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

A method and apparatus for allocating subcarriers in an orthogonal frequency division multiple access (OFDMA) system is described. In one embodiment, the method comprises allocating at least one diversity cluster of subcarriers to a first subscriber and allocating at least one coherence cluster to a second subscriber.

Ofdma with adaptive subcarrier-cluster configuration and selective loading

This paper deals with timing and frequency recovery for the uplink of an orthogonal frequency-division multiple access (OFDMA) system. The frequency estimator is derived from ad hoc reasoning, whereas the timing estimator is based on the maximum-likelihood criterion. Both schemes rely on the repetition of a fixed pilot symbol. Their main feature is that they provide feedforward estimates and allow synchronization in only two OFDM blocks. In contrast to other existing methods, they do not require that the subcarriers of a given user occupy adjacent positions in the signal bandwidth. This makes it possible to interleave subcarriers of different users so as to optimally exploit the frequency diversity of the channel. Theoretical analysis and computer simulations are used to assess the performance of the proposed synchronizers. It is found that the degradations due to residual frequency and timing errors are negligible at signal-to-noise ratios of practical interest.

Timing and frequency synchronization for the uplink of an OFDMA system

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

The multimedia era has started. The migration of computer (packet oriented) and telephony (circuit switched) networks in combination with an explosive development of new services, creates a mass market for public broadband communication. There are no reasons foreseen why these kinds of services should not go mobile. Future customers will not treat a mobile access differently than a fixed access to the network. The different types of services together with the demand of broadband mobile communication requires a new air interface. Limitations in the existing systems (flexibility and bit-rate) gives that they are not serious competitirs.

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A conceptual study of OFDM-based multiple access schemes : Part 1: Air interface requirements

This paper deals with the design of channel estimators in the uplink of an OFDM based multi-user system. First, we propose a user allocation scheme that is based on the aim to use the correlation of the radio channel for channel estimation purposes. Further, we present the linear MMSE channel estimator based on a few pilot tones. We present an error analysis that proves valuable in the evaluation of linear channel estimators under mismatch. Finally, we present some results on the design of linear channel estimators and pilot patterns, showing by means of examplesthe importance of careful pilot positioning.

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A conceptual study of OFDM-based multiple access schemes: Part 2 - Channel estimation in the uplink

This paper is part of a conceptual study of a multiuser OFDM system. In previous reports we have examined requirements for the air interface and presented uncoded performance (in terms of bit error rate) for the uplink of such systems. In this report we adopt the user allocation structure from the previous parts of the study and examine the role of channel coding. We present performance simulations that illustrate the effects of a convolutional code exploiting channel diversity, and channel estimator performance exploiting the channel correlation.

A conceptual study of OFDM-based multiple access schemes: Part 3 - Performance evaluation of a coded system

This document give an overview of Telias OFDM based proposal for UTRA and should be considered as an input to the OFDM concept group within SMG 2. The document describe the fundamental frame structure, principles of the detection, modulation, coding and synchronisation. Further on the channel structure, the resource management and handover principles are outlined.

Matthias Wahlqvist

Papers

A time and frequency synchronization scheme for multiuser OFDM

A conceptual study of OFDM-based multiple access schemes : Part 1: Air interface requirements

A conceptual study of OFDM-based multiple access schemes: Part 2 - Channel estimation in the uplink

A conceptual study of OFDM-based multiple access schemes: Part 3 - Performance evaluation of a coded system

Description of Telia's OFDM-based proposal