Phase Shift Keying (PSK) is a method for modulating an analog carrier signal with a digital data signal. In PSK the phase of a carrier signal, such as a Sine wave, changes depending on the input data information bits being transmitted. FIG. 1 illustrates a basic form of Phase Shift Keying commonly known as Binary Phase Shift Keying or BPSK. Referring to FIG. 1, waveform 101 represents a digital data signal. In the digital signal 101 a binary “1” is transmitted in time slot 105, a binary “1” is transmitted in time slot 107, a binary “0” is transmitted in time slot 109, a binary “1” is transmitted in time slot 111, a binary “0” is transmitted in time slot 113, a binary “0” is transmitted in time slot 115 and a binary “1” is transmitted in time slot 117.
The BPSK signal, derived from the 101 waveform, is shown in FIG. 1 as waveform 103. Between time periods 107 and 109 the data changes from a “1” to a “0” this transition is reflected in the 180° phase reversal, as seen at point 119, within the BPSK signal 103. Similarly the data transitions between period 109 & 111, 111 & 113, and 115 & 117 are reflected as 180° phase reversals at points 121, 123, and 125, within waveform 103, respectively.
A BPSK signal 207 may be generated by the mechanism of FIG. 2. In FIG. 2 a BPSK output 207, is generated by a switch which selects between a +1 203 and −1 205 and then multiplies either a +1 203 or −1 205 times the a Sin(x) carrier 201, producing a −Sin(x) carrier or a +Sin(x) carrier 205.
PSK is not limited to only two phases. Quadrature Phase Shift Keying (QPSK) uses four carrier phases, instead of two carrier phases, to create the modulated signal. In general a multitude of phases may used create Phase Shift Keying modulated signals, such multiphase phase shift keying signals are often referred to as Multiple Phase Shift Keying (MPSK) signals. In addition a signal may have further modulation, such as amplitude modulation, impressed on it. For example 16 QAM (Quadrature Amplitude Modulation) uses 12 carrier phases and 3 different amplitudes as signal modulation.
QPSK modulation is advantageous in that each phase change may be used to encode more than one bit of data. Commonly, in QPSK, a phase change represents two bits of data. The two bits of data encoded within a QPSK signal are commonly portrayed as quadrature vectors, as illustrated in FIG. 3. In FIG. 3 a phase shift of 45°, i.e. vector 301, represents bit pair 00, a phase shift of 135°, i.e. vector 303, represents bit pair 01, a phase shift of 255°, i.e. vector 305, represents bit pair 10, and a phase shift of 315°, i.e. vector 307, represents bit pair 01. These pairs of bits are commonly referred to as dibits or symbols. In general distinct information states of PSK signals are commonly referred to as symbols. A symbol may contain several bits of information.
In order to correctly recover binary data encoded using PSK techniques, the phase of the received signal may be compared to a reference signal. This can be accomplished by coupling the received signal into a balanced demodulator and providing a reference signal, whose frequency and phase is identical to the carrier, that was used to modulate the signal. Because of the necessity of providing a signal, which has a phase and frequency identical to the carrier signal used in the modulation process, balanced demodulator type circuitry can be complex and expensive. In order to remove the dependence on producing a carrier whose frequency and phase is identical to the carrier that was used to modulate the signal, differential PSK techniques may be used. In differential PSK (DPSK) there is no absolute phase, so there is no dependence on providing a carrier whose frequency and phase is identical to the carrier that was used to modulate the signal. The binary data is recovered from a DPSK signal by comparing the phase of the signal being received to the phase of the previous symbol received. Comparing a symbol to the previous symbol received removes the dependency on an absolute reference signal, that may be present with the non-differential methods of PSK signaling.
π/4 DQPSK uses phase shifts of ±π/4, ±3π/4 to avoid zero amplitude signals. π/4 DQPSK is used in cellular telephony. This invention can well be applied to π/4 DQPSK, for example. For the purpose of representing phase modulated signals, I/Q diagrams are often employed. I/Q diagrams commonly depict Q as a vertical axis and I as the horizontal axis. The I axis represents the in phase part of the signal vector and Q axis represents the quadrature portion of the signal vector. A signal can be represented as a vector rotating in the I/Q plane with the length of the vector representing the amplitude of the signal, as shown in FIGS. 3a, 3b, 3d, 5a, 5b. 
Linear amplifiers are commonly class A amplifiers. A class A amplifier is an amplifier in which a quiescent current is flowing, even when no signal is present. Generally class A amplifiers are significantly less efficient that class C amplifiers, which only consume power when they are amplifying a signal. Class C amplifiers are particularly suited to amplifying phase modulated signals, and it is often a common practice to employ Class C amplifiers when amplifying phase modulated signals. It is a common practice, when amplifying PSK signals, to separate the phase and amplitude components. If the amplitude and phase components are separated, the constant amplitude phase portion may be amplified in a class C type amplifier followed by an amplitude modulator to restore the envelope of the signal. In such implementations, where the phase and amplitude portion of the signals are processed separately, it is common to employ a delay in the amplitude signal in order to synchronize the phase and amplitude before recombining the phase and amplitude portions of the signal.
Generating PSK type signals presents several problems. One of the problems that can be encountered occurs during the modulation of the signal. If the transmit filtering is not done accurately, the spectral shape of the signal will be incorrect. Errors in transmit filtering will also degrade performance at the receiver owing to poorly matched filters. Also, the quality of the transmit signal is sensitive to timing synchronization errors when the phase and amplitude components are recombined.
Additionally, in implementations in which the phase and amplitude are separated and then recombined, various factors such as temperature, aging of components, and the like can introduce delay errors into the recombination of the signals. In addition such delay methods can involve delay lines which may be a significant expense. Improved methods of generating PSK signals are needed in the art.
USPTO Backgrounds
USPTO Backgrounds