Diversity Enhanced Multiuser and Multicarrier Mobile Radio Communications Systems

Kurzfassung

This thesis examines the transmission of multicarrier systems using Orthogonal Frequency Division Multiplexing (OFDM) modulated signals over wireless communication channels. Frequency and time diversity are the key components which are presented, combined and optimized without any a priori knowledge about the conditions between the transmitter and the receiver. These components are deliberately created and the creation of diversity should comply with different access schemes for a coded multiuser multicarrier system and the capabilities of the receiver. Therefore, diversity methods are implemented that allow to increase frequency and time diversity gradually. The potential diversity is exploited by a decoder at the receiver. In a CDMA system the artificially rotated complex data symbols allow a complex receiver to exploit the diversity even further. The need for artificial diversity results partly from the sectorized antenna arrays in today’s mobile radio systems. Adaptive antenna arrays form a beam towards specific users or a user group. Depending on the velocity of the users, it is possible to track them, and to reduce the interference to other users. Switched beams are used to ease the signal processing requirements and feedback information for tracking mobile users. This reduces intercellular interference by allocating spatially limited areas for different users, or user groups with low complexity. The advantage of focusing the desired signal towards a limited number of users comes at a price. The delay spread and the Doppler spread of the received signal is different from transmissions via an omnidirectional antenna. The channel transfer function of a Rayleigh fading channel shows a smaller delay spread due to the reduced set of transmission paths from the transmitter to the receiver. The Doppler spread still depends on the maximum velocity and its direction of arrival of e.g. the receiver, but it is no longer uniformly distributed [PRR97], [Jak94]. A reduced delay spread and a changed Doppler spread are beneficial, as e.g., the channel transfer function is less selective and easier to estimate. In mobile systems, multipath Rayleigh fading channels are common and slow or flat fading is fatal. Therefore, different concepts are presented to overcome the slow or flat fading by introducing time- and frequencyselective fading. Different frequency concepts enhance the performance together with space-block codes. In highly dynamic scenarios together with multiple receiver antennas increasing frequency diversity accumulates the robustness of the wireless system. We introduce Doppler diversity as an additional source for time diversity. This seems contradictory to the common belief that Doppler spread should be avoided, as it is the source of intercarrier interference. However, this is not necessarily true and depends on the system design, specifically on the subcarrier distance. Avoiding any intercarrier interference leads to the concept of discontinuous Doppler diversity. The discontinuous diversity scheme is transferred to the frequency domain, resulting in flexibility for CDMA and chunk-based access schemes. This concept provides an alternative to the detrimental behavior in line-of-sight conditions. The intention to focus on transmit antennas reduces the complex requirements on the mobile terminal and its power consumption. The reduced power consumption offers new alternatives for the mobile user and the network operator. The user could apply more complex systems that offer higher throughput or the running time of the mobile terminal could be extended. The network operator could use the benefits to reduce the transmission power as this lowers the inter-cellular interference. A reduced interference level allows the operator to take into account more users with the same infrastructure. In summary, this thesis provides a contribution to the application of transmit antennas both for existing standardized and for future mobile radio and broadcast systems.