Probability and Measure

When mobiles cannot support multiple antennas due to size or other constraints, conventional space-time coding cannot be used to provide uplink transmit diversity. To address this limitation, the concept of cooperation diversity has been introduced, where mobiles achieve uplink transmit diversity by relaying each other's messages. A particularly powerful variation of this principle is coded cooperation. Instead of a simple repetition relay, coded cooperation partitions the codewords of each mobile and transmits portions of each codeword through independent fading channels. This paper presents two extensions to the coded cooperation framework. First, we increase the diversity of coded cooperation in the fast-fading scenario via ideas borrowed from space-time codes. We calculate bounds for the bit- and block-error rates to demonstrate the resulting gains. Second, since cooperative coding contains two code components, it is natural to apply turbo codes to this framework. We investigate the application of turbo codes in coded cooperation and demonstrate the resulting gains via error bounds and simulations.

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Coded cooperation in wireless communications: space-time transmission and iterative decoding

Orthogonal frequency division multiple access (OFDMA) has recently attracted vast research attention from both academia and industry and has become part of new emerging standards for broadband wireless access. Even though the OFDMA concept is simple in its basic principle, the design of a practical OFDMA system is far from being a trivial task. Synchronization represents one of the most challenging issues and plays a major role in the physical layer design. The goal of this paper is to provide a comprehensive survey of the latest results in the field of synchronization for OFDMA systems, with tutorial objectives foremost. After quantifying the effects of synchronization errors on the system performance, we review some common methods to achieve timing and frequency alignment in a downlink transmission. We then consider the uplink case, where synchronization is made particularly difficult by the fact that each user's signal is characterized by different timing and frequency errors, and the base station has thus to estimate a relatively large number of unknown parameters. A second difficulty is related to how the estimated parameters must be employed to correct the uplink timing and frequency errors. The paper concludes with a comparison of the reviewed synchronization schemes in an OFDMA scenario inspired by the IEEE 802.16 standard for wireless metropolitan area networks.

Synchronization Techniques for Orthogonal Frequency Division Multiple Access (OFDMA): A Tutorial Review

In this paper, a fair scheme to allocate subcarrier, rate, and power for multiuser orthogonal frequency-division multiple-access systems is proposed. The problem is to maximize the overall system rate, under each user's maximal power and minimal rate constraints, while considering the fairness among users. The approach considers a new fairness criterion, which is a generalized proportional fairness based on Nash bargaining solutions and coalitions. First, a two-user algorithm is developed to bargain subcarrier usage between two users. Then a multiuser bargaining algorithm is developed based on optimal coalition pairs among users. The simulation results show that the proposed algorithms not only provide fair resource allocation among users, but also have a comparable overall system rate with the scheme maximizing the total rate without considering fairness. They also have much higher rates than that of the scheme with max-min fairness. Moreover, the proposed iterative fast implementation has the complexity for each iteration of only O(K/sup 2/Nlog/sub 2/N+K/sup 4/), where N is the number of subcarriers and K is the number of users.

Fair multiuser channel allocation for OFDMA networks using Nash bargaining solutions and coalitions

Two timing offset estimation methods for orthogonal frequency division multiplexing (OFDM) systems as modifications to Schmidl and Cox's method (see IEEE Trans. Commun., vol.45, p.1613-21, 1997) are presented. The performances of the timing offset estimators in additive white Gaussian noise channel and intersymbol interference channel are compared in terms of estimator variance obtained by simulation. Both proposed methods have significantly smaller estimator variance in both channel conditions.

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On timing offset estimation for OFDM systems

We present a multiuser synchronization scheme for tracking the mobile's uplink time and frequency offsets. It uses the redundancy introduced by the cyclic prefix and does not need additional pilots. We show performance results of an orthogonal frequency division multiplexing (OFDM)-based radio interface based on the universal mobile telecommunication system (UMTS) parameters. For a UMTS-typical mobile channel environment, the performance of a coherent system employing the scheme is virtually indistinguishable from the performance of a perfectly synchronized system. In a differentially modulated system, synchronization errors decrease the system performance by about 0.7 dB compared to a perfectly synchronized system.

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A time and frequency synchronization scheme for multiuser OFDM

This report is an introduction to orthogonal frequency-division multiplexing (OFDM). The focus is on signal processing areas pursued by our research group at Lulea University of Technology. We pres ...

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An introduction to orthogonal frequency-division multiplexing

This paper presents a symbol time offset estimator for coherent orthogonal frequency division multiplexing (OFDM) systems. The estimator exploits both the redundancy in the cyclic prefix and available pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the maximum-likelihood estimator for the additive white Gaussian noise channel. Simulations for an example system indicate a system performance as close as 0.6 dB to a perfectly synchronized system.

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Symbol time offset estimation in coherent OFDM systems

This paper presents a symbol time offset estimator for coherent OFDM systems. This estimator exploits both the redundancy in the cyclic prefix and the pilot symbols used for channel estimation. The estimator is robust against frequency offsets and is suitable for use in dispersive channels. We base the estimator on the ML estimator for the AWGN channel. Simulations of an example system indicate a system performance as close as 0.3 dB to a perfectly synchronized system. Compared to an estimator not using pilots, our estimator's performance could allow a shorter cyclic prefix and thus a more spectrally efficient system.

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Time and frequency offset estimation is addressed in an OFDM system that uses a cyclic prefix and pulse shaping. Earlier work has presented a method that avoids the use of pilots by exploiting the inherent correlation in the cyclically extended OFDM symbol. In this paper this technique is extended to include systems with pulse shaping. The joint maximum likelihood estimator of time and frequency offsets for such systems is presented and evaluated. An implementable structure of the extended estimator is given. Simulations show that it is possible to maintain the performance when introducing pulse shaping into an OFDM system.

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Daniel Landström

Papers

A time and frequency synchronization scheme for multiuser OFDM

An introduction to orthogonal frequency-division multiplexing

Symbol time offset estimation in coherent OFDM systems

Symbol time offset estimation in coherent OFDM systems

Time and frequency offset estimation in OFDM systems employing pulse shaping