a new microcell architecture using digital optical transport

7/30/2019 A New Microcell Architecture Using Digital Optical Transport

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A New M icrocell ArchitectureUsing Digital Optical Transport

PhilipM. WalaWEtseca Technology, Inc.

RR4Box 145,P.O. Box286Waseca, MN 56093USA

Abstract - The performance of microcells which rely onanalog modulation of an optical or microwave carr ier islimited by the dynamic range of the analog transport.This paper describesa new microcell design which relieson high speed, analog-to-digital conversion to digitize theentire cellular band, enabling transparent RF transportto be accomplished over a digitally modulated opticalfiber. L aboratory and field tests of an ADC K entroxmicrocell which uses this technique are describied. Theapplication of this technology to the future design of anall-digital cellular base station is also discussed.

I. INTRODUCTIONThe trend towards a "personal communications" environ-ment requires the ability to deliver sufficient coverage and

capacity to provide truly ubiquitous service. Coverage mustbe provided in areas not served by macrocell antenna instal-lations, including shadow areas, tunnels, and the interiors ofhigh-rise office buildings. Capacity must be sufficient to ac-commodate high concentrations of users in convention cen-ters and train stations. In addition, as cells continue to de-crease in size and increase in number, the cost of ireal estatebecomes significant, requiring that this coverage and capac-ity be delivered using equipment occupying a minimumamount of space. Finally, the plurality of potential modula-tion techniques, ncluding FM, TDMA and CDMA, providesincentive for an infrastructure configuration that maintainsflexibility for future system upgrades.

11 LIMITATIONS OF TRADITIONALOLUTIONSSolutions to the problem of delivering coverag,e and ca-

pacity fall into three general categories: cellular repeaters,o-78o3-1266-x/93~$3.00@19931EE E

distributed radio microcells, and centralized radio microcellsi l l .Cellular repeaters increase coverage by re-radiatingoff-the-air signals. Because existing channels arere-transmitted, there is no capacity increase. Also, systemdegradation can occur due to the fact that noise andinterference are re-radiated, along with the desired signals.

Distributed radio microcells or "mini-cells" are miniatureversions of cell sites, incorporating a separate receiver andexciter for each channel served. M i l e such a configurationcan add both capacity and coverage:, size and cost will in-crease proportionately to the number of channels. Such asystem also lacks the ability to upgrade to new modulationtechniques without replacing all of tlhe transceiver modules.Centralized radio microcells transport a broadbandspectrum of combined radio frequency (RF) signals betweenremote antenna sites and banks of centrally locatedsingle-channel transceivers. This configuration has theadvantage of transparency to modulation technique and theability to serve multiple channels without any change to theremote equipment.Most products of this type, however, suffer from the disad-vantage that the transport is accomplished by the analogmodulation of an optical or microwave carrier. This severelylimits the dynamic range of the system for two reasons.

First of all , there is a tradeoff between signal-to-noise ratioand third-order intermodulation performance, which is afunction of the modulation depth per channel [2], [3]. It canbe difficult to maintain the optimum operating point, espe-cially in a system where the number of active carriers canbeconstantly changing. Ultimately, in an optical system, thethird-order intermodulation performance of the diode laserbecomes the limiting factor [2], [4].

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Secondly, the nature of analog modulation is such that thesystem dynamic range is destined to degrade in proportion tothe attenuation on the transport link. In an optical system,this degradation is equal to twice the optical attenuation indB [5], yielding an optical budget of as little as 10 dE3 [6]and maximum fiber distances on the order of 10km [7].

1550 nm -

111 A NEWAPPROACH:IGITALW TRANSPORT

Opt ica l i z Dig i ta l -RF L inearRec e i ver C onver t o r- o w e r -mpl i f i er

Many of the problems associated with analog modulationcould be circumvented if it were possible to first digitize theinformation signal and then transport it digitally. Up untilnow that possibility has not been considered for the transportof the broadband "A " or "B" cellular spectrum because ofthe difficulty of digitizing the 12.5 MHz bandwidth withenough speed, resolution and linearity to accurately repro-duce the dynamic range of signals found in the mobileenvironment. Recently, however, analog-to-digital convert-ers with sufficient performance capabilities have becomeavailable [SI, and have been successfully incorporated into anewly announced fiber- fed microcell product [9].This digital microcell system consists of two main compo-nents: a donor or "host" site interface, and a remotemicrocell unit (Fig. 1)

552.96 M b p s ' .80 0 MHzerial : 30.72 MHzParallel ,RFI WDM

~

1310 nm Opt ica l 12 RF-Digitol

In the forward or "downlink" path, the host interfacetakes the combined channels from the cell site transmittersand digitizes the broadband signal at a sample rate of 30.72MHz. An optical transmitter adds overhead and signalinginformation and creates a 552.96 Mbps serial data streamwhch is used to digitally modulate an optical carrier. At theremote site, an optical receiver demodulates the incomingoptical carrier, and reconstructs the original 30.72 M Hz par-allel word, which is then converted back to RF by adigital-to-RF converter. At this point, the combined forwardlink channels have been reconstructed, and are ready to belinearly amplified for transmission.

In the reverse or "uplink" path, signals coming in throughthe antenna from mobiles or handhelds are filteredto removeout-of-band signals. The 12.5MHz spectrum is then digit-ized in much the same way as the forward link spectrum,and a similar serial data streamisconstructed for digital op-tical transport. At the host site, the optical carrier isdemodulated, and the original RF signal is reconstructed andsent on to the individual channel receivers.This particular configuration also incorporates the use oftwo different wavelength lasers and a wavelength divisionmultiplexer (WDM ) in order to enable bi-directional trans-port of forward and reverse path RF over a single fiber.

Dup l exer_ _

C E L L SITE RF E Q U I P M E N T M I C R O C E L L HOST I N T E R F A C E

Transmitter

80 0 MHz ; 30.72 MHzParallel WDM :RFDig i ta l -RF 12 Opt ica l 13io nm 1Receivereceivers : Sp l i t t e r , C onver t o r - *

Conver tor -

552.96 MbpsSerial1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .optical fiber (bi-direc tional digital link)

Fig.1. Digital RF transportmicrocell: systemblock diagram586


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The host interface for each microcell is contained in all7" x 7" x 8.5" rack mountable module. The remote cabinetis 34" x 20" x 11" (including power supply and backup bat-teries), and can be pole or wall mounted.Iv. DIGITAL? TRA ~TSPORT:ERFORMANCE EVALUATIONMicrocells employing the digital transport architecture

have been tested on the bench as well as in actual service,and have demonstrated all the advantages of other central-ized radio microcells, including transparency to modulationtechnique and channel selection, and a small remote unitsize that is independent of the number of channels being car-ried. The absolute group delay due to the filtering anddigitization process has been measured at less than 4 psacross the band, and should not adversely affect TDMA orCDMA signals.In addition, the digital microcell has proven to have somesignificant advantages over earlier generation analog ver-sions. Because the forward link carries a relatively narrowdynamic range of signals, with the path performance typi-cally l imited by the linearity of the power amplifier (as inanalog microcells), much of the following discussiion will fo-cus on the transport of the more hostile reverse link (mobileto cell site) spectrum.

Assuming that the digital transport can be accomplishedwithout significant errors, the reverse link dynamic rangewill be limited by signal-to-noise ratio and linearity of thedigitization and reconstruction process. In analog systems,the spurious free dynamic range is a function of the noiselevel in the signal bandwidth (No)and the third-lorder inter-cept point (IP3),both in a m :

SFDR = (2/3) x ( IP, - No (1)Neglecting fiber loss, distributed feedback (DFB) lasers withan output power of +3 a m ave achieved a s~rurious reedynamic range on the order of 82 d3 [4], with ;an absolutetheoretical maximum of approximately 88 dB, assuming a30kHz signal bandwidth [lo].

In the digitized system described here, (1) caiinot be ap-plied, because the third-order intermodulation products notonly fail to rise faster than the fundamentals, their level mayactually drop as the fundamental signals increase or addi-tional signals are added (due, in part, to the "ditheringeffect", which can add effective bits of resolution to an over-sampled system). Consequently, one must evaluate the

dynamic range by noting the behaviior of the noise floor andthe third-order products as a functialn of signal level in orderto determine the effect of any anomalies in the curves. Intheprototype system, third-order spurious free dynamic rangewas limited to about76dB.

The real advantage of the digital transport methodbecomes apparent when one examines the optical budgetsofthe digital and analog systems. First of all, although thedigital system third-order dynamic range is less than thetheoretical maximum attributed to analog systems, it shouldbe noted that this is achieved with a -4 dBm output laser,rather than the +3 dBm assumed earlier. An analog systemwith an equivalent laser power would be limited to amaximum dynamic rangeof about74dB.

Secondly, the nature of digital transport is such that, upuntil the point where the optical level approaches the sharpthreshold where synchronization is lost, there is no degrada-tion of the signal being transported whereas analog systemssuffer2dB of degradation for every dB of optical attenuation[5]. The digital transport can maintain the RF path at itsfull dynamic range for optical levels as low as -31 dBm(more than a 20 dB improvement. over analog transport).The dramatic nature of the improvement can be seen in Fig.2, with an arbitrary 65 dB dynamic range performance re-quirement marked to show the effect on the usable fiberdistance.

V. DIGITALF TRANSPORT:JTURE PPLICATIONSThe dynamic range and optical budget advantages already

demonstrated constitute only a portion of the potential bene-fitsof digital RF transport.

80

U- 70

40L I b ' 20 I j o ' 4b ' 5 b ' st01 I " . . , I I I Ifiber distance (km)

Fig.2. Dj"mic rangevs. fiber distance (-4dBm lasers)

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Further potential advantages stem from the fact that digi-tal signals are well suited to being time-&vision multiplexedwith other digtal signals. Although the current system usestime-division multiplexing to carry secondary digital chan-nels with alarm and control information, secondary channelscould serve a multitude of other purposes, including anuplink diversity path, an alternate RF service (such as pag-ing), or digitized cable TV signals.

Ultimately, the establishment of a standard for the &gitaltransport of RF could lead to a digital RF distribution sys-tem, carrying cellular or personal communications services,paging, trunking, or even broadcast signals, with drop andadd capability, to a widespread network of microcells situ-ated to provide seamless coverage of a service area.

Perhaps the biggest potential for the future of digitaltransport can be anticipated by another look at Fig. 1. Be-cause the voice channels between the mobile telephoneswitching office (M TSO) and the cell site are often alreadyin digitized form, and the interface to the host microcell in-terface optical transceiver is a parallel digtal word, onemight legtimately wonder why one should even bother tomake the intermediate conversion to RF. Conceivably, thecell site transceivers, combiners, splitters, and RF-digitalconverters could all be replaced by a digital signal process-ing unit, which would take the digitized voice signals andsynthesize a digitized RF spectrum, reversing the process onthe uplink path. In this configuration, RF would appear onlyat the remote microcell cabinet, and all cell site RF trans-ceivers, combiners, splitters and filters would be replaced byan all-digital base station.

VI. CONCLUSIONSThe application of new digital RF transport technology to

a fiber-fed microcell application has been described, and itsperformance compared with that of analog microcells. Digi-tal microcells have demonstrated dynamic range as good asor better than their analog counterparts, with no discernibledegradation for fiber distances in excess of 50 km. DigitalRF transport also has the potential for revolutionizing theway we think about distributing RF by making possible adigital RF transport infrastructure, in which a network of re-mote antenna locations are served, without degradation,from a centrally located all-digital base station.

ACKNOWLEDGMENTSThis work was performed under contract to the Wireless

Systems Group of ADC Kentrox, Portland, Oregon inassociation with Steinbrecher Corporation, Woburn,Massachusetts, and American Lightwave Systems, Meriden,Connecticut.

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a new microcell architecture using digital optical transport

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