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Multiple Antenna Technologies Multiple Antenna Technologies Manar Mohaisen | YuPeng Wang | KyungHi Chang The Graduate School of Information Technology and Telecommunications INHA University ABSTRACT the receiver. Alamouti code is considered as the simplest transmit diversity scheme while the receive diversity includes maximum ratio, equal gain and selection combining Multiple antenna technologies have methods. Recently, cooperative received high attention in the last few communication was deeply investigated as a decades for their capabilities to improve the mean of increasing the communication overall system performance. Multiple-input reliability by not only considering the multiple-output systems include a variety of mobile station as user but also as a base techniques capable of not only increase the station (or relay station). The idea behind reliability of the communication but also multiple antenna diversity is to supply the impressively boost the channel capacity. In receiver by multiple versions of the same addition, smart antenna systems can increase signal transmitted via independent channels. the link quality and lead to appreciable On the other hand, multiple antenna interference reduction. systems can tremendously increase the channel capacity by sending independent signals from different transmit antennas. I. Introduction BLAST spatial multiplexing schemes are a good example of such category of multiple Multiple antennas technologies proposed antenna technologies that boost the channel for communications systems have gained capacity. much attention in the last few years because In addition, smart antenna technique can of the huge gain they can introduce in the significantly increase the data rate and communication reliability and the channel improve the quality of wireless transmission, capacity levels. Furthermore, multiple which is limited by interference, local antenna systems can have a big contribution scattering and multipath propagation. to reduce the interference both in the uplink Through shaping the antenna radiation and the downlink by employing smart pattern and adaptively adjusting the antenna antenna technology. weight vector, smart antennas improve the To increase the reliability of the communication link quality by increasing communication systems, multiple antennas the received signal power and suppressing can be installed at the transmitter or/and at the interference. Fig. 1. Multiple antenna technologies. Besides, on-line calibration technique is II. MIMO Diversity also adopted to correct the errors due to the distortions and nonlinearity of the radio In communication systems, we have to frequency components in the antenna array increase the reliability of the communication system. operation between transmitter and receiver while maintaining a high spectral efficiency. Fig. 1 summarizes the different multiple The ultimate solution relies in the use of antenna technologies and gives some diversity, which can be viewed as a form of examples of these technologies. redundancy [1]. There are many diversity techniques that can be applied to This paper is organized as follows: in communication systems; we mention herein section II we present multiple antenna time diversity, frequency diversity, and diversity schemes employed at the spatial diversity or any combination of these transmitter or/and at the receiver. Spatial three diversities. In time diversity, the same multiplexing presented by BLAST schemes information-bearing signal is transmitted in is detailed in section III. Section IV is different time slots where a good gain can be dedicated to some advanced multiple input achieved when the duration between the two multiple-output (MIMO) systems including slots, in which the same symbol is multi-user MIMO and cooperative transmitted, is greater than the coherence communications. While techniques related time of the channel. In frequency diversity, to the smart antennas such as phased the same information-bearing signal is antenna array, switched beam antenna array, transmitted on different subcarriers where a and adaptive antenna array are described in good diversity gain can be achieved when Section V. Finally, we conclude in Section the separation between subcarriers is greater VI. than the coherence bandwidth. Finally, in spatial diversity, the same (MS) receiver complexity while improving information-bearing signal is transmitted the detection performance. or received via different antennas where The pioneering work in the transmit the maximum gain can be achieved when diversity was done by Alamouti where he the fading occurring in the channel is proposed his famous 2×1 space-time code. independent (or low correlated). In the Alamouti scheme achieves diversity gain receiver, diversity gain can be achieved by while requiring only a linear decoder. combining the redundant signals arriving Later on, Tarokh et al. proposed a via independent (or lowly correlated) generalized theory of the complex channels. orthogonal space-time codes. Based on Tarokh work, more than two antennas can Fig. 2 shows some possible combinations be used and the code rate can be fractional. of transmit diversity which can be In the following we present the two achieved when employing multiple different types of space-time codes. transmit antennas. 2.1.1 Complex Orthogonal Space- In the following section, we present some Time Codes famous space-time block codes applied at the transmitter side. We present also the For this type of space time codes, the combining techniques used when different following conditions must be satisfied versions of the information-bearing signal are received. Finally, we present a scheme Square transmission matrix (number that includes transmit and receive of transmit antennas Nt equal to diversities. number of used time slots m) A unity code rate (number of used time slots m equals to number of 2.1 Space Diversity at the transmitted symbols l) Transmit Side Orthogonality of the transmission matrix in the time and space The basic idea of the use of transmit domains ( SSHH =S S ) where SH is diversity is to reduce the mobile station the conjugate transpose of S . Fig. 2. Transmit diversity. ⎡ s12s ⎤ S= ⎢ **⎥ ⎣-s21s ⎦ Fig. 3. Alamouti scheme example with QPSK modulation. As said before, the simplest complex At the receiver the following signals are orthogonal space-time code is the received (with applying the complex Alamouti code which uses two transmit conjugate to the received signal at t2) antennas and one receive antenna. Furthermore, Alamouti scheme requires ⎡ yhhsn⎤⎡ ⎤⎡⎤⎡⎤ 11211=+ (1) that the fading channel envelope remains ⎢ ***⎥⎢ ⎥⎢⎥⎢⎥ * ⎣yhhsn22122⎦⎣− ⎦⎣⎦⎣⎦ constant over two time slots. The linear combiner multiplies the Fig. 3 shows an example of the encoding received symbols by the Hermitian process of Alamouti scheme with QPSK transpose of the channel matrix (for modulation [2], [3]. simplicity, we consider that channel is Fig. 4 shows the receiver structure used perfectly estimated). The output of the for decoding the combined received linear combiner is then given by symbols. ⎡x ⎤⎡⎤⎡⎤sw 111=+⎡⎤hh22 + (2) ⎢x ⎥⎢⎥⎢⎥⎣⎦12sw ⎣ 222⎦⎣⎦⎣⎦ Maximum-Likelihood (ML) decoder is then applied to get the transmitted symbols. As one can see, the simplicity of the receiver is due to the spatio-temporal orthogonality of the transmission matrix. A complex orthogonal space-time code hˆ 1 using 4 or 8 antennas was proposed by ˆ Tarokh et al. in [4]. h2 2.1.2 Generalized Complex Orthogonal Space-Time Codes sˆ sˆ 1 2 The search for space-time codes with more Fig. 4. Alamouti code receiver. than two antennas was started by Tarokh, Jafarkhani, and Calderbank. Their work has built the basis for a theory of frequency-Time domains. These coding generalized complex orthogonal designs. schemes are thus known as ST, SF, and Generalized complex orthogonal designs STF coding, respectively [7]. are distinguished from Alamouti code by the following 2.1.3 Cyclic Delay Diversity (CDD) A non-square transmission matrix CDD can be considered as a very simple (number of used time slots ≠ number transmit diversity scheme. CDD can of Tx antennas) achieve transmit artificial frequency A fractional code rate (number of diversity by selecting appropriate transmit transmitted symbols < number of delays. In this method, multiuser diversity, used time slots) obtained by scheduling based on Orthogonality of the transmission frequency domain channel response, can matrix is only guaranteed in the time be improved by adjusting the delay spread sense. (at the transmitter) which is done by controlling the delay values dependent on As a consequence of these characteristics, the channel condition [8], [9]. the spectral efficiency is reduced and the number of time slots over which the 2.2 Space Diversity at the channel should be constant is increased. Receive Side The transmission matrix of a generalized In space diversity at the receive side, complex space-time code with 3 antennas, multiple antennas are used in the receiver 4 transmitted symbols and 8 used time with sufficient spacing between antennas slots is given by [5] in such a way mutual correlation between antennas is reduced and as consequence ⎡⎤s13s 2 s diversity gain is increased [10]. To get ⎢⎥−−s s s diversity gain at the receiver, received ⎢⎥241 signals from different antennas are ⎢⎥−s31s 4 s ⎢⎥ combined. There are four combining −s −s s G = ⎢⎥423 (3) methods, namely, select combining (SC), 3 ⎢⎥*** s13s 2 s maximal-ratio combining (MRC), equal- ⎢⎥*** ⎢⎥−−s 24s1 s gain combining (EGC), and square-law
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