Abstract:
A communication system compensates pulse positioned modulated data signals for channel induced intersymbol interference and extracts pulse positioned encoded data from a received signal corrupted with the channel induced intersymbol interference. The communication system has a transmitter and a receiver. The transmitter includes a modulation apparatus that has a symbol mapping circuit, which receives data symbols to be transmitted and maps the data symbols to a transmission code. The receiver has a demodulation apparatus to recover data symbols in the presence of the channel induced intersymbol interference. The demodulation apparatus has a sampling circuit in communication with a signal receiving circuit within the receiver to sample at a regular period received data symbols acquired by the receiving circuit. The samples of the data samples are retained by a sample retaining circuit in communication with the sampling circuit. The sample retaining circuit transfers the retained samples to a symbol mapping circuit. The symbol mapping circuit then recovers the data symbols.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to transmission and reception of signals. More particularly, this invention relates to the encoding and modulation of transmission signals with digital data and the demodulation and decoding of received signals corrupted by channel induced intersymbol interference.  
           [0003]    2. Description of Related Art  
           [0004]    In the transmission and reception of digital data, channel induced intersymbol interference is the arrival of multiple copies of a transmitted signal that causes the information from one segment of information to corrupt a subsequent segment of information. The multiple copies of the transmitted signals occur generally because of reflections of the transmitted signals within the environment.  
           [0005]    The level of the reflection determines the level of degradation of the signal received and the probability of the intersymbol interference causing errors in the reception of the signals. Also, the amount of time required for the reflections to transit from the transmitter to the reflection point and thence to the receiver further determines the quantity of intersymbol interference.  
           [0006]    Refer now to FIG. 1 for a discussion of channel induced intersymbol interference in a wireless radio frequency (RF) transmission. In this instance the transmitter  5  is connected to the antenna  10 , which radiates an RF signal. Even though the RF signal emanates from the antenna  10  as a contiguous wavefront, it is shown as three separate wavefronts  35 ,  40 , and  50 . The wavefront  35  is travels through the atmosphere in a direct path. The wavefront  40  travels through the atmosphere and is reflected from a landmass  30  such as mountains and hills. The reflected wavefront  45  then arrives at the antenna  15 . The wavefront  50  travels through the atmosphere and is reflected by buildings  25  of a metropolitan area. The reflected wavefront  55 , likewise, then arrives at the antenna  15 .  
           [0007]    The antenna  15  is connected to the receiver  20 . The receiver acquires the transmitted signal and recovers the information modulated within the RF signal. It is apparent that the wavefront  35  travels the most direct route from the transmitting antenna  10  to the receiving antenna  15 . The reflected wavefronts  45  and  55 , on the other hand, arrive at the antenna  15  at some time later than the wavefront  35 . If the amplitude of the reflected wavefronts is larger than the environmental noise, the receiver will acquire the reflected wavefronts  45  and  55  and be indistinguishable from the wavefront  35 , except that they will be delayed in time.  
           [0008]    Refer now to FIG. 2 for a discussion of the channel induced intersymbol interference within a wireless RF data communication system. The digital data  105  and its synchronizing clock  100  is the input signal to the transmitter  5  of FIG. 1. A common method of modulating the RF signal with the digital data  105  is frequency shift keying (FSK) In frequency shift keying, the voltage level of the digital data representing a digit having a binary 0 causes the RF signal to have a first frequency f 0    135  and the digit having a binary 1 causes the RF signal to have a second frequency f 1    140 .  
           [0009]    The transmitter  5  transmits the RF signal  110  from the antenna  10  as described above. The RF signal transits the multiple paths as described with each signal being delayed in time. The direct RF signal λ 1    115  arrives at the antenna after a delay δ 1    145 . The reflected RF signal λ 2    120  arrives at the antenna after a delay δ 2    150 . The reflected RF signal λ 3    125  arrives at the antenna after a delay δ 3    155 .  
           [0010]    The direct RF signal λ 1    115  and the reflected RF signals λ 2    120  and λ 3    125  are superimposed upon each other at the antenna and transferred to the receiver  20 . The receiver  20  employs superheterodyne techniques to extract the digital data from the differences in frequency that defines the digital data. The data message  130  should change from the binary zero to the binary 1 at the time  170 . However, the delayed reflected signals λ 2    120  and λ 3    125  interfere with the direct received signal  115  and cause an uncertainty  160  of the digital data. The digital data may remain at the binary 0 and is not definitely changed until the time  175 . The magnitude of the delayed reflected signals λ 2    120  and λ 3    125  determines the impact of this intersymbol interference. Similarly, the data message  130  should change from the binary 1 to the binary 0 at the time  10 , however, the interference from the delayed reflected signals λ 2    120  and λ 3    125  might cause the data message  130  to remain at the binary 1 until the time  185 . This uncertainty time  165  may cause a misinterpretation of the digital data.  
           [0011]    Similar channel induced intersymbol interference can occur in wireless infrared light transmission as illustrated in FIG. 3. In this case the transmitter  205  and the receiver  220  are generally enclosed within a room  200 . The transmitter  205  excites the light emitting diode (LED) to emit infrared light. A typical type of transmission is on-off-keying (OOK), where the LED is excited for data having a binary one and turned off for data having a binary zero. The light is transmitted to a photodiode  215 . The photodiode  215  is connected to the receiver  220 , which recovers the received light signal and demodulates the received signal to extract the transmitted information.  
           [0012]    The light signal as transmitted from the LED  210  is a contiguous wavefront, but for illustration it is shown as three separate wavefronts  235 ,  240 , and  250 . The wavefront  235  represents the portion of the light signal that travels directly to the photodiode  215 . The wavefronts  240  and  250  are transmitted toward the sidewalls of the room  200  and the wavefronts  246  and  255  are reflected to the sidewalls to the photodiode  215 . The reflected wavefronts  246  and  255  travel a longer distance through the room  200  to arrive at the photodiode  215  and thus interfere with the wavefront  235  that arrive earlier.  
           [0013]    Referring to FIG. 4 for a discussion of the modulation of the light signal as transmitted from the transmitter  205  of FIG. 3. The data clock  300  has the frequency rate that the digital data message is gated within the transmitter  210 . The modulation clock (PPM CLK  305 ) that is used to generate the four-pulse position modulated signal of the transmitted signal  310 . Each time slot t1, t2, t3, t4 is divided into four sub-increments s 1 , s 2 , s 3 , s 4 . One sub-increment s 1 , s 2 , s 3 , or s 4  is set to a binary one, in this representation, to represent a two digit binary number. Since only one sub-segment may be occupied for any one time slot t1, t2, t3, t4, the coding can only represent the four possible combinations of the two digit binary numbers. The four-pulse position modulated signal of the data message illustrates the modulation of the four possible binary digit combinations of the dual-bit data and is explained as follows:  
                                                       Time Slot   Dual-Bit Code   PPM Encoding                           t1   00   1000           t2   01   0100           t3   10   0010           t4   11   0001                      
 
           [0014]    The pulse positioned modulated signal  310  is transmitted by activation of the LED  210 . The direct light signal λ 1    320  arrives at the photodiode  215  after a delay δ 1    340 . The reflected light signal λ 2    325  arrives at the photodiode  215  after a delay δ 2    345 . The reflected light signal λ 3    330  arrives at the photodiode  215  after a delay δ 3    350 .  
           [0015]    The direct light signal λ 1    320  and the reflected light signals λ 2    325  and λ 3    330  are superimposed upon each other at the photodiode  215  and transferred to the receiver  220 . The receiver  220  recovers the pulse positioned data message  335 . The pulse positioned data  335  changes from the voltage level of the binary zero to the voltage level of the binary one after the delay δ 1    340 . However, the delay δ 2    345  and δ 3    350  of the reflected signals λ 2    325  and λ 3    330  causes the photodiode to detect the binary one for a time longer than the length of a sub-increment s 1 , s 2 , s 3 , s 4  of a time slot t1, t2, t3, or t4. The magnitude of the reflection maybe insufficient to consistently determine this uncertainty time  355 . Further, if two symbols such as a 11 followed by a 00 are transmitted the detection of the binary one extending beyond the sub-increment s 1 , s 2 , s 3 , s 4  of a time slot t1, t2, t3, or t4 now interferes with a subsequent symbol. The severity of the channel induced intersymbol interference prevents the extension of the bandwidth of communication system beyond the ability of the communication to reliably detect and recover the received data message.  
           [0016]    U.S. Pat. No. 6,169,765 (Holcombe) describes an output signal pulse width error correction circuit and method wherein errors in a data signal conforming to a communications protocol having a prescribed duty cycle are corrected by monitoring a duty cycle of the data signal, comparing the duty cycle to a duty cycle reference voltage corresponding to the prescribed duty cycle, and adjusting a pulse width of the data signal to conform to the prescribed duty cycle of the protocol.  
           [0017]    U.S. Pat. No. 5,394,410 (Chen) explains a technique for encoding data for serial transmission and the correlative technique for decoding the transmitted data.  
           [0018]    U.S. Pat. No. 5,892,796 (Rypinski) illustrates frame format and method for adaptive equalization within an integrated services wireless local area network to prevent fading and intersymbol interference due multiple path radio propagation.  
           [0019]    U.S. Pat. No. 6,118,567 (Alameh, et al.) teaches a waveform encoding method and device provide for generating/receiving a power efficient binary intensity modulated optical data signal from a binary source signal which minimizes a time between adjacent pulse transitions and maximizes a pulse peak amplitude for transmission over a low-power wireless infrared link  
           [0020]    “Efficient Reconstruction of Sequences,” Levenshtein, et al., IEEE Transactions on Information Theory, January 2001, Volume: 47, Issue: 1, pp. 2-22 introduces and provides solutions for problems of efficient reconstruction of an unknown sequence from its versions distorted by errors of a certain type.  
           [0021]    “Spread-Response Precoding for Communication Over Fading Channels,” Wornell, IEEE Transactions on Information Theory, March 1996, Volume: 42, Issue: 2, pp. 488-501 presents “spread-response precoding” to effectively transform an arbitrary Rayleigh fading channel into a nonfading, simple white marginally Gaussian noise channel.  
           [0022]    “Linear Complexity Of A Sequence Obtained From A Periodic Sequence By Either Substituting, Inserting, Or Deleting K Symbols Within One Period,” Jiang et al., IEEE Transactions on Information Theory, May 2000, Volume: 46, Issue: 3, pp. 1174-1177 provides a unified derivation of the bounds of the linear complexity for a sequence obtained from a periodic sequence over GF(q) by either substituting, inserting, or deleting k symbols within one period.  
           [0023]    “On The Synchronizability And Detectability Of Random PPM Sequences,” Georghiades, IEEE Transactions on Information Theory, January 1989, Volume: 35, Issue: 1, pp. 146-156, investigates the problem of synchronization and detection of random pulse-position modulation (PPM) sequences under the assumption of perfect slot synchronization. Bounds on the symbol error probability and the probability of false synchronization that indicate the existence of a severe performance floor are derived. A way to eliminate the performance floor is suggested by inserting ‘special’ PPM symbols in the random data stream.  
         SUMMARY OF THE INVENTION  
         [0024]    An object of this invention is to provide a communication system where pulse positioned modulated data signals is compensated for channel induced intersymbol interference.  
           [0025]    An object of this invention is to extract pulse positioned encoded data from a received signal corrupted with channel induced intersymbol interference.  
           [0026]    To accomplish at least one of these objects and other objects a communication system has a transmitter and a receiver. The transmitter transmits a signal containing pulse positioned modulated and compensated data symbols The receiver acquires the signal containing the pulse positioned modulated data symbols, recovers the data symbols, and extracts the data.  
           [0027]    The transmitter includes a modulation apparatus connected to receive data and convert the data to pulse positioned modulated data symbols, the pulse positioned modulated data symbols encoded to compensate for the channel induced intersymbol interference The modulation apparatus has a symbol mapping circuit, which receives data symbols to be transmitted and maps the data symbols to a transmission code. The modulation apparatus compares two adjacent data symbol digits, if the two adjacent data symbol digits have a first data level, the data symbol digits are transmitted. If the first digits of the data symbol digits has a first data level and a second digit of the data symbol digits has a second data level, the data symbols are also transmitted. However, if the first and second data symbol digits have a second data level, transmitting a first of the two adjacent data symbol digits is transmitted, a second of the two adjacent data symbol digits is converted to the first data level and the second data symbol digits is then transmitted.  
           [0028]    The receiver has a demodulation apparatus to recover data symbols in the presence of the channel induced intersymbol interference. The demodulation apparatus has a sampling circuit in communication with a signal receiving circuit within the receiver to sample at a regular period received data symbols acquired by the receiving circuit. The samples of the data samples are retained by a sample retaining circuit in communication with the sampling circuit. The sample retaining circuit transfers the retained samples to a symbol mapping circuit. The symbol mapping circuit then recovers the data symbols.  
           [0029]    The symbol mapping circuit executes a method for recovering the data symbols by first determining from two adjacent data symbol digits if a first state transition is present in the data symbol digits. The first state transition indicates that a first of the two adjacent data symbol digits is at the first data level and a second of the two adjacent data symbol digits is the second data level. If the first state transition is present, a first state transition time is recorded. If the first state transition is not present the search to determine the first state transition continues until it is present.  
           [0030]    When the first state transition is determined, a second state transition is determined to be present in the data symbol digits The second state transition indicates that a first of the two adjacent data symbol digits is at the second data level and the first of the two adjacent data symbol digits is the second data level. When the second state transition is present, a second state transition time is recorded. The time difference between the first state transition time and the second state transition time is then calculated. If the time difference is less than a boundary time, at least one of any data symbol digits received subsequently to the second data symbol digit of the two adjacent symbol digits having the first state transitions is set to the first data level. If the time difference is greater than the boundary time, a first data symbol digit received subsequently to the second data symbol digit of the two adjacent symbol digits having the first state transition is set to the first data level and a second data symbol digit to the second data level.  
           [0031]    All data symbol digits received subsequent to the second data symbol digit of the two adjacent symbol digits having the first state transition and prior to a data symbol boundary are set to the first data level. The data symbol digit arriving subsequent to the data symbol boundary to a second data level and all remaining symbol digits of one data symbol subsequent to the data symbol boundary are set to the first data level.  
           [0032]    The method for recovering the data symbols continues by determining if a last data symbol digit of a symbol is at the second data level. If the last data symbol digit is at the second data level, all symbol digits of a following data symbol are determined if they are at the first data level. If all the data symbol digits of the following data symbol are at the first data level, the first symbol digit of the following data symbol is set to the second data level. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    [0033]FIG. 1 is a diagram of the operation of a wireless radio transmitter and receiver illustrating channel induced interference of the prior art.  
         [0034]    [0034]FIG. 2 is a timing diagram illustrating the effects of channel induced intersymbol interference on received digital data in wireless radio frequency channel of the prior art.  
         [0035]    [0035]FIG. 3 is a diagram of the operation of a wireless infrared transmitter and receiver illustrating channel induced interference of the prior art.  
         [0036]    [0036]FIG. 4 is a timing diagram illustrating the effects of channel induced intersymbol interference on received digital data in wireless infrared channel of the prior art.  
         [0037]    [0037]FIG. 5 is a plot of a stream of pulse positioned modulated symbols being mapped to compensate for intersymbol interference of this invention.  
         [0038]    [0038]FIG. 6 is a plot of a stream of pulse positioned modulated symbols transmitted and received demonstrating intersymbol interference of this invention.  
         [0039]    [0039]FIG. 7 is a plot illustrating the detection of state transitions of a received stream of pulse positioned modulated symbols and from the state transitions of the received pulse positioned modulated symbols recovering the transmitted pulse positioned modulated symbols in the presence of intersymbol interference of this invention.  
         [0040]    [0040]FIG. 8 is a plot illustrating the reconstruction of the pulse positioned modulated symbols from the received compensated symbol stream of this invention.  
         [0041]    [0041]FIG. 9 is a block diagram of a transmitter of the communication system of this invention.  
         [0042]    [0042]FIG. 10 is process flow diagram of the method for compensation of pulse positioned modulated signals of the transmitter of FIG. 9.  
         [0043]    [0043]FIG. 11 is a block diagram of a receiver of the communication system of this invention.  
         [0044]    [0044]FIGS. 12 a - 12   c  is process flow diagram of the method of for recovery of pulse positioned modulated signals of the receiver of FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0045]    The intersymbol interference as described above limits the frequency spectrum usable for the transfer of digital data. The transfer of long strings of zeros and ones or heavy toggling between zeros and ones causes corruption of the digital data due to the multipath effects that induce the intersymbol interference described. The use of pulse positioned modulation provides a relatively low switching rate. Once the receiving system is synchronized to the received signal, the recovery of the data is more reliable since the errors that occur are not position dependent and do not propagate. Certain symbol arrangements however, do cause limitation of the frequency due to the multipath effects that can create intersymbol interference. In symbol combinations (11) followed by (00), and (10) followed by a (00) of a four pulse position modulated encoded data, the intersymbol interference is sufficient to potentially corrupt the received data and make the transmitted data not recoverable. The method and system of this invention employs a recoding of the pulse positioned modulation to compensate for the symbol combinations (11) followed by a (00). The symbols combinations (10) followed by a (00) are determined as a result of the patterns detected.  
         [0046]    Refer now to FIG. 5 for a more detailed discussion of the recoding of the pulse positioned modulation to create a signal which then modulates the transmitted signal (either radio frequency or light signals). The PPM signal shows the digital data (0110001100010011) and formed partitioned to form the symbols SYM  1 , . . . , SYM  8 . The symbols SYM  2  and SYM  3  contain the symbols 10 and 00 that are the first of the potential intersymbol interference candidates. The symbols SYM  4  and SYM  5  contain the symbols 11 and 00 that are the second of the potential intersymbol interference candidates.  
         [0047]    In the case of the four pulse position modulation, as .described in FIG. 4, each symbol is divided into four different times slots s 1 , s 2 , s 3 , and s 4 . Each time slot represents a digit of the symbol and only one of the four different times slots s 1 , s 2 , s 3 , and s 4  may contain the voltage level representative of the binary one. The intersymbol interference for the symbols SYM  4  and SYM  5  is compensated by placing the symbol digit s 1  from a binary one for SYM  5  to a binary zero for the transmission signal. This is an illegal character for the digital data and can be corrected at the receiver as describe hereinafter. Thus all symbols having a data pattern as shown for symbols SYM  4  and SYM  5  are recoded or mapped to the format of the transmit signal (XMIT SIGNAL), where the symbols SYM  4  and SYM  5  are now coded as (0001), (0000).  
         [0048]    Referring to FIG. 6, the transmit signal (XMIT SIGNAL) modulates a wireless RF signal as described in FIG. 2 or a light signal as described in FIG. 4 for broadcast. The wireless RF or light signal is acquired at the receiver. The receiver amplifies wireless RF or light signal and recovers the received signal. The received signal is delayed as described in FIGS. 1 and 3 from the transmitted signal by a delay δ. However, the reflected signals λ 2  and λ 3  of FIGS. 1 and 3 delayed and are superpositioned on the direct signal λ 1 .  
         [0049]    The pulse width of each symbol digit of the pulse positioned modulated data is extended for at least a second symbol digit time slot s 1 , s 2 , s 3 , or s 4 . Thus the symbols SYM  2  and SYM  3  containing the symbols 10 and 00 are now merged to contain the symbol data (0011) (1100). If the reflected signals λ 1  and λ 3  of FIGS. 1 and 3 are delayed even longer, the symbol data could actually be (0011) (1110). The symbols SYM  4  and SYM  5  are similarly corrupted to become (0001) (1000). Thus the recoded data of SYM  5  is now interfered with from the previous symbol SYM  4 .  
         [0050]    The recovery of the received pulse positioned modulated signal is shown in FIG. 7. The rising transitions and the falling transitions of the received signal are recorded and the time difference τ from the rising transition and the falling transition is determined. If the time difference τ is less than a preset parameter, for instance the time of three symbol digits, the first symbol digit of the sequence of binary 1&#39;s is retained as the voltage level of the binary one and the remaining symbol digits are set to the voltage level of the binary zero. The symbols SYM  4  and SYM  5  illustrate this. The last symbol digit of symbol SYM  4  and the first symbol digit of the symbol SYM  5  have a voltage level of a binary one. The last symbol digit of symbol SYM  4  is retained at the voltage level of the binary one and the first symbol digit of the symbol SYM  5  is set to the voltage level of the binary zero. This retains the recoding described above.  
         [0051]    Alternately, if the time difference τ is greater the preset parameter the first three symbol digits are set the voltage levels of the binary one, followed by the binary zero, followed by the binary one (101) and the remaining symbol digits of a symbol are set to the voltage level of the binary zero. The symbols SYM  2  and SYM  3  illustrate this recovery The last two symbol digits s 3  and s 4  of the symbol SYM  2  and the first two symbol digits s 1  and s 2  of the symbol SYM  5  all have the voltage level of the binary one. The time from the rising transition between the second and third symbol digits of the symbol SYM  2  and the falling transition between the third and fourth symbol digits of the symbol SYM  3  is greater than the preset parameter (duration of 3 symbol digits). The third symbol digit s 3  of the symbol SYM  2  and the first symbol digit s 1  of the symbol SYM  3  are retained at the voltage level of the binary one and the last symbol digit s 4  of the symbol SYM  2  is set to the voltage level of the binary zero. The remaining symbol digits (s 3  and s 4 ) of the symbol SYM  3  are set to the voltage level of the binary zero.  
         [0052]    The recovered signal now reflects the transmitted signal of FIGS. 5 and 6. FIG. 8 illustrates the final decoding to recover the received version of the original pulse positioned modulated data. The recovered data is examined for the existence of a symbol code having a voltage level of a binary one at the fourth symbol digit s 4  and all the symbol digits s 1 , s 2 , s 3 , and s 4  of the following symbol digit have a voltage level of a binary zero. The first symbol digit s 1  of the following symbol digit is set to the voltage level of a binary one Examining the symbols SYM  4  and SYM  5 , the last symbol digit s 4  of the symbol SYM  4  is at the voltage level of a binary one and the symbol digits s 1 , s 2 , s 3 , and s 4  of the symbol SYM  5  are at the voltage level of the binary zero. The first symbol digit s 1  of the symbol SYM  5  is set to the voltage level of the binary one and the symbol codes for symbols SYM  4  and SYM  5  are recovered as (11) (00).  
         [0053]    Refer now to FIGS. 9 and 10 for a description of the structure and operation of the transmission subsystem of a communication system of this invention. Digital data D 0 , . . . , Dn  400  is acquired (Box  440 ) by the data input register  405 . In this illustration the digital data is originally parallel data such as would be created, transformed, and stored in a computing system The synchronizing clock circuit  410  provides the data clock  412  to gate the input digital data D 0 , . . . , Dn  400  to the data input register  405  at the data rate shown in FIG. 4. The data  407  retained by the data input register  405  is transferred to the pulse position modulator  415 . The pulse positioned modulator  415  groups the data  407  to form (Box  445 ) multiple bit or binary digit symbols as shown in FIG. 4. For a four pulse positioned modulation, the data  407  is grouped into two bit or binary digital symbols. The synchronizing clock circuit  410  provides a pulse positioned clock  413  to the pulse positioned modulator  415  to determine the pulse positioned modulation encoding (Box  450 ) for each of the formed symbols. The pulse positioned clock  413  is equivalent to the pulse positioned modulation clock  305  of FIG. 4. The pulse positioned modulation encoded symbols are then transmitted serially (Box  455 ) as the pulse positioned modulated data  417  to the pulse positioned modulation mapping circuit  420 . The pulse positioned modulation mapping circuit  420  compares adjacent symbol digits of the pulse positioned modulation data  417  to map (Box  460 ) the pulse positioned modulation data  417  to provide a compensation for the presence of channel induced intersymbol interference. When two adjacent symbol digits are compared (Box  465 ) and are both at a voltage level of the binary one, the pulse positioned modulation mapping circuit sets (Box  470 ) the second symbol digit of the pair or symbol digits to the voltage level of the binary zero. The mapping of the pulse positioned modulated data  417  is equivalent to the method described in FIG. 5 where a data symbol (11) is adjacent to a data symbol (00). The pulse positioned modulation mapping circuit  420  has formed the transmission signal  422 , which is transferred to the transmission signal modulation circuit  425 . The transmission signal modulation circuit  425  modulates (Box  475 ) a signal  427  that is to be transmitted, either Frequency Shift Keying an RF Signal or gating a light signal. The modulated signal  427  is the input signal to the transmission driver that excites a transducer such as the transmission antenna  15  of FIG. 1 or the LED  210  of FIG. 3. The modulated signal  435  is then broadcast (Box  480 ) through the transmission medium.  
         [0054]    An illustration of the structure and operation of the receiving subsystem of the communication system of this invention is shown in FIGS. 11 and 12 a - 12   c . The modulated signal  500  is acquired (Box  555 ) by a receiver  505  either through the antenna  20  of FIG. 1 or the LED  210  of FIG. 2. The amplifier and conditioning circuit  510  amplifies, demodulates, and conditions (Box  560 ) the received signal  500  to create the received pulse positioned modulated data  512 . Generally, the transmitter of FIG. 9 will have embedded a synchronization signal and a start and/or stop signal within the transmitted signal. The synchronization signal is detected by the clock synchronization circuit  515  and the clock synchronization circuit  515  generates (Box  565 ) a receiver system clock that is aligned to the embedded synchronization signal. The start signal indicates the beginning of the transmitted data follows immediately upon completion of the start signal. The stop signal indicates the completion of the data message and any following detected data signal is not part of the transmitted message. The start/stop recovery circuit detects the presence of the start and/or stop signals within the received pulse positioned modulated signal.  
         [0055]    Upon detection (Box  570 ) of the start signal, the received pulse positioned modulated data is sampled (Box  575 ) by the data sampling circuit  525 . The clock synchronizing circuit  515  provides a sampling dock to provide at least one sample during a symbol digit s 1 , s 2 , s 3 , and s 4  time to determine the voltage level of the binary digit being sampled. The sampled pulse positioned modulated data  527  is transferred to the sample register  530  where is it retained (Box  580 ) for extraction of the transmitted pulse position modulated data that is corrupted by intersymbol interference. The retained samples are transferred to the sample mapping circuit  540 , which performs the extraction of the transmitted pulse positioned modulated data.  
         [0056]    The current sample for a symbol digit is compared (Box  585 ) to a previous sample of a symbol digit. If the previous symbol digit is at the first voltage level indicating a binary zero and the current symbol digit is a the second voltage level indicating a binary one, a rising edge has occurred (Box  590 ). If the rising edge has not occurred, but the comparison indicates that the previous symbol digit has the second voltage level indicating a binary one and the current symbol digit has the first voltage level indicating a binary zero, then a falling edge has occurred (Box  620 ). However, if the previous and the current symbol digits are equal (either the first level indicating a binary zero or the second level indicating a binary one), then no transition has occurred and the next sample is taken (Box  575 ) and retained (Box  580 ) for comparison (Box  585 ) with the now previous sample.  
         [0057]    When a rising edge occurs (Box  590 ), the sample time at which the rising edge occurs is recorded (Box  595 ). The difference time Δ 1  between the recorded time for the rising edge and a previous falling edge is determined (Box  600 ). Upon comparison (Box  605 ) with a preset time P0 for instance 3×τs (τs being the time duration of a symbol digit) if the difference time Δ 1  is not greater than the preset time P0, the next sample is taken (Box  575 ) and retained (Box  580 ) for comparison (Box  585 ) with the now previous sample.  
         [0058]    When a falling edge occurs (Box  620 ), The sample time at which the falling edge occurs is recorded (Box  625 ). The difference time Δ 2  between the logged time of the rising edge and the logged time of the falling edge is calculated (Box  630 ). The difference time Δ 2  is compared (Box  635 ) to a preset time P1 (for instance 3×τs). If the difference time Δ 2  is less than the preset time P1, the symbol digit at the rising edge having the voltage level of the binary one is retained (Box  640 ) at the binary one and all symbol digits remaining in the symbol up to the falling edge are set (Box  645 ) to the voltage level of the binary zero. This provides the recovery of the symbol digits as discussed above for SYM  4  and SYM  5  of FIG. 7.  
         [0059]    If the difference time Δ 2  is greater than the preset time P1, the symbol digit (Slot  1 ) at the rising edge having the voltage level of the binary one is retained (Box  655 ) at the binary one. The adjacent symbol digit (Slot  2 ) is set (Box  660 ) to the voltage level of the binary zero and the next adjacent symbol digit (Slot  3 ) is retained (Box  665 ) at the voltage level of the binary one. This provides the recovery of the symbol digits as discussed above for SYM  2  and SYM  3  as discussed in FIG. 7.  
         [0060]    The difference time Δ 2  is then compared (Box  670 ) to an even longer preset time P2 (for instance 5×τs). If the difference time Δ 2  is greater the longer preset time P2, the final slot next to a symbol boundary is set (Box  675 ) to the voltage level of the binary zero. The first symbol digit (Slot  1 ) of the following symbol is retained (Box  680 ) at the voltage level of the binary one and all remaining slots of the symbol should be set (Box  685 ) to a binary zero. The sampling is skipped to the next symbol boundary (Box  690 ). The longer preset time P2 allows for the recovery of a set of symbol digits having a coding of (101001) from a received corrupted pulse position modulated data of (11111111). The worst incidence of this occurring would permit the reception of the received pulse position modulated data (0001) (1111) (1110) and then recovery of the transmitted (0001) (0100) (1000).  
         [0061]    Upon completion of the recovery of the transmitted pulse positioned modulated data, the next sample is taken (Box  575 ) and retained (Box  580 ) for comparison (Box  585 ) with the now previous sample. If a rising edge is determined (Box  590 ), the time of the rising edge is recorded (Box  595 ). The difference time Δ 1  between the previous falling edge and the present rising edge is determined (Box  600 ). The difference time Δ 1  is compared (Box  605 ) to the preset time 3×τs and if the difference time Δ 1  is greater than the preset time 3×τs, the first symbol digit (Slot  1 ) of the symbol subsequent to the falling edge is set (Box  610 ) to the voltage level of the binary one. The remaining symbol digits are retained at the voltage level of the binary zero and the sampling skips (Box  615 ) to the next symbol boundary. The sampling and recovery process then continues until the message is complete with the reception of a stop signal or synchronization signal.  
         [0062]    Returning to FIG. 11, the pulse position modulated data  540  recovered by the symbol mapping circuit  535  is transferred to the data extraction circuit  545 . The data extraction circuit  545  decodes the pulse position modulated data to extract the data symbols and assemble the data symbols to the originally encoded data.  
         [0063]    The symbol mapping circuit  535  and the data extraction circuit  545  are in the preferred embodiment logical state machines capable of extremely high speed recovery of the data symbols and extraction of the data. However, it is known in the art that the structure and method described above may be accomplished within a digital signal processor or similar computing system with the functions and processes being programs stored on data storage medium for execution by the processes.  
         [0064]    Further, the preferred embodiment illustrates a four pulse positioned modulated data signal. It is in keeping with the intent of this invention that any number of pulse position modulated symbol digits may be employed to encode the digital data. The structure and method of the communications system of this invention functions with a higher order pulse position modulated encoding.  
         [0065]    The communications systems as shown in FIGS. 1 and 3 illustrate wireless transmission of the broadcast modulated signal. It is keeping with the intention of this invention that the modulated signal be transmitted within a cable, either an electrical signal in a copper cabling or a light signal within a fiber optic cable.  
         [0066]    While this invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.