Note: Descriptions are shown in the official language in which they were submitted.
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SIMULCAST RESYNCHRONIZATION IMPROVEMENT USING GLOBAL POSITIONING SYSTEM
CRoss-RE~ERENcFs TO
Rl~LATED APPLICATIQNS AND PATENTS
This application is somewhat related to commonly-assi ~ned U.S. Palent
No. ~,1?7,396 to Rose et al.. issued on December lj. 199~. entitled "Public Ser~ice
Trunking Simulcast System," and U.S. Patent i~o. 4,90~,37 l to Hall et al.. issued on
February 20, 1990, entitled "Radio Trunking Fault Detection System". This application
is also somewhat related to the followin~J commonly-assigned copendino applications:
serial number 07/~'J4, 1~3 of Brown et al. entitled "Self Correction Of PST Simulcast
System Timin~", filed '~ January 19g~ (Attorney Docket Number ~6 444; Client
Reference No. 45-~ 64) and serial number 07/906,4, 3 of Thomas A. Brown entitled
"Control Channel Tirninc~ Detection and Self Correction For Dic~itally Trunked
Simulcast Radio Cornmunication System". filed 30 June 1997 (Attorney Docket
Number 46-59~. Client Reference ~o. 4~-~-711). The disclosures of each of the
above patents and applications are incorporated b,v reference as iI' e~pressly se~ forth
herein.
FIELl) OF TElE ~N~v~NTION
This invention relates to radio frequency (RF) si~nal transmission systems,
and in particular to "sirnulcastinc" systems for providin~ the simultaneous transmission
of the same information b,v two or more separatelv located RF transmitters. ~lore
particularly, the invemion relales to an improved method and apparatus for C~enerating
simulcast timin~ "resynch" Iresvnchronization~ reference si~nals at each transmitter site
to maintain coherency of transmissions.
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BACKC.RO~D AlND SUM~L~RY OF THE ~NrrloN
As is well known, due to FCC power l;mit~tions, geo;,raphical and/or other
factors, it is sometimes r ot possible for a single RF tr~ncmittin~ site to provide adequate
covera~e to a large desired coverage area. For e~ample, government entities commonly
use land-mobile radio communications systems to provide communications between aheadquarters and various mobile and portable radio users that rove throu._hout the
jurisdiction of the governmental entity. In some cases the _eographical area of
jurisdiction is so laroe that it is not possible for a single land-based RF tr~n.~mi~;n ~ site
to cover it. Even if the effective radiated power of the single tr~n.cmi.csion site was
sufficiently _reat to cover the entire area, users in outlying or fringe areas might receive
only spottv service because of the "line-of-site" nature of VHF tr~n.~mic~ions and/or due
to geographical obstructions (e.g., hills, bridges~ buildincTs, and the curvature of the
earth) interposed between the single tr~n~mitt~r site and various fringe locations within
the covera=e area.
One known way to expand the coverage area is to provide multiple,
"simulcasting" tr~n~mitlin~ sites. In order to simplify mobile radio operation and
conserve radio frequency spectrum, such "simulcasting" RF tr~n~mit~in g sites all
transmit substantially identical signals at substantialIy identical times on substantially
identical radio frequencies. Such "simulcasting" e~imin~tes control overhead and other
comple~ities associated with performing "hand offs" from one RF transmitting site
coverage area to another as is common, for example, in cellular and "multi-site" RF
communications systems. So-called "simulcasting" dic~itally trunked RF repeater
systems are generally known. The followinc~ is a listing (which is by no means
e~haustive) of prior documents that describe various aspects of R~ transmission
simulcasting and related issues:
U.S. Patent No. ~,17~,396 to Rose et al.;
U.S. Patent No. 4,903,,~1 to Hall et al.,
I T S. Patent l~io. 4,696,05' to Breeden;
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U.S. Patent No. 4,696,~51 to Breeden;
U.S. PatentNo. 5,~45,63 l to Averbuch;
U.S. Patenl: No. 5,287,550 to Fennell et al;
U.S. Patenl: No. 4,782,499 to Clendening;
U.S. Patenl:No. 5,052,028 to Zwack;
U.S. Paten~ No. 4,570,265 to Thro;
U.S. Patent No. 4,516,269 to Krinock;
U.S. Patemt No. 4,475,246 to Batlivala et al.;
U.S. Patent No. 4,~17,220 to Martin;
U.S. PatentNo. 4,97~,410 to Cohen et al.;
U.S. Patent No. 4,608,699 to Batlivala et al.;
U.S. PatentNo. 4,918,437 to Jasinski et al.;
U.S. Patent No. 4,578,515 to Persinotti;
U.S. PatentNo. 5,003,617 to Epsom et al.;
U.S. PatentNo. 4,939,746 to Childress;
U.S. Patent No. 4,903,26~ to Dissosway et al.;
U.S. Patent No. 4.~6,496 to Cole et al.;
U.S. Patent ~io. 4,96~,966 to Jasins~;i et al;
U.S. Patent~o. 3,90'?,161 to Kiowaski et al;
U.S. PatentNo. 4,'~18,65~ to Ogawa et al;
U.S. PatentNo. 4,~55,815 to Osborn;
U.S. Patent No. 4,~11,007 to Rodman et al;
U.S. Patent No. 4,414,661 to Karlstrom;
U.S. Patent No. 4,47~,80' to Pin et al.;
U.S. Patenti~o. 4,597.105 to Freebur~; and
Japanese E'a2ent Disclosure No. 61-1078 76.
U.S. Patent No. 5,17~,396, issued December 15, 199~ to Rose et al.,
entitled "Public Service Trunl;ing Simulcast Svstem", discloses a trunked radio
simulcast svstem havin~ control site and remote site architectures that include R~
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tr~n.~mi~.~ion timing synchrnni7~tinn fea~ures that are relevant to the presently preferred
exemplary embodiment. In addition, U.S. Patent No. 4,903,3~1, issued Februa~ 20,1990 to Hall et al., entitled "Radio Trunkin;, Fault Detection System," discloses a
trunked radio repeater system having a radio frequency repeater site architecture that
includP~ fault and caIl testing and failure detection features that are somewhat relevant
to the present inventiorl. These patents are both cr mmonly assigned to the assignee of
the present invention ;and are both incorporated by reference herein.
While simulcastin~ thus provides various advantages as compared to other
techniques for e~pandin_ covera~e area, it also in~roduces its own particular set of
comple~ities that must be dealt with. By way of illustration, please refer to FIGURE
1 -- which is a schematic diagram of an e~emplary three-site simulcasting digitally
trunked land-mobiIe RF cornmunications system 10. System 10 includes three
simulcastiIlg transmitting sites, S1, S and S3. The tr~n~mi~sions of site S1 cover the
coverage area Al, and similarly, the tr~n~mi~ions of sites S~ and S3 cover respective
coverage areas A~, AJ . A centraI control point C coupled to each of sites S 1, S2 and
S3 via respective communication links (L1-L3) delivers, in real time, substantially
identical si~n~llin~. (including digital control charmel sign~lling and associated timing
information) for tr~n~micsion by the various sites.
Each RF cha~nel at all sites is modulated w ith amplitude, phase and time
delay corrected information. To accomplish this, time, phase and amplitude stable
communication links must be provided between a main control point site and all other
simulcast transmit sites by means of a high quality phase-stable back-bone
communication systern arran~ement (e.~J., radio. microwave or fiber optic). In this
regard, commercial wire-common-carriers do not provide the degree of stability
required for simulcast; whereas, dedicated, user controlled, voice/data ~rade,
synchronous multiple~ used in conjunction with radio, microwave or fiber optic back-
bone distribution paths rnost effectively do provide the needed communications circuits
and stabilit~ for simulcast.
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E~emplary system 10 is preferably a digitally trunked simulcast
communications systerm of the type m~rket. d by Ericsson-GE Mobile Communications
Inc. (ECiE) under the ~rade name EDACS. This system provides a digital RF control
channel and plural RF workin~, channels. In such a digitally trunked system, an
exemplary mobile radio unit M within one (or more) of covera~e areas Al-A3
continuously monitors an "outbound" di~ital control channel when it is not actually
enga~ed in active communications on a workin~ channel with other units. Mobile Mmay request co~ u"ications by transmitting a channel acsi~nment request messa~Je on
an "inbound" control charmel. Upon receipt of such channel assignment request (and
presumin~ that at least one working channel is available for temporary assignment to
mobile unit ~1 and other units with which mobile unit ~I wishes to communicate),control point C responds by causing a control channel assi_nment messa~e to be
tr~ncmitted by each site S1-S3 over the outbound control channel. In simulcast system
10, this channel ~c~ignment message is trancmitt~d simultaneously by each of
tr~n~mittin~ sites S l-S3 over the sarne outbound control channel frequency (such that
mobile unit M and other mobile units "called" by the channel assi~nment messa~e will
receive the message regardless within which coverage areas Al-A3 they may happento be Located). Mobile unit ~vI (and other called mobile units) respond to the received
outbound trunl;ing control channel assignment message by ~h~nging frequencv to an RF
wor~ing channel and conducting communications on the working channel. Once the
wor~ing channel communications are concluded, the mobile unit ~ (and other called
mobile units) retum to monitoring the outbound control channel for additional messages
directed to them.
Referrinc once again to FIGU~E 1, suppose mc,bile unit M is located
within an overlap area X wherein covera~e areas A~ and .~3 overlap one another.
Within this overlap area X mobile unit ~L will receive (perhaps at approximately equal
si~nal strength levels) the outbound control channel transmission of site S2 and also the
outbound control channel transmission of site S3 . Simulcast system 10 is appropriately
desi~ned such that such outbound control channel transmissions from sites S~ and S3
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are on substantially the same RF frequency so that no heterodyning or other interference
occurs. Similarly, control point C sends, over links Ll-L3, substantially identical
outbound control çh~nnel messa~Jes for tr~n.~mis.~ion by each of sites S1-S~.
However, a problem can arise if the outbound controI channels are not
precisely synchronized to one another. A transceiver located within overlap region X
that receives outbound control channel synchronization signals delayed with respect to
one another by even a small time period (e.g., more than a one-half bit period, or about
~ microseconds for 9600 baud operation) could end up losing bits and~or temporarily
losing synchronization, bit recovery and error checkin_ capabilities.
Delays due to the limited speed at which electromagnetic waves propagate
must be taken into account in systems simulcasting data at high data tr~n~micsion rates
(an RF si_nal travels "only" about 300 meters in one microsecond). It is possible (and
usually necessary) to adjust the relative effective radiated power levels of the site
transmitters so that the distances across the overlap regions X are kept less than a
desired maximum distance--and thus, the difference in the RF propagation delav times
across an overlap recrion due to the different RF path len~ths berween the site and a
receiver within the overlap region is minimi7Pd Even with this optim i7~tion, however,
it has been found that (due to the additional differential delay caused by the different
RF path lengths) a maximum system differential delay stability of ' S microseconds
must be observed to guarantee that the transceiver in any arbitrary location within a
typical overlap region X will receive the corresponding digital signal bit edges within
5~ microseconds of one another.
Fortunately, it is typicallv possible to minimi7e time delay differences to
on the order of a microsecond throu~h various known techniques. For exarnple, it is
well known in the art to introduce adjustable delay networks (and phase equalization
networks) in line with some or all of in~er-site links L l-L3 to compensa._ for inherent
difrerential link del~y ~imes (see l~.S. patent 4~516,'~69 to Krinock, and ~.S. patent
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numbers 4,696,051 and ~,696,052 to Breeden, for e~ample). Conventional microwaveand fiberoptic link chamlels e~hibit arnplitude, phase and delay characteristics that are
extremely stable over long periods of time (e. ~., many months), so tnal such additional
delays, once ad~usted, guarantee that a signal input into all of the inter-site links L l-L~
at the same ~ime will arrive at the other ends of the links at almost exactIy the sarne
time. The same or additional delays can be used to compensate for differeM, constant
delay times introduced by signal processin~ e4uipment at the sites S l-S3 to provide
.~imlllt~n~ous coherent tr~n.~mi.~.cion of the signals by the different sites. For e~ample,
the above-identified Rose et al. patent application describes a techni~ue wherein
~n'~iti~n~l frequency and timina information is provided to each site over one or more
particular inter-site link channels so as to elimin~te timin~ ambiguities that may result
from the use of conven-tional multi-level, multi-phase protocol-type modems. In this
manner, the above mentioned simulcast system forces coherence at the start of data
tr~n~mission on a particular established communications path, thus correcting for any
multi-bit ambiguity created by the inter-site communication link modem.
Briefly, referring now to FIGURE 2 which generally depicts an Ericsson-
GE (EGE) multiple site simulcast tr~ncmics;on system of the type described in
accordance with the above mentioned Rose et al. patent, a "master!' resynch
(resynchroni~tion signal) circuit 100 located at control point site C produces reference
edges/tones, e.g., at 740~ Hz and 300 ~, t~nat are sent to each transrnit site ~S 1~ 7)
a dedicated channel over the inter-site communication links (L l-L2). Digital and voice
data aligned to these reference signals is also sent via the communication links (L 1 -I,2)
between control point C and the transmit sites (S1-S~). The lower (300 Hz) tone is used
as a "gating" reference (for read-out timing of a broadcast data buffer at the transmit
sites) and the higher (2400 Hz) tone is used as a data clockin~ frequency reference.
Each transmit site (S l-SX) in the simulcast system includes a "universal" (i. e., common
hardware) resynchronization circuit for recovering reference edges from the tones. By
performance of a periodic "resynch" operation the universal resynch circuit at each
simulcast system site re-ali~ns the broadcast data received via the inter-site links to
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these reference edges. Consequently, as previouslv mentioned above, it is reguired that
the signal paths for these reference tones (conventionally provided via the inter-site
links) be of high quality and very phase-stable as any variation or noise in these signals
will have an adverse affect on overall simulcast system performance.
Thus, it is known to resynchronize the control channel at each tr,~n~mi1ter
site periodically on a routine basis in order to correct any control channel timing errors
that may arise in simulcast system 10. Moreover, the above-identified U.S. patent
application serial no. 07/824,1~3 to Brown et al., filed Januarv 2~, 1992, describes
additional techniques (which have been in public use for more than a year and are
therefore prior art to the present application) for periodically "kicking" a site modem
in order to ensure that the modem uses a distributed common clocking signal; and for
"retraining" a communications link and associated site modems for a simulcast system
working channel if a routinely pe;fo;rned working channel "test call" (e.g., as described
in U.S. Patent 4,903,3'~1 to Hall et al.) fails.
With respect to the above mentioned EGE simulcast svstems, applicant has
disclosed in the instar~t specification an improved method and apparatus for generating
resynchronization tones at each transmit site that increases the reliability of
synchronized timing throughout a simulcast system and greatly simplifies the procedure
for simulcast system alignment. More specifically, an improvement in system
performance is accomplished by providinc~ a c~lobal positioning satellite (GPS) receiver
at each transmitting site to serve as the source for generating the precise, stable
frequency reference tones needed for periodic "resynch" operations. The GPS system,
traditionally used for navi ,~ti~n~,l pulposes, is a series of satellites svnchronized in time
and continuousl~ transmitting, inter alia, time, date and positioning information.
Althou_h, cellular radiotelephone system sites using the GPS for providing absolute
timing is kno~vn, such systems admit to numerous inherent problems which render these
systems expensive and unreliable (see, for example, U.S. Pa~ent 5, Z45,634 to
Averbuch). In accordance with the present in~rention. an improved multiple site RF
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simulcast system is achieved using GPS system broadcast si~nals in conjunction with
a p2rticular resynchronization circuitry arrangeme~t at each simulcast site which
overcomes many of the drawbacks of previous GPS synchroni~ed systems.
In accordance with the present invention, resynchronization reference
signals are not sent to transmit sites along with the broadcast signals via the site
interconnect links (L1, L~, etc.). Illstead, each simulcast site includes a "universal"
(_eneric) EGE ~imlllr~.~t system "resynch" circuit and simulcast data resynchronization
is periodically perforrned on a routine basis by each system site separately andindependently. In a preferred e~emplary embodiment, a stable, precise 9600 bps data
clock reference and the lower frequency ",~ating" sianal (e.g., 300Hz as described
above) are derived separately at each simulcast system site from a global positionina
satellite (GPS) broadcast transmission ac~uired by USiIl_ a GPS receiver at each site.
The control point site (C) includes a universal EGE "resynch" circuit and it is no longer
employed as a "master" circuit. The "resynch" circuit at each site utilizes the GPS
derived reference signal tones to align the RF broadcasting of simulcast data.
Consequently, dedicated stabilized channels on the site interconnect linL;s are no longer
needed for distributina "resynch" reference sianal tones. In addition, because the EGE
"resvnch" operation will force alignment to the correct simulcast system timina. any
variation in site interconnect link latency is automatically corrected whenever a
"res,vnch" operation is performed (so long as the latency variation is within the "gating"
sianal timing window) without any link latency measurement or correction.
Conventional non-EGE commercial simulcast systems if similarly outfitted with GPS
receivers would require an arithmetic calculation of iatency change and/or stora~e of
lirl~ timing parameters. Moreover, employing a GPS receiver in combination with an
EGE universal "resynch" circuit in an arranoement to generate "res,vnch" reference
tones for the resynch circuit and autonomously aliuriing the data broadcast at each site
in accordance with the present invention further improves simulcast operations by
reducing timing jitter and eliminatin,g the need for alignment checl~s on reference tone
po larit~ .
_ _ _ _ . _ . ... . . . .. ... ... _ _
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Because fine tuning a par~icular mlllt;ple site simulcast system a~angement
often requires specifIc ad3ustments of the broadcast timin;, at one or more sites off of
the "nomin~l" equal ~ming, a method of producing such timing offsets must provided.
In accordance with a fi~her aspect provided by the present invention, the ~PS receiver
utilizes a delay unit that provides adjustable sign~1 delays of +12511s in incremental
0.~1s steps and which is accessible via an RS-232 port. AccordingIy, an additional
advanta=e is achieved by the present invention in that simulcast system fine tuning
timing adjus~nents can be made remotely via an RS-232 link.
BRIEF DESCRIPTION OF TEIE DRAWINGS
These and other features and advantages of the present invention wilI
~ecome more cc~mrJIe~1y understood by referring to the following detailed description
of presently pre~erred exemplary embodiments in conjunction with the FIGURES in
which Iike reference numerals refer to like elements throughout:
FIGIJ~RE 1 is a general schematic illustration of a simplified e~emplary=
multiple site RF comrnunication simulcast system;
FIGURE 2 is a general schematic block dia~ram of the central control point
C and remote transceiver sites S I and S2 of an Ericsson-GE multiple site RF simulcast
communication system of a type on which operation of the present invention may be
particularly suited;
FIGURE 3 is a oeneral schematic block diagram of an e~emplaly
arran~ement of a modified multiple site RF simulcast communication system usin_ GPS
receivers to improve the resynchronization reference si(Jnal Feneration at every site.
DETAIlLED DESCRlPTTON OF TT~E DR~,VINGS
.
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In the following description, for purposes of e~cplanation and not limitation,
specific details are set forth, such as particular circuits, circuit components, irlterfaces,
techniques, etc. in order to provide a ~orough understanding of the present invention.
However, it will be a~ L to one skilled in the art that the present invention may be
practiced in other e:mbodiment~s that depart from these specific details. In other
instances, detailed descriptions of we~l known methods and progr~mming procedures,
devices, and circuits are omitted so not to obscure the description of the present
invention with unnecessary detail.
The basic: architecture of an Ericsson-GE simulcast syslem as described
above is shown in FIGURES 1 and 2 -- that is, it includes a central control point C and
plural tr~n~mitting sites S 1. .. Sn. Althou~h only two (remote) tr~n.sm ittin g sites S I -S''
are shown in FIGURES 2 and 3, it will be appreciated by one skilled in ~e art that
numerous remote sites participating in siml~lc~sting are likewise in communication with
control point C via identical microwave, fiber-optic, cable or land-line communication
path links L l -Ln. Moreover, the present invention is not limited to use solely with a
microwave or land-line link but may be used with any other type of appropriate
communication link such as radio wave.
In a multiple site radio frequency simulcasting RF tr~ncmi.csion system, data
provided via the inter-site communication links (L1, L2, etc.) from control point C to
the RF transmitter sites (S 1, S2, e~c.) e~hibits random time delav skew because multi-
phase link modems (MC, M1, ~, etc.) at each site recover clock signals from an
arbitrary one of multiple phases. The data stream outputs of modems are temporarily
stored at the sites in memory buffers Ml, ~, etc. associated with the modem at each
site. Timing information provided to each site from control point C via linL; channels
initially sets the memoly buffer output timing at each site to elimin~te transmission
timing ambiguities. On a continual basis, resynch circuitr,v at the sites periodically
resynchronizes memory buffer output timin~ in accordance ~hith a pair of reference
_ _ ,, , . . .. ... .. . . ... ... . . . . . _ .. .. .. . _
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frequency tones sontimlously provided to each site over the dedicated link channels
from control point C as previously discussed above.
Referring now to FIGURE 3, an exemplary embodiment of a modified
multiple site RF commllnication simulcast system in accordance with the present
invention is discussed. GPS re~eiv~ 301,301a and 301b at all sites (including control
point C) provide a reference si_nal ac~?uired from a common GPS timing si~nal
broadcast via satellite 300. Tone generator circuits 302, 30'~a, 302b at each site utilize
the received GPS timing reference signal to generate a hi ,h frequency 9600 bps data
clocking reference tone and a low frequency data "gating" (timing) tone that are used
by universal resynch circuits 30,, 303a, 303b for performing the periodic resynch
operations. In accordance with the present invention, a ~ating signal frequency much
lower than, for example, 300Hz may be used as long as the frequency chosen is anintegral submultiple of the data stream frarne timin_ (i.e., the data frame period divided
by the gating frequency period) and a submultiple of 9600 (e.g., using lOOHz provides
a gating period of 10 ms; 60Hz provides a gating period of 16.6 ms; etc). Gatingfrequency is preferably selected based on e~pected system link latency variations.
Resyn~ i~L,on reference tones are not sent to tr~n~mi~cion sites via the
inter-site communication links from a riesign~t~d control point site as in the e~emplary
EGE simulcast systems discussed above. Consequently, additional phase-stable, delay-
compensated ch~nnels ~or resynch signals are not required. Instead, a resynch operation
is periodically performed on a contim~inG basis by universal resynch circuits 303, 303a,
303b separately at each ~iml~lc~t system site using the resynch reference tones derived
from the received GPS signal by tone ~enerator circui~s 302, 302a, 302b. Retimed(realigned) data is thereby provided on a continuin~ basis to channel transmitter(s)
30~a, 304b at each site for simultaneous RF tr~n~micsion. In addition, because aresynch operation forces data strearn alignment to the correct simulcast system timin~g,
any inter-site intercom1ection link latency variation (within the "gating" si_nal timing
~indow) will be automatically corrected whenever the resvnch operation is performed.
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~ oreover, this modified arrangement for providing resynchronization reduces timing
jitter and el;min~t~ the need for alignment checks on reference tone polarity.
. ~
As also d~ picted in FIGURE 3, GPS receivers 301, 301a, 301b include a
delay unit that provides adjustable si~nal delays of +12511s in incremental steps of 0.5~1S
and which is controllable via an RS-232 port. Accordin~ly, an additional advanta_e is
achieved by the present invention in that simulcast system fine tuning timing
adjustments can be made remotely via an RS-~32 link.
W~ile the invention has been described in connection with what is presently
considered to be the most practical and preferred embodiment, it is to be understood
that the invention is not to be limited to the disclosed embodiment, but on the contrary,
is int~n~led to cover various modifications and equivalent arrangements included within
the spirit and scope of the appended clain~s.
