MIMO Spatial Multiplexing

A multiple-input multiple-output (MIMO) system is a transmission system equipped with multiple antennas both at the transmitter and receiver. It improves transmission efficiency by aggressively exploiting multipath which has been thought of as a factor of signal deterioration, and there has been an increase of research on it as one of the core technologies for future wireless communications all over the world since it appeared in the late 1990s.

Various transmission techniques have been proposed and studied in a MIMO system. In particular, "space time coding (STC)" for higher transmission quality and "spatial multiplexing (SM)" for higher data-rate transmission are the hottest topics among them. Our laboratory mainly focuses on SM. In MIMO-SM, the transmitter spatially multiplexes multiple data substreams by simultaneously transmitting the substreams in parallel according to the number of transmit antennas. The MIMO-SM can be roughly classified into two types: substream transmission per transmit antenna and substream transmission per transmit beam. We refer to the former as "space division multiplexing (SDM)," and a typical case of the latter is "eigenbeam-space division multiplexing (E-SDM)." Simple concepts of both two schemes are shown below. SDM is based on almost the same concept as SDMA; the transmitter sends an independent signal substream from each transmit antenna, and the receiver demultiplexes the received signal by a smart antenna technique. On the other hand, if the transmitter knows the channel state information (CSI), it can weight the transmitted substreams in advance so as to make it easier for the receiver to demultiplex the received signal and to improve the received substream quality. In other words, it is possible also for the transmitter to perform smart antenna processing. In particular, E-SDM is the optimum transmission that enables us to transmit substreams via spatially orthogonal beams, so that the receiver can demultiplex the received signal without any inter-substream interference.

In SDM, since the transmit control is easier than E-SDM, we can reduce the computational load at the transmitter. On the other hand, E-SDM is the optimum transmission in a MIMO system so that theoretically we can obtain the maximum channel capacity.

Concept of SDM.

Concept of E-SDM.

As stated above, a MIMO system is a technique beneficially using multipath. On the other hand, OFDM is an effective countermeasure to multipath because it can efficiently use the given frequency band. The system combining these two techniques, or MIMO-OFDM, has been extensively studied as a promising way to significantly improve transmission efficiency in multipath environments, and is already one of the core technologies for next wireless fields such as wireless LAN.

In our laboratory, studies on substream detection schemes in SM, channel estimation schemes specialized on MIMO-OFDM, schemes controlling transmit beams & resource allocation in E-SDM, etc., have also been advanced. We also conduct MIMO experiments using our own systems to examine the proposed schemes in practice.

Transmission experiment using a MIMO testbed.

Indoor MIMO channel measurement.


T. Ohgane, T. Nishimura, and Y. Ogawa, "Applications of Space Division Multiplexing and Those Performance in a MIMO Channel," IEICE Trans. Commun., vol. E88-B, no. 5, pp. 1843-1851, May 2005.

Y. Ogawa, K. Nishio, T. Nishimura, and T. Ohgane, "Channel and Frequency Offset Estimation for a MIMO-OFDM System," IEEE VTC2004-Fall, vol. 2, pp. 1523-1527, Sept. 2004.

K. Miyashita, T. Nishimura, T. Ohgane, Y. Ogawa, Y. Takatori, K. Cho, "High Data-Rate Transmission with Eigenbeam-Space Division Multiplexing (E-SDM) in a MIMO Channel," IEEE VTC2002-Fall, vol. 3, pp. 1302-1306, Sept. 2002.

H. Nishimoto, Y. Ogawa, T. Nishimura, and T. Ohgane, "Measurement-Based Performance Evaluation of MIMO Spatial Multiplexing in a Multipath-Rich Indoor Environment," IEEE Trans. Antennas and Propag., vol. 55, no. 12, pp. 3677-3689, Dec. 2007.