ITU Channel Research Articles

Performance evaluation of high mobility OFDM channel estimation techniques

In wireless communication, Orthogonal Frequency Division Multiplexing (OFDM) has been adopted due to its robustness to multipath fading and high data rate transmissions. At the other hand, the performance of OFDM systems severely degraded due to multi-path fading and Doppler frequency shifts in mobile systems, which causes inter-carrier-interference (ICI). Thus, Estimation of channel parameters is required at the receiver using a pre designed estimator where pilot tones are inserted in each OFDM symbol. In this paper, a random pilot data are generated and inserted in each OFDM symbol at equally spaced locations. The performance test of Least Square (LS) and Linear Minimum Mean Square (LMMSE) estimation methods are proposed with Discrete Fourier Transform (DFT) based on both LS and LMMSE, where different ITU channel models are considered in order to compare their performance for data transmission in high mobile systems with different Doppler frequencies exceeds 200 Hz and minimal number of pilots.

Environment-Aware Drone-Base-Station Placements in Modern Metropolitans

Unmanned aerial vehicles, i.e., drones, have recently caught attention for providing on-demand capacity to wireless networks as drone-base-stations (drone-BSs). Many studies assume simplified channel models based on average characteristics of the environment to estimate the placement of drone-BSs. However, especially in urban areas, positioning of drone-BSs with respect to intersections and roof-top heights of buildings can severely change the path loss characteristics. To address this issue, we adopt an ITU channel model utilizing more information about the environment, such as the shapes of the buildings. We optimize parameters of the selected ITU model, so that it can be used for altitudes both strictly lower and higher than building roof-tops. Using ray-tracing simulations as a benchmark, we compare the proposed model with a widely used simpler model. Results show that the proposed model can reduce the root-mean-squared error from 35 to 10 dB, which may have critical implications for drone-BS operations, such as planning for the required number of drone-BSs to cover outdoor urban users, as demonstrated with simulations.

A novel QoS aware medium access control scheduler for LTE-advanced network

LTE-A specification mandates Medium Access Control (MAC) scheduler entity to ensure strict guaranteed quality of service (QoS) both in downlink and uplink direction. To the best of our knowledge the LTE-A MAC schedulers proposed so far constitute of an objective function dependent on single constraints like number of radio resources, user throughput, user channel conditions, user data buffer or user's downlink power requirement. In real world scenario if all the above constraints are not simultaneously taken into consideration, then it would be difficult to meet LTE-A QoS requirements. In this regard, to the best found knowledge, for first time in this paper, we propose a Multi Objective QoS aware LTE-A Downlink-MAC Scheduler (MOQDS) algorithm which adhere to two level QoS and fairness requirements of LTE-A specification. MOQDS algorithm does scheduling at two levels with each level has its objective function with its multiple operational constraints. Every transmission time interval, MOQDS uses multi-objective optimization to selects right user(s) and its corresponding application(s) to meet LTE-A QoS requirements. Simulation results are being compared with well referred LTE-A schedulers like modified largest weighted delay first, exponential rule proportional fairness and log rule under various ITU channel models. MOQDS achieves an average of 50% reduction in packet drop rate and minimum three times increased cell throughput as compared to above mentioned schedulers’ types and also with respect to all the other MAC schedulers mentioned in the references.

Wireless Ranging Method for Vehicular Node based-on IEEE802.11p Short Preamble

Increase of the traffic flow and traffic accident has raised more and more demands on vehicular node positioning. Because GNSS (Global Navigation Satellite System) can only work in LOS (Line Of Sight) environment, a correlate-sum method is proposed, that is, firstly cross-correlate between the received signal and the short preamble, secondly sum up the correlation result in groups, and lastly find the maximum peak to estimate the time delay. Simulation results are illustrated which show that in the ITU channel model for vehicular test environment, the proposed method provides better precision, robustness and less time complexity, especially in low SNR environments.

IMPLEMENTATION AND COMPARISON OF WAVELET TRANSFORM AND FOURIER TRANSFORM IN WI-MAX OFDM SYSTEM

WIMAX wireless communication is based on ofdm technology which enables going towards 4G Based on IEEE 802.16d-2004. But the reliability of ofdm is limited with the problems of time varying nature of the channel. This can reduced by adding cyclic prefix or guard interval between each block of data symbols. This guard interval minimizes the spectral efficiency in ofdm system. Recently, it was proved that discrete wavelet transform in ofdm system will reduce the isi Inter carrier interference, which is produced by loss of ortho-gonality between carriers and also provides more spectrum efficiency. It is found that proposed wavelet design achieves much lower bit error rates, Increases signal to noise power ratio (SNR), and can be used as an alternative to the Conventional OFDM WIMAX. The proposed OFDM system was modeled tested, and its Performance was found under different International Telecommunications Union (ITU Channel models.

Least Squares Interpolation Methods for LTE SystemChannel Estimation over Extended ITU Channels

Abstract — Performance of pilot-aided channel estimation techniques such as the Least Squares (LS) method depends on not only on the signal-to-noise ratio (SNR), channel conditions and pilot ratio, but also on the choice of interpolation method for deriving channel estimates at non-pilot subcarriers. This paper investigates the bit-error-rate (BER) performance of linear, spline and Piecewise Cubic Hermite Interpolating Polynomial (PCHIP) interpolation methods in LS channel estimation over the three extended ITU channel profiles defined for Long Term Evolution (LTE) testing. Simulation results show that applying the linear interpolation method produces the best BER performance over the fading channel with the smallest multipath delay spread. It is also shown that the choice of best interpolation method actually depends on the SNR in some of the fading channel profiles. Index Terms—ITU, LS, LTE, PCHIP, spline. I. INTRODUCTION Orthogonal frequency division multiplexing (OFDM) technology has been adopted in many of the next generation communication systems such as Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX) [1]. OFDM is preferred not only because it offers high data rate transmission but also because of its flexibility and robustness against multipath fading [1]. Multipath channel fading exists when multiple delayed copies of a transmitted signal are received at the receiver, thereby resulting in the distortion of the transmitted signal. These distortions are collectively termed „channel fading‟, causing reception errors and hence increased bit-error-rate (BER). Channel fading can be broadly classified as time-selective and/or frequency selective, depending on the relationship between the fading channel and the transmitted signal parameters [2]. To combat multipath fading and improve BER, some form of channel estimation method is required in order to estimate the channel distortion effect for each of the active subcarriers within an OFDM symbol [3].

Multiwavelength fiber ring laser based on optical add-drop multiplexers and a photonic crystal fiber Sagnac interferometer

In this paper, a configuration for a tunable multiwavelength erbium doped fiber ring laser based on optical add-drop multiplexers and a highly birefringent photonic crystal fiber Sagnac interferometer is proposed. Four different laser lines were obtained simultaneously all with an optical signal to noise ratio higher than 65dB, the best to our knowledge when using photonic crystal fibers. This proposed configuration is based on commercial devices and is adapted to the ITU channel's normative.

Iterative power and subcarrier allocation in rate-constrained orthogonal multicarrier downlink systems based on hybrid harmony search heuristics

This paper presents a novel iterative hybrid algorithm for subcarrier and power allocation in a cognitive orthogonal frequency division multiple access (OFDMA) downlink. In the considered setup a primary base station forwards information to K distant receivers by using a single OFDM waveform, whereas a secondary base station – subject to stringent per-user rate constraints – interferes with the former by sending information from users to the same set of destinations. Power and user allocation at both base stations is jointly performed by the proposed algorithm to maximize the overall throughput of the setup while satisfying, at the same time, the imposed rate constraints. Our proposal, which stems from an hybridization of the harmony search (HS) and differential evolution (DE) algorithms along with a greedy local repair method, is shown – through computer simulations over the extended vehicular A ITU channel model – to be an effective and practical resource allocation procedure for cognitive OFDMA downlinks.

Analysis and selection of optimum driving current combinations for tunable wavelength laser

AbstractIn a distributed Bragg reflector (DBR) laser, driving currents in the tri‐electrode are adjusted to generate various channels in the ITU‐Band. Many driving‐current combinations can generate the same ITU channel. But when we take system performance into consideration, such as the chirping and the switching time when we switch DBR lasers between ITU channels, certain current combinations will generate better system performance than the others. In this paper, we develop the principle of how to select current combinations in order to obtain better system performance and also discuss the method of how to evaluate the system performance when different current combinations are selected. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1417–1423, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21643

Demonstration of stable wavelength switching on a fast tunable laser transmitter

We describe a new technique to investigate continuous wavelength switching between 32 ITU channels in a tunable laser transmitter by switching the laser once every microsecond. We examine the effect of different switching schedules on the switching of the laser, an important issue for commercial switch fabrics for optical packet routing. Switching to all 32 channels of the laser is characterized in terms of a "packet-loss rate" (PLR) due to wavelength switching. PLRs are studied as a function of the switching-time window for a laser to tune to an ITU channel. Error-free wavelength switching in less than 50 ns is demonstrated for 26 hours.

Control of widely tunable SSG-DBR lasers for dense wavelength division multiplexing

This paper presents a general discussion on the control of widely tunable super structure grating distributed Bragg reflector (SSG-DBR) lasers. A feedback control scheme is presented that ensures frequency stability and accuracy (better than /spl plusmn/0.5 GHz), as well as high side mode suppression ratio (>35 dB). The active section voltage is monitored to maintain mode stability and a highly stable Fabry-Perot etalon is used as a reference to lock the laser frequency to a specific ITU channel. It is shown that stability can even be maintained when directly modulating the laser at 1.244 Gb/s. Furthermore, a characterization scheme is demonstrated that uses the voltage monitoring to generate a look-up table of operation points very efficiently and accurately. For all operation points, the frequency accuracy is better than /spl plusmn/0.5 GHz and the side mode suppression ratio is above 35 dB.