ES2615807T3 - Procedure and receiver for the identification of transmitters that transmit digital terrestrial broadcast signals - Google Patents

Abstract

Procedure for identifying transmitters (5-8) belonging to a single frequency network that transmit respective terrestrial digital television signals, where each transmitter (5-8) transmits its own identifier, characterized in that said own identifier is encoded on the carriers of services of a plurality of N symbols (S40-S47) of a frame of the terrestrial digital television signal and is stored by a table previously compiled in memory means of a receiver (13), and because said method comprises the steps of : a) solve, in the frequency domain and with respect to H, a linear system Y> = X · H, where the indices i and j vary between 1 and N, and where: - Y is a vector of N elements, where a element Yj of vector Y represents a terrestrial digital television signal acquired through said receiver (13) and relative to the jth symbol of the plurality of N frame symbols; - X is a matrix with a size NxN, where an Xji element of matrix X represents a contribution to the signal that the receiver (13) would receive if the ith transmitter was the only one to transmit the signal through an ideal transmission channel of terrestrial digital television relative to the jth symbol; - H is a vector of N elements, where the Hi element of vector H represents the frequency response of the channel relative to the ith transmitter; b) identify each transmitter (5-8) through the respective identifier and the respective frequency response module of the channel of element Hi of vector H, which represents the level information, with respect to the strongest signal, to which it is received the signal of said transmitter.

Description

DESCRIPTION

Procedure and receiver for the identification of transmitters that transmit digital terrestrial broadcast signals.

[0001] The present invention relates to a method and a receiver for identifying transmitters that

they transmit digital terrestrial radiotelevision signals that can be received at a given geographical point of a single frequency DVB-T broadcast network.

[0002] The digital terrestrial television system adopted in Europe and in many other countries of the world is called DVB-T (acronym for digital video broadcasting - terrestrial) and has been described in the ETSI standard

ETS300744: "Digital Video Broadcasting; Framing structure, channel coding and modulation for digital terrestrial television (DVB-T)", ETSI, 1997.

[0003] Said system is based on a modulation of multi-carriers of the OFDM type (acronym in English multiplexed by orthogonal frequency division).

[0004] The number of carriers in the OFDM symbol may be 2048 or 8192; These two modes are usually called 2k mode and 8k mode.

20 [0005] As is known, traditional analog television (for example, PAL, SECAM or NTSC systems)

It is transmitted to users by means of geographic networks of “multi-frequency” circular transmitters, in which each transmitter uses a frequency different from those of adjacent transmitters in order to avoid any interference problem.

25 [0006] Now, with the introduction of digital terrestrial television DVB-T, networks of

"Single frequency", also known as SFN, in which all transmitters in the network emit in a single frequency signals that must be exactly equal and synchronous, so that all receivers capture a series of echoes of the same signal (in worst case scenario, with different power levels and different propagation delays).

30

[0007] This entails the use of a suitable mechanism to generate the data flow, to distribute it to the various transmitters of the network and to synchronize them, as described in the technical standard ETSI TS101191: "Digital Video Broadcasting (DVB ); DVB mega-frame for Single Frequency Network (SFN) synchronization ", ETSI, 2004.

35

[0008] Referring to Figure 1, a time / frequency map of a frame of an OFDM signal is shown, in which each row represents an OFDM symbol and each cell represents a subcarrier.

[0009] The frame structure of the OFDM signal consists of a sequence of OFDM symbols within 40 which are modulated data carriers, according to the chosen scheme (QPSK, 16QAM or 64QAM), and

Some service carriers.

[0010] Service carriers can be classified into two categories:

45 a) pilot carriers: they are transmitted at a higher power level (+2.5 dB) and used at the reception to estimate the channel response. They can be subdivided into:

a1) continuous pilot carriers (continuous pilots): they occupy fixed positions, that is, they are repeated at the same frequencies in each symbol;

50 a2) scattered pilot carriers (scattered pilots): occupy frequency positions that vary from one symbol

to another.

b) TPS carriers (acronym for serialization of transmission parameters): these are carriers of services modulated with information about transmission parameters; They use a very robust modulation 55 (differential modulation / coding 2PSK, with error correction code BCH), so that they can be received even in extreme conditions.

[0011] In more detail, TPS carriers are used to incorporate the parameters related to transmission systems, that is, for codification and channel modulation. 17 and 68 carriers are transmitted

TPS in 2k mode and 8k mode, respectively.

[0012] In a given OFDM symbol, all TPS carriers transmit the same bit, with an encoding

differential. The position of the TPS carriers within the symbol is static and has been chosen according to a pseudo-random rule, in order to avoid frequency periodicity. Therefore, the exact position of the TPS carriers is specified in a table defined in the ETS 300744 standard. The TPS carriers contain service information, which includes:

- type of modulation (QPSK, 16QAM and 64QAM);

10 - hierarchy information;

- guard interval;

- FEC code of the data carriers;

- transmission mode (2k or 8k).

[0013] TPS carriers are transmitted in 68 consecutive OFDM symbols, thereby defining the

called "OFDM frame".

[0014] Four consecutive OFDM frames constitute a "superframe". The reference sequence corresponding to the TPS carrier of the first symbol of each OFDM frame is used to initiate modulation.

20 differential.

[0015] Each OFDM symbol transmits a TPS bit. The group of 68 TPS bits contained in an OFDM frame is defined as follows:

25 - 1 start bit;

- 16 synchronization bits;

- 37 bits of information;

- 14 bits of redundancy as protection against errors.

[0016] At the time the ETS 300744 standard was published, among the 37 bits of information, in

In fact, only 23 bits of information were used, while the remaining 14 bits were reserved for future use and should be assigned a value of 0.

[0017] With the introduction of the DVB-H standard (acronym for digital video broadcasting - portable), that is, the system for adapting television to portable devices, some of these 14 bits have been assigned

remaining; specifically, 8 of them have been allocated for cell identification. The details of this assignment are specified in the updated DVB-T standard ETS 300744, that is, ETSI ETS300744: "Digital Video Broadcasting; Framing structure, channel coding and modulation for digital terrestrial television (DVB-T) '', ETSI, 2009 The information relating to the transmission parameters is sent in the manner shown in Figure 2. Bits S40-40 S47 carry the information related to the cell identifier (Cell-Id) used to identify the cell from which the transmission is derived. signal received.

[0018] The most significant byte of the cell identifier is transmitted in the odd frames of the superframe, while the least significant byte of the cell identifier is transmitted in the even frames.

Four. Five

[0019] Thus, using 8 bits, up to 65,535 cell identifiers can be assigned, which is equivalent to 16 bits.

[0020] The bit allocation is shown in fig. 3. If the cell identifier is not available, the eight 50 bits must be assigned a value of zero.

[0021] While in the DVB-H standard it is mandatory to use the cell identifier (which in that standard identifies a group of telephone signal transmitters), in the case of the DVB-T standard the cell identifier is a optional parameter. In any case, although not expressly specified in the DVB-T standard,

55 understands that all transmitters in an SFN network have the same cell identifier value.

[0022] In many countries of the world, including Italy, the analogue television system is being replaced by the digital television system. This entails a series of problems, some of which were already known in analog systems, while others are specific to the digital world.

[0023] In particular, during the process of installation and configuration of the network and the activities of supervision and detection of problems, it is very useful to be able to identify the incoming signals, that is to say, selectively identify the origin of each of the signals of terrestrial digital television that reach a point

5 determined from the network.

[0024] For example, US 2004/187162 refers to a transmitter identification system, which uses an identification signal inserted in a digital television signal, which makes it possible to identify the source transmitter in a receiving station. Ideally, the identification signal is a sequence

10 orthogonal pseudo-random time synchronized with the signal frame structure of the digital television signal. The identification of the various transmitted signals, designed especially for single frequency networks, allows the network to be adjusted to eliminate or minimize the effects of multipath in certain locations, which receive transmissions from various transmitters.

15 [0025] At present, measuring instruments used for this purpose can only identify the signal

more robust received and can only provide indications of delay and intensity in regards to the other contributions.

[0026] Because in an SFN network identical signals are transmitted at the same time from different transmitters, the resulting effect on the receiver is similar to receiving a single signal, over which they overlap

echoes generated by the reflection of the signal in obstacles (buildings, mountains and the like).

[0027] The OFDM receiver incorporates an echo equalization block based on the estimation of the frequency response of the channel, obtained through the pilot carriers.

25

[0028] As is known, the inverse Fourier transform of the frequency response of the channel is the impulse response: some measuring instruments take advantage of this property in order to provide indications relative to the received echoes (delay with respect to the direct signal and level). However, these instruments do not allow the selective identification of the transmitters that send the various contributions to the

30 received signal, corresponding to the various peaks highlighted by the impulse response, although it would be extremely useful when putting into operation, monitoring and maintaining an SFN network.

[0029] The use of different cell identifier values for each transmitter that is part of an SFN network is a great help in finding the origin of the main beam; however, with the current instruments

35 Information on the origin of the other signals cannot be obtained.

[0030] TPS carriers whose cell identifier bits are different generate a conflict that may result in a demodulation failure only for TPS carriers. However, because the modulation adopted for TPS carriers is very robust, this only occurs in especially unfavorable conditions, it is

40 say, when the signals received are of the same level. In practice, this situation is extremely unusual and, in any case, it has been proven that a correct reception can be achieved in the users' receivers even if the TPS carriers are not adequately demodulated.

[0031] However, experience has shown that some semi-professional devices, generally used by antenna installers, who use TPS carriers to obtain an estimate of the

Signal quality received based on geometric criteria of the constellation points (MER) is negatively affected by conflicts between cell identifiers.

[0032] Therefore, an object of the present invention is to provide a method and a receiver

to identify digital terrestrial radio television signal transmitters, which are adapted to identify all digital terrestrial radio television signal transmitters that can be received at a given geographical point of a single frequency DVB-T broadcast network.

[0033] The method of the present invention allows a unique identifier to be entered for each

transmitter on the TPS carriers of each digital terrestrial radio television signal.

[0034] Because it is introduced in the TPS carriers, said identifier does not affect the correct one

demodulation by the receiver of the audio / video / data streams contained in the signal. Thanks to the

presence of different signals transmitted by different transmitters, it is possible to go back to the single frequency responses of the transmission channels on the defined routes between each transmitter and the receiver.

[0035] From the frequency responses, it is possible to go back to each transmitter, which can be identified through its own identifier contained in the transmitted signal.

[0036] Such objects are achieved by the method and the receiver to identify transmitters of digital terrestrial radiotelevision signals with the features set forth in the appended claims, which are considered an integral part of the present specification.

10

[0037] The invention will now be described in detail in some of its preferred embodiments, which are provided herein by way of non-restrictive example, with reference to the accompanying drawings, in which:

- Figure 1 shows a time / frequency map of a frame of an OFDM signal;

15 - Figure 2 shows the use of TPS bits in an OFDM frame;

- Figure 3 shows an assignment of the TPS carriers that is useful for encoding an identifier of a transmitter;

- Figure 4 shows a geographical area comprising a plurality of digital terrestrial radiotelevision signal transmitters belonging to a single frequency network;

20 - Figure 5 shows an example of the result obtained from a receiver in which the procedure according to the present invention is applied.

[0038] Referring to fig. 4, a geographical area 1 is shown, in particular the Italian region of Piedmont, which comprises a plurality of 5-8 transmitters of digital terrestrial radio television signals

25 belonging to a single frequency network.

[0039] At a geographical point 3 of the geographical area 1, it is desired to know the intensity of each digital terrestrial radiotelevision signal that can be received from each 5-8 transmitter. In the example of fig. 4, transmitters 5-8 are called "Mottarone", "Trivero", "Eremo" and "Penice".

30

[0040] The transmitters 5-8 covering the geographical area 1 are configured to each transmit their own identifier, which is encoded in the TPS service carrier (see Fig. 2) of a plurality of OFDM symbols of a frame of the respective terrestrial digital radio television signal being transmitted.

[0041] In particular, the identification code can be advantageously transmitted in symbols S40-S47.

[0042] The OFDM signal generated in this way is perfectly consistent in the symbols in which the bits of the TPS service carriers are identical, for example the symbols S0-S39 and S48-S53, while in the symbols S40-S47 , in which different bits related to the identification codes of

40 different transmitters, the OFDM signal is not perfectly consistent. To be more precise, the OFDM signal is consistent in the cells that carry data (audio, video, etc.), which guarantees perfect reception, but is not consistent in TPS cells, in which collisions can occur .

[0043] Suppose now that a receiver 13 located at geographical point 3 receives signals from digital terrestrial radiotelevision from transmitters 5-8.

[0044] The signals transmitted by transmitters 5-8 are signals x (t) in the temporal domain, as is the signal y (t) received by a receiver 13, which is ideally displayed within transport means used for measuring bells

fifty

[0045] The signals x (t) and y (t) can be represented in the frequency domain, applying the Fourier transform, respectively as X (w) and Y (w).

[0046] Given that the transmission channel through which signals x (t) travel from 55 transmitters 5-8 to receiver 13 is affected by multipath propagation, it may be characterized by

impulse response h (t) or by the Fourier transform H (w) thereof, also called frequency response of the channel.

[0047] Therefore, the relation Y (w) = X (w) ■ H (w) is fulfilled, in which the variables X, Y and H are quantities

complex and the operator "■" indicates a complex product.

[0048] In addition, because OFDM modulation uses sampled data processing, with a fast Fourier FFT transform in the demodulator and the inverse of the same IFFT in the modulator, the variables

5 mentioned above are expressed using a first Index k for the position of the frequency and a second Index n for the position of the symbol within the frame.

[0049] It follows that, for an OFDM signal, the above relationship can be rephrased as follows: Y (n, k) = X (n, k) ■ H (n, k).

10

[0050] From an analytical point of view, signals Y (40, k) ... Y (47, k), received by receiver 13 in the TPS cells of the first Index k during symbols S40-S47 of the frame of the OFDM signal, they are due to the superposition of all the signals of the plurality of transmitters of the single frequency network that reach the reception area of the geographical point 3. It is the case that each contribution is multiplied by its own

15 channel frequency response. Therefore, in mathematical terms:

twenty

Y (40, k) = Xi (40, k) ■ Hi (40, k) + Y (41, k) = Xi (41, k) ■ Hi (41, k) + Y (47, k) = X1 (47, k) ■ H1 (47, k) +

X2 (40, k) ■ H2 (40, k) + X3 (40, k) ■ X2 (41, k) ■ H2 (41, k) + X3 (41, k) ■ X2 (47, k) ■ H2 (47, k) + X3 (47, k) ■

 Ha (40, k) + ..

 . + Xs (40, k) ■ Hs (40, k)

 Ha (41, k) + ..

 . + Xa (41, k) ■ Ha (41, k)

 H3 (47, k) + ..

 . + Xs (47, k) ■ Hs (47, k)

where the subscript of signals Xi identifies the transmitter 5-8 referred to by the signal. For each value of k, the previous equations define a linear system Y = XH, in which the vector H is the incognite, and in which:

25

- Y is a vector of N elements, in which an element Yj of the vector Y represents a digital terrestrial radiotelevision signal acquired through the receiver 13 and relative to the j-esimo symbol S40-S47 of the plurality of N symbols S40-S47 of the plot of the OFDM signal;

- X is a matrix with an NxN size, in which an Xji element of the matrix X represents a contribution to the signal that the receiver would receive if the i-th transmitter was the only one to transmit on a transmission channel

ideal, the digital terrestrial radiotelevision signal relative to the j-esimo symbol;

- H is a vector of N elements, in which the Hi element of vector H represents the frequency response of the channel relative to the i-th transmitter.

[0051] The linear system Y = X H admits a single solution, provided that the determinant of matrix X is

nonzero.

[0052] In practice, this is easily verified: otherwise, you can go back to the most favorable case by appropriately choosing the values of the identification codes assigned to the

40 transmitters 5-8.

[0053] Once said linear system has been solved, the frequency responses of each of the N transmission channels of each transmitter 5-8 are obtained; the module of each frequency response represents the level information, regarding the strongest signal, to which the transmitter is being received 5-8

45 corresponding.

[0054] The result is, therefore, a list of the individual levels of the received signal (with values normalized to 1), each individual level being associated with its own 5-8 transmitter by means of the respective identifier. Then, through a table previously compiled and stored in a media

50 receiver memory 13, the name and / or geographical location of each transmitter 5-8 can be highlighted.

[0055] The linear system shown above consists of eight equations with eight unknowns. Therefore, by resolving said system, up to eight transmitters received at a geographical point 3 can be identified through the respective identifier.

55

[0056] Normally, this value is more than sufficient for practical use; however, the procedure can be extended by having other received signals participate in the system of equations, for example the symbols S48-S67, and / or the symbols of the frames containing the other eight bits of the sixteen bits that constitute the code of identification of each transmitter 5-8.

[0057] In this way, the number of equations and, therefore, of

incognitas

5 [0058] As another possibility, since the transmitter identifier consists of a binary word of 16

bits and, therefore, admits 65,535 values in addition to zero, a trial and error process can be used, constructing with each attempt the matrix X by the procedure described above, but each time considering a set of N identification code values new and different, different from the previous case.

[0059] It is assumed that the operations described herein are only carried out on carriers

TPS, that is, only in the values of Index k that belong to the set of said carriers.

[0060] In principle, because in a DVB-T OFDM symbol all TPS carriers are modulated with

the same bit, the procedure according to the present invention can be applied even using a single carrier and, therefore, only a value of the first Index k. However, if it is desired to increase the already high ratio between robustness and noise of the process of the present invention, the procedure can be repeated for more than one value of the first Index k and, preferably after equalization, calculate an average of the results obtained from this way.

20 [0061] To make the procedure fit in the best possible way as stipulated in the aforementioned

ETSI document, according to which the Cell-Id cell identifier values of the transmitters of an SFN network should all be equal within the same cell (transmitter group), it is conceivable that the identifiers of each transmitter operating in the geographical area in which it is desired to carry out a measuring bell only during the strictly necessary time and, later, to restore the predefined value, equal to all the transmitters, previously decided by the network operator.

[0062]

In summary, the procedure according to the present invention includes the following steps:

- construct the matrix X, in which each column consists of the signal [Xi (40, k), ... Xi (47, k)] T that the receiver 13 receives it 30 if it were only transmitting the i-th transmitter by an ideal channel;

- acquire, through receiver 13, the received signal Y (40, k) ... Y (47, k), and construct the vector Y with said column elements;

- solve the linear system Y = XH with respect to H and obtain the vector H of the channel responses: each element of the vector H represents the channel response of each transmitter 5-8, in the same order as that of the

35 columns of the matrix X;

- calculate the module of the elements of vector H; these values are the levels, with respect to that of the most intense signal, that is to say normalized to 1, of the various transmitters 5-8 in the order indicated above;

- from the vector obtained in the previous stage, compile an ordered list in which said respective levels are associated with the identification codes of the transmitters 5-8 participating in the procedure;

40 - from a predefined table, in which the names and / or geographical locations of the transmitters are associated with the respective transmitters 5-8 that are being used, register the list obtained in the previous stage;

- use the list as! obtained to label the various components of the impulse response that the measurement receiver provides in graphic format; alternatively or in addition to the above, present the list in a table format.

Four. Five

[0063] Referring to fig. 5, a list is shown in a table format that represents an example of the result provided by a receiver 13 in which the method of the present invention is applied. The receiver 13 comprises at least one antenna adapted to receive a digital terrestrial radiotelevision signal, a DVB-T demodulator, a processor and memory means.

fifty

[0064] Receiver 13, located at geographical point 3, acquires the received signal, processes it according to the method of the present invention, using the processor and the memory means, and presents it on its own screen, or on a screen associated with it, both the strongest signal transmitter identifier (in this case, the transmitter signal 7 "Eremo") and the weakest signal identifiers, indicating

55 for each of them, for example, the power of the signal, with respect to that of the strongest signal, and the delay. Naturally, the absolute values of the received signals or other types of processed values can be represented on the screen. Optionally, in order to carry out the aforementioned process, the identifiers of each of the transmitters operating in the geographical area in which it is desired to carry out the measurement bell can be adjusted only for the time strictly necessary. , and then reassign them

its predefined value, equal for all, previously decided by the network operator.

[0065] The above description shows the characteristics of the present invention, as well! As its advantages.

5

[0066] A first advantage of the present invention is that all digital terrestrial radiotelevision signal transmitters that can be received at a given geographical point of a single frequency network are identified.

[0067] A second advantage is that the application of the method according to the present invention does not

It affects the robustness of the signal being transmitted.

[0068] The method and receiver for identifying digital terrestrial radio television signal transmitters described herein by way of example may be subject to many possible variations without

15 depart from the novel character of the idea of the invention; It is also evident that in the practical application of the invention, the illustrated details may have different shapes or be substituted by other technically equivalent elements.

[0069] Therefore, it is readily understood that the present invention is not limited to a method and a receiver for identifying digital terrestrial radio television signal transmitters, but can admit many

modifications, improvements or substitutions of parts and equivalent elements without departing from the idea of the invention, as clearly stated in the following claims.

Claims (10)

1. Procedure to identify transmitters (5-8) belonging to a single frequency network that transmit respective digital terrestrial radio television signals, where each transmitter (5-8) transmits its 5 own identifier, characterized in that said own identifier is encoded in the service carriers of a plurality of N symbols (S40-S47) of a frame of the terrestrial digital radiotelevision signal and is stored by a table previously compiled in memory means of a receiver (13), and because said method comprises the steps of: 10 a) solve, in the frequency domain and with respect to H, a linear system Y = XH, where indices i and j vary between 1 and N, and where: - Y is a vector of N elements, where an element Yj of the vector Y represents a digital terrestrial radiotelevision signal acquired through said receiver (13) and relative to the jth symbol of the plurality of N 15 plot symbols; - X is a matrix with an NxN size, where an Xji element of matrix X represents a contribution to the signal that the receiver (13) would receive if the i-esimo transmitter was the only one to transmit the signal through an ideal transmission channel digital terrestrial radiotelevision relative to the j-esimo symbol; - H is a vector of N elements, where the Hi element of vector H represents the frequency response of the 20 channel relative to the i-esimo transmitter; b) identify each transmitter (5-8) through the respective identifier and the respective frequency response module of the channel of element Hi of vector H, which represents the level information, with respect to the strongest signal, to which it is received the signal of said transmitter. 25 2. Method according to claim 1, wherein said identifiers of said transmitters (5-8) are selected such that the determinant of matrix X is nonzero. 3. Method according to claim 1, which further comprises the step of repeating step 30 a) for at least one more frequency value of the signal frame and calculating the average of the results obtained for each frequency value. 4. Method according to claim 1, wherein said identifier is encoded in bits S40-S47 of the service carriers of said frame. 35 5. Method according to claim 4, wherein said identifier is also encoded in one or more of the S48-S67 bits of said frame. 6. Method according to claim 1, wherein during each attempt said matrix X is constructed each time using a new set of different identification code (N) values. 7. Method according to one or more of the preceding claims, wherein a screen associated with said receiver (13) displays the following: said identifier and / or the name of said transmitter (5-8); the level and / or the delay, whether relative or absolute, with which the respective signal is received from said 45 transmitter (5-8). Method according to one or more of the preceding claims, wherein the identifiers of each of said transmitters (5-8) are reset by assigning them a predetermined value, equal for all transmitters and previously defined by the network operator. fifty 9. Receiver (13) adapted to identify transmitters (5-8) belonging to a single frequency network, which transmit respective digital terrestrial radio television signals, where each transmitter (5-8) transmits its own identifier, characterized in that said identifier itself is encoded in the service carriers of a plurality of N symbols (S40-S47) of a frame of the digital terrestrial radiotelevision signal, 55 and because said receiver comprises: a) memory means for storing said own identifier of each transmitter (5-8) through a previously compiled table; b) means to solve, in the frequency domain and with respect to H, a linear system Y = XH, where the indices i and j vary between 1 and N, and where: - Y is a vector of N elements, where an element Yj of the vector Y represents a radiotelevision signal 5 digital terrestrial acquired through said receiver (13) and relative to the jth symbol of the plurality of N plot symbols; - X is a matrix with an NxN size, where an Xji element of matrix X represents a contribution to the signal that the receiver (13) would receive if the i-esimo transmitter was the only one to transmit the signal through an ideal transmission channel digital terrestrial radiotelevision relative to the j-esimo symbol; 10 - H is a vector of N elements, where the Hi element of vector H represents the frequency response of the channel relative to the i-esimo transmitter; c) means for identifying each transmitter (5-8) through the respective identifier and the respective frequency response module of the channel of element Hi of vector H, which represents the level information, with respect to the 15 strongest signal, to which the signal of said transmitter is received. 10. Receiver according to claim 9, comprising means for presenting in a screen associated with said receiver (13) the following: said identifier and / or the name of said transmitter (5-8); the level and / or the delay, whether relative or absolute, with which the respective signal is received from said transmitter (5-8).