Thomas Zemen

Improved channel estimation for iterative receivers

In iterative receiver structures, soft information becomes available after the decoding stage. This information is used to enhance the quality of the channel estimates for the next iteration. We derive a generalized estimator based on the linear minimum mean square error (LMMSE) principle for deterministic pilot information combined with soft information. We present the special case of multi-carrier code division multiple access (MC-CDMA) in detail and provide simulation results. The presented channel estimation algorithm can be also applied to direct sequence (DS)-CDMA and multiple-input multiple-output (MIMO) systems.

Iterative detection and channel estimation for MC-CDMA

Multi-carrier code division multiple access (MC-CDMA) systems are under intense investigation for high bit rate wireless communications systems. Their equalization is based on the fast Fourier transform, allowing for an efficient implementation. Iterative receivers with joint detection and decoding have been shown to achieve very good performance for direct-sequence (DS)-CDMA systems. We apply this concept to MC-CDMA, the multiuser detector is implemented as parallel interference canceller with post-minimum mean squared error filtering. In this contribution a new pilot based channel estimation scheme based on random time sequences is developed. The presented simulation results for a multi path scenario show, that in a fully loaded system with 64 users the single user bound can be approached up to 1 dB. A bit error rate (BER) of 10/sup -3/ is reached already at an E/sub b//N/sub 0/ of 12 dB with a 4 state, rate 1/2 convolutional code.

Time Variant Channel Equalization for MC-CDMA via Fourier Basis Functions

When users move at vehicular speed the radio channel is time variant. Sayeed et al. proposed a Fourier basis expansion model for time variant channels. We apply this concept to MC-CDMA for channel equalization and give simulation results for the forward link. To gain further insights we give a detailed discussion of the benefits and weaknesses of the Fourier basis expansion channel model.

Relaying for IEEE 802.11p at road intersection using a vehicular non-stationary channel model

Traffic s afety at road intersections can be improved by establishing reliable communications between vehicles. For vehicle-to-vehicle communications, this requires information exchange in non line-of-sight (NLOS) conditions due to the obstruction by buildings. In order to overcome the low receive signal-to-noise ratio (SNR) due to NLOS, we consider to place a relay at road intersections to enhance the reliability of communication links. In this paper, we implement a vehicular non-stationary geometry based stochastic channel model for road intersections, which is an extension of an existing highway channel model. The model is verified by comparison with vehicular channel measurements. Using the proposed channel model, we present link level simulation results for IEEE 802.11p relaying at varying transmitter/receiver locations using different channel estimation techniques. The results show that a relay at the intersection is able to greatly extend the reliable communications region. Besides, in the high SNR regime with moderate or high mobility transmitter and receiver, the block type least square channel estimator is the bottleneck that limits the relaying performance. An advanced iterative channel estimator is also simulated, which exhibits robustness against increased vehicle velocities.