This paper presents fast-synchronized noncoherent limiter discriminator detection of standardized FQPSK (Feher quadrature phase-shift-keying) and filtered OQPSK (offset quadrature phase-shift-keying) suitable for nonlinearly amplified (NLA) second and third generation wireless communication systems with fast fading, cochannel interference, or strong phase noise. Based on the phase characteristics of FQPSK and OQPSK, noncoherent limiter-discriminator detection is presented. Then we apply maximum-likelihood sequence estimation (MLSE) with lookup tables to improve the bit error rate (BER) performance.

A reconfigurable Global Mobile Standard (GSM) compatible radio modem interface which doubles the number of simultaneous video and voice transmissions per channel is presented for personal communication systems (PCS). The result is that an average video frame rate of up 14 fps can be supported within a single TDMA time slot, doubling the number of conventional GMSK multimedia transmissions per GSM channel. The design employs an in-circuit reconfigurable cross-correlated FQPSK signal processing technique. This innovative network modem is reconfigured in-circuit for high bit rate data transmission or GSM compatible operation. Spectrum efficiency /spl nu//sub f/ (b/s/Hz) is investigated in a nonlinear amplified (NLA) environment-providing a 6 to 9 dB advantage in power efficiency. Our results show that the GSM compatible configuration of a specific cross correlated FQPSK-KF implementation offers up to 5.1 dB improvement in E/sub b//N/sub o/, power efficiency, and network capacity /spl nu//sub T/ (erlangs/Hz m/sup 2/) may be increased 76% over GMSK BT/sub b/=0.3.

A reconfigurable global mobile standard (GSM) compatible radio modem interface which doubles the number of simultaneous video and voice transmissions per channel is presented for personal communication systems (PCSs). The result is that an average video frame rate of up 14 frames per second (fps) can be supported within a single TDMA time slot, doubling the number of conventional GMSK multimedia transmissions per GSM channel. The design employs an in-circuit reconfigurable (ICR) cross-correlated Feher's (see Englewood Cliffs, NJ: Prentice-Hall, 1995). quadrature phase shift keyed (FQPSK) signal processing technique to support a data rate of 357.5 kb/s in a 200 kHz bandlimited GSM channel. This innovative new network modem is reconfigured in-circuit for high bit rate data transmission or voice operation that is compatible with existing GSM equipment. Spectrum efficiency /spl eta//sub f/ (b/s/Hz) is investigated in a nonlinear amplified (NLA) environment-providing a 6 to 9 dB advantage in power efficiency for increased battery life of hand-held terminals. Results show that RF power efficient nonlinear amplified spectrum efficiency is increased from 1.35 b/s/Hz to 1.85 b/s/Hz. Bit error rate (BER) performance is evaluated in a Gaussian and Rayleigh fading channel, and the merits of coherent demodulation in microcellular PCS are examined. Our results show that the GSM compatible configuration of a specific cross correlated FQPSK-KF implementation offers up to 2 dB improvement in E/sub b//N/sub 0/ over conventional GMSK BT/sub b/=0.3. The PCS network capacity /spl eta//sub T/ (Erlangs/Hz m/sup 2/) may be increased 37% over GMSK BT/sub b/=0.3.

A newly developed constant envelope FQPSK modem/radio architecture, which employs a modified double-jump (DJ) filter in the cross-correlated FQPSK system, is proposed for personal communications systems (PCS) and mobile satellite applications. The power efficiency and spectrum efficiency of this system are investigated in a nonlinear amplified (NLA) environment. The bit error rate (BER) performance is evaluated in a noisy Rayleigh fading channel. We demonstrate that with the simplest threshold detectors (binary robust eye diagrams in I and Q channels), this system is 4-7 dB more power efficient than the US digital cellular and Japanese Handyphone standard /spl pi//4-QPSK (50%-100% more spectrally efficient than the recently adopted wireless local area network (LAN) standard GFSK and the European standard GMSK). The results indicate that the proposed DJ filtered FQPSK is a power and spectrally efficient modem/radio technique. By selecting different system parameters, this system can be optimized for a wide range of applications in PCS and mobile satellite communications.>

An adaptive Viterbi algorithm, derived from a dynamic estimate of the fading channel is used for the decoding of a convolutional coded 16 QAM system in a mobile environment. The estimates are obtained by a sequence of known pilot symbols embedded in the data stream, and perform compensation for Rayleigh fading. The likelihood criterion in the Viterbi decoder is also modified by these channel estimates through a metric weighting function. We demonstrate through computer simulations, that our new technique achieves a BER improvement of 7-10 dB at P/sub e/=10/sup -3/ in a fast flat Rayleigh fading environment compared to an uncoded system. The BER performance of our new technique in a co-channel interference (CCI) controlled environment is also studied, and the results show that it may achieve a 40% to 85% improvement in capacity over the standard modem scheme for the new US digital cellular system, /spl pi//4-QPSK.>

A family of power and spectrally efficient modulations is proposed for wireless mobile, and personal communications services (PCS). This family of modulations is based on the FQPSK processor. The main focus of this paper is to present the latest developments and results of FQPSK modulation with 4 signal states. FQPSK which is a constant envelope modulation has been shown to increase the spectral efficiency and capacity by up to 92% over GMSK which is the current modulation for the DECT European standard in mobile cellular communications. It also outperforms PI/4-DEQPSK which is the current modulation standard used in the USA, under IS-54, and Japan, in power efficiency, battery lifetime, and reduced radiation while maintaining equal capacity. To improve further the spectral efficiency of FQPSK, simple baseband processing and filtering strategies are applied. Such baseband processing leads to a spectral efficiency of 1.6 b/s/Hz, and has BER performance of 1*10-2 for a C/I of 18.5 dB in a hardlimited Rayleigh fading channel. To achieve higher spectral efficiency, FQPSK-4/spl times/4 which is a 16 state modulation is proposed. This modulation can be nonlinearly amplified, and is compatible with 16 QAM. FQPSK has been also shown to be compatible with OQPSK, GMSK, and can be demodulated with their receivers with less than 0.5 dB degradation.>

Pilot-symbol aided coherent M-ary PSK modems in digital cellular mobile radio systems are analyzed theoretically. The error-floors caused by the Doppler spread in a fast fading channel are removed in both flat and selective fading channels. However, the error-floors caused by the delay spread are lower-bounded by those that exist in the ideal coherent detection. The systems are modeled as frequency-selective fast Rayleigh fading channels, corrupted by co-channel interference (CCI) and additive white Gaussian noise (AWGN). In the proposed scheme, pilot symbols are inserted periodically to monitor the channel characteristics. The fading processes experienced by the pilot symbols are used to estimate those suffered by the data symbols using interpolation or filtering. The estimated fade characteristics are used to compensate the random phase variation caused by the Doppler spread, so that the signals can be demodulated coherently. The theoretical performances of the fade compensated coherent modems are evaluated. The results show that the fade compensated coherent demodulation with the least redundancy achieves the same performance as the ideal differential detection in a fading channel. The performance approaches that of the ideal coherent demodulation as more redundancy is allowed. The pilot-symbol-insertion (PSI) scheme is also applicable to M-ary QAM modems and Rician channels. The residual frequency offset can also be compensated by the PSI technique.>

We apply baseband pulse shaping techniques for /spl pi//4 QPSK in order to reduce the spectral regeneration of the bandlimited carrier after nonlinear amplification. These Feher's patented techniques, namely, /spl pi//4 FQPSK (superposed QPSK) and /spl pi//4 CTPSK (controlled transition PSK), may also be noncoherently demodulated. Application of these techniques is in fast fading, power efficient channels, typical of the mobile radio environment. Computer simulation and experimental studies demonstrate that with these baseband waveshaping techniques, carrier envelope fluctuations are significantly reduced, and the out-of-band power after nonlinear amplification is suppressed by up to 20 dB compared to /spl pi//4 QPSK. In frequency noninterleaved land or satellite mobile radio systems operating in a nonlinear, fading and ACI (adjacent channel interference) environment, these techniques may achieve 20%-50% higher spectral efficiency compared to /spl pi//4 QPSK. In mobile cellular systems using /spl pi//4 QPSK, such as the new North American and the Japanese digital cellular systems, the application of these baseband pulse shapes may allow more convenient and less costly amplifier linearization.>

In order to increase the channel capacity of CDMA (code division multiple access) mobile/personal communication systems, an inter-channel interference cancellation (IIC) technique is proposed. By employing one or few additional interference cancellation circuits, it makes it possible to increase allowable error of the TXPC (transmission power control) at the mobile/personal terminals. The experimental results show that by employing the proposed IIC scheme, Pe performance degradation due to a desired and an undesired signal power ratio of -20 dB (20 dB of TXPC control error) can be improved to only 1.5 dB at Pe=10/sup -3/.>

Two low-redundancy codes for in-band pilot insertion are introduced. KF10 suppresses the baseband PAM and the modulated QAM signal power at the Nyquist frequency so that the dual-pilot tone calibration technique of recent interest can be arranged to convey both a symbol timing and a carrier reference to the receiver. OF00 suppresses the signal power at zero as well as the Nyquist frequency and allows the utilization of a triple-pilot scheme which provides, in addition to a symbol timing, a carrier reference with no 180 degrees phase ambiguity.>

An improved low-frequency spectral suppression (LOFS) code is introduced and analyzed as a method to suppress low-frequency energy in a digital baseband signal with significantly lower redundancy than required methods. Alternatively, the LOFS code will suppress more energy near DC with a given redundancy when compared to current coding methods, allowing insertion of a pilot tone which leads to a simple unambiguous carrier recovery subsystem that can track system-induced noise. The LOFS code reduces redundancy by adding control bits in word format for multilevel signals, that is, control bits for multiple frames are inserted in one symbol duration. Use of this LOFS code could reduce the redundancy of currently used digital transmission systems from 4% to 1%. Analysis of computer simulations shows that a premodulation high-pass filter, while significantly degrading the uncoded PAM signal, causes little degradation of the coded data and keeps data to pilot interference to a minimum. Experimental hardware results are included to verify simulation results.>

A novel block inversion code (BIC) which is both simple and effective in controlling low-frequency components of multilevel data signals is introduced, and its power spectrum is analyzed. The BIC's effect on the performance of QAM modems is studied, and an expression for the coded modem's symbol error rate is obtained. The code allows M-ary QAM system designs to incorporate unmodulated carrier-pilot insertion techniques using high-pass filters, for more robust, unambiguous carrier recovery and automatic gain control. The effect of BIC coding on the performance of a 256 QAM modem with carrier-pilot insertion using high-pass filters is investigated. It is found that, under some circumstances, BIC coding can significantly reduce the ill-effect of the high-pass filter on the modem's performance.>

The performance of 16-state SQAM (superposed-QAM) system, in a nonlinearly amplified multichannel environment, in the presence of additive white Gaussian noise (AWGN), intersymbol interference (ISI), and adjacent channel interference (ACI), is experimentally analyzed via computer simulation. Various channel conditions, such as channel spacings between the main and adjacent channels, and fade depth on the desired main channel, are examined. Baseband signal waveshape of 16-SQAM as well as the receive low-pass filter bandwidth are optimized to achieve a good P(e) performance. The result shows that 16-SQAM system is more efficient than MAMSK (multiamplitude MSK) in the frequency utilization. >

Differential detector structures that use decision feedback to partially remove the effect of destructive intersymbol interference from minimum-shift keying (MSK) are introduced. In comparison to the conventional two-bit differential detection of GMSK (Gaussian MSK), the proposed structures provide about 4-dB E/sub b//N/sub 0/ improvement when the premodulation filter time-bandwidth product is 0.25. As the premodulation filter becomes narrower, the advantages of the proposed receivers are more pronounced. Thus the bit-error-rate performance of differential detection is brought closer to coherent detection while avoiding the problems associated with the carrier recovery.>

Low redundancy FEC coded 1024-QAM modems, staggered 1024-QAM, and 256-QAM modems for spectrally efficient (up to 8.84 bits/s/Hz) microwave and cable systems applications are described. Such a high spectral efficiency is required for CEPT-1 (2.048 Mbit/s) rate digital transmission in a single analog supergroup (SG` band as well as for other emerging systems applications. Practical constraints of operational analog FDM systems are presented and taken into account in the choice of the low redundancy FEC codec and the coded 1024-QAM modem. Theoretical, computer simulation and experimental results of 256QAM modems have been extended to the feasibility study of 512-QAM, 961-QPRS, and 1024-QAM modems. Our experience with 256-QAM modems which have a T-1 (1.544 Mbit/s) rate in a 240 kHz analog supergroup (SG) band, i.e., an efficiency of 6.66 bits/s/Hz, demonstrates that a regenerative span over 1000 km is feasible over FDM radio systems. A significantly increased spectral efficiency of 8.84 bits/s/Hz is required for CEPT-1/SG system applications. Our R&D results, presented in this paper, demonstrate the feasibility of FEC coded 1024QAM modems, equipped with powerful digital adaptive equalizers, carrier phase noise, and symbol clock jitter cancellation subsystems, for the transmission of CEPT-1 rate signals in a single SG band.

The performance of new bandwidth compressive 16 SQAM (Superposed-QAM) modems, operating in a highly power-efficient saturation mode, is analyzed. Nonlinearly amplified 16 SQAM retains the out-of-band energy much lower than 4-level rectangular (i.e., 16 QAM) or sinusoidal shaped (i.e., MAMSK) QAM, and operates within 0.7 dB of theoretical (i.e., 16 QAM in Nyquist linear channel) performance atP(e) = 10^{-6}using simple 4th-order Butterworth LPF's. Effects of baseband pulse shaping, receive filter BTs, power level variations and propagation time difference (or static phase shift) of transmit HPA's are analyzed in terms of BER.

An efficient modem is introduced for system applications which favor differential detection techniques. In differentially detected controlled transition PSK (DCTPSK), pulse-shaping techniques are used to achieve bandwidth and power efficiency in nonlinear channels. The baseband waveshapes, eye diagrams, and signal space diagram of DCTPSK are given; its bit error rate performance is calculated and its power spectral density is obtained. In a multicarrier adjacent channel interference environment, the compact spectrum of DCTPSK enables it to utilize the available bandwidth and power more efficiently than some well-known modulation techniques such as DQPSK and DMSK.

The performance of differential minimum shift keyed (DMSK) systems, with and without nonredundant error correction, is studied. Previous publications described DMSK systems in an additive white Gaussian noise (AWGN) environment. This correspondence presents a study and original performance results of DMSK systems operated in a combined adjacent-channel-interference (ACI) and AWGN environment. Our experimental (hardware) and computer simulation results are found to be in close agreement.

Improved efficiency multilevel partial response systems have recently been discovered. They are superior to conventional partial response systems when operated above the Nyquist signaling rate. This behavior has not as yet been explained and this correspondence gives a concise analysis, which shows the reason for the superior performance.

To enable differential detection of coincident, hard-limited, and reduced sidelobe QPSK type of signals, we introduce asymmetrical pulse shapes. This asymmetry in theIandQbaseband channels leads to coincident (unstaggered) QPSK system applications. We consider the bit error probability performance of these modulation schemes on a hardlimited channel in the presence of uplink and downlink additive Gaussian noise. It is found that the unstaggered QPSK modulation with the asymmetrical pulse has better bit error probability performance than QPSK and staggered QORC.

A spectral and power efficient modulation techniquesuperposed quadrature amplitude modulation (SQAM-is introduced. In SQAM, the premodulation baseband signal is a double-interval (2T_{s}) raised-cosine pulse superposed with weighted single-interval (Ts) raisedcosine pulses. Our results indicate that SQAM has spectral advantages over OQPSK, QBL, and MSK, and betterP(e)performance than MSK, IJF-OQPSK (or SQORC), and TFM.

Performance degradations of 256-QAM modems in distorted channels are studied in this correspondence. Illustrative linear, parabolic, and sinusoidal amplitude and group delay channel distortions, caused by filter imperfections and/or by selective radio fades, are investigated. To enable an easy comparison with 64-QAM systems, we present the degradations of these systems in the same figures as those of 256-QAM modems. It is shown that linear (slope) group delay and sinusoidal (ripple) amplitude distortions cause the most significant performance degradations.

An improved efficiency partial-response signaling (PRS) and quadrature partial-response signaling (QPRS) data filtering strategy, which enables a higher transmission rate (more symbols/s/Hz) than feasible with conventional PRS and QPRS systems, is introduced. With the use of our technique, a smaller number of signal levels (states) can be chosen for specified spectrally efficient applications; thus, an increase inS/Nimmunity is obtained with a lower cost and simpler hardware implementation.

The performance of TSI-OQPSK (two-symbol-interval-offset QPSK) modems in nonlinearly amplified (saturated or hard-limited), adjacent channel interference (ACI) satellite earth station environment is studied. In closely spaced mobile satellite channels and higher frequency (≥ 14 GHz) earth stations, the effect of ACI on theP_{e} = f(E_{b}/N_{0})performance of faded systems may become very significant. Simulation results indicate that for the frequently used close channel spacing case (≤ 77 percent of the bit rate) and a practical fade depth (≥ 12 dB), our new TSI-OQPSK signal(n = 2)has 1 dB less degradation than IJF-OQPSK and 3 dB less than OQPSK, and that conventional QPSK could not operate in such an environment.

The principles and applications of a new baseband linearizer are described. It is demonstrated that, with the use of our linearizer, significant improvements in the effective radiated power of the earth station high-power amplifier can be attained. In a particular example, the characteristics of the recommended INTELSAT V satellite simulator model are used. For the 120 Mbit/s QPSK transmission system operating in a complex interference environment, we show that with the new iinearizer the earth station high-power amplifier may be operated almost in full saturation while meeting the current INTELSAT requirements. This is a significant improvement in power utilization efficiency. Similar improvements could be obtained for small digital singlechannel-per-carrier earth stations.

The performance of intersymbol-interference and jitterfree offset-QPSK (IJF-OQPSK) and partial-response (PR) IJFOQPSK modems in a nonlinearly amplified multichannel environment is studied and compared to that of conventional QPSK and OQPSK modems. Power efficiency is maximized by operating the high-power amplifier (HPA) in full saturation. The effect of adjacent-channel interference and flat fade on the performance of illustrative nonlinear satellite systems is investigated. Simulation and measurement results are reported. It is shown that both IJF-OQPSK and PR-IJF-OQPSK modems exhibit a significant performance improvement compared to QPSK and OQPSK modems, in illustrative SCPC and on-board regenerative satellite systems, they provide an improvement of more than 4 dB compared to QPSK.

The performance of a modulation technique which is both power and bandwidth efficient, termed nonlinearly amplified 64-state quadrature amplitude modulation (NLA 64-state QAM), is studied. To assess the effects of selective fading and/or system filter imperfections, computer simulated results are presented which show the sensitivity of the modulation technique to typical group delay and amplitude distortions. In addition to the above-mentioned results, the effects of modulator imperfections in the power level combining on the Peperformance are evaluated.

A new modulation technique, cross-correlated phase-shift keying (XPSK), is introduced. XPSK is a band-limited offset QPSK modulation technique which has an almost constant envelope. In XPSK modulators, a controlled amount of cross correlation between the in-phase (I) and quadrature (Q) channels is introduced.IandQcross correlation reduce the envelope fluctuation Of the intersymbol-interference and jitter-free OQPSK (IJF-OQPSK) modulation scheme, introduced by Feher et al. [1], [2], from 3 dB to approximately 0 dB, thus further improving the performance of IJF-OQPSK systems in nonlinear radio systems [7], [14]. It is shown that the baseband signal of the modulator, theP_{e} = f(E_{b}/N_{0})performance, and the spectral characteristics of nonlinearly amplified (hard-limited or saturated) radio systems of XPSK and tamed frequency modems (TFM) are practically the same. The XPSK demodulator is a conventional OQPSK demodulator, the TFM demodulator requires a somewhat more complex signal processor. For this reason, the XPSK approach may lead to significant demodulator hardware cost savings, particularly in point-to-multipoint distribution systems such as broadcast systems. Simulation results for linear and nonlinear (saturated amplifier) systems operated in an adjacent-channel interference environment (in addition to thermal noise) are presented. Measurement results performed on a 128 kbit/s rate hardware-prototype modem are also reported. Experimental eye diagram and power spectrum density measurement results are in close agreement with the computer simulation results.

The error probability (Pe) performance of intersymbolinterference and jitter-free offset-QPSK (IJF-OQPSK) modems in a cochannel and adjacent-channel interference environment is evaluated using a computer simulation technique. Hardware design and experiments have been completed to verify the simulation results. The results indicate that a spectral efficiency of 1.5 bits/s/Hz can be obtained with hard-limited IJF-OQPSK channels. This is a significant improvement compared to hard-limited QPSK, OQPSK, and MSK systems. It is expected that the IJF-OQPSK scheme will have applications in low-cost power and bandwidth efficient earth-stations and terrestrial radio systems with transmit high-power amplifiers operating in saturation.

The probability of error (Pe) performance of an intersymbol-interference and jitter-free-offset QPSK (IJF-OQPSK) modem in conventional and regenerative satellite channels is studied and compared to that of a QPSK modem using computer simulation techniques. The results show that the IJF-OQPSK modem performs better than QPSK in both cascaded nonlinear channels and regenerative satellite links, especially with saturated earth station HPA and transponder TWTA.

Based on Feher's nonlinear filter (NLF) invention, we developed a class of intersymbol-interference and jitter-free (IJF) baseband signals and JJF-OQPSK systems. In [14] and [18] we demonstrated the performance advantages of these systems over QPSK, OQPSK, and MSK systems. In this paper a new class of IJF two-symbol-interval (TSI) baseband pulse shapes is introduced. The spectral properties and the Peperformance of these TSI-OQPSK modulated systems is investigated. We show that in a hard-limited radio system the main lobe of then \geq 2, TSI-OQPSK is narrower and that the sidelobes, in the critical|(f-f_{c})T_{s}| < 2region, are lower than that of QPSK, OQPSK, MSK, andN = 1, IJF-OQPSK systems. A new filtering strategy, the sum of approximate eye (SAE), is also introduced. It is shown that SAE filters may reduce theE_{b}/N_{0}requirements of band-limited and hard-limited TSI-OQPSK modems.

New power and bandwidth efficient modulation techniques, named intersymbol interference and jitter-free (IJF)-QPSK and IJF-OQPSK, are presented. The properties of the IJF-QPSK and IJF-OQPSK signals in linear and nonlinear earth-station-satellite systems are studied. A finite-state Markov chain model is used to calculate power spectra of hard-limited IJF-QPSK and IJF-OQPSK. The model provides insights into the spectral spreading action of the ideal hard-limiter on the IJF-QPSK and IJF-OQPSK signals, Experimental, simulation, and theoretical results are in good agreement. The results indicate that the IJF-OQPSK modulated signal exhibits much less spectrum spreading than QPSK, OQPSK, and MSK. The error probability (Pe) performance of IJF-QPSK and IJFOQPSK in an additive white Gaussian noise and adjacent-channel interference environment is evaluated. The results show that the Peperformance of the IJF-OQPSK is superior to that of QPSK, OQPSK, and MSK in narrow-band nonlinear channels.

A generalized technique for generating quadrature amplitude modulated (QAM) signals that permits nonlinear amplification is presented. With this technique a high-power2^{2n}-state QAM signal is generated by combiningnunfiltered, nonlinear amplified, QPSK signals,nbeing a positive integer. The specific methods of generating 16- and 64-state signals using this technique are presented. An attractive feature of the technique is that despite significant difference with the conventional method of generating QAM signals, the same straightforward demodulation techniques apply to both, and the PeversusS/Nperformances are essentially identical. In the 16-state version employing traveling wave tubes (TWT's) or GaAsFET amplifiers, the technique is shown to result in a transmitter output power advantage that is on the order of 5 dB compared to the conventional method. This advantage is achieved, however, at the expense of an additional output power amplifier.

The general probability of error expressions of coherentM-ary PSK (M = 2, 4, 8, 16) systems in the presence of both impulsive and Gaussian noise are derived. This work is an extension of our paper related to BPSK and DPSK systems in a complex interference environment [1]. The presented impulsive and Gaussian channel disturbance is a first-order approximation to operational multichannel satellite and terrestrial microwave systems in which, in addition to front-end Gaussian noise, out-of-band intermodulation noise may be present. Our analysis contains an extension of the method developed by Bello and Esposito [2], [3], to include the Gaussian as well as the impulsive noise environment. The numerical results show that a well defined threshold region exists, above which the effect of the impulsive noise on the system performance is predominant.

In this paper we investigate the combined effect of sampling offset and phase error on the probability of error Peperformance of quadrature partial response (QPR) and offset QPR systems. The Gram-Charlier series expansion method is used to get accurate probability of error expressions. As an example, class 1 and class 4 partial response signaling schemes are examined.

General probability of error expressions of coherent PSK and differentially coherent DPSK systems in the presence of both impulsive and Gaussian noise are derived. The presented impulsive and Gaussian channel disturbance is a first-order approximation to operational multichannel satellite and terrestrial microwave systems in which in addition to front-end Gaussian noise, out-of-band intermodulation noise is also present. Our analysis contains an extension of the method developed by Bello and Esposito [1], [2] to include the Gaussian as well as the impulsive noise environment. Computed results are presented for a one-pole bandpass filter and a lognormal impulse amplitude distribution. The numerical results show that a welldefined threshold region exists, above which the effect of the impulsive noise on the system performance is predominant. A comparative study of system performance having integrate-sample-and-dump receivers with sample-only receivers is presented.

An integrated view of the performance of a rage 3/4, forwardacting, error-correcting convolutional codec, as used on the Intelsat single channel per carrier (SCPC) satellite communication system, is presented with respect to various transmission problems. The available literature does not indicate clearly whether such a codec would have acceptable performance in an environment involving detrimental factors such as sinusoidal interference (SI) or intersymbol interference (ISI). Experimental performance evaluations are given for the cases of an additive white Gaussian noise (AWGN) environment, a pure sinusoidal interference signal, AWGN combined with SI, and finally for AWGN in an ISI environment. The results confirm that this type of codec is designed to combat random errors effectively, and not burst-type errors such as those generated by SL Further observations indicate that it is preferable to measure a codec performance in terms of bit error rate (BER) reduction for a given signal-to-noise (S/N) ratio rather than SIN reduction for a given BER since this latter method is too susceptible to the operating conditions to which the codec is submitted.

An analysis of a symbol timing recovery (STR) technique using digital processing is presented. The ratio of the discrete spectral power at the symbol rate frequency to the nearby continuous spectral power is used as a criterion of STR performance. It is shown that this power ratio equals the quality factor of the narrow bandpass filter and that it does not depend on the value of the delay element. The performance of the STR subsystem is consequently determined by the quality factor of the bandpass filter rather than by the specific delay. In addition to this, some experimental evidence that the additive channel noise has little effect on the power ratio is given. A modified phase-locked loop with anLCprefiiter is proposed to extract the symbol timing clock. This prefilter improves the acquisition and synchronization performance of the PLL. The STR technique presented here has the advantages of lower cost and simpler hardware implementation over other serial STR techniques.

The effects on spectral densities, symbol waveshapes and PeversusS/Nperformances, resulting from the addition of filtering followed by hard limiting on raised cosine filtered QPSK, offset QPSK, and MSK point-to-point radio systems are studied. A mathematical model and physical insight are presented into the crosstalk phenomenon between quadrature channels, created in the systems by the effect of limiting on filtered signals. This crosstalk is shown to result whether the filtering is ideal or otherwise. Computergenerated and measured eye diagrams showing crosstalk as predicted on a filtered, then limited, offset QPSK signal are given. Measured and computed spectral density results are given which are in close agreement with each other, indicating that the computer model provides a good representation of the real system. In addition, an explanation of the shape of the power spectra associated with filtered, then limited, modulated signals is provided by studying the symbol waveshapes of these signals. Using the spectral density results and theP_{e} (S/N)performance findings, it is shown that for a microwave system which: 1) incorporates an amplitude limiting amplifying device in the transmitter, 2) must operate within the FCC limits for radiated spectrum, and 3) must operate at a spectral efficiency greater than 1 bit/s/Hz, offset QPSK modulation is the best choice of the three modulation methods studied.

A modified BPSK modulation technique using premodulation filtering is presented. The modified BPSK modulator achieves a smooth phase transition from 0 to π rad by going through an intermediate value of\pi/2rad. The resultant PSK signal has a very low AM component, particularly suitable for data transmission through nonlinear channels. Experimental tests performed in a channel having a high degree of AM nonlinearity (AM-to-AM conversion), have shown that the modified BPSK signal, when passed through a hard-limiter, has a spectrum with greatly reduced spectral sidelobes compared to the conventional BPSK signal. The modified BPSK signal has spectral lines at the carrier frequency plus multiples of the signaling frequency, which in some cases might be useful for timing recovery in the demodulation process.

Lender's partial response baseband system model is analyzed, extended and compared with two other partial response models. The feasibility of each system model in practical implementation is assessed. The effects of filter imperfections and of channel nonlinearities on the performance of QPRS digital radio systems is analyzed by means of computer simulation.

An analytical method to predict the data pattern dependent jitter is presented. Results obtained are shown to be in excellent agreement with those obtained by computer simulation and laboratory measurement. In order to find methods that can generate an ISI (intersymbol interference) and jitter-free bandlimited Nyquist signal, a double-intervaled raised cosine and its spectral characteristics are studied. Based on this, a search for new double-intervaled pulses having low spectral sidelobes is conducted. Pulses having desirable spectral characteristics and their application in communication systems are discussed. Two methods for the generation of ISI and jitter-free Nyquist signals are described. The first one uses the concept of pulse overlapping while the second one incorporates the nonlinear switching concept. These two methods are equivalent as they can generate the same output signals. Hardware implementation and measured results on this nonlinear switching filter are given. The simplicity of its design and its low cost will enable this type of nonlinear switching filter suitable for use in low bit rate satellite communication systems.

In this paper a model of Minimum Shift Keying (MSK) is introduced, an analysis of which shows that the theoretical maximum bandwidth efficiency of MSK is 2 bits/s/Hz, the same as for Quadrature Phase Shift Keying (QPSK) and Offset Quadrature Phase Shift Keying (Offset QPSK). The filtering necessary to achieve minimum spectrum width as well as a raised cosine spectrum while maintaining the condition of zero intersymbol interference for Offset QPSK is reviewed, and the equivalent filtering for MSK derived. In addition, partioning of this filtering between transmitter and receiver for optimum probability of error (Pe) versus receiver input signal to noise (S/N) ratio is given. Computer simulation results are presented for a MSK system and an Offset QPSK system, both filtered for 0%, 50% and 100% excess bandwidth raised cosine spectrum. The results show fully open eyes at the decision threshold input for all cases. They also show that when filtered to give the same spectrum the different system have identical PeversusS/Nperformance,S/Nbeing defined at the input to the receiver decision threshold, i.e., after all the filtering for the desired raised cosine spectrum. This performance is best for 100% excess bandwidth filtering and worst for 0% excess bandwidth filtering.

Amplitude and delay slope are significant components of inband distortion resulting from multipath fading. In this paper, we present and compare computer simulated results of signal to noise (S/N) degradation for probability of error (Pe) of 10-4, due to these distortion components on Quadrature Phase Shift Keying (QPSK), Offset Quadrature Phase Shift Keying (Offset QPSK) and 8 Phase Shift Keying(8 PSK)systems. For all systems the transmitter and receiver filtering is modeled so as to provide a 50% excess bandwidth raised cosine spectrum at the receiver detector input. The QPSK and Offset QPSK systems are studied with bit rates of 44.7 Mbits/s, the so-called T3 rate, and the 8 PSK system is studied with bit rates of 44.7 Mbits/s and 67 Mbits/s, a 67 Mbit/s 8 PSK system having the same modulated frequency spectrum as 44.7 Mbit/s QPSK and Offset QPSK systems. When the frequency spectrum is the same for all systems, then for a given amplitude slope, results show that QPSK is the least degraded, followed by Offset QPSK, then 8 PSK, and for a given delay slope, Offset QPSK is the least degraded, followed by QPSK, than 8 PSK. When the bit rate is the same for all systems, then for a given amplitude slope, QPSK is the least degraded. 8 PSK is less degraded than Offset QPSK for values of amplitude slope less than 0.47 dB/MHz, but more degraded for higher values. For equal bit rates and a given delay slope, Offset QPSK is the least degraded, followed by 8 PSK, then QPSK. By relating degradation results to published information on the occurrence of amplitude and delay slope distortion components during multipath fading, estimates ofS/Ndegradation for a Peof 10-4due to these distortion components as a function of frequency selective fade depth are made. Finally, measuredS/Ndegradations for a Peof 10-4due to amplitude and delay slope on a 44.7 Mbit/s Offset QPSK system with filtering similar to the computer model are presented and compared to the computer simulation results.

A fast symbol timing recovery technique for PSK signals is presented. For the carrier recovery and for the symbol timing recovery subsystems, a parallel processing of the received modulated signal is employed. Theoretically it is shown that the optimum delay is2/f_{c}and notT/2as published in the literature, where fcandTare the carrier frequency and the symbol interval, respectively. This technique has the advantage over the currently employed serial processing technique in faster acquisition of symbol timing and with it in faster overall system synchronization. Experimental verification of theoretical results is presented and applications in satellite and terrestrial microwave communication systems are discussed.

Pseudo-error detectors are devices which show great potential for the measurement of the bit error rate of an on-line digital communications link. They are implemented in the form of a second detector (in addition and in parallel to the traffic data bit detector) which is very perturbation-sensitive. They do not compromise the traffic handling capacity of the system. Four methods of generating the pseudo-error characteristic are described: i) shifted detection threshold; ii) intersymbol interference enhancement; iii) noise addition; iv) sampling phase offset. Practical considerations generally govern the choice of method. Experimental results of pseudo-error detector behavior in the presence of Gaussian noise show that stable characteristics can be achieved to estimate a wide range of bit error rate (BER's) in very modest time intervals. In addition, experimental results in a real complex environment consisting of a 1.544 Mbits/s T1 capacity digital link using QPSK modulation techniques, including both Gaussian and non-Gaussian perturbations, show pseudo-error detection to be reliable for the measurement of BER and for controlling channel switching.

A brief literature survey of transversal filters is followed by a design example of a binary transversal filter. A particular application in the modulator of the single channel per carrier data transmission equipment is described. Computer simulations and COS/MOS hardware implementation proved the validity and the feasibility of the theory described by Mueller. Design and evaluation of a linear phase filter approaching the Nyquist channel is the end result of our study.

The signal-to-noise(S/N)ratio of digital signals can be improved by using diversity transmission, followed by combining at the receive end, resulting in a lower probability of errorP(e). Second-order combining schemes for data signals are presented in this paper. The effect of delay difference in the two channels of a diversity system is analyzed, using four-phase coherent, phase-shift keying (PSK) as an example. For comparison with predicted performance, experimental results are included.

Methods are presented for the transmission of wideband digital signals above and below the frequency division multiplexing (FDM) baseband spectrum of analog microwave systems. In particular, it is shown that a Data Above Voice (DAV) system employing a 1.544 Mbit/s data channel above FDM channels and a Data Under Voice (DUV) system employing 1.544 Mbit/s channel below 720 FDM channels, are practical hybrid systems. Theoretical models of hybrid transmission are disucssed and the DAV/DUV methods are compared. A conceptual hybrid system design procedure is introduced and field measurement results on an 8 hop 6 GHz system are presented, comparing favorably with theoretical predictions.

A method for transmission of a 1.544-Mbit/s data channel in a frequency band which is above the baseband of sound and vision signals of color television in FM microwave systems is described. The impact of the interfering video signal on phaseshift keyed (PSK) performance is evaluated. The probability of error as a function of the receiver carrier-to-noise ratio in the data channel for the case of Gaussian noise, sinusoidal interference, and various video test waveform (color bar, stair step, etc.) interference and combinations of these is measured. Analog and also novel digital methods of insertion of the television sound subcarrier signal are described. Data above video (DAVID) transmission measurement results which have been performed on an RCA 6-GHz microwave hop are presented.

A hybrid information carrier is defined as a carrier for partly analog and partly digital source information. Some of the reported schemes transmit 1.5 Mbits/s and 1200 frequency division multiplexing (FDM) channels, others 6.3 Mbits/s and 600 FDM channels, in the shared spectrum of the microwave baseband (BB). Baseband diversity (BBD) is frequently used in this type of system. As hybrid transmission is getting wider popularity, it is important to determine the requirements on hybrid BBD systems. Different block diagrams are considered and performance of realizable hybrid BBD units is discussed. The BBD has different configurations for different data rates. We discuss the reasons for this within the theoretical performance and practical limit of technology. The important functions of signal presence and noise evaluation occur in the BBD. A novel approach to noise evaluation in the BB of microwave systems is feasible with digital techniques. The analog noise and pilot detection are in its threshold for high noise condition and fast pilot sensing time requirement. The proposed scheme evaluates digitally the error probabilityP(e), based on error detections, and works well in high noise conditions.

Control, supervisory, and order wire signal transmission, i.e., service channel (SC) is a requirement for most microwave systems. In digital radio the data spectrum extends to the lowest baseband (BB) frequencies. Frequency translation of the SC above the data spectrum is one possibility. Another method, in frequent use, is the double modulation technique, e.g., the SC amplitude modulates the carrier that is already phase modulated by the data spectrum. A novel approach that results in significant equipment cost savings and good performance is proposed. The low-frequency power density spectrum of the random multilevel channel coded data is removed. The vacated spectrum is available for the SC. For different bit rates, the error probabilityP(e)degradation due to SC is calculated. The analysis is performed for various channel coded messages and SC spectrum utilizations. The high-pass filter that assures low-frequency data spectrum removal is optimized for the SC signal to noise ratio and dataP(e)degradation. The analysis is verified with computer simulations and actual multilevel frequency-shift keying (FSK) 6-GHz microwave hop measurements. The proposed approach is also applicable to analog microwave systems when carrying hybrid information. The hybrid system carries an SC, 6.3-Mb/s data, and 900 frequency division multiplexed voice channels in a shared BB.

Performance evaluations and measurement principles of hybrid information carriers, that is, carriers for partly analog and partly digital source infomation, are established. Simulation requirements, which are independent from a particular hybrid configuration or system capacity, for the analog and digital sources are presented. In order to perform the analysis of multilevel digital streams, a simple formula for the power spectrum density of the randomn-level Gray-coded stream is derived. With normalization of the formula, the same power density spectrum for any number of levels is achieved. The required length of the pseudorandom sequence as a function of the transmission characteristics and hybrid environment is established. The measurement difficulties, when the noise source is not Gaussian, are discussed.