Daniel Aronsson

Channel estimation and prediction for adaptive OFDM downlinks [vehicular applications]

Channel estimation and prediction algorithms are developed and evaluated for use in broadband adaptive OFDM downlinks over fading channels for vehicular users. Accurate channel estimation may be obtained by using a combined pilot-aided and decision-directed approach based on Kalman filtering and prediction. The correlation properties of the channel in both time and space are taken into account. Kalman performance at much lower computational complexity is attained with recently developed constant gain adaptation laws. We present and evaluate a state-space realization of such an adaptation law, with computational complexity of the order of the square of the number of parallel tracked pilot subcarriers. In an adaptive OFDM system, prediction of the channel power a few milliseconds ahead are also required. Frequency-domain channel estimates can be transformed to the time domain, and used as regressors in channel predictors based on linear regression. We also make a preliminary evaluation of the direct use of complex channel prediction in the frequency domain for channel power prediction.

Using “predictor antennas” for long-range prediction of fast fading for moving relays

Channel state information at transmitters is important for advanced transmission schemes. However, feedback and transmission control delays of multiple milliseconds in LTE systems result in severe outdating of this information at vehicular velocities. Channel prediction based on extrapolation of the short-term fading is inadequate in LTE systems at vehicular velocities and high carrier frequencies. We here propose and evaluate a simple scheme which may extend the prediction horizon when used on vehicles: Use an additional antenna, a “predictor antenna”, placed in front of the transmission antennas in the direction of travel. This is of particular interest for use with moving relays: Local access points placed on e.g. buses or trams. A measurement-based study for 20 MHz downlink channels at 2.68 GHz is reported here for both line-of-sight and non-line-of-sight conditions.

Channel estimation and prediction for adaptive OFDMA/TDMA uplinks, based on overlapping pilots

In adaptive wireless packet transmission for multiple users, resources are allocated based on measurement and feedback of the channel qualities. Uplink channels of adaptive OFDM systems that use FDD have to be estimated and predicted based on uplink pilots transmitted by all active users. To prevent a prohibitive pilot overhead, the use of overlapping (simultaneously transmitted) pilots is considered. Kalman estimators and predictors can efficiently utilize the channel correlation in time and frequency to obtain estimates. The estimates have higher error than in the downlink case where overlapping pilots are not needed. However, the estimation and prediction MSE increases rather slowly with the number of simultaneous users. The results indicate that the accuracy is adequate for control of an adaptive transmission loop.

Peak-to-average power ratio reduction in OFDM using cyclically shifted phase sequences

Two approaches for reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) are proposed that are relied on a set of cyclically shifted phase sequences (CSPS) and implemented using the time domain circular convolution. After multiplying CSPS with the frequency domain data, the signal candidates can be expressed as weighted sum of the circularly shifted OFDM time domain data in the first method, which is called CSPS method. In the second method, weighted coefficients for generating the signal candidates in CSPS method are optimally selected to improve its performance; thus, the second method is referred to as optimised CSPS (OCSPS) method. The performances of the CSPS and OCSPS methods are evaluated using simulated data and compared with those of selective mapping (SLM) and partial transmit sequences (PTS). The simulation results show that both the CSPS and OCSPS methods can reduce the PAPR effectively, and that the OCSPS performs even better than the CSPS. The OCSPS can achieve the same performance as compared to the PTS. A distinct feature of the proposed methods is that only one inverse discrete Fourier transform is needed, and thus, the candidates can be calculated in time domain directly.

Block interleaved frequency division multiple access for power efficiency, robustness, flexibility, and scalability

The multiple access solution in an IMT-Advanced mobile radio system has to meet challenging requirements such as high throughput, low delays, high flexibility, good robustness, low computational complexity, and a high power efficiency, especially in the uplink. In this paper, a novel multiple access scheme for uplinks denoted as B-IFDMA is presented. We show that this scheme is able to provide equal or better error rate performance than the Single-Carrier Frequency Division Multiple Access (SCFDMA) schemes IFDMA and LFDMA, when considering realistic channel estimation performance at the receiver and no reliable channel state information at the transmitter. We also show that B-IFDMA provides better amplifier efficiency than OFDMA and can provide better end-to-end energy efficiency than IFDMA and LFDMA. Moreover, the scheme shows a promisingly high robustness to frequency-offsets and Doppler spread. Thus, this scheme can be regarded as a promising solution for the uplink of future mobile radio systems.

Kalman Predictor Design for Frequency-Adaptive Scheduling of FDD OFDMA Uplinks

Frequency-adaptive multiuser scheduling in OFDM utilizes the frequency-selective small-scale fading to allocate subcarriers with advantageous signal-to-noise ratio to each user. Due to channel time-variability and delays of the transmission control loop, this will in general require channel prediction. FDD (Frequency Division Duplex) uplinks pose the most challenging prediction problem: All sub-bands that may potentially be allocated must here be predicted for all involved user terminals, based on pilots transmitted from all terminals. This poses challenges with respect to prediction accuracy, estimator complexity and pilot overhead. This paper explores the design, performance and complexity of Kalman predictors used for uplink prediction, in the context of the EU WINNER project baseline design system. One conclusion is that uplink prediction that is useful at vehicular velocities in 4G systems operating at 3-5 GHz is indeed feasible. However, the channel predictability depends crucially on the local fading environment, so predictors should be based on models of the Doppler spectrum for each terminal.

Performance Evaluation of Memory-Less and Kalman-Based Channel Estimation for OFDMA

The next generation wireless systems based on Orthogonal Frequency Division Multiple Access (OFDMA) need to operate in widely different deployment and usage scenarios. Thus, support for flexible resource allocation is important. In this paper we investigate the performance of different memory-less and memory-based channel estimators with different OFDMA subcarrier allocation schemes and different pilot patterns. We evaluate the performance in various fading environments and for different user terminal velocities. The results show that channel estimation can perform well enough for time-frequency localized resources as small as 22 channel symbols with two pilots in many important scenarios. The results provided can be used to identify appropriate subcarrier allocations for the next generation OFDMA based wireless systems.

Estimation and detection of a periodic signal

We consider detection and estimation of a periodic signal with an additive disturbance. We study estimation of both the frequency and the shape of the waveform and develop a method based on Fourier series modelling. The method has an advantage over time domain methods such as epoch folding, in that the hypothesis space becomes continuous. Using uninformative priors, the noise variance and the signal shape can be marginalised analytically, and we show that the resulting expression can be evaluated in real time when the data is evenly sampled and does not contain any low frequencies.We compare our method with other frequency domain methods. Although derived in various different ways, most of these, including our method, have in common that the so called harmogram plays a central role in the estimation. But there are important differences. Most notable are the different penalty terms on the number of harmonic frequencies. In our case, these enter the equations automatically through the use of probability the...