Ronald Nissel

On pilot-symbol aided channel estimation in FBMC-OQAM

Filter bank multicarrier modulation is considered as a possible candidate for 5G. In this paper, we consider pilot-symbol aided channel estimation and address the problem of canceling the imaginary interference at the pilot positions. We develop a matrix formulation for the transmission system which allows us to formulate general conditions on the auxiliary pilot symbols, capturing also the interde-pendency of closely spaced pilots and an arbitrary number of auxiliary pilot symbols. By using two auxiliary symbols per pilot instead of one, we are able to improve the peak-to-average power ratio as well as the achievable capacity for small to medium signal-to-noise ratios. The achievable capacity can further be increased by interference cancellation based on linear precoding for which we propose an algorithm to find the coding matrix required. Finally, we compare auxiliary pilot symbols and linear precoding in terms of complexity and performance.

Bit error probability for pilot-symbol aided channel estimation in FBMC-OQAM

Filter Bank MultiCarrier (FBMC) might replace Orthogonal Frequency Division Multiplexing (OFDM) in 5G. For such FBMC system, we derive closed-form expressions for the Bit Error Probability (BEP) including channel estimation, whereas we focus on the comparison of FBMC with OFDM. We assume additive white Gaussian noise and a Rayleigh fading channel with low delay spread and low Doppler spread, so that the channel induced interference can be neglected compared to the noise. Our channel estimation is based on pilot symbols whereby the imaginary interference, inherently caused in FBMC, is canceled at the pilot positions either by auxiliary symbols or through coding. Moreover, we propose an optimal power allocation between pilot symbols and data symbols to minimize the BEP.

Doubly-selective MMSE channel estimation and ICI mitigation for OFDM systems

In high mobility orthogonal frequency division multiplexing systems, subcarriers are no longer orthogonal, causing Inter-Carrier Interference (ICI). Equalization then becomes more challenging and requires an accurate estimate of the time-variant channel. In this paper, we propose a novel Minimum Mean Squared Error (MMSE) estimation of the sampled time-variant transfer function. Based on such channel estimation, we propose an iterative three step ICI mitigation technique whereby each step increases the channel estimation accuracy and, consequently, the performance of our MMSE equalization. In Step 1, we consider ICI as an additional noise term. In Step 2, we reduce the ICI at pilot positions and finally, in Step 3, we treat all estimated data symbols, obtained from Step 2, as if they were pilot symbols. We evaluate the Bit Error Ratio (BER) of our ICI mitigation technique by means of simulation and testbed measurements (up to 400 km/h). In both cases, we achieve a BER close to perfect channel knowledge and zero ICI.

OFDM and FBMC-OQAM in Doubly-Selective Channels: Calculating the Bit Error Probability

Filter bank multi-carrier (FBMC) is a modulation technique with enhanced spectral properties compared with orthogonal frequency division multiplexing (OFDM). In this letter, we investigate the performance degeneration of OFDM and FBMC in doubly-selective channels, that is, time-selectivity and frequency-selectivity. For that, we derive closed-form bit error probability (BEP) expressions for arbitrary linear modulation methods based on one-tap equalizers, with OFDM and FBMC being special cases, covered by our general BEP expressions. We validate our calculations by Monte Carlo simulations and investigate the BEP error if the interference is approximated as Gaussian noise.

Experimental evaluation of FBMC-OQAM channel estimation based on multiple auxiliary symbols

Filter Bank Multi-Carrier (FBMC) has been identified by many authors as a possible successor for orthogonal frequency-division multiplexing in 5G. In this paper, we consider pilot-symbol aided channel estimation in FBMC. To deal with the imaginary interference, inherently caused in FBMC, we employ auxiliary symbols. In contrast to previous works, we propose to use multiple auxiliary symbols per pilot which decreases the peak-to-power average ratio and, for certain operation points, also increases the achievable capacity. The applicability of our channel estimation method is then validated through real world measurements, where we show that multiple auxiliary symbols lead to a higher throughput for practical relevant signal-to-noise ratios.

Enabling Low-Complexity MIMO in FBMC-OQAM

Filter Bank Multi-Carrier (FBMC) offers superior spectral properties compared to Orthogonal Frequency Division Multiplexing (OFDM), at the cost of imaginary interference, which makes the application of Multiple-Input and Multiple-Output (MIMO) more challenging. By spreading symbols in time (or frequency), we can completely eliminate the imaginary interference, so that all MIMO techniques known in OFDM can be straightforwardly applied in FBMC. The spreading process itself has low complexity because it is based on Hadamard matrices. Although spreading allows to restore complex orthogonality in FBMC within one transmission block, we observe interference from neighboring blocks. By including a guard timeslot, the signal-to-interference ratio can be further improved. Furthermore, we investigate the effect of a time-variant channel on such spreading approach. Finally, testbed measurements show the applicability of our FBMC based MIMO transmission scheme in real world environments.

Experimental Validation of the OFDM Bit Error Probability for a Moving Receive Antenna

For an orthogonal frequency-division multiplexing system with pilot-symbol aided channel estimation, we compare the measured bit error ratio to the theoretical bit error probability. In order to measure mobile physical systems, we utilize the Vienna Wireless Testbed which has been augmented by a rotation wheel unit. The analytical solution assumes Rayleigh fading, additive Gaussian noise, and an arbitrary linear interpolation method to estimate the unknown channel taps. Our measurements confirm our assumptions and demonstrate convincingly that our theoretical expressions accurately model the true physical behavior, even for speeds of up to 100\,km/h.

Low-latency MISO FBMC-OQAM: It works for millimeter waves!

A key enabler for high data rates in future wireless systems will be the usage of millimeter Waves (mmWaves). Furthermore, Filter Bank Multi-Carrier (FBMC) with its good spectral properties has also been considered as a possible future transmission technique. However, many authors claim that multiple antennas and low-latency transmissions, two of the key requirements in 5G, cannot be efficiently supported by FBMC. This is not true in general, as we will show in this paper. We investigate FBMC transmissions over real world channels at 60 GHz and show that Alamoutis space time block code works perfectly fine once we spread (code) symbols in time. Although it is true that spreading increases the transmission time, the overall transmission time is still very low due to the high subcarrier spacing employed in mmWaves. Therefore, coded FBMC in combination with mmWaves enables high spectral efficiency, low-latency and allows the straightforward usage of multiple antennas.

Closed-form capacity expression for low complexity BICM with uniform inputs

In this paper, we derive closed-form capacity expressions for a low complexity bit-interleaved coded modulation system with uniform inputs in a Rayleigh fading channel with additive white Gaussian noise. Additionally, we include pilot-symbol assisted channel estimation in our considerations. Finding a closed-form solution is enabled by assuming quantization and that the decoder has no channel state information. The effects of these assumptions on the capacity are investigated separately. To verify our closed-form expressions and to show the applicability of our system model to real world physical channels, we perform measurements using the Vienna Wireless Testbed.

On the Influence of Doubly-Selectivity in Pilot-Aided Channel Estimation for FBMC-OQAM

Superior spectral properties of Filter Bank Multi-Carrier (FBMC) techniques make them an interesting choice for future wireless systems. In order to directly apply well known pilot-symbol aided channel estimation in FBMC, however, the intrinsic imaginary interference has to be canceled. Such cancellation method is usually designed for a doubly-flat channel. In this paper, we investigate the effects of doubly-selectivity, that is, time-selectivity and frequency-selectivity, on the channel estimation Mean Squared Error (MSE) in FBMC. The calculation of the MSE is based on a compact matrix description, allowing us also to find an MSE minimizing channel estimation method that takes the underlying waveform into account.