14.6 Implementation: Realizing the MTMR Potential

In order to realize the bandwidth efficiency potential promised by the use of MTMR technology in multipath environments, a number of practical approaches have been proposed in recent years. These approaches can be naturally grouped in two distinct categories:

Space-time coding schemes, wherein the signals radiated from the various transmit antennas are jointly encoded and must, therefore, be jointly decoded. ^[41], ^[42], ^[43], ^[44], ^[45] These schemes tend to be more robust, but the joint decoding process required for good performance suffers rapid increases in complexity as the number of antennas grows. Additionally, new (vector) coding formats may have to be devised. It appears, however, that both these shortcomings may have remedies. Recent results appear to indicate that conventional (scalar) codes may be used to build good vector codes, ^[46], ^[47] while at the same time some reduced-complexity decoding strategies are emerging. ^[48]
An alternative approach is that of layered architectures, wherein each transmit antenna radiates a separately encoded signal. At the receiver, these signals can be successively decoded and their interference canceled. ^[49], ^[50] The decoding complexity of these architectures increases more gracefully with the number of antennas. Furthermore, they make direct use of existing scalar coding formats. As an added benefit, layered architectures may offer interesting synergies with upper layers on the data communication protocol. ^[51],^[52] These incentives, however, come at the expense of reduced robustness because now each signal must be decoded without support from the others, which are conveying independent data. Furthermore, errors in the detection of each of the signals may propagate through the interference cancellation process and adversely impact the detection of other signals. Chief among these layered architectures is the original Bell Labs layered space-time (BLAST) scheme proposed by Foschini and co-workers ^[53], ^[54] and later refined by other authors. Extensions of the BLAST concept to frequency-selective environments have also been put forth. ^[55] Also, because the detection problem in a layered architecture bears close resemblance to the more-general problem of multiuser detection, the reader is referred also to the abundant literature on this topic. ^[56]

Needless to say, a number of hurdles must be overcome before these new concepts can be widely implemented. First of all, it is necessary to assess the antenna arrangement and spacings that are required, as well as the multipath richness of the environments of interest. In that respect, very encouraging experimental data — both indoor and outdoor — has been surfacing. ^[57], ^[58], ^[59], ^[60], ^[61] Second, the historical opposition to installing multiple antennas on a terminal must be conquered. It is to be expected that terminals requiring increasingly higher throughputs will tend to be naturally larger in size and, as a result, they will offer additional room for multiple, closely spaced antennas.

^[41]Guey, J.-C. et al., Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels, Proc. IEEE Vehicular Technology Conference (VTC'96), Atlanta, 1996, pp. 136–140.

^[42]Tarokh, V., Seshadri, N., and Calderbank, A.R., Space-time codes for high data rate wireless communications: performance criterion and code construction, IEEE Trans. Inf. Theory, 44, 744–765, 1998.

^[43]Hammons, A.R. Jr. and El Gamal, H., On the theory of space-time codes for PSK modulation, IEEE Trans. Inf. Theory, 46 (2), 524–542, 2000.

^[44]Tao, M. and Cheng, R.S., Improved design criteria and new trellis codes for space-time coded modulation in slow flat-fading channels, IEEE Commun. Lett., 5 (7), 313–315, 2001.

^[45]Byun, M.-K. and Lee, B.G., New Bounds of Pairwise Error Probability for Space-Time Codes in Rayleigh Fading Channels, Proc. Wireless Communication and Networking Conference (WCNC'02), March 2002, 89–93.

^[46]Hochwald, B.M. and ten Brink, S., Achieving Near-Capacity on a Multiple-Antenna Channel, Proc. Allerton Conference on Communication, Control, and Computing, Oct. 2001, 815–824.

^[47]Biglieri, E., Tulino, A.M., and Taricco, G., Performance of space-time codes for a large number of antennas, IEEE Trans. Inf. Theory, 48 (7), 1794–1803, 2002.

^[48]Vikalo, H. and Hassibi, B., Maximum-likelihood sequence detection of multiple antenna systems over dispersive channels via sphere decoding, EURASIP J. Appl. Signal Process., Special issue on space-time coding and its applications, Part II, 2002.

^[49]Foschini, G.J., Layered space-time architecture for wireless communications in a fading environment when using multielement antennas, Bell Labs Tech. J., 41–59, 1996.

^[50]Foschini, G.J. et al., Simplified processing for high spectral efficiency wireless communication employing multielement arrays, J. Selected Areas Commun., 17 (11), 1841–1852, 1999.

^[51]Zheng, H., Lozano, A., and Haleem, M., Multiple ARQ Processes for MIMO Systems, 13th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'2000), Lisbon, Portugal, Sept. 15–18, 2002.

^[52]Ariyavisitakul, S.L., Turbo space-time processing to improve wireless channel capacity, IEEE Trans. Commun., 48 (8), 1347–1358, 2000.

^[53]Foschini, G.J., Layered space-time architecture for wireless communications in a fading environment when using multielement antennas, Bell Labs Tech. J., 41–59, 1996.

^[54]Foschini, G.J. et al., Simplified processing for high spectral efficiency wireless communication employing multielement arrays, J. Selected Areas Commun., 17 (11), 1841–1852, 1999.

^[55]Lozano, A. and Papadias, C.B., Layered space-time receivers for frequency-selective wireless channels, IEEE Trans. Commun., 50 (1), 65–73, 2002.

^[56]Verd , S., Multiuser Detection, Cambridge University Press, New York, 1998.

^[57]Kermoal, J. P. et al., Experimental Investigation of Multipath Richness for Multielement Transmit-and-Receive Antenna Arrays, Proc. IEEE Vehicular Technology Conference (VTC'00 Spring), Tokyo, May 2000.

^[58]Martin, C.C., Winters, J. H., and Sollenberger, N.R., Multiple-Input Multiple-Output (MIMO) Radio Channel Measurements, Proc. IEEE Vehicular Technology Conference (VTC'00 Fall), Boston, Sept. 2000.

^[59]Xu, H. et al., Experimental verification of MTMR system capacity in a controlled propagation environment, IEEE Electron. Lett., July 2001.

^[60]Ling, J. et al., Multiple transmit multiple receive (MTMR) capacity survey in Manhattan, IEEE Electron. Lett., 37 (16), 1041–1042, 2001.

^[61]Erceg, V. et al., Capacity Obtained from Multiple-Input Multiple-Output Channel Measurements in Fixed Wireless Environments at 2.5 GHz, Int. Conf. on Communications (ICC'02), New York, Apr. 2002.