RU2574080C2 - Method for parallel multi-frequency transmission of digital information using combined multi-position frequency and phase modulation in frequency subchannels - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: invention relates to digital communication engineering and can be used for the multi-frequency transmission of digital information via communication links. The transmission method includes the combination of digital multi-frequency and multi-position phase modulation of OFDM subcarriers, the number of which is equal to N, divided for modulation into clusters (groups) of L subcarriers. In each cluster, multi-frequency modulation symbols are transmitted by selecting R from L subcarriers, of which P are then phase-modulated, while R-P subcarriers remain unmodulated. At the receiving side, after the selection of quadrature subcarrier components in the OFDM circuit, demodulation is performed in two steps: first, multi-frequency modulation symbols are incoherently demodulated by determining R subcarriers; at the second step, using reference oscillations generated from the quadrature components of the modulated R-P subcarriers, P phase-modulated subcarriers are coherently demodulated.

EFFECT: high spectral efficiency and noise-immunity of the methods for the multi-frequency transmission of digital information, which do not require the inclusion, in the transmitted signal, of special training signals or pilot tones for channel state estimation.

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Description

Description of the invention

The invention relates to techniques for multi-frequency transmission of digital information over communication channels. The proposed method is advisable to use for the implementation of modems in digital communication systems. The transmission method includes combining digital multi-frequency and multi-position phase modulation of N OFDM subcarriers, divided for modulation into clusters (groups) of L subcarriers. In each cluster, multi-frequency modulation symbols are transmitted by selecting R from L subcarriers, of which P is then phase modulated, and R-P subcarriers remain unmodulated. On the receiving side, after quadrature subcarrier components are selected in the OFDM scheme, demodulation is performed in two stages: first, multi-frequency modulation symbols are incoherently demodulated by defining R subcarriers, in the second stage, using reference oscillations formed from quadrature components of unmodulated RP subcarriers, coherently demodulate P phase modulated subcarriers. The technical result consists in increasing the spectral efficiency and noise immunity of the methods of multi-frequency transmission of digital information that do not require the inclusion of special training signals or pilot tones in the composition of the transmitted signal to assess the state of the channel. The advantages of the proposed method are fully realized with reliable and stable transmission of digital information over channels with time-varying parameters (fast fading).

The present invention relates to techniques for transmitting digital information and can be used to implement modems in digital communication systems. Including for the simultaneous transmission of two or more digital information flows for various purposes. From the current level of technology there are many systems for transmitting digital information, which carry out multi-frequency parallel transmission of characters representing digital information obtained by multi-position modulation of multi-frequency OFDM subcarriers. A block diagram of such a system is shown in FIG. one.

A widely known example is the digital television system [1]. Other examples are contained in US patent 7702045 B2, class H04L 27/06 "Method for estimating wireless channel parameters", patent date 04/20/2010, in patent US 665442901, class H04L 25/02, published on 11/25/2008, "Pilot -aided channel estimation for OFDM in wireless systems ". In all of the above examples, methods for assessing the channel state are based on the inclusion of training symbols or pilot tones, the parameters of which are known on the receiving side, and, therefore, do not transfer useful ones, i.e. information data, and serve to determine the characteristics of the channel, using statistical estimation methods for this. The disadvantages of the above technical solutions are the significant cost of both computational resources and bandwidth resources of OFDM systems for the transmission of pilot tones and obtaining reliable estimates of the channel state with rapidly changing (in the scale of the OFDM symbol duration) parameters.

Closest to the claimed technical solution is the combined (hybrid) modulation method described in [2], in which N subcarriers of the OFDM system, for implementing multi-position frequency modulation (MFSK), are divided into K clusters containing 4 subcarriers in each of them. The incoming bit information stream is distributed into clusters: a pair of consecutive bits (dibits) of the information stream is addressed to each cluster, the dibit values, in accordance with the Gray code, determine the number of one of the subcarriers in the cluster, which is actually transmitted as part of the OFDM group signal (see Fig. 2), thereby producing 4FSK modulation.

To increase the spectral efficiency of the system, the phases of the subcarriers are modulated according to the law of relative (differential) phase modulation (DPSK): the value of the phase of the subcarrier in the k-th cluster changes relative to the phase value of the subcarrier in the (k-1) -th cluster in accordance with the bit values of the additional information stream . A block diagram explaining the principle of operation of the prototype is shown in FIG. 3.

а на втором, используя поднесущие, определенные на первом этапе, демодулируются $${\widehat{q}}_{DPSK}$$

биты дополнительного информационного потока, переданные методом DPSK. Для демодуляции в способе-прототипе нет необходимости в знании состояния канала и, следовательно, в его оценке, что делает его надежным способом для передачи по каналам с быстрыми замираниями. Недостатком данного технического решения является использование относительной фазовой модуляции, уступающей фазовой модуляции (PSK) по характеристикам помехоустойчивости и надежности.The demodulation process is carried out in two stages: the first determines the transmitted subcarriers and their corresponding dibits $${\widehat{q}}_{MFSK},$$

and in the second, using subcarriers defined in the first stage, they are demodulated $${\widehat{q}}_{DPSK}$$

bits of the additional information stream transmitted by the DPSK method. For demodulation in the prototype method, there is no need to know the state of the channel and, therefore, to evaluate it, which makes it a reliable way to transmit over channels with fast fading. The disadvantage of this technical solution is the use of relative phase modulation, inferior to phase modulation (PSK) in terms of noise immunity and reliability.

, где $$C_L^R$$

- число сочетаний по R из L символов. Часть из переданных R поднесущих, равная P, модулируется по фазе B-позиционным кодом. R-P поднесущих не подвергаются фазовой модуляции и используются для формирования опорных колебаний при фазовой демодуляции. Правило выбора этих поднесущих определяется принципом «наиболее близкого размещения по частоте к модулированным по фазе поднесущим», минимизируя среднее значение расстояния между колебаниями для сохранения максимально возможной корреляции между поднесущими обоих видов. Поясним вышеприведенный принцип на примерах: в кластере с размером L=6 с использованием R=3 тональных поднесущих и дополнительной фазовой модуляцией Р=2 поднесущих при передаче днесущих с частотами f_n1<f_n2<f_n3 в качестве опорного колебания назначается поднесущая с частотой f_n2. Таблица, приведенная на фиг. 4, иллюстрирует этот пример, в которой фазомодулированные поднесущие обозначены mpsk.The inventive method for transmitting digital information receives the main technical result in the form of a combination of increasing spectral efficiency and noise immunity of reliable and stable methods for transmitting digital information that do not require the inclusion of special training signals or pilot tones in the transmitted signal for the implementation on the receiving side of the use of special procedures for estimating channel parameters . The proposed method can be used to transmit two streams of data coming from two sources. For this, the N-total number of frequency subchannels of a multi-frequency system is divided into N / L clusters (groups), each of which contains L subchannels. In each cluster, a combination of R tonal subcarriers is simultaneously transmitted simultaneously along R subchannels, displaying one of the possible symbols of a multi-tone multi-position frequency modulation. The total number of characters possible for transmission in a separate cluster is $$C_L^R$$

where $$C_L^R$$

- the number of combinations of R of L characters. A portion of the transmitted R subcarriers equal to P is phase modulated by a B-position code. RP subcarriers are not phase modulated and are used to form reference oscillations during phase demodulation. The rule for choosing these subcarriers is determined by the principle of "closest frequency distribution to phase modulated subcarriers", minimizing the average value of the distance between oscillations to maintain the highest possible correlation between subcarriers of both types. Let us explain the above principle with examples: in a cluster with size L = 6 using R = 3 tonal subcarriers and additional phase modulation P = 2 subcarriers when transmitting subcarriers with frequencies f _n1 <f _n2 <f _{n3, the} subcarrier with frequency f _n2 . The table shown in FIG. 4 illustrates this example in which phase-modulated subcarriers are designated mpsk.

Another example of a cluster with the size L = 8 and R = 4 tonal subcarriers P = 2 phase-modulated subcarriers when transmitting subcarriers with frequencies f _n1 <f _n2 <f _n3 <f _n4 , the subcarriers with frequencies f _n2 are selected as reference; f _n4 . If an integer number of N / L clusters is organized, then the number of bits transmitted simultaneously is equal to:

where . - means the whole part.

The block diagram of the modulator, the operation of which is described above is shown in FIG. 5.

вариантов, составленных из комбинаций, содержащих по R элементов. Каждый из которых получен возведением в квадрат значений квадратурных компонент отдельных поднесущих в кластере. Значения квадратурных компонент демодулированных R поднесущих сохраняются в буферном запоминающем устройстве (ЗУ) для выполнения на втором этапе демодуляции P фазомодулированных поднесущих с использованием R-P поднесущих для формирования опорных колебаний. В предлагаемом способе передачи цифровой информации для детектирования этих R-P поднесущих, участвующих в передаче части данных полезной цифровой информации, передаваемых методом многотональной многочастотной модуляции, используют некогерентный прием. Использование немодулированных по фазе поднесущих позволяет, в отличие от прототипа, при модуляции поднесущих по фазе применить не относительные методы модуляции, а абсолютные, что дает сочетание повышения спектральной эффективности и помехоустойчивости приема части данных цифровой информации, передаваемых методом фазовой модуляции.On the receiving side, the demodulator in each cluster, after dividing the OFDM signal into separate subcarriers and recording the quadrature components of the subcarriers in the buffer memory, performs operations in two stages sequentially: at the first stage, R subcarriers are incoherently demodulated by comparing and choosing the R largest values from $$C_L^R$$

options made up of combinations containing R elements. Each of which is obtained by squaring the values of the quadrature components of the individual subcarriers in the cluster. The values of the quadrature components of the demodulated R subcarriers are stored in a buffer memory (ROM) for performing, in a second step of demodulating, P phase modulated subcarriers using RP subcarriers to generate reference oscillations. In the proposed method for transmitting digital information for detecting these RP subcarriers involved in transmitting a portion of the useful digital information data transmitted by the multi-tone multi-frequency modulation method, incoherent reception is used. The use of unmodulated phase subcarriers allows, in contrast to the prototype, when modulating subcarriers in phase, absolute rather than relative modulation methods are used, which gives a combination of increasing spectral efficiency and noise immunity of receiving a portion of digital information transmitted by the phase modulation method.

A block diagram illustrating the operation of a subcarrier demodulator of one cluster is shown in FIG. 6.

Phase modulation and, accordingly, demodulation can be implemented in the hierarchical modulation mode [1, 3]. To do this, on the transmitting side, the information data stream subject to multiple phase modulation is divided into low and high priority substreams and in the MPSK scheme the symbols of these streams are displayed with phase values in accordance with the data hierarchy [1, 3]. The offset due to a change in the phase angles during hierarchical modulation is taken into account in the demodulator scheme, which separates the high and low priority sub-flows.

Information sources

1. Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2), DVB Document A122, June 2008.

2. M. Wetz, I. PeriǺ¡a, W. Teich and J. Lindner. OFDM-MFSK with Differentially Encoded Phases for Robust Transmission over Fast Fading Channels. 11th International OFDM-Workshop, Hamburg, Germany, August 2006.

3. A. Schertz, Ch. Weck. Hierarchical modulation - the transmission of two independent DVB-T multiplexes on a single frequency. EBU TECHNICAL REVIEW, April 2003 p / 1-13.

Claims (1)

A method for transmitting digital data that is received at the input of the transmitting part of a communication system in two separate streams and then transmitted over N parallel channels of a multi-frequency communication system, using the grouping of L separate frequency subchannels into N / L clusters (groups), in each of which digital data is displayed in in the form of modulation symbols obtained by the combined modulation method sequentially in two stages: initially, according to the law of multi-tone multi-frequency modulation, a subset R of L active subnets is generated the constituents displaying in each cluster the symbols of this modulation for the data of one of the information streams, displaying a combination of R tonal subcarriers, selecting the modulation symbol, and then, in the second stage, the digital data of the second stream is modulated according to the phase modulation law P from R active subcarriers and at the receiving On the side, the demodulation process is carried out respectively in each cluster in two stages sequentially: first, in the incoherent demodulator, the received symbols of the tonal multi-frequency modulation are demodulated, detecting the R asset ith subcarriers, the totality of which determines the values of the carrier symbols of the digital data of the first stream, and at the second stage, choosing from the R phase-modulated subcarriers R demodulated at the first stage, the carrier symbols of the second stream are demodulated, characterized in that during transmission at the second stage in each a cluster of R active subcarriers defined by the law of multi-tone multi-frequency modulation is modulated by the method of multi-position phase modulation of a part of subcarriers P, and the remaining RP active subcarriers, pos The positions of which in the cluster are uniquely determined by a given rule during transmission with respect to P by phase-manipulated subcarriers are used on the receiving side to form the reference oscillations necessary for coherent demodulation of the symbols of the second information stream.

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