28 research outputs found
On Channel Estimation for 802.11p in Highly Time-Varying Vehicular Channels
Vehicular wireless channels are highly time-varying and the pilot pattern in
the 802.11p orthogonal frequency-division multiplexing frame has been shown to
be ill suited for long data packets. The high frame error rate in off-the-shelf
chipsets with noniterative receiver configurations is mostly due to the use of
outdated channel estimates for equalization. This paper deals with improving
the channel estimation in 802.11p systems using a cross layered approach, where
known data bits are inserted in the higher layers and a modified receiver makes
use of these bits as training data for improved channel estimation. We also
describe a noniterative receiver configuration for utilizing the additional
training bits and show through simulations that frame error rates close to the
case with perfect channel knowledge can be achieved.Comment: 6 pages, 11 figures, conferenc
All-to-all Broadcast for Vehicular Networks Based on Coded Slotted ALOHA
We propose an uncoordinated all-to-all broadcast protocol for periodic
messages in vehicular networks based on coded slotted ALOHA (CSA). Unlike
classical CSA, each user acts as both transmitter and receiver in a half-duplex
mode. As in CSA, each user transmits its packet several times. The half-duplex
mode gives rise to an interesting design trade-off: the more the user repeats
its packet, the higher the probability that this packet is decoded by other
users, but the lower the probability for this user to decode packets from
others. We compare the proposed protocol with carrier sense multiple access
with collision avoidance, currently adopted as a multiple access protocol for
vehicular networks. The results show that the proposed protocol greatly
increases the number of users in the network that reliably communicate with
each other. We also provide analytical tools to predict the performance of the
proposed protocol.Comment: v2: small typos fixe
Time- and Frequency-Varying -Factor of Non-Stationary Vehicular Channels for Safety Relevant Scenarios
Vehicular communication channels are characterized by a non-stationary time-
and frequency-selective fading process due to fast changes in the environment.
We characterize the distribution of the envelope of the first delay bin in
vehicle-to-vehicle channels by means of its Rician -factor. We analyze the
time-frequency variability of this channel parameter using vehicular channel
measurements at 5.6 GHz with a bandwidth of 240 MHz for safety-relevant
scenarios in intelligent transportation systems (ITS). This data enables a
frequency-variability analysis from an IEEE 802.11p system point of view, which
uses 10 MHz channels. We show that the small-scale fading of the envelope of
the first delay bin is Ricean distributed with a varying -factor. The later
delay bins are Rayleigh distributed. We demonstrate that the -factor cannot
be assumed to be constant in time and frequency. The causes of these variations
are the frequency-varying antenna radiation patterns as well as the
time-varying number of active scatterers, and the effects of vegetation. We
also present a simple but accurate bi-modal Gaussian mixture model, that allows
to capture the -factor variability in time for safety-relevant ITS
scenarios.Comment: 26 pages, 12 figures, submitted to IEEE Transactions on Intelligent
Transportation Systems for possible publicatio
Pilot Design for Enhanced Channel Estimation in Doubly Selective Channels
This paper investigates pilot design for enhanced channel estimation in single carrier communication systems over doubly-selective channels (DSC). Our contribution is twofold: first, we propose to use Huffman sequences as pilot clusters with low peak-to-average power ratio (PAPR), yet with good channel estimation performance when periodic pilot placement is adopted; second, we propose a low-complexity pilot placement strategy based on the analysis of the complex-exponential basis expansion model (CE-BEM) of the DSC. The latter leads to improved channel estimation performance with useful insights for pilot placement
Low Complexity Scalable Iterative Algorithms for IEEE 802.11p Receivers
In this paper, we investigate receivers for Vehicular to Vehicular (V2V) and Vehicular to Infrastructure (V2I) communications. Vehicular channels are characterized by multiple paths and time variations, which introduces challenges in the design of receivers. We propose an algorithm for IEEE 802.11p compliant receivers, based on Orthogonal Frequency Division Multiplexing (OFDM). We employ iterative structures in the receiver as a way to estimate the channel despite variations within a frame. The channel estimator is based on factor graphs, which allow the design of soft iterative receivers while keeping an acceptable computational complexity. Throughout this work, we focus on designing a receiver offering a good complexity performance trade-off. Moreover, we propose a scalable algorithm in order to be able to tune the trade-off depending on the channel conditions. Our algorithm allows reliable communications while offering a considerable decrease in computational complexity. In particular, numerical results show the trade-off between complexity and performance measured in computational time and BER as well as FER achieved by various interpolation lengths used by the estimator which both outperform by decades the standard least square solution. Furthermore our adaptive algorithm shows a considerable improvement in terms of computational time and complexity against state of the art and classical receptors whilst showing acceptable BER and FER performance
A novel delay dictionary design for compressive sensing-based time varying channel estimation in OFDM systems
Compressive sensing (CS) is a new attractive technique adopted for Linear Time Varying channel estimation. orthogonal frequency division multiplexing (OFDM) was proposed to be used in 4G and 5G which supports high data rate requirements. Different pilot aided channel estimation techniques were proposed to better track the channel conditions, which consumes bandwidth, thus, considerable data rate reduced. In order to estimate the channel with minimum number of pilots, compressive sensing CS was proposed to efficiently estimate the channel variations. In this paper, a novel delay dictionary-based CS was designed and simulated to estimate the linear time varying (LTV) channel. The proposed dictionary shows the suitability of estimating the channel impulse response (CIR) with low to moderate Doppler frequency shifts with acceptable bit error rate (BER) performance