IMPROVED SPECTRAL COMPLIANCE FOR FM HD RADIO USING DIGITAL ADAPTIVE PRE-CORRECTION

Transcription

1 IMPROED SPECTRAL COMPLIANCE FOR FM HD RADIO USING DIGITAL ADAPTIE PRE-CORRECTION ABSTRACT HD Rado mplementaton has ntroduced a great deal of dscusson about spectral re-growth problems when dgtal carrers ntermodulate wth the prmary FM carrer, causng spurous emssons on adjacent channels. Pre-correcton systems may be mplemented to mtgate the effects of transmtter system nonlnearty gvng rse to the out-of-band emssons. TIM HARDY Nautel Mane Inc. Bangor, Mane Conventonal fxed pre-correcton technques have not provded a suffcent soluton to ensure spectral ntegrty n a changng envronment. Changes n SWR, an adjustment n the output power of the transmtter, a change n amplfer temperature, or agng and falures of RF amplfers can result n serous transgressons of the HD Rado mask and nterference wth other statons. Ths paper presents theory and measured performance of dgtal adaptve pre-correcton under unstable envronmental condtons. Comparson s made between fxed pre-correcton curves and adaptve precorrecton under typcal condtons at broadcast stes. BACKGROUND The trend n the broadcast ndustry s now toward dgtal communcatons standards. The Bquty FM HD Rado system employs a dgtal modulaton technology known as Orthogonal Frequency Dvson Multplexng (OFDM. Ths modern communcatons technque provdes both excellent bandwdth effcency and hgh tolerance to the mult-path fadng envronment common n urban settngs. All HD Rado systems gong on the ar today are hybrd systems, whch means the transmtted sgnal conssts of both an analog FM modulated porton and a dgtally modulated OFDM. The dgtally modulated porton conssts of many QPSK modulated OFDM carrers spaced 363 Hz apart. The carrer spacng and carrer symbol rate are such that they are orthogonal,.e. they do not nterfere wth each other. In the basc hybrd mode the man OFDM spectrum contans 382 carrers from approxmately 130 khz to 200 khz, both above and below the channel center frequency. Fgure 1 Hybrd FM HD Rado Spectrum In the extended hybrd modes the spectrum contans up to 534 carrers from approxmately 100 khz to 200 khz, both above and below the channel center frequency (Fgure 1. Wth the ntroducton of OFDM, transmtter desgns have had to evolve due to a new requrement: lnearty. Tradtonal FM transmtters operated hghly non-lnear class C amplfers. Ths was acceptable due to the constant envelope nature of the FM sgnal. The OFDM envelope has large scale varatons due to the changng constructve and destructve nterference of the ndvdual RF carrers. Whle the answer to the lnearty problem les n part n the desgn of the amplfers themselves, correcton technques are also requred n a practcal mplementaton. LINEARITY AND THE EFFECT ON SPECTRUM: Interference s one of the most sgnfcant potental problems when workng wth the HD Rado FM system. The sgnal broadcast by any transmtter may be broadly categorzed nto two classes: ntentonal and unntentonal emssons. Intentonal emssons nclude the FM sgnal and the OFDM sgnal. Unntentonal emssons nclude RF harmoncs, spurous emssons and nose. Whle some nterference problems may have been caused by ntentonal emssons, the focus of ths paper s on the mtgaton of unntentonal emssons due to amplfer non-lnearty. All real amplfers

2 devate to some degree from the deal requrement that the gan s ndependent of nput voltage. Generally, gan s reduced as nput levels ncrease toward amplfer saturaton (Fgure 2. Ths s referred to as AM-AM dstorton. AM-AM dstorton may be descrbed by a functon that relates the magntude of the nstantaneous gan to the nstantaneous nput magntude. IM3 occurs n the spectrum at the followng frequences: cos( 2 * w1 w2 cos( 2 * w2 w1 The ffth order term of the Taylor seres produces ffth order ntermodulaton products, IM5, that occur at these frequences: o = g m ( (Eq 1 cos( 3* w1 2 * w2 cos( 3* w2 2 * w1 Note that the hgher order terms of the Taylor seres also contrbute to the lower order ntermodulaton products. In ths case, the IM3 products also have a contrbuton from the ffth order polynomal term expanson. Fgure 3 shows the two tones, IM3 and IM5 dstorton products. Fgure 2 AM-AM Characterstc Alternately, the AM-AM characterstc can be modeled by a Taylor seres expanson relatng the nstantaneous nput voltage magntude to the nstantaneous output voltage magntude. Both equaton 1 and 2 may accurately represent the AM-AM characterstc of a typcal amplfer. o a3 + = a a... a (Eq 2 The frst term of the expanson s the lnear term. The coeffcent a 1 s the small sgnal gan of the amplfer. For an deal amplfer, all hgher order coeffcents would be zero. Even order terms of the Taylor seres are often not used as they cannot produce any dstorton terms n the band of the fundamental RF term. The effect of ths non-lnear model can be llustrated by drvng ths functon wth a two tone nput sgnal. = cos( w t + cos( w2 1 t (Eq 3 The response of the Taylor seres model to a two tone nput sgnal can be determned by expandng and usng trgonometrc denttes. In addton to fundamental and harmoncs of the nput tones, dstorton products are produced. Thrd order ntermodulaton products, IM3, result from the thrd order term of the Taylor seres. 5 5 n n Fgure 3 Output of Ffth Order Model wth Two Tone Input The non-lnear response of amplfers may be measured usng a network analyzer by dong what s referred to as a power sweep. In ths type of measurement, the amplfer s characterzed usng a sngle RF nput tone of varyng voltage. As the voltage s stepped over a wde range, the output of the amplfer s measured. If the output voltage magntude s dvded by the nput voltage magntude, a gan characterstc descrbng the amplfer AM-AM dstorton s determned. For example, the ffth order Taylor seres would have the followng gan characterstc. Gan + o 2 ( = = a1 + a3 a5 (Eq 4 Addtonally the network analyzer wll tell us that the phase measured between the nput and output of the amplfers was not constant durng the power sweep. Ths phase characterstc s called AM-PM dstorton (Fgure 4. (Eq 5 o Phase = g φ ( 4

3 Fgure 4 AM-PM Characterstc Usng the functons g m and g Ø representng the AM-AM and AM-PM dstortons, the relatonshp of the nput voltage to the output voltage s shown. o = g( = g m ( jg ( φ e (Eq 6 Equaton 6 s useful because t suggests the amplfer characterstc s essentally a complex valued gan defned solely by the nput sgnal magntude. The amplfer complex gan s also defned by the AM-AM and AM-PM dstortons that may be measured and tabulated by a network analyzer. As was descrbed by the two tone analyss shown above, the amplfer non-lnearty wll ntroduce dstorton to the sgnal f not corrected for. Fgure 5 shows the output spectrum for a typcal FM amplfer operatng wth the HD Rado sgnal. Fgure 5 Un-Corrected Amplfer Spectrum Ths sgnal contans the upper and lower OFDM sdebands that nter-modulate n a smlar manner as the two tone test descrbed above. The IM3 and IM5 products can be clearly seen. In addton the carrers of the ndvdual OFDM sdebands ntermodulate causng skrts. The IM products are unntentonal emssons from the transmtter that may cause several undesrable effects f they are not controlled: Out of band nterference wth adjacent Broadcast channels In band nterference wth your HD Rado OFDM sgnal In band nterference wth your FM sgnal Out of band nterference can happen when the transmtter output contans sgnals n another broadcast channel. The acceptable out of band emssons levels are currently governed by an emssons mask proposed by Bquty (Fgure 6. Fgure 6 Proposed HD Rado Emssons Mask

4 In band nterference wth the dgtal sgnal occurs when the unntentonal emssons nterfere wth the OFDM carrers at the recever. Ths s generally less of a concern because the QPSK modulated carrers are very robust and relatvely tolerant of nose and nterference. It s very unlkely that any transmtter lnear enough to meet the proposed emssons mask wll have a negatve effect on the OFDM sgnal to nose rato at the recever. In band nterference wth the FM Sgnal may also occur. Ths may be more sgnfcant as the analog sgnal can be degraded by lower levels of emssons than the dgtal sgnal. The problem worsens f the unntentonal emssons are close to the center frequency where the FM recever s most senstve. Because the FM system s analog, ths nterference could be perceved as nose or hss when usng analog recevers. Extended hybrd operaton may exacerbate ths stuaton as the addtonal OFDM carrers and ther local IM products are closer to the FM carrer. by Nautel and other HD Rado transmtter manufacturers s known as pre-dstorton or precorrecton. Pre-correcton s the preferred technque for Nautel because of ts hgh performance, stablty, low cost and low mpact on the transmtter desgn tself. PRE-CORRECTION: Pre-Correcton can be defned as placng a complementary non-lnear system at the nput of the Amplfer stages such that the overall system s lnear (Fgure 7. Because the pre-correcton system s at the amplfer nput, small sgnal technques may be used makng ths a more practcal soluton. SOLUTIONS TO THE SPECTRUM PROBLEM: The amplfer lnearty may be mproved for HD Rado operaton by changng the class of operaton from class C to A/B or even class A. Ths generally has the effect of mprovng the amplfer characterstc at low levels at the expense of effcency. Hghly lnear operaton may be obtaned n ths way by ncreasng the amplfer bas currents and reducng the peak output power. Typcally a 3 db mprovement n IM3 products s obtaned for a 1 db reducton n output power. Unfortunately, ths cannot be the complete soluton due to the ncreased number of amplfers requred. Other ndustres before broadcast rado have been copng wth the lnearty problem for some tme. Many technques for mprovng amplfer lnearty have been nvestgated ncludng feed forward, feedback and cartesan loop to name a few. The technque employed Fgure 7 Complementary Gan Curves - Pre-Correcton Prncple Mathematcally, a non-lnear amplfer characterstc g(x may be corrected for wth a complementary characterstc h(x such that g(h(x = Gx where G s the constant lnear gan resultng from the cascade of the two systems. For ths to be true, G h(x = g -1 (x. Fgure 8 Fxed Dgtal Pre-Correcton Block Dagram

5 Fgure 9 Adaptve Dgtal Pre-Correcton Block Dagram Ths technque may correct for both AM-AM and AM-PM dstorton. Pre-correcton systems may be mplemented n ether the analog or dgtal doman. Analog pre-correcton may be lmted n ts ablty to fathfully produce the requred nverse characterstc. The degree of mprovement made by pre-correcton s determned by how well the two non-lnear characterstcs are matched. When properly adjusted, dgtal precorrecton generally yelds excellent results. Even f careful manual matchng of the pre-correcton curve s done, the amplfer characterstc wll generally vary over tme. The causes of varyng amplfer nonlnearty nclude temperature effects (seasonal or warmup, load mpedance changes due to antenna msmatch or cng and changes n the operatng frequency or power level. ADAPTIE PRE-CORRECTION: Desgnng a system that learns the non-lnear characterstc wll mtgate the senstvty of fxed precorrecton lnearzaton systems. Ths s known as adaptve pre-correcton and was frst proposed n 1983 by Saleh and Salz. The Nautel system s mplemented completely n the dgtal doman (Fgure 9. In the forward path, an deal dgtal sgnal contanng the HD Rado carrers and the FM sgnal (n the case of common amplfcaton s syntheszed. The deal sgnal s then pre-corrected usng a correcton curve stored n a look up table (LUT. Ths LUT stores a large number of dscrete correcton vectors. Each correcton vector can correct for the amplfer gan and phase error for a gven amplfer nput voltage range. As the sgnal ampltude changes over tme, many dfferent correcton vectors are used.

6 Stll n the dgtal doman, the pre-corrected sgnal s then converted to the correct FM channel frequency and fed to a dgtal to analog converter (DAC. The analog sgnal s then fed to the transmtter for amplfcaton. In the reverse path, the transmtter output s sampled usng a drectonal coupler and dgtzed wth an analog to dgtal converter (ADC. The output sample s then shfted to the same frequency as the deal reference sgnal at the system nput. The reference sgnal s fed through a delay regster to tme algn t wth the sampled transmtter output sgnal. By subtractng the deal sgnal from the actual transmtter output, an error sgnal s obtaned. Ths error sgnal descrbes sgnal dstorton at the transmtter output. Usng a recursve algorthm, the error sgnal s used to update the pre-correcton curve stored n memory. After each teraton of the recursve algorthm, the correcton vectors n the LUT converge on the deal pre-correcton soluton such that dstorton at the transmtter output s mnmzed. Utlzng modern dgtal hardware, the converson tme s on the order of a few seconds, easly fast enough to correct for real world varatons n the amplfer nonlnear characterstc. Adaptve Pre-Correcton Lmtatons: Saturaton: At some pont, the amplfer s output power cannot be made to ncrease when the nput power s ncreased. As a result, the desred output power must be set so that the amplfer saturaton pont does not sgnfcantly dstort the sgnal. For the HD Rado sgnal, Bquty requres a mnmum peak to average rato of 5.5 db at the transmtter output. For example, a 10 kw transmtter capable of 11,000 W peak power should not be drven beyond 3,100 W average dgtal power n a separate amplfcaton system. If the dgtal power s ncreased sgnfcantly beyond ths pont, amplfer saturaton may ntroduce unacceptable emssons outsde the emssons mask. Amplfer memory: The relatvely smple adaptve precorrecton system descrbed assumes that the amplfer s memoryless. Amplfer memory occurs when the gan at any one nstant n tme s dependent not only on the current amplfer nput but also on prevous amplfer nputs. For sgnals of relatvely low bandwdth, usng careful amplfer desgn technques, memory effects can be mnmzed such that the effect s not troublesome. Comparson of Fxed and Adaptve Pre- Correcton: The followng experments were conducted to determne the mprovement obtaned by usng an adaptve pre-correcton system. In each experment a reasonable devaton of the amplfers operatonal envronment was made. These changes were ntended to represent the real world condtons that mght be found at a typcal broadcast ste. These measurements were made on a Nautel 10 operatng n all-dgtal mode as requred by the dgtal transmtter n a separate amplfcaton system. The output power n every case s 3 kw and the operatng frequency s 98 MHz. The spectrum analyzer was set to 1 khz RBW and BW, sample detecton wth a 30 sweep average. Each spectrum plot shows the spectrum resultng from makng a change n the operatonal envronment wth the adaptaton dsabled. After the ntal measurement s made, the adaptaton was enabled and a second measurement was made llustratng the mprovement due to the adaptve pre-correcton. SWR senstvty: In ths experment, a 1.5:1 SWR was ntroduced by means of a short crcut stub on the transmtter output. Fgure 10: 1.5:1 SWR wth fxed and adaptve pre-correcton The SWR test shows that adaptve correcton acheved an mprovement of 7 db over fxed correcton on the lower or worse IM3 sdeband (Fgure 10. Also note that the fxed pre-correcton dd not ensure complance wth the emssons mask wth a 1.5:1 SWR.

7 Temperature Senstvty: In ths test the ar ntake and exhaust ports were mpeded untl the exhaust ar temperature rose by 50ºF. Ths was ntended to smulate a change n the room temperature that mght be found wth seasonal varatons at a ste wthout heatng or ar-condtonng. Fgure 11: 29ºC Temperature Rse wth Fxed and Adaptve Pre-correcton The temperature test shows that adaptve correcton acheved an mprovement of 8 db over fxed correcton on the lower or worse IM3 sdeband (Fgure 11. Also note that the fxed pre-correcton dd not ensure complance wth the emssons mask wth a 50ºF temperature change. Frequency Senstvty (N+1 capablty: In ths test the frequency was changed from 88 MHz to 108 MHz wth fxed and adaptve pre-correcton. Whle ths test s not relevant to most statons because they always operate on a fxed frequency, t s sgnfcant n nstallatons where there s a sngle backup transmtter for multple statons. Fgure 12: Frequency Change From 88 MHz to 108 MHz wth Fxed and Adaptve Pre-correcton The frequency change test shows that adaptve correcton acheved an mprovement of 12 db over fxed correcton on the lower or worse IM3 sdeband (Fgure 12. Also note that the fxed pre-correcton dd not ensure complance wth the emssons mask wth frequency change across the band. CONCLUSIONS Pre-correcton s the accepted lnearzaton technque used by FM HD Rado transmtter manufacturers. Precorrecton can correct for AM-AM and AM-PM characterstcs that would otherwse result n unacceptable emssons. Fxed pre-correcton systems are unable to correct for the effect of a varyng envronment on amplfer nonlnear characterstcs. Expermental results show that SWR, temperature and frequency changes typcal of many broadcast stes may result n unacceptable emssons as defned by the proposed HD-Rado emssons mask. Dgtal adaptve pre-correcton dd not suffer a smlar degradaton. Emssons were mantaned wthn the mask at all tmes.