Abstract:
The signaling overhead required for pilot symbol aided modulation is significantly reduced by recognizing that the residual uncertainty in the carrier frequency decreases after an initial carrier frequency estimate is made during an initial signal interval. This allows a commensurate reduction in channel process sampling rate during the remainder of the message; i.e., the frequency uncertainty of the pilot symbols can be decreased. This technique may be particularly effective when the rate of change of the received carrier frequency and phase is low as in fixed satellite terminal equipment. The resultant increase in spectral efficiency makes reduced-overhead pilot symbol aided modulation attractive for applications in low-cost/low-complexity terminal equipment.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to modulation and demodulation in communications systems and, more particularly, to pilot symbol aided modulation techniques and associated receiver signal processing. 
     Pilot symbol aided modulation has been investigated for mobile communications applications since it facilitates estimation of the carrier signal, which is needed for coherent demodulation at the receiver. One technique is to periodically insert known pilot symbols (i.e., known symbols) into the sequence of modulated data symbols comprising the signal to be transmitted. These pilot symbols essentially sample the channel process. The receiver recovers the pilot symbols from the sequence of symbols received from the channel and uses a standard interpolation technique to reconstruct a coherent carrier for use in demodulation. 
     In the mobile application, pilot symbol aided modulation&#39;s main drawback is that it requires relatively high overhead because the carrier&#39;s frequency and phase can vary quite rapidly due to the Doppler effect. However, in a geosynchronous satellite communications system with stationary terminals, the apparent carrier frequency changes much more slowly. Therefore, although it is tempting to utilize pilot symbol aided modulation in order to simplify the terminal&#39;s receiver, the required overhead is still quite high when the channel process is sampled above the Nyquist rate based on the initial carrier frequency uncertainty. One cause of this carrier frequency uncertainty is the satellite&#39;s motion. Second, the use of low-cost oscillators in the system can result in unacceptable shorter-term frequency stability and significant longer-term frequency drift. Thus, while the receiver becomes less expensive to build, the high overhead required reduces system capacity and, therefore, nullifies any cost advantage in the system. 
     Accordingly, it is desirable to provide a pilot symbol aided modulation and demodulation signal processing which overcome the shortcomings described hereinabove. It is furthermore desirable to provide such modulation and demodulation techniques particularly as applicable to very small aperture satellite (VSAT) communications terminals. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with preferred embodiments of the invention described herein, variable-interval pilot symbol aided modulation and demodulation techniques significantly reduce the signaling overhead required for pilot symbol aided modulation. Variable-interval pilot symbol aided modulation involves partitioning a transmitted message burst into at least two segments, generally of unequal length, such that the interval between pilot symbols differs; and corresponding demodulation signal processing involves reducing the channel process sampling rate (i.e., reducing the frequency of pilot symbols) after an initial carrier frequency estimate is made during an initial signal interval. A receiver recovers all of the pilot symbols in the received burst&#39;s shorter pilot inter-symbol interval segment and reconstructs an estimate of the received signal&#39;s carrier therefrom, the shorter pilot inter-symbol interval segment of the received burst being demodulated such that the estimated carrier is derived as in standard pilot symbol aided modulation techniques. The receiver calculates an estimate of the received signal&#39;s carrier frequency and phase from the pilot symbols in the shorter pilot inter-symbol interval segment of the received burst. In the longer pilot inter-symbol interval segment of the burst, the received carrier process is sampled by the pilot symbols at a lower rate such that frequency-shifted pilot symbols are used to obtain a local carrier for demodulation of the shorter pilot inter-symbol interval segment of the received burst. Switching circuitry is used to alternate between demodulating the shorter and longer pilot inter-symbol interval segments of the burst. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will become apparent from the following detailed description of the invention when read with the accompanying drawings in which: 
     FIGS. 1-4 illustrate exemplary transmitted bursts useful in describing a receiver&#39;s synchronization and demodulation signal processing in accordance with preferred embodiments of the present invention as follows: FIG. 1 represents a basic burst structure for variable-interval pilot symbol aided modulation; FIG. 2 illustrates an exemplary burst structure for short messages; FIG. 3 illustrates an exemplary burst structure useful for minimizing delay in transmission and demodulation; and FIG. 4 represents a second exemplary burst structure for short messages; and 
     FIG. 5 is a block diagram illustrating a receiver in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Variable-interval pilot symbol aided modulation in accordance with preferred embodiments of the present invention is based on recognition of the concept that residual uncertainty in the carrier frequency decreases after an initial carrier frequency estimate is made during an initial signal interval, allowing a commensurate reduction in channel process sampling rate during the remainder of the message. FIGS. 1-4 illustrate several message burst structures that utilize this concept. As used herein, the term burst refers to the duration of a transmission, regardless of its length. That is, the term burst is to be construed broadly to include both long and short transmission intervals. And the transmitted bursts specifically described herein are provided by way of example only. 
     In FIGS. 1-4, the transmitted symbols labeled P denote pilot symbols, and those labeled D are data symbols. As shown in FIGS. 1-4, the message burst that is transmitted via variable-interval pilot symbol aided modulation is partitioned into at least two segments in which the interval between pilot symbols differs. The order of segments in FIGS. 1-4 is given with reference to the order of their occurrence in time which may or may not be different from their order of processing in the receiver. The order of processing in the receiver depends on system requirements, channel characteristics, and burst length. For example, for FIGS. 1 and 3, the first segment is processed first in the receiver; but in FIGS. 2 and 4, the middle (or second) segment is processed first in the receiver for purposes of synchronization. In these cases, the first segment of the received burst is buffered for later processing. Further, the interval between pilot symbols is designated in FIGS. 1-4 using the symbol T Pn , where n denotes the order of processing in the receiver. Note that T P1  falls in the first segment of the burst of FIG.  1  and in the middle segment of the burst of FIG.  2 . 
     In the following description, the burst structure of FIG. 1 is used as an example in describing the receiver&#39;s synchronization and demodulation signal processing. The transmitted burst of modulated symbols in FIG. 1 is segmented into two parts or segments, generally of unequal length, as illustrated. The shorter pilot inter-symbol interval segment is referred to as the T P1  segment; and the longer pilot inter-symbol interval segment is referred to as the T P2  segment. That is, T P1 &lt;T P2 . That is, the T P1  segment refers to the burst segment having the pilot symbols occurring most frequently (the shorter pilot inter-symbol interval segment). The shorter pilot inter-symbol interval segment is the burst segment that is processed first in the receiver. In FIG. 1, the shorter pilot inter-symbol interval segment (i.e., the T P1  segment) comprises the first transmitted segment. 
     A receiver  10  shown in the embodiment of FIG. 5 recovers all of the pilot symbols in the received burst in block  12  and reconstructs an estimate of the received signal&#39;s carrier from the pilot symbols comprising the shorter pilot inter-symbol interval segment (i.e., the T P1  segment) in block  14 . The T P1  segment of the received burst is demodulated with the carrier derived from this step as in standard pilot symbol aided modulation techniques, the complex conjugate of the estimate of the carrier over the T P1  segment being provided in block  16 . It should be noted that pilot symbols from T P1  segment of a burst are used advantageously in the recovery of the carrier for the T P2  segment. 
     In block  18 , the receiver calculates an estimate of the received signal&#39;s carrier frequency (and phase, if deemed desirable for the particular application) from the pilot symbols in the T P1  segment of the received burst. (This step can be done in parallel with the demodulation of the T P1  segment.) These carrier frequency and phase estimates are used in block  20  to generate an initial estimate of the received carrier during the T P2  segment, the complex conjugate of which is provided in block  22 . In particular, the output of block  20  is an estimate of the carrier over the T P2  segment based on the estimate of the carrier frequency that is derived from pilot symbols in the T P1  segment. 
     The pilot symbols for the T P2  segment of the burst, via selection block  24 , are multiplied in multiplier  26  by the appropriate samples of the complex conjugate of this estimated carrier in order to reduce the frequency uncertainty in the received carrier in the T P2  segment. Since the received carrier process is sampled (by the pilot symbols) at a lower rate in the T P2  segment of the burst, this multiplication step prevents aliasing during carrier signal reconstruction. These frequency shifted pilot symbols are then used to reconstruct the residual carrier in block  28  for the T P2  segment of the burst via interpolation. The complex conjugate of the residual carrier for the TP 2  segment of the burst is then provided in block  30 . The output of block  30  is multiplied in multiplier  32  by the initial carrier estimate from block  22  to obtain a local carrier for demodulation of the TP 2  segment of the received burst. 
     The sampled received burst is suitably delayed in delay block  34  to account for latency in the generation of the coherent reference for demodulation. The delayed received signal samples are multiplied in multiplier  36  by the complex conjugate of the carrier generated by the pilot-symbol-aided synchronization algorithm, via a switch  40 , and filtered in a filter block  42  to produce the demodulator&#39;s soft output data. Switch  40  is in Position  1  for the demodulating the T P1  segment of the burst and in Position  2  for the T P2  segment. 
     Adaptation of the pilot symbol aided carrier recovery method described hereinabove to other burst structures, such as those shown in FIGS. 2,  3  and  4 , is straightforward. For the burst structure of FIG. 3, carrier reconstruction for the first segment of the burst (as it occurs in time as shown in FIG. 3) is eliminated since it comprises only pilot symbols; i.e., only the second segment bears data. 
     For the burst structure of FIG. 2, the middle (second) segment of the burst is processed like that of the first segment in FIG. 1 because it is there that the pilot symbols occur with greater frequency. The interpolation which produces the carrier for demodulating the first and third burst segments utilizes the pilot symbols from both segments and may include one or more of the middle segment&#39;s pilot symbols in this case. Note that the interval between pilot symbols is the same in segments one and three and that the receiver signal processing for these is similar to that for the second segment in FIG. 1. A variation of the method for bursts of the type of FIG. 2 is to recover the carrier for demodulation of the first and third segments in two distinct steps. The first utilizes the pilot symbols from the middle burst segment to the beginning of the burst. The second utilizes the pilot symbols from the middle segment to the end of the burst. 
     Pilot symbol aided carrier recovery for bursts of the type shown in FIG. 4 combines the modifications summarized for the cases illustrated in FIGS. 2 and 3. Namely, carrier reconstruction for the middle segment of the burst is not necessary since it comprises only pilot symbols. The two variations for first-and-third-segment carrier recovery described above for the burst of FIG. 2 apply directly. 
     Exemplary modulation schemes for which the techniques described herein are suitable are binary phase shift keying and quadrature phase shift keying. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.