The 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’06) HYBRID MIMO TRANSCEIVER SCHEME WITH ANTENNA ALLOCATION AND PARTIAL CSI ATTRANSMITTER SIDE W. C. Freitas Jr. F. R. P. Cavalcanti R. R. Lopes Nokia Technology Institute Wireless Telecom Research Group Communications Department Bras´ ılia-DF, Brazil Fortaleza-CE, Brazil Campinas-SP, Brazil ext-walter.freitas@nokia.com rodrigo@gtel.ufc.br rlopes@decom.fee.unicamp.br ABSTRACT Hybrid multiple-input multiple-output (MIMO) transceiver scheme (HMTS) combines transmit diversity and spatial multiplexing, thus achieving at the same time the two possible spatial gains offered by MIMO channels. In the design of HMTS spatial diversity and spatial multiplexing branches are disposed in parallel in order to achieve diversity and multiplexing gains at the same time. Since the spatial multiplexing branches have no protection, they are more susceptible to the fading effect becoming the bottleneck in the performance of the whole transceiver. In this paper, we propose a solution to this bottleneck in the hybrid MIMO transceiver scheme using a partial channel state information at the transmitter side. The idea is to perform an antenna allocation. Thus, the most powerful subchannels are allocated to the most susceptible layers (spatial multiplexing branches). Through this solution we decrease the performance imbalance between the two layers of the HMTS G3+1, increasing the whole transceiver performance with low complexity feedback requirements. I. I NTRODUCTION The use of the multiple antennas has proliferated now in wireless systems as a possible solution to the capacity limitation of the current wireless systems. With the use of multiple antennas over certain scenarios we can achieve an increase in the capacity almost linear with the number of antennas [1]. The idea is that the use of multiple antennas creates a multiple-input multiple-output (MIMO) linear system in which the MIMO channel linking the transmitter and receiver antennas can be seen as multiple single-antenna subchannels with no additional power consumption, time transmission and bandwidth. These multiple subchannels can be separated through their spatial signatures in an environment rich in multipaths. Another well-known advantage of multiple antennas is in providing spatial diversity through the multiple links created by the multiple antennas. The idea is that with multiple links there exists a lower probability that all of them experiment deep fading. More specifically, if there are M transmit and N receive antennas, generically denoted as (M Tx-N Rx), with sufficient signal scattering and antenna spacing, there are MN independent links between the transmitter and the receiver. In this situation it is possible to provide an MN -fold protection against channel fading. This protection is called diversity gain and the number of independent links is the diversity order. On the other hand, there are min(M,N ) degrees of freedom, which can be used to spatially multiplex data for increase spectral efficiency. This gain in multiplexing symbols through the MIMO wireless channel is known as spatial multiplexing gain. MIMO schemes are known to provide these two main types of gains: spatial multiplexing gain and diversity gain. Spatial multiplexing gain describes the higher data rates that can be obtained using the spatial subchannels created by the MIMO channel. An example of a pure multiplexing scheme is the vertical Bell laboratories layered space-time (VBLAST) [1]. On the other hand, pure diversity schemes, like space-time block codes (STBC) [2, 3], are concerned with diversity gain. In other words, their objective is to increase the link reliability against fading. Apart from these two gains, it is also possible to achieve coding gain, as in the case of space-time trellis codes (STTC). This topic will be left to a future investigation. In this article, we focus on the spatial multiplexing and diversity gains. It was shown in [4] that there exists a tradeoff between these two gains in the MIMO wireless channel. Zheng and Tse have shown that when one tries to maximize one possible gain of the MIMO wireless channels, this leads to a degradation of the other gain. For example, space-time block codes (STBC), well-known schemes in providing diversity gain, have no concerns about the capacity gain. On the other hand, vertical Bell laboratories layered space-time (VBLAST) schemes were designed aiming multiplexing as many different symbols as possible, but does not provide any diversity gain. Zheng and Tse just characterized the tradeoff, not proposing any scheme capable of achieving it. A solution in this direction was proposed with a modification in the VBLAST scheme, called Diagonal BLAST [1], in which the transmitted symbols are multiplexed in all the transmit antennas available, but in different time instants. Unfortunately, this solution brings a considerable delay in order to achieve a diversity gain, and thus is not very practical. Tarokh et al. in [5] presented the idea of combining array processing in the receiver and space-time coding strategies through multiple transmit antennas to reach at the same time reliable and very high data rate communication. Here we present a similar structure to that in [5], denoted Hybrid MIMO transmission schemes (HMTS). HMTS arise as a solution to jointly achieve spatial multiplexing and diversity gains. With HMTS, it is possible to considerably increase the data rate while keeping a satisfactory link quality in terms of bit error rate (BER). In fact, HMTS apply pure diversity schemes (e.g. STBC) jointly with pure spatial multiplexing schemes (e.g. VBLAST), so that parts of the data are space-time coded across 1-4244-0330-8/06/$20.00 c 2006 IEEE