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A Muiti-Mode Modulation and Demodulation System and Method

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A Muiti-Mode Modulation and Demodulation System and Method United States Patent [191 [11] Patent Number: 4,726,069 Stevenson [45] Date of Patent: Feb. 16, 1988 [54] A MUITI-MODE MODULATION AND Primary Examiner-Benedict V. Safourek DEMODULATION SYSTEM AND METHOD Attorney, Agent, or Firm-David O'Reilly [76] Inventor: Carl R. Stevenson, 845 N. Woods [571 ABSTRACT Ave., Fullerton, Calif. 92632 An improved system and method for modulation, de- [21] Appl. No.: 6l2,092 modulation and signal processing for single sideband communications systems which provides correction for [22] Filed. May 18,l984 the adverse effects of rapid fading characteristics in a [51] Int. c1.4 ............................................... H04B 7/00 mobile environment. The system provides modulation [52] US. c1. ........................................ 455/46; 455/47; through a modified Weaver modulator in which the 455/71; 455/202; 375/50; 375/61; 375/77; audio input is processed to produce an output in the 375/97; 329/50 form of an upper sideband having a pilot tone in a spec- [58] Field of Search ...................... 332/44, 45; 375/43, tral gap at approximately midband. The receiver in- 375/50,97, 100, 102; 455/46,47, 71, 202, 203, cludes a modified Weaver demodulator and a correc- 305, 306, 312,234, 235,245,296; 329/50 tion signal generating circuit which processes the re- t561 References Cited ceived faded audio input and pilot tone to produce a U.S. PATENT DOCUMENTS correcting signal. The correcting signal is mixed with the received signal to regenerate unfaded versions of 3,271,681 9/1966 MCN& ................................. 455147 both the signal and pilot by removing random ampli- 3,271,682 9/1966 Bucher, Jr. ........................... 455/47 tude and phase modulations imposed on them by the 3,611,144 10/1971 Harmon, Jr. et al. ................455/46 3,792,364 2/1914 Adas ............................... 455/202 fading. 4,313,211 1/1982 Leland ................................ 455/235 4,328,590 511982 ~ee...................................... 455/203 21 Claims, 6 Drawing Figures USPatent Feb. 16,1988 Sheet 1 of 4 4,726,069 AUDIO PROCESSOR t- AUDIO INPUT Fig. L U.S. Patent Feb. 16,1988 Sheet 2 of 4 4,726,069 (a) BASEBAND AUDIO (b) 0 BASEBAND AUDIO WITH SPECTRAL GAP (d) 0 SIGNAL AT B AND 8' WITH UPPER SIDEBAND REMOVED (t) 0 SIGNAL AT E' WITH PILOT CARRIER I II I INSERTED IN UNFOLDED REVERSED PHASE LOWER SIDEBAND I I11 Le-& ,J L-/-'I UPPER SIDEBAND I F OUTPUT WITH (9) 0 nln PILOT CARRIER (h) 0 FADED UPPER SIDEBAND AND PILOT WC CARRIER AT RECEIVER Fig. 3. U.S. Patent Feb. 16,1988 Sheet 3 of 4 4,726,069 122 1121 S AUDIO OUTPUT M PU SIGNAL PROCESSOR S' o--- U.S. Patent Feb. 16,1988 Sheet 4 of 4 4,726,069 SIGNAL inn.-- I r58 I I 54 - 62 T I04 60 64 H I L BERT INPUT TRANSFORMER -I I I I I I. I I I I I I I I I I I I 4,726,069 1 2 (0 Weaver, D. K., Jr.: “A Third Method of Generation A MUITI-MODE MODULATION AND and Detection of Single-Sideband Signals”, Proc. DEMODULATION SYSTEM AND METHOD IRE, Vol. 44, pp. 1703-1705, December 1956. (g) Goatcher, J. K. et al.: “Noise Considerations in an This invention was made with government support, 5 Integrated Circuit VHF Radio Receiver”, Proc. under contract No. NAS 7-918, awarded by NASA. IERE Conference on Radio Receivers and Associ- The government has certain rights in this invention. ated Systems”, Leeds, England, July 1981, pp. 49-59. (h) Darlington, Sidney: “On Digital Single-Sideband FIELD OF THE INVENTION Modulators”, IEEE Transactions on Circuit Theory, This invention relates to single sideband communica- 10 vola cT17, NO. 3, August 1970. tion systems and more particularly relates to methods of To date, however, ACSB has not been developed to correcting for the adverse effects of propagation- its fullest potential. The equipment currently undergo- induced fading in mobile communications systems. ing developmental field trials in the U.S. is designed for operation exclusively in VHF “High Band” (approxi- BACKGROUND OF THE INVENTION I5 mately 150 MHz). At higher frequencies, such as the For many years the availability of frequencies for 800 MHz bandy the Performance of current technoh3Y radio transmissions was not a problem. H~~~~~~,re- is dubious, due to the rapid increase in the severity of cently spectral have become and adverse channel conditions resulting from multipath Scarce due to a marked escalation in the public’s need fading, as well as the combination of Doppler shift and for additional channel capacity. neFM modulation 20 the stringent tuning requirement for accurate demodu- techniques in use today require bandwidths which are lation of ACSB signals. Several different modulation wide to permit the allocation of a number formats have been investigated by others on an experi- of additional channels in existing bands with present mental basis in search of reasonable and effective solu- technology. due to a general shortage of tions to these implementation problems, some of which suitable spectral resources, the assignment of additional 25 are described in the above references, but all seem to have serious disadvantages in either a technical or ‘spectrum to these uses appears quite unlikely, despite an the significant, demonstrable need. facts have economic These Since the primary impairments to effective mobile Of the dictated the need to make more communications systems utilizing ACSB are the ran- for audio communications’ 30 dom amplitude and phase caused by the vehi- Thus attention has turned to the Of sing1e sideband cle’s motion through a qua&stationary pattern of multi- (SSB) si@a1s for transmissions which can path signals, an efficient system which can detect and reduce the spectra1 space required, particularly for land automatically correct for these unwanted variations is mobile communications. The use of amplitude-compan- needed. dored signal sideband (ACSB) signals alternative as an 35 McGeehan et reference rreyr listed above, de- to the more traditional “narrowband” FM thus has been scribes experiments in which a pilot tone is inserted into proposed as a possible means of relieving the Severe a centrally-located notch in the audio baseband prior to spectrum congestion which hampers effective land mo- modulation, and have reported very encouraging re- bile communications throughout the world. sults. They also argued quite convincingly that such a The following references describe the potential for 40 modulation format carries with it a number of signifi- sing1e communications (SSB) and propose cant advantages over methods which place the pilot at System for implementing it and are incorporated herein the upper or lower edge of the channel. Their imple- by reference: mentation of Feedforward Signal Regeneration (FFSR) REFERENCES circuitry as an “add-on convenience circuit” to a con- 45 ventional SSB receiver does, however, impose some (a) Future Private Land Mobile Telecommunications Re- very difficult filtering requirements at baseband for quirements, Prepared by the Planning Staff of the properly separating the pilot from the voice spectrum Private Radio Bureau, Federal Communications for processing and requires the receiver to employ Commission, Washington, D.C. costly SSB crystal filters at IF to achieve the required (b) Lusignan, B.B.: “Single-Sideband Transmissions for 50 adjacent channel selectivity. Land Mobile Radio”, IEEE Spectrum, July 1978. For ACSB to gain unqualified, permanent regulatory (c) McGeehan, J.P. et al.: “Theoretical and Ewerimen- and commercial acceptance on a large scale, a standard tal Investigation of Feedforward Signal Regeneration modulation format (with regard to pilot placement) and as a Means of Combating Multipath Propagation an effective means for correcting for the adverse effects Effects in Pilot-Based SSB Mobile Radio Systems”, 55 of fading in the mobile environment is essential. Such a IEEE Transactions on Vehicular Technology, Vol. standard must provide acceptable performance not only VT-32, No. 1, February 1983. in the VHF region, but also into the higher UHF fre- (d) McGeehan, J. P. et al.: “Speech Communications quency bands, such as the 800 MHz and 1500 MHz Over a 942 MHz Tone-Above-Band Single Sideband bands, since long-term growth in mobile communica- Mobile Radio Channel (6.25 kHz) incorporating 60 tions will almost certainly result in increased use of Feedforward Signal Regeneration:, (a paper pres- these higher-frequency regions. Furthermore, the most ented to the IEEE Vehicular Technology Society acceptable standard from the stand-point of both regu- Conference, 1983). latory and commercial acceptance would also be com- (e) McGeehan, J. P. et al.: “Pilot Tone Single Sideband patible and interoperable with existing FM technology for Mobile Satellite Communications”, Proc. IEEE 65 and capable of reliable transmission of digital data in Third Intl. Conference on Satellite Systems for Mo- either SSB or FM modes without a requirement for bile Communications and Navigation, London, June expensive duplication of hardware (Le. effectively re- 1983 quiring two different radios in one box). This would 4.726.069 3 4 permit the technology to be gradually and economi- cal alternative to the more conventional method of cally “phased in” to bands which are presently occu- employing narrow, steep-skirted IF (intermediate fre- pied by FM systems, allowing practical implementation quency) crystal filters for the rejection of the unwanted of spectrally-efficient technology with a minimal sideband and adjacent channel signals. amount of cost and disruption to existing services. No 5 The transmitter employs a modified Weaver modula- system or modulation format proposed to date has ad- tor to generate a single sideband signal having a central dressed this requirement for multi-mode capability in a spectral gap or notch and an pilot carrier located in the cost-effective and technically practical manner. gap. The initial transmitter stage is an audio processor Research conducted during the development of the which provides a band-limited, compressed, preempha- invention disclosed herein strongly support the conclu- 10 sized signal, processed in a manner which will be sion that virtually all .of the major disadvantages of readily apparent to those knowledgeable in this art.
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