Patent Publication Number: US-2009225741-A1

Title: Wireless system using a new type of preamble for  a burst frame

Description:
FIELD OF INVENTION 
     The present invention relates to the field of wireless communication, in particular to a new type of preamble for burst frame timing and peak-distance detection. 
     STATE OF THE ART 
     For example for high rate indoor wireless systems beyond 1 Gbps, the wireless channel delay spread might be over tens of symbols, which makes conventional adaptive equalizers including linear, decision feedback or maximum likelihood sequence estimation (MLSE) equalizer unrealistic.
         The adaptive linear equalizer including either linear or decision feedback equalizer is difficult to converge with short training period because of many of the taps for covering the wireless channel delay spread, which is over tens of symbols.   The complexity of maximum likelihood sequence estimation (MLSE) or of a Viterbi equalizer grows exponentially with the number of symbols included in a wireless channel delay spread and becomes extremely complex when a wireless channel delay spread is over tens of symbols.       

     A conventional single carrier wireless system with a frequency domain equalizer uses a cyclic prefix for carrier synchronization and a burst preamble for frame synchronization and coarse timing synchronization. Normally the channel estimation is realized by introducing an additional pilot frame and the frame adopts a constant amplitude zero auto-correlation sequence (CAZAC). 
     The disadvantages of the state of the art technology for single carrier wireless systems using frequency domain equalizer are as follows:
         A long burst preamble is used for packet/burst frame detection, automatic gain control, coarse timing synchronization and coarse frequency synchronization. However, only a rough timing for a FFT block can be obtained due to a flatness of the autocorrelation peak and the effect of noise.   Additional circuits have to be used for packet/burst frame detection, automatic gain control, coarse timing synchronization and coarse frequency synchronization.       

     SUMMARY OF THE INVENTION 
     The present invention relates to a method for generating a wireless communication signal, whereby said communication signal is based on a temporal frame structure with burst frames, each burst frame comprising at least one combination of a guard interval and a data frame, said method comprising the step of inserting a preamble before a first of said at least one combination, said preamble and said guard interval each comprising at least one pseudorandom-noise, PN, sequence, whereby said at least one PN sequence of the guard interval is identical to said at least one PN sequence of the preamble. 
     Favourably, said preamble comprises a plurality of PN sequences. 
     Favourably, at least one PN sequence of said plurality of PN sequences of said preamble is inverted in relation to the other PN sequences of said preamble. 
     Favourably, at least two adjacent PN sequences of said plurality of PN sequences of said preamble are arranged with a distance to each other, wherein control information is encoded in said distance. 
     Favourably, said at least one PN sequence is a maximum length sequence. 
     Favourably, said wireless communication signal is a single carrier or a multi carrier communication signal. 
     The present invention also relates to a signal generator operable to generate a wireless communication signal, whereby said communication signal is based on a temporal frame structure with burst frames, each burst frame comprising at least one combination of a guard interval and a data frame, said generator comprising a preamble insertion module operable to insert a preamble before a first of said at least one combination, said preamble and said guard interval comprising at least one pseudorandom-noise, PN, sequence, whereby said at least one PN sequence of the guard interval is identical to said at least one PN sequence of the preamble. 
     Favourably, said preamble comprises a plurality of PN sequences. 
     Favourably, at least one PN sequence of said plurality of PN sequences of said preamble is inverted in relation to the other PN sequences of said preamble 
     Favourably, at least two adjacent PN sequences of said plurality of PN sequences of said preamble are arranged with a distance to each other, wherein control information is encoded in said distance. 
     Favourably, said at least one PN sequence is a maximum length sequence. 
     Favourably, said wireless communication signal is a single carrier or a multi carrier communication signal. 
     The present invention further relates to a method for processing a received wireless communication signal, whereby said communication signal is based on a temporal frame structure with burst frames, each burst frame comprising at least one combination of a guard interval and a data frame and a preamble preceding said combination, said preamble comprising at least one pseudorandom-noise, PN, sequence, said method comprising the steps of correlating said at least one PN sequence of the preamble, and outputting a correlation function. 
     Advantageously, said correlation function from said at least one PN sequence of said preamble is used to perform burst frame detection, automatic gain control, coarse timing synchronization and/or coarse frequency synchronisation of said wireless communication signal. 
     Further advantageously, said guard interval comprises at least one PN sequence, said at least one PN sequence being identical to or inverted in relation to said at least one PN sequence of said preamble, whereby said at least one PN sequence of the guard interval is correlated in order to obtain a correlation function, whereby said correlation function is used to perform channel estimation and/or equalization of said carrier wireless communication signal. 
     Further advantageously, the method comprises the detection of a correlation peak in said correlation function(s). 
     Further advantageously, said burst frame comprises at least two guard intervals with respective PN sequences and at least two PN sequences in said preamble, whereby timing information is detected from the correlation functions of said at least two PN sequences of the preamble and correlation functions of the PN sequences of the guard intervals. 
     Further advantageously, a predetermined time duration between the correlation functions of said at least two PN sequences of the preamble identifies the presence of a preamble. Hereby, the preamble can be identified easily on the basis of the time duration between the correlation functions, particularly if the time duration between the correlation functions of the PN sequences of the guard intervals is different. Further advantageously, a predetermined time duration between the correlation functions of said PN sequences of the guard intervals identifies the presence of a data frame. Hereby, the data frames can be identified easily on the basis of the time duration between the correlation functions, particularly if the time duration between the correlation functions of the PN sequences of the preamble is different. Further advantageously, said preamble comprises a plurality of PN sequences, wherein a detection of a variation in time durations between the correlation functions of said PN sequences of the preamble is performed in order to obtain control information. Thus, if the some of the PN sequences of the preamble have a different distance from each other as compared to other PN sequences of the preamble, this variation could contain encoded control information, which could be decoded and used on the receiver side. 
     The present invention further relates to a signal processor operable to process a received single carrier wireless communication signal, whereby said communication signal is based on a temporal frame structure with burst frames, each burst frame comprising at least one combination of a guard interval and a data frame and a preamble preceding said combination, said preamble comprising at least one pseudorandom-noise, PN, sequence said processor comprising a correlation module operable to correlate at least a part of said at least one PN sequence of the preamble and to output a correlation function. 
     Advantageously, said signal processor is operable to use said correlation function from said at least one PN sequence of said preamble to perform burst frame detection, automatic gain control, coarse timing synchronization and/or coarse frequency synchronisation of said wireless communication signal. 
     Further advantageously, said guard interval comprises at least one PN sequence, said at least one PN sequence being identical to or inverted in relation to said at least one PN sequence of said preamble, whereby said correlation module is operable to correlate said at least one PN sequence of the guard interval in order to obtain a correlation function, whereby said signal processor is operable to use said correlation function to perform channel estimation and/or equalization of said carrier wireless communication signal. 
     Further advantageously, said signal processor comprises a detection module operable to detect a correlation peak in said correlation function(s). 
     Further advantageously, said burst frame comprises at least two guard intervals with respective PN sequences and at least two PN sequences in said preamble, whereby said signal processor is operable to detect timing information from the correlation functions of said at least two PN sequences of the preamble and correlation functions of the PN sequences of the guard intervals. 
     Further advantageously, a predetermined time duration between the correlation functions of said at least two PN sequences of the preamble identifies the presence of a preamble. 
     Further advantageously, a predetermined time duration between the correlation functions of said PN sequences of the guard intervals identifies the presence of a data frame. 
     Further advantageously, said preamble comprises a plurality of PN sequences, wherein said signal processor is operable to perform a detection of a variation in time durations between the correlation functions of said PN sequences of the preamble in order to obtain control information. 
     The present invention concentrates on the areas of multi carrier or single carrier wireless systems with a frequency domain equalizer and simultaneously provides a coarse frame timing, a carrier synchronization and a channel estimation without an additional overhead. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The features, objects and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, wherein: 
         FIG. 1  shows an example of a frame structure of an OFDM system or single carrier system using a frequency domain equalizer, 
         FIG. 2  shows an example of a block diagram of an OFDM system, 
         FIG. 3  shows an example of a block diagram of a single carrier system using a frequency domain equalizer, 
         FIG. 4  shows an example of a frame structure using PN sequences, 
         FIG. 5  shows an example of a burst frame comprising a burst preamble and a frame structure, 
         FIG. 6  shows a detailed view of a state of the art example of a burst preamble, 
         FIG. 7  shows another detailed view of a state of the art example of a burst preamble with a succeeding frame structure, 
         FIG. 8  shows another example of a burst frame, 
         FIG. 9  shows another example of a burst frame, 
         FIG. 10  shows a further example of a burst frame, 
         FIG. 11  shows another example of a frame structure using PN sequences as well as an example of the coarse frame timing and the carrier synchronization based on the auto-correlation peak of a PN sequence, 
         FIG. 12  shows an example of an apparatus for a channel equalization based on a Fast Fourier Transformation (FFT), 
         FIG. 13  shows an example of an apparatus for a channel equalization based on a Discrete Fourier Transformation (DFT), and 
         FIG. 14  shows an example of a frame structure with an additional guard interval. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a multi or single carrier wireless communication system with a frequency domain equalization, whereby at least one PN sequence is used and/or inserted in a preamble of a frame, burst, packet or the like of a transmitter signal; thus the PN sequence, favourably having a high correlation peak and a low correlation sidelobe, is used to indicate the beginning of a burst frame and for automatic gain control, coarse frame timing, time and/or frequency synchronization and the like in a receiver, whereby each burst frame is composed of at least one or a plurality of data blocks. 
     Throughout the invention the term “PN sequence” can be replaced by the term “M sequence”, whereby a M sequence is a special case of a PN sequence as explained in detail further below. In addition at least one characteristic of a M sequence can correspond to at least one characteristic of a PN sequence. 
     In the following, the term burst frame will be used to describe the fact that the wireless signal is based on a temporal structure in which data, information etc. is transmitted in packets, frames, data bursts or the like, but is in no way intended to restrict the scope of the invention. 
     According to the invention, at least one PN sequence is also used in a guard interval between or favourably as cyclic prefix of data frames of the frame, burst, packet etc. Hereby, the PN sequence can be used for coarse FFT block timing, carrier synchronization and/or channel estimation on the receiver side. When a PN sequence is used in a guard interval for channel estimation and equalization, the same complex (I/Q) matched filter for the PN sequence which is anyway required in the receiver can be also used to detect the position of and the distance between two consecutive correlation peaks to differentiate the timing of each FFT block and the timing of the burst frame. 
     The advantages of the proposed new burst frame structure are low complexity and low overhead; thus, there is no need for additional circuits for packet/burst frame detection in the receiver, and the proposed burst preamble for the burst frame can be shorter than the one of the state of the art. 
     For example, for high rate indoor wireless systems beyond 1 Gbps, the wireless channel delay spread might be over tens of symbols, which makes a conventional linear equalizer, a decision feedback equalizer and a maximum likelihood sequence estimation (MLSE) equalizer unrealistic. 
     One possible solution is to adopt an orthogonal frequency division multiplexing (OFDM) technique, which is an example of a multi-carrier wireless communication technique. The main advantage of OFDM is the low complexity frequency domain equalization. This is achieved with the introduction of a guard interval or a cyclic prefix between data frames, which enables the receiver to cope with time dispersive channels, as long as the channel impulse response is shorter than the cyclic prefix. 
     A similar approach using frequency domain equalization can be adopted for single carrier wireless systems. Here time domain guard intervals and/or cyclic prefixes inserted between data frames (cf.  FIG. 1 ) are used to cope with the multi-path fading channels and to suppress inter-frame interference if the channel impulse response is shorter than the cyclic prefix. A single carrier wireless system with a frequency domain equalizer may also use such a cyclic prefix for frequency synchronization. When a PN sequence is used in or as a guard interval between data frames instead of a cyclic prefix, the correlation peak (auto- or cross-correlation peak) of the PN sequence can be used for channel estimation and equalization and/or carrier synchronisation in the receiver. 
     For single carrier systems with a frequency domain equalizer, an additional burst preamble is usually added for packet/burst frame detection, automatic gain control, coarse timing synchronization and/or coarse frequency synchronization and the like in the receiver. 
     The present invention is directed to a single or multi carrier system with a frequency domain equalizer, whereby at least one PN sequence is used in a guard interval, and whereby the identical PN sequence is used in the burst preamble to indicate the beginning of a burst frame. The term “identical” is intended to include that one of the PN sequences has the same real value but is inverted in relation to the other PN sequence. 
     As mentioned, the guard interval allows the receiver to cope with a time dispersive multi-path fading channel, as long as the channel impulse response is shorter than the time duration of the guard interval. Otherwise there might be an inter-frame interference. The channel estimation accuracy can be improved using consecutive PN sequences in the preamble. In the following the basics of a PN sequence and a M sequence, as used in the present invention, as well as their characteristics are explained in more detail. 
     Generally speaking, a signal comprising a message unknown to a receiver has a random nature and is called a stochastic signal. In case the signal would not have a random nature, the receiver would be capable to reconstruct the message from the already sent signal due to the deterministic nature of the signal. That means, that the signal with a deterministic nature can be periodic, so that the value of the signal can be predicted. 
     Regarding specific definitions, a signal of deterministic nature is a signal, which has a value x as a real number for every time t. A signal of stochastic nature is a signal, which has a random number y for every time t, whereby said number y can be presented in a probability density function. The generation and the characteristics of said sequences are known to a person skilled in the art. 
     An ideal auto-correlation function is defined as: 
     
       
         
           
             
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     A non-ideal auto-correlation function comprises several values more, whereby an almost ideal auto-correlation function of a periodic consecutive function comprises distinct high auto-correlation peaks and a constant low auto-correlation value and a non-ideal auto-correlation function comprises less distinct high auto-correlation peaks and noisy low auto-correlation values. 
     A PN sequence is a pseudo-random noise signal, which displays some deterministic features like periodic behaviour. A periodic cycle within the sequence can recur at least once. In case that the periodic cycle is as long as the PN sequence, meaning exactly one period cycle is available, said sequence is also called a M sequence, standing for “maximum length sequence”. These sequences are pseudo-random number sequences and are known by a person skilled in the art. 
     The main advantages of the present invention are that:
         A PN sequence with a good correlation peak, e.g. auto-correlation peak, and a small (auto-)correlation side-lobe is used at least once in a preamble of a data burst and in a guard interval between data frames of the data burst. Compared with conventional single carrier wireless systems with frequency domain equalization, the overhead introduced by the guard interval does not change. Since the PN sequence will be used for coarse frame timing and channel estimation, there is no need for additional pilot frames. The total overhead is thus reduced.   A reliable carrier synchronization can be achieved using an auto-correlation peak of the PN sequence instead of a conventional cyclic prefix, which is sensitive to a channel impulse response.   A reliable coarse timing can be achieved using a PN sequence instead of an additional pilot frame.   Reliable channel estimation can be achieved using the auto-correlation peak of a PN sequence.   A MMSE (minimum mean square error) channel equalization can be achieved to improve the performance using the auto-correlation side-lobe information of a PN sequence.   The channel estimation accuracy can be further improved using the consecutive PN sequences.   When a PN sequence with a good auto-correlation peak and a small auto-correlation sidelobe is used for a cyclic prefix, channel estimation and equalization, the same matched filter circuits in the receiver are re-used for packet/burst frame detection, automatic gain control, coarse timing synchronization and coarse frequency synchronization. No additional circuits are required.   The proposed burst preamble, whereby the concatenated identical M sequences or the other PN sequences used for a channel estimation are adopted and a distance detection between the concatenated correlation peaks enable the low overhead burst preamble for burst frame based transmission systems.       

       FIG. 1  shows a general example of a frame structure  13  of a single carrier system (or an OFDM system) using a frequency domain equalizer. 
     One key principle of OFDM is that since low symbol rate modulation schemes (i.e. where the symbols are relatively long compared to the channel time characteristics) suffer less from intersymbol interference caused by multipath, it is advantageous to transmit a number of low-rate streams in parallel instead of a single high-rate stream. Since the duration of each symbol is long, it is feasible to insert a guard interval between the OFDM symbols, thus eliminating the intersymbol interference. The guard-interval also reduces the sensitivity to time synchronization problems. 
     An example of a frame structure  13  of a single or multi carrier system is shown in  FIG. 1  in the time domain and comprises three cyclic prefixes  10   a ,  10   b ,  10   c  and three data frames  12   a ,  12   b ,  12   c . The basic frame structure comprises one cyclic prefix  10   a  and one data frame  12   a  and can be chained successively with other basic frame structures. The cyclic prefixes  10   a ,  10   b ,  10   c  are embedded in the guard intervals  14   a ,  14   b ,  14   c , respectively. At the chronological end of the respective data frames  12   a ,  12   b ,  12   c , a respective end  11   a ,  11   b ,  11   c  is designated, said ends  11   a ,  11   b ,  11   c  being part of the respective data frames  12   a ,  12   b ,  12   c.    
     In OFDM, a data frame is processed by a FFT (Fast Fourier Transformation), whereby the FFT window is as long as the data frame, said FFT window determining the time when said data is being processed by the system and/or the size of the data to be transformed by FFT step by step or at once. In an OFDM symbol the cyclic prefix  10   a  is a repetition of the end of the symbol  11   a  whereby said cyclic prefix  10   a  is placed at the beginning of said data frame  12   a . So the cyclic prefix  1   a  is equal to  10   a ,  11   b  is equal to  10   b  and  11   c  is equal to  10   c.    
       FIG. 2  shows an example of a block diagram of an OFDM system comprising a transmitter  33  and a receiver  34 , which can be embodied according to the present invention, whereby said transmitter  33  is operable to modulate and transmit electromagnetic waves which are orthogonal frequency division multiplexed, eventually. Said receiver  34  is operable to receive electromagnetic waves and also demodulate said waves which are orthogonal frequency division multiplexed. Said OFDM system is operable to establish a wireless connection and exchange data between its transmitter  33  and its receiver  34 . 
     The transmitter  33  comprises a modulator  20 , e.g. a quadrature amplitude modulation (QAM) modulator, an Inverse Fast Fourier Transformation (FFT) module  21 , a insertion module  22 , a Radio frequency transmitter  23  and an antenna  35 . The modulator  20  is connected to the Inverse FFT module  21 , the Inverse FFT module  21  is connected to the insertion module  22 , the insertion module  22  is connected to the Radio frequency transmitter  23  and the Radio frequency transmitter  23  is connected to the antenna  35 . 
     First an input signal to be modulated and transmitted is sent to the modulator  20 . The modulator  20  is operable to modulate an input signal according to the respective modulation scheme, e.g. QAM. The Inverse FFT module  21  is operable to apply an inverse FFT transformation on the signal received from the modulator  20 . The insertion module  22  is operable to insert PN sequences into preambles of data bursts as well as PN sequences as guard intervals in between data frames of data bursts according to the present invention as explained in further detail below into the signal received from the Inverse FFT module  21 . The Radio frequency transmitter  23  is operable to convert the signal received from the insertion module  22  into a signal which is transmittable by the antenna  35 , said antenna  35  being operable to transmit electromagnetic waves carrying data based on said input signal. 
     The receiver  34  comprises an antenna  36 , a Radio frequency receiver  24 , a Remove module  25 , a FFT module  26 , a Channel equalizer  27 , a Channel estimation module  28  and a demodulator  29 , e.g. a QAM demodulator. The antenna  36  is connected to the Radio frequency receiver  24 , the Radio frequency receiver  24  is connected to the Remove cyclic prefix module  25 , the Remove module  25  is connected to the FFT module  26 , the FFT module  26  is connected to both the Channel equalizer  27  and the Channel estimation module  28 , the Channel estimation module  28  is additionally connected to the Channel equalizer  27  and the Channel equalizer  27  is eventually connected to the demodulator  29 . 
     Finally an output signal sent out by the demodulator  29  can now be further processed. 
     The antenna  36  is operable to receive the signal sent by the antenna  35  and convert said electromagnetic signal into an electric signal. The Radio frequency receiver  24  is operable to receive the electric signal from the antenna  36  and convert said signal into a baseband signal. The Remove module  25  is operable to receive the signal from the Radio frequency receiver  24  and detect and remove the preamble of a received data burst in order to enable frequency and time synchronization, frequency estimation and so forth as well as operable to remove the PN sequences comprised in the guard intervals between data frames for further processing as required by the present invention as explained in further detail below. The FFT module  26  is operable to transform the signal received from the Remove module  25  according to a Fast Fourier Transformation. The Channel estimation module  28  is operable to receive the signal from the FFT module  26  and estimate the channel quality and other characteristics based on the channel, said channel corresponding to the wireless connection between the transmitter and the receiver. The channel quality might also describe the background and/or receiver noise. The Channel equalizer  27  is operable to receive one signal sent by the FFT module  26  and one signal sent by the Channel estimation module  28 . Then the Channel equalizer  27  compensates for the dynamic frequency response of the wireless channel. The demodulator  29  is operable to demodulate the signal sent by the Channel equalizer  27  and output a demodulated output signal. 
       FIG. 3  shows an example of a block diagram of a transmitter  31  and a receiver  32  of a single carrier system using a frequency domain equalizer according to the present invention. 
     The transmitter  31  is operable to modulate and transmit electromagnetic waves which are modulated onto a single carrier. The receiver  32  is operable to receive electromagnetic waves as transmitted from the transmitter  31  and to demodulate received waves which were modulated onto a single carrier. Obviously, the transmitter  31  and the receiver  32  are adapted to establish a wireless connection and to exchange data and control information. 
     The transmitter  31  comprises a modulator  120 , which is for example a quadrature amplitude modulation (QAM) modulator or any other suitable modulator, which is adapted to modulate an input signal according to the implemented modulation scheme. The modulated signals are forwarded to an insertion module  122 , which is operable to insert PN sequences into preambles of data bursts as well as PN sequences as guard intervals in between data frames of data bursts according to the present invention as explained in further detail below. Therefore generated data bursts, data packets or the like are then forwarded to a radio frequency transmitter  123  which is operable to transform the data bursts into radio frequency signals which are then transmitted via an antenna  135 . 
     The receiver  32  comprises an antenna  136  adapted to receive the signals transmitted from the transmitter  31 . The received signals are then converted by a radio frequency receiver  124  into the base band signal and forwarded to a remove module  125  which is operable to detect and remove the preamble of a received data burst in order to enable frequency and time synchronization, frequency estimation and so forth as well as operable to remove the PN sequences being part of the preamble and/or of the guard intervals between data frames for further processing as required by the present invention as explained in further detail below. 
     Before being removed, the PN sequences of the preamble are correlated in the receiver  32  with an identical or a similar PN sequence to execute the tasks of the receiver  32  mentioned above based on the outputted correlation function. 
     The data frames without the preambles and the guard intervals are then forwarded to a fast Fourier transformation module  126  and transformed from the time domain into the frequency domain. The frequency domain signals are then forwarded to a channel equalizer  127  as well as to a channel estimation module  128 . The channel estimation module  128  is operable to estimate the channel quality and other channel characteristics. The channel equalizer is adapted to receive a corresponding information from the channel estimation module  128  and is operable to compensate the received signals for the dynamic frequency response of the wireless channel. The compensated signal is then forwarded to an inverse fast Fourier transformation module  121  which transforms the signal back to the time-domain and forwards the time domain signal to a demodulator which demodulates the signal in correspondence to the modulation scheme used in the modulator  120  of the transmitter  31 . For example, the demodulator  129  is a quadrature amplitude modulation demodulator or any other suitable demodulator. 
     It has to be understood, that the insertion of PN sequences into the guard intervals and into the preamble of a data burst can be implemented in separate modules instead of the one insertion module  21  and  122  of the transmitters  33  and  31 , respectively. Similarly, instead of one remove module  25  and  125  of the receivers  34  and  32 , respectively, separate modules can be implemented in the receivers  34  and  32  in order to remove the PN sequences from the guard intervals as well as the preamble. 
     The receivers  34  and  32 , respectively, and the transmitters  33  and  31 , respectively, could be part of a mobile wireless device, like e.g. a cell phone, a pda, a notebook, an electronic organizer and so on. Moreover the receivers  34  and  32 , respectively, and the transmitters  33  and  31 , respectively, might be integrated in a semiconductor chip and comprise additional modules operable to extend the operability of the said receiver and/or transmitter, which are not shown in the  FIGS. 2 and 3  for the sake of clarity. Furthermore, the modules can be realized by respective external and separate devices, which can be connected via wires. 
     When compared with an OFDM system, the main advantages of single carrier wireless systems with a frequency domain equalizer of the present invention can be summarized as follows
         The energy of individual symbols is transmitted over the whole available frequency spectrum. Therefore, narrow band notches within the channel transfer function have only small impact on the performance. For OFDM systems, narrow band notches would degrade the performance of transmitted symbols assigned over the relevant sub-carriers. Of course, the diversity can be regained partly by utilizing an error control decoder with some performance loss.   A low peak to average ratio for the radiated signal, which makes the power amplifier (PA) from the transmitted side more efficient and cheaper, especially for the millimetre wave wireless systems.   Robust to the effect of phase noise, which makes the local oscillator (LO) simpler, especially for the millimetre wave wireless systems.   The number of analogue-digital-converter (ADC) bits for the receiver side can be reduced, which is critical for high rate communications because of the power consumption and chip size.   The carrier frequency error between the transmitter side and the receiver side can destroy the orthogonality between subcarriers and introduce the inter-subcarrier interference for OFDM systems. However, it has no effect on single carrier systems with a frequency domain equalizer.   It is more suitable for the user scenario, when the transmitter side would be simple or have low power consumption and the receiver side would be complex or have relatively high power consumption, like high definition television.       

       FIG. 4  shows an example of the structure of a part of a burst frame  43  (or packet frame etc.) of the present invention. 
     This frame structure  43  comprises three PN sequences  40   a ,  40   b ,  40   c  and three data frames  42   a ,  42   b ,  42   c  and is shown in the time domain. Eventually each PN sequence is embedded in a respective guard interval  44   a ,  44   b ,  44   c  and completely fills out said interval, said guard intervals  44   a ,  44   b ,  44   c  being the respective time periods before the data frame periods  42   a ,  42   b ,  42   c . All PN sequences  40   a  to  40   c  are favourably identical to each other. 
     In  FIG. 4 , one data frame and one guard interval comprising at least one PN sequence, for example  42   a + 40   b , are processed by the FFT module  126  of the receiver  32  or the FFT module  26  of the receiver  34 , whereby the FFT window (or FFT frame) is e.g. as long as the length of a data frame plus the length of a guard interval. The data frames  42   a ,  42   b ,  42   c  can have all the same length; this applies also to the PN sequences  40   a ,  40   b ,  40   c . The frame structure is different from  FIG. 1 ; while in  FIG. 1  only the data frame is processed by a FFT, in  FIG. 4  the FFT processes at least a data frame and a PN sequence. Alternatively, the FFT window could have a different length. Since the PN sequence  40   a  is the same as the PN sequence  40   b , based on the same principle regarding OFDM systems, the inter-frame interference introduced by the time disperse multi-path fading channel can be eliminated when the wireless channel delay is less then the length of PN sequence. 
     The PN sequence  40   a ,  40   b ,  40   c  also helps the receiver  34  or the receiver  32  to correctly place the FFT frames and indicates the beginning of the respective data frames  42   a ,  42   b ,  42   c  being processed during a respective FFT frame, when a PN sequence is used in a respective guard interval. 
     The guard interval  44   a ,  44   b ,  44   c  is operable to provide a guard time in case of e.g. a propagation delay and to clearly separate the respective data frames  42   a ,  42   b ,  42   c  from each other, so that the data of one data frame does not overlap with data of an adjacent data frame in case of a multipath propagation during a transmission. 
     The data frame  42   a ,  42   b ,  42   c  is operable to provide data and/or information of any kind, which is based on or corresponds to the content of a conversation like e.g. a phone call or other data meant to be transmitted and received by another communication participant. These data might comprise for example video data, audio data, emails, pictures, control data and the like. The data frames  42   a ,  42   b ,  42   c  are favourably of the same size, whereby their data does not necessarily fill out said data frames completely. The data can also be stored in a scattered pattern in the data frame. 
     The sequence or alternatively said the time flow of the frame structure starts with the first PN sequence  40   a , continues with adjacent first data frame  42   a , then the second PN sequence  40   b , the second data frame  42   b , the third PN sequence  40   c  and ends with the third data frame  42   c . Of course, the frame structure is not limited to these three data frames and three PN sequences, but can comprise a higher or lower number of data frames and PN sequences. Also, each guard interval may comprise more than one PN sequence, whereby it is advantageous if each guard interval has the same number of sequences. 
     A FFT frame, whose operability was already explained in  FIG. 1 , might be as long as the combination of at least one data frame  42   a ,  42   b ,  42   c  and of at least one guard interval  44   a ,  44   b ,  44   c . This is different to the example of  FIG. 1 , wherein one FFT frame has the length of one data frame. 
     Regarding the PN sequence, the guard interval might comprise either a single PN sequence or a plurality of identical PN sequences, whereby said plurality of PN sequences is formed as a continuous string of sequences. 
     It is emphasized that all the characteristics and features of the PN sequence described and used in the guard interval may also apply for the PN sequence used in the frame structures of the other figures. 
     The correlation of the guard interval with a predetermined and/or controllable function comprising one or a plurality of identical PN sequences is performed in a correlator of the receiver  34  of  FIG. 2  or the receiver  32  of  FIG. 3 , operable to receive the signal sent from the transmitter  33  or  31 , respectively. The choice regarding the predetermined function and its amount of PN sequences is dependent on the characteristics of the correlation to be determined. 
       FIG. 5  shows a burst frame  101  according to the present invention comprising a burst preamble  100  and a frame structure  43   a . Said structure  43   a  is similar to the frame structure  43  shown in  FIG. 4  and shows five PN sequences  40   a ,  40   b ,  40   c ,  40   y ,  40   z  as cyclic prefix and three data frames  42   a ,  42   b  and  42   y . The PN sequences  40   a  to  40   c  and  40   y  are part of the guard intervals  44   a ,  44   b ,  44   c  and  44   y , respectively. The burst preamble  100  comprises at least one PN sequence, which may be the same PN sequence as used in one or all guard intervals. 
     At least one further data frame may exist between the guard intervals  44   c  and  44   y . But the frame structure  43   a  could also only comprise one data frame  42   a  and the adjacent PN sequences  40   a  and  40   b , eventually meaning that the present invention is not restricted to a specific number of data frames and/or guard intervals. 
       FIG. 6  shows a detailed view of a state of the art example of a burst preamble  100  comprising a first preamble section  103  and a second preamble section  104 . This burst preamble  100  can e.g. be used as the burst preamble  100  of the frame structure  101  of  FIG. 5  or as preamble of the frame structure  43  of  FIG. 4 . 
     The first preamble section  103  is used in the receiver for frame synchronisation, course timing synchronisation, course frequency offset estimation and/or automatic game control (AGC). The first preamble section  103  comprises ten training symbols which are arranged next to each other, without gaps. 
     In another example, gaps may be implemented between the training symbols. This can be done
         by keeping the duration of the training symbols constant and extending the duration of the preamble section or   by keeping the duration of the preamble section constant and reducing the duration of the training symbols or   by keeping the duration of the preamble section constant and reducing the number of training symbols.       

     The second preamble section  104  comprises a long guard interval  106 , a first long symbol  107   a  and a second long symbol  107   b . The two long symbols  107   a  and  107   b  are used in the receiver for fine frequency offset estimation and for channel estimation. 
       FIG. 7  shows another detailed view of a state of the art example of a burst preamble  100  which comprises all the features shown in  FIG. 6 , which are eventually the first preamble section  103 , the second preamble section  104 , the plurality of training symbols  105 , the long guard interval  106 , the first long symbol  107   a  and the second long symbol  107   b.    
     In addition, it is shown, that the time duration of the burst preamble is sixteen microseconds long and both the first and the second preamble section take each eight microseconds; thus splitting said sixteen microseconds in two even portions. One training symbol  105  has a time duration of 0.8 microseconds, the long guard interval  106  has a duration of 1.6 microseconds and both the first and the second long symbol  107   a  and  107   b  have each a time duration of 3.2 microseconds, respectively. 
     Three data frames  115 ,  116 ,  117  are succeeding the burst preamble  100 , whereby the first data frame  115  comprises a preceding guard interval of 0.8 microseconds and a signal of 3.2 microseconds afterwards, the second data frame  116  comprises a guard interval of 0.8 microseconds and a data block of 3.2 microseconds. The third data frame  117  corresponds to the second data frame  116 , whereby the two data blocks can be different to each other. 
     In another example, the guard intervals of  FIG. 7  preceding the data frames  115 ,  116 ,  117  may correspond to the guard intervals  14   a - 14   c  of  FIG. 1 , respectively. 
       FIG. 8  shows another example of a burst frame comprising an embodiment of the present invention, said embodiment comprising a preamble  110  and in addition a succeeding frame structure  43   b , said frame structure  43   b  being similar to or might even correspond to the frame structure  43   a  shown in  FIG. 5  or the frame structure  43  of  FIG. 4 . 
     The preamble  110  comprises a single PN sequence  111 , which exhibits a correlation function  112  shown below which is the result after processing it in a correlator of the receiver  32 . Also the PN sequences  40   a ,  40   b ,  40   c ,  40   z  show the same correlation functions  53   a ,  53   b ,  53   c ,  53   z , respectively. The PN sequence  40   y  has, of course, also a correlation function, but said function is not shown in  FIG. 8 . 
     Due to the different time spaces between the correlation functions from e.g. peak to peak or minimum to minimum, the time duration D 1  between the correlation functions  112  of the preamble and the correlation function  53   a  of the PN sequence of the first guard interval  44   a  is shorter than the time duration D 2  between the correlation functions  53   a  and  53   b  or  53   b  and  53   c  of the PN sequences of the guard intervals between the data frames. Thus, by detecting a time duration, such as D 1  or D 2 , and comparing said time duration with an already identified time duration or a predefined, stored and/or predetermined time duration, the receiver  34  or  32  can determine valuable information, e.g. at which position of the burst frame the receiver is reading the burst frame at the moment. 
     The detection method can be summarized as follows:
         1. When the distance between two autocorrelation peaks is roughly equal or equal to D 2 , said part will be treated as the data part.   2. When the distance between two autocorrelation peaks is roughly equal or equal to D 1 , which is different to D 2 , the coarse frame timing is acquired, which indicates the beginning of the burst frame.       

     Advantageously, at least one PN sequence is proposed to be used as the part of the burst preamble  110 , which is identical to the succeeding PN sequences of the guard intervals between the data frames. 
       FIG. 9  shows another detailed view of an example of a burst frame comprising a preamble  110 ′ and a frame structure  43   b , said frame structure  43   b  corresponding to the one shown in  FIG. 8 . 
     The preamble  110 ′ in this example comprises three PN sequences  111   a ,  111   b  and  111   c , which exhibit the correlation functions  112   a ,  112   b  and  112   c , respectively as shown below. The time duration D 1  between the correlation graphs  112   c  and  112   b  as well as between  112   b  and  112   a  and  112   a  and  53   a  are the same, respectively. The time duration D 2  between the correlation graphs  53   a  and  53   b  is larger than the time duration D 1 . 
     The idea of detecting the current position within the burst frame and/or of detecting different synchronizations can be extended to more than one PN sequence in the burst preamble to improve the detection reliability and reduce the false alarm probability. When more than one PN sequence is used as a part of a burst preamble, a −PN sequence can be used instead of a PN sequence. −PN stands for an inverse sequence of PN, respectively, whereby e.g. +1 is set to −1 and −1 is set to +1. The same applies to M sequences. 
     In another example, at least one PN sequence which is used as cyclic prefix is exactly the same one used to construct the preambles. Favourably, the preamble comprises only one or up to three of said PN sequences, whereby in case of three PN sequences, the one in the middle has to be inverted. 
       FIG. 10  shows another detailed view of an example of a burst frame which comprises a preamble  110 ″ as well as a frame structure similar or equal to the one  43   b  shown in  FIG. 9  or  8 . The preamble  110 ″ comprises three PN sequences  111   a ,  111   b ,  111   c  and a free space  113  between the first PN sequence  111   a  and the second PN sequence  111   b  of the preamble burst  110 ″. Due to this free space  113 , the time duration D 3  between the correlation graphs  112   b  and  112   a  is larger than the time duration D 1  between the correlation graphs  112   c  and  112   b  or between  112   a  and  53   a . Except for the preamble  110 ″, the burst frame corresponds to the one shown in  FIG. 9 . 
       FIG. 11  shows another example of a frame structure  43   b  of a burst frame of the present invention and the coarse frame timing and the carrier synchronization based on the correlation peak of a PN sequence in the receiver  32 . 
     This frame structure  43   b  corresponds to the frame structure  43  shown in  FIG. 4  and comprises four PN sequences  40   a ,  40   b ,  40   c ,  40   d  and three data frames  42   a ,  42   b ,  42   c , whereby the first three PN sequences are used in guard intervals. Below each of these PN sequences  40   a ,  40   b ,  40   c  the correlation function of said PN sequences is shown as a graph  53   a ,  53   b ,  53   c , respectively. 
     The correlation graphs  53   a ,  53   b ,  53   c  of the PN sequences comprises a high correlation peak and a low correlation side-lobe, respectively. This correlation function is created in a correlator of the receiver  32 , when the signal with the frame structure comprising the PN sequence is received and correlated with an identical PN sequence (cross-correlation) or with itself (auto-correlation). 
     In case the guard interval  44   a  comprises a plurality of identical PN sequences formed as a continuous string and is correlated with one identical PN sequence at a receiver, the correlation graph of the PN sequence will comprise a plurality of high correlation peaks and low correlation side-lobes. 
     In another example, the guard interval  44   a  comprises a plurality of e.g. different PN sequences formed as a continuous string and is correlated with one PN sequence being part of said string, it is possible to locate the exact position within the guard interval, when the high correlation peak appears in the graph. 
     Instead of one single PN sequence, a correlation sequence of identical or different PN sequences can be used for correlating with said received correlation sequence. 
     Due to the characteristics of the correlation graphs  53   a ,  53   b ,  53   c  of the PN sequences  40   a ,  40   b ,  40   c , the receiver  32  can achieve coarse timing, channel estimation carrier synchronization, obtain signal-noise-ratio (SNR) estimation and/or implement minimum mean-square error (MMSE) channel equalization on the basis of the PN sequences. The MMSE channel equalization is described more in detail in relation to  FIGS. 12 and 13 . 
     Based on the characteristics of the graphs  53   a ,  53   b ,  53   c , it is possible to determine the beginning of the FFT frame. The FFT frame might start from the beginning or at the end of the graphs  53   a ,  53   b ,  53   c . Also the high correlation peak or the low correlation side-lobe might be the starting point of the FFT frame. The FFT frame, which is already explained in  FIG. 4 , comprises at least the data frame succeeding the respective PN sequence. Alternatively the beginning of the FFT frame is independent from the guard interval, but at least comprises the complete succeeding data frame. 
     In particular the coarse frame timing can be determined by the correlation peak of the graph  53   a ,  53   b ,  53   c  of the PN sequence as shown in  FIG. 11 . 
     The carrier synchronization can be implemented based on the I/Q constellation rotation of the strongest correlation peak of two nearby PN sequences. Below the correlation graphs  53   a  and  53   b  the respective constellation points  51  and  52  are shown in Cartesian coordinates. The phase difference between these two constellation points and the time period between the two PN sequences  40   a  and  40   b  can be used for carrier synchronization. 
     The guard interval might comprise at least one PN sequence, whereby said one PN sequence has a complex value and comprises one I-channel PN sequence and one Q-channel PN sequence. In alternative embodiments the I-channel sequence and the Q-channel sequence could either be identical or different from each other. Alternatively, the correlation value has a complex value comprising a real and an imaginary part like I and Q. 
     The channel transfer function can be estimated based on several correlation peaks of the graphs  53   a ,  53   b ,  53   c  of the respective PN sequences, whereby the correlation side-lobe from the PN sequence can be used for signal to noise ratio (SNR) calculation. The acquired information can be used for MMSE channel equalization, as shown in  FIGS. 12 and 13 . 
       FIG. 12  shows an device for channel equalization based on FFT, which can be an additional part of the receiver  32  or the receiver  34  of the present invention. 
     This device comprises a FFT module  65 , a SNR estimation module  62 , a FFT module  63  and a MMSE channel equalization module  64 , whereby said device is adapted to perform for channel equalization. At least a part of said device can be implemented into the receiver  34  of  FIG. 2  or the receiver  32  of  FIG. 3  e.g. as a channel equalizer  27  or  127 ; in particular the MMSE channel equalizer  64  can be implemented as said equalizer  27  or  127 . 
     The FFT module  65  is operable to receive a signal which represents a channel transfer function in the time domain, convert said signal into a signal representing a channel transfer function in the frequency domain and output said signal. The SNR estimation module  62  is operable to receive the same channel transfer function in the time domain, which was received by the FFT module  65  and calculate and/or estimate the signal-noise-ratio of said function on the basis of the correlation side-lobe. The FFT module  63  is operable to receive a signal comprising the data frame and apply the FFT to said signal. The MMSE channel equalization module  64  is operable to receive the channel transfer function in the frequency domain provided by the FFT module  65 , the SNR estimation signal provided by the SNR estimation module  62  and the signal provided by the FFT module  63  and eventually calculate and demodulate the output signal. 
     It has to be ensured that the channel transfer function  53  comprises the PN sequence with a main high (auto-)correlation lobe and a smaller (auto-)correlation side-lobe. Other correlation characteristics are, of course, possible. 
       FIG. 13  shows an example of an apparatus for a channel equalization based on a Discrete Fourier Transformation (DFT). 
     This device comprises a Discrete Fourier Transformation (DFT) module  61 , a SNR estimation module  62 , a FFT module  63  and a MMSE channel equalization module  64 , whereby said apparatus is operable for channel equalization. 
     Except for the FFT module  65 , the device of  FIG. 13  corresponds to the apparatus of  FIG. 12 . The device of  FIG. 13  can be implemented into said receiver  34  or said receiver  32 . 
     Like in  FIG. 12 , it has to be ensured in  FIG. 13  that the channel transfer function  53  comprises the PN sequence with a main high (auto-)correlation lobe and a smaller (auto-)correlation side-lobe. 
     As shown in  FIG. 13 , FFT can be used instead of DFT to reduce the calculation complexity for obtaining the channel transfer function from frequency domain, which will be adopted for channel equalization. 
       FIG. 14  shows another example of a frame structure with an additional guard interval. 
     This frame structure is based on the frame structure  43  shown in  FIG. 4  and comprises three guard intervals  83   a ,  83   b ,  83   c  and three data frames  82   a ,  82   b ,  82   c , whereby said guard intervals are or comprise at least one PN sequence, respectively. In each of said guard intervals  83   a ,  83   b ,  83   c  a respective PN sequence is embedded. Since the guard intervals  83   a ,  83   b ,  83   c  are in this example larger than the PN sequences  80   a ,  80   b ,  80   c , some free space is left on the right and left side of the PN sequences. For example and in detail an additional guard interval or free space  84   a  is located between the data frame  82   a  and the PN sequence  80   b  and an additional guard interval or second free space  84   b  is located between the PN sequence  80   b  and the data frame  82   b.    
     Thus, the guard interval  83   a ,  83   b ,  83   c  between the data frames is extended in this example. If the length of guard interval  83   a ,  83   b ,  83   c  is longer than the wireless channel delay spread, there is no effect on the correlation peak from the data frame part and more accurate channel estimation can be obtained. 
     The additional guard intervals between the PN sequences and the data frames, meaning the free spaces  84   a  and  84   b  can comprise a sequence of zeros. The two free spaces  84   a  and  84   b  might be of different or equal size, respectively. 
     The invention is not limited to the features shown and described above by way of example, but can instead undergo modifications within the scope of the patent claims attached and the inventive concept. 
     Further embodiments of the invention are possible, but not shown in the drawings for the sake of clarity.
     REFERENCE NUMBERS     1  First position in cyclic prefix     2  Second position in cyclic prefix     3  Third position in cyclic prefix     4  Fourth position in cyclic prefix     5  Fifth position in cyclic prefix     6  Sixth position in cyclic prefix     7  Seventh position in cyclic prefix     8  Eighth position in cyclic prefix     10   a - c  Cyclic prefix     11   a - c  End of data frame  1 - 3       12   a - c  Data frame  1 - 3       13  Frame structure of state of the art     14   a - c  Guard intervals     20  Quadrature amplitude modulation (QAM) modulator     21  Inverse Fast Fourier Transformation (Inverse FFT) module     22  Cyclic prefix insertion module     23  Radio frequency transmitter (Tx RF)     24  Radio frequency receiver (Rx RF)     25  Remove cyclic prefix module     26  Fast Fourier Transformation (FFT) module     27  Channel equalizer     28  Channel estimation module     29  QAM demodulator     31  Transmitter of single carrier system     32  Receiver of single carrier system     33  Transmitter of OFDM system (orthogonal frequency division multiplex)     34  Receiver of OFDM system (orthogonal frequency division multiplex)     35  Antenna of transmitter     36  Antenna of receiver     40   a - d, y, z  PN sequence as cyclic prefix     42   a - c, y  Data frame  1 - 3       43 ,  43   a ,  43   b  Frame structure     44   a - d, y  Guard intervals     51  Constellation point of PN sequence of data frame  1       52  Constellation point of PN sequence of data frame  2       53  Channel transfer function (time domain)     53   a - c, z  Correlation function of PN sequence as graph     61  Discrete Fourier Transformation module     62  Signal-Noise-Ratio estimation module     63  Fast Fourier Transformation module     64  Minimum mean-square error estimation module     65  Fast Fourier Transformation module     80   a - c  PN sequence as cyclic prefix     82   a - c  Data frame  1 - 3       83   a - c  Guard interval     84   a  First free space     84   b  Second free space     90  Cyclic prefix     91   a  First free space     91   b  Second free space     93  Symmetric axis of cyclic prefix     94  Guard interval     95   a  Boarder between preceding data frame and guard interval     95   b  Boarder between succeeding data frame and guard interval     100  Burst preamble     101  Burst frame     103  Preamble section  1       104  Preamble section  2       105  Training symbol     106  Long guard interval     107   a  Long symbol  1       107   b  Long symbol  2       110  Preamble     111  PN sequence of preamble     111   a - c  PN sequence  1 - 3  of preamble     112  Correlation function of PN sequence of preamble as graph     112   a - c  Correlation function of PN sequence  1 - 3  of preamble as graph     113  Free space in preamble     115  Data frame with guard interval     116  Data frame with guard interval     117  Data frame with guard interval