Small Delay Spread Research Articles

Communication Rate Increase in Drill Strings of Oil and Gas Wells Using Multiple Actuators.

Wireless data communication and telemetry during drilling deep oil and gas wells are important enablers for safe and timely drilling operations. The transmission of information through drill strings and pipes using sound waves is a useful and practical approach. However, given the limited available bandwidth, transmission rates are typically smaller than what is needed. In this paper, a new method and system are proposed to increase the transmission rate over the same bandwidth, by deploying more than one actuator. Upon using multiple actuators, several data streams can be transmitted simultaneously. This increases the data rate without the need for additional bandwidth. The experimental results of a testbed with two actuators are presented, where the transmission rate is doubled with no bandwidth increase. A strain sensor receiver and accelerometer receivers are used to separate and demodulate the two data streams. It is demonstrated that it is possible to recover the data in the new faster system benefiting from two actuators, while having about the same bit error probability performance as a one-actuator system. Various combinations of strain and acceleration sensors are considered at the receive side. Due to some properties of strain channels (e.g., smaller delay spreads and their less-frequency-selective behavior) presented in this paper, it appears that a strain sensor receiver and an accelerometer receiver together can offer a good performance when separating and demodulating the two actuators’ data in the testbed. Overall, the experimental results from the proposed system suggest that upon using more than one actuator, it is feasible to increase the data rate over the limited bandwidth of pipes and drill strings.

Sum Rate Maximization With Shortened Cyclic Prefix in a MIMO-OFDM System

This correspondence paper addresses linear precoding design for sum rate maximization in low-latency orthogonal frequency-division multiplexing systems. In order to mitigate the overhead of cyclic prefix (CP) originating from shortened symbol duration for low-latency communications, 5G wireless systems need to adopt short CP lengths. As channel delay spread must be less than the CP length, we first derive the effective root-mean-square (RMS) delay spread and the achievable rate using the zero-forcing assumption. We construct a sum rate optimization problem for each user subject to delay spread constraints and then convert the problem into a solvable convex problem along with a semi-definite relaxation technique. The precoding matrix is finally obtained by solving optimization problems for all users. Numerical results reveal that the proposed scheme attains superior performance to the conventional sum rate optimization, as well as small RMS delay spread.

Propagation Path Loss Modeling and Outdoor Coverage Measurements Review in Millimeter Wave Bands for 5G Cellular Communications

The global bandwidth inadequacy facing wireless carriers has motivated the exploration of the underutilized millimeter wave (mm-wave) frequency spectrum for future broadband cellular communication networks, and mmWave band is one of the promising candidates due to wide spectrum. This paper presents propagation path loss and outdoor coverage and link budget measurements for frequencies above 6 GHz (mm-wave bands) using directional horn antennas at the transmitter and omnidirectional antennas at the receiver. This work presents measurements showing the propagation time delay spread and path loss as a function of separation distance for different frequencies and antenna pointing angles for many types of real-world environments. The data presented here show that at 28 GHz, 38 GHz and 60 GHz, unobstructed Line of Site (LOS) channels obey free space propagation path loss while non-LOS (NLOS) channels have large multipath delay spreads and can utilize many different pointing angles to provide propagation links. At 60 GHz, there is more path loss and smaller delay spreads. Power delay profiles PDPs were measured at every individual pointing angle for each TX and RX location, and integrating each of the PDPs to obtain received power as a function of pointing angle. The result shows that the mean RMS delay spread varies between 7.2 ns and 74.4 ns for 60 GHz and 28 GHz respectively in NLOS scenario.

Design and performance evaluation of a covert timing channel

In a model-based covert timing channel (CTC), the (covert) sender modulates the inter-packet delays (IPDs) of the packet stream generated by the overt application (source) by following a well-known statistical model of the application traffic. Implementing a CTC system that operates on real application traffic such as Skype requires addressing several challenges. First, packets generated by Skype must meet a maximum end-to-end delay requirement, which imposes limits on how long a packet can be buffered by the sender. Second, buffer overrun and underrun may occur because of transient mismatches between the rate at which packets are generated by the source and the rate at which the sender can service the covert buffer. As a single IPD of Skype traffic is small and has a small delay spread, we propose to use delay of multiple IPDs to modulate the encoded symbols. To minimize buffer overruns and underruns, we partition the delay of multiple IPDs so that each encoded symbol can be mapped to multiple partitions of the delay distribution. We then provide a mathematical model to choose the appropriate partition for a given encoded symbol based on the state of the buffer. We evaluate the performance of a users-space implementation of the CTC system in real network settings for Skype traffic. We show that the covert channel based on the proposed design can be established even when the source is connected to a public WiFi network and it is largely non-detectable under well-known statistical tests including the entropy test, the Kolmogorov–Smirnov test, and the Kullback–Leibler divergence test. Copyright © 2015 John Wiley & Sons, Ltd.

Semiblind Channel Estimation for IFDMA in Case of Channels with Large Delay Spreads

Discrete Fourier Transform- (DFT-) precoded Orthogonal Frequency Division Multiple Access (OFDMA) with interleaved subcarrier allocation per user is considered which is denoted as Interleaved Frequency Division Multiple Access (IFDMA). For IFDMA, the received signal exhibits a cyclostationarity with the parameter which defines the number of allocated subcarriers per user. In previous works, we have shown that this cyclostationarity can be exploited for the application of subspace-based semiblind channel estimation to IFDMA. Nevertheless, the formerly proposed algorithm is restricted to channels with small delay spreads, meaning that the number of channel delay taps needs to be smaller than . In this paper, we introduce a new semiblind subspace-based channel estimation algorithm which identifies a subspace of the received signal that is spanned by the elements of the so-called cyclic channel impulse response. By doing so, the number of elements to be estimated is reduced and the proposed algorithm is able to cope with channels with large delay spreads.

Statistical Prefiltering for OFDM Systems Using Multiple Transmit Antennas

The incorporation of the orthogonal frequency-(multiplexing (OFDM) with the space-time or space-frequency coding is a robust and effective method to achieve a transmit-diversity gain and to suppress an intersymbol interference for the broadband wireless transmissions. This paper is concerned with the downlink-performance improvement using multiple transmit antennas for the OFDM systems with a large delay spread. A delay-spread reduction method using space-frequency block coding and statistical prefiltering (SPF) was proposed, which combines signal prealignment and multiple transmit beamformers designed with the statistical knowledge of the downlink channel state information. The proposed method, which is called the SPF-OFDM, transforms a large delay-spread channel into multiple channels, each with small delay spread. Thus, it does not only shorten the effective excess delay but also preserves the path diversity through the use of the space-frequency coding. Computer simulations have evaluated the effectiveness of the proposed scheme, and comparisons have been made with the conventional solutions

A reduced complexity channel estimation for OFDM systems with transmit diversity in mobile wireless channels

A reduced complexity channel estimation for OFDM systems with transmit diversity is proposed by exploiting the correlation of the adjacent subchannel responses. The sizes of the matrix inverse and the FFTs required in the channel estimation at every OFDM data symbol are reduced by half of the existing method for OFDM systems with nonconstant modulus subcarrier symbols or constant modulus subcarrier symbols with some guard tones. The complexity reduction of half FFTs size and some matrix multiplications is still achieved for constant modulus subcarrier symbols with no guard tones. The price for the complexity reduction is a slight BER degradation and for the channels with small relative delay spreads, the BER performance of the reduced complexity method becomes quite comparable to the existing method. An alternative approach for the number of significant taps required in the channel estimation is described which achieves a comparable performance to the case with the known suitable number of significant taps. A simple modification which reduces the lost leakage of the nonsample-spaced channel paths is also proposed. This modification achieves a substantial performance improvement over the existing method without any added complexity.

Identifiability results for blind beamforming in incoherent multipath with small delay spread

Several explicit identifiability results are derived for deterministic blind beamforming in incoherent multipath with small delay spread. For example, it is shown that if the sum of spatial and fractional sampling diversities exceeds two times the total number of paths, then identifiability can be guaranteed even for one symbol snapshot. The tools come from the theory of low-rank three-way array decomposition (commonly referred to as parallel factor analysis (PARAFAC) and data smoothing in one and two dimensions. New results regarding the Kruskal-rank of certain structured matrices are also included, and they are of interest in their own right.

Reduction of the error floor of MSK by selection diversity

We examine the error floor of minimum shift keying (MSK) when the receiver uses selection diversity. Both received signal strength indication (RSSI)-driven diversity and bit error rate (BER)-driven diversity are analyzed. The considered channel is a two-delay Rayleigh-fading channel; detection is done by differential detectors with either fixed or adaptive determination of the sampling time. Analysis is based on the method of error regions, where the instantaneous impulse response is represented by phasors in the complex plane. We first compute the joint probability density function (pdf) of the phasors of the two diversity branches. Depending on the selection criterion, the error probability is then computed as an integral over certain functions of this pdf. In the limit of small delay spreads S, the integrals are evaluated analytically. For fixed sampling, the admissible delay spread for BER=10/sup -3/ is increased by about a factor of three (as compared to the no-diversity case) for RSSI-driven diversity and a factor of four for BER-driven diversity. For adaptive sampling, RSSI-driven diversity gives little improvement in the admissible S for BER=10/sup -3/ while BER-driven diversity increases it by some 50%. Results are confirmed by comparison with Monte Carlo simulations and measurements.

Equalization of linearly frequency-selective fading channels

A simple technique for removing intersymbol interference (ISI) introduced by linearly frequency-selective fading channels is presented. The technique involves the optimization of the overall impulse response of the transmit and receive filters and effectively reduces the channel to one which is flat fading. Computer simulation results show that this equalization method works for channels with small delay spreads.

Error performance of differentially-detected MSK in small delay-spread Rayleigh channel

A first order approximation is introduced for the irreducible intersymbol-interference error probability of differentially detected MSK in a Rayleigh fading channel with small delay spread. The error probability is derived from the PDF of the group delay and depends only on the normalised delay spread. DECT-like systems demand receiver synchronisation on the instantaneous mean excess delay.

Measured performance of DECT transmission in low dispersive indoor radio channel

The effect of indoor radio channels with small delay spreads on the performance of a DECT-like radio link were measured and compared to previous analytical and simulation results. It was found that delay spreads less than 4.5% of the bit length without antenna diversity, and less than 20% of the bit length with instantaneous (slot-by-slot) error-rate driven antenna diversity are tolerable to obtain an average bit error rate below 10−3.

Error Distribution and Diversity Performance of a Frequency-Differential PSK HF Modem

A 4800-bit/s digital data modem is operated over a simulated HF channel to determine the bit-error rate and error distributions of the received serial binary data stream. The measured error distributions for multipath-limited conditions are compared with back-to-back operation and theoretically determined random distributions, and are also used to evaluate forward acting error correction assuming half-rate random error-correcting block codes. This improvement in bit-error rate is compared with that measured by operating the modem at 2400 bit/s with dual in-band frequency diversity. The error distributions for back-to-back operation approach random except for small block lengths where the systematic error resulting from the frequency-differential PSK coding appears. The error distributions for multipath-limited operation deviate from random especially for small multipath delay spreads (low biterror rates) where the correlation bandwidth is large. Forward error correction produces larger improvement factors as the code block length is increased. However, the improvement factor barely exceeds two orders of magnitude for the largest block considered. In fact, the burst nature of the errors is such that improvement factors actually decrease in most cases for multipath-limited operation as the raw bit-error rate decreases. On the other hand, inband diversity is more effective for small multipath delay spreads than for large ones. However, forward error correction appears to offer more potential especially if the bit errors can be spread over a greater number of bits.