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

The paper studies the problem of finding an optimal subcarrier and power allocation strategy for downlink communication to multiple users in an orthogonal-frequency-division multiplexing-based wireless system. The problem of minimizing total power consumption with constraints on bit-error rate and transmission rate for users requiring different classes of service is formulated and simple algorithms with good performance are derived. The problem of joint allocation is divided into two steps. In the first step, the number of subcarriers that each user gets is determined based on the users' average signal-to-noise ratio. The algorithm is shown to find the distribution of subcarriers that minimizes the total power required when every user experiences a flat-fading channel. In the second stage of the algorithm, it finds the best assignment of subcarriers to users. Two different approaches are presented, the rate-craving greedy algorithm and the amplitude-craving greedy algorithm. A single cell with one base station and many mobile stations is considered. Numerical results demonstrate that the proposed low complexity algorithms offer comparable performance with an existing iterative algorithm.

Computationally efficient bandwidth allocation and power control for OFDMA

Carrier Interferometry (CI) provides wideband transmission protocols with frequency-band selectivity to improve interference rejection, reduce multipath fading, and enable operation across non-continuous frequency bands. Direct-sequence protocols, such as DS-CDMA, are provided with CI to greatly improve performance and reduce transceiver complexity. CI introduces families of orthogonal polyphase codes that can be used for channel coding, spreading, and/or multiple access. Unlike conventional DS-CDMA, CI coding is not necessary for energy spreading because a set of CI carriers has an inherently wide aggregate bandwidth. Instead, CI codes are used for channelization, energy smoothing in the frequency domain, and interference suppression. CI-based ultra-wideband protocols are implemented via frequency-domain processing to reduce synchronization problems, transceiver complexity, and poor multipath performance of conventional ultra-wideband systems. CI allows wideband protocols to be implemented with space-frequency processing and other array-processing techniques to provide either or both diversity combining and sub-space processing. CI also enables spatial processing without antenna arrays. Even the bandwidth efficiency of multicarrier protocols is greatly enhanced with CI. CI-based wavelets avoid time and frequency resolution trade-offs associated with conventional wavelet processing. CI-based Fourier transforms eliminate all multiplications, which greatly simplifies multi-frequency processing. The quantum-wave principles of CI improve all types of baseband and radio processing.

Multicarrier sub-layer for direct sequence channel and multiple-access coding

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

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

A random access protocol system and method for communicating between a mobile station and a base station in a multi-access digital radio communication system, including the steps of cyclically repeating a random access sequence and transmitting each cyclically repeated random access sequence in an orthogonal frequency division multiplex format from the mobile station as an uplink transmission.

Random access orthogonal frequency division multiplex system and method

The present invention relates to a method at mobile radio system for synchronization of transmitter and receiver. The system relates to MC/DS-CDMA-system. In the system information is transmitted digitally in frames. For the purpose a data channel, a pilot channel and a synchronization channel are created. In the data channel the data blocks (D0, D1, D2, etc.) are separated by a guard space Δ. In the pilot channel, respective the synchronization channel, the information is arranged in blocks which are synchronous with each other. The repeating distance, (A), of the pilot channel, corresponds to the length of one of the data blocks (D0, D1, D2, etc.). The repeating distance for the synchronization channel is S, corresponding to a number of blocks in the data channel. In the synchronization channel information is introduced into the blocks which indicate their relation to the data channel. The method in this way allows an identification of the position of the pilot channel, which indicates the position of the synchronization channel, at which a decoding of the information of the synchronization channel appoints the position of the data channel.

Method for synchronization of transmitter and receiver at mobile radio system

A future mobile communication system must support not only the traditional speech service but also other services such as multimedia, circuit switched and packet based data. Though no "killer application" is known today the system design must be flexible enough to simultaneously handle different types of services (circuit switched and packet based), with different demands on quality of service (blocking, delay and throughput). Efficient handling of different mixtures of services must be a key issue, for a high capacity system. This work has been done in connection with the evaluation of candidates for the European, Universal Mobile Telecommunication System (UMTS). The paper studies what effects a mixed service scenario would give on the capacity of the cellular system, using different types of resource allocation. The different profiles of the services, in combination with the large amount of freedom regarding user behaviour, motivates scenario simulations. Two scenarios (speech and circuit switched data dominated) are studied. The performance is compared for different types of resource allocation schemes, in an OFDM based multiple access system. In OFDM, all resources are available in all cells as the FFT works on the total available bandwidth. This makes a multiple access system based on OFDM suitable for using together with dynamic resource allocation.

Capacity comparison of an OFDM based multiple access system using different dynamic resource allocation

Utilisation of frequency divided OFDM (FD OFDM) in the uplink of a multi-access system requires that all mobile stations meet the requirements for orthogonality between sub-carriers in the time and frequency domain. Alternatively, another access technique can be used with OFDM, which can tolerate some relaxation in orthogonality requirements, e.g. code division OFDM (CD OFDM, or COFDM). The use of FD OFDM thus requires a random access technique that does not disturb the orthogonality between sub-carriers. Because time synchronisation is essential if orthogonality is to be preserved between sub-carriers, it is vital that delay difference between a mobile station and a base station be estimated during a random access protocol. In a random access protocol, a mobile station transmits a known signal sequence to a base station. The base station can lock onto the known sequence, detect it and estimate the time delay. According to the present invention, the random access sequence can be repeated cyclically and may comprise a m-sequence, a Gold sequence, or a four phase sequence with good cross-correlation properties, which is modulated onto the sub-carriers in the frequency domain. The random access sequence is transmitted without guard spaces. The random access sequence may be modulated onto all sub-carriers, or alternatively onto selected sub-carriers only. Alternatively, the random access sequence may be modulated onto all sub-carriers, but some sub-carriers may be transmitted at higher power than others.

Christer Östberg

Papers

A time and frequency synchronization scheme for multiuser OFDM

Random access orthogonal frequency division multiplex system and method

Method for synchronization of transmitter and receiver at mobile radio system

Capacity comparison of an OFDM based multiple access system using different dynamic resource allocation

Random access in multicarrier systems