Kombination adaptiver und nichadaptiver OFDMA-Übertragungsverfahren unter der Berücksichtigung realistisch erzielbarer Kanalkenntnis
Zusammenfassung der Projektergebnisse
The OFDMA (Orthogonal Frequency Division Multiple Access) transmission scheme is a promising candidate for future mobile radio networks. Besides the beneficial properties concerning implementation and combatting the negative effects of multipath propagation, OFDMA provides an efficient adaptation to the current channel conditions by adaptively allocating the different resources to the different users in time and frequency. Having perfect channel knowledge at the transmitter, adaptive OFDMA schemes achieve very good performances. In case that no channel knowledge is available at the transmitter, the use of non-adaptive OFDMA which does not rely on instantaneous channel knowledge but exploits frequency, time or spatial diversity is the best strategy. Hybrid OFDMA schemes offer the opportunity to use both adaptive and non-adaptive transmission strategies. Besides aspects like different data traffic and Signal-to-Noise-Ratio (SNR) conditions, the mobility of the users is the most crucial criterion to select the applied multiple access scheme since adaptive OFDMA requires accurate channel knowledge which can only be provided in scenarios with slowly moving users. Here, the question arises which multiple access scheme shall be applied to serve the different users and which resources shall be allocated to which user, especially in scenarios where the quality of the channel knowledge differs from user to user. In the literature, the selection of the applied multiple access scheme is based on rather heuristic approaches as the performance of the two access schemes in the presence of imperfect channel knowledge is not analytically derived. Furthermore, the impact of the number of adaptively served users on the achievable system performance due to multi-user diversity is completely neglected allowing to perform an access scheme selection on a user-by-user basis. Therefore, these approaches cannot guarantee that for each user, certain requirements in terms of Bit Error Rate (BER) and minimum data rate are always fulfilled as the performance of each user strongly depends on the exploited multiuser diversity and, thus, on the number of adaptively served users. In this work, the access scheme selection is based on analytical expressions of the achievable data rates for both adaptive and non-adaptive transmissions taking into account imperfect Channel Quality Information (CQl). The access scheme selection in this work is considered for hybrid OFDMA systems applying multiple antenna techniques, namely Orthogonal Space Time Block Coding (OSTBC), Transmit Antenna Selection (TAS) and Maximum Ratio Combining (MRC). Furthermore, different user demands in terms of allocated resources are considered. Finally, also the overhead in terms of pilot transmissions and signaling is taken into consideration. The resources of the adaptive and non-adaptive transmissions are multiplexed in frequency, i.e., different resources in frequency direction are either reserved for non-adaptive or adaptive transmission over several time slots. The aim is to maximize the system data rate while fulfilling a given requirement in terms of BER and user data rate. In this work, the different user demands for the adaptive transmission mode of the hybrid OFDMA system are realized by applying a Weighted Proportional Fair Scheduling (WPFS) where the SNR is applied as CQl assuming either continuous-valued or quantized CQl. To determine the proper WPFS weights for the considered multiple antenna schemes, analytical expressions for the channel access probability are derived and it is shown how to adjust the WPFS weights to meet the different user demands. Next, for both the adaptive and non-adaptive access scheme, analytical expressions for the user data rate and BER are derived taking into account different user demands and imperfect CQl assuming four different sources of error: time delays, estimation errors, quantization and an imperfect feedback link. Furthermore, two types of hybrid adaptive/non-adaptive resource allocation schemes are considered which differ in the order of allocating the resources to the non-adaptive and adaptive users. With the help of the derived expressions, the achievable data rates can be determined as a function of the CQl impairment parameters and the number of adaptively served and non-adaptively served users, respectively. Now, the access scheme selection can be performed such that the system data rate is maximized while the target BER and minimum data rate is fulfilled for each user. Hereby, it has been shown that this combinatorial user serving problem can be solved without testing all possible user serving combinations to find the best solution. In a next step, also the overhead in terms of pilot transmissions and signaling is taken in account. To do so, a frame structure for the hybrid OFDMA system is developed where both Time and Frequency Division Duplex are considered. With the help of these structures, the overhead concerning pilot transmission and signaling is incorporated in the analysis of the system performance. Finally, the performance of hybrid OFDMA systems in a scenario with user-dependent imperfect CQl is evaluated and compared to the performance of conventional pure adaptive or non-adaptive OFDMA systems. It is shown that for low to medium number of active users in the cell, hybrid systems outperform the conventional ones for increasing CQl inaccuracy even if the overhead due to pilot transmissions and signaling is considered. Future work could also consider multiple antenna techniques that employ spatial multiplexing. The spatial domain could be used to increase the data rate of the adaptive access scheme by transmitting several data streams in parallel. Another possibility is to use multiple antennas to spatially multiplex the resources for the adaptive and non-adaptive transmissions, for instance applying a grid of beams where one beam serves rather static users while another beam serves more dynamic users. Furthermore, future work could consider hybrid OFDMA schemes with multi-hop relay transmissions. For all mentioned proposals, the impact of imperfect channel knowledge has to be taken into account in order to select the proper multiple access scheme such that the quality of service constraints are fulfilled for each user.
Projektbezogene Publikationen (Auswahl)
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"Transmit Antenna Selection and OSTBC in adaptive multiuser OFDMA systems with different user priorities and imperfect CQl," Proc. International ITG Workshop on Smart Antennas (WSA '10), Bremen, Germany, February 2010
A. Kühne and A. Klein
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"Combination of adaptive and non-adaptive multi-user OFD-MA schemes in the presence of user-dependent imperfect CSI," in OFDM - Concepts for Future Communication Systems, Hermann Rohling (Eds.), chapter 4, pp. 150-153, Springer-Verlag (2011)
A. Kühne and A. Klein
