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Exh. E

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IEEE 'l'R:\NS.\C'l'IONS ON COMMUNIC.\TIONECHNOLOGY VOL. COM-14,O. 5 OCTOBEI~ 1966

Transmission Facilities for General PumoseJ.Wideband Services on Analog Carrier Systems

Absiraci-Three newgeneral purpose wideband data facilitiesprovide for the transmission of synchronous and nonsynchronous,2-level data at bit ra tes ranging rom 19.2 to 25 0 kb/s.Thesefacilities make use of half-group, group, and supergroup bandwidthtelephone facilities. The majorcomponents involved are he type303 wideband data station, the exchange area transmission systems(local loop, N-Carrier, TI Carrier), and the long-haul transmissionsystems [L-type multiplex (LMX) terminals, and coaxial cable andmicrowave radio circuits]. The group band system is representativeand is discussed in some detail.

The analog carrier systems were designed principally to handlevoice frequency ransmission and they present special problemswhen they are adapted to accommodatewideband signals. Forexample, up to eight LMX group connector filters may be encoun-tered na 4000 mile circuit and each connector introduces over150 ps of delay distortion in the transmission band. Special delayequalizers are added to each connector which reduce the distortionto f3 ps in the band 64 to 104 kHz.

The modulation schemes utilized in the various wideband modemswere carefully tailored to the respective carrierystems. TheLMX modem, usedasan example in this paper,utilizes ves-tigial sideband, suppressedarrier,mplitude modulation.A carrier requency pilot is added to the ransmitted signal tofacilitatecarrier recovery in the receiver. The pilot isseparatedfrom he wideband signal in the receiver by acrystal bandpassfilter. The filtered pilot isused for two functions; it isused tocontrol an AGC circuit and hence the level of the received signal,and it is used in amultiple loop APC circuit to coherently demodulatethe wideband signal. The APC circuit is new and promises improvedperformance over the conventional phase-locked oscillator whenused inacoherentdetector. It s capable of maintainingsmallerphase errors in the presence of phase perturbations and, further-more, the recoveredcarrier is always at exactly the correct re-quency and hence the circuit is incapable of breaking lock .

T INTRODUCTIONHE ANALOG carrier systems that play a prominentrole in the transmission of wideband (da ta) signals inthe Bell System are theN-Carrier for short-haul transmis-sion and -the L-Multiplex for coast-to-coast transmission.Local loops, which interconnect the da ta et on the custom-er's premises and the centraloffice equipment, make use ofordinary pairs in the telephone ables. In addition, there isa ncw digital time-multiplexed transmission system, he T1Carrier, th at is used for exchange area service. The ana-log facilities were designed principally tohandle voicetransmission, and they present special problems when heyare adopted toacconmlodate wideband signals.

Threedistinctgeneralpurposewideband services areavailable: supergroup, having a maximum channel capac-

Paper CP66-1031 presented at th e 1966 IEEE International Con-Manuscript received January 26, 1966; revised May 27, 1966.vention, New York, N. Y.The author is with the Bell Telephone Laboratories, Inc., NorthAndover, Mass.

ity of 250 kb/s, full-group, which can handle up to 0 kb/s,and half-group, capable of 19.2 kb/s operation. These sys-tems aredesignated general p w p o s e in that they can andlesynchronous or nonsynchronous, 2-level da ta at any bitspeed up to their respective design maxima. Furthermore,the transmission facilities described in this paper are ana-log and can accept any signal tha t falls within their signalspectrum and power handling capabilities [I] .

TI~ANSMISSIONACILITIESFigure 1shows a possible coast-to-coast wideband con-nection, although all carriersystemsdo not necessarily

appcar n one circuit [2]. The ype 303 data set rans-mitterattenuates t,he low-frequency components in the2-level da ta signal by means of a simple high-pass filter.The low-frequency components are reinserted at th e da taset receiver bya egenerative eedbackcircuit [3] . Th econsequent eduction in owfrequency ontent n thetransmitted signal alleviates low-frequency transmissionrequirements. A baseband repeater system (local loop) isavailable which permits transmission of t he restored polarline signal over ordinary pairs n telephone cable. The localloop is currently used for wideband transmissionup to20 miles, although most circuits are less than 10 miles inlength.

The N-Carrier transmission system (Fig. 1) is used onshort-haul routes up to 200 miles. The N-Carrier widebandterminals, as well as those for L-Carrier and T1 Carrier,incorporate modulation schemes that are carefully tailoredto the facilities. The bandwidth of the N-Carrier line issufficient, or half-group or full-group wideband service, butis insufficient for upergroup service. The at ter is re-stricted to t he use of a baseband local loop or T1 Carrierfor exchange area transmission.

The TI Carrier system utilized pulse code modulationand time-multiplexing techniques to provideshort-haultransmission of 24 voice frequency signals. The system hasasynchronousbit speed of 1..544 mb/s. New wideband

LOCAL LOW N-CARRIER COAXIAL CABLEILMXlI\-\ --OL LOFFICE TOLLOFZlCC

OFFICE OFFICE

Fig. 1 . Wideband transmission circuit.655

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656 IEEE T R A N SA C T I O N S ON C OMMUN I C A T I O NE C H N O LO G YC TO B E R

terminals permit it to handle either eight 50-kb/s or two250-db/s da ta channels n place of the voice channels[4]. The half-group dat a service: is in a format that is notreadily handled by the T1 Carrier [ 3 ] .

The long-haul transmission plant is made up of LMXterminals together withrepeatercd oaxial cables and point-to-point microwave radio chancds. The terminals includeseveral steps of single sideband, suppressed carrier, ampli-tude modulation.Twelve voice channels which are nor-mally multiplexed in a group band extending from 60 to108 kHz can be replaced with either two half-group wide-band channels or one full-group channel. Five group chan-nels aremodulated nto he supergroupband and ensupergroups are combined in a mastergroupband. The250 kb/s service replaces five grllup channels and operatesdirect ly nto he 312 to 552 kHzsupergroupband. Thevariousradiosystemscarry on e to hreemastergroups;the L3 Carrier multiplexes thret: mastergroups into a 7.97MHz band; and, shortly, L4 Carrier will introduce a 16.98M H z broadband channel composed of six mastergroups.The typ e 303 da ta service involves only the group andsupergroup bands, but the signa,ls can be multiplexed intoany position in the L3, L4, or ra,dio spectra.

The restored polar line signal is recovered at the outputof each of th e transmission facilities depicted n Fig. 1.Thus, the type 303 data set can operate directly into anyone of them, and, furthermore, the order of interconnec-tion of the various systems is unimportant.

The modulation techniques utjlized in the various wide-bandmodems were carefully sselected for optimum per-formance considering the particular da ta service and carrierfacilitynvolved. Each combination enerates niqueproblems and constraints. The I M X group-band terminalis representative, however, and itherefore th e remainder ofthe paper will be devoted to a discussion of this system.

LM X GROUPBAND YSTEMWideband Equipment

The new equipment tha t has to be added to a LMXcircuit to adapt it for dat a transmission is shown in Fig.2. The upperdiagramdepictsastandard voice circuitconsisting of aelephone et, local cablepairs, LMXterminals, and coaxial cable an'd microwave facilities. Inthe lower figure, the system has been adapted for 50 libwideband service. The 303 data set requires an additionalcable pair to provide 4-wire transmission.' Special ampli-fiers and loss equalizers are spliced into the local cable toadapt i t for wideband signals. .A t the telephone office, adataransmi tte r nd receiver (wideband modem) areadded to modulate the restored polar line signal into thegroup band. FinaIly, special delay and loss equalizers areadded a t connector points to correct the delay and attenua-tion distortion introduced by he LMX bandpass filters.

The delay distortion introduced by the LMX terminalequipment, is depicted in Fig. 3 . The upper characteristicstandard wideband channel and requires two additional cable pairsA voice frequencyoordinationhannel accompanies everyno t shown i n the figure.

represents the relative envelope delay of the terminal re-ceiver and transmitter lone and the ower delay cha,racter-istic is of the group connector filter. The otal relativeenvelope delay distort ion (consisting of the sum of the twocharacteristics in Fig. 3 ) is indicated by the dotted curvein Fig. 4. The delay equalizer that is added to each con-nector reduces the distortion to approximately . t 3 ps asrepresented by the solid curve.A new connector and equal-izer have recently been designed that will further improvethe equalization to +1.5 ps per link so that the residualdelay distortion will be =t12 ps for a ypical 4000 milecircuit.50 kb Wideband Modem Channel ChaTacteristics

Th e 50 kb modem basebandchannelcharacteristic isshown in Fig. 5.It is designed to have a 50 Hz to 37 kHz ,nearlysquarebandandall n-bandshaping is done nthe type 303 data set [3]. The square band characteristic

Fig, 2. LMX ermi nal and 1ocal.loop modifications for widebandservlce.

u-sEc R E L A T I V EN V E L O P EL L A YELAY LM X R E C E I V E R A N D T R A N S M I T T E R

F R E Q U E N C Y - KH z

'"T \ R E L A T I V EN V E L O P EE L A Y 1R O U PC O N N E C T O RU - S E CDELAY 100

60 m 80 90 10 0 IUBF R E Q U E N C Y - K H z

Fig. 3. Delay a t junctions between L MX systems.

DELAY EQUALIZEDGROUP AND 7

60 70 80 90 100 108F R E Q U E N C Y - k H z

Fig. 4. Residual delay distortion of the group band for one link ofLR4X.

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1966 RONNE:IDEBANDERVICESN ANALOG CARRIER SYSTEMS 657

permitsseveralmodems to be connected in andem ifdesired. The high-pass filter located in the type 303 dataset transmitter shapes the spectrumt low frequencies anda rolloff network in t he da ta set receiver shapes the high-frequencyband.

The group band data channel after modulation is showninFig. 6. The wideband nlodem for the LMX systemincorporates vestigial sideband, suppressed carrier, ampli-tude modulation with coherent demodulation [5].Vestigialsidebandoperationpermitsoptimumutilization of theavailablebandwidth. Suppressing the carrier esults inmaximum signal-to-noise performance since the LMXsystem is both total-power and single-tone limited [a].The basic group extends from 60 to 108 kHz. T he available

CHANNEL CHARACTERISTIC

0 I0 25 kH z 3 7 k H z

Fig. 5 . Baseband 50 kb/s channel.

VOICEFREQUENCYCHANNEL4 k H z 1ESTIGIALSIDEBAND7

6 O k H z WIOEBANOFREQUENCYCARRIER

Fig. 6. Group frequency 50 kb/s channel.

Y F ~IN - I

Fig. 7. Fifty kb/s wideband ransmitter.

CARRIERIO Bk Hz . OU TVF

SIGNALIGNAL

Fig.8. Fift y kb/s wideband receiver.

bandwidth is somewhat restricted by a 104.08 kHz pilottone which is used for automatic gain regulation of theLMX broadband terminals. Th e frequency band betweenthe pilot and the 108 kHz band edge is available, however,for the voice frequency coordination hannel. The widebandmodem carrier frequency is 100 kHz, which divides theband into a 4 kHz vestigial upper sideband and a 37 kHzlower sideband, resulting in an equivalent baseband of 37kHz.Wideband Transmit terA block diagram of the transmi tter is shown in Fig. 7.The baseband signal first passes through a line equalizerand low-pass filter. The line equalizer can be adjusted tocompensate for variations in th e loss and delay slope ofthe intra-office wiring. T he low-pass filter limits the noisebandwidth and suppresses high-frequency signal compo-nents which would cause frequency foldover upon modu-lation. A crystal-controlled oscillator generates a 200 kHzsine wave which is then divided to 100 kHz by a singlestage counter. I n this way a square wave carrier is gen-erated with a precise 50 percent duty cycle for modulatingthe wideband signal. The carrier drives a balanced, wotransistor, product modulator. The signal at the ou tputof the modulator is double sideband, amplitude modulated.Power in the vicinity of the carrier frequency is small be-cause of the high-pass filtering carried out in the data set .A carrier frequency pilot tone which is in-phase with themodulating carrier is added to the modulated signal at apower 9 dB below the average power in one sideband. Thebandpass filter following the modulator passes the lowersideband and a 4 kHz vestige of the upper sideband. Thefilter suppresses all modulation products th at fall outsidethe dat a channel toprevent interferencewithadjacentgroup channels that are multiplexed on either side of t hewideband channel. The signal is then delay and loss equal-ized and combined with the voice frequency coordinationchannel or ransmission over the LMX facilities. Theequalizer corrects the delay and amplitude distortion in-troduced by all transmission networks in the transmitte rand receiver and also for the residual delay distortion inone link of LMX.Wideband Receiver

A block diagram of the wideband receiver is shown inFig. 8. The voice frequency and wideband signal are first,separated by bandpass filters for demodulation. The re-ceiving bandpass filters also limit the noise bandwidth andpreventadjacentchannelsfrom nterferingwith he re-ceived signals.

The 100 kHz carrier pilot is separated from the datasignal by a narrow band crystal filter. Th e pilot is used tocontrol the gain of the receiving amplifier and hence thepower level of the received signal. Th e AGC circuit is fast-acting o supplement the slow-acting regulators of theLMX terminals. Impairments due to signal level varia-tions arise in two ways: 1) if the signal level is allowed todrop at the inpu t of a ink, the S /N ratio in that linkwill be reduced, and 2) a signal level variation at the input

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655 lEEE 'l'H.\NS:\CTIONS ON COMMUNICA'I'IONECHNOLOGYCTOBER

of a slicer is equivalent to a variation in the lice level andhence a decrease in noise immunity.

The filtered 100 kHz pilot is slso used as a carrier tocoherently demodulate the wideband signals. Phase errorsintroducedby the crystal pick-off

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coNcLusloNThe telephone carrier systems werc originally designed

for voice frequency transmission but have proven adapt-able to high-quality wideband service. This has been ac-complished by the careful selection of a general data format(restoredpolar)and of modulation schemes tailored foreach of th e facilities. This paper describes the widebandterminal equipment designed for the LMX group band.Other signal processing schemes are required for the wide-band terminals designed for th e X-carrier and TI Carrierfacilities.

The phase-locked filter promises improved performanceover the phase-locked oscillator when used in a coherentdemodulator. It is not only capable of maintaining smallerphase errors in the presence of phase perturbations but th erecovered carrier is always at exactly the correct frequency

and hence the circuit is incapable of bf,eaking ock. LOSS ofpilot automatically shuts off the receiver, preventing tlhegeneration of unintelligible signals due to noncoherentdemodulation.

REFERENCES[ l ] 1. T. James, The evolution of wideband services in the U~ ril, ed[2]J., J. Mahoney, Jr., Transmissionplan for general purpose[ 3 ] R. . 11. Fracassi and F. E. Froehlich, A wideband data station,[4]R. Tarbox, T1 carrier transmission systems for general purposewideband services. 1966 IEEE Intematl Conv. Rec., pt,. 1,

States, this issue, pp. (736-640.wtdeband services, this i s sue , pp. 641-648.this i s s ~ e , p. 648-654.pp. 210-218.[5 ] F. K. Becker, J. R. Davey,and B. R. Saltzberg, A n A Mvestigial sideband data transmission set using synchronoltsdetection for serial tmnsmission up to 3,000 bit,s per second,i l IEE Trans. Communication and Electronics), vol. 81, ptj. I,

Concise PapersThe Covariance of the Frequency of a Narrowband

Gaussian Random Process

Abstract-The mean andcovariance of the nstantaneous re-quency of a narrowband, zero-mean, stationary, Gaussian randomprocess are found when the quadrature components of the processare denticallydistributed.When he quadrature componentsarestationaryand statistically ndependent, the mean andcovarianceexpressions are identical o well-known expressions.

The mean and covariance of t,he instantaneous freqllency of anarrowband, zero-mean, stationary Gaussian random processhave been found by Lawson and Uhlenheck [ l ] , nd Rice [2],Ilnder t,he const.raint that the quadratmureomponents: of t,he processare st,at.istically independent and identically distributed.Anothermethod of finding the mean and covariance of the instantaneousfrequency of a zero-mean, narrowband, Gaussian random process ispresented here. The process need not be st,ationary, and the qnad-rature components need not, be statistically independent,; however,the quadrat,n re components mus t be ident,ically distributed. Whent,he process is not stationary, or t.he qua dra ture component,s are not,st.atist,ically independent, t,he covariance of the frequency containsa term that is not, present in Lawson and Uhlenbecks [ I ] or Rices[2 ] expressions.

Th e mean and covariance of the inst,antaneous frequency are ofint,erest in the analysis of F M systems ,hat, use a fading channel.

renearch Derformed while th e author was witkthe Communication aboratory ofI\iIanuscript received Apr i l 14.1966; rev ised June 15, 1966. This paper presents~ ~.Stanforditesearch Institute.versity, Ithaca, N. Y .The author is with heDepartmentofElectricalEngineering, Cornell Uni-

When tshe multiplicative disturbalrces of the channel are modeledas a random, time-variant linear filter, with a time-variant transferfunction that is a realization of a zero-mean, Gaussian randomfield [3] , he channel output is a realization of a zero-mean, non-stat,ionary, Gallssian random process. The resttlts given here aredirectly applicable to a F M system wing s11ch a channel.

The complex representation [4]-[6] is used for all narrowbandGaussian random processes throughout his note, thus all narrow-band Gaussian random processes are zero-mean and ointly circu-larly complex, i.e.,

E[ z ( t ) ]= 0 and E[ z (~ ) ! / (u ) ] 0. (1)The statisticsof two such processes are thlls cornplelely det.erminedby the cross-covariance fnnction

czb,(t,u)= EML)Y*(?L)I (2)and the two auto-covariance functions

c.( t ,u) = E[z( t )z*(u)]nd cY(l,u) = E[u(l)y*(u)]. (# )[The well-known expectation operator is E [ ] nd y* s the complexconjugate (and transpose, if appropriate) of 11.1 Other well-knownproperties of th e complex representation are ha t he magnit,ndeof the complex representation Iz(t)l is the envelope of the narrow-band process, and ha t he phase of th e complex representat,ioll+ [ z ( t ) ] s the phase of the narrowband process.

This note is also concerned with s ( t ) , the instantaneous complcxfrequency of a process, which is defined as the derivative, wit,h re-spect to ime, of t,he natural logarithm of t.he process

s ( t ) a- In[z(t)] = Ih [z ( t ) ]= $(t) /z( t ) . (4)aat659