A method and system for simplified evaluation of the digital representation and precoding of encoded communication data in a system using phase-cyclic precoding.

Abstract

This invention discloses a "geometric shift transform" structure and processing that enables the use of a symbol alphabet codeword representation of a user's binary data stream as a signal transmission format from a central unit (modem) to remote units (RF devices) in a precoded OFDM system. This invention simplifies the calculation of the Discrete Fourier Transform (DFT), enables secure communication, and assumes the execution of layer mapping and resource block mapping processing in the central unit.

Description

[Technical Field]

[0001] This invention relates to the field of digital data communications, including mobile communications, terrestrial digital radio communications, and digital satellite communications. In this specification, it is useful to distinguish between signal formats transmitted wirelessly (analog coding of digital signals) and signal representations used within a transmission system connected to an air interface, i.e., in-system signal transmission. More specifically, this invention relates to the latter, i.e., in-system digital interfaces between encoders, modulators, and RF transmitters (e.g., block-up converters), and to the evaluation of precoded signals in precoded orthogonal frequency division multiplexing (OFDM) systems.

[0002] Related applications This application claims the benefits of U.S. Patent Application No. 18 / 334,321, filed on 13 June 2023. [Background technology]

[0003] In conventional digital electronic communication systems, such as digital satellite transmission systems, the signal input to the central unit (CU) of the transmission system (i.e., the information to be transmitted wirelessly to the receiver) is generated by the data source as a binary digital data stream, also called a user data stream. This stream, consisting of "1s" and "0s," needs to be converted into a format that can be effectively transmitted via electromagnetic signals. This format conversion is called "modulation." Traditionally, modulation was performed in the CU, converting the binary digital stream into an analog signal that encoded the underlying data bits. This analog signal is usually transmitted analogously to a remote unit (RU) equipped with RF equipment and a transmitting antenna. In the case of digital radio, such as satellite communication systems, this modulated analog signal can be superimposed on a carrier wave at a predetermined other radio transmission frequency.

[0004] However, it is desirable to be able to transmit the modulated signal from the CU to the RU (connected to the transmitting antenna) as a digital signal. Digital signal transmission offers many advantages over analog, including improved noise immunity, greater flexibility in frequency planning, improved signal quality, easier signal manipulation, and enhanced security (encryption).

[0005] As will be described in a more detailed description of the present invention, the digital representation of a transmittable signal in a system includes a symbol alphabet (also called a symbol codeword) and a sampled waveform. To minimize the computational power required by the RU, the most commonly used system partitioning in terrestrial systems (e.g., O-RAN "Split" 7.2 and 7.2X) uses a sampled waveform format to transmit signals between the CU and the RU.

[0006] However, the data rate required for digital signal transmission using sampled waveforms is extremely high, reaching more than 10 times the bit rate of the user data stream. For example, in an LTE system, a 75 Mbps user data stream requires an in-system digital bit rate of 1.23 Gbps. Therefore, an alternative system approach that can securely digitally transmit in-system signals from the CU to the RU without requiring high data rates or significant computing power at the RU would be extremely beneficial.

[0007] This invention addresses those needs and other related issues. [Overview of the Initiative]

[0008] The present invention can satisfy the above needs by enabling the use of symbol alphabet (codeword) transmission between an encoder (central unit) and an RF device (remote unit) in a precoded OFDM system, and by simplifying precode conversion calculations. The present invention can be implemented in satellite or ground (e.g., O-RAN) systems.

[0009] The present invention discloses a novel method for implementing an M-point phase cyclic transform used for precoding in a precoded OFDM system for a digital signal stream represented as a sequence of M k-bit symbolic codewords. Each codeword is a 2-point phase cyclic transform in the base constellation of points on the complex plane. k Represents any of the points. In a preferred embodiment, the method includes the step of generating a geometric shift table of stored values, wherein the table is 2 k The table has rows and M columns, Defined by TIFF2026521728000002.tif28170, each column is referenced by index n, and each row is 2 k It is referenced by one of the k-bit codewords, where n ranges from 0 in the first column to n=M-1 in the Mth column, and the exponent φ m represents the phase factor specific to the particular phase cyclic transformation being applied. The entries in the first column of the table are the complex values ​​corresponding to each point in the base constellation, arranged according to the order of the corresponding codewords: The file TIFF2026521728000003.tif12170 contains the following entries in each subsequent column: the complex values ​​of each point in the base constellation, and the corresponding phase factor φ that characterizes the phase cyclic transformation. n The value is equal to the result of multiplying by , and the entries in each row are arranged in the order of the corresponding codewords. The method further comprises the steps of: associating each of the M symbol codewords in a sequence of M codewords with an index m, where m ranges from 0 to M-1; forming a set of M outputs referenced by an index l, where l ranges from 0 to M-1; and evaluating each of the M outputs of the transformation as the sum of the M table entries, where each entry is selected from the nth column of the table (where n is evaluated as the remainder when the product of l and m is divided by M) and selected from the row corresponding to the codeword assigned to index m (where m ranges from 0 to M-1).

[0010] In a preferred embodiment, the phase cyclic transformation is a discrete Fourier transform (DFT), and the phase factor φ n is -2πn / M. The method according to the present invention can further include the step of performing layer mapping processing and resource block mapping processing on the signal codeword stream at the first position, and the forming step and the evaluating step are called geometric shift precoding and are performed at a position away from the first position. This novel transformation is preferably performed in a wireless transmission system, the layer mapping and resource block mapping processing are performed by a modem at the first position, and the preparation of the signal for geometric shift precoding and transmission via the air interface is performed at a remote position.

[0011] In other embodiments, a method of transmitting a digital signal within a pre-coded OFDM transmission system is disclosed. The steps of this embodiment are, in a modem, encoding a signal stream as a sequence of M k-bit symbol codewords, each codeword representing one of 2 k points in a base constellation of points in the complex plane, and transmitting the symbol codeword stream from the modem to a remote unit. In such an embodiment, in the remote unit, the symbol codeword stream is input into an M-point phase cyclic precoding transformation, and the transformation is i.) generating a geometric shift table of stored values, the table having 2 k rows and M columns, the table being defined by TIFF2026521728000004.tif28170, each column being referenced by an index n, each row being referenced by one of 2 k k-bit codewords, n ranging from 0 in the first column to n = M - 1 in the Mth column, the exponent φ m representing a phase factor specific to the particular phase cyclic transformation being applied, the entries in the first column corresponding to complex numerical values for each point in the base constellation arranged in the order of the corresponding codewords: The file TIFF2026521728000005.tif12170 contains the following entries in each subsequent column: the complex values ​​of each point in the base constellation, and the corresponding phase factor φ that characterizes the phase cyclic transformation. n The process is implemented by: ii) a step equal to the value multiplied by and the entries in each row being arranged in the order of the corresponding codewords; ii) a step associating each of the M symbol codewords in a sequence of M codewords with an index m, where m ranges from 0 to M-1; iii) a step forming a set of M outputs referenced by an index l, where l ranges from 0 to M-1; and iv) a step evaluating each of the M outputs of the transformation as the sum of M table entries, where each entry is selected from the nth column of the table (where n is evaluated as the remainder when the product of l and m is divided by M) and selected from the row corresponding to the codeword assigned to index m (where m ranges from 0 to M-1).

[0012] In yet another embodiment, a non-temporary computer-readable storage medium is disclosed for storing one or more computer-readable programs, which, when executed by a processor, perform a method for computationally performing an M-point phase cyclic transform used for precoding in a precoded OFDM system. This transform (which may be a DFT) is performed on a digital signal stream represented as a sequence of M k-bit symbolic codewords, each codeword being 2 in the base constellation of points on the complex plane. k Represents one of the points. The method performed by the processor is a.) generating a geometric shift table of the stored values, wherein the table is 2 k The table has rows and M columns, Defined by TIFF2026521728000006.tif28170, each column is referenced by index n, and each row is 2 k It is referenced by one of the k-bit codewords, where n ranges from 0 in the first column to n=M-1 in the Mth column, and the exponent φm represents a phase factor specific to the particular phase cycle transformation being applied, and the entries in the first column are complex numerical values corresponding to each point of the base constellation arranged according to the order of the corresponding codewords: including TIFF2026521728000007.tif12170, and the entries in each subsequent column are equal to the complex numerical value of each point of the base constellation multiplied by the corresponding phase factor φ characterizing the phase cycle transformation, and the entries in each row are arranged according to the order of the corresponding codewords, step, and b.) a step of associating each of the M symbol codewords within the sequence of M codewords with an index m, where m ranges from 0 to M - 1, step, and c.) a step of forming a set of M outputs referenced by an index l, where l ranges from 0 to M - 1, step, and d.) a step of evaluating each of the M outputs of the transformation as the sum of the M table entries, where each entry is selected from the nth column of the table (evaluated as the remainder when the product of l and m is divided by M) and from the row corresponding to the codeword assigned to the index m (where m ranges from 0 to M - 1), and can comprise. n It should be understood that the present invention is not limited to the details of the configurations and the arrangement of components described herein and shown in the drawings and photographs. Those skilled in the art will recognize that various modifications can be made without departing from the scope of the present invention.

[0013] It should be understood that the present invention is not limited to the details of the configurations and the arrangement of components described herein and shown in the drawings and photographs. Those skilled in the art will recognize that various modifications can be made without departing from the scope of the present invention.

Brief Description of the Drawings

[0014] Further advantages of the present invention will become apparent to those skilled in the art by reference to the following detailed description of the preferred embodiments and the accompanying drawings. [Figure 1] FIG. 1 is a representative conventional time - domain voltage waveform plot of a representative digital radio signal segmented into symbol periods, where the digital numerical values are encoded in the amplitude and phase of the sine - wave segments. [Figure 2]Figure 2 is an exemplary conventional two-dimensional diagram of a 16QAM IQ constellation, showing an arbitrary binary coded symbol alphabet assigned to each constellation point. [Figure 3] Figure 3 is a simplified block diagram of a modern satellite communications signal transmission chain, illustrating the conventional functional division in the modem and BUC. [Figure 4] Figure 4 is a simplified block diagram showing a portion of the transmission path of a conventional OFDM communication system that uses phase-cyclic precoding such as the Discrete Fourier Transform. [Figure 5] Figure 5 is a simplified block diagram showing a conventional configuration of a DFT-s-OFDMA system, illustrating the signal format along the signal chain. [Figure 6] Figure 6 shows an exemplary 16QAM constellation illustrating the rotation of selected constellation points / alphabet according to the present invention. [Figure 7] Figure 7a shows a 1024-bit encoded string (generated from a user data stream) at the bottom, and above it, a 4x256 register to which 1024 bits can be arbitrarily assigned, which functions as input to 32 phase-cyclic precoding (PCPT) processes. Figure 7b is an explanatory diagram showing how the encoded data string shown in Figure 7a is divided into 4-bit symbol codewords in series, and above it, a 4x256 array to which the codewords are assigned in series. [Figure 8] Figure 8a is a magnified view of the first 16 columns of the buffer shown in Figure 7b, illustrating a simple sequential layer mapping of codewords to the PCPT input. Figure 8b is an explanatory diagram illustrating an alternative layer mapping in which codewords are assigned to the PCPT input in a non-sequential manner. [Figure 9]Figure 9a is an explanatory diagram showing the resource block mapping of individual PCPT outputs to a contiguous block of IFFT inputs. Figure 9b shows that the resource block mapping shown in Figure 9a can be equivalently implemented in a layer mapping step, thereby allowing the use of a simple sequential resource block mapping from PCPT outputs to IFFT inputs. [Figure 10] Figure 10a shows a simple sequential intrablock mapping from the PCPT output to the corresponding IFFT block input. Figure 10b shows a "scrambled" intrablock mapping as an alternative mapping to Figure 10a. [Figure 11] Figure 11 is a block diagram showing a preferred embodiment of the DFT-s-OFDMA or SC-FDMA system of the present invention using symbol codeword transmission and geometric DFT, in which the DFT input mapping (layer mapping and resource block mapping) functions are separated from the encoder in the central unit. [Modes for carrying out the invention]

[0015] As described above, in conventional digital radio systems, signal modulation is performed by the transmitting portion of a modem located in the central unit (CU) of the transmission system, and the superposition (and subsequent amplification) of this modulated signal onto the radio frequency carrier is performed by an RF device (e.g., a block-up converter (BUC)) located in the remote unit (RU). In such conventional systems, the signal is transmitted from the CU to the RU as an analog signal. In contrast, in modern terrestrial radio systems, the signal is typically transmitted digitally between the CU and the RU.

[0016] The present invention improves precoded OFDM systems that use phase-cyclic precoding transforms (e.g., discrete Fourier transform, discrete Hartley transform, or generally phase-shifting linear canonical transforms), such as systems using DFT-s-OFDMA or SCFDMA. The invention disclosed herein can significantly reduce the data rate required for digital communication between CU and RU (to one-tenth compared to the complex-valued IQ symbolic representation of the precoded signal, depending on the specific waveform configuration), significantly reduce the computational load for evaluating the precode, and further enable the simultaneous allocation of important functions related to frequency and time allocation (resource element mapping) and scrambling functions (for signal obfuscation) on the CU.

[0017] Referring to the drawings here, similar reference numerals indicate the same or corresponding features across multiple drawings.

[0018] The following is a brief description of conventional digital radio transmission. Conventionally, transmission consists of a series of time blocks called symbol periods, during which the transmission is a sinusoidal segment whose amplitude and phase are set to one of a set of predetermined values. Figure 1 shows a typical time domain and voltage plot 10 of such a typical digital radio signal, using methods that modulate both the amplitude and phase of the signal simultaneously ("Quaternary Amplitude Modulation (QAM)" and "Amplitude-Shift Modulation (APSK)"). Thus, both the amplitude and phase encode the information of the digital bits transmitted by each symbol. As shown in this example, the voltage waveform 12 in the transmission time domain is divided into eight symbol periods shown in Figure 1. The sinusoidal segments in each symbol period 1 to 8 shown in this figure have discrete signal voltage amplitudes indicated by horizontal bars and discrete phases indicated by vertical dotted lines. Therefore, the waveform 12 with symbol period 1 has the voltage amplitude specified by line 21 and the phase specified by line 31, while the signal with symbol period 8 has the amplitude specified by line 28 and the phase specified by line 38.

[0019] Each predefined amplitude-phase pair in a symbol can be represented as a set of points on the complex in-phase / orthogonal (IQ) plane. Such a set of predefined points is called an orthogonal amplitude modulation (QAM) constellation, in which the points are arranged at approximately equal intervals on a square grid. This representation is often denoted as xQAM, where x is the number of points on the complex plane corresponding to the predefined amplitude-phase pair. While such a set of points on the complex plane can, in principle, constitute a constellation, in practice the number of points is a power of 2, i.e., 2 k It is generally understood that k is the number of bits in the "codeword," and a one-to-one correspondence is possible from a k-bit binary "codeword" to individual points in the constellation. Such a correspondence is also called the "alphabet" of the constellation. Therefore, for example, k=4 represents a 4-bit binary codeword, and 2 4 A QAM or 16QAM constellation is obtained.

[0020] APSK is a commonly used constellation structure that replaces QAM. In this structure, the constellation points are arranged in essentially circular concentric circles centered on the origin of the complex IQ plane. Although the APSK constellation is not square, it is usually 2 k It is constructed using 2 points, and thus, k A one-to-one correspondence is possible between each k-bit word and a constellation point.

[0021] Figure 2 shows an example of codeword assignments and alphabets for a 16QAM constellation. Here, each of the 16 points is designated by a codeword (0000-1111) and a single letter of the alphabet (A-P). Thus, in this example, the voltage amplitude r and phase θ of the modulated signal at a given symbol period are represented by a single point in the constellation (e.g., codeword 1001 / alphabetical letter O). It will be understood that the information can be transmitted digitally along the signal path in the system as a "sampled waveform" (a numerical sample of the voltage versus time of the sinusoidal segment of each symbol), or as a "symbol alphabet" (or "codeword"), i.e., as a stream of characters or binary numbers on the constellation that uniquely represent the combination of sinusoidal amplitude and phase, respectively.

[0022] Next, the signal transmission architecture will be described. Figure 3 is a simplified functional block diagram showing a conventional transmit signal processing configuration 100 commonly used for satellite transmission. This diagram shows the signal chain from the moment the signal enters the modem 110, passes through the modem, is output from the modem, is transmitted as an analog signal to an RF device, such as a BUC / SSPA 130, undergoes upconversion and amplification, and is then transmitted via the air interface (via an antenna connected to a BUS, not shown). Data source 1 generates a user digital data stream 50. Encoder 112 in modem 110 receives the user digital data stream 50 as input and adds data framing bits (header, trailer, pilot symbol), error correction bits, and encryption to it. It also divides the resulting encoded stream into k-bit blocks for constellation coding, as described above. These k-bit blocks correspond to sequences of alphabetic characters of a selected constellation, shown as a symbol codeword stream 52.

[0023] A conventional digital modulator block 114 is located following the encoder block 112. The modulator 114 converts a stream of symbol codewords or alphabetic characters 52 into a signal in the format shown in Figure 1. In modern modems, this is done in two steps. First, a sampled waveform digital representation 54 of the signal to be transmitted is generated. This digital representation of the baseband waveform is typically in the form of I and Q components, and is then converted into an analog signal by a digital-to-analog converter DAC 116. Thus, in this data path, we see that there are two different digital signals within the modem: a digital symbol codeword representation 52 and a digital waveform representation 54. Each contains the same information but has different characteristics and offers different advantages and disadvantages as the basis for transmitting information. The output of the DAC 116 is filtered by a filter 118 and then sent to the RF device RU 130 via a frequency converter 120 as an analog signal with an intermediate carrier frequency, typically in the L band, as shown here.

[0024] However, as mentioned above, there are advantages to transmitting signals from the modem to the RF equipment in digital format. Digital signals are less susceptible to interference and distortion. By supplying signals to the RF equipment in digital format, digital signal processing can be easily used to mitigate signal degradation in the RF equipment (non-flat frequency response, nonlinear distortion). Furthermore, and this is extremely important in certain applications, the use of digital representation enables the use of remote, virtualized modems.

[0025] A simple form of digital transmission is to use the sampled waveform output 54 of a digital modulator 114 as the digital transmission format. While this is a simple approach, it has the drawback that this sampled waveform digital representation 54 may require more than 10 times the data rate of the underlying user digital data stream 50.

[0026] Alternatively, transmitting the symbol alphabet (codeword) stream 52 from the encoder block to the modulator located in the radio unit 130 (antenna) would be preferable, as it would require only a data rate slightly higher than the underlying user data rate. However, the drawback of this approach is that the RF equipment must have a modulator that generates time-domain waveforms. Therefore, the modulator must "know" all the constellations used by the modem. Such a modulator may be a proprietary modulator specific to certain waveforms containing sensitive information.

[0027] Therefore, for any given set of constellations, the required modulator would likely be a custom-designed piece of hardware with unique specifications. However, the situation is different with OFDM modulation systems. This is because the time-domain waveform is generated by an inverse Fourier transform block, which is typically implemented as an IFFT. (For example, in an ASIC or a high-speed field-programmable gate array (FPGA), the only information required to construct such an IFFT modulator is the number of subcarriers and the bit depth.) This makes simple (non-pre-coded) OFDM systems particularly suitable for applications involving the transmission of codewords to RF equipment. In this case, the RF equipment can use a simple lookup table to convert the symbol codeword into a complex input to the IFFT modulator. However, simple OFDM waveforms suffer from a very high peak-to-average power ratio (PAPR), which is an indicator of the ratio of the signal's maximum instantaneous power to its average power. High PAPR is undesirable in communication systems because it increases costs and power consumption by requiring amplifiers with peak power capacities much larger than the average transmitted power.

[0028] OFDM systems employing phase-cyclic precoding, such as Discrete Fourier Transform Spread Orthogonal Frequency Division Multiple Access (DFT-s-OFDMA), also known as Single Carrier FDMA (SC-FDMA), address the challenge of high PAPR. This approach essentially adds a DFT block between the inverse Fourier transform block that generates the time-domain output and the (digital) modulator. While this additional precoding process reduces PAPR, it complicates the application of codeword signal transmission compared to a simple (non-precoding) OFDM configuration, as shown in the block diagrams of Figures 4 and 5. In this case, each input to the IFFT block is not an individual symbol, but rather an aggregated output of a mathematical transformation (DFT) process that takes multiple symbols as inputs to generate each output to the IFFT block.

[0029] The output of the precoding transform, and therefore the input to the IFFT block, is not a set of individual symbols, but the result of complex mathematical operations on multiple symbol inputs. Thus, the input to the IFFT modulator is not a simple codeword, and an initial analysis of this configuration suggests that representing the output of the phase-cyclic transform of multiple symbol inputs to the precoding transform requires a sampled waveform approach rather than a symbol alphabet representation.

[0030] However, as described below, the present invention enables the use of symbol alphabet (codeword) transmission between the encoder and the RF device in a DFT-s-OFDMA architecture, and simplifies the calculation of precoding conversion.

[0031] Rotation factor-As further background, in digital signal processing (DSP), the rotation factor is a complex constant used in the calculation of the Discrete Fourier Transform (DFT) and its inverse, the Inverse Discrete Fourier Transform (IDFT). The rotation factor is expressed as a complex exponent, the argument of which is a multiple of 2π / N, where N is the number of points in the DFT or IDFT. The rotation factor is used to perform the "butterfly operation," which is a fundamental building block of the FFT (Fast Fourier Transform) algorithm. The butterfly operation involves multiplying two complex numbers and adding the result with two other complex numbers, where y[k] is the output of the butterfly operation, x[k] and x[k+N / 2] are the input values, and ω N k-1 If the argument is a rotation factor of 2π(k-1) / N, then y[k]=ω N k-1 This is expressed as x[k+N / 2]+x[k]. The use of a rotation factor in the FFT algorithm reduces the number of complex multiplications required to compute the DFT, thereby making the algorithm more efficient than directly computed the DFT. Nevertheless, the FFT still requires complex multiplication, and these operations are computationally intensive.

[0032] Focusing on the fact that multiplication by rotational phase factors in precoding transformations can be represented as a rotation of constellation points around the origin of the complex plane, the inventors found that it is not necessary to perform DFT on arbitrary numerical inputs, but only on specific sets of complex numbers corresponding to the constellation points. Therefore, the computationally intensive multiplication by rotational factors can be achieved by storing a rotated version of the constellation lookup table corresponding to each of the required rotational factors. This simplifies the evaluation of the transformation to a set of complex additions of the values ​​stored in those "rotated" lookup tables. As shown in the following concrete example, this finding makes it possible to use symbolic alphabet (codeword) transmission of information to reduce the required data rate capacity, and significantly reduces the computational load in precoding evaluation.

[0033] The following describes a specific example implemented using a conventional DFT application, followed by an explanation of how the present invention enables the use of symbol codeword transmission and simplifies DFT evaluation.

[0034] The simplest form of the conventional OFDM system of this type is shown in Figure 4. Figure 4 is a simplified diagram 200 showing part of the transmission path of a 16QAM phase-cyclic precoding OFDM modem. Modulator 204 receives a bit string 202 encoded with 4 bits per symbol as input and modulates the symbol codewords to symbol complex IQ values. The number of points in IFFT modulator 212 (which is equal to the original number of OFDM subcarriers) is "N", and the number of points in precoding FFT transform 208 is "M". Specifically, this example deals with the case where M=8 and N=256. As shown in the figure, there are two mappings in this block diagram. The first mapping 205 maps the symbol IQ representation (output of modulator block 204) to the input of phase-cyclic precoding block 208, and the second mapping 210 maps the output of phase-cyclic transform 208 to the input of IFFT block 212. The input to the second mapping 210 is the serialized complex IQ output of the precoding transform 208.

[0035] In the configuration described above, the encoded binary bitstream 202 is divided into 32-bit blocks, which are further subdivided into eight 4-bit alphabet characters (corresponding to 4 bits per symbol in the 16QAM constellation used). The eight 4-bit subblocks (each corresponding to one point in the constellation and its corresponding complex number) are sequentially indexed from 0 to 7. The amplitude r of the eight symbols represented by the alphabet characters contained in subblocks m=0~7 m and phase θ m This becomes the input to the Discrete Fourier Transform (DFT) defined by the following equation. TIFF2026521728000008.tif17170 Here, j is the imaginary unit, and there are 8 X l∫

[0036] In a simple configuration, the process of mapping the output of the Fourier transform 208 to the IFFT212 input is a simple mapping that sequentially associates the DFT output with the IFFT input. That is, the l-th output of IFFT208 is applied to the l+pM input of IFFT212, where l is the index of the DFT output and p is the index of the set of (N / M) bit input blocks. The input to each point of IFFT212 is given as the sum of complex numbers represented by each symbol alphabet character specified by each of the eight subblocks, multiplied by their corresponding phase "rotation factor".

[0037] It should be noted that the simplest sequential mapping used in the aforementioned example is only one of many possible mapping permutations. In general, this mapping is an integral part of the subcarrier allocation and bandwidth management functions in a modem. In O-RAN terminology, the first and second mappings 205 and 210 are called “layer mapping” and “resource element mapping,” respectively, and these mappings control where each user’s data is sent on the time-frequency resource grid. Therefore, in system partitioning, it is most desirable to leave this mapping function on the modem or CU side. Furthermore, the details of this mapping may be the proprietary intellectual property of the modem manufacturer, and therefore the modem manufacturer may want to “confidentially” the details of the subcarrier mapping from the RF equipment manufacturer. From this point of view as well, it is advantageous to partition the system so that the subcarrier mapping function is located in the modem.

[0038] Figure 5 shows one embodiment of a more generalized conventional DFT-s-OFDMA system that can take any mapping. Here, the data rate of the sampled waveform format 203 may be more than 10 times that of the symbol alphabet ("codeword") format 202. The mapping can also be used to conceal the underlying data. If the first and second mappings are unknown to a third party attempting to intercept the signal, even if M and N are reasonable values, the number of possible permutations becomes so large that it would be nearly impossible to properly decode the signal. For example, in the example in Figure 4, if M=8 and N=256, then 10 40 This means that there are permutations that exceed the number of possible combinations.

[0039] While the Fast Fourier Transform (FFT) implementation of the DFT is computationally efficient, it still involves a considerable number of complex number multiplications. In contrast, the present invention utilizes the fact that the DFT implementation required in this application does not need to handle arbitrary complex numbers as input to the Phase Cyclic Precoding Transform (PCPT), but only complex numbers representing points within the IQ constellation being used as input. That is, r in equation (1) m and θ m It is not an arbitrary value, but 2 k A specific pair of values ​​selected from the constellation points (r α , θ α ) is limited to only that. In the specific example above, index α takes the range from A to P.

[0040] Next, the inventors discovered that the multiplication of points on the complex plane corresponding to a constellation by a unit amplitude phase factor, which is required in PCPT (e.g., DFT) calculations, can be geometrically represented as a rotation of the constellation points by the corresponding phase angle around the origin of the complex plane. Therefore, the evaluation of the inputs to each input point on the IFFT block (corresponding to the output of the DFT) can be simplified to evaluating the complex sum of eight points represented by the alphabetical letters of each of the eight subblocks of the corresponding rotated constellation. In other words, no complex multiplication is performed in this calculation. In this example, there are a total of eight constellation rotations. Figure 6 shows a 16QAM constellation map showing the original state and the first two of these rotations, along with the alphabetical assignments after rotation. In the figure, black circles represent zero rotation, i.e., the original state of the constellation. The entirely gray dots represent a rotation of π / 4 (45 degrees), and the striped dots represent a rotation of π / 2 (90 degrees). The alphabetical assignments after rotation are also shown. For example, the rotational assignment of the letter A is as follows: In the original state, point A is shown in the upper left corner of the 16-point grid. The first 45-degree constellation rotation places A (shown here as the entire gray dot) at the top of the Q axis. The second rotation places point A (the striped pattern) in the upper right corner of the grid, and the third rotation places point A at the right end of the I axis. Note that the remaining five rotations, namely 3π / 4, π, 5π / 4, 3π / 2, and 7π / 4, have been omitted for clarity in the diagram.

[0041] In this specification, performing a Discrete Fourier Transform (or alternative PCPT) by simply summing the contents of a lookup table containing the rotated constellation points for a symbolic alphabet input is referred to as a “geometric transformation.” An exemplary lookup table for the 16QAM case with k=4 and M=8 (for an 8-point DFT) is an 8×16 complex matrix, as shown in equation (2) (where each of the 16 points A-P has 8 possible rotational positions on the constellation). Each entry in the table in equation (2) is the product of the following: • Represents two codewords (corresponding to alphabet letters A-P) in a constellation in its non-rotated state. k (16) one of the complex numbers, and ·Phase factor: TIFF2026521728000009.tif9170 Here, n takes the range from 0 to M-1 (a total of 8 states, including the unrotated state of the constellation and the 7 rotated states). TIFF2026521728000010.tif39170

[0042] The complex values ​​corresponding to the unrotated constellation (n=0) constitute the first column of the stored value table and are hereafter referred to as the "base constellation." The remaining columns of the table are each referred to as the "nth rotation constellation." k A more general form of the table of stored values ​​for an M-point DFT, which can take on any number of values, is shown in equation (3). TIFF2026521728000011.tif32170

[0043] Furthermore, most constellations exhibit some degree of rotational symmetry. In the case of the QAM constellation, there is quadruple rotational symmetry, resulting in M / 4 unique column values ​​in the lookup table for the M-point precoding transformation. Therefore, it is possible to reduce the size of the lookup table by reusing duplicate values.

[0044] A more general expression for the PCPT of the generalized M point is shown in equation (4). φ m The value is a phase factor specific to a particular PCPT implementation. TIFF2026521728000012.tif30170

[0045] As described in relation to Figure 4, a complete precoded OFDMA system has two distinct mapping steps: the mapping of symbolic codewords to the PCPT inputs and the mapping of PCPT outputs to the IFFT inputs. These are referred to as "layer mapping" and "resource element mapping" in O-RAN terminology. However, it should be understood that these mapping functions described for O-RAN are required in any precoded OFDMA system. Therefore, the use of the terms "layer mapping" and "resource element mapping" in this specification is not intended to limit the scope of the present invention to O-RAN systems.

[0046] Figure 7a shows the layer mapping for the case where M=8 and N=256, as described above. The lower section shows a sequence of 1024 encoded data bits used as the input for one symbol period, and above it is a diagram of registers holding 256 4-bit codewords. This mapping is, in principle, arbitrary. In practice, it is also possible to use strings of encoded bits larger than 1024 bits and cross-map to multiple symbols. Here, for simplicity, we will limit the explanation to the case where an N×k bit string (1024 in this case) is associated with the input for a single symbol period. However, it is also arbitrary to assign 1024 bits to 256 4-bit symbol codewords, but here, for simplicity, we will only consider the case where 1024 bits are sequentially divided into 4-bit symbol codewords. This is shown in Figure 7b.

[0047] Figure 8a shows a magnified view of Figure 7b (the first two PCPT inputs), illustrating the simplest layer mapping process in which sequentially divided codewords are sequentially mapped to the PCPT inputs. This layer mapping of symbol codewords to PCPT inputs is arbitrary, as previously mentioned. One reason to consider alternatives to this simple sequential mapping is to mitigate the effects of multipath. By mapping temporally adjacent symbols to different PCPT inputs, those temporally adjacent symbols are transmitted at separate frequencies, reducing the likelihood that they will be simultaneously affected by multipath-induced nulls, thereby improving the effectiveness of error correction algorithms. This type of improved layer mapping is shown in Figure 8b. Another reason to use non-sequential mapping is to scramble and obfuscate the data. To reconstruct the data, the receiver must use reverse mapping, and even with only eight input PCPT points, there are over 40,000 possible permutations.

[0048] As is well known, a second mapping exists between the output of a PCPT and the input of an IFFT. In the O-RAN standard, this is called resource element mapping. While this specification uses O-RAN terminology, it should be understood that equivalent functionality is required in pre-coded OFDMA systems other than O-RAN, such as digital satellite communication systems. Generally, the output of a PCPT is mapped in blocks to a continuous range of inputs of an IFFT. These are known as resource blocks. As shown in the diagram, resource element mapping can be broken down into two separate processes: resource block mapping and intra-block mapping.

[0049] In mobile communications, resource element mapping is used to allocate data to available frequency-time slots (known as resource elements in the O-RAN standard). These mappings are performed dynamically to effectively utilize the allocation of individual time increments (symbol periods) and subcarrier frequencies. These mappings can also be used to obfuscate signals by scrambling the mapping in a way known only to specific senders and receivers. Figure 9a shows an example of resource block mapping. Figure 9b shows that the resource block mapping shown in Figure 9a can be arbitrarily implemented in a first mapping.

[0050] Within a block, the simplest intrablock assignment, mapping PCPT outputs to IFFT inputs, is a sequential assignment, as shown in Figure 10a. However, assigning PCPT outputs to the range of corresponding IFFT inputs does not necessarily have to be sequential. Non-sequential intrablock mappings can primarily function as scrambling for signal obfuscation purposes. Figure 10b shows an example of a non-sequential mapping. For an 8-point PCPT, there are over 40,000 such non-sequential mappings.

[0051] In summary, a preferred embodiment of the "geometric DFT shift" of the present invention is shown in block diagram 1000 of Figure 11. The figure shows a simplified central unit (CU) or modem (1100) and remote unit (RU) or BUC / SSPA (1200) that can be implemented in a digital satellite system. The following functions are included in the CU 1100: The encoder 1110 adds error correction bits and optional headers, trailers, or other auxiliary bits to the user's binary data stream 1001. The stream is divided into streams 1002 as k-bit symbol codewords. Blocks of M symbol words are mapped to M-point blocks for input to the DFT 1120 (layer mapping process). (N ÷ M) M-word blocks of the mapped symbols are mapped to (N ÷ M) M-word IFFT input segments (resource block mapping). This remapped codeword stream 1003 is transmitted from CU1100 to RU(BUC)1200 as symbolic codewords, without any concern that a malicious actor could intercept and understand the scrambled transmission information.

[0052] Upon arrival at RU1200, each M-word block in transmission undergoes the "geometric transformation" process according to the present invention in block 1202, as described above. Due to the "pre-scrambling" of the resource blocks of the M-word blocks, the output of the geometric transformation can be sequentially applied to the input of IFFT1206 via the in-block mapping block 1204.

[0053] In block 1208, the outputs of IFFT1206 are connected in series and a cyclic prefix is ​​added. Then, pulse shaping filtering is applied as needed to form a sampled digital version 1004 of the baseband signal for transmission. This signal is converted to an analog signal by I DAC and Q DAC blocks 1210 and 1212, upconverted to the transmission frequency in block 1220, amplified by RF power amplifier 1230, and then transmitted via the antenna. In some applications, it may be preferable to perform numerical upconversion to an intermediate frequency before digital-to-analog conversion, and the illustrated examples are not intended to limit this or other embodiments.

[0054] The method described herein offers two distinct advantages over conventional methods: (1) a reduction in the data rate of the signal transmitted between the modem and the RF device, and (2) a reduction in the computational load in evaluating the signal precoding. The data is transmitted in symbol alphabet format rather than sampled waveform format, which significantly reduces the data rate in the digital connection between the modem and the RF device to less than one-tenth. Layer mapping is implemented in the CU, and resource block mapping is implemented in the CU by ordering subblocks of symbol codewords in the symbol stream. Intrablock mapping is implemented in the RU.

[0055] The geometric DFT of the present invention evaluates phase-cyclic precoding by calculating the DFT of M points. 2 This simplifies the process to 1 table lookup and M complex number addition operations, completely eliminating the need for complex number multiplication. It should be understood that embodiments of this method other than those shown in Figure 11 are also possible and are included within the scope of the present invention.

[0056] While embodiments of the present invention have been illustrated and described, these embodiments are not intended to illustrate and describe all possible forms of the invention. Various modifications, alterations, and alternatives can be conceived by those skilled in the art without departing from the intended spirit and scope of the teachings of the invention. The invention is intended to encompass such modifications and alterations.

Claims

1. A method for implementing an M-point phase cyclic transform used for precoding in a precoded OFDM system for a digital signal stream represented as a sequence of M k-bit symbol codewords, wherein each codeword is a base constellation of points on the complex plane. ｋ In a method of representing any of the individual points, a. A step of generating a geometric shift table of stored values, wherein the table is 2 ｋ The table has rows and M columns, Defined by, Here, each column is referenced by index n, and each row is 2 ｋ It is referenced by one of the k-bit codewords, where n ranges from 0 in the first column to n=M-1 in the Mth column, and the exponent φ ｍ This represents a phase factor specific to the particular phase cyclic transformation being applied, The entries in the first column are complex values ​​corresponding to each point in the base constellation, arranged according to the order of the corresponding codewords: The entries in each subsequent column include the complex value of each point in the base constellation, and the corresponding phase factor φ that characterizes the phase cyclic transformation. ｎ The step is equal to the value obtained by multiplying by , and the entries in each row are arranged according to the order of the corresponding codewords, b. A step of associating each of the M symbol codewords in a sequence of M codewords with an index m, wherein m takes the range from 0 to M-1. c. A step of forming a set of M outputs referenced by index l, wherein l takes the range from 0 to M-1. d. A method comprising the step of evaluating each of the M outputs of a transformation as the sum of M table entries, wherein each entry is selected from the nth column of the table (where n is evaluated as the remainder when the product of l and m is divided by M) and from the row corresponding to a codeword assigned to index m (where m ranges from 0 to M-1).

2. In the method of claim 1, The phase cyclic transform is a discrete Fourier transform, and the phase factor φ ｎ A method characterized by having -2πn / M.

3. In the method of claim 1, A method further comprising the steps of performing layer mapping and resource block mapping on a signal codeword stream at a first position, wherein the forming and evaluation steps are referred to as geometric shift precoding and are performed at a position away from the first position.

4. In the method of claim 3, A method characterized in that the conversion is performed in a wireless transmission system, the layer mapping and resource block mapping processes are performed in a modem at the first location, and the geometric shift precoding and signal preparation for transmission via the air interface are performed at the remote location.

5. A method for transmitting a digital signal stream within a precoded OFDM transmission system, a. A step in a modem to encode a signal stream as a sequence of M k-bit symbol codewords, wherein each codeword is a base constellation of points on the complex plane. ｋ A step represents one of the individual points, b. A step of transmitting a symbol codeword stream from the modem to a remote unit, the remote unit inputting the symbol codeword stream to an M-point phase cyclic precoding transform, The aforementioned M-point phase cyclic precoding transformation proceeds through the following steps i to iv, i.e., i. A step of generating a geometric shift table of stored values, wherein the table is 2 ｋ The table has rows and M columns, Defined by, Here, each column is referenced by an index n, and each row is referenced by one of two ｋ k-bit codewords, where n ranges from 0 in the first column to n = M - 1 in the Mth column, and the exponent φ ｍ represents a phase factor specific to the particular phase cycle transformation being applied. The entries in the first column are complex values ​​corresponding to each point in the base constellation, arranged according to the order of the corresponding codewords: The entries in each subsequent column include the complex value of each point in the base constellation, and the corresponding phase factor φ that characterizes the phase cyclic transformation. ｎ The step is equal to the value obtained by multiplying by , and the entries in each row are arranged according to the order of the corresponding codewords, ii. A step of associating each of the M symbol codewords in a sequence of M codewords with an index m, wherein m takes the range from 0 to M-1. iii. A step of forming a set of M outputs referenced by index l, wherein l takes the range from 0 to M-1, iv. A step of evaluating each of the M outputs of a transformation as the sum of M table entries, wherein each entry is selected from the nth column of the table (where n is evaluated as the remainder of the product of l and m divided by M) and from the row corresponding to the codeword assigned to index m (where m ranges from 0 to M-1) A method characterized by being implemented by

6. A non-temporary computer-readable storage medium for storing one or more computer-readable programs, wherein the computer-readable programs are executed by a processor, and the processor is configured to perform a method for computationally performing an M-point phase cyclic transform used for precoding in a precoded OFDM system on a digital signal stream represented as a sequence of M k-bit symbol codewords, where each codeword is a base constellation of points on the complex plane. ｋ Represents any of the individual points, and the method described above a. A step of generating a geometric shift table of stored values, wherein the table is 2 ｋ The table has rows and M columns, Defined by, Here, each column is referenced by index n, and each row is 2 ｋ It is referenced by one of the k-bit codewords, where n ranges from 0 in the first column to n=M-1 in the Mth column, and the exponent φ ｍ This represents a phase factor specific to the particular phase cyclic transformation being applied, The entries in the first column are complex values ​​corresponding to each point in the base constellation, arranged according to the order of the corresponding codewords: The entries in each subsequent column include the complex value of each point in the base constellation, and the corresponding phase factor φ that characterizes the phase cyclic transformation. ｎ The step is equal to the value obtained by multiplying by , and the entries in each row are arranged according to the order of the corresponding codewords, b. A step of associating each of the M symbol codewords in a sequence of M codewords with an index m, wherein m takes the range from 0 to M-1. c. A step of forming a set of M outputs referenced by index l, wherein l takes the range from 0 to M-1. d. A non-temporary computer-readable storage medium characterized by the step of evaluating each of the M outputs of a transformation as the sum of M table entries, wherein each entry is selected from the nth column of a table (where n is evaluated as the remainder of the product of l and m divided by M) and from the row corresponding to a codeword assigned to index m (where m ranges from 0 to M-1).

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