Ruly Lai-U Choi

New transmit schemes and simplified receivers for MIMO wireless communication systems

In this paper, optimized transmit schemes for multiple-input multiple-output (MIMO) systems with simplified receivers are proposed for the downlink of high-speed wireless communication systems. In these systems, MIMO signal preprocessing is performed at the transmitter or base station with the receiver at the mobile station having a simplified structure that requires only limited signal processing. An important potential application for our transmit MIMO techniques is in the downlink of high-speed wireless communication systems with Vertical Bell Laboratories Layered Space-Time (V-BLAST) or a similar technique utilized in the uplink, creating a high-speed duplex system with a simplified mobile station transceiver structure. Two approaches are introduced and these depend on whether or not receive diversity is employed at the receiver. Both methods require that channel state information be available at the transmitter. In addition, some important associated issues such as peak-to-average power ratio requirements at the transmitter and robustness to downlink channel errors are also investigated and various solutions are proposed. Simulation results are provided and these show that performance improvement can be achieved when compared with other MIMO transmit schemes.

MISO CDMA transmission with simplified receiver for wireless communication handsets

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors.

MIMO CDMA antenna system for SINR enhancement

We present a system to enhance signal-to-interference plus noise ratio (SINR) for multiple-input-multiple-output (MIMO) direct-sequence code-division multiple-access (DS/CDMA) communications in the downlink for frequency-selective fading environments. The proposed system utilizes a transmit antenna array at the base station and a receive antenna array at the mobile station with finite-impulse response filters at both the transmitter and receiver. We arrive at our system by attempting to find the optimal solution to a general MIMO antenna system. A single user joint optimum scenario and a multiuser SINR enhancement scenario are derived. In addition, a simplified one-finger receiver structure is introduced. Numerical results reveal that significant system performance and capacity improvement over conventional approaches are possible. We also investigate the sensitivity of the proposed system to channel estimation errors.

On strategies of multiuser MIMO transmit signal processing

In this letter, we introduce five different strategies of linear transmit signal processing for multiuser multiple-input multiple-output (MIMO) systems and provide performance comparisons in terms of maximum throughput in both uncorrelated and correlated channels when the number of transmit antennas is much larger than the number of receive antennas. It is shown that the multiuser MIMO schemes are preferable to time-division multiple-access (TDMA)-based MIMO schemes, hence demonstrating the power of multiuser MIMO signal processing. Our work also indicates possibilities for future research in finding efficient suboptimal algorithms. As an example, we show that our multiuser MIMO decomposition scheme can improve the maximum throughput compared to TDMA-based MIMO schemes for large number of transmit antennas or high transmit power.

MIMO transmit optimization for wireless communication systems

In this paper we introduce a new transmit and receive structure for multiple input multiple output (MIMO) wireless communications. The major advantage of our structure is that no MIMO signal processing is required at the receiver and therefore a simplified receiver structure is possible. We envisage that our technique could be utilized in the downlink of a wireless communications system, with V-BLAST or similar technique utilized in the uplink, creating a high-speed wireless duplex system with a simplified mobile station transceiver structure. Simulation results reveal that performance is as good or better as receiver based MIMO techniques under realistic conditions.

Effective use of long-term transmit channel state information in multi-user MIMO communication systems

We propose a novel approach to the design of linear transmit processing for the downlink of multi-user multi-input multi-output (MU-MIMO) communication over fading wireless channels. Our purpose is to address the open problem of downlink transmit processing in MU-MIMO systems with only long-term CSI at the transmitter. In contrast to existing schemes, only long-term channel state information is required at the transmitter. For this to work, the MU-MIMO channel must experience correlated fading. The approach consists of two steps: 1) design of transmit processing which can deal with correlated fading channels but is allowed to use full CSI at first; 2) conversion of the transmit processing to use long-term CSI only. As a solution for the first step, we propose a joint receive and transmit mean square error minimization scheme, while the second step is performed by an innovative subspace-based procedure. Performance evaluation shows the significant potential of the proposed approach.

Transmit MMSE pre-RAKE pre-processing with simplified receivers for the downlink of MISO TDD-CDMA systems

In this paper, we introduce an optimised transmit pre-processing technique to reduce multiple access interference in the downlink of TDD-CDMA systems with multiple transmit antennas employed at the base station. In this system, signal pre-processing is performed at the base station, so that a simplified receiver structure that consists of a one-ringer correlator can be utilized at the mobile station. Analytic solution for the optimised transmit pre-processing technique is derived by minimizing the transmit mean square error (MMSE). Numerical results are also provided and these demonstrate significant performance improvement when compared to the conventional RAKE system and the pre-RAKE MRC transmit diversity system.

Joint transmit and receive multi-user MIMO decomposition approach for the downlink of multi-user MIMO systems

We propose a joint transmit and receive decomposition approach for the downlink of multi-user MIMO systems. This approach makes use of the assumption of part of the receiver structure in computing the solution for the transmit processing. With this assumption, this approach decomposes a multi-user MIMO downlink channel into multiple parallel independent single-user MIMO downlink channels. A new degree of freedom is obtained by making the number of data streams variable. Sample simulation results are provided and these show the potential of the proposed scheme.

Frequency domain pre-equalization with transmit diversity for MISO broadband wireless communications

In this paper, we introduce a frequency domain pre-equalization scheme with multiple transmit antennas to combat intersymbol interference (ISI) and fading in the downlink of broadband wireless communications. MIMO processing in the frequency domain is performed at the base station so that the mobile station can have a simplified receiver. The optimal transmit frequency domain coefficient matrix is derived by minimizing the mean square errors (MMSE). Moreover, we derive an expression for the signal to interference-plus-noise ratio (SINR) and error probability based on the Gaussian approximation of the interference term. Improved performance is demonstrated by analysis and simulation results. In particular, it is shown that this pre-equalization scheme can equalize the channel and attain multipath or frequency diversity. Therefore, our proposed scheme can obtain transmit antenna diversity as well as frequency diversity.

MIMO CDMA antenna systems

We investigate the performance of a DS/CDMA system with multiple-input multiple-output (MIMO) antenna processing in the downlink. We formulate the weights for the transmit and receive antennas by trying to obtain the optimum signal-to-interference-plus-noise ratio (SINR) in a frequency selective fading channel. The gain of our MIMO CDMA system with the formulated weights is illustrated by employing computer simulations for various configurations of antennas to determine the average bit error rate (BER) performance. The simulation results reveal that our system can reduce the average BER significantly.