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Multifunctional Mimo Systems: a Combined Diversity and Multiplexing Design Perspective HANZO LAYOUT 4/8/10 12:55 PM Page 73 ACCEPTED FROM OPEN CALL MULTIFUNCTIONAL MIMO SYSTEMS: A COMBINED DIVERSITY AND MULTIPLEXING DESIGN PERSPECTIVE MOHAMMED EL-HAJJAR AND LAJOS HANZO, UNIVERSITY OF SOUTHAMPTON ABSTRACT in demand for high-speed multimedia wireless X services. Advances in channel coding made it W 11 A In this treatise we investigate the design alter- feasible to approach Shannon’s capacity limit in natives of different multiple-input multiple-out- systems equipped with a single antenna [1], but put schemes while considering the attainable fortunately these capacity limits can be further X diversity gains, multiplexing gains, and beam- extended with the aid of multiple antennas. W 1LAA forming gains. Following a brief classification of Recently, multiple-input multiple-output Beamformer different MIMO schemes, where the different (MIMO) systems have attracted considerable MIMO schemes are categorized as diversity research attention and it is considered as one of techniques, multiplexing schemes, multiple the most significant technical breakthroughs in AA m1 access arrangements, and beamforming tech- contemporary communications. niques, we introduce the family of multifunction- Explicitly, MIMO schemes can be categorized al MIMOs. These multifunctional MIMOs are as diversity techniques, multiplexing schemes, AA (Nt-mK) capable of combining the benefits of several multiple access methods, and beamforming as MIMO schemes and hence attaining improved well as multifunctional MIMO arrangements, as performance in terms of both their bit error rate shown in Fig. 1. Diversity is the technique where X as well as throughput. The family of multifunc- the receiver receives several replicas of the same WN 1 tional MIMOs combines the benefits of both transmitted signal, while assuming that at least space-time coding and the Bell Labs layered some of them are not severely attenuated. Spa- The authors space-time scheme as well as those of beam- tial diversity can be attained by employing multi- forming. We also introduce the idea of layered ple antennas at the transmitter or the receiver. investigate the steered space-time spreading, which combines Multiple antennas can be used to transmit and design alternatives of the benefits of space-time spreading, V-BLAST, receive appropriately encoded replicas of the and beamforming with those of the generalized same information sequence in order to achieve different multiple- multicarrier direct sequence code-division multi- diversity and hence obtain an improved bit error ple access concept. Additionally, we compare the rate (BER) performance. On the other hand, input multiple-output attainable diversity, multiplexing, and beamform- multiplexing techniques were designed in order ing gains of the different MIMO schemes in to improve the attainable spectral efficiency of schemes while order to document the advantages of multifunc- the system by transmitting the signals indepen- tional MIMOs over conventional MIMO dent of each of the transmit antennas. In the considering the schemes. context of diversity and multiplexing techniques, attainable diversity the antennas are spaced as far apart as possible CLASSIFICATION OF MULTIPLE-INPUT so that the signals transmitted to or received by gains, multiplexing the different antennas experience independent MULTIPLE-OUTPUT SYSTEMS fading, and hence we attain the highest possible gains, and beam- diversity or multiplexing gain. Additionally, mul- Recently, there has been an urgent demand for tiple antennas can also be used in order to forming gains. flexible and bandwidth-efficient transceivers improve the signal-to-noise ratio (SNR) at the capable of supporting the explosive expansion of receiver or the signal-to-interference-plus-noise the Internet and the continued dramatic increase ratio (SINR) in a multi-user scenario. This can be achieved by employing beamforming tech- niques, where the antenna gain is increased in The financial support of Vodafone under the auspices of the direction of the desired user, whilst reducing the Dorothy Hodgkin postgraduate award and that of the the gain towards the interfering users. European Community under Seventh Framework Pro- A simple spatial diversity technique, which gramme grant agreement ICT OPTIMIX nINFSO-ICT- does not involve any loss of bandwidth, is consti- 214625 is gratefully acknowledged. tuted by the employmexnt of multiple antennas IEEE Wireless Communications • April 2010 1536-1284/10/$25.00 © 2010 IEEE 73 HANZO LAYOUT 4/8/10 12:55 PM Page 74 at the receiver, where several techniques can be alize the concept of transmit diversity schemes employed for combining the independently fad- to more than two transmit antennas, contriving ing signal replicas. In case of narrowband fre- the generalized concept of orthogonal space- quency-flat fading, the optimum combining time block codes (OSTBCs). The family of OST- strategy in terms of maximizing the SNR at the BCs is capable of attaining the same diversity combiner output is maximum ratio combining gain as space-time trellis codes (STTC) [4] at (MRC). Additionally, other combining tech- lower decoding complexity when employing the niques have been proposed in the literature, as same number of transmit antennas. However, a shown in Fig. 1, including equal gain combining disadvantage of OSTBCs when compared to (EGC) and selection combining (SC). All three STTCs is that they employ unsophisticated repe- combining techniques are said to achieve full tition coding and hence provide no coding gain. diversity order, which is equal to the number of Furthermore, inspired by the philosophy of receive antennas. STBCs, Hochwald et al. [6] proposed the trans- On the other hand, several transmit — rather mit diversity concept known as space-time than receive — diversity techniques have also spreading (STS) for the downlink of wideband been proposed in the literature [2–5], as shown code-division multiple access (WCDMA) that is in Fig. 1. In [2] Alamouti proposed a witty trans- capable of achieving the highest possible trans- mit diversity technique using two transmit anten- mit diversity gain. nas, the key advantage of which was the Regretfully, the OSTBC and STS designs of employment of low-complexity single-receive- [3, 6] contrived for more than two transmit antenna-based detection, which avoids the more antennas result in a reduction of the achievable complex joint detection of multiple symbols. The throughput per channel use when complex-val- decoding algorithm proposed in [2] can be gen- ued constellations are used. An alternative idea eralized to an arbitrary number of receive anten- invoked for constructing full-rate STBCs for nas using MRC, EGC, or SC. Alamouti’s complex-valued modulation schemes and more achievement inspired Tarokh et al. [3] to gener- than two antennas was suggested in [7]. Here the strict constraint of perfect orthogonality was relaxed in favor of achieving a higher data rate. The resultant STBCs were referred to as quasi- MIMO techniques orthogonal STBCs [7]. The OSTBC and STS designs offer — at best — the same data rate as an uncoded single- Diversity techniques antenna system, but they provide improved BER performance compared to the family of single- Receive diversity antenna-aided systems, since they attain a diver- Maximum ratio combining (MRC) sity gain. In contrast to this, several high-rate space-time transmission schemes having a nor- Equal gain combining (EGC) malized rate higher than unity have been pro- Selection combining (SC) posed in the literature. For example, high-rate space-time codes that are linear in both space Transmit diversity and time, the family of so-called linear disper- sion codes (LDCs), was proposed in [5]. LDCs STBC strike a flexible trade-off between emulating STS space-time coding and/or spatial multiplexing. STTC The attractive concept of LDCs invokes a matrix- based linear modulation framework, where each LDC space-time transmission matrix is generated by a Quasi-orthogonal STBC linear combination of so-called dispersion matri- ces, and the weights of the components are determined by the transmitted symbol vector. Multiplexing techniques OSTBCs and STTCs are capable of providing an attractive diversity gain for the sake of BLAST improving the achievable system performance. However, this BER performance improvement is Multiple access techniques often achieved at the expense of a rate loss, since OSTBCs and STTCs may result in a SDMA throughput loss compared to single-antenna- Beamforming techniques aided systems. As a design alternative, a specific class of MIMO systems was designed for improv- ing the attainable spectral efficiency of the sys- Beamformers designed for SNR gain tem by transmitting different parallel signal Beamformers designed for interference suppression streams independently over each of the transmit antennas, hence resulting in a multiplexing gain. Beamforming techniques This class of MIMOs subsumes Bell Labs’ lay- ered dpace-yime (BLAST) scheme and its rela- LSSTC tives [8]. The BLAST scheme aims to increasie LSSTS the system throughput in terms of the number of bits per symbol that can be transmitted in a given bandwidth at a given integrity. Figure 1. Classification of MIMO techniques. In contrast to the family of BLAST schemes, 74 IEEE Wireless Communications • April 2010 HANZO LAYOUT 4/8/10 12:55 PM Page 75 where multiple antennas are activated by a sin- gle user for increasing the user’s throughput, space-division multiple access (SDMA) [9] X W11 employs multiple antennas for the sake of sup- AA1 porting multiple users. SDMA exploits the unique user-specific channel impulse responses X (CIRs) of different users to separate their W received signals. 1LAA On the other hand, in beamforming arrange- Beamformer DOA B1 1 ments [9] typically λ/2-spaced antenna elements Rx1 are used for the sake of creating a spatially STC AA Rx2 selective transmitter/receiver beam, where λ rep- m1 resents the carrier’s wavelength. Beamforming is B employed for providing angular receiver selectiv- LSSTC AA decoder ity by mitigating the effects of various interfering (Nt-mK) signals, provided that they arrive from sufficient- Serial-to-parallel converter RxN ly different angular directions.
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