Showing papers on "Phase-shift keying published in 1993"

Coded modulation with unequal error protection

Abstract: It is always desirable to maintain communications in difficult situations, even though fewer messages can get across. The author has developed such capabilities for one-way broadcast media, such as the envisioned terrestrial broadcasting of digital high-definition television signals. In this television broadcasting, the data from video source encoders are not equally important. It is desirable that the important data be recovered by each receiver even under poor receiving conditions. Two approaches for providing such unequal error protection to different classes of data are presented. Power-efficient and bandwidth-efficient coded modulation is used in both approaches. The first approach is based on novel signal constellations with nonuniformly spaced signal points. The second uses time division multiplexing of different conventional coded modulation schemes. Both approaches can provide error protection for the important data to an extent that can hardly be achieved using conventional coded modulation with equal error protection. For modest amounts of important data, the first approach has, additionally, the potential of providing immunity from impulse noise through simple bit or signal-point interleaving. >

Performance of CDMA/OFDM for mobile communication system

Abstract: The OFDM technique is an interesting approach in mobile communications in order to achieve a high spectral efficiency and to combat the frequency selectivity of the channel. Another interesting technique is a CDMA system with Rake-receiver. The main advantages of CDMA are well known, but its capacity is limited by multiuser interference. The aim of this study is to combine the principle of a CDMA system with OFDM. This combination allows one to perform a maximum-likelihood detection (MLD), to use the spectrum in an efficient way, to exploit frequency diversity, and to retain many advantages of a CDMA system. In addition, it permits simple cell-separation by using frequency hopping, and a simple hardware realization. Two examples for mobile communication system using CDMA/OFDM with Walsh-Hadamard code-spreading for the downlink (Base-Mobile) are studied. Different coherent detection algorithms such as conventional detection, MLD, and iterative detection are analyzed. The analytical performance of MLD is evaluated. It is shown that by using BPSK modulation, one can transmit 64 active users at rate of about 16.664 kbit/sec in a 1.28 MHz bandwidth, resulting in a spectral efficiency of 0.8332 bit/sec/Hz. By using QPSK modulation half of this bandwidth will be used. It is also analytically shown that the performance of a CDMA/OFDM with MLD, using frequency-diversity is comparable to an optimum CDMA system with Rake-receiver using multipath diversity. However, in practice providing diversity with OFDM is much simpler than a Rake-receiver. In addition, the OFDM technique simplifies the channel-estimation problem. Hence using CDMA/OFDM for the downlink is a promising approach since the receiver at the mobile station should be very simple.

Atmospheric optical communication system using subcarrier PSK modulation

Abstract: An atmospheric optical communication (AOC) system using subcarrier phase shift keying (PSK) is proposed, and its superiority to OOK in the presence of scintillation is discussed theoretically. An experimental AOC setup using a subcarrier modulated by 155.52 Mb/s phase differential shift keying (DPSK) at light wavelength /spl lambda/ = 0.83 /spl mu/m over an 1.8 km outdoor path is employed to show the performance. Theoretical and experimental results are compared under scintillation in clear weather. Good agreement is observed. BER in various kinds of weather is given. AOC using subcarrier M-ary PSK and multiple subcarriers is discussed. >

Differential detection of pi /4-shifted-DQPSK for digital cellular radio

Abstract: The detection of pi /4-shifted-DQPSK modulation using a tangent-type differential detector with an integrated symbol timing and carrier frequency offset correction algorithm is discussed. pi /4-shifted-DQPSK modulation has been proposed for use in a high-capacity, TDMA-based digital cellular system being developed in the US; differential detection could potentially allow the production of low-complexity mobile units. Results obtained using the proposed IS-54 TDMA frame structure for base to mobile transmissions are presented. Theoretical and simulation bit-error-rate (BER) results are presented for static and Rayleigh fading channels. BER results are provided as a function of E/sub b//N/sub 0 /and C/I, where the interferer is a second pi /4-shifted-DQPSK signal. Additional results are provided which show the BER sensitivity to Doppler frequency shifts, time delay spread, and carrier frequency offsets. >

Near-quantum optimum receivers for the phase-quadrature coherent-state channel.

Abstract: A theoretical analysis of two simple implementations for optical receivers that achieve near-quantum optimum performance for phase-quadrature coherent-state signaling is carried out. For a large received average photon count per symbol, Ns, the error probability is proportional to exp(−2Ns) as opposed to the conventional heterodyne performance of exp(−Ns/2).

Bit error simulation for pi /4 DQPSK mobile radio communications using two-ray and measurement-based impulse response models

Abstract: An accurate software/hardware bit-by-bit error simulator for mobile radio communications is described. Simulation results in indoor and outdoor channels are compared with theoretical results. Bit error rate (BER) results in simulated frequency-selective fading channels generated by several channel models such as two-ray, constant amplitude, and simulated indoor radio channel impulse models (SIRCIMs) are presented. It is shown that BER is not only dependent on the RMS delay spread, but also on the distribution of temporal and spatial multipath components in local areas. An important result is that a two-ray Rayleigh fading model is a poor fit for indoor wireless channels and, if used, can underestimate the BER by orders of magnitude. A real-time bit error simulation of video transmission using the bit-by-bit error simulator is described. The simulator, called BERSIM, is shown to be a useful tool for evaluating emerging data transmission products for digital mobile communications. >

F-QPSK-a superior modulation technique for mobile and personal communications

Abstract: A brief review of the principles and properties of F-QPSK (quadrature phase shift keying) modulation is given. Its spectral efficiency is compared with that of Gaussian minimum-shift keying (GMSK) using adjacent channel interference (ACI) as a parameter. It is established that in a noncellular environment, hardlimited F-QPSK has a spectral efficiency of 1.42 b/s/Hz, which is up to 51% more spectrally efficient than GMSK BT=0.5 for ACI=20 dB. F-QPSK's BER performance in AWGN and Rayleigh fading channels is shown to be superior to that of GMSK. The spectral efficiency of the modulations in cellular/microcellular environments where frequency is reused in geographically separate cells to achieve higher capacity is compared. It is shown that the application of F-QPSK in such environment would leads to a 95% increase of system capacity compared to GMSK. It is concluded that the power and spectrally efficient F-QPSK make it an excellent candidate for future high-capacity personal communication system (PCS) networks. >

Mobile cellular radio systems

Abstract: A frequency hopped cellular mobile radio system, comprising a plurality of base stations each affording a cellular region, or cell, within which communication is facilitated with a plurality of mobile units, each of which mobile units is free to move from cell to cell, the system having in combination the following features; a synchronization system which serves to bring all mobile units within a cell into synchronization, as observed from the base station of that cell, whereby orthogonal frequency hopping between mobile units of the cell is facilitated, frequency hopping between mobile units in different cells being non-orthogonal; an affiliation strategy which tends to constrain each mobile unit to maintain communication with a base station selected in dependence upon received signal strength; mobile unit transmission power control in dependence upon received signal/noise ratio at a base station, whereby transmitted mobile power tends towards the minimum necessary for acceptable communication; a frequency hopping strategy, wherein the number of bits per hop is limited to a few bits per hop, the actual number of hops being chosen having regard to performance and overall framing delay constraints; Nyquist filtering of signals effective to constrain interference between hops both in the time domain and in the frequency domain, hopped dwells being arranged to extend in time beyond a nominal bit transmission period (before and after the period) so as to overlap; differential phase shift keying (DPSK) modulation combined with randomized quadrature phase shift keyed (QPSK) modulation to de-correlate interference between cells; and, a system of error control coding.

Performance analysis of an all-digital BPSK direct-sequence spread-spectrum IF receiver architecture

Abstract: A VLSI architecture for an all-digital binary phase shift keying (BPSK) direct-sequence (DS) spread spectrum (SS) intermediate frequency (IF) receiver is presented, and an in-depth performance analysis is given. The all-digital architecture incorporates a Costas loop for carrier recovery and a delay-locked loop for clock recovery. For the pseudorandom noise (PN) acquisition block, a robust energy detection scheme is proposed to reduce false PN locks over a broad range of signal-to-noise ratios. The proposed architecture is intended for use in the 902-928 MHz unlicensed spread spectrum radio band. A 100 kbs information rate and a 12.7 Mchips/second PN code rate are assumed. The IF center frequency is 12.7 MHz and the IF sampling rate is 50.8 Msamples/second, which is the Nyquist rate for the 25.4 MHz bandwidth signal. Finite wordlength effects have been simulated to optimize the architecture, thereby minimizing the chip area, and results of the finite wordlength simulations demonstrate that the chip architecture achieves a bit error rate performance within 1 dB of theory in an additive white Gaussian noise channel. >

Coded Modulations for Fading Channels: An Overview

Abstract: It is by now well known that combined channel coding and digital modulation (in short coded modulation) yield power and bandwidth efficient digital transmission schemes for the Gaussian channel. These schemes perform closer to channel capacity than those where coding and modulation are treated separately. More recently, coded modulation methods have been investigated for transmission over Rayleigh fading channels. It has been discovered that systems optimized for the Gaussian channel are not optimum for fading channels. This is primarily due to the importance of built-in time diversity of the coded modulation scheme for fading channels. In this paper we give a brief overview of recent developments in signal design for Rayleigh fading channels. Mainly schemes based on phase shift keying are presented. Theoretical guidance for design and identification of the most important system parameters are given. A couple of detailed examples of code designs and their error probability performance are also given. Optimum and suboptimum receivers are discussed. An overview of analytical performance analysis tools, as well as code designs for both trellis coded modulation and block coded modulation is included in the paper. Methods for unequal error protection are given. Potential application areas are identified. An extensive reference list is supplied.

Maximum likelihood open loop carrier synchronizer for digital radio

Abstract: The problem of carrier synchronization frequency offset is considered. New frequency estimation algorithms for the data-aided (DA), the decision directed (DD), and the nondata-aided (NDA) mode are derived from the maximum likelihood principle. Their performance is assessed by both analysis and simulation, and the error variance is compared to the Cramer-Rao bound. The resulting open-loop system for estimating and correcting frequency and carrier phase for a coherent M-ary phase shift keying (MPSK) transmission system is presented and verified. Implementation issues like filter functions and nonlinearities are addressed. It is found that the proposed structures provide optimal synchronization at high SNR and robust synchronization even at low SNR. >

Coded M-DPSK with built-in time diversity for fading channels

Abstract: Good coded modulation for fading channels requires built-in time diversity. Under a constraint on the interleaving delay, the authors construct and compare three categories of coded M-DPSK (M-ary differential phase-shift keying) schemes with 4 >

A 0.7-3 GHz GaAs QPSK/QAM direct modulator

Abstract: A single chip I/Q direct modulator for use in digital radio links is presented. This device translates directly the phase and quadrature baseband signals to a microwave frequency that can be chosen between 700 MHz and 3 GHz avoiding the use of IF circuits. It is able to generate any type of digital modulation as QPSK, n-PSK, n-QAM, GMSK, etc., with band-limited spectrum. The bandwidth of the I/Q modulating signals is more than 500 MHz allowing the use of the circuit even in the highest capacity systems. The device has 120 components in a 2.2*2.4 mm chip and has been fabricated using 0.5 mu m GaAs MESFET process. >

Performance evaluation for band-limited DS-CDMA communication system

Abstract: The model uses a coherent binary phase shift keying (BPSK) modulation scheme operated in the asynchronous mode. For ease of interpretation of results, raised cosine spectra for filters is assumed. A standard Gaussian approximation is used to derive approximated error probabilities, P/sub e/, in simple form. The effect of imperfect despreading synchronization is shown.

Performance of microcellular mobile radio in a cochannel interference, natural, and man-made noise environment

Abstract: A model is developed for performance analysis of a microcellular digital mobile radio system with Rayleigh-faded cochannel interference, Gaussian noise, and narrowband impulsive noise (Middleton's Class A noise) using the differential phase shift keying (DPSK) modulation technique. The desired signal is assumed to be Rician faded. The effects of selection diversity on the performance are investigated and compared to the nondiversity case. The performance is measured in terms of bit error probability and spectrum efficiency. The influence of reuse distance (i.e. cluster size), traffic intensity (i.e. blocking probability), impulsive index, Rician parameter, and turning point of the dual path loss law characteristic of microcells on the performance parameters is investigated in detail. >

A soft-output bidirectional decision feedback equalization technique for TDMA cellular ratio

Abstract: Issues encountered in the design of reliable narrowband time-division multiple access (TDMA) digital cellular mobile communication systems are considered. In particular, the problem of compensating for the harsh multipath fading environment in systems whose transmission bandwidth is commensurate with the coherence bandwidth of the fading channel is considered. A TDMA channel characterization parameter, the slot-normalized fade rate, is introduced, and an adaptive bidirectional equalization technique, which estimates the location of a deep fade within a time slot, is proposed. The simulation results show that the carrier-to-noise ratio requirement is only 15.5 dB when this equalization technique is used. This is achieved without diversity, and with low complexity. An equivalent equalized land mobile radio channel model and the analytical solution for the optimal bit likelihood calculation for pi /4-shift quadrature differential phase-shift keying (QDPSK) modulation are also derived under certain channel conditions. The results are used as soft decisions for the convolutional decoder. >

A micro-cellular CDMA system over slow and fast Rician fading radio channels with forward error correcting coding and diversity

Abstract: The microcellular radio environment is characterized by a Rician fading channel. The use of a slotted code division multiple access (CDMA) scheme is considered in single- and multi-microcell systems. The throughput and delay performance of a slotted CDMA network are analyzed for slow and fast Rician fading radio channels using differential phase shift keying (DPSK) modulation. The application of selection diversity (SD) and maximal ratio combining (MRC) improve the performance for both slow and fast fading. It is also shown that the use of forward error correcting (FEC) codes enhances the system performance. Computational results are presented for maximum rms delay spread in the order of 2 mu s and data rates of 32 and 64 kbit/s. A comparative analysis of macro-, micro- and pico-cellular CDMA systems is also presented. >

Postdetection optimal diversity combiner for DPSK differential detection

Abstract: Postdetection diversity reception weights and combines all the detector outputs before symbol decision to combat the effects of multipath fading. A theoretical analysis of a postdetection optimal diversity combiner that can minimize the symbol error probability for differential phase shift keying (DPSK) differential detection in the presence of multiplicative Rayleigh fading, and co-channel interference (CCI) is presented. The effect of unequal average powers among diversity branches is taken into account. It is shown that the postdetection maximal-ratio combiner (MRC) described previously by the author is not optimal unless all branches have the same average power. It is also found that the combiner optimized for the effect of CCI (fading induced random FM noise) should weight each branch detector output in inverse proportion to the average CCI power (desired signal power). Assuming two-branch diversity, calculated BER (bit-error-rate) performance of pi /4-shift QDPSK due to AWGN, CCI, and random FM is presented. In addition, the BER due to multipath channel delay spread (which is not treated in the theoretical analysis) is also computed to find the optimal combiner. >

Approximate results for the bit error probability of binary phase shift keying with noisy phase reference

Abstract: Approximate results for the bit error probability (BEP) of binary phase shift keying (BPSK) in the presence of a noisy carrier phase reference are presented. The results show correctly the behavior or the BEP as a function of SNR. The accuracy of the approximations is verified by simulations and numerical integration of the BEP formulas. The results are compared with existing bounds. >

Decision feedback equalization of the indoor radio channel

Abstract: The measured multipath profiles from five different indoor areas are used for the performance analysis of a binary phase shift keying (BPSK) modem with a decision feedback equalizer (DFE). The performance from the measured multipath profiles is compared with the performance predictions based on a computer simulated channel model. Both average probability of error and probability of outage are calculated for a DFE with three fractionally spaced forward and three feedback taps. An equivalent delay power spectrum function, determined from the ensemble of the measured channel impulse responses, is defined. Using this function, analytical lower bounds on the average probability of error and the probability of outage of the BPSK/DFE modem with an infinite number of feedback taps and three forward taps are determined and compared with the results based on measured data and the computer generated channel impulse responses. >

Quantization loss in convolutional decoding

Abstract: The loss in quantizing coded symbols in the additive white Gaussian noise (AWGN) channel with binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK) modulation is discussed. A quantization scheme and branch metric calculation method are presented. For the uniformly quantized AWGN channel, cutoff rate is used to determine the step size and the smallest number of quantization bits needed for a given bit-signal-to-noise ratio (E/sub b//N/sub 0/) loss. A nine-level quantizer is presented, along with 3-b branch metrics for a rate-1/2 code, which causes an E/sub b//N/sub 0/ loss of only 0.14 dB. These results also apply to soft-decision decoding of block codes. A tight upper bound is derived for the range of path metrics in a Viterbi decoder. The calculations are verified by simulations of several convolutional codes, including the memory-14, rate-1/4 or -1/6 codes used by the big Viterbi decoders at JPL. >

Wide-band measurement and analysis techniques for the mobile radio channel

Abstract: The appearance of high bit rate digital radio systems would appear to demand the existence of wide-band channel models, planning tools, and simulation capabilities. The authors present an overview of various approaches to wide-band channel data acquisition (channel sounding) and data analysis for the locally QWSSUS channel. They outline how high-speed digital signal processing techniques can be employed to construct a nearly ideal real-time wide-band channel sounder. The digital sounder described employs a TMS32050A to implement a matched filter type receiver. The probe signal is a carrier BPSK modulated with a binary maximal length sequence; the received signal is processed in real time, yielding a complex time variant channel impulse response. A dynamic range in excess of 40 dB is achieved. They also briefly discuss techniques to use this data for stored channel simulation. Finally, by way of pointing out that much more information can be extracted from channel sounding data than is generally appreciated, they briefly discuss the use of probability theory (Bayesian techniques) in model selection. >

Multistage decoding of multilevel block M-PSK modulation codes and its performance analysis

Abstract: Multistage decoding of multilevel block multilevel phase-shift keying (M-PSK) modulation codes for the additive white Gaussian noise (AWGN) channel is investigated. Several types of multistage decoding, including a suboptimum soft-decision decoding scheme, are devised and analyzed. Upper bounds on the probability of an incorrect decoding of a code are derived for the proposed multistage decoding schemes. Error probabilities of some specific multilevel block 8-PSK modulation codes are evaluated and simulated. The computation and simulation results for these codes show that with multistage decoding, significant coding gains can be achieved with large reduction in decoding complexity. In one example, it is shown that the difference in performance between the proposed suboptimum multistage soft-decision decoding and the single-stage optimum decoding is small, only a fraction of a dB loss in SNR at the block error probability of 10/sup -6/. >

Simulation performance of a blind adaptive array for a realistic mobile channel

Abstract: A least squares constant modulus algorithm (LSCMA) is used to blindly adapt a narrowband beamformer to capture a Doppler-shifted and fading binary phase shift keying (BPSK) signal corrupted by noise and multipath. The multipath components are modeled as having equal amplitude and uniformly distributed phase, as well as directions of arrival in three dimensions (the Parsons and Turkmani channel model). Two implementations of the LSCMA are evaluated by computer simulation for a 4-element antenna array moving at 60 mph and 120 mph in noisy environments. In a comparison of bit error rates (BERs) between the two LSCMA implementations and the single antenna, the LSCMAs are found to provide 8 to 9 dB improvement in the link budget for a BER of 1%.

Low complexity adaptive equalizer radio receiver employing reduced complexity branch metric calculation

Abstract: A low complexity adaptive equalizer for use in U.S. digital cellular radios demodulates π/4-shifted differentially encoded quadrature phase shift keyed (DQPSK) encoding in the presence of intersymbol interference (ISI) with reduced decoding complexity by employing an estimated received constellation which takes into account channel changes over time and ISI. The complexity is reduced by tracking a reduced number of estimated reference symbol constellation points and taking advantage of the geometry to estimate the remaining symbol constellation points. Branch metrics are also determined with a reduced number of computations.

Spread-spectrum path diversity in a shadowed Rician fading land-mobile satellite channel

Abstract: The effects of two types of path diversity techniques, namely selection diversity and maximal ratio combining, on the bit error probability are investigated for direct-sequence spread-spectrum (DS/SS) transmission in a land mobile satellite channel using coherent binary phase-shift keying (BPSK) modulation. It is assumed that the channel consists of a log-normally shadowed line-of-sight signal plus Rayleigh distributed multipath signals. The bit error probability is evaluated for light, average, and heavy shadowing. The performance is also measured in terms of the outage probability. >

Atmospheric Optical Communication System Using Subcarrier PSK Modulation

Abstract: An atmospheric optical communication (AOC) system using subcarrier phase shift keying (PSK) is proposed, and its superiority to OOK in the presence of scintillation is discussed theoretically. An experimental AOC setup using a subcarrier modulated by 155.52 Mb/s phase differential shift keying (DPSK) at light wavelength /spl lambda/ = 0.83 /spl mu/m over an 1.8 km outdoor path is employed to show the performance. Theoretical and experimental results are compared under scintillation in clear weather. Good agreement is observed. BER in various kinds of weather is given. AOC using subcarrier M-ary PSK and multiple subcarriers is discussed. >

Quadrature compensation for orthogonal signal channels

Abstract: A system for processing inphase and quadrature data channels, such as a Costas loop QPSK demodulator, employs an additional feedback loop for adjustment of phase offset between carrier reference signals, this loop being in addition to the Costas error loop for control of frequency and phase of an oscillator which provides the regenerated carrier signal. The additional loop employs cross-channel products of demodulated inphase and quadrature data signals as does the Costas loop. The regenerated carrier is applied via an adjustable phase-offset unit to provide quadrature carrier reference signals to phase detectors of inphase and quadrature data channels. The phase offset unit includes a 90 degree hybrid circuit energized by the carrier signal at a main input port plus an adjustable fraction of the carrier power applied to an auxiliary input port. Adjustment of the relative magnitudes of the signals at the two input ports of the hybrid circuit in accordance with the sum of the cross-channel products provides a desired amount of phase offset between the two carrier reference signals in correspondence with the deviation from orthogonality of the components of the composite input signal. This removes crosstalk resulting from the deviation in orthogonality of the components of the composite input signal.

The effects of constellation density on trellis-coded modulation in fading channels

Abstract: Dense constellations such as 16-QAM (quadrature amplitude modulation) have not seen much use in mobile communication because of their greater peak-to-average power ratio and their seemingly greater sensitivity to noise and channel interference. It is demonstrated that dense constellations can actually improve performance. Using a completely analytical method, three constellations are compared with the same net throughput of 2 bits/symbol: uncoded QPSK (quadrature phase-shift keying), rate 2/3 TCM (trellis-coded modulation) 8-PSK and rate 1/2 TCM 16-QAM. Comparison on the basis of average power puts TCM 16-QAM 5-dB ahead of TCM 8-PSK (phase-shift keying). Even comparison on the basis of peak power gives TCM 16-QAM a 2.44-dB advantage over TCM 8-PSK. QPSK is much poorer than either. >

Defining, designing, and evaluating digital communication systems

Abstract: The fundamental relationships in defining, designing, and evaluating digital communication system performance are reviewed. The concept of bandwidth-limited and power-limited systems and how such conditions influence the design when the choices are confined to M-ary phase-shift keying (MPSK) and M-ary frequency-shift keying (MFSK) modulation are examined. The definitions and computations involved in transforming from data bits to channel bits to symbols to chips are discussed. It is suggested that since most digital communication systems share these concepts, understanding them should enable one to evaluate other such systems in a similar way. >