Advances in MIMO Techniques for Mobile Communications—A Survey <s> Turbo MUD <s> This paper presents an overview of progress in the area of multiple input multiple output (MIMO) space-time coded wireless systems. After some background on the research leading to the discovery of the enormous potential of MIMO wireless links, we highlight the different classes of techniques and algorithms proposed which attempt to realize the various benefits of MIMO including spatial multiplexing and space-time coding schemes. These algorithms are often derived and analyzed under ideal independent fading conditions. We present the state of the art in channel modeling and measurements, leading to a better understanding of actual MIMO gains. Finally, the paper addresses current questions regarding the integration of MIMO links in practical wireless systems and standards. <s> BIB001 </s> Advances in MIMO Techniques for Mobile Communications—A Survey <s> Turbo MUD <s> Iterative multiuser detection in a single-carrier broadband multiple-input multiple-output (MIMO) system is studied in this paper. A minimum mean squared error (MMSE) low-complexity multiuser receiver is derived for space-division multiple-access (SDMA) space-time trellis-coded (STTrC) systems in frequency-selective fading channels. The receiver uses MMSE filtering to jointly detect several transmit antennas of the user of interest, while the interference from the undetected transmit antennas, cochannel interference (CCI), and intersymbol interference (ISI) are all cancelled by the soft cancellation. The performances of two extreme receiver cases are evaluated. In the first case, only one transmit antenna of the user of interest is detected at a time and the remaining ones are cancelled by soft cancellation. In the second case, all the transmit antennas are detected jointly. The comparison of the two cases shows improvement with the latter one, both in single-user and multiuser communications and in the presence of unknown cochannel interference (UCCI). It is further shown that in the multiuser case, the proposed receivers approach the corresponding single-user bounds. The number of receive antenna elements required to achieve single-user bound is thereby equal to the number of users and not to the total number of transmit antennas. <s> BIB002
An iterative Turbo MMSE MUD scheme is presented in BIB002 for single-carrier (SC) space-time trellis-coded (ST-TC) SDMA MIMO systems in frequency-selective fading channels. This scheme can jointly detects multiple UE transmit antennas while cancelling the MUI from undetected users along with co-channel interference (CCI) and ISI through soft cancellation. Unknown co-channel interference (UCCI) from other interferers not known to the system is also considered. It is also shown that the no. of BS antennas required to achieve the corresponding lower performance bound of single-user detection is equal to the no. of users rather than the total no. of transmit antennas. The receiver derivations in BIB002 are provided for M-PSK modulation but can be extended to QAM as well. Figure 36 shows the system model while the UE transmitter block diagram is given in Figure 37 . The system has a total of simultaneous users, each indexed by BIB001 , K where the first K are the users to be detected at the BS while the remaining I K are unknown interfering users representing the source of UCCI. Each UE is equipped with transmit antennas. However, the system can also support unequal no. of antennas at the UEs. Each UE encodes the bit sequence for using a rate STTC code, where B represents the frame length in symbols. The encoded sequences , are grouped into B blocks of symbols, where represents the modulation alphabet of M-PSK, and then interleaved by user-specific permutation of blocks of length within a frame, such that the positions within each block remain unchanged. This interleaving process preserves the rank properties of the STTC code. User-specific training sequences of length are then attached at the start of the interleaved sequences. After serial-to-parallel (S/P) conversion of the entire frame, the resulting sequences are transmitted over the frequency-selective MIMO channel using the transmit antennas.  which can also be represented as where each vector  where the vectors , , is given by (46) is the AWGN vector with covariance matrix V I . The Turbo receiver block diagram is shown in Figure 38 . For the k-th user, the receiver first associates the signals from the user's transmit antennas to sets of equal size such that the antennas represent the first set and so on. The training sequence is used to obtain an estimate of the channel matrix H. The UCCI-plus-noise covariance matrix R is then estimated. The estimate for the first iteration is given by Two special cases of the proposed receiver architecture are considered in BIB002 . Receiver 1, with 0 1 n , detects the transmit antennas of user k one by one s receiver type has the lowest complexity since the complexity depends exponentially on 0 n . Receiver 2, with and (b) FER performance, (K, K , N ) = (2, 1, 3) Copyright © 2010 SciRes. IJCNS twice the ithm (GA) Assisted MUDs ry of transmit power of a desired user's antenna. The performance of both receivers is obviously better for the 3 dB SIR case. However, the UCCI has a considerable impact on performance and more iterations are needed to achieve a reasonably low SER or FER. The performance will degrade further in case of multiple UCCI antennas and also as the UCCI sources i.e. the no. of unknown interferers increase. .