency carrier by rotating the phase smoothly through either +90° of −90° from its previous value according to the polarity of the information bit being transmitted. Thus, the phase nominally lies at 0 to 180°at the end of even bits and 90 or −90 ° at the end of odd bits. With suitable precoding, it may be arranged that an even bit B(2i) is represented always by a terminal phase of 0° for a ‘1’ and 180 °for a ‘0’, and that odd bits B(2i+1) are represented by 90° for a ‘1’ and −90° for a ‘0’.
In MSK, the phase rotates smoothly at a constant rate in either a clockwise or anticlockwise direction. The constant rate of change of phase represents either a positive frequency offset or a negative frequency offset from the nominal radio carrier frequency. The frequency offset changes abruptly when the data polarity changes the direction of phase rotation.
In a known variant of MSK, called Gaussian Filtered Minimum Shift Keying (GMSK), a Gaussian filter is used to smooth the frequency transitions so that the phase rotation does not exhibit abrupt changes of direction. This smoothing effect by the Gaussian filter reduces the spectral energy in neighboring radio frequency channels and improves adjacent channel interference characteristics, at the expense of rounding the data transitions so that after a data ‘1’ or ‘0’ the phase may not quite reach the expected end points, with a consequent slight loss of noise immunity. A Gaussian filter was found expirically in the past to reduce adjacent channel energy the most for a given amount of rounding loss. The GMSK modulation technique is used in the European GSM cellular phone system.
GMSK is a constant amplitude modulation where the signal only varies in phase. Better spectral containment may be obtained by a so-called linear modulation where the amplitude is permitted to vary. The spectral efficiency may be measured in bits per second per Hertz of transmission bandwidth. The spectral efficiency may be increased by using quaternary modulation instead of binary modulation. For example, two bits at a time may be combined to form quaternary symbols with a value of 0, 1, 2, or 3, which are conveyed by transmitting a signal phase of 0, 90, 270, or 180°, respectively. Such a modulation is called Quadrature Phase Shift Keying (QPSK). Alternatively, the four phases, also known as constellation points, may be systematically shifted through 45 degrees between successive symbol periods so that even symbols are represented by 0, 90, 270, or 180° while odd symbols are represented by 45, 135, −135, or −45°. This alternative modulation is called Pi/4-QPSK. When in Pi/4-QPSK, the data symbol is represented by the change in phase from the previous value to the next value, being one of the four rotations +/−45 or +/−135°, it is known as Differential Pi/4-QPSK or Pi/4-DQPSK. QPSK, Pi/4-QPSK and Pi/4-DQPSK may all be regarded as time varying vectors in the two-dimensional complex plane that have a time varying real coordinate (I) and a time varying imaginary coordinate (Q). If the I and Q waveforms are separately linearly filtered, the spectral containment can be as good as the filter characteristics can be made but at the expense of introducing amplitude modulation, which is harder to transmit than pure phase modulation. Nevertheless, I-Q filtered Pi/4-DQPSK is the modulation used in the US digital cellular system IS-54. The IS-54 modulation achieves 1.62 bits per second per Hertz of channel bandwidth while GSM achieves 1.35 bits per second per Hertz.
Yet another modulation uses the four phase shifts +/−45 and +/−135° to represent a quaternary symbol (bit pair), but smoothes the phase changes in the same way as GMSK to provide spectral containment without introducing amplitude modulation. The technique, called 4-ary CPM, is neither as spectrally efficient nor power efficient as Pi/4-DQPSK, however.
The current invention differs from all of the above in first producing two binary-modulated, constant envelope GMSK signals using a first half A(
1
), A(
2
). . . A(n) of the total number of information bits to be conveyed for the first GMSK signal and a second half B(
1
), B(
2
). . . B(n) of the total information bits to modulate the second GMSK signal.
SUMMARY OF THE INVENTION
A first binary data stream of B bits per second is impressed on a radio carrier using GMSK to provide a transmission which alone would achieve a spectral efficiency of about 1.35 bits per second per Hertz. A second binary data stream of B bits per second is modulated in a similar fashion on the same carrier frequency but with a 90° phase shift from said first radio carrier. The two modulated carriers are then linearly added to form a radio signal modulated with 2B bits per second achieving a spectral efficiency of 2.7 bits per second per Hertz. The sum signal is a non-constant envelope signal that carries information largely in its phase but also in its amplitude changes. The signal resembles a four-phase signal and may be received using a Viterbi equalizer adapted for resolving bit pairs.
An information stream or bit-block to be transmitted is divided into a first half comprising bits labelled A(
1
), A(
2
) . . . A(n) and a second half comprising bits labelled B(
1
), B(
2
) . . . B(n). The A-half bits are used to modulate a first GMSK signal and the B-half bits are used to modulate a second GMSK signal on the same carrier frequency, but having a 90° phase difference to the first carrier. The two GMSK signals are then added to form the inventive modulated signal,which will be referred to hereinafter as “Quadrature Overlapped GMSK” or QO-GMSK for short.
Even numbered bits of the A-half, A(2i), cause the first GMSK signal to adopt nominal terminal phase values of 0 to 180° while odd numbered bits A(2i+1) causes nominal terminal phases of +/−90°. On the other hand, even numbered bits B(2i) of the B-half cause the second GMSK signal to adopt terminal phases of nominally +/−90° at the same time as the first GMSK signal is at nominally 0 to 180°. Likewise, odd numbered bits B(2i+1) of the B-half cause the second GMSK signal to adopt nominally 0 to 180° terminal phases at the same time as the first GMSK signal is at nominally +/−90°. In this way, the two GMSK signals are made distinct from one another and thus both the A-bits and the B-bits can be distinguished in the received signal.
The Gaussian filtering of each individual GMSK signal causes departures from the nominal phase values and thus the two GMSK signals are not perfectly separated. Nevertheless, an equalizer may be used to resolve the interference of one GMSK signal to the other GMSK signal. The preferred type of equalizer for use with the invention is a Viterbi maximum likelihood sequence estimator (MLSE), which postulates sequences of bit pairs each comprising one A-bit and one B-bit. Typically, three consecutive bit-pairs are postulated to predict the instantaneous complex vector value of the QO-GMSK signal. The actual signal values are compared with predicted signal values in a Viterbi MSLE processor to compute a cumulative mismatch value or path metric for each postulated sequence bit pairs, the sequence finally having the lowest path metric being outputted as the most likely sequence of bits pairs, thus demodulating the QO-GMSK signal.
When practicing the present invention, a communications system achieves a spectral efficiency of 2.7 data bits per second
Burns Doane Swecker & Mathis L.L.P.
Ericsson Inc.
Vo Don N.
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