Minimization of detection of symbol-rate spectral lines by delayand multiply receivers

The detectability of the symbol rate of a digitally modulated signal by a delay and multiply receiver is investigated. Two signal pulse types are considered: Nyquist pulses and rectangular pulses. Since many digital transmission systems use rectangular pulse shapes, symbol rate-line detectability is considered for receivers optimized for rectangular pulses. For a receiver configuration that is reasonable for rectangular pulses, it is found that with a Nyquist pulse parameter α=0.25, the rate-line signal-to-noise ratio is down about 20 dB from that for a rectangular pulse signal     

FFT-based interpolation for multipath detection in GPS/GLONASS receivers

A zero-stuffing interpolation technique using fast Fourier transforms is proposed for excellent resampling of correlation-pulse signals, departing from a small number of Nyquist samples. Potentially, the method offers, at the earliest possible stage, detection (integrity control) and eventually correction of time-dispersion effects by multipath in navigational receivers.<>     

The effects of multipath delay spread on timing recovery

Equations for the recovered timing for a squaring timing recovery circuit under multipath radio propagation are derived. Both coherent and differential detections are studied. If delay spread is much smaller than the symbol duration, the recovered timing can be approximated by the centroid of the power delay profile, p(t). Two cases of timing loop bandwidth are considered. If the fading frequency is much lower than the bandwidth of the timing loop, the instantaneous sample of p(t) is used to generate the timing clock. If the fading frequency is much higher than the loop bandwidth, the ensemble average of p(t) over fading samples is used to recover the timing. A computer simulation is performed for a system operating in a frequency-selective, slowly fading environment. It is found that for root mean square (rms) delay spread less than or equal to 0.1 of the symbol duration, a squaring timing loop with either narrow or wide bandwidth can properly determine the timing detection. The main mechanism of the "irreducible bit error rate" in this case is the closure of the eye-pattern instead of timing error.     

多径对雷达探测性能的影响分析

分析了自由空间和多径2种环境下雷达探测概率与信噪比的关系,雷达探测范围以及不同反射系数和不同载频下的多径传输因子。研究结果表明,反射系数越小,探测概率曲线越接近自由空间下的变化曲线。信噪比较大时,多径衰落造成其检测性能逐渐下降。当性噪比较低时,为得到相同的检测概率,多径所需要的信噪比比自由空间下约小7 dB~8 dB。多径传输因子随着距离起伏较大,反射因子越大,多径传输因子起伏越大,其相应的暴露区裂化程度越严重。     

BER for optical heterodyne DPSK receivers using delay demodulationand integration detection

Previous work on the bit error rate (BER) performance of differential phase-shift keying (DPSK) including the effects of additive white Gaussian noise (AWGN) and phase noise has concentrated on the delay demodulator with narrow-band intermediate frequency (IF) bandpass filtering (BPF) and sampling detection. No similar analysis has yet been performed for the delay demodulator with wideband IF bandpass filtering and integration detection. Phase noise is an important consideration in coherent optical communication systems and the most widely accepted model is a Brownian motion process. A closed-form BER expression along with detailed Monte Carlo simulation results are presented for the DPSK delay demodulator with wideband IF bandpass filtering and integration detection filtering including phase noise effects using the Brownian motion model. Analytic expressions are also obtained for the moments of the phase noise component of the decision variable. Using these moments, estimates of the phase noise BER floor are produced. It is found that this receiver has noise performance comparable to receivers with narrowband IF bandpass filtering and sampling detectors for signal-to-noise ratio (SNR) and phase noise in the range of practical interest, but with potentially less degradation due to intersymbol interference (ISI)     

Superresolution of multipath delay profiles measured by PNcorrelation method

Time resolution of multipath delay profiles measured by using the autocorrelation of a pseudonoise (PN) code sequence is generally limited by the chip interval of the PN code sequence. A superresolution PN correlation method (SPM) is proposed which improves the time resolution of delay profiles measured by the conventional PN correlation method. The SPM is based on a decomposition of the eigenvector space of the correlation matrix of the delay profile data vector and gives the number of paths and their delay times with higher resolution. It is verified by computer simulations and experiments using coaxial delay lines that the SPM can resolve two paths with a delay difference of a few tenths of the chip interval. The applicability of the SPM to the analysis of an indoor multipath environment in which many delayed waves arrive with short delay differences is demonstrated by an indoor radio propagation experiment at 2.3 GHz     

Pilot-based estimation of time-varying multipath channels forcoherent CDMA receivers

Reliable coherent wireless communication requires accurate estimation of the time-varying multipath channel. This paper addresses two issues in the context of direct-sequence code-division multiple access (CDMA) systems: (i) linear minimum-mean-squared-error (MMSE) channel estimation based on a pilot transmission and (ii) impact of channel estimation errors on coherent receiver performance. A simple characterization of the MMSE estimator in terms of a bank of filters is derived. A key channel characteristic controlling system performance is the normalized coherence time, which is approximately the number of symbols over which the channel remains strongly correlated. It is shown that the estimator performance is characterized by an effective signal-to-noise ratio (SNR)-the product of the pilot SNR and the normalized coherence time. A simple uniform averaging estimator is also proposed that is easy to implement and delivers near-optimal performance if properly designed. The receivers analyzed in this paper are based on a time-frequency RAKE structure that exploits joint multipath-Doppler diversity. It is shown that the overall receiver performance is controlled by two competing effects: shorter coherence times lead to degraded channel estimation but improved inherent receiver performance due to Doppler diversity, with opposite effects for longer coherence times. Our results demonstrate that exploiting Doppler diversity can significantly mitigate the error probability floors that plague conventional CDMA receivers under fast fading due to errors in channel estimation     

The effect of multipath interference on the performance of packetradios

The effect of multipath interference on the performance of packet radios is investigated. The protocols considered include pure ALOHA and slotted ALOHA. Signal reception with and without capture effect is considered in the analysis. Mathematical expressions of system throughput and packet delay have been successfully obtained for each protocol. Numerical calculations are used to demonstrate when the multipath interference can be ignored and when it can be disastrous     

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.     

Joint application of STBC and RAKE receivers in multipath Rayleigh fading channels

This letter investigates the issues on joint application of STBC systems and RAKE receivers in a CDMA system. We show the advantages of using STBC-RAKE receivers in multipath Rayleigh fading channels via error probability analysis and extensive computer simulations. Specifically, this study leads to a better understanding of the interplay between spatial diversity by STBC and multipath diversity via RAKE receivers.     

The effect of multipath interference on the performance of random accessing

This paper studies the effect of multipath interference on the performance of random access techniques in which the power level of a packet depends on its time of arrival. The protocols considered are pure ALOHA and slotted ALOHA. Key performance measures such as channel throughout and mean packet delay are obtained. These results are derived partially using minislot approximation and are verified via computer simulations. For practical purposes in evaluating the successful packet reception probability the signal capture effect is also taken into consideration. How seriously the multipath interference affects the two protocols are witnessed through numerical examples. Copyright © 1999 John Wiley & Sons, Ltd.     

Performance evaluation of DS-CDMA systems on multipath propagationchannels with multilevel spreading sequences and RAKE receivers

Performance of small-cell wireless direct sequence code-division multiple access (DS-CDMA) systems with multilevel spreading sequences is investigated. An analytical methodology to evaluate both outage probability and mean bit error probability is presented; and its results are compared to those obtained with a semianalytical Monte Carlo based approach. Multipath propagation, the exact correlation properties of spreading codes, and both the simple correlator and RAKE receivers are taken into account in the analysis, which is oriented to asynchronous and synchronous environments. The spreading sequences investigated include multilevel sequences with complex isomorphic mapping, as well as some well-known two-level sequences. The numerical results show the impact of multilevel sequences on system performance, the improvements found in the absence of multiple access interference by using them rather than binary sequences, and the agreement between analytical and semianalytical evaluation     

Reduce delay of multipath TCP in IoT networks

Wireless Networks - Multi-homed devices such as smartphones, tablets and laptops are enabled with multiple heterogeneous interfaces available for transmission. Those interfaces can be utilized for...     

Super-resolution of pulsed multipath channels for delay spreadcharacterization

Frequency selective channels can be measured by either a continuous wave frequency sweep, which gives directly the transfer function, or by sounding the channel with pulses, which results in the impulse response. When the sounding pulse becomes a chirp-type waveform, the two approaches become similar. Simple rectangular pulses can be used for partial, but usually sufficient, channel characterization. In this paper, the process is described for resolving impulse response components into bins smaller than the duration of the sounding pulse and smaller than the reciprocal of the channel bandwidth. From such “super-resolution”, the delay spread of the propagation channel can be established accurately even when it is much less than the sounding pulse duration, allowing longer, higher energy pulses to be used for channel characterization. The process is demonstrated using subtractive deconvolution where a loop gain of unity is shown to be stable; and a modified inverse filter technique, in which the modification caters for the spectral zeros of the sounding pulse     

A comparison of GSM receivers for fading multipath channels withadjacent and co-channel interference

We evaluate and compare several data detection schemes used in GSM systems. In particular, we compare the performance of decision feedback equalization (DFE) and nonlinear data directed estimation (NDDE) to that of maximum likelihood sequence estimation (MLSE). Establishing the performance of the basic NDDE detector is a first step in investigating the applicability of block transmission techniques to GSM systems. Our simulation results, obtained both for fading multipath channels and adjacent- and co-channel interference scenarios, suggest that the NDDE offers certain performance advantages over the DFE, and the performance of both detectors is comparable to that of the MLSE for the SNR region of interest in practical systems. Thus, they both represent viable alternatives to the MLSE detector     

The design and multiplier-less realization of software radio receivers with reduced system delay

This work studies the design and multiplier-less realization of a new software radio receiver (SRR) with reduced system delay. It employs low-delay finite-impulse response (FIR) and digital allpass filters to effectively reduce the system delay of the multistage decimators in SRRs. The optimal least-square and minimax designs of these low-delay FIR and allpass-based filters are formulated as a semi-definite programming (SDP) problem, which allows zero magnitude constraint at /spl omega/=/spl pi/ to be incorporated readily as additional linear matrix inequalities (LMIs). By implementing the sampling rate converter (SRC) using a variable digital filter (VDF) immediately after the integer decimators, the needs for an expensive programmable FIR filter in the traditional SRR is avoided. A new method for the optimal minimax design of this VDF-based SRC using SDP is also proposed and compared with traditional weight least squares method. Other implementation issues including the multiplier-less and digital signal processor (DSP) realizations of the SRR and the generation of the clock signal in the SRC are also studied. Design results show that the system delay and implementation complexities (especially in terms of high-speed variable multipliers) of the proposed architecture are considerably reduced as compared with conventional approaches.     

Effects of multipath transmission on the measured propagation delay of an FM signal

Position location techniques based on propagation delay have been proposed previously. A narrow-band version of this technique involves the transmission of RF carrier, modulated by a single audio frequency. At a receiver, the audio phase is a measure of distance provided the propagation delay is less than one quarter cycle. The transmission medium introduces multipath distortion and the received signal consists of a set of signals, each an attenuated and delayed replica of the transmitted signal, having traversed a different path. When FM is employed, the phase of the demodulated composite is a nonlinear function of the parameters of the multipath structure of the channel. In this paper, this functional relationship is derived.     

Joint estimation of PN code delay and multipath using the extendedKalman filter

The problem of delay estimation in the presence of multipath is considered. It is shown that the extended Kalman filter (EKF) can be used to obtain joint estimates of time-of-arrival and multipath coefficients for deterministic signals when the channel can be modeled as a tapped-delay line. Simulation results are presented for the EKF joint estimator used for synchronization in a direct-sequence spread-spectrum system operating over a frequency-selective fading channel. A simplified model of the EKF joint estimator is considered for analysis purposes. The evolution in time of the tracking error probability density function and the nonlinear tracking error variance are examined through numerical solution of the Chapman-Kolmogorov equation. The nonlinear tracking error variance is compared to both the linear error variance estimate directly provided by the EKF and the Cramer-Rao lower bound     

The effect of mixer mount loss on the performance of SIS receivers

Tucker's quantum theory of mixing (in the 3-port approximation) is employed with Eisenhart and Khan's equivalent circuit for a junction mounted in waveguide to predict the gain of an SIS mixer as a function of guide impedance, series inductance, junction capacitance, IF load impedance and backshort loos. The improvements which will result from optimisation of these parameters are quantified. It is shown that for optimum performance a backshort VSWR>100 is required, which is hard to realise at high frequencies     

Comparison of EML and DOT discriminator DLL multipath performancein GPS/GLONASS navigation receivers

The impact of multipath signals on the code phase tracking in digital GPS/GLONASS navigation receivers is analysed. As an interesting result it is for example, shown that for moderate correlator spacings a DOT discriminator DLL offers a larger potential of multipath error reduction by code phase smoothing than an EML discriminator DLL due to significantly smaller mean code phase tracking errors