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`O Fficc of Business l_".|1tcrpt-lacs
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`stamped pages and continuation of contents page and article "256 QAM Modern for
`Multicarrier 400 Mbit/s Digital Radio" pages 329-335 - are a true and accurate
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`

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`fi¢:73RIL 1987
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`VOLUME SAC-5
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`NUMBER 3
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`(ESSN 0733-8716}
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`A PUBLICATION OF THE IEEE COMMUNICATIONS SOCIETY
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`G1-csl Editorial
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`. J. C. Y. Huang, K. Kmlriyrmia, A. .Lc*(‘iw'I, and F. Si'rIi)eii.v1k
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`31?
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`.‘igii Lei.':-i' Mridiiiariaii Ta:’(‘}'miqm*.i‘ for 1)igim.l mm’ Aiinfog Raa'i'r1.v
`‘. diagonal Ccichannel Operaiiion for High Spectrum iiflicieney Based on the Example of 16-QAM--I40 Mbit/s Systems Using Rolioi'i'O l9
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`.-‘C. Vogei
`QA M Modem for Mliliiczirrier 400 Mbii/5 Digiial Radio ,
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`iiilcvei QAM Me-dulziiion Techniques for Dig-iini Microwave Radios .
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`. Y. Drzido, S. Trtkenmlm. E. ."'ir/<uda, T. Saimrii’. mm’ H. Nakrrmnrrti
`icm Puraniclcrs and PCi‘iOI'I'I'I:li1CC of High Speed Modems on Analog Radio .
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`«gt: and Field Test oia 256—QAM DIV Modem .
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`. .1 W. C-7fflPa‘i'?7£’J"1’i'J‘1. C. -'3. HRS-'vi’I'. J. J. M0-‘(’J".i, T. C. ilfock. F. /I. fl:f00tL}’\ R. S. Simons, HI, E. B. D. Ha.l'mi, mid J. A, Ritchie, Jr.
`. 4-{JAM and 256—Q;‘\M Coded Modems for Microwave and Cabie System Applications .
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`: ‘.‘liCIillL)Il of Coded Modulation to 1.544 Mbil/5 Data—in—'Voice Modems For FDM FM and SSH Analog Radio Sysicins. .
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`. B. E. (.'crHirr.s', T. R. Fisdier, S. A. GJ"0.’i(‘J'?i\’_’}’(’l", and R. J. M<‘Gm'rt?
`mprigaiion and Uiamm‘ Mcirim'i'ii_i;
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`. J. Lmsrr-gram mm’ M. .5‘yii.~ai'n
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`‘alive Fading Radio ChanncJ.~‘.: Modeiingaind Prediction
`iisiicul Analysis/Siinulzilion of 21 Three Ray Model for Mullipiitli Fading with Applications to Outage Prediction .
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`.="!io Clianncl Cliaraclerizzilion by Three Tones .
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`. .13. H. Lin wire‘ /1. .4’. Giger
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`fl‘ éCI'\’illl0IlS and Conclusions from 21 Three Year Digilail Radio Field Experiment in Australia .
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`. J. C. Campbeli. R. P. COl4l'!.$', A. L. Mwmi. and R.
`.L. Reid
`.;ic1‘:a‘ioii Sigiinturcs, A Sinlislieaily Based. Dynnniic, Digital Microwave Radio System Measurement Technique .
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`"'7 v Operation of Dual Polarized QAM Sysieins in the Preseiice of Depolarization Crosstalk and Diliei'eiilia| Fading .
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`P. F. Duvoisiii, 5'. T. I-fsirir, and E. P. WiHi'rrim'o:I
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`i {L11prme C01!fl!£’J'HJ(?(I.$'i'l'.l"(’
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`'.i‘L=c‘!iniqiie.v
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`iinccd "i"imc— and l—'requeney—|‘)oinain Adaptive liquaiization in Mu]iiievc1QAM Digital Radio Systems _
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`_ __ _ ._ G. St-fmid, B. Imiki. riiid J. A. [V-:.-.r.mlc
`-iplivc iiquaiiralion of iiigli (.'apncity M-QAM Radio Systems on Muiiipatli Fading Channels -
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`. . 0. Andn'smm, G. Bi'arimm'. mm’ L. (’m'amiriiio
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`'l Digital Adaptive Equaiimliuii in 54—QAM Radio Systems .
`_i_orm:uice of Fiziclioncd and Noniraclioned Equalizers with i-ligli-Level QAM .
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`. R. Agusri, F. C1:.9ar."a.'aH,ririd J’. J. Oiiiius
`1_’Cw Class of Adaptive Cr0sS—Pol£iriZ211ion interference Canccllers for Digital Radio Systems . .. . .. . _. .
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`E "'.|li1il?.(:d Cross-l’0l21riZa1ion Interference Cnnccller for Mullilcvcl Digiial Radio .
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`11" Cr0s.:‘.~P0l Cancellur for Mierow-.wc Radio Syslcnis .
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`_ J. W. Cr.-rim, Y. Bm-—N._w5s, S, Gross, M. L. .S'.'efnberger,mir1 W. E. Srriddiforzf
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`‘.3-:cisi0ri—Dirccled Divttrsity Combiners~—Prineiple:: and Siiiiulaiion Results .
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`.‘.fialJ.sysIz'rii Peiforiiiarice
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`"H13 '|iimes~Four Carrier Iiecovci-y in M—QAM Digital Radio “Receive”; .
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`. A. J. Rusiaka, Jr., 1_,.}. G:'eeri.i'r:%!'n, R. S. Rmiirrn. and A. A. M. Safeili
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`*"-:>’furn1:uice Analysis of Digital Radio Links with Nonlinear Transmit Amplifiers .
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`.S.PrqJoiii1mru' L. J. Grin:-ii.cic=ixi
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`524
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`(C'0im'ni«'d on oii.r.rir:l(? bark cover)
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`Page 00002
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`

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`4:
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`IEEE COMMUNICATIONS S0(‘lI~;'I‘Y 3'“
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`
`Page 00003
`
`

`
`3 IOL
`
`(AL ON SELECTED AREAS IN COMMUNICATIONS, VOL. SAC-5, NO. 3, APRIL I987
`
`256 QAM Modem for Multicarricr 400 Mbit/s
`Digital Radio
`
`YASUHISA NAKAMURA, YOICHI SAITO, MEMBER,
`
`IEEE, AND SATORU AI-KAWA
`
`Absl‘ract—-This paper describes the performance of a 256 QAM Ino-
`dem with 400 Mbit /5 transmission capacity. A variety of novel tech-
`niques are introduced as ways to achieve good performance. Key tech-
`niques include 1) an accurate 256 QAM modulator employing a new
`]Lnm.mit|iic multiplier EC, 2) a carrier recovery circuit which satisfies
`such requirements: good phase jitter performance and no false lock
`plienomenon, 3) a highly stable high-level decision circuit, and 4) a
`forward error correcting code. As an overall modem performance, BER
`characteristics and signatures are presented. The equivalent CNR deg-
`radations ol‘ 1 dB (at BER of 10“) and 2 dB (at BER of IV’) are
`obtained using a single l.-ee—error correcting code and n seven-tap
`baseband transversal equalizer. The residual bit errors are decreased
`below the order of 10"". The performance of a 256 QAM multicarrier
`modem has given prospect for the development of 400 Mbit / s digital
`microwave radio system.
`
`sion threshold control (AGTC) circuits.” Due to these
`circuits, an excellent 256 QAM BER performance can be
`obtained. Forward crror correction (FEC) is one of the
`key techniques for high-level modulation systems, bo-
`causc it eliminates residual bit errors. A single Lcc—error
`correcting code with low redundancy is employed.
`Finally, the 256 QAM BER performance, and signature
`with and without adjacent channel interference are pre-
`sented. The equivalent CNR degradations of 1 dB at BER
`of 10”‘ and of 2 dB at BER of 10-9 are obtained. The
`residual bit errors are decreased below the order of 10‘1°.
`
`11. GENERAL DESCRIPTION
`
`I.
`
`INTRODUCTION
`
`'
`
`‘:
`
`NE of the most important criteria in the design of
`digital radio systems is the transmission capacity per
`i.o., bits/second/Hertz. High—levcl
`H" RF bandwidth,
`modulation schemes for increasing‘ spectrum utilization cf-
`ficicncy are now a major subject in the development of.
`digital microwave radio. In recent years, several digital
`radio systems with 16 QAM, and even with as high as 64
`QAM modulation have been developed and are in opera-
`tion in the microwave frequency band [l]—[4]. The trend
`to increase spectrum utilization cfiicicncy may continue
`[5]-
`As the modulation level increases, the system becomes
`more sensitive to multipath induced waveform distortion
`and interference noise. It was already demonstrated [8]
`that the multicarricr transmission method is effective for
`high—lcvcl modulation schemes in a multipath environ-
`ment. On the basis of the above considerations, this paper
`describes a 256 QAM rnulticarricr modem for the 400
`Mbit / s digital microwave radio (DM—400M) system [6],
`[7].
`First, the performance of a newly developed monolithic
`fllultiplicr IC for 2.56 QAM modulation and demodulation
`15 described. Next, two principal techniques employed in
`3 demodulator are stated. One is “a carrier recovery PLL
`With control mode selection function” which satisfies such
`Tequircrncnts as good phascjittcr performance and no false -
`lock phenomenon. Another is “automatic gain and deci-
`
`Manuscript received September 15, 1986; revised December 12, 1986.
`The authors are with NTI‘ Electrical Communication Laboratories, Nip-
`P°Il Telegraph and Telephone Corporation, Kanagawa, 238-03 Japan.
`IEEE Log Number 8613242.
`
`A. Outline of 256 QAM Four—Carr£er Modem
`For the realization of a digital radio system having a
`transmission capacity of 400 Mbits/s within 80 MHZ
`bandwidth, a 2.56 QAM modulation using the Nyquist
`spectral shaping (or = 0.5) is required. This enables the
`frequency utilization efliciency of 10 bits / s /Hz when the
`orthogonal dual polarization is employed.
`In a high-level modulation system such as 64 QAM or
`256 QAM, the multipath fading causes large degradation
`of BER performance. A multicarricr system is considered
`to be a promising method for high-level signal transmis-
`sion in a fading channel. From the 256 QAM transmission
`characteristics estimation, a four—carrier system with 12.5
`MBaud data rate and rolloif factor of 0.5 was found nec-
`cssary to achieve 400 Mbits / s [8]. In this situation, the
`frequency spacing between adjacent carriers is 20 MHz
`and the radio channel is composed of four modems. The
`modem block-diagram and major system parameters are
`shown in Fig. 1 and Table 1.
`The transmitting terminal equipment converts 400
`Mbit/ s data into 32 rails of 12.5 MBaud binary signals.
`These 32 binary bit streams are fed to the four modulator
`circuitry. In each modulator, eight binary streams are dif-
`ferentially encoded (quadrant symmetry encoding) and re-
`dundant bits for error correcting are added by an FEC
`coder circuit. Those streams are converted by DIA con-
`vertcrs to form in-phase and quadrature 16 level signals.
`Each 161evcl signal modulates a local oscillator. The 256
`QAM signals with cosine rolloff spectrum shaping (ct =
`0.5) are then combined by a hybrid circuit and supplied
`to the transmitter. The 256 QAM f0ur—carrier spectrum is
`shown in Fig. 2.
`At a demodulator, the 256 QAM four-carrier signals are
`
`0733-8716f87f0400—0329$01.00 © 1987 IEEE
`
`Page 00004
`
`

`
`IEEE JOURNAL ON SELECTED AREAS [N COMMUNICATIONS, VOL. SAC—5, NO. 3, APRIL [987
`
`:.n::’.-*'.'r=7-*5"I"
`
`Demodulator
`
`Fig. 1. Block diagram of 256 QAM four-carrier modem.
`
`v : l0dB/div.
`H :
`l0MHz/div‘.
`IF aw : i0l]KHz
`
`Video BW : 30.0Hz
`
`Fig. 2. Four-carrier spectrum.
`
`in [5]. Concerning the modulation section, it was revealed
`that the allowable maximum modulation phase error is
`i0.5° to satisfy the requirement for an equivalent CNR
`degradation of 0.6 as at a BER of 10-6 for 256 QAM.
`Therefore, a new monolithic multiplier IC capable of
`reducing the modulation phase and amplitude errors has
`been developed for the 256 QAM [9]. The main perfor-
`mance of the multiplier itself is presented in Table II. The
`new IC has a baseband input voltage linearity of more
`than 1.5 V and modulation phase error is less than 0.2‘,
`which are extremely superior to the conventional ring
`modulators and to multiplier IC developed for a 16 QAM
`system. Third-order interrnodulation products (IM3) of
`more than 55 dB at the average output power level is ob-
`tained. These performances are achieved with the aid of
`the latest device technology: SST (super seif—aligned pro-
`cess technology) [10]. The modulation phase error ob-
`
`TABLE 1
`MAIN PARAMETERS or: 256 QAM 400 Mbit / s MULTICARRIER MODEM
`
`J (
`
`hiflwith arthogunnl dual polarization
`
`-
`
`distributed by a hybrid circuit and coherently detected to
`produce two orthogonal 16 level baseband signals. The
`seven-tap baseband transversal equalizers are employed
`to equalize both in-phase and quadrature waveform dis-
`tortions. In order to improve pull-in performance, ZF
`(zero forcing) with MLE (maximum level error) algo-
`rithm is employed [6}. Denrodulated 16 level signals are
`regenerated by AID converters to produce eight rails of
`12.5 MBaud binary signals. Error correction is then car-
`ried out in the FEC decoder circuit.
`
`B. Key Techniques for an Accurate 256 QAM Modem-
`In order to realize an accurate 256 QAM modem, the
`following novel techniques are applied.
`1) monolithic multiplier IC for the modulator and the
`phase detector,
`2) high—level decision circuit with automatic gain and
`decision threshold control (AGTC circuit),
`3) carrier recovery with control mode selection func-
`tion,
`'
`
`4) forward error correction (FEC) technique.
`
`Ill. CIRCUIT DESCRIPTION
`A. Modulation Section
`
`The degradation factors arising at various parts in a mo-
`dern, such as waveform distortion, phase error. carrier jit-
`ter, etc. , were categorized and the effects of these factors
`on 256 QAM equivalent CNR degradation were presented
`
`Page 00005
`
`

`
`NAKAMURA e: «L: 256 QAM MODEM FOR MULTICARRIFR 400 Mbit/s I)IGI'1‘Al. RADIO.
`MI:9
`
`Gto
`
`
`
`NumberofSignalPoints 33
`
`New Accurate -
`~'°E'.°;.'.9
`Conventions!
`modulator‘
`
`"
`
`0.5
`.5
`[deal
`Phase err-or
`‘(developed for IE QAM
`
`1
`
`CNFu‘*5l]dE
`
`0
`
`In 15 20 253:: as 40 -‘.5
`'-a 5
`Phase error [deg.}
`
`Fig. 5. Phase comparator characteristic.
`
`Fig. 3. Modulation phase error distributions comparing new monolithic 1C
`and conventional modulator.
`
`Fig. 4. 256 QAM signal constellation.
`
`TABLE 1i
`MONOLITHIC MuLrn>t.I1=,R IC PERFORMANCE.
`
`_
`
`modulation phase error"
`modulation .9_-mplitudc error
`
`less than 0.2 degree
`less than [I.2dB
`
`amplitude deviation between
`ti
`te .
`h
`I.
`more than 1_5v
`bascband inputlinearity voltage
`IM3 (‘third-order inlierruodulation more than 55:13
`products}
`Where the eutputbacimffis 'l'dB
`
`fabrication process
`
`SST (Sun 21' Sell’-aligned process
`technology}
`
`tained from the new monolithic multiplier IC and conven-
`tional one developed for a 16 QAM system are measured.
`Fig. 3 shows the number of signal points versus modu-
`lation phase error comparing the two multiplier [C5, The
`no niber-of signal points of the quadrature multiplier with
`phase error of less than :O.5° are 238 by using the new
`monolithic IC’s. The measured maximum phase error of
`less than i1° has been obtained.
`it is concluded from experimental results that the newly
`developed monolithic multiplier IC almost satisfies the re-
`quirements for 256 QAM modulator. Fig. 4 shows the
`measured 256 QAM signal space diagram.
`
`3. Demodulation Section
`
`1 1) Carrier Recovery with Central Mode Selection
`}"Ltnetion.' A variety of carrier tracking loops for the QAM
`Signal have been proposed [11], [12]. One of the most
`Effective methods for 16 QAM carrier recovery was a se-
`lect“-’e gated phase locked loop (PLL), which uses only
`tilt‘: error signal derived from the same phases of a 4'-PSK
`Signal. The recovered carrier jitter of more than 35 dB for
`3 15 QAM signal was obtained by this method [11]. How-
`
`ever, the required carrier jitter for 256 QAM is more than
`45 (113 when the equivalent CNR degradation of 0.3 dB is
`permitted. Therefore, it is necessary to design a carrier
`recovery circuit which satisfies such requirements: a) good
`phase jitter performance and b) no false lock phenome-
`non.
`
`The received 256 QAM signal is demodulated into 16-
`level baseband signals at in—phase and quadrature chan-
`nels. The regenerated first-bit signal sets (a1, bl), which
`are the most significant bit (MSB) of AID converters, and
`the fifth-bit signal sets (a5, 175), which are error polarity
`signals, are multiplied and is used as the VCO control
`signal. The phase control voltage V(ti) is obtained as fol-
`lows.
`
`: 6-155 "' 51 (£5.
`
`(1)
`
`The reduction of a PLL noise bandwidth and the im-
`
`provement of VCO phase jitter are necessary to obtain a
`recovered carrier jitter of more than 45 dB. The carrier
`jitter of more than 45 dB has been achieved by designing
`RF local oscillator frequency stability of the order of 10"
`and employing a voltage controlled crystal oscillator
`(VCXO) in the demodulator. In spite of a good carrier
`jitter performance,
`it has been clarified from the phase
`comparator characteristic that the carrier recovery circuit
`has a false-lock point in the same frequency when input
`CNR is sufliciently high [5].
`The selective gating of VCXO control signal during the
`course of an acquisition is effective in order to prevent the
`false-lock phenomenon.
`After locking into a normal phase, the operation of se-
`lecting the control signal is inhibited. The switching is
`performed by monitoring the intersymbol
`interference
`which is easily estimated from the multiplication of the
`fifth and the sixth bits of AID converter. This technique
`simultaneously enables the improvement of pull«in and
`carrier jitter performance.
`The phase comparator characteristic in a selective gat-
`ing mode is obtained as follows.
`Let Dsi be the probability that the signal point 1' is in-
`volved in a selective area. D31’ is shown as
`
`Dsi =
`
`H
`
`RsUEs
`
`Pi(x,y)dx dy
`
`(2)
`
`where, P.i(x, y) is a Gaussian pdf
`
`(i = 1 -- 256).
`
`Page 00006
`
`

`
`332
`
`IEBE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS. VOL. SAC-5, N0. 3, APRIL 1937
`
`Pi(x, y) = Pt'[x) - Pi(y)
`
`.
`
`2
`
`.
`
`2
`
`= .r“21....2°‘Pi‘ iiigifll - as l” ‘mizyl
`
`(3)
`
`inwphase component of signal point 1'
`Six
`quadrature component of signal point If
`Say
`white Gaussian noise power
`02
`26 = minimum distance between signal points.
`
`The average CNR (carrier to noise ratio) of 256 QAM
`can be obtained as
`
`CNR = 35 two? = kg.
`
`(4)
`
`In (2), Rs. means the positive area producing correct
`control voltage “+1” and E5 means the negative area
`producing error control voltage “-1.” When the signal
`point i is involved in a selective area, the phase control
`voltage Vi(G) is shown as
`
`14(9) = “R5 Pi(x, 3’) Cir dy
`
`—
`
`Pstx. y) «my.
`
`(5)
`
`When the signal point t' is involved in a nonselective
`area, the phase control voltage Vi(9) is put into “hold”
`condition by a sample and hold circuit. Then, the follow-
`ing equations are introduced.
`256
`
`Pe(ti) = phase comparator characteristic.
`
`From (6), Pe(3) can be written as
`
`Pe(6) = 2} V(a)Ds;'[§1Dst.
`
`2E6
`i=1
`
`256
`
`(6)
`
`(7)
`
`The phase comparator characteristic in a selective gat~ _
`ing mode is shown in Fig. 5. The number of error signals
`used in the selective gating in Fig. 5 is 96 out of 256.
`Note that the loop exhibits no false lock point. The re—
`covered carrier spectrum is shown in Fig. 6. The mea-
`sured carrier jitter was 45.5 dB and the pu1l—in frequency
`range of more than 8 kHz was obtained.
`2) High-Level Decision Circuit with Automatic Gain
`and Decision Threshold Control
`(AGTC): In order to
`achieve a good BER performance, an automatic gain and
`decision threshold control (AGTC) circuit is employed
`[14]. It uses the first and fifth bits of A.-‘D converter as the
`feedback signals for the amplitude variation and dc drift
`of demodulated signals. These degradations are mainly
`caused by a local leak of the modulator, AGC level vari-
`ation and temperature characteristic of amplifiers. The
`feedback signals are fed to the dc amplifier which is lo-
`cated before the A/Dconverter. Fig. 7 shows the circuit
`configuration.
`
`IF BWZ l_K|-lz
`Video BW1 l{)0Hz
`
`Fig. 6. Recovered carrier spectrum.
`
`dc level
`monitor
`
`Fig. 7. Feedback circuit configuration.
`
`pno cbntrnl
`
`I’IIIIII
`
`i:1
`
`
`
`EquivalentUNdegradationI581
`
`_
`
`Input
`
`level variation can}
`
`Fig. 8. Equivalent C /N degradation due to input level variation".
`
`Let the input signal to the AID converter be 11(1) and
`applying Laplacian “s" to u(I), in dc drift compensation
`feedback loop [5]:
`
`U(s) = I/(s) + 5(3)/(1+ Kd.'F(s)).
`
`(8)
`
`When F (.9) is supposed to be a perfect integrator,
`
`'
`
`F(s) = Kn/s,
`
`(9)
`
`then,
`
`U“) 3 Vi‘) “
`
`’
`
`19(5):
`
`(10)
`
`In amplitude variation compensation feedback loop (866
`Appendix),
`
`Page 00007
`
`

`
`NM-(AM-URA et n.i.: 256 QAM MODEM FOR MULTICARRIER 400 Mbit/s DIGITAL RADIO
`
`o—-o Iwithout FEC
`-----a Iwith
`FEC
`Modern back to back
`Clock I
`|2.5MB
`
`l |
`
`:,\
`‘
`‘.
`'n\\
`
`38
`
`C N R (dB}
`
`Fig. 9. 256 QAM BER performance.
`
`gle Lee-error correcting code (72, '70} is shown as
`Pa = 107 - P3
`
`(13)
`
`Pa = SER after FEC
`
`P = SER before FEC.
`
`The rate overhead of this code is only about 3 percent.
`The differential decoding is performed after error correc-
`tion.
`
`IV. OVERALL PERFORMANCE
`
`The 256 QAM signal has 16 baseband levels. The
`baseband signal is obtained by D.’ A converters as
`
`S[=23'fl|+22'a2+2'fl3'l'I5l4
`
`s,=2“-b,+2*-b,+2-b,+b...
`
`- (ad, b4) are binary codes and
`-
`Signal sets (al, bi), -
`categorized as “Path 1’? to “Path 4" indicating the first
`to fourth bit. The BER of 256 QAM is obtained as the
`average of the BER’s of each path. Considering a quad-
`rant symmetry differential encoding, the average BER of
`256 QAM becomes
`‘
`
`Pr: = 19/64 errc(a/x/E 0')
`
`= 19/54 errc(k,/«fi%)
`
`(14)
`
`minimum distance between signal points,
`' 25
`02 .= white Gaussian noise power.
`
`The 256 QAM BER’s for Path 4 {modern back to back)
`are shown in Fig. 9. The single Lee-error correcting code
`and a seven-tap baseband transversal equalizer are imple-
`mented in this system. The equivalent CNR degradation
`of 1 dB {at a BER of 1o“‘) and 2 as (at a BER of 10-”)
`are obtained. The measured coding gain by the FEC at a