Switching Between Diversity and Multiplexing
Wireless communication systems work better when the transmitter can respond to changes in the propagation channel. In a system with a single transmit and receive antenna this often means adjusting the rate or the power of the transmitted signal. In a multi-antenna system it is also possible to adjust the spatial mode. In particular, it is possible to switch between diversity modes (sending a single reliable data stream) and multiplexing modes (sending multiple streams for higher data rates). We were among the first to recognize the importance of switching between diversity and multiplexing modes of operation.
The main idea is as follows. In the late 1990’s there were two competing approaches to communication in a MIMO wireless channel. The first was known as transmit diversity or space-time coding. The idea here was to send redundant information across all the transmit antennas to improve the quality in the received signal. This is a diversity scheme because it exploits the diversity of possible transmission paths between transmitter and receiver to improve reliability. The other scheme proposed for MIMO communication was called spatial multiplexing. The idea in spatial multiplexing is to send an independent data stream on each antenna thereby effectively sending multiple data streams in parallel. This approach allows the communication system to transmit data generally at a higher rate but typically less reliably than with transmit diversity. In our early work, we proposed the simple idea of switching between transmit diversity and spatial multiplexing based on the channel state. This has the benefits of both reliable signal transmission and high data rates.
In the past several years we have enhanced the concept of switching between diversity and multiplexing in a number of ways. For example, we combined this approach with limited feedback. The ideas is to dynamically adjust the number of transmitted streams based on the channel. When the channel could support only one stream, the transmitter would use beamforming and the receiver would indicate which beamforming vector to use via the limited feedback beamforming framework I developed. When the channel could support additional modes, this would be signaled to the transmitter along with appropriate limited feedback information. This work, called multi-mode precoding, allows the communication link to operate near the theoretical link capacity while at the same time only requires low receiver processing complexity. In later work we investigated the idea of switching between diversity and multiplexing based on spatial correlation information. This has the advantage of reducing the amount of overhead signaling required since the correlation of the channel varies more slowly than the channel itself.
C. B. Chae, A. Forenza, R. W. Heath, Jr., M. R. McKay, and I. B. Collings, “Adaptive MIMO Transmission Techniques for Broadband Wireless Communication Systems,” IEEE Communications Magazine, May 2010, pp. 112-118.
This paper reviews a framework for MIMO adaptation based on choosing the spatial mode on spatial correlation information, while choosing the rate based on instantaneous channel state information. In this way, the number of data streams varies more slowly than the rate on the streams. The paper is based in part on a tutorial given at several major conferences.
H. Kim, C. B. Chae, G. de Veciana, and R. W. Heath, Jr., “A Cross-Layer Approach to Energy Efficiency for Adaptive MIMO Systems Exploiting Spare Capacity,” IEEE Trans. on Wireless, vol. 8, no. 8, pp. 4264-4275, August 2009.
This is recent work that proposes to switch between MIMO and SIMO modes of operation, on the uplink, to save RF energy, including circuit power in the optimization. The motivation is that multiple transmit antenna RF chains consume more power than a single chain. When the system is lightly loaded (has spare capacity) it is good to send to users at potentially lower rates to save energy.
A. Forenza, M. R. McKay, A. Pandharipande, R. W. Heath, Jr., and I. B. Collings, “Adaptive MIMO Transmission for Exploiting the Capacity of Spatially Correlated Channels,” IEEE Trans. on Veh. Tech., vol. 56, no. 2, pp. 619-630, March 2007.
This paper proposes spatial mode adaptation based on spatial correlation. The idea is to show how different transmission strategies like space-time block coding, hybrid transmission, and spatial multiplexing, all have a different dependence on the transmit spatial correlation matrix. Depending on the eigenvalues of the spatial correlation matrix, switching points can be derived as a function of SNR that show when different transmission techniques are optimum.
M. R. McKay, I. B. Collings, A. Forenza, R. W. Heath, Jr., “Multiplexing / Beamforming Switching for Coded-MIMO in Spatially-Correlated Rayleigh Channels,” IEEE Trans. on Veh. Tech., vol. 56, no. 5, part 1, pp. 2555-2567, Sept. 2007.
This paper derives criteria for switching between diversity and multiplexing in systems that employ bit interleaved coded modulations. Of particular interest, this paper uses various accurate approximations for the coded bit error rate to derive the switching points.
N. Khaled, B. Mondal, R. W. Heath, Jr., G. Leus, and F. Petre, “Interpolation- Based Multi-Mode Precoding for MIMO-OFDM Systems with Limited Feedback,” IEEE Trans. on Wireless, vol. 6., no. 3, pp. 1003-1013, March 2007.
In this paper we established the concept of multi-mode precoding for MIMO-OFDM systems. The idea is to send back quantized versions of the right singular vectors and the preferred modes separately. Then the right singular vectors can be interpolated at the transmitter followed by mode selection. The proposed approach gives very good error rate performance with little feedback overhead.
Here we the concept of multi-mode limited feedback precoding. The idea is to have multiple limited feedback precoding codebooks, each for a different number of modes (one-stream, two-stream, etc). Suboptimum selection algorithms were proposed to enable selecting the number of spatial modes first, followed by the appropriate codeword index.
R. W. Heath, Jr. and D. J. Love, “Multi-Mode Antenna Selection for Spatial Multiplexing with Linear Receivers,” IEEE Trans. on Signal Processing, vol. 53, no. 8, part 2, pp. 3042-3056, August 2005.
This paper introduces the concept of multi-mode antenna selection. The idea is to use antenna subset selection as a form of limited feedback precoding. The number of spatial modes, and the mapping of streams to antenna, are communicated from the receiver to the transmitter via a low rate feedback link. This paper shows that it is possible to achieve full diversity and near capacity performance with just N bits of feedback where N is the number of transmit antennas.
R. W. Heath, Jr. and A. J. Paulraj, “Switching Between Diversity and Multiplexing in MIMO Systems,” IEEE Trans. on Communications, vol. 53, no. 6, pp. 962-968, June 2005.
This is the original paper on switching between multiplexing and diversity. Don’t be deceived by the date, the original manuscript was submitted in 2001. The idea here is to switch between spatial multiplexing (2 streams) and space-time block coding (1 stream) with only a single bit of feedback. Some analysis of switching thresholds is provided, motivating the use of the Demmel condition number as a metric for MIMO channel quality.
S. Catreux, V. Erceg, D. Gesbert, and R. W. Heath, Jr., “Adaptive Modulation and MIMO Coding for Broadband Wireless Data Networks,” IEEE Communications Magazine, pp.108-115, June 2002.
This paper describes the key ideas behind the link adaptation algorithm developed at Iospan Wireless for one of the first MIMO-OFDM wireless systems. The insight was that the mean and variance of the post-processing SNR in the frequency domain could be used to build lookup tables for choosing between diversity and multiplexing with different amounts of spatial and frequency selectivity. Realistic system level simulations show that MIMO provides spectral efficiency improvements even in interference limited cellular systems.
Received the best student paper award (given to A. Forenza and M. R. McKay) for the paper:
A. Forenza, M. R. McKay, R. W. Heath, Jr., and I. B. Collings, “Switching between OSTBC and spatial multiplexing with linear receivers in spatially correlated MIMO channels,” Proc. of the IEEE Vehic. Tech. Conference, vol. 3, pp. 1387-1391, Melbourne, Australia, 7-10 May 2006.
We have been fortunate to have several sponsors of our work including the Office of Naval Research and the National Science Foundation. We have also had several industrial sponsors in the past including Motorola, Samsung, and Freescale. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of any sponsors of this work.