QPSK Data Research Articles

Asymptotic Analysis of General Multiuser Detectors in MIMO DS-CDMA Channels

We analyze a MIMO DS-CDMA channel with a general multiuser detector including a nonlinear multiuser detector, using the replica method. In the many-user, limit the MIMO DS-CDMA channel with the multiuser detector is decoupled into a bank of single-user SIMO Gaussian channels if a spatial spreading scheme is employed. On the other hand, it is decoupled into a bank of single-user MIMO Gaussian channels if a spatial spreading scheme is not employed. The spectral efficiency of the MIMO DS-CDMA channel with the spatial spreading scheme is comparable with that of the MIMO DS-CDMA channel using an optimal space-time block code without the spatial spreading scheme. In the case of the QPSK data modulation scheme the spectral efficiency of the MIMO DS-CDMA channel with the MMSE detector shows {\it waterfall} behavior and is very close to the corresponding sum capacity when the system load is just below the transition point of the {\it waterfall} behavior. Our result implies that the performance of a multiuser detector taking the data modulation scheme into consideration can be far superior to that of linear multiuser detectors.

Electronic compensation of chromatic dispersion using a digital coherent receiver

Digital signal processing (DSP) combined with a phase and polarization diverse coherent receiver is a promising technology for future optical networks. Not only can the DSP be used to remove the need for dynamic polarization control, but also it may be utilized to compensate for nonlinear and linear transmission impairments. In this paper we present results of a 42.8Gbit/s nonlinear transmission experiment, using polarization multiplexed QPSK data at 10.7GBaud, with 4 bits per symbol. The digital coherent receiver allows 107,424 ps/nm of chromatic dispersion to be compensated digitally after transmission over 6400km of standard single mode fiber.

Variable Spreading Factor-OFCDM with Two Dimensional Spreading that Prioritizes Time Domain Spreading for Forward Link Broadband Wireless Access

This paper proposes the optimum design for adaptively controlling the spreading factor in Orthogonal Frequency and Code Division Multiplexing (OFCDM) with two-dimensional spreading according to the cell configuration, channel load, and propagation channel conditions, assuming the adaptive modulation and channel coding (AMC) scheme employing QPSK and 16QAM data modulation. Furthermore, we propose a two-dimensional orthogonal channelization code assignment scheme to achieve skillfully orthogonal multiplexing of multiple physical channels. We first demonstrate the reduction effect of inter-code interference by the proposed two-dimensional orthogonal channelization code assignment. Then, computer simulation results show that in time domain spreading, the optimum spreading factor, except for an extremely high mobility case such as for the fading maximum Doppler frequency of f D = 1500 Hz, becomes SF Time = 16. Furthermore, it should be decreased to SF Time = 8 for such a very fast fading environment using 16QAM data modulation. We also clarify when the channel load is light such as C mux /SF = 0.25 (C mux and SF denote the number of multiplexed codes and total spreading factor, respectively), the required average received signal energy per symbol-to-noise power spectrum density ratio (E s /N 0 ) is reduced as the spreading factor in the frequency domain is increased up to say SF Freq = 32 for QPSK and 16QAM data modulation. When the channel load is close to full such as when C mux /SF = 0.94, the optimum spreading factor in the frequency domain is SF Freq = 1 for 16QAM data modulation and SF Freq = 1 to 8 for QPSK data modulation according to the delay spread. Consequently, by setting several combinations of spreading factors in the time and frequency domains, the near maximum link capacity is achieved both in cellular and hotspot cell configurations assuming various channel conditions.

Comparison of Throughput Employing Hybrid ARQ Packet Combining in Forward Link OFCDM Broadband Packet Wireless Access

This paper compares the throughput performance employing hybrid automatic repeat request (ARQ) packet combining, i.e., Chase combining, and Incremental redundancy, considering the frequency diversity effect in the broadband forward-link channel for Orthogonal Frequency and Code Division Multiplexing (OFCDM) packet wireless access achieving a peak throughput above lOOMbps. Simulation results show that the achievable throughput at the average received signal energy per symbol-to-background noise power spectrum density ratio (E s /N 0 ) of 0 and 6 dB employing Incremental redundancy is increased by approximately 35 and 30% compared to that using Chase combining for QPSK and 16QAM data modulation schemes with the coding rate of R = 1/2, respectively, considering a large frequency diversity effect in a 12-path exponential decayed Rayleigh fading channel, since the reduced variations in the received signal level in a broadband channel bring about a larger coding gain in Incremental redundancy. We also show that when adaptive modulation and channel coding (AMC) is applied, Incremental redundancy is superior to Chase combining since the large coding gain is effective in achieving a large time diversity gain for a low number of retransmissions such as M = 1 or 2 for a maximum Doppler frequency up to f D = 400 Hz. It is demonstrated, nevertheless, that the total throughput when employing Incremental redundancy associated with a near optimum MCS set according to the channel conditions becomes almost identical to that using Chase combining when a large number of retransmissions, M, is allowed, such as M = 10, owing to time diversity alone with frequency diversity.

A More Accurate Analysis of Interference for Rake Combining on DS-CDMA Forward Link in Mobile Radio

In this paper, we improve the performance analysis of the Rake receiver for the DS-CDMA forward link using long random spreading sequences (RSS's) [15] by more accurately evaluating the correlation between the various interference terms. We also extend the analysis to the case of short (periodic) RSS. The accuracy of the expressions obtained in our analysis is verified by computer simulation. We show that for a given normalized spreading factor, the bit error rate (BER) performance of the Rake receiver is the same for BPSK and QPSK data modulation. We also show that when the channel delay spread is smaller than a data symbol duration, the CDMA receiver has similar BER performance for long and short RSS's. However, for large delay spread, the employment of short RSS's may result in severe performance degradation.

COHERENT SS DEMODULATION WITH ESTIMATION AND COMPENSATION OF PHASE ROTATION USING A PILOT CHANNEL

This paper describes and clarifies a configuration and its performance of estimation and compensation of phase rotation in radio propagation using a pilot channel for the SS (spread spectrum) modulation and demodulation. Coherent radio transmission scheme is composed as follows: (i) Continuous pilot signal is transmitted through a pilot channel, (ii) the pilot channel is composed of O-QPSK and QPSK for reverse and forward links, respectively, (iii) pilot signal is added in each I channel of O-QPSK and QPSK, and (iv) coherent demodulation is achieved by estimation and compensation of phase rotation using the pilot channel. Phase rotation and its linearity has been estimated for QPSK and O-QPSK data modulation methods. The constellation on the complex plane is used for the evaluation of phase rotation and the Eye pattern for the demodulated waveforms. The bit-error-rate (BER) versus Eb/N0 is used for the characteristics of data modulation/demodulation and SS modulation/demodulation of the proposed radio system. A proposed scheme proved that (i) estimation of rapid phase rotation caused by multipath radio propagation with high-speed Doppler shift in frequency is well realized by the use of continuous pilot transmission (pilot channel), (ii) compensation of rapid phase rotation is well done by simplified logic operations, and (iii) high performance for high speed data transmission is realized by coherent demodulation. This system will be applied to wireless LANs with high performances and simplified configurations.

A Bayesian network (BN) decoder for direct‐conversion microwave and millimeter‐wave six‐port receivers (SPRs)

AbstractIn this paper, learning algorithms are used to decode QPSK data in a direct‐conversion hardware (QPSK‐unimode) microwave/millimeter‐wave receiver using an application‐specific six‐port module. Two different algorithms, the K‐means and Bayesian network, are considered for operation of the decoder to recover IQ data from QPSK modulated signals. Bit‐error‐rate (BER) results versus noise level (Eb/N0) are presented after a brief data‐collecting period involving fewer than 500 data points. Results of the Bayesian network for Eb/N0=−3 dB and Eb/N0=−9 dB are presented, using the same data‐gathering period. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 33–38, 2001.

Adaptive PTS approach for reduction of peak-to-averagepower ratio of OFDM signal

An adaptive partial transmit sequence (PTS) technique is presented for the reduction of the peak-to-average power ratio (PAP) of an orthogonal frequency division multiplexing (OFDM) signal. The main problem for PTS is how to minimise the number of iterations necessary for locating the optimal weighting factors (which increases exponentially with the number of sub-blocks). In the proposed adaptive PTS approach, the iterations are stopped once the PAP drops below a preset threshold. For an OFDM system with 256 subcarriers and QPSK data symbols, adaptive PTS reduces the 0.1% PAP by 4 dB, while PTS (non-adaptive) reduces it by 4.1 dB. The complexity of adaptive PTS is just 2% of that of PTS.

Coherent Detection of Non-Differentially Phase-Modulated Data Bursts on Rician Fading Channels

This work presents the application of themaximum likelihood block phase estimation concept to thecoherent demodulation of non-differentiallyphase-modulated π/4 QPSK data on Rician fadingchannels typical in land-mobile satellite communication. Aderivation of the maximum likelihood block phaseestimation is given and then applied to designalgorithms for the efficient estimation of carrier phaseand frequency offsets for the coherent demodulation andfrequency tracking purposes in the receiver. The basicapproach taken in the removal of modulation prior tophase estimation is based on the nonlinear operation of phase quadruppling. The resulting phaseambiguities of ±90 or 180° are removedthrough a combination of phase unwrapping and referencesymbols known to the receiver. This approach is seen to result in significant improvement over theuse of differential coding of data used in previousworks.

A wideband DS-CDMA modem for a mobile station

This paper presents the design, implementation and test results of a complete wideband CDMA modem with a modulator and a demodulator for use in next-generation mobile stations. The modem is based on NTT DoCoMo's proposal for the next generation W-CDMA air interface. The modulator supports multicode transmission capability, which enables three dedicated physical channels to be transmitted under a single radio link identification. The receiver consists of a searcher, three fingers and combiner and each finger can demodulate three dedicated physical channels simultaneously. For coherent QPSK data demodulation, a novel phase estimation algorithm has been employed. In this algorithm, a finger selects the minimum value after comparing four phase prediction values to the previous one to compute the correct phase. It has been shown that the proposed algorithm outperforms the WMSA proposed by NTT DoCoMo. The proposed mobile station modem was implemented with FPGAs and verified by the board-level experiment. Experiments have verified that the modem fully complies with the specification and is capable of up to 384 kbps data transmission when using multicode transmission.

120 Mbit/S QPSK data and multi-channel TV transmissionover 13 km fibre to a 60 GHz mobile radio link using an electroabsorption modulator as a transceiver

A broadband analogue fibre-radio millimetre-wave system has been constructed. The system is capable of downstream 120 Mbit/s QPSK data transmission over 13 km of fibre and onward radio transmission at 60 GHz. Simultaneous transmission of downstream data, broadcast video and upstream data over fibre is also incorporated using sub-carrier multiplexing.

Full-duplex, 25 km analogue fibre transmission at120 Mbit/s with simultaneous modulation and detection in an electroabsorptionmodulator

Analogue fibre-optic transmission of full-duplex 120 Mbit/s QPSK data, plus multichannel video, is demonstrated over 25 km using a single electroabsorption modulator as simultaneous optical transmitter and receiver at the remote terminal.

High speed optical mixer for distribution of 16QAMor QPSK data signals over 38 GHz

The authors demonstrate that an optoelectronic millimetre-wave (mm-wave) laser mixer operating at 38 GHz is capable of managing different modulation formats at different rates with the same identical operating conditions. This mixer is a relatively simple and compact optical source which integrates a wideband distributed transimpedance amplifier with a fast MQW VUG DFB laser. The mixer is fed with an electrical power of only 3 dBm at 7.6 or 9.5 GHz to provide the 38 GHz carrier. The device is capable of handling generated harmonics with either 16 QAM at 5 Mbit/s, QPSK at 60 Mbit/s or NRZ at 622 Mbit/s signals over 15 km fibre fed mm-wave radio systems without optical amplification.

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

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.

Adaptive equalization of indoor radio channels for 156-Mb/s, QPSK data transmission

Indoor radio communication in the 20–60 GHz band using TDMA with differentially encoded QPSK is consideed. A burst-type transmission, based on a basic time slot consisting of a preamble for synchronization and equalizer training and of an information data section, is adopted. We employ fractionally spaced decision feedback equalization and give the relevant analytical and simulated performance results in terms of Doppler frequency. An upper limit is determined for the channel variations which can be tracked by the equalizer. Performance results are obtained for equalization with and without carrier phase recovery. Calculated and simulated probability of error show that error propagation degrades the performance by about 6 dB at a bit error rate of 10−3 for a channel without any diversity. However, the effect of past decision errors is negligible for dual diversity. Numerical stability, required accuracy, and hardware complexity are also discussed.

Millimeter-wave fiber optics systems for personal radio communication

System concepts for millimeter-wave personal communication systems and the advantages of millimeter-wave band usage are briefly described. Demonstration of broadband millimeter-wave subcarrier transmission concepts over fiber-optic links is performed. Several fiber-optic link architectures, including one using a combination of direct laser modulation and indirect (external) optical modulation, are outlined with respect to signal transmission at millimeter-wave frequencies. Several configurations are experimentally investigated using 70-MHz, 300-MHz, and 26-GHz subcarriers which transmit either FM or QPSK data signals. Additionally, the use of optical MMIC technology, which can result in the design of compact and cost-effective optical receivers, is described with respect to personal communication radio base station equipment. MMIC HEMTs operating as photodetectors are newly characterized in terms of digital and analog signal reception with excellent performance being observed. >

A generalized least squares fast transversal filter algorithm for the decision feedback equalization of dispersive channels

Recently computationally efficient recursive least squares adaptive filtering algorithms have been introduced in transversal filter form as well as in lattice form. Fast transversal filter implementation of the generalized recursive least squares adaptive algorithm, which is appropriate for multichannel adaptive filtering and estimation, is presented in this paper. The above algorithm is derived in particular for the decision feedback equalization of dispersive channels, when the number of stages in the forward and backward filters are different. The algorithm considered in this paper is advantageous over the generalized least squares lattice algorithm considered by Ling and Proakis, because of the less computational effort required. The convergence rate of the mean square error for the decision feedback equalization of QPSK data transmitted through the dispersive channel using the generalized fast transversal filter algorithm is obtained using computer simulations.

Techniques for simplifying some near-maximum-likelihood detectors

The paper studies some novel techniques for simplifying a near-maximum-likelihood detector that operates on a sequence of received samples, each of which carries two new data symbols. The two data symbols may originate from independent sources, and one of these may be an unwanted interfering symbol. In a received sample, the two new data symbols have been multiplied by arbitrary quantities that may vary independently, but smoothly, from one sample to the next. The new techniques are studied for a particular application of near-maximum-likelihood detection in a land-mobile-radio system, which introduces flat fading into the transmitted QPSK data signals. The receiver here employs an arrangement of combined detection and estimation that has recently been developed for use with the given signals. The arrangement enables two QPSK signals to be transmitted simultaneously over any one channel, in place of the one QPSK signal conventionally used, leading to a more efficient use of bandwidth. The paper shows how substantial simplifications may be introduced into the detection process without, however, significantly changing its resultant operation. The new techniques described here are of quite general application to many near-maximum-likelihood detectors, in applications over flat fading or frequency-selective fading channels, or over time invariant channels that introduce intersymbol interference.

A BPSK/QPSK Timing-Error Detector for Sampled Receivers

A simple algorithm for detection of timing error of a synchronous, band-limited, BPSK or QPSK data stream is proposed. The algorithm requires only two samples per symbol for its operation. One of the two samples is also used for the symbol decision. Derivation of the s curve reveals a sinusoidal shape.

A new approach to multipath correction of constant modulus signals

An adaptive digital filtering algorithm that can compensate for both frequency-selective multipath and interference on constant envelope modulated signals is presented. The method exploits the fact that multipath reception and various interference sources generate incidental amplitude modulation on the received signal. A class of so-called constant modulus performance functions is developed which sense this AM term but are insensitive to the angle modulation. Simple adaptive algorithms for finite-impulse-response (FIR) digital filters are developed which employ a gradient search of the performance function. One of the resulting algorithms is simulated for the example of an FM signal degraded by specular multipath propagation. Substantial improvements in noise power ratio (NPR) are observed (e.g., 25 dB) with moderately rapid convergence time. These results are then extended to include tonal interference on a FM signal and intersymbol interference on a QPSK data signal.