Classical and Bayesian Linear Data Estimators for Unique Word OFDM

Unique word - orthogonal frequency division multiplexing (UW-OFDM) is a novel OFDM signaling concept, where the guard interval is built of a deterministic sequence - the so-called unique word - instead of the conventional random cyclic prefix. In contrast to previous attempts with deterministic sequences in the guard interval the addressed UW-OFDM signaling approach introduces correlations between the subcarrier symbols, which can be exploited by the receiver in order to improve the bit error ratio performance. In this paper we develop several linear data estimators specifically designed for UW-OFDM, some based on classical and some based on Bayesian estimation theory. Furthermore, we derive complexity optimized versions of these estimators, and we study their individual complex multiplication count in detail. Finally, we evaluate the estimators' performance for the additive white Gaussian noise channel as well as for selected indoor multipath channel scenarios.Comment: Preprint, 13 page

Peak-to-Average-Power-Ratio (PAPR) Reduction Techniques for Orthogonal-Frequency-Division- Multiplexing (OFDM) Transmission

Wireless communication has experienced an incredible growth in the last decade. Two decades ago,the number of mobile subscribers was less than 1% of the world\u27s population. As of 2011, the number of mobile subscribers has increased tremendously to 79.86% of the world\u27s population. Robust and high-rate data transmission in mobile environments faces severe problems due to the time-variant channel conditions, multipath fading and shadow fading. Fading is the main limitation on wireless communication channels. Frequency selective interference and fading, such as multipath fading, is a bandwidth bottleneck in the last mile which runs from the access point to the user. The last mile problem in wireless communication networks is caused by the environment of free space channels through which the signal propagates. Orthogonal Frequency Division Multiplexing (OFDM) is a promising modulation and multiplexing technique due to its robustness against multipath fading. Nevertheless, OFDM suffers from high Peak-to-Average- Power-Ratio (PAPR), which results in a complex OFDM signal. In this research, reduction of PAPR considering the out-of-band radiation and the regeneration of the time-domain signal peaks caused by filtering has been studied and is presented. Our PAPR reduction was 30% of the Discrete Fourier Transform (DFT) with Interleaved Frequency Division Multiple Access (IFDMA) utilizing Quadrature Phase Shift Keying (QPSK) and varying the roll-off factor. We show that pulse shaping does not affect the PAPR of Localized Frequency Division Multiple Access (LFDMA) as much as it affects the PAPR of IFDMA. Therefore, IFDMA has an important trade-off relationship between excess bandwidth and PAPR performance, since excess bandwidth increases as the roll-off factor increases. In addition, we studied a low complexity clipping scheme, applicable to IFDMA uplink and OFDM downlink systems for PAPR reduction. We show that the performance of the PAPR of the Interleaved-FDMA scheme is better than traditional OFDMA for the uplink transmission system. Our reduction of PAPR is 53% when IFDMA is used instead of OFDMA in the uplink direction. Furthermore, we also examined an important trade-off relationship between clipping distortion and quantization noise when the clipping scheme is used for OFDM downlink systems. Our results show a significant reduction in the PAPR and the out-of-band radiation caused by clipping for OFDM downlink transmission system

AirSync: Enabling Distributed Multiuser MIMO with Full Spatial Multiplexing

The enormous success of advanced wireless devices is pushing the demand for higher wireless data rates. Denser spectrum reuse through the deployment of more access points per square mile has the potential to successfully meet the increasing demand for more bandwidth. In theory, the best approach to density increase is via distributed multiuser MIMO, where several access points are connected to a central server and operate as a large distributed multi-antenna access point, ensuring that all transmitted signal power serves the purpose of data transmission, rather than creating "interference." In practice, while enterprise networks offer a natural setup in which distributed MIMO might be possible, there are serious implementation difficulties, the primary one being the need to eliminate phase and timing offsets between the jointly coordinated access points. In this paper we propose AirSync, a novel scheme which provides not only time but also phase synchronization, thus enabling distributed MIMO with full spatial multiplexing gains. AirSync locks the phase of all access points using a common reference broadcasted over the air in conjunction with a Kalman filter which closely tracks the phase drift. We have implemented AirSync as a digital circuit in the FPGA of the WARP radio platform. Our experimental testbed, comprised of two access points and two clients, shows that AirSync is able to achieve phase synchronization within a few degrees, and allows the system to nearly achieve the theoretical optimal multiplexing gain. We also discuss MAC and higher layer aspects of a practical deployment. To the best of our knowledge, AirSync offers the first ever realization of the full multiuser MIMO gain, namely the ability to increase the number of wireless clients linearly with the number of jointly coordinated access points, without reducing the per client rate.Comment: Submitted to Transactions on Networkin

Adjustable dynamic range for paper reduction schemes in large-scale MIMO-OFDM systems

In a multi-input-multi-output (MIMO) communication system there is a necessity to limit the power that the output antenna amplifiers can deliver. Their signal is a combination of many independent channels, so the demanded amplitude can peak to many times the average value. The orthogonal frequency division multiplexing (OFDM) system causes high peak signals to occur because many subcarrier components are added by an inverse discrete Fourier transformation process at the base station. This causes out-of-band spectral regrowth. If simple clipping of the input signal is used, there will be in-band distortions in the transmitted signals and the bit error rate will increase substantially. This work presents a novel technique that reduces the peak-to-average power ratio (PAPR). It is a combination of two main stages, a variable clipping level and an Adaptive Optimizer that takes advantage of the channel state information sent from all users in the cell. Simulation results show that the proposed method achieves a better overall system performance than that of conventional peak reduction systems in terms of the symbol error rate. As a result, the linear output of the power amplifiers can be minimized with a great saving in cost

Distributed Digital Radios for Land Mobile Radio Applications

The main objective of this dissertation is to develop the second generation of Distributed Digital Radio (DDR) technology. A DDR II modem provides an integrated voice/data service platform, higher data rates and better throughput performance as compared to a DDR I modem. In order to improve the physical layer performance of DDR modems an analytical framework is first developed to model the Bit Error Rate (BER) performance of Orthogonal Frequency Division Multiplexing over Frequency Modulation (OFDM/FM) systems. The use of OFDM provides a spectrally efficient method of transmitting data over LMR channels. However, the high Peak-to-Average (PAR) of OFDM signals results in either a low Signal-to-Noise Ratio (SNR) at FM receiver or a high non-linear distortion of baseband signal in the FM transmitter. This dissertation presents an analytical framework to highlight the impact of high PAR of OFDM signal on OFDM/FM systems. A novel technique for reduction of PAR of OFDM called Linear Scaling Technique (LST) is developed. The use of LST mitigates the signal distortion occurring in OFDM over FM systems. Another important factor which affects the throughput of LMR networks is the Push-to-Talk (PTT) delay. A PTT delay refers to the delay between the instant when a PTT switch on a conventional LMR radio is keyed/unkeyed and a response is observed at the radio output. It can be separated into a Receive-To-Transmit Switch Interval (RTSI) or a Transmit-To-Receive Switch Interval (TRSI). This dissertation presents the typical RTSI delay values, distributions and their impact on throughput performance of LMR networks. An analytical model is developed to highlight the asymmetric throughput problem and the unintentional denial of service (UDOS) occurring in heterogeneous LMR networks consisting of radios with different PTT delay profiles. This information will be useful in performance and capacity planning of LMR networks in future