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`USO05659891A
`[11] Patent Number:
`[45] Date of Patent:
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`I
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`5,659,891
`Aug. 19, 1997
`
`1/1981 Perkins .................................... .. 370/69
`4,244,047
`5,163,181 11/1992 Koontz . . . . . .
`. . . . .. 455/103
`5,343,499
`8/1994 Jasper et a1.
`...... .. 375/39
`5,392,452 2/1995"Davis ................................... .. 455/38.1
`OTHER PUBLICATIONS
`
`47 C.F.R. § 22.106 (1994).
`
`Primary Examiner—Reinhard J. Eisenzopf
`Assistant Examiner-Lee Nguyen
`Attorney, Agent, or F irm-Finnegan, Henderson, Farabow,
`Garrett and Dunner
`
`[57]
`
`ABSTRACT
`
`United States Patent [191
`Hays et a].
`
`[54] MULTICARRIER TECHNIQUES IN
`BANDLIMITED CHANNELS
`
`[75] Inventors; William D_ Hays; Dennis Cameronq
`both of Jackson, Miss; Walter Roehr,
`Reston, Va.
`
`[73] Assignee: Mobile Telecommunication
`Technologies, Jackson, Miss.
`
`[21] APPL Na; 480,718
`
`[22] Filed:
`
`Jun. 7, 1995
`
`[51] Int. Cl.“ ..................................................... .. H04B 1/02
`Amcthod 9f multicam'er modulation using wlocated trans
`[52] US. Cl. .......................... .. 455/103; 455/45; 370/339;
`mitters to achieve higher transmission capacity for mobile
`370/343
`Pagi{1g and FWO-WaY digitél c°mmunicé?on in a manner
`[58] Field of Search ................................... .. 455/103, 104,
`consistent w1th FCC emiss1on mask limits. Co-1ocat1on of
`455/17, 46’ 59‘ 6O, 6L 62~ 491’ 45; 375/260;
`'70’
`40, 344 thC transmitters obviates the need for SlIiIlgCHt, symmetrical
`subchannel interference protection and provides for a wider
`range of operating parameters, including peak frequency
`deviation, bit rate, and carrier frequencies, to obtain optimal
`transmission performance.
`
`[56]
`
`References Cited
`
`U-S- PATENT DOCUMENTS
`
`1/1970 Chang .................................. .. 370/691
`3,488,445
`3,914,554 10/1975
`
`. . . .. 179/1555 R
`
`Seidel . . . . . . . .
`
`5 Claims, 11 Drawing Sheets
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`14
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`LINEAR
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`RF AMPLIFIER
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`SUMMATION
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`1217
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`MODULATOR
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`DATA SOURCE
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`12a
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`7 MODULATOR
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`DATA SOURCE
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`113
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`1
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`1
`MULTICARRIER TECHNIQUES IN
`BANDLINIITED CHANNELS
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`5,659,891
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`FIELD OF THE INVENTION
`The present invention relates generally to multicarrier
`modulation techniques, and more particularly, to a method
`for operating more than one carrier in a single mask-de?ned,
`bandlimited channel assigned to mobile paging use.
`
`DISCUSSION OF RELATED ART
`The rising popularity of mobile paging services has
`resulted in increased competition for air time on the limited
`number of radio-frequency channels allocated by the Federal
`Communications Commission (FCC) for mobile paging use.
`As demand begins to approach and even exceed the capacity
`of assigned channels to handle transmission tra?ic, delays in
`service and deterioration of transmission quality are becom
`ing a major concern to mobile paging users and providers.
`The ability of mobile paging providers to successfully
`address the problem of transmission saturation is limited by
`the ?nite range of air space dedicated to mobile paging use.
`Channels assigned by the FCC to radio paging providers
`typically have narrow bandwidths (eg 25 kHz) and are
`subject to stringent emission mask limitations.
`One method that can be used to allow greater access to a
`I particular channel is to increase the number of messages
`transmitted over the channel in a given period. This can be
`achieved. for example, by increasing the data rate of the
`transmission or by reducing the length of transmitted mes
`sages. U.S. Pat. No. 5,392,452 issued to Davis, for example,
`describes a high data rate transmission scheme for handling
`lengthy messages in a paging system Systems employing
`techniques to increase transmission rates, however, are
`prone to higher error rates. In addition, high data rates tend
`to generate greater transmission interference.
`Another approach to addressing the saturation problem is
`to increase the transmission capacity of the channel itself.
`Traditionally, mobile paging providers have operated only
`one transmission signal, or carrier, within an assigned chan
`nel. While this mode ensures simplicity of operation and
`compliance with FCC mask requirements, it does not pro
`vide the most e?icient use of the limited frequency band
`width available. Successful multicarrier modulation,
`however, is difficult to achieve without incurring unaccept
`able levels of interference.
`The problem of interference is compounded when a
`receiver is attempting to acquire a signal from a distant
`transmitter while in close proximity to a transmitter oper
`ating on an adjacent .channel. In this environment, the
`receiver may experience di?iculty in detecting the signal
`from the distant source due to interference from the signal
`transmitted on the adjacent channel from the closer source.
`This is known as the “near-far” problem. This problem can
`be avoided by co-locating the transmitters at essentially the
`same geographic location.
`The FCC requires signals to be con?ned within emission
`limit masks in order to prevent interference caused by
`signals straying or spilling into adjacent channels. FCC
`masks typically require the power spectral density of a
`signal to be attenuated at least 70 dB at the band edge.
`Despite these stringent constraints, some carrier overlap can
`be expected. even when the maximum carrier spacing con
`sistent with the FCC mask requirements is utilized. Such
`overlap can result in unacceptable interference of the
`carriers, making it difficult for the receivers to acquire the
`proper carrier.
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`Thus. a traditional multicarrier design would commonly
`require the same stringent protection levels between sub
`channels. Speci?cally, when more than one carrier is oper
`ating within a single channel, each carrier is traditionally
`con?ned to a submask de?ning a subchannel internal to the
`channel. The carriers are symmetrically located within the
`channel such that they are evenly spaced relative to each
`other and to the band edges of the primary mask de?ning the
`primary channel. Although such symmetry achieves maxi
`mum inter-carrier spacing and reduces the opportunity for
`interference among adjacent carriers, it often necessitates
`the need for sophisticated receiver and transmitter schemes.
`
`SUMMARY OF THE INVENTION
`It is an object of this invention to achieve higher capacity
`over a bandlimited channel for paging without the need for
`stringent subchannel interference protection. Additional
`objects, advantages, and features of the invention will be set
`forth in part in the description that follows, and in part will
`be obvious from the description, or may be learned by
`practice of the invention. The advantages of the invention
`will be realized and attained by means of the instrumentali
`ties and combinations particularly pointed out in the written
`description and claims herein, as well as the appended
`drawings.
`To achieve these and other objects, advantages, and
`features in accordance with the purpose of the invention, as
`embodied and broadly described herein, the invention pro
`vides a method of operating a plurality of paging carriers in
`a single mask-de?ned, bandlimited channel comprising the
`step of transmitting the carriers from the same location at
`center frequencies within the channel such that the fre
`quency ditference between the center frequency of the outer
`most caniers and the band edge of the mask de?ning said
`channel is more than half the frequency difference between
`the center frequencies of each adjacent carrier.
`In another aspect, the invention provides a method of
`operating at least two paging carriers each in a correspond
`ing subchannel of a single mask-de?ned. bandlimited chan
`nel comprising the step of transmitting the carriers from the
`same location with each carrier centrally located in a cor
`responding subchannel wherein the frequency difference
`between the center frequency of the outer most subchannels
`and the band edge ofthe mask de?ning said channel is more
`than half the frequency di?erence between the center fre
`quencies of each adjacent carrier.
`in another aspect, the invention provides a method of
`operating a plurality of carriers in a single mask-de?ned,
`bandlimited channel to achieve higher transmission capacity
`over the channel in a mobile paging system having a
`plurality of transmitters generating a plurality of modulated
`carriers over a single bandlimited channel and a plurality of
`mobile, independent receiving units. capable of receiving
`one of said plurality of carriers. The method comprises the
`steps of co-locating the plurality of transmitters such that the
`plurality of carriers can be emanated from the same trans
`mission source, and transmitting the carriers over a plurality
`of subchannels spaced asymmetrically within the mask
`de?ning the channel.
`It is to be understood that both the foregoing general
`description and the following detailed description are exem
`plary and explanatory only and are not restrictive of the
`invention, as claimed.
`BRIEF DESCRIPTION OF THE DRAWINGS
`The accompanying drawings. which are incorporated in
`and constitute a part of this speci?cation, illustrate several
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`embodiments of the invention and together with the
`description. serve to explain the principles of the invention.
`In the drawings,
`FIG. 1 is a block diagram of a co-located multicarrier
`transmitter system in a linear ampli?er con?guration for
`using the present invention;
`FIG. 2 is a block diagram of a co-located multicarrier
`transmitter system in a composite transmitter con?guration
`for using the present invention;
`FIG. 3A is a graph depicting two submasks de?ning two
`subchannels in a single. mask-de?ned bandlimited channel.
`FIG. 3B is a graph depicting the power spectra of two
`carriers asymmetrically located within a single mask
`de?ned. bandlimited channel.
`FIG. 4 is a graph depicting an exemplary FCC emissions
`mask that requires the power spectral density to be attenu
`ated at least 70 dB within 10 kHz from center ?'equency.
`FIG. 5A is a graph depicting the power spectra of a system
`with peak deviation of 2400 Hz and data rate of 6000 bits per
`second (bps);
`FIG. 5B is a graph depicting the performance of the
`system of FIG. 5A in terms of bit error rate versus the signal
`noise ratio (SNR);
`FIG. 6A is a graph depicting the power spectra of a system
`with peak deviation of 1800 Hz and a data rate of 6400 bps;
`FIG. 6B is a graph depicting the performance of the
`system of FIG. 6A in terms of bit error rate versus SNR;
`FIG. 7A is a graph depicting the power spectra of a system
`with peak deviation of 2100 Hz and a data rate of 6400 bps;
`and
`FIG. 7B is a graph depicting the performance of the
`system of FIG. 7A in terms of bit error rate versus SNR.
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`nal or carrier. The outputs of the ?rst and second modulators
`are fed into ?rst and second RF ampli?ers. 23a and 23b,
`respectively. The outputs of the RF ampli?ers are combined
`into a single output signal by summation circuit 24. the
`output of which is applied to antenna 25 for transmission in
`a bandlimited channel.
`Alternative embodiments of co-located transmitter sys
`tems are also possible. For example. the co-located trans
`mitter con?gurations discussed above can be expanded to
`support more than two data sources and transmit more than
`two carriers in the bandlimited channel.
`Because transmitter co-location does not give rise to the
`near-far problem to which the FCC mask requirements are
`directed. carrier spacings far closer than would ordinarily be
`allowed (e.g.. 5 to 10 kHz) are achievable. Moreover. the
`carriers need not be symmetrically or evenly spaced within
`the mask de?ning the channel. That is. the ?'equency spac
`ings between adjacent carriers. while symmetric to each
`other. can be smaller than the frequency spacings between
`the band edges of the mask and the nearest respective carrier.
`Indeed. carrier spacings may be irregular such that the
`carriers are asymmetrically located within the mask without
`incurring undue interference.
`Referring to FIG. 3A. two submasks de?ning two
`subchannels. 30a and 30b, are asymmetrically located
`within a single mask-de?ned. bandlimited channel 31,
`resulting in some subchannel overlap. FIG. 3B depicts two
`carriers, 32a and 32b, operating respectively over two
`asymmetrically-located subchannels, resulting in some car
`rier overlap. In accordance with this asymmetry. the fre
`quency diiference between the center frequency of each
`carrier and the nearest band edge of the mask is greater than
`half the frequency difference between the center frequencies
`of the two carriers.
`The practical implication of transmitter co-location is that
`a greater range of operating parameters. including the peak
`frequency. deviation bit rate. and carrier frequencies. are
`available so that multicarrier modulation in a standard
`bandlimited channel can be obtained without the need for
`stringent subchannel interference protection. In accordance
`with the present invention. these and other parameters can be
`adjusted so that the carriers generated and transmitted
`according to the present invention will remain within the
`FCC emission limits while providing optimal transmission
`performance.
`FIG. 4 shows a FCC emissions mask which requires the
`power spectral density to be attenuated at least 70 dB within
`10 kHz from center frequency. Co-location of the transmit
`ters allows for signals with a greater range of deviation and
`band rates to be carried in the bandlimited channel than has
`been otherwise thought possible in view of FCC 70 dB
`cuto?’ requirements. The present invention will be further
`clari?ed by the following examples. which are intended to
`be purely exemplary of the invention.
`FIG. 5A is a spectra graph of a two-carrier system in
`which the following parameter values were selected: a peak
`frequency deviation of 2400 Hz. a bit rate of 6000 bps.
`premodulation ?lter cutotf frequency of 3000 Hz. and carrier
`frequencies set to within 4590 Hz of the center frequency.
`Using this operative parameter combination. the carriers
`remained within the FCC mask while providing an accept
`able error-rate versus signal strength performance (FIG. 5B).
`Alternatively. FIG. 6A is a spectra graph of a two-carrier
`system using a peak frequency deviation of 1800 Hz. a bit
`rate of 6400 bps. a premodulation ?lter cutoff frequency of
`3200. and carrier frequencies set to within 5150 Hz of the
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
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`Reference will now be made in detail to the presently
`preferred embodiments of the invention. examples of which
`are illustrated in the accompanying drawings. Wherever
`possible. the same reference numbers will be used through
`out the drawings to refer to the same or like parts.
`Referring to FIG. 1. a co-located multicarrier transmitter
`system in a linear ampli?er con?guration 10 comprises a
`?rst and second data source. 11a and 11b, a ?rst and second
`modulator. 12a and 12b, a summation circuit 13. a linear RF
`ampli?er 14. and an antenna 15. The ?rst and second data
`sources. 11a and 11b, generate a respective ?rst and second
`digital bit stream which are provided to respective ?rst and
`second modulators. 12a and 12b. Each modulator converts
`the incoming digital information into a representative modu
`lated signal or carrier. The outputs of each modulator are
`then combined into a single output signal by summation
`circuit 13. the output of which is fed into linear RF ampli?er
`14. The combined output signal is then applied to antenna 15
`for transmission in a bandlimited channel.
`Alternatively. referring to FIG. 2. a co-located multicar
`rier transmitter in a composite ampli?er con?guration 20
`comprises a ?rst and second data source. 210 and 21b, a ?rst
`60
`and second modulator. 22a and 22b, a ?rst and second RF
`ampli?er. 23a and 23b, a summation circuit 24, and an
`antenna 25. The ?rst and second digital bit streams gener
`ated respectively by the ?rst and second data sources. 21a
`and 21b, are provided to ?rst and second modulators. 22a
`and 22b, respectively. Each modulator converts the incom
`ing digital information into a representative modulated sig
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`center frequency. As depicted in FIG. 6B, this combination
`of operative parameters exhibited a higher bit error rate than
`the system of FIG. 5A.
`FIG. 7A is a spectra graph of a two-carrier system using
`a peak frequency deviation of 2100 Hz. a bit rate of 6400
`bps. a premodulation ?lter cutoff frequency of 3200, and
`carrier frequencies set to within 4750 Hz of the center
`frequency. As depicted in FIG. 7B, this combination gives
`signi?cantly better performance than the system of FIG. 6A
`but performs slightly worse than the system of FIG. 5A.
`Thus, according to the present invention, increased trans
`mission capacity is achieved by operating more than one
`carrier in a standard bandlimited channel assigned for
`mobile paging use, such as in the Narrowband Personal
`Communications Service or the Part 22 Service. In the
`modulation technique of the present invention, caniers oper
`ating at different frequencies are ?t within a single band
`width allocation in a manner consistent with FCC mask
`requirements. This is achieved through the use of co-located
`transmitters and the selection of an optimal combination of
`operating parameters, including peak ?equency deviation,
`bit rate, and carrier separation frequencies. Through the
`multicarrier modulation technique of the present invention,
`the normal transmission capacity of a standard channel can
`be increased without the need for stringent subchannel
`protection levels and complicated receiver and transmitter
`schemes.
`The modulation technique of the present invention has
`particular application in metropolitan areas where the vol
`ume and concentration of transmission tra?ic is high and
`where the need for increased transmission capacity is acute.
`In addition, the modulation technique of the present inven
`tion may also be suited for use in areas where the incidence
`of unacceptable interference is high, such as international
`border regions. In that type of environment. transmissions
`from the respective bordering countries can be assigned to
`one of the carriers operating within the channel to reduce the
`risk of interference.
`It will be apparent to those sln'lled in the art that various
`modi?cation and variations can be made to present invention
`without departing from the spirit or scope of the invention.
`Other embodiments of the invention will be apparent to
`those skilled in the art from consideration of the speci?ca
`tion and practice of the invention disclosed herein. Thus, it
`is intended that the speci?cation and examples be considered
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`as exemplary only, with a true scope and spirit of the
`invention being indicated by the following claims.
`What is claimed is:
`1. Amethod of operating a plurality of paging carriers in
`a single mask-de?ned, bandlimited channel comprising the
`step of transmitting said carriers from the same location with
`said carriers having center frequencies within said channel
`such that the frequency difference between the center fre
`quency of the outer most of said carriers and the band edge
`of the mask de?ning said channel is more than half the
`frequency difference between the center frequencies of each
`adjacent carrier.
`2. The method of claim 1 wherein adjacent carriers
`overlap with each other.
`3. A method of operating at least two paging carriers each
`in a corresponding subchannel of a single mask-de?ned,
`bandlimited channel comprising the step of transmitting said
`caniers from the same location with each carrier centrally
`located in said corresponding subchannel wherein the fre
`quency difference between the center frequency of the outer
`most of said corresponding subchannels and the band edge
`of the mask de?ning said channel is more than half the
`frequency difference between the center frequencies of each
`adjacent carrier.
`4. The method of claim 3 wherein adjacent subchannels
`overlap with each other.
`5. In a paging system having a plurality of transmitters
`transmitting a plurality of modulated carriers over a single
`mask-de?ned, bandlimited channel and a plurality of mobile
`receiving units independently receiving one of said plurality
`of caniers, a method of operating said plurality of carriers in
`said channel to achieve higher transmission capacity com
`prising the steps of:
`co-locating said plurality of transmitters such that said
`plurality of caniers can be emanated from the same
`transmission source; and
`transmitting said plurality of carriers over a plurality of
`subchannels spaced within the mask de?ning said chan
`nel wherein the frequency di?erence between the cen
`ter frequency of the outer most caniers and the band
`edge of said mask is greater than half the frequency
`difference between the center frequencies of each adj a
`cent carrier.
`
`15