Pilot Frequencies Research Articles

Beam Acquisition and Training in Millimeter Wave Networks With Narrowband Pilots

This paper studies initial beam acquisition in a millimeter wave network consisting of multiple access points (APs) and mobile devices. A training protocol for joint estimation of transmit and receive beams is presented with a general frame structure consisting of an initial access sub-frame followed by data transmission sub-frames. During the initial subframe, APs and mobiles sweep through a set of beams and determine the best transmit and receive beams via a handshake. All pilot signals are narrowband (tones), and the mobiles are distinguished by their assigned pilot frequencies. Both non-coherent and coherent beam estimation methods based on, respectively, power detection and maximum likelihood (ML) are presented. To avoid exchanging information about beamforming vectors between APs and mobiles, a local maximum likelihood (LML) algorithm is also presented. An efficient fast Fourier transform implementation is proposed for ML and LML to achieve high-resolution. A system-level optimization is performed in which the frame length, training time, and training bandwidth are selected to maximize a rate objective taking into account blockage and mobility. Simulation results based on a realistic network topology are presented to compare the performance of different estimation methods and training codebooks, and demonstrate the effectiveness of the proposed protocol.

On improved DFT-based low-complexity channel estimation algorithms for LTE-based uplink NB-IoT systems

Channel estimation is crucial to achieving wide-area coverage for ultra-low-cost and low-power narrowband Internet of Things (NB-IoT) devices that are in coverage extremities. Radio coverage can be extended by repeatedly transmitting the same signal over a protracted period. In repetition dominated NB-IoT systems, existing channel estimators extensively used in the orthogonal frequency-division multiplexing (OFDM) system may be no longer applicable due to their considerable computational complexity and power consumption. In this paper, we propose narrowband demodulation reference signal (NDMRS)-assisted transform-domain low-complexity channel estimation algorithms named random sorting least squares (RS-LS), and de-noising LS (D-LS). Another sub-optimal estimator, stemming from the filtered channel estimates called linear minimum mean square error-approximation (LMMSE-A) is also studied. We first estimate initial channel response at pilot frequencies using the conventional LS method; and then, apply several additional operations in time-domain to suppress LS estimation error without exploiting extra frequency-band resources, and increasing significant computational complexity. Finally, channel estimates for the remaining OFDM symbols within an NB-IoT subframe are obtained by employing the time dimensional linear interpolation. Through several simulation examples, the viability of the proposed estimators is verified in comparison with the conventional LS, denoise, and optimal LMMSE estimators in terms of channel mean square error (MSE), block error rate (BLER), and throughput against signal-to-noise ratio (SNR) for Long Term Evolution (LTE)-based uplink NB-IoT systems.

Photonic Microwave Frequency Measurement Based on Frequency-Configurable Pilot Tones

A photonic technique for microwave frequency measurement with flexible range and high resolution is proposed based on frequency-configurable pilot tones. The method extracts the remote unknown microwave frequency by measuring electrical power at two known pilot frequencies, avoiding dual-channel photodetection and broadband microwave power measurement over the whole interested frequency range. Moreover, it enables flexible measurement range by configuring two pilot frequencies. A proof-of-concept experiment is successfully demonstrated with the frequency range of 6–18.9 GHz and the resolution of 0.1 GHz.

A Pioneering Perusal of Mimo- OFDM Realy Guesstimate Disposition

A multiple-input multiple-output (MIMO) communication system combined with the orthogonal frequency partition multiplexing (OFDM) modulation technique can achieve reliable high data rate show over broadband wireless channels. Channel state information for both single-input single-output (SISO) and MIMO systems based on pilot aided arrangement is investigated in this paper. The estimate of channel at pilot frequencies with conventional Least Square (LS) and Minimum Mean Square (MMSE) estimation algorithms is carried out through Matlab simulation. For outdoor communicationscenarios, where wireless channels are sparse in nature, pathinterruptions of different transmit-receive antenna couples share a jointsparse decoration due to the spatial correlation of MIMO channels.By simultaneously exploiting those MIMOchannel characteristics, the proposed scheme performs better thanpresent state-of-the-art systems. Furthermore, by joint processingof signals associated with different antennas, the pilot overheadcan be reduced the performance of MIMO OFDM and SISO OFDM are assessed on the basis of Bit Error Rate (BER) and Mean Square Error (MSE) level. Further enhancement of presentation can be attained through maximum diversity Space Time Block Coding and Maximum Likelihood Detection at transmission and reception ends respectively. MMSE estimation has been shown to perform much better than LS but is more complex than LS for the MIMO system using pilot carriers.

Spectrum Efficiency Evaluation with Multiuser Scheduling and MRC Antenna Diversity with Effects of Combining Errors for Rayleigh Fading Channels with Feedback

In this paper, we present an analytical framework to jointly investigate MRC diversity and multiuser scheduling under Rayleigh fading channels with the effect of combining errors under a low SNR regime. We derive spectrum efficiency expressions for multiuser scheduling systems with two different cases of multiuser scheduling, (1) Full feedback user scheduling (2) Limited feedback user scheduling with effects of combining errors. A signal to noise ratio (SNR) based user selection scheme is considered. For combining errors, correlation is defined as the magnitude of the complex cross-correlation between the transmission coefficients of the medium associated with pilot and message frequencies. We derive the probability density function and cumulative density function for the above cases. Adaptation policies like (1) Optimal power and rate adaptation policy (OPRA), (2) Optimal rate adaptation policy, (3) Channel inversion with fixed rate policy, (4) Truncated channel inversion with fixed rate policy are employed to improve system performance. It is observed that limited feedback user scheduling along with employment of adaptation policies yields better system performance compared to full feedback user scheduling. Further, OPRA policy yields maximum capacity among the four policies.

Complex Amplitudes Tracking Loop for multipath channel estimation in OFDM systems under slow to moderate fading

This paper deals with multipath channel estimation for Orthogonal Frequency-Division Multiplexing systems under slow to moderate fading conditions. Most of the conventional methods exploit only the frequency-domain correlation by estimating the channel at pilot frequencies, and then interpolating the channel frequency response. More advanced algorithms exploit in addition the time-domain correlation, by employing Kalman filters based on the approximation of the time-varying channel. Adopting a parametric approach and assuming a primary acquisition of the path delays, channel estimators have to track the complex amplitudes of the paths. In this perspective, we propose a less complex algorithm than the Kalman methods, inspired by second-order Phase-Locked Loops. An error signal is created from the pilot-aided Least-Squares estimates of the complex amplitudes, and is integrated by the loop to carry out the final estimates. We derive closed-form expressions of the mean squared error of the algorithm and of the optimal loop coefficients versus the channel state, assuming a Rayleigh channel with Jakes' Doppler spectrum. The efficiency of our reduced complexity algorithm is demonstrated, with an asymptotic mean squared error lower than the first-order auto-regressive Kalman filters reported in the literature, and almost the same as a second-order Kalman-based algorithm.

Block-Type Pilot Arrangement with Alternating Polarity for ICI Mitigation in Mobile OFDM Systems

Improvement on Inter Carrier Interference (ICI)mitigation techniques for OFDM caused by Doppler effectsthrough minimizing channel estimation error and decreasingchannel time varying rate is investigated. The performanceof pilot-aided channel estimation techniques depends on pilot placement and arrangement and also on the channel time varying rate. The block-type and comb-type pilot arrangements are studied through different numbers of guard bands, with or without the involvement of the Doppler shift compensation. The estimation of channel at mid-point of each OFDM symbol is derived from pilot frequencies based on the least square algorithm while the channel interpolation is done using piecewise linear approximation. For ICI mitigation technique we implement frequency domain zero forcing equalizer. We compare the performance of schemes with different pilot arrangementsand Doppler shift compensations by measuring bit error rate with QPSK as sub-channel modulation scheme and with mobileto-fixed of single ring scattering as channel model. The results are in favour of block-type pilot arrangements with alternating polarity and Doppler compensation of 0:55 times the maximum Doppler shift.

Channels estimation in OFDM system over Rician fading channel based on comb-type pilots arrangement

Orthogonal frequency-division multiplexing (OFDM) is a transmission technique that is based on many orthogonal carriers that are transmitted simultaneously. Channel estimation techniques for OFDM systems, based on comb-type pilot arrangement, over frequency-selective Rician and time-varying fading channel are investigated. The advantage of comb-type pilot arrangement, in channel estimation, is the ability to track the variation in the channel, which is the main reason for inter-carrier interference modelled as an additive white Gaussian noise, leading to an increase in the noise level. The estimation of the channel at the pilot frequencies is based on least-square (LS) method. Several interpolation methods have been used to estimate the channel response at the data frequency. In this study, simulation results for Rician channel model were examined and bit error rate/signal-to-noise ratio performance, for various conditions, were considered, and the pilot arrangement was used with the LS estimation and low-pass interpolation techniques, leading to a reduction in the effect of Doppler frequency.

MIMO-OFDM Channel Estimation Using Pilot Carries

A multiple-input multiple-output (MIMO) communication system combined with the orthogonal frequency division multiplexing (OFDM) modulation technique can achieve reliable high data rate transmission over broadband wireless channels. Channel state information for both single-input single-output (SISO) and MIMO systems based on pilot aided arrangement is investigated in this paper. The estimation of channel at pilot frequencies with conventional Least Square (LS) and Minimum Mean Square (MMSE) estimation algorithms is carried out through Matlab simulation. The performance of MIMO OFDM and SISO OFDM are evaluated on the basis of Bit Error Rate (BER) and Mean Square Error (MSE) level. Further enhancement of performance can be achieved through maximum diversity Space Time Block Coding (STBC) and Maximum Likelihood Detection at transmission and reception ends respectively. MMSE estimation has been shown to perform much better than LS but is more complex than LS for the MIMO system using pilot carriers.

Channel estimation techniques based on pilot arrangement in OFDM systems

Channel estimation techniques for OFDM systems based on a pilot arrangement are investigated. Channel estimation based on a comb type pilot arrangement is studied through different algorithms for both estimating the channel at pilot frequencies and interpolating the channel. Channel estimation at pilot frequencies is based on LS and LMS methods while channel interpolation is done using linear interpolation, second order interpolation, low-pass interpolation, spline cubic interpolation, and time domain interpolation. Time-domain interpolation is obtained by passing to the time domain by means of IDFT (inverse discrete Fourier transform), zero padding and going back to the frequency domain by DFT (discrete Fourier transform). In addition, channel estimation based on a block type pilot arrangement is performed by sending pilots in every sub-channel and using this estimation for a specific number of following symbols. We have also implemented a decision feedback equalizer for all sub-channels followed by periodic block-type pilots. We have compared the performances of all schemes by measuring bit error rates with 16QAM, QPSK, DQPSK and BPSK as modulation schemes, and multipath Rayleigh fading and AR based fading channels as channel models.

Pilot spacing in orthogonal frequency division multiplexing systems on practical channels

The performance of two different channel interpolation methods to be used with orthogonal frequency division multiplexing systems are investigated. The considered schemes use constant pilot frequencies for channel response estimation. The interpolation techniques are piecewise-constant and piecewise-linear methods which due to their inherent simplicity are straightforward to implement. The results are given as error probability vs. pilot separation for a channel with exponential type power-delay profile and M-ary quadrature amplitude (MQAM) submodulation with M=4, 16 and 64.

Frequency and time in communications

Modern communications systems make use of sophisticated multiplexing techniques in order to achieve efficient spectrum utilization and thereby realize the maximum number of message channels through a particular transmission medium. Synchronization of transmit/receive carrier (or clock) generators is essential in such systems to avoid distortion; also, close tolerances on the accuracy of carrier and pilot frequencies must be imposed to maintain transmission through minimal bandwidth channel selection filters. Typical requirements on both synchronization and accuracy for several categories of commercial communications systems range from several parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> to a few parts in 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> .