Abstract:
A channel sounding system employs orthogonal sequences to meet the Cramer-Rao bound in estimating the channel and achieves considerable simplification of the structure necessary to perform the channel sounding. These advantages are achieved by developing orthogonal sequences of substantially arbitrary length as a function of first and second existing orthogonal sequences and using such orthogonal sequences for channel sounding in lieu of M-sequences. The techniques of the invention are especially suited to systems that use multiple antennas at the transmitter and multiple antennas at the receiver, so called multiple-input multiple-output (MIMO) systems.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to the art of channel sounding, and more particularly, to channel sounding where the channel is linear.  
         BACKGROUND OF THE INVENTION  
         [0002]    Channel sounding is the process of measuring the characteristics of a channel so as to design a communication system that best takes advantage of the determined characteristics of the channel. This is typically done by having a transmitter transmit a signal that is made up of a repeating known training sequence and then processing the signal after it has passed through the channel at a receiver to develop an estimate of the channel characteristics. It is well known in the art that the Cramer-Rao bound is the limit to which channel characteristics may be estimated using linear channel sounding techniques. One method of estimating the channel characteristics is to employ the so-called “least squares method”. Doing so, gives conditions that the training sequence must meet in order to achieve the Cramer-Rao bound.  
           [0003]    One condition that, if met, will yield a channel estimate at the Cramer-Rao bound is that the training sequence be orthogonal. However, the art was typically only aware of orthogonal sequences that were very short, e.g., no greater than 16 symbols, or several sequences that were much longer, at lengths of (2′) 2 , where n in an integer. The use of the known short sequences was of no value, because they cannot be used to measure channels with large delay spread, such as is required for wideband communication. The use of the known long sequences was also of no value, because they require complicated modulation schemes that are not practical to implement. Thus orthogonal sequences were not used in the art and no practical study was devoted to the use of orthogonal sequences for channel sounding.  
           [0004]    Instead, so-called “M-sequences”, which are pseudo-orthogonal sequences, have become the pervasive sequences that are employed for channel sounding. These Msequences have been extensively studied, with much literature being devoted to them and their use in channel sounding. Thus, the entire mindset of the art was to not employ orthogonal sequences, which were dismissed as impractical.  
           [0005]    The signals conveyed in data transmission systems generally are not periodic and occupy infinite bandwidth. In order to limit the occupied bandwidth, the transmitted signals are filtered by a channel or pulse shaping filter. Channel filters are also referred to by those of skill in the art as pulse shaping filters.  
           [0006]    Because M-sequences are not actually orthogonal, it is common practice in the art to apply a channel filter similar to those applied to data transmission systems when using M-sequences for channel sounding applications. Disadvantageously, this adds to the cost of the system.  
           [0007]    A popular channel filter that is often used as the channel filter in channel sounding systems that employ M-sequences is the raised cosine filter, a filter that is usually applied in matched pairs, i.e., one at the transmitter and one at the receiver, for various well known technical reasons. In practice these channel filters can only be approximated, and thus the ultimate performance of the communication system is limited.  
           [0008]    Channel filtering should not be confused with the reconstruction filtering which takes place when discrete time signals, e.g., digital signals, are converted to continuous time, e.g., analog, signals. The reconstruction filter eliminates the spectral replicas appearing in the continuous time reconstruction of a discrete time signal. The technical requirements placed on the reconstruction filter, e.g., cut off rate and ultimate attenuation can be quite severe. Therefore it is common practice to interpolate the discrete time signal to ease the requirements on the reconstruction filter.  
           [0009]    Clearly then, in general, it is the role of the channel filter to define the signal bandwidth while the reconstruction filter assists in the digital to analog conversion process.  
         SUMMARY OF THE INVENTION  
         [0010]    We have recognized that the use of periodic orthogonal sequences can not only allow the Cramer-Rao bound to be met in estimating the channel, but they permit considerable simplification of the structure necessary to perform the channel sounding. For example the implementation of the least-square method can be simplified, and post filtering to improve the quality of the estimate can also be simplified.  
           [0011]    We have also recognized that the channel filtering, e.g., raised cosine filtering, performed in the prior art when M-sequences were transmitted is unnecessary when using orthogonal sequences according to the invention due to the periodic nature of the orthogonal sequences employed. This is because a periodic sequence is represented perfectly by a Fourier series without approximation. Therefore the interpolation performed to facilitate reconstruction to a continuous time, e.g. analog, signal can be computed perfectly in the frequency domain as an extension of the Fourier series describing the sequence, followed by an inverse Fourier series transform to express the sequence in the time domain. The interpolation may also be done just as accurately in the time domain because of the duality between the frequency and time domains.  
           [0012]    The periodic orthogonal sequences are generated by using the techniques disclosed in the previously filed copending application Ser. No. 09/648,983—which is incorporated by reference as if fully set forth herein—to develop orthogonal sequences of substantially arbitrary length as a function of first and second existing orthogonal sequences and using such orthogonal sequences for channel sounding in lieu of M-sequences.  
           [0013]    The techniques of the invention are especially suited to systems that use multiple antennas at the transmitter and multiple antennas at the receiver, these being so called multiple-input multiple-output (MIMO) systems. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    In the drawing:  
         [0015]    [0015]FIG. 1 shows a channel sounding system in accordance with the principles of the invention;  
         [0016]    [0016]FIG. 2 shows an exemplary structure of FIR filter of FIG. 1, in accordance with the principles of the invention;  
         [0017]    [0017]FIG. 3 shows an exemplary embodiment of the receiver of FIG. 1 but in which a band-limiting filter is interposed between the demodulator and the FIR filter;  
         [0018]    [0018]FIG. 4 shows an exemplary embodiment of the receiver of FIG. 1 but in which an averager is coupled to receive the output of the FIR filter; and  
         [0019]    [0019]FIG. 5 shows an exemplary embodiment of the invention for a MIMO system. 
     
    
     DETAILED DESCRIPTION  
       [0020]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.  
         [0021]    Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams, pseudocode, and the like represent various processes which may be substantially represented in computer readable medium and so executed by a computer or processor, whether or not such computer or processor is explicitly shown.  
         [0022]    The functions of the various elements shown in the FIGs., including functional blocks labeled as “processors”, may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” or “controller” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, read-only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. Similarly, any switches shown in the FIGs., are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the implementor as more specifically understood from the context.  
         [0023]    In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein.  
         [0024]    Unless otherwise explicitly specified herein, the drawings are not drawn to scale.  
         [0025]    [0025]FIG. 1 shows a channel sounding system, including transmitter  131  and receiver  133 , in accordance with the principles of the invention. Transmitter  131  includes a) orthogonal sequence source  101 , b) modulator  103 , and c) optional antenna  105 . Receiver  133  includes a) optional antenna  107 , b) demodulator  109 , and c) finite impulse response (FIR) filter  111 .  
         [0026]    Orthogonal sequence source  101  supplies a training signal that is made up of a repeating base orthogonal sequence. In accordance with the principles of the invention, the base orthogonal sequence that is repeated may have a substantially arbitrary length so long as it is least as long as the channel length, e.g., it is generated by using the techniques disclosed in the previously filed copending application Ser. No. 09/648,983—which is incorporated by reference as if fully set forth herein—, i.e., the orthogonal sequence is a function of first and second existing orthogonal sequences. In the discrete time domain, the base orthogonal sequence has a perfectly white spectrum within the bandwidth of the sequence. Once interpolated the spectral energy outside the bandwidth of the sequence is, ideally, zero. When converted into the continuous time domain the spectrum appears as a line spectrum as the sequence is repeated an infinite number of times. Note that the interpolated sequence is still a periodic orthogonal sequence. Orthogonal sequence source  101  may be a memory that stores the base orthogonal sequence and supplies it as an output in a repetitious manner. Alternatively, orthogonal sequence source  101  may continuously generate the orthogonal sequence using computation techniques.  
         [0027]    Modulator  103  receives as input the training signal as it is supplied by orthogonal sequence source  101 . In accordance with an aspect of the invention, the training signal is supplied to modulator  103  directly from orthogonal sequence source  101  without any filtering being performed between orthogonal sequence source  101  and modulator  103 . Modulator  103  modulates the training signal using a carrier signal e jwk  that is also received as an input by modulator  103 . The modulated training signal is supplied as an output by modulator  103 , e.g., to optional antenna  105 . Optional antenna  105  broadcasts the modulated training signal as a wireless signal.  
         [0028]    Optional antenna  107  receives the wireless signal broadcast by optional antenna  105  and converts it to a modulated received training signal in electrical form. The modulated received training signal in electrical form is supplied to demodulator  109 , which demodulates it to baseband using a local carrier signal e jwk  that is also received as an input by demodulator  109 . The baseband demodulated received training signal is supplied by demodulator  109  as an output to FIR filter  111 . In accordance with an aspect of the invention, there is no filtering performed between demodulator  109  and FIR filter that corresponds to any filtering performed in transmitter  131 . However, as will be further described hereinbelow, filtering that is independent of any filtering performed in transmitter  131  may be performed between demodulator  109  and FIR filter  111 .  
         [0029]    In accordance with an aspect of the invention, FIR filter  111  performs least-squares processing on the baseband demodulated received training signal. FIR filter  111  supplies as an output a channel estimate, e.g., a sequence of values that specify the channel&#39;s impulse response. Advantageously, the channel estimate is the best estimate that can be achieved using linear estimation techniques since it meets the Cramer-Rao bound given the observation period.  
         [0030]    [0030]FIG. 2 shows an exemplary structure of FIR filter  111  for performing least-squares processing on the baseband demodulated received training signal, in accordance with the principles of the invention. Shown in FIG. 2 are a) N−1 delay elements  201 , including delay elements  201 - 2  through  201 -N; b) N multipliers  203 , including multipliers  203 - 1  through  203 -N; and c) N−1 adders  205 , including adders  205 - 2  through  205 -N, where N is the number of symbols in the base orthogonal sequence. Note that FIR filter  111  has a conventional structure of an FIR filter. Each symbol of the baseband demodulated received training signal is initially supplied to the first delay element, delay element  201 - 2 , as well as to the first multiplier, multiplier  203 - 1 . For each clock cycle the symbol stored in each delay element is supplied to the next delay element in the delay chain formed by delay elements  201  until delay element  201 -N, after which the symbol exits the filter. The symbol stored in each of delay elements  201  is multiplied, using a corresponding one of multipliers  203 , by a coefficient value. Additionally, the symbol currently being supplied as an output by demodulator  109  is multiplied by a coefficient using multiplier  203 - 1 . The various products produced by multipliers  203  are summed using adders  205  and an output of the FIR filter is supplied from adder  205 -N.  
         [0031]    In accordance with an aspect of the invention, the coefficient values C, through C N  supplied to each of multipliers  203  corresponds to the complex conjugate of one of the symbols of the base orthogonal sequence. More specifically, multiplier  203 - 1  is supplied with the complex conjugate of the last symbol of the base orthogonal sequence, multiplier  203 - 2  is supplied with the complex conjugate of the next to last symbol of the base orthogonal sequence, and so on, with multiplier  203 -N being supplied with the complex conjugate of the first symbol of the base orthogonal sequence. Advantageously, the structure of FIR filter  111  is considerably simpler than the structure required to implement a general purpose least-squares process.  
         [0032]    [0032]FIG. 3 shows an exemplary embodiment of receiver  133  but in which band-limiting filter  301  is interposed between demodulator  109  and FIR filter  111 . Note that in accordance with the principles of the invention, no such bandlimiting filter, or any corresponding filter for that matter, exists in the transmitter between orthogonal sequence source  101  and modulator  103 , i.e., the transmitter is exactly the same as transmitter  131  as shown in FIG. 1. The purpose of band-limiting filter  301  is to eliminate any out-of-band noise that may have been introduced into the baseband demodulated received training signal at receiver  131  prior to being supplied to FIR filter  111 . Theoretically such a filter is not required but as a matter of practicality it is good practice to include such a filter. In other words, band-limiting filter  301  reduces out-of-band emissions that creep into the system but are not inherently caused by operation of orthogonal sequence source  101  or modulator  103 .  
         [0033]    [0033]FIG. 4 shows an exemplary embodiment of receiver  133  but in which averager  401  is coupled to receive the output of FIR filter  111 . Averager  401  develops the average of channel estimates produced by FIR filter  111 . Such an average estimate tends to be more accurate. Furthermore, since the observation time is increased, the Cramer-Rao bound is still met.  
         [0034]    Note that once the modulated orthogonal sequence is developed, it may be recorded and played back at a later time for channel sounding purposes. This recording and playback may be performed either before or after the modulated orthogonal signal passes through the channel.  
         [0035]    The techniques of the invention are especially suited to systems that uses multiple antennas at the transmitter and multiple antennas at the receiver, so called multiple-input multiple-output (MIMO) systems. In particular, once a base orthogonal sequence is developed, the sequence is offset by a different amount for each transmit antenna. For example, each sequence could be offset by a multiple of the channel length for each transmit antenna, where the multiple ranges from 0 to M−1, where M is the number of transmit antennas. Furthermore, by not using exactly the same amount of offset shifting for each transmit antenna, e.g., not having each signal offset by exactly the channel length, but keeping the overall total shifting the same, e.g., the average of all of the shifts is the channel length, it is possible to determine at a receiver from which transmit antenna a particular signal originated.  
         [0036]    More specifically, FIG. 5 shows an exemplary embodiment of the invention for a MIMO system. FIG. 5 shows transmitter  531  and receiver  533 , where transmitter  531  has  4  transmit elements and  2  receive elements, to effectively create  8  channels C 11  through C 41  and C 12  through C 42 , only the former of which are shown being transmitted for the sake of clarity. Similar to transmitter  131  (FIG. 1) transmitter  531  includes a) orthogonal sequence source  501 , b) modulators  503 , c) optional antennas  505 , and d) delay elements  523 . Receiver  533  includes a) optional antennas  507 , b) demodulators  509 , c) finite impulse response (FIR) filters  511 , d) demultiplexers  525 , and e) synchronization controller (SYNCH)  527 .  
         [0037]    Essentially the same as orthogonal sequence source  101  (FIG. 1), orthogonal sequence source  501  supplies a training signal that is made up of a repeating base orthogonal sequence. In accordance with the principles of the invention, the base orthogonal sequence that is repeated may have a substantially arbitrary length and should be as least as long as the channel length times the number of transmit elements.  
         [0038]    Delay elements  523  form a delay chain, so that the output of each of delay elements  523  is a delayed version of the orthogonal sequence supplied by orthogonal sequence source  501 . The delays of each of delay elements  523  may be identical, but they need not be so. In fact, if identical delays are not employed, then, as noted above, it is possible to determine at receiver  533  from which transmit element a particular signal originated  
         [0039]    Each of modulators  503  receives as input respective one of either the training signal as it is supplied by orthogonal sequence source  101  or one of the delayed versions supplied by delay elements  503 . In accordance with an aspect of the invention, the training signal is supplied to modulators  503  without any filtering being performed between orthogonal sequence source  101  and modulator  103 . Modulator  503  modulates each of the original or delayed training signals using a carrier signal e jwk  that is also received as an input by each of modulators  503  to produce modulated training signals. The modulated training signals are supplied as an output by each modulator  503 , e.g., to a respective one of optional antennas  505 . Each of optional antenna  505  broadcasts its modulated training signal as a wireless signal.  
         [0040]    Each of optional antennas  507  receives the wireless signals broadcast by each of optional antennas  505  and converts it to a respective modulated received training signal in electrical form. Each of the modulated received training signals in electrical form is supplied to the one of demodulators  509  associated with the antenna, which demodulates it to baseband using a local carrier signal e jwk  that is also received as an input by demodulators  509 . The baseband demodulated received training signals are each supplied by demodulators  509  as an output to respective associated FIR filters  511 . In accordance with an aspect of the invention, there is no filtering performed between demodulators  509  and FIR filters  511  that corresponds to any filtering performed in transmitter  531 .  
         [0041]    In accordance with an aspect of the invention, each of FIR filters  511  performs least-squares processing on the baseband demodulated received training signal it receives. To this end, each of FIR filters  511  uses a coefficients the complex conjugates of the base orthogonal sequence in the same manner as described above in connection with FIG. 2. Each of FIR filters  511  supplies as an output a series of channel estimates, one for each of the transmit elements. The channel estimates repeat in a pattern corresponding to the values of delays  523 . Advantageously, the channel estimate is the best estimate that can be achieved using linear estimation techniques since it meets the Cramer-Rao bound given the observation period.  
         [0042]    Demultiplexers  525  are responsive to timing signals from synchronization controller  527  so as to separate and supply as separate outputs each of the channel estimates produced by each of FIR filters  511 .