HIGH PERFORMANCE MODEM USING PILOTE SYMBOLS FOR EQUALIZATION AND FRAME SYNCHRONIZATION

Abstract: A modulator (fig. 2) that modulates and encodes data using data symbols (52), interspersed with pilot symbols (56), for transmission to another modem, and a demodulator (fig. 3) that decodes data symobls or a signal received from another modem. In the demodulator, a received signal is sampled at four times the symbol rate (104). The sampled signal is converted to quadrature signals and low-pass filtered to produce a complex baseband signal (120). Data symbols in the complex baseband signal are sampled (146) in synchronization with the start of each symbol based on the pilot symbols that are interspersed in the signal. By processing the pilot symbols, full modem synchronization is maintained down to a very low signal-to-noise ratio, through noise bursts, or even when the data symbols representing data are replaced by predefined sequences of data symbols that convey the dots and dashes of Morse code identifying the station transmitting the modulated signal. The continued presence of pilot symbols interspersed with the sequences of data symbols conveying Morse code prevents loss of synchronization between the transmitting station and the receiving station. The pilot symbols are also used to update equalizer tap coefficients in an equalizer (142) of the demodulator, thereby minimizing errors in the output data caused by channel fading, noise, and other effects. In addition, the sequence of pilot symbols is used to delimit data frames, so that bit synchronization and frame synchronization in the data are not required.

Description: HIGH PERFORMANCE MODEM USING PILOT SYMBOLS FOR EQUALIZATION AND FRAME SYNCHRONIZATION Field of the Invention This invention generally relates to modulating and demodulating data for transmission between two locations, and more specifically, to synchronizing frames of data that are modulated at one location and transmitted to another, where the frames of data are demodulated. Background of the Invention The term "modem" is a concatenation of portions of two words, "modulator" and "- emodulator," and is applied to devices that are capable of modulating data for transmission over a telephone or radio frequency (RF) link and demodulating data received from another location that was modulated by a similar device. The use of modems for interconnecting personal computers so that users can exchange data and communicate has become widespread. Improvements in technology have reduced the cost of modems currently available while increasing the rates at which data can be transmitted. In achieving each successive level of improvement in data rate, modem manufacturers have tended to adopt and conform to industry-wide standards that define the operation of the modem, thereby ensuring at least some degree of compatibility between modems. For example, the Comitέ Consultatif Internationale de Tέlέgraphie et Tέlέphonie (CCITT) V.32 Standard defines a type of modem with one of the fastest available data rates, 9600 bits/sec. (ignoring standards such as CCITT V.42bis that employ data compression to achieve higher effective data rates, and proprietary systems). The next higher data transfer standard will be provided with the release of the V-fast Standard, with speeds beyond 14.4 Kbits/sec. The use of modems is not limited to data exchange between computers, and in certain applications, proprietary modem modulation protocol designs can provide improved performance. In paging systems, modems are often used to transmit data between a central paging terminal and a plurality of simulcast paging transmitters for retransmission as a radio signal to a pager unit carried by a subscriber of the paging service. The data received by the paging unit can then be read by the subscriber on a display screen that is part of the paging unit. An asynchronous modem suitable for use in a simulcast paging system is disclosed in commonly assigned U.S. Patent No. 5,227,741, entitled "Variable Speed Asynchronous Modem." Although the modem disclosed in the patent successfully deals with problems of isochronous distortion (system "jitter") to achieve data transfer rates over radio frequency links in excess of 2400 bits/sec, it can not achieve the 9600 bits/sec. performance of the V.32 Standard that is more typically used in connection with telephone line modems. There are several problems that are encountered in the application of a modem to simulcast paging systems that typically do not arise in telephone line modem applications and are not very well addressed by the V.32 Standard. Telephone line signal-to-noise ratio (SNR), particularly on some unconditioned lines, can cause errors to arise in the data transmitted between two V.32 modems. In response to a SNR of about 24 dB, the V.32 Standard calls for the modems that are linked to drop to a lower data rate, possibly 4800 bits/sec. or even 2400 bits/sec. As a result, the actual or real data rate of a modem conforming to the V.32 Standard in a simulcast link network is much less than 9600 bits/sec, especially after applying forward error correction techniques. It would be preferable to maintain the higher 9600 bits/sec. rate, even at the lower SNR. Furthermore, it would be desirable to improve data throughput by having low overhead forward error correction and control information. When two V.32 modems establish a link, there is an initial period of training and synchronization, which may require repeating if interrupted by a brief line outage or noise burst. At the receiving modem, the process of synchronization requires identifying the timing of each symbol in the data to accurately determine when each symbol occurs. Once synchronization at the symbol level is achieved, the receiving modem must find the start and end of each data frame from the demodulated data stream. Further, the synchronization of a V.32 modem requires a return path or alarm return in order to request the retransmission of training sequences any time that synchronization is lost, thereby significantly reducing data throughput. It would be preferable to provide automatic synchronization for two modems establishing a communication link and to provide for automatic recovery from brief line outages and noise bursts, without using a return path. In addition, the V.32 Standard is very sensitive to low SNR. For example, demodulator synchronization in a modem conforming to the V.32 Standard is typically lost at a SNR range of about 22 to 24 dB. Ideally, synchronization should be maintained to a SNR less than about 10 dB in a paging link system, to provide adequate system performance. Another significant limitation of the V.32 standard for use in an RF linked communications system is the Federal Communications Commission (FCC) requirement of periodic identification of data link transmitters using discernible Morse code. Under the V.32 Standard, sending a station identifier in Morse code causes the demodulator to completely lose synchronization. Therefore, after the Morse code is sent, the two modems previously communicating must again engage in the training and synchronization sequence that extends over about one second. In a busy paging system, loss of data throughput during this retraining and synchronization interval can significantly limit the system. The V.32 Standard provides for equalizing amplitude and group delay distortion, but the equalizers typically used for this purpose introduce about 2 dB degradation in the modem performance, which represents an undesirable modem degradation. Equalizer performance of modems used for simulcast paging, particularly in links having a poor SNR, should be improved over the current state, to at least meet, if not exceed, the V.32 Standard, and to be able to withstand substantial impulse noise. The present invention addresses the above-noted problems in the V.32 Standard and in other prior art modems not discussed above. Although it was developed to meet the needs of a specific simulcast paging system, this invention has applicability to almost any communications system in which data must be efficiently transferred via modem between two geographically disparate points at relatively high rates. Summary of the Invention In accordance with the present invention, apparatus for demodulating an input signal are defined. The analog input signal specifically includes both data symbols that are used to encode data and pilot symbols (also known as reference symbols). The apparatus comprises sample means that are coupled to receive the input signal and periodically sample it to produce a corresponding digital sample signal. Quadrature conversion means convert the digital sample signal into a complex baseband signal. The complex baseband signal is then processed by symbol synchronization means to determine symbol timing for the data symbols in the complex baseband signal, as a function of the pilot symbols and data symbols that are also contained within it, producing a synchronized signal in which the data symbols are properly aligned in synchronization with their symbol times. Also provided are means for separating the pilot symbols from the data symbols in the synchronized signal, producing a data symbol signal. Decoding means decode the data symbol signal to recover data corresponding to the data symbols that are contained therein. The pilot symbols comprise a repeating pattern of a pre-defined number of pilot symbols interspersed at pre-defined intervals with the data symbols. The symbol synchronization means comprise means for determining differential phase relationships for successive pilot symbols in the complex baseband signal, where a 180° phase shift represents a decoded pilot symbol equal to a binary " 1 " and a 0° phase shift represents a decoded pilot symbol equal to a binary "0." This sequence of decoded pilot symbols is then compared with the expected sequence of pilot symbols to determine the start of the pilot symbol sequence. Equalizer means, coupled to receive the synchronized signal and the equalizer coefficients, equalize the synchronized signal as a function of the equalizer coefficients to compensate for any degradation of the input signal that would inhibit decoding the data symbols to recover the data. Preferably, the decoding means comprise a trellis decoder. The symbol synchronization means also preferably include means for sampling the complex baseband signal at times that synchronize the data symbols with their corresponding symbol times. One of the features of the present invention is that the symbol synchronization means include means for maintaining complete modem synchronization if Morse code replaces the data symbols in the input signal. This feature prevents loss of synchroni¬ zation if Morse code that identifies a source of the input signal is periodically input to the sample means. Modem synchronization includes: (a) symbol timing synchronization, (b) pilot symbol synchronization, (c) phase and frequency tracking synchronization, (d) forward error correction framing synchronization, and (e) equalizer convergence. A method for demodulating an input signal that includes data symbols used to encode data and pilot symbols is another aspect of the present invention. The steps of this method are generally consistent with the functions of the elements comprising the apparatus discussed above. Brief Description of the Drawings The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: FIGURE 1 is a schematic block diagram illustrating a simulcast paging system in which the present invention is used; FIGURE 2 is a functional block diagram of a modulator that maps input data to data symbols periodically interspersed with pilot symbols; FIGURE 2 A is a convolutional encoder used to encode input data in the modulator; FIGURE 2B is a block diagram of a station identifier that periodically produces data symbols representing the Morse code, which is used to identify a transmitting site; FIGURE 3 is a functional block diagram of a demodulator in accordance with the present invention; FIGURE 4 is a flow chart of the logic employed by the modulator in an interrupt routine that encodes the input data as data symbols and converts the data symbols and pilot symbols into a passband signal for transmission to the demodulator; FIGURE 5 is a flow chart showing the steps carried out by the demodulator to demodulate the data from the received signal; FIGURE 6 is a flow chart of a routine for adjusting symbol timing in the demodulator; FIGURE 7 is a flow chart of a routine for processing the demodulated symbols in the demodulator of the present invention; FIGURE 8 is a schematic block diagram of the modulator portion of the high speed modem; and FIGURE 9 is a schematic block diagram of the demodulator portion of the high speed modem. Detailed Description of the Preferred Embodiment A simulcast paging system 20 in which the present invention is used for demodulating data is shown in FIGURE 1. Simulcast paging system 20 includes a paging terminal 22, which is a source of data messages that are transmitted to specific pager units carried by customers subscribing to the simulcast paging system. Such data messages are typically generated by other equipment (not shown) that is connected to paging terminal 22, as will be apparent to those of ordinary skill in the art. Associated with paging terminal 22 and connected to it by a data line 24 is a modem 25. In simulcast paging system 20, paging terminal 22 most often transmits modulated data messages to a plurality of paging transmitters 34 rather than receiving such messages, and the modulator portion of modem 25 receives much more use than the demodulator portion. Therefore, while both the modulator and demodulator functions of the modem are available to paging terminal 22, modem 25 is hereafter referred to simply as modulator 26. The data messages input to modulator 26 from paging terminal 22 are typically in the form of non-return-to-zero (NRZ) data; however, other data formats can also be modulated by modulator 26, including digital data in the form of compressed voice communications. As explained below, modulator 26 produces a modulated signal that conveys the data supplied from paging terminal 22 to each of the plurality of paging transmitters 34. In FIGURE 1, only two such paging transmitters 34a and 34b are shown for purposes of illustration, but it will be understood that simulcast paging system 20 typically includes many more such paging transmitters, some of which may be connected to paging terminal 22 by an RF link 33, as is paging transmitter 34b, instead of by telephone lines 28, as is paging transmitter 34a. The modulated signal produced by modulator 26 is conveyed over telephone lines 28 and transmitted by a transmitter 27 from an antenna 31 to corresponding modems 29a and 29b, respectively. In the case of paging transmitter 34b, an antenna 35 receives the modulated signal and supplies it to a receiver 37. Transmitter controllers 43 located at each paging transmitter site include modems 29 and other components that are not shown. The modulated signal conveyed by telephone lines 28 is input to modem 29a on a line 45a. Similarly, a received modulated signal output from receiver 37 is coupled by a line 39 to transmitter controller 43b, and the modulated signal is input over a line 45b to modem 29b. The paging transmitters primarily use the demodulator portion 30 of modems 29 for demodulating data messages transmitted from paging terminal 22 and have only an occasional need of the modulator portion of the modem. Accordingly, modems 29a and 29b are hereafter referred to simply as demodulators 30a and 30b. Nevertheless, the modulator capability of the modems installed at the paging transmitters is available to paging transmitters 34a and 34b when needed and is periodically used by them for transmission of system-related data messages back to paging terminal 22, for example, in responding if the paging transmitter is poled by the paging terminal. Demodulators 30 process the modulated signal transmitted via telephone lines 28 and RF link 33 from paging terminal 22, demodulating the signal to produce the data message conveyed thereby, which is input to their corresponding paging transmitter 34. Paging transmitters 34a and 34b each modulate an RF carrier at the same frequency, transmitting the data message through antennas 36a and 36b, respectively, as radio signals 38a and 38b. The radio transmissions are received by, for example, a pager unit 40, which is carried by a subscriber to the paging service who is the intended recipient. The pager may be disposed within a reception zone 44a of paging transmitter 34a, or in a reception zone 44b of paging transmitter 34b, or in both reception zones. It should be noted at this point that the present invention is not in any way limited to use in a paging system, but has simply been disclosed in connection with such a system, since that is one of the first contemplated commercial uses of the modem. Modulator 26 and demodulator 30 could also equally well be used to transfer data between two personal computers (not shown), e.g., interconnected via telephone line 28. Turning now to FIGURE 2, a functional block diagram 50 illustrates how modulator 26 processes input data for transmission to and demodulation by demodulator 30, in accordance with the present invention. The modulator produces a series of data symbols corresponding to the input data, as shown in a block 52. In the preferred embodiment, the data symbols are produced by trellis encoding an input signal using a 16-state trellis coded modulation (TCM) scheme, which employs a 32-cross constellation set. This TCM scheme provides a gain of 4.77 dB over conventional unencoded 16-level quadrature amplitude modulated (QAM) signals. The V.32 TCM specification of the prior art employs a 90° phase invariant, eight state TCM scheme, with 3.98 dB of gain compared to unencoded 16-QAM. Moreover, unlike prior art standards such as the V.32, the present invention provides for periodically inserting pilot symbols into the stream of data symbols. As shown in block 56 of FIGURE 2, the pilot symbols are produced by a pilot symbol source, which generates a predefined pseudo-random sequence of the pilot symbols (or preferably, accesses a look-up table in which a predetermined randomly generated sequence of pilot symbols is stored). In the preferred embodiment, there are 63 pilot symbols in the repeating pseudo-random sequence. This pseudo-random sequence should have a sufficient number of pilot symbols so that an auto-correlation function of the sequence has a well defined peak, i.e., a ratio of a maximum peak to that of the false peaks (background) should be at least two-to-one in the auto-correlation function. In addition, the particular sequence should have a frequency domain spectrum that simulates random white noise; this characteristic permits data symbols conveying Morse code transmissions to be discernible above any audible "noise" of the pilot symbols, while maintaining the pilot symbols in the modulated data stream for synchronization purposes. Pilot symbols comprising the sequence are inserted one at a time, interspersed into the stream of data symbols, with the sequence repeating as necessary until all data symbols are transmitted. The same predefined randomly generated sequence of pilot symbols is used for all modems constructed in accordance with the present invention, so that at any modem 29, the demodulator can monitor the pilot symbols extracted from the received signal to synchronize the demodulation process and update equalizer coefficients used to equalize the received signal. As shown in FIGURE 2A, the data symbol source that is used for conveying data (as opposed to Morse code) preferably comprises a convolutional encoder 53, which processes four bits of binary input data at a time. The four bits of binary input data are represented by Yl n , Y2 n , Q3 n and Q4 n , where the subscript "n" identifies the encoder clock time, beginning at a time n = 0. Only two of the bits, Yl n and Y2 n , are used to encode Y0 n by encoder 53; bits Yl n , Y2 n , Q3 n , and Q4 n are passed through the encoder unchanged. Bits Yl n and Y2 n are encoded by performing a series of XOR operations on the data, as indicated at three nodes 55a, 55b, and 55c. A series of time delays, T, are effected at blocks 57a, 57b, 57c, and 57d by providing buffered storage for data that are shifted out with each encoder clock pulse. Each time delay of block 57 preferably comprises a one-bit delay buffer. In this scheme, the previous value of Y0 n is XORed with the current value of Y2 n and the resulting value is buffered in block 57c. At the same time, a new value is entered into block 57b that is the result of XORing the current values for Yl n and Y2 n with the value previously buffered in block 57c. Similarly, a new value is entered into the buffer of block 57a that is the result of XORing the current value for Y2 n with the value buffered in block 57b, and the current value of Y0 n . Convolutional encoder 53 thus successively encodes bits Yl n and Y2 n to produce a fifth bit Y0 n (for the clock time n=0, Y0 n is initialized to 0), which is redundant The 16-state trellis encoding scheme has a constraint length equal to four, compared to the V.32 Standard, which uses an eight state scheme having a constraint length of three. When Morse code is to be transmitted instead of data symbols corresponding to actual data, data symbol source 52 is switched to a station identifier 47 (shown in FIGURE 2B) that includes a source of Morse code 49, which specifies the actual dots and dashes to be transmitted, e.g., using a look-up table providing the Morse code dots and dashes required to identify the modulator site, and a translator 51 that produces a sequence A of predefined data symbols that corresponds to a dot of the Morse code, or a sequence B of predefined data symbols that corresponds to a dash of the Morse code. As is well known to those of ordinary skill in the art, frequency depends on rate of change of phase, i.e., phase rotation. There is a fixed incremental phase rotation between the successive predefined data symbols comprising sequence A, so that inclusion of sequence A in the transmitted signal, when heard as a received signal on a loudspeaker (not shown), is audible as a first frequency, e.g. 1200 Hz. Similarly, there is a fixed incremental phase rotation between successive data symbols comprising sequence B that is different than the relative phase rotation between successive data symbols in sequence A, selected so that when sequence B is included in the transmitted signal, it provides a received signal audible on the loudspeaker as a second frequency, e.g., as an 1800 Hz tone, which is different from the first frequency. The sequences A and B of data symbols, interspersed with pilot symbols (which are audible in the received signal only as low amplitude white noise), thus convey the specific Morse code provided by Morse code source 49 as a series of first and second frequency tones corresponding to dots and dashes. With reference back to FIGURE 2, a logic driven switch 54 selects between the data symbol source in block 52 and the pilot symbol source in block 56. A line 60 couples the pilot symbol source to logic driven switch 54. In the preferred embodiment, every 13th symbol selected by logic driven switch 54 for input to symbol mapping in block 62 is a pilot symbol. Each pilot symbol also comprises five bits. Data are encoded and mapped to symbols at a rate of 2400 symbols per second in this embodiment. The five bits output from data symbol source in block 52 or the five bits from the pilot symbol source in block 56, depending upon the position of logic switch 54, are mapped into a 32-point signal constellation, producing a complex baseband signal conveyed on lines 64 and 66, as shown in FIGURE 2. The I portion of the signal conveyed on line 64 is applied to a low-pass filter 68, which eliminates out-of-band components. Similarly, the Q signal conveyed on line 66 is input to a corresponding matched filter 70. The filtered signals output from these two filters are respectively applied to a multiplier 72 (from filter 68) and to a multiplier 74 (from filter 70). Multiplier 72 multiplies the filtered I portion of the signal by a function cos(2πf c t), as shown in a block 76. Similarly, the filtered Q portion of the signal input to multiplier 74 is multiplied by a quadrature function sin(2πf c t), as shown in a block 78. The resulting passband components at the output of multipliers 72 and 74 are then summed together in a summing junction 80, producing a passband signal s(t) that is conveyed over telephone line 28a or in the form of RF signal 33, as shown in FIGURE 1, for receipt and demodulation by demodulator 30. FIGURE 8 illustrates the components used to implement the modulation functions shown in FIGURE 2. The NRZ signal from the paging terminal (or other source) is conveyed on line 24 to a digital signal processor (DSP) 366. Virtually all of the encoding and mapping functions discussed above are implemented by the DSP in accordance with a programmed set of instructions. The passband signal that is thus produced is conveyed on lines 372 to a digital-to-analog converter (DAC) 374, which produces a corresponding analog signal. The analog signal is applied over a line 376 to a post filter 378, and a filtered output signal is delivered from the post filter to the radio transmitter or to a telephone link (shown in FIGURE 1), on a line 360, for transmission to the corresponding modem at the other end of the communication link, where the received signal will be demodulated by the present invention. Referring to FIGURE 3, a block diagram 100 illustrates the functional aspects of demodulator 30. A received signal, indicated by r(t) in a block 102, generally corresponds to the modulator output signal s(t) in block 82 of FIGURE 2. However, due to line noise and/or other sources of signal degradation in the channel that can affect the signal during its propagation between modulator 26 and demodulator 30, the received signal r(t) may have been altered slightly so that it does not correspond exactly to the modulated output signal s(t). The present invention uses the pilot symbols that were included in the transmitted signal to recover the data transmitted, even though the received signal has been affected by its transmission to the demodulator. It is also contemplated that in certain applications, the received signal r(t) may be a digital signal instead of analog. For example, a pulse code modulated (PCM) signal could be input to the modulator. The received signal r(t), whether analog or digital, is sampled at a fixed rate f s equal to 9600 samples/sec. If the received signal is analog, it is converted to digital by an analog-to-digital converter (ADC), which is not specifically shown. Instead, since either analog or digital signals can be processed by the demodulator and either type of signal is sampled at 9600 samples/sec, the input stage of the demodulator is generally represented in FIGURE 3 as a switch 104. Sampling of the received analog signal is not synchronized with any reference clock, but simply occurs arbitrarily at the fixed rate. The digital samples (or sampled signal) produced by sampling the received signal are input to multipliers 106 and 108, which are connected in parallel, producing quadrature signals. In multiplier 106, the sampled signal is multiplied by a function cos(2πf c t) provided by a block 110 , producing an output that is applied to a low-pass filter 114. In this filter, a function p(t) acts on the signal to provide anti-aliasing and to remove out-of-band noise. Similarly, multiplier 108 multiplies the digital samples of the received signal by a function sin(2πf c t) as indicated in a block 112. The product the multiplication is input to a low-pass filter 116, which provides a filter function p(t) matching that of low-pass filter 114, for anti-aliasing and for removal of out-of-band noise. The outputs of low-pass filters 114 and 116 jointly comprise a complex baseband signal derived from the received signal. A switch 122 samples the complex baseband signal at a rate of 2400 samples/sec. The time at which these samples are taken by switch 122 is controlled by a timing signal output from a symbol timing block 126 on a line 124. This timing signal, which is determined as a function of the data and pilot symbols included in the received signal, controls the sampling of the complex baseband signal so as to synchronize with the occurrence of each data symbol. Samples of the complex baseband signal are conveyed from symbol timing block 126 on a line 128 to a differential pilot symbol decode and synchronization block 132, which derives a synchronization signal that is output on a line 134. Differential pilot symbol decode and synchronization block 132 searches for the start (or end) of the 63 pilot symbols comprising the repeating pseudo-random pilot symbol sequence to determine the timing of the synchronization signal for each frame of data symbols. This synchronization signal is conveyed on line 134 into a pilot symbol processing block 136, which controls the majority of the demodulator synchronization functions. It should be noted that in the claims following this description, the term "symbol synchronization means" is intended to include the functions of symbol timing block 126, differential pilot symbol decode and synchronization block 132, and pilot symbol processing block 136. The samples of the complex baseband signal are input on line 128 to a pre- gain block 130, which includes an automatic gain control. The automatic gain control ensures that the sampled signal has an amplitude within a desired range. Gain of the automatic gain control in pre-gain block 130 is set in response to a feedback signal that is input on a line 160 from pilot symbol processing block 136. The amplified signal from block 130 is input to a multiplier 140, which multiplies the complex baseband signal by a function e"J (2πf o t+ Φ ) , where f 0 corresponds to a frequency offset correction determined by pilot symbol processing block 136, t is time, and φ corresponds to a phase tracking correction of the complex baseband signal. The frequency offset correction is used to correct for frequency offsets of up to 7 Hz in the complex baseband signal derived from the received signal. If the frequency offset correction were not applied, errors in the demodulation process would occur. Pilot symbol processing block 136 determines the frequency offset correction based upon the rate of phase rotation of the pilot symbols contained in the complex baseband signal. Phase corrected samples of the complex baseband signal comprising a signal x(k) are input to an equalizer in a block 142, which equalizes the signal to correct for degradation that may have occurred during its transmission from the modulator. The equalizing function implemented in this block is as follows, where x(k) ' is the equalized signal. iV-I The coefficients cj- in the above equation correspond to equalizer tap coefficients, which are continually updated by pilot symbol processing block 136 and supplied to the equalizer in block 142 on lines 144. These coefficients are determined as a function of the received pilot symbols, i. e., based on a comparison of the received pilot symbols with the expected pilot symbols in the pre-defined sequence that was randomly produced and made available to both the modulator and demodulator. The equalized signal from block 142 is selectively applied either to a line 150 to select each pilot symbol in the signal or to a line 152 to select successive data symbols in the signal, using a logic switch 146, which is controlled by a select signal produced by pilot symbol processing block 136 and conveyed on a line 148. Since in the preferred embodiment, every 13 th symbol in the equalized output is a pilot symbol, pilot symbol processing block 136 simply uses a timing function controlled by the synchronization signal, to determine when the signal output from the equalizer in block 142 is a pilot symbol or a data symbol and then switches the output signal accordingly. Successive symbols are output from block 142 at a rate of 2400 symbols/second. Logic switch 146 couples to line 152 to pass 12 successive data symbols to a trellis decoder block 154, and then switches to line 150 to convey the 13th symbol, which is a pilot symbol, to pilot symbol processing block 136. The signal output from the equalizer in block 142 is switched between lines 150 and 152 in this manner until all data symbols in the received signal have been processed. The data symbols input to trellis decoder block 154 are decoded using a Viterbi decoder, and the received pilot symbols are used to determine the equalizer tap coefficients c j -, to adjust the gain and phase of the complex baseband signal, and to correct the frequency offset, as described above. Because of the use of pilot symbols in this manner, data produced by trellis decoder 154 and input over a line 156 to an output data buffer block 162, are much less affected by channel noise and other sources of signal degradation than data transmitted between conventional modems. Pilot symbol processing block 136 determines frame synchronization by controlling a switch 166. When switch 166 is closed, a frame of output data that is stored in output data buffer 162 is supplied to an output block 167. Output block 167 provides for data frame processing using forward error correction (FEC), as indicated in a block 169. The FEC applied to the data frame employs a conventional cyclic- redundancy check (CRC) algorithm and a Reed Solomon decoder to identify and correct errors in the data. The decoded and corrected data are finally output on a line 168. If a temporary line outage (or interruption) occurs in the middle of a transmission between modulator 26 and demodulator 30, for example, due to a very long noise burst or a momentary line disconnect that causes the demodulator to lose synchronization, then the demodulator will immediately resynchronize when the channel returns to normal operation. This automatic resynchronization capability is not provided in the V.32 Standard (or equivalent telephone line modem schemes that do not use pilot symbols), because such schemes require the sending modulator to stop transmission of data and then to retransmit a synchronization sequence before data transmission can again resume. In comparison, a communication system in which demodulator 30 is used is much more reliable and can recover from significant line outages much more quickly. A block diagram of demodulator 30 in FIGURE 9 illustrates the relative simplicity of its hardware components, which include an analog-to-digital converter (ADC) 382 to convert (sample) the received analog or audio signal input on a line 380 into the digital signal at the 9600 Hz rate, for input on a line 384 into a DSP 386. ADC 382 corresponds to switch 104 in FIGURE 3. As was the case in the modulator, most of the functions illustrated in FIGURE 3 are implemented by DSP 382, in accordance with programmed instructions. Since both the modulation and demodulation of data are carried out by DSPs programmed to provide the desired functions, it is appropriate at this time to discuss the logic employed in each of the devices. Turning now to FIGURE 4, a flow chart 170 illustrates the steps carried out by the modulator in encoding data for transmission to the demodulator. Beginning with a block 172, the routine starts by transferring output data that have been mapped to data symbols with the periodically interspersed pilot symbols to DAC 374 at a rate four times the symbol rate. Thus, 9600 times a second, an interrupt routine causes the modulated data to be input to the DAC, as illustrated in a block 174. At each clock cycle, an interrupt number is incremented, as shown in a block 176. Thereafter, a decision block 178 determines if (interrupt number MOD 4) equals zero, i.e., whether the interrupt number is an integer multiple of four, since the symbol rate (2400 symbols per second) is equal to 1/4 the sample rate. In other words, the output signal is sampled four times for each symbol transmitted. An affirmative response to decision block 178 leads to a block 180 in which an input is obtained for the p(t) filter function applied for both the I and Q channels using data output from the symbol mapper in block 62 (shown in FIGURE 2). If four samples of the output signal have not yet been taken to complete the current symbol, a negative response to decision block 178 leads to a block 182, which indicates that the p(t) filter input is equal to zero. From both blocks 180 and 182, the logic proceeds to a block 184, in which the p(t) filter function is applied to both the I and Q channels. Thereafter, in a block 186, the functions sin(2πf c t) and cos(2πf c t) are updated for the current time t. In a block 188, the I channel output of the filter is multiplied by the updated cosine function, and the Q channel output of the filter is multiplied by the updated sine function. The resulting two products are summed to provide the transmitted signal s(t), which is stored (in a buffer that is not shown), as provided in a block 190, for transmission at the start of the next interrupt. The sample interrupt routine is then terminated in a block 192. In FIGURE 5, the steps carried out by the demodulator are illustrated in a flow chart 200, beginning at a block 202, wherein the received signal r(t) is sampled 9600 times/second. The resulting digital sample r(t) provided by the ADC is read in a block 204. In a block 206, values for I and Q, the complex baseband components are determined by respectively multiplying the samples of the received signal by cosine and sine functions of (2πf c t). The two components of the complex baseband signal are then low-pass filtered, as shown in a block 208, using pulse shaping filter function p(t). The filtered I and Q components are stored in a buffer, as shown in a block 210. In a block 212, a variable referred to as SA-VlPLEj lJMBER_is set equal to (SAMPLE_NUMBER+1) MOD 4. The purpose of this step is to ensure that at every fourth sample, a routine is called for processing the demodulated symbol. Thus, in a decision block 214, a determination is made as to whether SAMPLE_NUMBER is equal to zero and if so, in a block 216, a PROCESS DEMOD SYMBOL routine is called. Details of this subroutine are explained below in connection with FIGURE 7. Alternatively, if the new value of SAMPLE_NUMBER is not equal to zero, a decision block 222 determines if the value of SAMPLE_NUMBER is one and if so, calls an ADJUST SYMBOL TIMING routine in a block 220. Details of the ADJUST SYMBOL TIMING routine are discussed below with regard to FIGURE 6. After returning from either the PROCESS DEMOD SYMBOL routine or the ADJUST SYMBOL TIMING routine, or if the result of decision block 222 is negative, the program logic proceeds to a block 218, which returns from the interrupt initiated in block 202. A flow chart 270 illustrated in FIGURE 7 discloses the steps implemented in carrying out the PROCESS DEMOD SYMBOL routine, which starts in a block 272. In the first step shown in a block 274, a received symbol s(n) is interpolated and a fractionally spaced symbol s(n - T/2) is determined. This step is required because the received signal r(t) is sampled at 9600 times/second, but symbols likely start at times intermediate the points in time at which the received signal is sampled (and are thus referred to as fractionally spaced symbols). In this interpolation process, the received symbol s(n) is interpolated between the current sample and the sample immediately preceding the current sample. The fractionally spaced symbol is therefore the symbol that was received two samples prior to the current sample. In a block 276, a SYMBOL TIMING ERROR is determined, based upon the difference in time at which the current symbol was initially sampled and the beginning of that sample. Clearly, the SYMBOL TIMING ERROR is equal to zero only if switch 122 was closed concurrently with the beginning of the symbol instead of at some time before or after the beginning of the symbol. The difference in time between the beginning of the symbol and the time that the switch closed corresponds to the SYMBOL TIMING ERROR. This value is used in the ADJUST SYMBOL TIMING ROUTINE of FIGURE 6. A block 278 labeled DIFFERENTIAL PILOT SYMBOL DECODE AND UPDATE PILOT SYMBOL CORRELATION FOR SYNCHRONIZATION refers to the function of determining the phase difference between successive pilot symbols and as a function of the pilot symbols thus decoded, updating the synchronization signal (i.e., carrying out the previously described function of differential pilot symbol decode and synchronization block 132 in FIGURE 3). The pilot symbol phase difference is expressed as either a binary 1 or 0, depending upon whether the phase difference is 180° or 0°, respectively. Since the pilot symbols transmitted are in a pre-defined pseudo-random sequence, a corresponding sequence of decoded pilot symbol phase differences (expressed as binary values) is expected in the complex baseband signal derived from the received signal. The correlation value that identifies the end of a sequence of the pilot symbols (and thus the start of the next sequence), which is used as a basis for determining the synchronization, is identified using a comb filter and is a function of the comparison of the differential pilot symbol phase values that are decoded and the expected differential pilot symbol phase values that correspond to the pre-defined sequence of pseudo-random pilot symbols. In a decision block 280, a determination is made as to whether the demodulator is synchronized with the modulator. The demodulator is considered to be synchronized if the correlation value is greater than about 60% of its maximum value. However, if this value is less than 60% of its maximum value, the demodulator is consider unsynchronized, and the logic proceeds to a return block 282. Once nominal synchronization is obtained (i.e., within the acceptable range), the logic proceeds to a decision block 284, which determines if the current symbol is a pilot symbol or a data symbol, based upon timing relative to the synchronization signal. As noted above, every 13th symbol is a pilot symbol and the intervening 12 symbols are data symbols. At a block 285, the logic determines if the current pilot symbol is the last pilot symbol of the pre-defined repeating sequence of pilot symbols, based on the correlation of the phase differences with the expected phase differences, and if so, proceeds to a block 287, which enables output of the buffered data frame that is stored by output data buffer 162 (shown in FIGURE 3). At a block 286, the equalizer input signal is updated with the pilot symbol. Thereafter, a block 288 determines the equalizer output in accordance with the equation described above in connection with block 142 (in FIGURE 3). In a block 290, the equalizer tap coefficients c j - are adjusted by comparing the expected pilot symbol to the equalized received pilot symbol provided by the equalizer. In other words, if the pilot symbol output from the equalizer does not match the expected pilot symbol in the pre-defined sequence, the equalizer tap coefficients are adjusted to provide the expected pilot symbol. Frequency offsets and phase errors in the received signal are compensated by adjusting the demodulator's frequency offset tracking and phase error in a block 292. This step corresponds to a determination of the value f 0 that provides the required frequency offset correction of the signal input on a line 138 to multiplier 140 in FIGURE 3. Finally, in a block 294, phase tracking is adjusted in response to the phase error φ, which is determined by the phase difference between the received and expected pilot symbol from block 136 (in FIGURE 3). The logic then continues on the demodulator flow diagram illustrated in FIGURE 5. If a data symbol is currently being processed after synchronization is achieved between the demodulator and modulator, the flow of logic proceeds from decision block 284 to a block 296, in which the equalizer input is updated. Thereafter, the equalizer output is determined in a block 298 using the current equalizer tap coefficients in the summation of each of the samples defining the data symbol. The resulting equalized data symbol is then input to the trellis decoder, as indicated in a block 300 in flow chart 270, for trellis decoding of the data symbol, using a conventional Viterbi decoding algorithm. Since the encoder module configuration used for encoding the data transmitted is known, the shortest path through the trellis diagram from a current state (i.e., the encoded symbol) back to the original state of the data encoded has the highest probability associated with it. Accordingly, that path through the Trellis determines the data most likely transmitted. Thus, the data bits mentioned in a block 302 in flow chart 270 correspond with a high probability to the data originally transmitted. After the output data are stored in a buffer in block 302, the logic returns at block 282, as noted above. The ADJUST SYMBOL TIMING routine that is called in block 220 (FIGURE 5) is illustrated in a flow chart 230 in FIGURE 6, starting at a block 232. The first step of this routine in a block 234 is to set a variable sym error equal to the value of SYMBOL TIMING ERROR that was determined in block 276 in FIGURE 7. Next, in a block 236, a variable Ipfjsymjerror, corresponding to the low- pass filter symbol error is set equal to the filtered sym error, i.e., the value sym_error is low-pass filtered in symbol timing block 126 to obtain a running average of the sym_error variable. A decision block 238 determines if an absolute value of the variable lpf_sym_error is greater than a threshold value. In the preferred embodiment, the threshold is proportional to an average of |r(t)| 2 . If this condition is not met, the logic proceeds to a return block 240. Conversely, if the condition is met, the value Ipfjsymjerror is set equal to its previous value divided by two in a block 242; this step effectively resets the running average to a substantially lower value, so that the next time a determination is made in decision block 238, the logic will likely proceed to return block 240, because the average error is less than the threshold value. Next, a decision block 244 determines if the value of Ipfsym error is greater than zero, i.e., if this value is positive and, if so, a block 246 retards the symbol time by delaying the sampling by switch 122 (shown in FIGURE 3). Conversely, if the value of Ipfjsymjerror is negative, a block 248 advances the symbol timing by advancing the sampling by switch 122. Methods for delaying or advancing the sampling of a signal are well known to those of ordinary skill in this art and need not be further discussed. Morse code can periodically be transmitted by the modulator site for identification purposes, using the two predefined sequences of data symbols that are selected specifically to represent the dots and dashes of the Morse code by virtue of their audible characteristics resulting from the phase rotation between successive data symbols in the sequences. However, the present invention maintains synchronization of the demodulator during such transmissions, and any data symbols representing data (not Morse code) that are transmitted after the transmission of Morse code ceases can be immediately demodulated, because the demodulator uses the pilot symbols that are transmitted with the data symbols representing the Morse code to stay in synchronization. Although the disclosed preferred embodiment of the demodulator process a signal received as a passband signal by converting the passband signal to a complex baseband signal, it is also contemplated that many of the same benefits can be achieved by processing the passband signal directly, without such conversion. To directly process the received signal in the passband instead of the complex baseband, low-pass filters 114 and 116 must be changed to passband filters, and the equalizer shown in block 142 must be a passband equalizer instead of a complex baseband equalizer. These and other changes necessary to process the received signal in the passband so as to realize the benefits of the present invention discussed above in connection with the preferred embodiment will be apparent to one of ordinary skill in the art. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. Apparatus for demodulating an input signal, said input signal including data symbols used to encode data, and pilot symbols, said apparatus comprising: (a) sample means, coupled to receive the input signal, periodically sampling it to produce a corresponding digital sampled signal; (b) quadrature conversion means, for converting the digital sampled signal into a complex baseband signal; (c) symbol synchronization means, for processing the complex baseband signal to determine symbol timing for the data symbols in the complex baseband signal as a function of the pilot symbols and data symbols contained therein, producing a synchronized signal in which the data symbols are properly aligned in synchronization with their corresponding symbol times; (d) means for separating the pilot symbols from the data symbols in the synchronized signal, producing a data symbol signal; and (e) decoding means for decoding the data symbol signal, and recovering the data corresponding to the data symbols contained therein. 2. The apparatus of Claim 1, wherein the pilot symbols comprise a pre-defined repeating pattern of a pre-defined number of pilot symbols interspersed at pre-defined intervals with the data symbols, said symbol synchronization means comprising means for determining phase relationships for successive pilot symbols in the complex baseband signal, phase relationships between successive pilot symbols determining decoded binary values for the pilot symbols to define a sequence of pilot symbols in the complex baseband signal. 3. The apparatus of Claim 2, further comprising means for comparing the sequence of pilot symbols defined with said pre-defined repeating pattern of pilot symbols, to determine a start time for the sequence of pilot symbols in the complex baseband signal and to determine equalizer coefficients. 4 The apparatus of Claim 3, further comprising equalizer means, coupled to the symbol synchronization means to receive the synchronized signal and to the means for comparing to receive the equalizer coefficients, for equalizing the synchronized signal as a fiinction of the equalizer coefficients and compensating thereby for any degradation of the input signal that would inhibit decoding the data symbols to recover the data. 5. The apparatus of Claim 1, wherein the decoding means comprise a trellis decoder. 6. The apparatus of Claim 1, wherein said symbol synchronization means further comprise means for sampling the complex baseband signal at times that synchronize the data symbols with their corresponding symbol times. 7. The apparatus of Claim 1, wherein the quadrature conversion means include an anti-aliasing filter. 8. The apparatus of Claim 1, wherein the symbol synchronization means include means for maintaining synchronization when Morse code replaces the data symbols. 9. A method for demodulating an input signal, said input signal including data symbols used to encode data, and pilot symbols, said method comprising the steps of: (a) periodically sampling the input signal to produce a corresponding digital sampled signal; (b) converting the digital sampled signal into a complex baseband signal; (c) processing the complex baseband signal to determine symbol times for the data symbols in the complex baseband signal as a function of the data symbols and pilot symbols contained therein, producing a synchronized signal in which the data symbols are properly aligned in synchronization with their corresponding symbol times; (d) separating the pilot symbols from the data symbols in the synchronized signal, producing a data symbol signal; and (e) decoding the data symbol signal to recover the data corresponding to the data symbols contained therein. 10. The method of Claim 9, wherein the pilot symbols comprise a repeating pattern of a pre-defined number of pilot symbols periodically interspersed with the data symbols, said step of processing including the steps of determining differential phase relationships for successive pilot symbols in the complex baseband signal, a phase shift between the successive pilot symbols in the complex baseband signal determining a binary value for the pilot symbols that defines a sequence of pilot symbols in the complex baseband signal. 11. The method of Claim 10, further comprising the step of comparing the sequence of pilot symbols from the synchronized signal with the repeating pattern of pilot symbols that is expected to determine a start of the sequence of pilot symbols and a plurality of equalizer coefficients. 12. The method of Claim 11, further comprising the step of equalizing the synchronized signal as a function of the equalizer coefficients to compensate for any degradation of the input signal that would inhibit decoding the data symbols to recover the data. 13. The method of Claim 10, wherein the step of processing further comprises the step of sampling the complex baseband signal at times that synchronize the data symbols with their corresponding symbol times. 14. The method of Claim 9, further comprising the step of maintaining synchronization when Morse code replaces the data symbols. 15. A method for demodulating an input signal, said input signal including data symbols used to encode data, and pilot symbols, said method comprising the steps of: (a) sampling the input signal at a first pre-defined sample rate, producing a sampled signal; (b) converting the sampled signal into a complex baseband signal; (c) sampling the complex baseband signal at second pre-defined rate that is synchronized to a symbol timing signal, producing a synchronized complex baseband signal, said symbol timing signal being determined by: (i) determining symbol timing error for the sampling of a symbol; and (ii) producing the symbol timing signal at a time that minimizes the symbol timing error; (d) equalizing the synchronized complex baseband signal, producing an equalized signal; (e) separating the pilot symbols from the equalized signal; (f) as a function of the pilot symbols separated from the equalized signal, updating equalizer coefficients used for equalizing the synchronized complex baseband signal so that errors in the data symbols contained in the equalized signal are reduced; and (g) decoding the data symbols in the equalized signal to produce a data signal. 16. The method of Claim 15, further comprising the step of correcting any errors in the data signal to produce corrected output data. 17. The method of Claim 16, wherein the step of decoding uses a trellis decoder and the data signal comprises a trellis decoder output signal, and the step of correcting errors comprises the steps of establishing data frames from the data signal using the pilot symbols; and then detecting and correcting any errors in the data frames. 18. The method of Claim 15, wherein the step of converting the sampled signal into a complex baseband signal comprises the steps of down converting the sampled signal to obtain quadrature components; and then filtering the quadrature components with pulse shaping filters to obtain the complex baseband signal. 19. The method of Claim 15, further comprising the step of modifying an amplitude of the synchronized complex baseband signal by a gain determined as a function of the pilot symbols. 20. The method of Claim 15, wherein the step of updating equalizer coefficients comprises the step of comparing the pilot symbols contained in the equalized signal with an expected sequence of pilot symbols to determine the updated equalizer coefficients. 21. The method of Claim 15, further comprising the step of adjusting a phase of the synchronized complex baseband signal as a function of a sequence of pilot symbol phases. 22. Apparatus for demodulating an input signal that includes data symbols used to encode data, and pilot symbols, said demodulator comprising: (a) means, coupled to receive the input signal, for sampling the input signal and converting it into a corresponding digital sampled signal; (b) a quadrature converter, coupled to receive the digital sampled signal, said quadrature converter producing a complex baseband signal corresponding to the digital sampled signal; (c) symbol synchronization means, for processing the complex baseband signal as a function of the pilot symbols and the data symbols contained within the complex baseband signal, to produce a synchronized signal in which the data symbols are synchronized with their corresponding symbol times; (d) equalizer means, coupled to the symbol synchronization means to receive the synchronized signal, for equalizing the synchronized signal to produce an equalized signal in which degradation of the input signal that would impair decoding the data symbols is compensated; (e) means coupled to the equalizer means to receive the equalized signal, for separating the pilot symbols from the data symbols in the equalized signal, and for determining updated equalizer coefficients that are supplied to the equalizer means to reduce errors in the data symbols contained in the equalized signal, by comparing the pilot symbols in the equalized signal with an expected pre-defined sequence of pilot symbols; and (f) decoding means, coupled to the means to receive the equalized symbol, for decoding the data symbols in the equalized signal to recover the data encoded at the modulator. 23. The apparatus of Claim 22, further comprising means coupled to the signal synchronization means and to the means for separating, for determining a phase rotation of the pilot symbols in the equalized signal; and means for determining a frequency offset correction for the synchronized signal, as a function of the phase rotation. 24. Apparatus for demodulating an input signal that includes data symbols used to encode data, and pilot symbols, said demodulator comprising: (a) means, coupled to receive the input signal, for sampling the input signal and converting it into a corresponding digital sampled signal; (b) a quadrature converter, coupled to receive the digital sample signal, said quadrature converter producing a complex baseband signal corresponding to the digital sample signal; (c) synchronization means, for processing the complex baseband signal to determine a symbol timing error and in response to the symbol timing error, adjusting a symbol sampling time to produce a synchronized signal in which the data symbols are synchronized with their corresponding symbol times; (d) means coupled to the synchronization means to receive the synchronized signal, for: (i) separating the pilot symbols from the data symbols; (ii) comparing the pilot symbols separated to an expected sequence of pilot symbols to determine each time that the sequence of pilot symbols starts to repeat; and (iii) based upon a result of the comparison, determining a frame synchronization; (e) decoding means, coupled to receive the data symbols from the synchronized signal, for decoding the data symbols to recover data; (f) storage means, coupled to the decoding means to receive the data, for storing the data until the frame synchronization enables its output as a data frame; and (g) error correcting means, coupled to the storage means to receive the data frame, for correcting errors in the data frame, producing corrected output data. 25. Apparatus for demodulating a signal that conveys data using data symbols, said data symbols being interspersed with pilot symbols, and said signal at times conveying a Morse code by selectively using specific data symbols, said apparatus comprising: (a) sampling means for sampling the signal, producing a corresponding digital sample signal; (b) a quadrature converter, coupled to the sampling means to receive the digital sample signal, said quadrature converter producing a complex baseband signal corresponding to the digital sample signal; and (c) synchronization means, for processing the complex baseband signal to determine pilot symbols included therein and for producing a synchronized signal in which the data symbols are synchronized with their corresponding symbol times, wherein said pilot symbols are used by the synchronization means to prevent loss of synchronization of the signal being demodulated during the transmission of the Morse code. 26. The apparatus of Claim 25, wherein the pilot symbols comprise a pseudo-random sequence so that any audible sound caused by the pilot symbols does not audibly obscure the Morse code. 27. A method for demodulating an input signal that includes data symbols used to encode data, and pilot symbols, said method comprising the steps of: (a) sampling the input signal and converting it into corresponding digital samples; (b) producing a complex baseband signal corresponding to the digital samples; (c) processing the complex baseband signal as a function of the pilot symbols and the data symbols contained within the complex baseband signal, to produce a synchronized signal in which the data symbols are synchronized with their corresponding symbol times; (d) equalizing the synchronized signal to produce an equalized signal in which errors in the data symbols contained therein reduced; (e) separating the pilot symbols from the data symbols in the equalized signal; (f) determining updated equalizer coefficients by comparing the pilot symbols separated from the equalized signal with an expected pre-defined sequence of pilot symbols, said updated equalizer coefficients being used in the step of equalizing; and (g) decoding means, coupled to the means to receive the equalized signal, for decoding the data symbols contained in the equalized signal to recover the data encoded at the modulator. 28. The method of Claim 27, further comprising the steps of determining a phase rotation of the pilot symbols in the equalized signal; and determining a frequency offset correction for the synchronized signal, as a function of the phase rotation. 29. A method for demodulating an input signal that includes data symbols used to encode data, and pilot symbols, said method comprising the steps of: (a) sampling the input signal and converting it into corresponding a digital sampled signal; (b) producing a complex baseband signal corresponding to the digital sampled signal; (c) processing the complex baseband signal to determine a symbol timing error; (d) in response to the symbol timing error, adjusting a symbol sampling time to produce a synchronized signal in which the data symbols are synchronized with their corresponding symbol times; (e) processing the synchronized signal by: (i) separating the pilot symbols from the data symbols; (ii) comparing the pilot symbols separated with an expected sequence of pilot symbols to determine when the sequence of pilot symbols starts to repeat; and (iii) based upon a result of the comparison, determining a frame synchronization corresponding to the sequence of pilot symbols repeating; (f) decoding the data symbols to recover data; (g) storing the data until, as a function of the frame synchronization, output of the data as a data frame is enabled; and (h) correcting errors in the data frame, producing corrected output data. 30. A method for demodulating a signal that conveys data using data symbols, said data symbols being interspersed with pilot symbols and said signal at times conveying Morse code by selectively using specific data symbols, said method comprising the steps of: (a) sampling the signal producing a corresponding digital sampled signal; (b) producing a complex baseband signal corresponding to the digital sampled signal; (c) processing the complex baseband signal to determine pilot symbols included therein; and (d) producing a synchronized signal in which the data symbols are synchronized with their corresponding symbol times, said pilot symbols being used to prevent loss of synchronization of the signal being demodulated during the transmission of the Morse code. 31. The method of Claim 30, wherein the pilot symbols comprise a pseudo-random sequence so that any audible sound caused by the pilot symbols does not audibly obscure the Morse code. 32. A modulator for conveying data, comprising: (a) a data symbol source that converts the data into a plurality of data symbols; (b) a pilot symbol source that produces a plurality of different pilot symbols, said plurality of different pilot symbols comprising a predefined random sequence; (c) a switch that selects between the data symbol source and the pilot symbol source so that successive pilot symbols comprising the predefined random sequence are interspersed with the data symbols from the data symbol source, producing a pattern of the data symbols and interspersed pilot symbols; and (d) means, coupled to the switch, for producing a passband signal as a function of the pattern of the data symbols and the interspersed pilot symbols produced by the switch. 33. The modulator of Claim 32, wherein the plurality of pilot symbols comprising the predefined sequence are characterized by having a substantially white frequency domain spectrum and are sufficient in number such that an auto-correlation function of the predefined sequence has a well-defined peak. 34. The modulator of Claim 32, wherein the means for producing a passband signal comprise: (a) mapping means for producing a complex baseband signal corresponding to the pattern of data symbols and interspersed pilot symbols; (b) a plurality of low-pass filters, coupled to the mapping means, for low-pass filtering components of the complex baseband signal, producing corresponding filtered signals; (c) multipliers, coupled to the low-pass filters, for multiplying the filtered signals by quadrature functions, producing passband components; and (d) a summer, for adding the passband components, producing the passband signal. 35. A method for modulating data, comprising the steps of: (a) converting the data into a plurality of data symbols; (b) producing a plurality of different pilot symbols, said plurality of different pilot symbols comprising a predefined random sequence; (c) selecting between the data symbols and the pilot symbols so that successive pilot symbols comprising the predefined random sequence are interspersed with the data symbols, producing a pattern of the data symbols and interspersed pilot symbols; and (d) producing a passband signal as a function of the pattern of the data symbols and the interspersed pilot symbols. 36. The method of Claim 35, wherein the plurality of pilot symbols comprising the predefined sequence are characterized by having a substantially white frequency domain spectrum and are sufficient in number such that an auto-correlation function of the predefined sequence has a well-defined peak. 37. The method of Claim 35, wherein the step of producing the passband signal comprises the steps of: (a) producing a complex baseband signal corresponding to the pattern of data symbols and interspersed pilot symbols; (b) low-pass filtering components of the complex baseband signal, producing corresponding filtered signals; (c) multiplying the filtered signals by quadrature functions, producing passband components; and (d) adding the passband components, producing the passband signal. 38. A modulator for conveying Morse code as data symbols, comprising: (a) a source of Morse code, the Morse code using dots and dashes to represent characters; (b) a translator for producing a first sequence of selected data symbols to represent a dot of the Morse code and a second sequence of selected data symbols to represent a dash of the Morse code, said first sequence differing from the second sequence of data symbols; (c) a pilot symbol source that produces a plurality of different pilot symbols, said plurality of different pilot symbols comprising a predefined random sequence; (d) a switch that selects between the translator and the pilot symbol source so that successive pilot symbols comprising the predefined random sequence are interspersed with the first and the second sequences of selected data symbols from the translator, producing a pattern of the first and the second sequences of data symbols and interspersed pilot symbols; and (e) means, coupled to the switch, for producing a passband signal as a function of the pattern of the first and second sequences of data symbols and the interspersed pilot symbols produced by the switch, said first and second sequences of data symbols in the passband signal producing corresponding first and second audible tones that respectively represent the dots and dashes of the Morse code, when received, and said pilot symbols serving to maintain synchronization of the passband signal, when received. 39. A method for modulating Morse code so that the Morse code is conveyed as data symbols, the Morse code using dots and dashes to represent characters, said method comprising the steps of: (a) producing a first data symbol sequence to represent a dot of the Morse code and a second data symbol sequence to represent a dash of the Morse code, the first data symbol sequence being different from the second data symbol sequence; (b) producing a plurality of different pilot symbols, said plurality of different pilot symbols comprising a predefined random sequence; (c) selecting between the data symbol sequences and the pilot symbols so that successive pilot symbols comprising the predefined random sequence are interspersed with the first and the second data symbol sequences, producing a pattern of the first and the second data symbol sequences and interspersed pilot symbols; and (d) producing a passband signal as a function of the pattern of the data symbol sequences and the interspersed pilot symbols, said first and second data symbol sequences in the passband signal producing corresponding first and second audible tones that respectively represent the dots and dashes of the Morse code, when received, and said pilot symbols serving to maintain synchronization of the passband signal, when received.

Inventors: FAWCETT GLENN S, MARCHETTO ROBERT F, STEWART TODD A, FAWCETT, GLENN, S., MARCHETTO, ROBERT, F., STEWART, TODD, A.
Applicants: GLENAYRE ELECTRONICS INC, GLENAYRE ELECTRONICS, INC.
Application year: 1995
Main classification: H04B1
IPC codes: H04H20, H04L1, H04L7, H04L27