MXPA00008389A - Ofdm receiving system - Google Patents

Abstract

A method of frame synchronization for OFDM point-to-multipoint channels is provided according to a first aspect of the invention, wherein sequences of reference frames are inserted at prescribed intervals between sequences of information frames. Each frame comprises a cyclic prefix, composed of two identical segments of data, which is used for locating a frame synchronization starting point by applying a subtraction function on the two segments of data for locating the starting point at the location of the minimum of the function. A method of channel equalization for OFDM channels is also provided according to a second aspect of the invention, wherein the reference frames and data known to a receiver are used for creating an equalization vector for correcting broadcast errors of the information frames received at the receiver side.

Description

SYNCHRONIZATION BY OFDM PICTURES AND COMPENSATION SYSTEM Field of the Invention The present invention relates to channel compensation and frame synchronization for data transmission of Orthogonal Frequency Division Multiplexing (OFDM) for fixed transmitters and receivers.

Background of the Invention In recent years, telecommunications technologies have become an industry field that has one of the largest expansions it has ever had. The above has been caused by the growing demand to transmit greater amounts of data that require greater bandwidth of the telecommunication channel. One of the proposed wireless technologies that provides greater bandwidth is a modulation technique called Orthogonal Frequency Division Multiplexing (OFDM), which has recently been suggested for use in a non-mobile digital video transmission. OFDM appears to be a powerful element to provide efficient energy signaling for a large number of users in the same channel. The basic idea of OFDM is to transmit blocks of information in parallel by using a large number of orthogonal subcarriers. Although OFDM transmission is an exact modulation technique compared to other transmission techniques, there are two problems associated with it: one of them is frame synchronization and the other is channel compensation. In the OFDM the data is divided into individually modulated sections called tables. These frames are strung together in a continuous manner and transmitted over an OFDM channel. It is very important for the receiver to know precisely where the tables begin and end, with the purpose of recovering valuable data. Finding these beginnings and endings of the frames is what is known as frame synchronization. There are many techniques for performing synchronization by frame, but usually these involve the addition of data flags to mark the beginning and end of frames. The above is done at the expense of adding portions of data that reduce the speed of transmission of valuable data. Since OFDM channels sometimes distort the data they carry to the point that they are totally unrecognizable, a scheme is required to correct any distortion of the channel. Such compensation techniques exist, but frequently these require that the correction of the data be made on the side of the transmitter so that the transmitter can know the state of the channel. In addition, these techniques do not apply to multipoint OFDM transmission since multiple routes are involved, each of which has its own different response. It should be noted that through the present application the term OFDM channel is used to designate the air path of the electromagnetic waves representing OFDM frames that are transmitted from a transmission system to a receiving system. When a channel involves multiple receivers that capture the same transmitted signal, the present application refers to a point-to-multipoint channel.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method and an apparatus for performing the exact OFDM frame synchronization by means of attaching a small portion of each of the OFDM frames to be transmitted to its own start, which in this way creates a cyclic prefix for each frame. Then this cyclic prefix is used on the receiver side to find the start of the received OFDM frames. Another objective of the present invention is to provide a powerful method of accurately compensating the OFDM frames received by an OFDM channel. This method does not require the transmitter to know anything of the conditions of the OFDM channel in such a way that it can be used for the transmission of point-to-multipoint OFDM data. In a preferred embodiment of the present invention, transmitted OFDM information frames include reference frames, inserted at previously determined intervals, and these reference frames contain known data for the receiver. Upon receipt of these tables, the receiver uses them for the purpose of collecting data on the status of the OFDM channel. Once this information is available, the receiver creates a correction or compensation vector, which is then multiplied with each received information frame in order to eliminate the undesired effect of the distortions of the OFDM channel. The resulting frames are verified in relation to the accuracy by multiplying the correction vector by a distorted reference frame and if the final result is sufficiently accurate, the correction vector is considered reliable and can be used to correct all Subsequent information that the tables receive. If the correction vector is inaccurate, another compensation vector can be created by using other reference frames. If the vector is still inaccurate, then some pilot tones, which consist of very small samples of data that the receiver knows and which are contained in the information tables, can also be used to update the correction vector, for better results.

For the purpose of channel compensation, an accurate OFDM frame synchronization is needed. Thus, another objective of the present invention is to provide an OFDM frame synchronization method that uses a cyclic prefix and that consists of attaching to the beginning of each frame sent by the transmitter a small part of its last part. The above serves two functions, the first of which is to protect the tables from the impulse response spill generated by the previous table, so that in this way it fortifies the system against the ISI (Intermediate Symbol Interference) and the second, to provide an exact frame synchronization method by using these cyclic prefixes to determine the exact locations of the beginning of the received OFDM frames. In accordance with the first broad aspect of the present invention, a frame synchronization method for OFDM channels is provided, wherein the starting point of frame synchronization is located within an input data bitstream signal that it has repeated portions of data at previously determined intervals, wherein the method comprises the steps of: a) subtracting the collected data from two points of the previous bitstream signal separated by a predetermined number of bits; b) detecting when the above data are identical in the two previous points of the previous bit stream; c) obtain the starting point of the synchronization of that frame from a result of step b). In accordance with a second broad aspect of the present invention, a method of channel compensation for point-to-multipoint OFDM channels is provided, wherein the method is carried out on one side of a receiver of an OFDM channel and it comprises the steps of: a) receiving an input data bitstream signal comprising sequences interspersed by reference and information frames, each reference frame containing known data for the receiver; b) detecting those reference frames in that input data bitstream signal of the OFDM frames; c) transforming that input stream by OFDM frames from a time domain to a frequency domain; d) creating a correction vector by using the above sequence by reference frames and data known to a receiver to correct transmission errors created by the distortions caused by that OFDM channel; e) correcting each information frame of that sequence by information frames received in that receiver by means of using that correction vector to counteract the distortion effect of that OFDM channel in those frames.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better described and understood with reference to the following drawings, in which: Figure 1 shows a preferred embodiment of the present invention related to frame synchronization. Figure 2 shows the cyclic prefix for two consecutive frames. Figure 3 represents a typical sequence for information frames preceded by a sequence of three reference frames that are used for frame synchronization on the receiver side. Figure 4 shows the application of the function of the window or function of the subtracter in a sequence of input OFDM frames to approximately detect the sequence of reference frames. Figure 5 shows a preferred embodiment of the invention related to the search of fine reference frames. Figure 6 illustrates a flow chart of the receiver initialization routine, in accordance with the preferred embodiment of the present invention. Figure 7 shows the process of the information table according to a preferred embodiment of the invention. Figure 8 shows a detailed view of a preferred embodiment of the present invention related to channel compensation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment of the present invention, a method for OFDM frame synchronization and channel compensation is provided which allows accurate transmission of OFDM data without the need for the transmitter to know anything of the state of the channel. This method is especially suitable for the wireless transmission of non-mobile point-to-multipoint broadband data, where multiple channels are involved, each of which has its own different response. A preferred embodiment of the present invention related to frame synchronization is partially illustrated in Figure 1. This figure shows the prior art of the modulation of the OFDM on the transmitter side and the demodulation of the data on the receiver side. However, Figure 1 also comprises objects of the present invention that will be described in the following lines. First, on the transmitter side, a common string of bits 10 are fed which represent the valuable data to be transmitted from one point to another in a digital chain splitter 12 which fractionizes the bit string 10 into word bits. 14 separate, each of which has a predetermined length. In our example, 6-bit words are used. Then, each word 14 is processed by the digital word mapping module 16 which maps that word into the complex plane of one of the possible states of the word. In view of the fact that the word 14 is 6 bits long, 26 = 64 states are possible, such that each of the four quadrants of the complex plane comprises 16 possibilities. The result of this step is a complex number 18 of the form A + Bj representing the word 14 of 6 bits processed. The words of the data are successively placed in a configuration 19, which thus forms a configuration 19 of a word dimension 18 having the dimension of an information frame. Then, a module of Reverse Fast Fourier Transformation (IFFT) 20 receives as input a series comprising the real parts of the complex numbers 18 and the imaginary parts of those same complex numbers 18 and performs in that It would be an IFFT. The result is a Fourier transform that creates a series of interleaved time data, such that the input of the series is converted from a frequency domain to a time domain. The output of the IFFT module 20 is a time series of complex numbers 22 of the form C + Dj where the first index presents the real parts C and the second the imaginary parts D. Each of these indices C and D represent the portion value of a data frame 30, as shown in Figure 1. As indicated above, a complex configuration of a dimension 23 contains all the complex numbers 22 resulting from the IFFT operation. Since the time series represents 1024 consecutive words, the output of the IFFT module 20 is a pair of data frames 30, each of which represents 1024 words 14. The frame 30 has the form of an analogous curve as shown in FIG. Figure 1, but is in fact represented by a series of discrete points, each of which has its coordinates in a digital representation. This is a common OFDM process and is known in the prior art.

Synchronization by Frames The next step of the method shown in Figure 1 is a preferred embodiment of the present invention and relates to frame synchronization. Synchronization by frames means that the receiver has to detect a reference through the bit signal of the input data indicating the beginning of the frames. Therefore, in a preferred embodiment of the present invention, on the transmitter side each frame 30 comprising the valuable data to be transmitted is input to a transmitter of the frame synchronization module 24 whose function is to add a reference in front of each frame with the purpose of allowing the receiver to detect the start of the frames. Figure 2 shows a better view of the frames comprising a cyclic prefix composed of two identical segments 26. One of the novel features of the present invention resides in the fact that each frame 30 to be transmitted, it is transformed in such a way that a data segment 26 of the table is copied and annexed at the beginning of the frame which in this way creates a longer frame 28, than the valuable data segment 30 which contains useful data. This valuable data segment 30 contains the useful data to be transmitted from the transmitter to one or more receivers. When the cyclic prefix is created, a small data segment 26 that is preferably approximately 10 percent of the length of the valuable data portion 30 is copied and appended to the beginning of the portion 30, thereby increasing the length of the frame by 10 percent only. The process of creating a cyclic prefix 25 is carried out in both the reference frames and the data frames and the result is that each transmitted frame starts and ends by means of the same data segment 26. These identical segments 26 in of a box 28 are used on the receiver side for the purpose of detecting the start of the frames, in a way that is revealed in detail later in this writing. In a preferred embodiment of the present invention, preferably the transmitter sends at prescribed sequential intervals of three consecutive reference frames 34 between the sequences of approximately 100 information frames 36. All the reference frames contain identical data that are known to the receiver and These reference frames are used for frame synchronization purposes. An example of the frame sequences sent by the transmitter 8 is illustrated in Figure 3. In a preferred embodiment of the present invention, the module 32 of frame synchronization of the receiver must perform two tasks, the first is to locate the sequence of the reference frames 34 between two sequences of information frames 36 and the second one is precisely locating the first sample of the data of the first reference frame 34 of that sequence of frames of reference. The exact detection of the start of the reference frames is needed in order to subsequently carry out the compensation of the channel. As mentioned above, the reference frames 34 are known information frames for the receiver 40 and are preferably sent by the transmitter 8 in sequences of three consecutive frames, at prescribed intervals between the information frames 36, as shown in the Figure 3. Other sequences comprising more or less reference frames can also be used. The receiver 40 can use a frame synchronization module 38 for the purpose of detecting the location of the reference frames 34 by comparing the sequence of the data from the beginning and the end of the frame, separated by an equivalent length or greater. to the length of a painting. The principle, shown in Figure 4, is to apply a subtraction between the two data sections, one at the beginning of reference frame 34 and the other at the end of it, which are assumed to be identical. In fact, the two sequences are no longer identical, but they are almost identical, since the OFDM channel has distorted the data during transmission. The previous distortion is a difference in the shape of the data between the transmitted data and the received data. Most of the distortion occurs during the transmission of electromagnetic waves to the air, while a small part of the distortion is due to the local oscillators of the IQ 35 modulator and the IQ 37 demodulator as well as other electronic components. The subtraction function is applied in all ranges of the sequence of three reference frames, as shown in Figure 4, and produces a minimum at the location of the first two reference frames of a sequence of three reference frames. First, an approximate detection of the reference frame is carried out. The presence of the sequence of three reference frames 34 is detected by means of using an algorithm that is based on the following window function, or subtraction function: W (x) [D (x + i) - D (x + i + N)] i = 0 where x is a sample index, k is an arbitrary "window size", D is the data set, and N is the number of samples in one or more data frames. When the routine locates the location of the falling edge of the function (x), it only approximates the location of the sequence of reference frame 34. The purpose of this first search routine is only to approximate the edge descending 39 of the curve of the subtraction function. Then another routine is used to precisely locate the location of the first data sample of reference frames 34. The above is carried out by means of using a process similar to that which was used at the beginning. As mentioned above, on the transmitter side, a termination segment 26 of each frame 22 is copied and appended to the beginning of that frame 22, to thereby create a cyclic prefix 25 as shown in Figure 2. cyclic prefix 25 comprises two identical data segments 26, one located at the beginning of the frame while the other is located at the end of the frame. The cyclic prefix 25 serves two functions: the first is to protect the spill tables from the impulse response that generated the previous frame, to thereby strengthen the system against ISI (Intermediate Symbol Interference); the second and the most crucial for frame synchronization is to provide a method for applying a variant of the function (x), called w (x), so that w (x) finds a minimum at the beginning of the box reference 10. Then w (x) is the same form as W (x), but has different parameters: k is now the length in samples of the cyclic prefix, D is the input data set, N is the length in samples of a box minus the extra part donated by the cyclic prefix and x remains as the data index. This second routine is illustrated in Figure 5 and resembles the first routine with the difference that it only applies to the first reference frame 34 of the sequence of reference frames. Its purpose is to trace a curve of the comparison between the two segments 24 of the cyclic prefix 25 of that frame. Since these two segments 26 are assumed to be identical, the routine would have a minimum 43 at the location of the best match. That location is detected and used as a starting point for synchronization by frames to read the subsequent data. The two routines, when used consecutively in combination with each other, accurately detect the true start of the first reference frame 34 in a sequence of frames. In a preferred embodiment of the present invention, the approximate search of the reference frame and the fine search of the reference frame are carried out only when the receiver is turned on, until the starting point of the frame synchronization is found. Once the above has been done, the processor can track the beginning of each subsequent frame, since all the frames have a previously determined length, so that you can skip these steps after the initialization of the receiver. However, when the distortions reach a point where it is no longer possible to keep track of the beginnings of the frames, the restart process of the receiver can be started again and the start of the reference frames can be searched again. After the approximate search of the reference frame and the fine search of the reference frame has been performed, a time domain correlation algorithm can be applied to the reference data in order to correct the frequency shift, which it is a phenomenon of data distortion caused by small variations of the local oscillators on the sides of the transmitter and the receiver. The principle is to perform a correlation between the first and the second reference frame in the time domain, by using a correlation coefficient e in the time domain. The quantity e is defined as: where / [a] is the angle of the complex value to; ? = SiD (x2 + i) D * (x2 + i + N //) (i = 0, 1 ..., Nfft-l); Nfft is the size of the FFT; the asterisk denotes the complex conjugate; x2 is the starting point obtained from the fine search of the reference frame; In a variant of this point of the art, it may be useful to average two values of e in order to obtain a more accurate value. A first value of e is obtained by calculating the first and the second reference frame while the second value is obtained by calculating the second and third reference frame of the sequence of the three reference frames. This value of e best represents the correction of the frequency shift for an average of three consecutive reference frames received over a longer period of time. The purpose of the receiver detecting the three reference frames 34 is to use them in order to acquire information on the status of the OFDM channel related to the current degree of data distortion in that channel. Figure 6 can be considered as a continuation of Figure 1 and mainly shows a channel flow graph of the compensation module 50 of the receiver. As the data arrives for the receiver, the frame detection is carried out by using the window function as described above and its variant. The next step after finding the start of the reference frame and therefore of all the frames in view of the CPU being able to keep track of the input index, is to transform the data into the frequency domain. The above is carried out by means of using an FFT algorithm. You can use the root-4 method and the process is done in real time. It is understood that the above text only describes the preferred embodiment of the present invention. However, other variants of the present frame synchronization method are also covered by a broad scope of the invention. For example, it is obvious that the sequences of other reference frame numbers can also be used or instead of conducting two consecutive searches of the starting point of synchronization by frames, also fewer or more searches can be done with the present method. In a broad aspect of the invention, it is possible that reference frames are no longer needed and that frame synchronization is performed by means of using the same subtraction method that is only applied in segments 26 of the cyclic prefix. A minimum is then located that gives the starting point to synchronization by frames. For best results, synchronization by frames can be improved as frames are received by applying the subtraction function in each input cyclic prefix over a period of time.

Channel Compensation The preferred embodiment of the present invention is a compensation routine for OFDM channels operating on the frequency domain data. A compensation vector used to compensate for the input data bit signal can be updated at each occurrence of the three reference frames that occurs approximately every 100 frames. It was already mentioned above that the input data bit signal comprises reference frames 34 which are data known to the receiver and a sample 45 of the data of a reference frame is wired on the receiver side to a memory 52, such as an EEPROM, in the circuit board of the receiver. Figure 8 shows a detailed flow chart of the channel compensation of the receiver. After the reference frames 34 have been located by means of the frame detection routines, the first step is to return these to the frequency domain, by means of using an FFT 54 module. Then, each of the two frames reference 34 received through the OFDM channel are divided by the exact reference frame sample 45 contained in the memory 52 and the two results are the inverse of the frequency response of the OFDM channel. The results are averaged and the result is a more accurate correction of the compensation vector which represents the average distortion caused by the OFDM channel in two consecutive frames. In a preferred embodiment of the present invention, instead of computing an average between the two vectors, the correlation coefficient in the mathematical operation is also involved in order to counteract the frequency shift problem described in detail earlier in this application . The formula used is: See = - eal + Veq2 * EXP (2? Ti £) 2 where Veq is the averaged compensation vector; Veql is the first compensation vector; Veq2 is the second compensation vector; i is the imaginary unit; e is the correlation coefficient. This operation results in a more accurate correction of the vector that represents the inverse of the frequency response of the communication channel. Other smoothing algorithms can be applied to this vector for the purpose of filtering illegitimate nails and noise. The correction vector, component by component, is then multiplied to the samples extracted from the third reference frame received within the sequence of three reference frames, so that in this way the distortion effect of the channel is compensated and eliminated. Then this corrected third reference frame must match the exact reference frame 45 which is contained in the memory 52 of the receiver. Verification can be performed more than once before the routine continues, and each time the operation has been done and the result is not accurate enough, the FFT start point of the third reference frame in the sequence can be return a sample of the data. When a location is found that results in an exact compensation vector and a compensation vector is generated, that starting point is used with a minimum of error for the entire process and reading of the subsequent data. The final compensation vector is calculated and each subsequent information frame 36 is multiplied by the same compensation vector in accordance with the following algorithm: '• eq i * v', eq- * EXP (2 2 pe) where Ieq is the table gives corrected data, I is the distorted data table, Veq is the compensation vector, e is the correlation factor.The purpose of this operation is both to correct the distortion caused by the OFDM channel in the data tables (by multiplication with the compensation vector Veq) and to correct the frequency shift for the data frames (by multiplication with EXP (i2e7r)) Figure 6 shows a flow diagram of the general operation of the receiver's start routine which is carried out each time the receiver is turned on.The purpose of this routine is to locate a better point to start reading the input data from that point, to continue on the track of the starting points of each input data box. As can be seen in Figure 6, after performing the two search steps of the reference frames and when the starting point of the first reference frame is found within a sequence of reference frames, the reference frames are correlated within the time domain to correct the effect of the frequency shift. Then, a FFT in the reference frames and a first compensation vector in accordance with the method shown in Figure 8. This vector goes through a plurality of checks involving the comparison of the result with the exact reference frame. , until the best starting point for the subsequent reading of the input data is found. Figure 7 shows the process of the information table, once the exact compensation vector has been generated. All the input information frames are subjected to a process wherein the cyclic prefix 25 is removed and then correlated in the time domain to eliminate the effect of the frequency shift. Then, all are passed through an FFT operator and multiplied by the compensation vector that has been better explained above in the text of the present, to remove the distortion caused in them by the OFDM channel. Finally, the tables are mapped and the digital data is fed to the data processing unit of the receiver. In another preferred embodiment of the present invention, additional verification data can be carried out on the receiver side to ensure that the data has been restored in an accurate manner. A check of the condition of the restored information frames can be carried out by using very small samples of information that each of the information frames 36 contains and which are called pilot tones. These pilot tones are very small sequences of data that contain information known to the receiver, just like the reference frames. Therefore, each of the information boxes is corrected by means of using the correction vector, these pilot tones can be verified if they start to drift from their expected values, a feedback signal can be sent with object to update the correction vector by using the deviated values of the pilot tones. As mentioned earlier in the present application, once the exact location of the reference frames has been found by means of the approximate and fine search module of the reference frames (when the receiver is turned on for the first time), then updates the compensation vector only in each incident of the reference frames, which occurs approximately once in every 100 reference frames, and the data tables are updated by using the compensation vector, as described above. In some cases, when the distortion of the OFDM channel is small and constant, the updating of the compensation vector can be performed even more rarely, for example at every third occurrence of the sequence of the reference frames. Neither the approximate search of the reference frame nor the fine search of the reference frame will be necessary, since the exact location of the reference frames is known. The above saves time and increases receiver performance.

Claims (25)

1. A frame synchronization method for OFDM channels, wherein a starting point of frame synchronization has to be located within a current signal of the input data bit having repeated portions of data at intervals previously determined by the data subtractors from two points of the previous current signal of the bit separated by a number of previously determined bits, which detect when the data are identical at the two points of that bit stream, and which obtain the previous starting point of the synchronization by tables of a result of that detection, which method is characterized in that: that subtraction comprises one of: a) subtractor bits separated by an integral number of frames, wherein the subtracted bits come from identical synchronization frames and a result of the previous subtraction provides an extended minimum that lasts approximately what lasts at least one frame, that search to detect that extended minimum and which determines the beginning of that extended minimum, the previous starting point obtained from the previous start of that extended minimum, which is a rough estimate of that starting point; and b) data subtracting bits collected from two points of a separate frame by a length of a valuable portion of data in that frame, where those subtracted bits come from the identical segments of first and second data in that frame forming a cyclic prefix individual, and a result of that subtraction provides substantially an instantaneous minimum, where the first segment of data is part of that valuable portion of data from the table and is copied and appended to the previous valuable portion of data to generate the previous second segment of data, that detection that determines the instantaneous minimum, the previous starting point obtained which is a fine estimate of the starting point; that in this way the detection and the obtaining of the starting point of synchronization by frames is simplified.

2. The frame synchronization method for OFDM channels according to claim 1, wherein the data bitstream signal comprises alternate sequences of reference frames and information frames.

3. The method of frame synchronization for OFDM channels according to claim 2, wherein each of the above sequences of reference frames comprises three consecutive reference frames, each reference frame comprising identical data.

4. The method of frame synchronization for OFDM channels according to claim 3, wherein each reference frame and information frame comprises the above individual cyclic prefix composed of the above first and the previous second identical segments of data.

The method of synchronization by frames according to claim 4, wherein an approximate search of reference frame is carried out by approximately locating a starting point of the synchronization by frame, wherein step a) includes: making a first subtraction in the data collected from the two points that were separated by an integral number of tables; detecting a falling edge of a minimum of a result of an application of the former first subtraction in a series of data samples ranging from the beginning of a first reference frame of that sequence of reference frames to at least a second frame of reference of that sequence, that minimum that begins with that descending bank and ends with an ascending bank; and locate the approximate starting point of the synchronization by frames in a location on that descending bank.

The method of frame synchronization according to claim 5, further comprising the steps of: performing a second subtraction on the data collected from the two points of a separate frame by a length of the valuable portion of data; detect a minimum of a result of the previous second subtraction applied to a series of pairs of those points; obtain a precise location of that minimum detected in the previous step as the starting point of synchronization by frames.

The method of frame synchronization according to claim 1, wherein an approximate frame synchronization is performed to roughly locate that starting point of frame synchronization in that input bit stream signal , that stream comprising OFDM frames, wherein step a) comprises: performing a first subtraction of those data collected from the two points that are separated by an integral number of frames; detect a falling edge of a minimum of a result of an application of that first subtraction in a series of data samples ranging from a beginning of a first four to at least a second frame, that minimum beginning with that descending edge and ending with an ascending edge; and to locate in an approximate way that point of beginning of the synchronization by pictures in a location of that descending bank.

The method of frame synchronization according to claim 1 or 7, wherein said bit stream signal comprises OFDM frames, each of those frames comprising that individual cyclic prefix composed of that first and said second identical segments of data, that method that also includes the steps of: performing a second subtraction on the data collected from the two points of a table separated by a length of that valuable portion of data; detect a minimum of a result of that second subtraction applied to a series of pairs of those points; obtain an accurate location of that minimum detected in that previous step as an accurate starting point for synchronization by frames.

9. The method of frame synchronization for OFDM channels according to claim 2, further comprising the steps of: incorporating on the transmitter side, between the sequences of the information frames to be transmitted, frames sequences reference at prescribed regular intervals, each of those reference frame sequences comprising at least one reference frame of known data to a receiver, each of those at least one reference frame comprising identical data known to that receiver; create on that side of the transmitter, those individual cyclic prefixes for each frame that is transmitted, those cyclic prefixes that are composed of those identical segments of data, where either a final portion of each frame is copied and appended to the beginning of each frame, or a starting portion of that table is copied and appended to the end of each of those tables.

10. A channel compensation method for point-to-multipoint OFDM channels performed on one side of the receiver of an OFDM channel, comprising the steps of: a) receiving an input data bitstream signal comprising interleaved sequences of reference and information tables, each reference frame containing known data for the receiver; b) detecting those reference frames in that OFDM frame input data bitstream signal, and correlating each pair of input reference frames to obtain a factor e, - c) transforming that frame current signal OFDM input from a time domain to a frequency domain; d) creating a correction vector by using that sequence of frames of reference and known data for a receiver to correct the transmission errors created by the distortions caused by that OFDM channel, that correction vector that corrects an effect of displacement frequency of that OFDM channel that uses that £ factor; e) correcting each information frame of those sequences of information frames received in that receiver by means of using that correction vector to counteract the distortion effect of that OFDM channel in those frames.

The method of channel compensation according to claim 10, wherein said interleaved sequences of reference and information frames are alternated and have a predetermined number of frames, and step b) is carried out by means of track a starting point of synchronization by previously located frames.

The channel compensation method according to claim 11, wherein the frame synchronization operation is carried out before step b) to locate a synchronization start point by frames within the frame input stream OFDM, that synchronization by frames comprising the steps of: f) subtracting the data collected from the two points of that input data bitstream signal, those two points that are separated by a predetermined number of bits: g) detect when those data are identical at those two points of that data bit stream signal; h) obtain that starting point of synchronization by frames as a result of step g).

13. The channel compensation method according to claim 12, wherein each of said reference frame sequences comprises three consecutive reference frames, each reference frame comprising identical data.

14. The channel compensation method according to claim 13, wherein each of said reference frames and said information frames comprises an individual cyclic prefix composed of first and second identical segments of data, wherein said first segment The data is part of a valuable portion of data from that table and is copied and appended to that valuable piece of data to generate that second data segment.

The method of channel compensation according to claim 14, wherein an approximate reference frame is carried out to approximate a starting point of a synchronization by frames, wherein said step f) comprises performing a first subtraction in the data collected from the two points that were separated by an integral number of tables; step g) comprises detecting a falling edge of a minimum of a result of an application of the above first subtraction in a series of data samples ranging from the beginning of a first reference frame of that sequence of reference frames to when minus a second reference frame of that sequence, that minimum that begins with that descending bank and ends with an ascending bank; and that point h) comprising locating the approximate starting point of the synchronization by frames at a location on that descending bank.

16. The channel compensation method according to claim 15, wherein said bit stream signal comprises OFDM frames, each of those frames comprising an individual cyclic prefix composed of that first and said second identical segments of data , where that first segment of data is part of a valuable portion of data from that table and is copied and appended to that valuable portion of data to generate that second segment of data, that method that also includes the steps of: i) perform a second subtraction on the data collected from the two points of a separate table by a length of that valuable portion of data; j) detect a minimum of one result of that second subtraction applied to a series of pairs of those points; k) obtain an accurate location of that minimum detected in that step j) as the precise starting point of synchronization by frames.

17. The channel compensation method according to claim 10, wherein said reference frame sequences comprise at least one reference frame and wherein step d) comprises performing for at least one reference frame of that sequence of reference frames a division of a first reference frame of that sequence by that data known to the receiver stored on the side of that receiver and which obtains at least one quotient representing at least one compensation vector representing an inverse of a response frequency of that OFDM channel.

18. The channel compensation method according to claim 17, wherein when more than one compensation vector is obtained, an average of the compensation vectors is calculated to obtain an averaged compensation vector.

19. The channel compensation method according to claim 18, wherein each of said reference and information frames comprises an individual cyclic prefix composed of said first and said second identical segments of data, wherein said first segment of Data is part of a valuable portion of data from that table and is copied and appended to that valuable piece of data to generate that second segment of data; step c) uses a frame synchronization start point within that cyclic prefix to transform that OFDM frame input stream from a time domain to a frequency domain.

20. The channel compensation method according to claim 19, further comprising the steps of: verifying an accuracy of that average of all compensation vectors by means of the following steps of: performing a multiplication of that compensation vector averaged with a last reference frame of that sequence of reference frames, that last reference frame that has not been used to calculate previously those compensation vectors, where that result represents a last compensated or corrected reference frame, according to which the corrected reference frame must be equal to that known data for the receiver; compare that result with those data known to the receiver to calculate an accuracy of that averaged compensation vector; move that starting point of synchronization by frames within that cyclic prefix by using the new data read in a new location to create an averaged compensation vector, by using the previous steps; select one that is more accurate from that new averaged compensation vector and that averaged compensation vector.

21. The channel compensation method according to claim 11, wherein said reference frame sequence comprises a cyclic prefix composed of a first and a second identical data segments, wherein said first data segment is part of a valuable portion of data from that table and it is copied and appended to that valuable portion of data to generate that second data segment and where that step c) uses a starting point of the synchronization by frames within that cyclic prefix for transform that OFDM frame input stream from a time domain to a frequency domain.

22. The channel compensation method according to claim 21, wherein step d) comprises the steps of: dividing a first frame of reference from that sequence of reference frames by means of that known data for the receiver which is stored on that side of the receiver and to obtain in a first quotient that represents a first inverted of a frequency response of that OFDM channel that is a first compensation vector of that OFDM channel; divide a second reference frame of that reference frame sequence by means of that known data for the receiver that are stored on that side of the receiver and to obtain in a second quotient that represents an inverted second of a frequency response of that channel OFDM which is a first compensation vector of that OFDM channel; calculate an average of that first quotient and second quotient, that average that represents a compensation vector of that OFDM channel; verify an accuracy of that compensation vector by comparing a result of a multiplication of that vector with a third reference frame of that sequence of frames of reference, with that data known to the receiver, that result representing compensation or correction of a reference frame, by means of which that corrected reference frame must be equal to that known data for the receiver; move that starting point of synchronization by frames within that cyclic prefix by using the new data read in a new location to create an averaged compensation vector, by using the previous steps; and select one that is more accurate from that new averaged compensation vector and that averaged compensation vector.

23. A method of transmitting a point-to-multipoint OFDM signal comprising: preparing transformed data frames for transmission; repeat a portion of these tables and add that portion to those tables; transmit those frames that include that portion that is added to them, by means of which that repeated portion allows the recipients to identify a starting point of synchronization by subtracting a signal received from it at a later time determined previously .

24. An OFDM receiver comprising: a received signal subtracting circuit for subtracting a received signal collected from two points of that received signal separated by a predetermined time interval and which generally outputs an output subtraction signal; and an element for generating a signal from a starting point of frame synchronization by means of using that output subtraction signal.

25. An OFDM receiver comprising: a receiver circuit that detects an input data signal comprising interleaved sequences of reference and information frames, each of said reference frames containing known data for the receiver; a circuit that detects those reference frames in that input data bit stream of the OFDM frames; a circuit that transforms that input current of OFDM frames from a time domain to a frequency domain; a circuit that creates a correction vector by using that sequence of frames of reference and the known data for the receiver to correct the transmission errors created by the distortions caused by that OFDM channel; and a circuit that corrects each information frame of those sequences of information frames received in the receiver by means of using this correction vector to counteract a distortion effect of that OFDM channel in those frames.