Patent Application
20050047496
1. Field of the Invention
This invention relates to a construction of a modem employing an inserted pilot symbol(s) for improved synchronization and equalization of demodulation circuitry within the modem and, more particularly, to the insertion of pilot symbols at a location within the transmitted symbol stream permitting use of the pilot symbols independently of the type of modulation applied to payload data.
2. Brief Description of Related Developments
Communication systems are widely used in many situations including communication between persons, as in cellular telephony, and between various forms of equipment, such as between a satellite and a ground station. Various data formats and protocols have evolved to facilitate communication in differing situations. Communication may involve multiple access technologies such as CDMA (code division multiple access), TDMA (time division multiple access), FDMA (frequency division multiple access), modulation technologies such as PSK (phase shift keying), QAM (quadrature amplitude modulation), and FEC (forward error correction) such as Reed Solomon coding, convolutional encoding, and turbo coding, by way of example. Detection of such signals may require a highly accurate time base for observation of relatively small differences in phase of a carrier signal, phase of the symbol and phase of the multiplexer frame. Furthermore, the time base employed in a receiver of a signal must be the same as the time base (synchronous) employed in a transmitter of the signal in order to enable successful operation of receiving processes (demodulation operation), such as matched filtering, by way of example. In the case of a communication system employing a modem at each end of a communication link, such as a link connecting two computers for enabling communication between the two computers, it is necessary to include within each of the modems circuitry for transmitting synchronization or time-frame signals and circuitry for recognizing received synchronization or time-frame signals.
A further consideration in the design of a communication system is the capacity of the system to reacquire phase synchronization, symbol timing and frame timing in the event of a momentary loss of transmitted signal as might occur if an obstacle, which can block transmission, momentarily passes across the communication link. It is of considerable advantage to provide for a rapid reacquisition so as to minimize any interruption in the communicated data. As a further consideration, a communication system may include a specific form of pilot symbol with the transmitted signal as an aide to acquisition of the aforementioned phase synchronization, symbol timing and frame timing. Additionally the communication system may have the feature of adaptive modulation wherein the modulation of the data symbols may be altered from time to time. This would present a problem if the data modulation is employed also for modulation of pilot symbols because the receiver might not be able to locate the pilot symbols due to the changing modulation. Therefore, it is advantageous to provide for pilot symbols which are constant and independent of modulation employed for the data symbols so that the receiver can track the pilot symbols independently of changes in the data modulation.
In order to obtain accurate reception of data transmitted by a communication system, presently available communication equipment may employ elaborate circuitry in a receiver of the communication system to regenerate a time frame employed in a transmitter of the communication system. In some cases, a synchronization pulse may be transmitted along with the data to serve as a time base for reception of the data. This presents a problem in the case of a noisy communication link because the precise location, in time, of the synchronization pulse may be difficult to ascertain with a resulting degradation in the quality of reception of the data.
The aforementioned problem is overcome and other advantages are provided by transmission of a sequence of pilot symbols, or a pilot word, along a communication link to accompany the transmission of data along the communication link, in accordance with the invention, to establish an accurate and precise time base to which the receiving circuitry can synchronize the receiver timing. Thereby, the receiving equipment can be precisely synchronized with the sequence of bits which constitutes the received data. The invention can also be implemented in the construction of modems, as may be employed at the terminals of the communication link. In such modem, the pilot symbols are inserted into the transmission (modulation) section for combining with data symbols, and are employed in the reception (demodulation) section of the modem as a reference for detection of the data symbols.
The disclosed embodiments enable use of the pilot symbols in a communication system in a manner which is compatible with operation of the communication system with any one of a plurality of modulation protocols and formats, such as those noted above. This is accomplished, in accordance with a feature of the invention, by including in the transmitter a location for insertion of the pilot symbols at a point subsequent to the conclusion of the specific form of modulation or formatting, such as subsequently to the implementation of a QAM or a PSK, by way of example.
In a typical construction of the transmission section of a communication system, digitized data, obtained from a source of data, is encoded by a suitable forward error correction code and then applied to a constellation mapper that provides any one of well known mappings for modulations such as BPSK, QPSK, QAM and FSK, by way of example. The data source, by way of example, may be a computer outputting digitized data, or speech-processing circuitry that converts voice into a digitized signal. A sequence of pilot symbols constituting a pilot word is stored in a memory of the transmission section, and the pilot word is also outputted to a constellation mapper providing one of the foregoing modulations. The modulated data and the modulated pilot symbols are multiplexed together to provide output sequences of pilot symbols interposed among sequences of data symbols. Thus, the multiplexing takes place after the pilot symbols and the data symbols are separately modulated. Therefore, an interleaving of sequences of pilot symbols with sequences of data symbols does not interfere with the formatting and modulation of the data signals. Thus, the present invention can be employed without derogating from the usual performance of the communication system.
At the receiver, the sequence of the pilot symbols is detected, as by use of a matched filter or a correlation process employing a replica of the sequence of the pilot symbols. In the practice of the invention, the length of the sequence of pilot symbols affects acquisition time such that a longer sequence provides for a faster determination of carrier phase, symbol timing and block or frame timing. The resulting estimates of carrier phase, symbol timing and block or frame timing are less noisy and more reliable. An increase in the length of the sequence of pilot symbols may detract from the amount of space available for sequences of data symbols so that, in practice, a trade-off may be necessary. In particular, the timing accuracy provided by the sequence of pilot symbols is sufficient for determining unambiguous carrier phase, symbol timing, block timing, and other parameters of the data transmission. The pilot symbol may also include codes or information useful for resetting a receiver, if desired. By way of example, this may include informing a receiver of switching data rate or modulation type. Thus, the invention rapidly and reliably establishes a time base with carrier phase and timing synchronization useful for equalization filter training, modem configuration switching, and turbo code synchronization, by way of example.
The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein:
Identically labeled elements appearing in different ones of the figures refer to the same element but may not be referenced in the description for all figures.
With reference to
In accordance with a feature of the invention, a sequence of pilot symbols is transmitted along with the sequence of data symbols via the communication link 22 to establish a time base, as well as provision for carrier estimation and equalizer training, for improved accuracy and fast acquisition in the operation of the demodulation and decoding circuitry 36. At the transmit side 26, pilot symbols are inserted into a train of formatted data symbols by insertion circuitry 40 located between the formatting circuitry 32 and the transmitter 24. The insertion circuitry 40 operates to interleave sequences of pilot symbols among sequences of data symbols as is shown in
Operation of the detector 42 is based on matched filtering of the received signal or of correlation of the received signal against a reference sequence of the pilot symbols or, alternatively, other pattern recognition techniques using absolute difference or MSE. Successful correlation requires that the reference sequence employed in the correlation operation of the detector 42 be the same as the reference sequence employed by the insertion circuitry 40. This is indicated diagrammatically by providing the sequence of pilot symbols from a common reference 46. In practice, this means communicating the reference sequence of pilot symbols, which is employed at the insertion circuitry 40, to the receive side 30 of the communication system 20 prior to a communication of data via the link 22. Alternatively, the reference sequences of pilot symbols can be stored in memories, indicated in phantom at 46A and 46B, wherein the reference sequences of pilot symbols are provided during the construction of the system 20.
The description of the communication system 20 in
With reference to
A sequence of pilot symbols constituting a pilot word is stored in the memory 62 and is outputted via the mapper 58 to the multiplexer 60. The mapper 58 is operative in the same manner as the mapper 56 to provide any of a plurality of modulations. The choice of pilot symbol mapping for the mapper 58 is independent of the type of mapping chosen for the data at the mapper 56. A timing unit 68 provides timing signals for synchronizing operations of the data source 66 with the memory 62 and the multiplexer 60, the timing signals including a data clock applied to the data source 66 and a frame timing applied to the multiplexer 60. The encoder 54, the mapper 56, the mapper 58, and the pilot-word memory 62 include respective terminals 70, 72, 74 and 76 by which, respectively, the encoder 54 is enabled to select one of a plurality of codes, the mapper 56 is enabled to select one of a plurality of modulations, the mapper 58 is enabled to select one of a plurality of modulations, and the memory 62 and is enabled to select one of a plurality of previously stored pilot words. In the operation of the modem 48, the timing unit 68 strobes alternately the data source 66 and the pilot word memory 62 to output from the multiplexer 60 the alternating sequence of data symbols and pilot symbols, shown in
Also included in the modem 48 is an up-conversion unit 78 comprising a numerically controlled oscillator (NCO) 80, a complex multiplier 82, a digital-to-analog converter 84, and a filter 86. The up-conversion unit 78 is operative to translate the signal outputted via the multiplexer 60 up to an RF (radio frequency) signal to be outputted by the modulation section 50 of the modem 48. In the operation of the up-conversion unit 78, the oscillator 80 outputs a signal at a predesignated frequency to the multiplier 82. The multiplier 82 multiplies the in-phase and quadrature (I and Q) components of the symbols outputted via the multiplexer by the signal outputted via the oscillator 80 to produce the digital equivalent of the RF output signal. The digitized signal at the output of the multiplier 82 is then converted to an analog signal by the converter 84 and filtered by the bandpass filter 86 to produce a sinusoidal waveform with modulation corresponding to the modulation imparted by the constellation mappers 56 and 58, and with a carrier frequency corresponding to the frequency of the oscillator 80.
In the demodulation section 52, the modem 48 has a down-conversion subsystem 88 with an analog-to-digital converter 90 which, upon receipt of an input RF signal, converts the input RF signal from analog format to digital format. The down-conversion subsystem 88 includes digital components, as will be described hereinafter, for outputting on line 92 a baseband digital signal having the general form of the signal produced by the multiplexer 60 and described in
Thus, in the operation of the demodulation section 52, the detector 94 is able to detect the presence of the pilot symbols on line 92. The time base circuitry 98 establishes a time base, based on the presence of the pilot symbols, for extraction of the data symbols from line 92. The reference sequence of pilot symbols to be employed by the detector 94 is to be the same as that employed in a distant modem communicating with the modem 48. By use of the time base, the inverse mapper 100 and the decoder 102 are able to demodulate and to decode the data symbols so as to recover the data and to output the data to the user of the modem.
Upon comparing the operation of the modem 48 in
The loop 120 is the carrier phase and frequency recovery loop. In the operation of the loop 120, a synchronization generator 122 outputs a signal for control of the delay of the delay unit 110, and outputs a further signal which serves as a reference signal for operation of the phase error detector 112. The delay of the unit 110 is adjusted to provide for alignment of the signal outputted by the oscillator 116 with the signal outputted by the converter 102. The delayed signal, outputted by the delay unit 110, is compared with the phase reference at the detector 112 which outputs a signal to the loop filter 114 indicating the error in phase or alignment of the two signals applied to the detector 112. The loop filter 114 applies the phase error to a control terminal of the oscillator 116 to adjust the frequency and phase of its output signal. The loop filter 114 operates in a well-known fashion to control the dynamic stability of the loop 120.
The output signal of the delay unit 110 is applied via an equalizer 124 to a mapper 126 and to the synchronization generator 122. The function of the equalizer 124 is to remove distortion in the signals received by the matched filter 108, this function being particularly useful in the case of received signals having the 16-QAM format. Upon receipt of the signal from the equalizer 124, the synchronization generator 122 is able to generate various timing signals, synchronized with the signal of the equalizer 124, the timing signals being indicative of carrier phase, symbol timing and frame timing.
The demodulation section 52 further comprises a time error detector 128, a further loop filter 130 and a phase-locked loop (PLL) 132. The signal outputted by the delay unit 110 is applied, along with a timing reference signal from the generator 122, as input signals to the detector 128. The detector 128 uses these two signals to compute a timing error, and outputs a signal via the loop filter 130 to the PLL 132 indicative of the time error between the signals of the delay unit 110 and the generator 122. The PLL 132 outputs a periodic waveform, such as a sine wave or a square wave, that serves as a clock signal for operation of the analog-to-digital converter 90. The detector 128, the filter 130 and the PLL 132 are part of a further loop 134 which functions as a timing synchronization loop. In the preferred embodiment of the invention, the pilot symbols have the same symbol rate as do the data symbols so that, upon a locking of the PLL 132 to the input signal at terminal 118, the strobing of the converter 102 is operative equally for recovery of both the data symbols and the pilot symbols.
The synchronization generator 122 is provided also with the pilot symbols reference, which may be provided by the pilot memory 96 of
Also, it is noted that certain portions of the equipment can be fabricated by FPGAs. Thus, all digital processing can be accomplished in the FPGA. The use of the FPGA is preferred in the construction of the invention because it enables one piece of equipment to be employed for handling any one of several possible formatting options. Alternatively, a DSP may be employed for a reduced throughput speed but increased programming capability. An ASIC may also be employed for maximum throughput speed in the situation wherein only a single format is anticipated, or also in any of a plurality of formats if the ASIC is constructed with the additional circuitry required for carrying forth the additional formats. In the cases of the FPGA and the DSP, optional coding and modulation may be provided for by including in memories of the FPGA and of the DSP instructions for the optional coding and modulation. By way of example, in
With reference to
It is to be understood that the above described embodiments of the invention are illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined by the appended claims.
