Patent Application
20110158352
The technical field relates generally to multi-bit phase encoded modulation/demodulation.
Digital signal communication exists in a vast range of consumer services, business operations, academic, governmental, medical and military services and operations. The range and scope of activities in which digital communication is used is, in fact, not susceptible to a concise description. Instead of categorizing and classifying the technical universe of digital signal communications according to the services, activities or operations to which the communications apply, though, there are two characteristics that are practical for many classification or rating purposes, which are: the “number of bits per symbol” and the “bit rate” in bits per second.
The term “bit” may be defined, for purposes of understanding concepts of the embodiments described herein, according to its information theory meaning as a two-state unit of information (logical “0” and logical “1”) and, therefore, the “bit rate” may be viewed as the information rate. As is well known to persons of ordinary skill in the art of digital communications, such communications may be performed by transmitting an energy having a state selectively movable, i.e., capable of being modulated, along each of N orthogonal axis that form an N-dimensional state space. The state vector of the signal represents a symbol and, therefore, for a symbol to represent M bits, the signal must be capable of being modulated to any one of 2M possible states (i.e., a 1-bit signal may be modulated to any one of two states; a 2-bit signal may be modulated to any one of four states). An example communication system provides M bits per symbol by modulating an energy signal, such as a sinusoidal wave, to a selected one of 2M different state positions within the N-dimensional state space.
In general, there is a correlation between the number of bits M encoded in a modulated signal and the computational complexity of the demodulation process. For example, binary phase shift keying (BPSK), encoding a single bit of data, is technically much simpler than quadrature phase shift keying (QPSK), which encodes two bits of data. A three bit per symbol phase shift keying (PSK) coding scheme requires a demodulation process of even higher technical complexity. This technical complexity has, in the known art, limited the scope of modulation schemes usable in portable, low power, low cost devices and, concurrently, limited the bit rate achievable by such devices.
One example system according to various exemplary embodiments includes a multi-stage demodulation circuit, structured to receive a given communication signal modulated by a superposition of a first modulation scheme to represent Q bits of a given S bit symbol and by a second modulation scheme to represent R bits of the remaining S bits of the symbol, having a first demodulation stage to demodulate the received communication signal according to the first demodulation scheme to extract the Q bits and a second demodulation stage to demodulate the received communication signal according to the second demodulation scheme to extract the R bits of the remaining S bits.
One example apparatus according to various exemplary embodiments includes a first demodulation circuit to receive a given communication signal modulated by a superposition of a first modulation scheme to represent Q bits of a given S bit symbol and by a second modulation scheme to represent R bits of the remaining S bits of the symbol, the first demodulation circuit having structure to detect the first modulation state and to generate a corresponding Q bits based on the detected first modulation state, arranged with an intermediate demodulation circuit to remove the first demodulation state from the received communication signal and to generate a corresponding intermediate demodulation signal having the second modulation state, and a second demodulation circuit to detect the second modulation state and to generate a corresponding R bits of the remaining S bits based on said detected second modulation state.
According to one aspect of one or more exemplary embodiments, the first demodulation stage is constructed to perform a phase shift keying demodulation of the received communication signal to extract and generate the Q bits.
According to one aspect of one or more exemplary embodiments, the first demodulation stage is constructed to perform a binary phase shift keying (BPSK) demodulation of the received communication signal to extract and generate one bit as the Q bits, with Q being one of a 0 state and 1 state depending to a BPSK phase state of the received communication signal.
According to one aspect of one or more exemplary embodiments, a first demodulation circuit is constructed to perform a first modulation scheme PSK demodulation of the received communication signal to extract and generate the Q bits, an intermediate demodulation stage to remove a PSK state from the received communication signal based on the Q bits and generate a corresponding intermediate demodulation signal having a second modulation state according to a phase jitter modulation (PJM) scheme, and a second demodulation circuit constructed to detect the intermediate demodulation signal's phase state within the PJM scheme and to generate a corresponding R bits based on the detected phase state.
According to one aspect of various exemplary embodiments, the second demodulation stage extracts R bits such that R is equal to S−Q, wherein the first demodulation stage and the second demodulation stage extract all S bits.
According to another aspect of various exemplary embodiments, the multi-stage demodulation system comprises another demodulation stage, wherein the second demodulation stage extracts R bits such that R is less than S−Q, wherein the another demodulation stage extracts a remaining S−(R+Q) bits of the S bits.
The above-summarized illustrative examples of embodiments and of illustrations, as well as the above illustrative advantages, features and benefits of each are not intended to be exhaustive or limiting. Other advantages will be apparent from the various embodiments and aspects that are further described with illustrative detail, and persons of ordinary skill in the art will, upon reading this disclosure, readily identify further variations within the scope of the appended claims.
Various illustrative examples of multi-stage demodulation circuits and systems according to various exemplary embodiments as well a various examples of various sub-sections and sub-structures, are described. The specific examples are only for illustrative purposes, to assist a person of ordinary skill in the art of digital communications and signal phase modulation in forming an understanding of the concepts sufficient for such a person, applying the knowledge and skills such persons possess, to practice the invention. The scope of structures, arrangements and materials capable of practicing and implementing one or more of the embodiments, though, is not limited to these specific illustrative examples.
The figures are included to assist, by way of graphical illustration, persons of ordinary skill in the art in forming a clear understanding of the subject matter. It will be understood that the figures are not hardware specifications or fabrication drawings; graphical symbols may, for example, be sized and placed for ease of viewing and not to represent a structural quantity.
To avoid obscuring novel features and aspects of the embodiments, unnecessary details of various technical background that is known to persons of ordinary skill in the art, are omitted.
Example embodiments and aspects may be described separately, and as having certain differences. Separate description or description of differences, however, does not necessarily mean the respective embodiments or aspects are mutually exclusive. For example, a particular feature, function, or characteristic described in relation to one embodiment may be included in, or adapted for other embodiments.
Various embodiments disclosed herein relate to a method for demodulating a received signal embodying at least a second modulation state superposed on a first modulation state. The method comprises a first step of demodulating the received signal to detect the first demodulation state and generate Q bits representing the first demodulation state; a second step of removing the first demodulation state from the received signal to generate an intermediate signal having the second modulation state; and a third step of demodulating the intermediate signal to generate R bits based on the second modulation state.
Further embodiments disclosed herein relate to a system for demodulating the received signal comprising at least a first stage or circuit that demodulates the received communication signal to detect the first modulation state and generates Q bits representing that state, an intermediate demodulation circuit that removes the first demodulation state from the received communication signal to generate an intermediate signal having the second modulation state, and a second stage or circuit that demodulates the intermediate signal to generate the remaining bits R bits based on the second modulation state.
Although the first demodulation stage may apply any demodulation scheme, in accordance with whatever first stage modulation is embodied by the received communication signal, various embodiments include a first stage that applies phase shift keying demodulation. The selection of the particular phase shift keying modulation and demodulation that is employed by the first stage may depend on the needs of the user. For example, quadrature phase shift keying (QPSK) allows more data to be encoded than binary phase shift keying (BPSK); however, BPSK is less complicated to implement and more robust.
Although the second demodulation stage may apply any demodulation scheme, various embodiments use phase jitter modulation (PJM) or frequency shift keying. The selection of the particular modulation may depend on the needs of the user. In various example embodiments, particular those using PSK in the first stage, PJM is preferred because it operates in the same signal domain as that PSK, which may make design considerations easier when implementing and setting parameters for the first stage and the second stage in view of one another.
Demodulation systems and apparatuses according to the various examples and embodiments may employ, in a unique arrangement, various well-known types of circuits for performing, as a feature of the novel arrangement and combination, demodulation of signals carrying two, three, or more bits per symbol. These circuits are readily implemented with analog circuitry and therefore readily incorporated into smart cards, electronic passports, and other electronic documents. Such devices can therefore attain the high bit rate normally obtained using Quadrature Amplitude Modulation (QAM), without incurring the high processing overhead, and hence cost, required for QAM demodulation.
As one illustrative example, in the known art three (3) bit symbols can be transmitted using an 8-state QAM coding scheme. QAM does this by modulating the phase or both phase and the amplitude of a sine wave carrier in a manner defined by a constellation of 8 states defined on the I-Q chart. Decoding is typically done by digitizing the received signal, with a high speed analog-to-digital decoder, and then feeding the digital samples to a processor chip, which extracts the phase and amplitude of the carrier and, hence, the three bits. Such processing is typically not feasible in small portable devices such as smart cards and other electronic documents.
The embodiments disclosed herein, however, accomplish this demodulation of a signal having three or more bits per symbol with, for example, simple analog circuits. Further among the carious features and benefits, the various embodiments provide a bit rate improvement, compared to conventional BPSK, of, for example, approximately 4× in examples using BPSK for the first stage and four-state PJM for the second stage, to 8× in examples using QPSK for the first stage and four-state PJM for the second stage.
The concepts of the present embodiments, further, are not limited to short range communications, e.g., smart cards, but may also be applied in long range communications, e.g., cell phone and satellite phone systems.
Referring now to the figures, illustrative examples of and among the various arrangements, architectures, systems and structures for practicing one or more of the various example embodiments will be described.
In various embodiments, phase jitter modulation (PJM) is used for the second modulation state. PJM is a form of Phase Shift Keying (PSK) that employs very small phase deviations, on the order of, for example, ±1 deg to ±2 deg between adjacent positions. As seen in the example phase jitter symbol constellation of
Methods and systems according to various examples or embodiments may use more than two stages of demodulation. For example, three stages of modulation may be superposed on a given communication signal. As one illustrative example, BPSK may be superposed with phase shift modulation and with an additional modulation scheme, such as, for example, another binary frequency shift keying modulation. One further illustrative example of a three stage method and system according to various exemplary embodiments may comprise a first BPSK stage applying, for example, 0 and 180 degree phase states, followed by a second BPSK stage applying, or example, ±90 degrees, followed by a four-phase PJM stage. The superposition of the first and the second BPSK stages therefore provides a total phase space of 0, 90, 180 and 270 degrees, which effectively provides a QPSK phase space, but using two BPSK circuits instead of a typically more complex QPSK demodulation circuit. The four-phase PJM third stage yields a total system phase space of 16 phases, i.e., 4 bits. Another illustrative example of a three stage method and system according to various exemplary embodiments may comprise a first BPSK stage applying, for example, ±90 degree phase states, followed by a second BPSK stage applying, or example, ±45 degrees, followed by a four-phase PJM stage.
In various embodiments, each of the first modulation state and the superimposed second modulation state may encode the same number of bits or a different number of bits. For example, the first phase modulation state may be a QPSK phase modulation state carrying two bits of data (Q=2) and the second first phase modulation state may be a phase jitter phase modulation state also carrying two bits of data (R=2); the resulting signal may therefore carry any of sixteen possible values. For an example additional flexibility, a Costas loop QPSK demodulator may be used to decode a BPSK as well. Thus, a single Costas loop device may be used at, for example, the first stage of a multi-stage demodulation apparatus according to the various exemplary embodiments and therefore provide a flexibility to provide either BPSK or QPSK demodulation. As will be understood from this disclosure, various exemplary embodiments provide demodulation of, for illustrative example, either a four bit per symbol scheme with QPSK modulation or a three bit per symbol scheme with BPSK modulation.
The Costas loop uses the in-phase mixer 110 to generate an in-phase-baseband signal by mixing a received signal with sin ωo t from the voltage-controlled oscillator, and a quadrature-phase mixer 120,130 to generate a quadrature-phase-baseband signal by mixing the received signal with cos ωo t from the voltage-controlled oscillator.
Referring to
The Correlation-Type Demodulator generates multiple delayed versions of an in phase carrier (e.g. 0°, 90°, 180° and 270°). Each phase shifted signal is multiplied together with the incoming signal. The squaring loop squares the input signal and uses a PLL to lock on the double frequency component (resulting from squaring the input signal) to reconstruct an in-phase clock signal. The reconstructed in-phase clock signal can then be used to detect the symbol.
The branch with the highest average value corresponds to the detected phase.
The in-phase-baseband signal is fed to a data sampler section 200 to determine the phase of the in-phase-baseband signal, and hence, whether the first stage of demodulation corresponds to a 0 or a 1. The data sampler section 200 includes a data sampler 210, and a timing recovery circuit 220 which provides the sampling time signal for the data sampler section 200. Data sampler 210 determines the phase of the in-phase-baseband signal, and hence the phase of the BPSK modulation imposed on the received signal. Based on the phase of the BPSK modulation, it is determined whether the first bit of the data carried by the received signal is 0 or 1.
Based on the phase of the BPSK modulation, the received signal SIN is delayed and inverted or not inverted prior to input to the PJM demodulator. As will be understood, selectively inverting the received signal SIN removes the BPSK modulation from the signal SIN, leaving only the phase jitter modulation in the signal that is input to the PJM demodulator. More particularly, the second signal input to a time delay circuit 300 which delays demodulation of the PJM modulation of the second signal by a sufficient time to complete demodulation of the BPSK signal in the Costas loop. After leaving delay circuit 300, the second signal is fed to phase inverter 400. Phase inverter 400 includes a phase inversion circuit 410 and a switch 420 which allows the second signal to be inverted, or to bypass the inverter 410. The switch is operated in response to the value of the first bit of data from BPSK demodulation, or the presence or absence of phase inversion in the BPSK signal. This allows phase inversion from BPSK modulation to be removed from the second signal by passing phase inverted signals through the inverter. The second PJM-modulated signal then enters the PJM phase demodulator 500.
With continuing reference to
Continuing to refer to
The actual value of ΔθM is a design choice, readily made by persons of ordinary skill in the phase modulation arts upon reading the present disclosure. As will be understood, a large ΔθM increases the influence of the nonlinearities, including higher signal amplitudes within the circuit phase detector circuit. Also, if an in phase clock signal is to be derived from the phase locked loop of the PJM demodulator a higher value of ΔθM also increases the amount that the VCO is deflected, which in turn results in a less clean clock signal. The lower limit of ΔθM is given by the internal phase noise of the circuit. The phase noise must be well below the actual modulation ΔθM. If ΔθM is made to small the error rate will increase.
The combination of BPSK demodulation and PJM demodulation concept of this example according to one or more various exemplary embodiments provides demodulation of a phase modulated signal carrying, for example, 3 or more bits of data per symbol—without requiring an analog-to-digital converter, and without requiring a digital data processing capability that is often infeasible to implement in, for example, smart cards and other portable electronic documents. The combination of BPSK demodulation and PJM demodulation concept of this example provides double the amount of data carried by the example PJM modulation alone, and quadruples the data carried by simple BPSK modulation. Therefore, the combination enhances the transmission rate based on analog elements.
Various exemplary embodiments relate to a method for multistage modulation. In various exemplary embodiments, a two stage modulation is carried out in a first stage and a second stage. According to one aspect, the first stage modulation is BPSK and, accordingly, in the first stage the signal gets either inverted or not to apply a BPSK modulation to the signal. In the example employing BPSK first stage modulation, the inversion may be done using either an analog phase inverter or a digital phase inverter. According to one example, a phase jitter modulation may be applied for a second stage modulation. In the second stage of such an example, a small phase excursion or phase jitter is applied to the BPSK-modulated signal.
Although the various exemplary embodiments have been described in detail with particular reference to certain exemplary aspects thereof, it should be understood that the invention is capable of other embodiments and its details are capable of modifications in various obvious respects. As is readily apparent to those skilled in the art, variations and modifications can be affected while remaining within the spirit and scope of the invention.
Accordingly, the foregoing disclosure, description, and figures are for illustrative purposes only and do not in any way limit the invention, which is defined only by the claims.
