Showing papers on "Phase-shift keying published in 1997"

Bit-interleaved coded modulation

Abstract: It has been recognized by Zehavi (1992) that the performance of coded modulation over a Rayleigh fading channel can be improved by bit-wise interleaving at the encoder output, and by using an appropriate soft-decision metric as an input to a Viterbi (1990) decoder. The paper presents in a comprehensive fashion the theory underlying bit-interleaved coded modulation, provides tools for evaluating its performance, and gives guidelines for its design.

Bit-interleaved coded modulation with iterative decoding

Abstract: A simple iterative decoding technique using hard-decision feedback is presented for bit-interleaved coded modulation (BICM). With an 8-state, rate-2/3 convolutional code, and 8-PSK modulation, the improvement over the conventional BICM scheme exceeds 1 dB for a fully-interleaved Rayleigh flat-fading channel and exceeds 1.5 dB for a channel with additive white Gaussian noise. This robust performance makes BICM with iterative decoding suitable for both types of channels.

Data-aided frequency estimation for burst digital transmission

Abstract: Burst transmission of digital data is employed in several applications such as satellite time-division multiple access (TDMA) systems and terrestrial mobile cellular radio. We propose a new algorithm for carrier frequency estimation in burst-mode phase shift keying (PSK) transmissions. The algorithm is data-aided and clock-aided and has a feedforward structure that is easy to implement in digital form. Its estimation range is large, about /spl plusmn/20% of the symbol rate and its accuracy is close to the Cramer-Rao bound (CRB) for a signal-to-noise ratio (SNR) as low as 0 dB. Comparisons with earlier methods are discussed.

Novel techniques for high-capacity 60-GHz fiber-radio transmission systems

Abstract: A broad-band 60-GHz fiber-radio transmission experiment has been performed using a combination of novel techniques. The 60-GHz carrier signal was generated using a master/slave (M/S) distributed-feedback (DFB) laser configuration, which gave high purity and high power with very wide frequency tunability. The data path was separated in the wavelength domain from the carrier path so that a remote upconversion scheme could be used to provide a fully transparent link. An electroabsorption modulator (EAM) was used as a full duplex transceiver so that bidirectional optical transmission could be implemented without the need for a laser at the remote site. Transmission of a 120-Mb/s QPSK signal over a fiber span of 13 km and a radio path of 5 m was demonstrated. Furthermore, the downstream optical signal contained the 120-Mb/s QPSK signal multiplexed with 20 channels of TV. The upstream optical signal consisted of 120-Mb/s QPSK data only. Good error performance was simultaneously achieved in both directions.

Analysis of M-ary phase-shift keying with diversity reception for land-mobile satellite channels

Abstract: An analytical technique well suited to numerical analysis is presented for computing the average bit-error rate (BER) and outage probability of M-ary phase-shift keying (PSK) in the land-mobile satellite channel (LMSC) with microdiversity reception. Closed-form expressions are found for L-branch microdiversity using both selection diversity combining (SDC) and maximal ratio combining (MRC). These expressions are extended to include both M-ary coherent PSK (M-PSK) and differential PSK [M-differential PSK (DPSK)]. Following previous empirical studies, the LMSC is modeled as a weighted sum of Rice and Suzuki distributions. Numerical results are provided illustrating the achievable performance of both M-PSK and M-DPSK with diversity reception. Using measured channel parameters, the performance in various mobile environments for various satellite elevation angles is also found.

Fading-resistant modulation using several transmitter antennas

Abstract: This paper proposes a bandwidth-efficient fading-resistant transmission scheme which implements transmitter diversity using L antennas at the base station. When the antennas are spaced sufficiently far apart, the transmission from each antenna undergoes a different degree of fading. These transmissions are coordinated to mitigate the effects of Rayleigh fading, and the mobile receiver can recover the entire L-dimensional transmitted vector signal as long as the signal energy of at least one coordinate is large enough. L-dimensional fading-resistant signal constellations are generated by maximizing a figure of merit for the Rayleigh fading channel. This scheme offers a significant performance improvement over a conventional single-antenna binary phase-shift keying (BPSK) scheme when coding is ineffective due to slow fading.

Symbol-by-symbol MAP demodulation of CPM and PSK signals on Rayleigh flat-fading channels

Abstract: Demodulation using the symbol-by-symbol maximum a posteriori probability (MAP) algorithm is presented. The algorithm is derived for the case of continuous phase modulation (CPM) signals transmitted over Rayleigh flat-fading channels, and a corresponding receiver structure is specified. It is shown that the MAP algorithm requires computing, for each trellis branch, the sum of the products of the weights of all paths through the trellis which pass through that branch, and that this generic computational problem can be solved efficiently by an approach that uses a forward and backward recursion through the trellis. Simulation results are presented which show both the hard and soft decision performance of the MAP receiver to be robust, even in the presence of fade rates of up to 30% of the symbol rate. The application of the receiver concept to phase-shift keying (PSK) signals is also discussed, and then evaluated via simulation. The concept of joint demodulation and decoding using iterative processing techniques is introduced. It is shown that the MAP receiver is well suited for iterative processing applications due to its use of a priori symbol probabilities and its production of optimal soft decisions. Simulation results for the reception of quaternary PSK (QPSK) show that the bit error rate (BER) performance of the iterative MAP receiver can approach that of a receiver operating with perfect knowledge of the fading process.

Combined turbo equalization and turbo decoding

Abstract: In this paper, the subject of turbo coding in the presence of intersymbol interference channel is investigated. An iterative decoder structure is presented, which combines the channel equalization and the turbo decoding. At each iteration extrinsic information from the channel detector is fed into the turbo decoders, and then their extrinsic information is fed back to the channel detector. Simulation results are presented for turbo code with BPSK and and PSK modulations, transmitted over several ISI channels having severe frequency distortion. The performances are about 1 dB from the ISI channel capacity at bit error rate of 10/sup -5/.

A differentially coherent PN code acquisition receiver for CDMA systems

Abstract: New differentially coherent detectors for acquisition of direct sequence spread-spectrum signals are introduced. These detectors are alternatives to the noncoherent detectors that have been considered almost exclusively in the past. The proposed detectors are suitable for commercial code-division multiple-access (CDMA) systems which operate with a relatively large noise floor and provide a surprisingly large signal-to-noise ratio (SNR) improvement over the noncoherent detectors of approximately 5 dB. Under the random code sequence assumption, an exact analysis of the differentially coherent detection performance for both full period correlation (FPC) and partial period correlation (PPC) is carried out. The detector performance in terms of detection and false alarm probabilities for both partial and full period correlations is investigated, and the results are compared with those of classical noncoherent detection. The mean acquisition time for both single-dwell and multiple-dwell acquisition schemes are compared with their noncoherent counterparts.

Performance of direct-sequence spread-spectrum RAKE receivers with random spreading sequences

Abstract: In this paper, we derive error probability expressions for binary phase-shift keying (BPSK) and quaternary phase-shift keying (QPSK) spread direct-sequence spread-spectrum (DSSS) systems employing random spreading sequences with RAKE receivers. The derived expressions accurately take into account the effect of interpath interference which usually has been neglected in previous analyses. Selection, equal gain, and maximal ratio techniques are considered for diversity combining. Two possible finger assignment strategies, one based on the instantaneous amplitudes and another based on the average powers of the multipath components, are considered for the assignment of multipath components to the available demodulating fingers in the RAKE receiver. Also, various simple, and in many cases, closed-form approximations for the error probabilities are derived and their accuracies are assessed.

Performance analysis of coherent DS-CDMA systems in a Nakagami fading channel with arbitrary parameters

Abstract: A new expression for the bit error rate (BER) of an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system with coherent BPSK demodulation in a multipath Nakagami (1960) fading channel is derived. The analysis assumes an arbitrary number of independent, but nonidentical resolvable paths combined by the RAKE receiver. The results obtained show that in such systems, the effect of nonidentical fading on system performance cannot be ignored.

Channel reuse strategies for indoor infrared wireless communications

Abstract: We examine systems of fixed-channel reuse for base stations in an indoor infrared wireless communication system. The following techniques are compared: time-division multiple access (TDMA) using on-off keying (OOK) or pulse-position modulation (PPM); frequency-division multiple access (FDMA) using binary phase-shift keying (BPSK) or quadrature phase-shift keying (QPSK); code-division multiple access (CDMA) using OOK with direct-sequence spreading by m-sequences or optical orthogonal codes (OOCs). We define a parameter /spl gamma/, which equals the signal-to-noise ratio (SNR) for unit optical path gain and is proportional to the square of the transmitted average optical power. Using measured pathloss data, it is found that in a system using hexagonal cells and a reuse factor of three, for cell radii above 3 m, TDMA with OOK or 2-PPM, and CDMA using OOCs all require approximately the same /spl gamma/ to achieve a worst-case bit-error rate (BER) of 10/sup -9/ within a cell. Using TDMA with 4-PPM results in a 6-dB decrease in the required value of /spl gamma/. CDMA using m-sequences requires an increase in /spl gamma/ of 5 dB over TDMA using OOK, and FDMA with BPSK requires an increase of 12 dB. For a given reuse factor N in the noise-limited regime, the required value of /spl gamma/ decreases in inverse proportion to N/sup 2/ for TDMA schemes and inversely with N for FDMA and CDMA schemes. For cell radii below 3 m, cochannel interference dominates the systems using TDMA, FDMA, and CDMA with an OOC, resulting in an irreducible BER above 10/sup -9/ at cell radii below 1.5 m. Only CDMA with m-sequences does not develop an irreducible BER, making it the only choice for cell radii below 1.5 m.

Optimum and suboptimum frame synchronization for pilot-symbol-assisted modulation

Abstract: Pilot-symbol-assisted modulation (PSAM) is a method to reduce the effects of fading in mobile communications by periodically inserting known symbols in the data stream. The receiver uses these pilot symbols to derive its amplitude and phase reference. One aspect of this procedure which has not received much attention in the literature is the method used by the receiver to locate the pilot symbols. This paper uses optimum frame synchronization techniques to develop two synchronizers for PSAM systems; one is based on a standard maximum likelihood (ML) estimation formulation, and the other is a sequential testing algorithm. Both methods use a simple quadratic correlation filter with an energy correction factor. Simulation results and a theoretical analysis are presented.

Blind carrier phase acquisition for QAM constellations

Abstract: A constant need for ever-increasing throughputs through fixed bandwidths, fueled by several high-speed applications (such as digital TV), has pushed system designers toward more throughput-efficient modulation schemes. Because of their relatively good performance, large quadrature amplitude modulation (QAM) constellations are being used in many of these applications. One of the problems associated with the use of large QAM constellations is that of carrier acquisition, which, for efficiency reasons, must often be done without the use of a preamble. The problem is further complicated for cross constellations, for which the high signal-to-noise ratio (SNR) corner points used by some simple carrier phase estimators are not available. We derive simple algorithms for carrier phase acquisition that can be used for both square and cross constellations, and compare their performance to those of the maximum-likelihood (ML), the fourth-power estimator, and a modified fourth-power estimator, obtained by considering a reduced constellation. The introduced algorithms convert the problem of carrier phase estimation into one of estimating the mode of an underlying distribution. An expression for this underlying distribution is also obtained. The results obtained indicate that the introduced algorithms significantly outperform the fourth-power estimator for moderate to high SNRs, especially when cross constellations are used.

Feedforward ML-based timing estimation with PSK signals

Abstract: We propose a new nondata-aided clock recovery scheme for phase-shift keying (PSK) modulated signals. Its derivation is based on maximum-likelihood (ML) methods and leads to a feedforward structure that can be easily implemented in digital form. The algorithm exhibits improvements with respect to other existing circuits, especially with small excess bandwidth factors. Its performance is assessed by simulation for quaternary PSK (QPSK) and 8PSK formats.

Methods and apparatus for correcting amplitude and phase imbalances in demodulators

Abstract: Methods and apparatus for detecting and correcting phase and amplitude imbalances existing between I (in-phase) and Q (quadrature phase) signal components of a complex signal, e.g., QAM or OPSK signal, that is being demodulated are described. The phase and amplitude imbalance and correction circuitry of the present invention are implemented as decision directed control loops which can be used in conjunction with an overall decision directed gain control loop. Amplitude imbalance is corrected by adjusting the gain of one of the I and Q signal components. Phase imbalance is corrected by adding a portion of one of the I and Q signal components to the other one of the I and Q signal components. Overall amplitude control is achieved by adjusting the gain of both the I and Q signal components by the same amount. Various embodiments of the present invention utilize I/Q differential amplification and cross coupling to compensate for amplitude or phase imbalance with amplitude and phase correction factors being calculated in an automatic closed loop fashion by examining sliced errors and the location of their corresponding target symbols. The present invention is well suited for use, in, e.g., QPSK and/or QAM demodulators.

Method and apparatus for demodulating trellis coded direct sequence spread spectrum communication signals

Abstract: A method and an apparatus for decoding trellis coded direct sequence spread spectrum communication signals are provided. A transmitted QPSK signal is received. Binary phase-shift keyed (BPSK) despreading is performed on the QPSK signal. The BPSK despreading comprises correlating the in-phase and the quadrature components of the received QPSK signal with independent PN sequences for the each of the in-phase and the quadrature components. The despread signal is then demultiplexed, and the pilot signal and the BPSK data signal are recovered. The recovered pilot signal is used to provide a channel phase estimate and a channel magnitude estimate. The recovered BPSK data signal is demodulated, despread, and decoded. The despreading and decoding comprises determining a number of cross-correlation terms of the received signal using a number of transmitted biorthogonal Walsh sequences. The cross-correlation terms are used as the branch metrics in a maximum likelihood decoding algorithm. The branch metrics may be computed with a Fast Walsh Transform scaled in response to the channel phase estimate from the recovered pilot signal. The maximum likelihood decoding algorithm may be a Viterbi algorithm in which the optimum path is the path with the maximum accumulated branch metric. The decoding algorithm recovers and outputs a transmitted data bit sequence.

Efficient evaluation of the error probabilities of spread-spectrum multiple-access communications

Abstract: We consider the performance of code-division multiple-access (CDMA) communications. In particular, we illustrate the use of a semianalytical approach which is combined with importance sampling for the efficient evaluation of the average bit-error rates (BERs) of asynchronous direct-sequence (DS) based CDMA systems employing binary phase shift keying (BPSK) modulation along with specific signature sequences for a variety of system parameters.

Serial concatenated trellis coded modulation with iterative decoding

Abstract: We propose a design approach for serial concatenation of an outer convolutional code and an inner trellis code with multilevel amplitude/phase modulations using a bit-by-bit iterative decoding scheme. An example is given for throughput of 2 bits/sec/Hz with 2/spl times/8PSK modulation to clarify the approach. In this example, an 8-state outer code with rate 4/5 and a 2-state inner trellis code with 5 inputs and 2/spl times/8PSK outputs per trellis branch were used. The performance of this code with input block of 16384 bits is within 1.1 dB from the Shannon limit for 8PSK at a bit error probability of 5/spl times/10/sup -8/ for 2 bits/sec/Hz with 10 iterations,.

VPSK and VMSK modulation transmit digital audio and video at 15 bits/sec/Hz

Abstract: Bandwidth efficiencies that have not been possible in the past are now being achieved using variable PSK (VPSK) and variable MSK (VMSK) modulation without the loss of signal power that normally accompanies high bandwidth efficiency methods. Theoretically, these two methods do not lose any signal energy with increasing bandwidth compression. Efficiencies up to 15 bits/sec/Hz are now being achieved in usable hardware with C/N ratios better than that obtainable using FM, BPSK or QPSK. This paper explains how this is accomplished and gives a full mathematical analysis of the method. FM-SCA, VSAT, STL and RPU equipment are now undergoing beta testing prior to FCC Type Acceptance submission. Installation is expected on a large network during 1997.

Transmitter for qam encoded data

Abstract: Quadrature Amplitude Modulated signals are generated from data bits by using a first Quadrature Phase Shift Keying (QPSK) modulator for encoding a first pair of the data bits into one of four carrier signal phases, thereby producing a firs QPSK signal. A second QPSK modulator encodes a second pair of the data bits into one of four carrier signal phases, thereby producing a second QPSK signal. The first QPSK signal is amplified to a first power level, and the second QPSK signal is amplified to a second power level. The first and second amplified signals are then combined to produce a signal in which four data bits are encoded. In another aspect of the invention, a new type of modulation, called Offset Quadrature Phase Shift Keying (OQPSK), is used in place of the first and second QPSK modulators, so that an Offset Quadrature Amplitude Modulation (OQAM) transmitter is formed. An OQPSK modulator encodes data bits by encoding a first sub-group of the data bits into a real part of a complex signal at an odd instant of a clock, and by encoding a second sub-group of the data bits into an imaginary part of the complex signal at an even instant of the clock. OQPSK modulation provides the benefit of having all signal transitions being constrained to trajectories around constant radius circles, thereby producing spectral efficiency.

Space-time coded modulation for high data rate wireless communications

Abstract: This paper presents the theory and practice of a new advanced modem technology suitable for high data rate wireless communications and presents its performance over a frequency-flat Rayleigh fading channel. The new technology is based on space-time coded modulation (STCM) with multiple transmit and/or multiple receive antennas and orthogonal pilot sequence insertion (O-PSI). In this approach data is encoded by a space-time channel encoder and the output of the encoder is split into N streams to be simultaneously transmitted using N transmit antennas. The transmitter inserts periodic orthogonal pilot sequences in each of the simultaneously transmitted bursts. The receiver uses those pilot sequences to estimate the fading channel. When combined with an appropriately designed interpolation filter, accurate channel state information (CSI) can be estimated for the decoding process. Simulation results of the proposed modem as applied to the IS-136 cellular standard are presented. We present the frame error rate (FER) performance as a function of the signal to noise ratio (SNR) and Doppler spread in the presence of timing and frequency offset errors. Simulation results show that, for example, for 10% FER, data rates up to 54 kbps per a 30 kHz channel can be supported at a SNR of 11.7 dB and a Doppler spread of 180 Hz using a 32-state 8-PSK space-time code with 2 transmit and 2 receive antennas. Simulation results for other cases are also provided.

Carrier recovery method and apparatus

Abstract: Improved carrier recovery methods and apparatus suitable for use with QAM, QPSK and a wide variety of other modulation formats is described. In accordance with the invention, the phase error between received symbols, representing a frequency error, is determined using one of a plurality of techniques. The estimated frequency error is used to adjust the phase and/or frequency of a received carrier signal to achieve a frequency lock. The methods and apparatus of the present invention can be easily integrated into existing carrier recovery designs to supplement known frequency In accordance with a first embodiment of the present invention, the receipt of pairs of consecutive outer symbols is detected, a frequency error associated with each pair of consecutive symbols is generated, and the frequency error is compared to a selected threshold value to determine if it is a non-ambiguous estimate of the frequency error. If the frequency error is non-ambiguous and from a pair of consecutive outer symbols, it is used to adjust the frequency and/or phase of a received carrier signal. In the second embodiment, the receipt of pairs of consecutive outer symbols are detected. An estimate of the frequency error, determined as the phase error between received symbols, is made in the second embodiment by doing a symbol to symbol, as opposed to a symbol to target comparison.

Impact of diversity reception on fading channels with coded modulation. II. Differential block detection

Abstract: For pt. I see ibid., vol.45, no.6, p.563-572, 1997. We study coded modulation with block differential detection in an arbitrarily correlated Rician fading channel with space diversity. Coded differential q-PSK is included in our analysis as a special case. A metric is chosen that is optimum for perfect interleaving, slow fading, and independent diversity branches. For slow fading, we compare the the cutoff rates of the channels resulting from different choices of block length N and diversity index M. Specifically, we show that block detection with diversity may or may not generate a better coding channel than usual differential detection, according to the code selected and the combination of values of M and N. In particular, for low-diversity orders (M=1,2) and for low-to-medium code rates, differential detection is still an optimal or near-optimal solution, while for high-diversity orders (M/spl ges/2) and medium-to-high code rates (up to uncoded modulation) block detection with N>2 can provide a significant gain. An error floor always exists when fading is fast. It decreases exponentially with the product of code diversity and space diversity, so that the latter emerges as a very effective technique for lowering the error floor of a system affected by fast fading. Performance examples based on actual coding schemes are also shown.

Array processing techniques for multiuser detection

Abstract: Techniques often used in the area of adaptive array signal processing are applied to the multiuser detection problem. The results of this effort include a robust detector, suitable for use in the presence of modeling errors, and a reduced-rank detector with improved transient behavior relative to full-rank detectors. Algorithm performance is presented in the form of bit-error-rate (BER) curves and least mean square (LMS)-like learning curves.

Method for the reception of multicarrier signals and related apparatus

Abstract: There are various methods for terrestrial transmission of digital multicarrier broadcast signals, having modulation types like OFDM, QPSK and QAM. A main concern in connection with such systems is synchronisation in case a receiver is switched on or tuned to another channel. Present receiver tuning concepts are based on the assumption that an acceptable number only of fixed center frequencies exists. Now, some broadcasters and national organisations are considering a lot of exceptions or, in general, additional large offset frequency positions in order to reduce the influence from and to other services in the same or neighbouring frequency sections. This will increase the amount of fixed channel numbers to be tuned and subsequently the size of the receiver memory and the scanning time for the entire set of frequencies. According to the invention, the time needed by the receiver to reach a given tuning point with adequate accuracy and to exclude the possibility that there is a system-compliant signal is reduced significantly by a special evaluation (EV2) of correlation values. After such decision the receiver can go ahead to the next tuning point. If, however, a system-compliant signal has been found - even with an offset - the multicarrier signal is further decoded in the normal way. The conformity check can also be carried out in the subsequent normal reception.

Performance evaluation of optimum combining and maximal ratio combining in the presence of co-channel interference and channel correlation for wireless communication systems

Abstract: Antenna diversity is an important technique to combat fading and reduce co-channel interference (CCI). In this paper, we present an analytical approach to derive bit error rate (BER) for Optimum Combining (OC) and Maximal Ratio Combining (MRC) in the presence of CCI. The paper has two parts. In the first part, the analysis of BER for OC with two co-channel interferers and MRC with an arbitrary number of interferers is presented for M-array antenna systems under the assumption that the channels of users are independent of each other. In the second part, the analysis of BER for OC and MRC in the presence of one dominant co-channel interferer is presented for dual antenna systems by assuming that the channels of the desired user or co-channel interferer are correlated. For DPSK signal, an exact BER expression is derived. The work presented here also yields an upper bound for BPSK or QAM signal based on the results of Foschini and Salz (1983).

Chip-level differential encoding/detection of spread-spectrum signals for CDMA radio transmission over fading channels

Abstract: This paper is concerned with the design and performance evaluation of a direct-sequence spread-spectrum transmission and signal-detection technique for application to code-division multiple-access mobile radio networks. Unlike the conventional differential encoding/detection of data symbols, such a technique envisages differential encoding/detection of the spreading code chips to counteract the fast time-selective fading encountered in mobile radio transmissions. Binary PSK modulation on a Rician frequency-flat-fading channel is assumed, with Gaussian and Rayleigh channels as particular cases. The bit-error rate performance of the receiver is analytically evaluated as a function of the relevant system parameters, and the impact on the receiver performance of a carrier frequency offset is also investigated.

A suboptimal algorithm for modulation classification

Abstract: In this paper we present a suboptimal algorithm for modulation classification to classify the general M-ary phase-shifted keying (MPSK) signal buried in additive white Gaussian noise (AWGN). We first derive the phase density functions of MPSK signals, then develop the required statistics for modulation classification and demonstrate a classifier for CW, binary phase-shifted keying (BPSK), quadrature phase-shifted keying (QPSK), and 8PSK. The structure of the proposed classifier is flexible and is easy to expand. The performance of classifier is evaluated in terms of the probability of successful classification. An example (BPSK/QPSK case) is provided to demonstrate the capabilities of the proposed classifier. The performance is evaluated through the theoretical approach and the Monte Carlo computer simulations and is compared with that previously published in 1992. It is shown that the performance of the proposed classifier is better. Further improvement in performance can be obtained by increasing the length of observation interval.

Quality estimation of PSK modulated signals

Abstract: The use of the Ka band (30/20 GHz) for satellite communication systems raises the problem of dealing with rain attenuation. A very important problem in fade countermeasure systems is the need to detect signal quality quickly and accurately. In fact, the countermeasure has to be initiated before the signal degradation effect an the bit error rate is detected by the user. This article presents an overview of different methods to evaluate signal quality, which are based on the availability of soft quantized levels of PSK demodulated signals at the receiver. It is shown that this class of methods has a good theoretical performance. Also, an innovative procedure is presented which adapts one of the methods to some existing hardware, and tunes up a set of parameters in order to compensate for the fact that the hardware is to same extent impaired.