Göran Klang

The WINNER B3G System MAC Concept

The European IST research projects WINNER and WINNER II aim at developing a single new ubiquitous radio access system concept that can be adapted for use in a wide variety of mobile communication scenarios, from wide area coverage to hot spots. This cellular packet data system uses a multicarrier-based air interface. Its medium access control (MAC) system layer is designed for either FDD or TDD cellular transmission. It supports adaptive transmission by tight interaction with the physical layer, and provides efficient resource allocation and scheduling within relay-enhanced cells. It also supports separate allocation of antenna resources per packet flow and furthermore provides support for spectrum sharing and flexible spectrum use. The paper outlines the resource allocation within a super-frame and the use of either opportunistic scheduling or diversity-based transmission within frames.

Integration of Spatial Processing in the WINNER B3G Air Interface Design

Numerous investigations of spatial processing algorithms are available in literature showing the benefit of MIMO systems in particular situations. From a system perspective, however, spatial processing has to provide adaptivity and scalability to a wide range of scenarios and has also to be seen in the context of its impact on other system services and functions, its enabling requirements, overhead, and robustness. This paper summarizes recent work towards integrated spatial processing services and functions for an OFDM-based B3G air interface, developed in the European IST research project WINNER. The current status of the air interface and the multi-user spatial domain link adaptation concept is summarized. A baseline spatial scheme selection process is developed as part of the overall MAC radio resource control

Transmit diversity based on space-time block codes in frequency selective Rayleigh fading DS-CDMA systems

Exploring the temporal and spatial domains both at the receiver and transmitter offers important means for considerably improving the system performance in cellular networks. In this paper, the impact of using open-loop transmit diversity based on space-time block codes in a multi-user DS-CDMA system operating in a frequency selective fading environment is explored. A non-joint linear multi-user detection and space-time decoding approach is proposed. It is shown that by introducing a simple constraint with respect to the spatio-temporal cross-correlation properties of the space-time code-corresponding to introducing interleaving-any space-time block code can be used as the base for the transmit diversity scheme. Simulation results using the complex (2,2,2) space-time block code known as Alamoutis (see IEEE Trans. on Select Areas in Communications, vol.16, p.1451-58, 1998) code, indicates that large diversity gains are achievable and that the amount of overhead originating from training data can be reduced considerably using the proposed approach.

Space-time interference rejection cancellation in transmit diversity systems [cancellation read combining]

We consider the use of space diversity methods in conjunction with interference rejection combining (IRC) to mitigate the impact of interference and noise for interference limited scenarios. The use of transmit diversity schemes in a system generally limits the interference rejection capabilities of IRC techniques. We introduce a novel space-time IRC (STIRC) scheme that exploits the spatio-temporal correlation induced by cochannel users employing transmit diversity in the form of space-time block codes (STBC). Thus, the system can draw upon the benefits of interference rejection methods although transmit diversity is being employed. Simulation examples are presented to illustrate the discussion and to highlight the importance of taking both the spatial and temporal structure of the interference into account.

Interference robustness aspects of space-time block code-based transmit diversity

Transmit diversity, which was initially developed for noise-limited environments, has been promoted as a viable candidate for improving the link quality in both existing and future systems for wireless communication. However, to ensure efficient spectrum utilization, receivers operating within wireless multiuser networks must be robust not only to fading and noise but to interference from other system users as well. This work considers interference robustness aspects when transmit diversity, in the form of space-time block coding, is used in multiuser systems. Properties of the space-time block encoded signals such as code rate, block structure, diversity order, etc., and their implications on detection and interference rejection by means of noise whitening are discussed. To handle the presence of space-time block encoded interference, a space-time processing-based extension of an interference rejection combining algorithm is proposed. Results are presented indicating that transmit diversity based on space-time block codes (STBCs) of the linear dispersion type improve robustness against interference in terms of an increased diversity advantage. This can be achieved either by increasing the number of transmit antennas or by reducing the rate of the code. It is also shown, by analysis and by simulation examples, that the performance improvements obtained by using transmit diversity in multiuser systems may rapidly subside as the signal-to-interference ratio decreases. However, by using the proposed interference rejection scheme tailored to the space-time encoded structure, performance improvements of transmit diversity are also obtained in a multiuser environment.

Structured semi-blind interference rejection in dispersive multichannel systems

The impact of interference is one of the most capacity limiting factors in cellular networks today. In order to host more users in existing systems and maximize the capacity of future networks, the use of interference suppression techniques are instrumental. We propose a structured semi-blind parameter estimation procedure to support rejection of unknown cochannel interference (CCI) in systems facing multipath channels with non-negligible delay spread. Employing spatio-temporal processing, the proposed scheme exploits the temporal correlation present in the output signal from an antenna array and the training data of a desired user in order to find parameter estimates that fit the range space of a structured CCI model to the signal subspace of the interfering signals. Simulation results illustrating the behavior of a spatio-temporal receiver using the proposed scheme with respect to both signal-to-noise and signal-to-interference ratios (SNRs and SIRs) are presented. In addition, results highlighting the impact of the training sequence length as well as the impact of the channel model order of the CCI users are also included.

Structured spatio-temporal interference rejection with antenna arrays

We propose a structured, training sequence based method to find the parameters of a spatio-temporal co-channel interference (CCI) rejection filter. By exploiting the inherent structure of the CCI, it is shown that the number of parameters that are to be estimated can be made independent of the spatio-temporal filter order. It is further shown that by taking advantage of the desired users known protocol structure, the parameters can be estimated using a semi-blind technique. Simulation results indicate that by taking the structure into account, for short training sequences, considerable gains are achievable compared to an unstructured method.

Channel estimation and interference rejection for multichannel systems

A number of schemes for blind channel estimation have been proposed to mitigate the problem of intersymbol (ISI) and co-channel interference (CCI) in mobile communication environments. In many of the proposed schemes no attention is paid to the fact that in most existing and scheduled future systems, training sequences will be available within the transmission protocol of the desired users. In this paper a semi-blind approach utilizing the rich structure inherent in the base station antenna array output and the knowledge of the desired users training sequence is proposed in order to improve the transmission link performance. In the described approach cancellation of unknown interfering sources is performed based on blind estimation of their corresponding channels allowing the channels of the desired users to be estimated using a conventional training sequence based channel estimation algorithm.