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The performed complexity evaluation studied the computational requirements of the most relevant advanced receivers applying multiple receive antennas. The obtained arithmetic operation rates closely followed the actual algorithms used in the link simulations. Utilisation of channel equalisation is a key factor in applying higher-order modulation and MIMO transmission techniques. The results indicated very high relative complexities for multi-antenna equalisers when the channel delay spread is high.
A conclusion is that two receive antennas in a mobile terminal should be considered as maximum in the near future, while more than two antennas may become feasible after receiver processing power and battery technology have improved. This fact should reflect to future development of 3G and later standards; the performance requirements set for the terminal receivers in different environments should be a compromise between user equipment cost and performance. The potential of MIMO channels in terms of capacity is well known but it cannot be utilised efficiently in many practical environments without very complex receivers. The results also indicate that new air interface techniques, such as OFDM, which has been proposed for future frequency bands, may decrease the receiver complexity when compared to WCDMA.
Nokia Mobile Phones has emphasised in its studies receiver implementability in addition to performance. Several practical algorithms have been developed and simulated taking into account aspects like available processing power and limitations set by the standard.
One of the studied advanced algorithms has been implemented in VHDL in Nokia Mobile Phones to obtain accurate complexity and power consumption figures. This has improved the companys knowledge of practical implementation of complex advanced algorithms. Recently, an invention report has been written about a new MIMO receiver and its patenting is under consideration in Nokia.
A full range of multi-antenna techniques for application to HSDPA have been studied. First of all the link-level performance was evaluated by Nokia Mobile Phones and Technical University of Catalonia (UPC) and these results used within the comprehensive, dynamic system simulator developed by Vodafone. Importantly the complexity of different algorithms has been studied, which provides an important factor in determining the applicability of the different algorithms studied. These results will be disseminated widely within Vodafone to both the R&D organisation, global technology team and the operating companies and allow Vodafone to define a strategy in relation to this evolution path of HSDPA. The results will help us determine our position in standardisation fora and if appropriate Vodafone will make contributions to drive the standardisation process in its preferred direction. Furthermore this evaluation allows Vodafone to drive its vendor (both infrastructure and terminal) roadmaps in a direction in alignment with its strategy.
Several multi-antenna receiver algorithms and transmission techniques have been simulated by following the current High-Speed Downlink Packet Access specifications.
The tested methods include both standardised and recently proposed transmission schemes using 1, 2 3 or 4 transmit antennas and receiver algorithms utilising 1, 2 or 4 antennas. For example, altogether four different dual-antenna receivers have been implemented for V-BLAST HSDPA transmission. In many cases, the simulations have been done by using complete HSDPA simulator including actual channel estimation, actual packet retransmission with hybrid-ARQ combining, channel coding and antenna correlations. An essential part of the simulation campaign has been post-processing of the link simulation results so that they have been usable in system simulations. Due to packet transmission and scheduling, the final conclusion can be made only based on system level results.
The results indicate large potential performance capacity improvement when advanced receivers are used with or even without utilising MIMO techniques. Both system capacity and, even more significantly, user peak data rates increase due to advanced multi-antenna techniques. Parts of the results have already been submitted to international conferences. Currently, the results have been and are being disseminated also within Nokia where they already have had an impact on the company's standardisation strategy.
I-METRA has contributed to the common framework developed by the Broadband Air Interface cluster and participated in the process of identifying projects studying similar scenarios. This framework and process will allow comparison of different proposals against standard measures. The results of this work will be important input to the on-going evaluation of options for systems beyond 3G, be that an evolution of existing systems or the development of a new air interface. This will both inform the wider research community (for example through the BAI cluster workshop in December 9th 2003) and dissemination within the industrial partners.
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