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:
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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]
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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.
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