This paper studies the optimal use of limited-rate feedback of channel state information (CSI) in the case of a wideband multicarrier channel with multiple transmit antennas and a single receive antenna. With full knowledge of the CSI, the receiver should compute the optimal beamforming vectors, jointly quantize them, and feed them back to the transmitter. The achievable forward data rate depends on the rate of the feedback link. The optimal tradeoff between the forward and feedback rates is characterized using rate distortion theory in the limit of infinite number of subcarriers with fixed amount of feedback per subcarrier. The distortion metric is the difference between the forward rate achieved with limited feedback and the capacity with perfect CSI at the transmitter. The rate distortion function gives the forward rate as a function of the feedback rate. Numerical results show that to achieve a target forward rate, the required feedback rate can be substantially reduced by joint quantization of the beamformers across subcarriers. A simple quantizer amenable to practical implementation is shown to approach the rate distortion bound with near-linear computational complexity.

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base transceiver stations (BTSs) so that the signals superpose coherently at each receiver, which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission scheme for the downlink is studied, which does not require phase alignment. The focus is on jointly serving two users in adjacent cells sharing the same resource block. The two BTSs partially share their messages through a backhaul link, and each BTS can transmit a superposition of two codewords, one for each receiver. Each receiver decodes its own message, and treats the signals for the other receiver as background noise. With narrowband transmissions the achievable rate region and maximum achievable weighted sum rate are characterized by optimizing the power allocation (and the beamforming vectors in the case of multiple transmit antennas) at each BTS between its two codewords. For a wideband (multicarrier) system, a dual formulation of the optimal power allocation problem across sub-carriers is presented, which can be efficiently solved by numerical methods. Results show that the proposed cooperation scheme can improve the sum rate substantially in the low to moderate signal-to-noise ratio (SNR) range.

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base stations (BSTs), which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission scheme for the downlink is studied, which does not require phase alignment. We consider two adjacent cells each with a single user, and assume that the BSTs share their messages through a dedicated link. Each BST transmits a superposition of two codewords, one for each receiver. Each receiver decodes its own message, and treats the signals for the other receiver as background noise. With narrowband transmissions the achievable rate region and maximum achievable weighted sum rate are characterized by optimizing the power allocation at each BST between its two codewords. For a wideband (multicarrier) system, a dual formulation of the optimal power allocation problem across subcarriers is presented, which admits efficient numerical solution. Results show that the proposed cooperation scheme can improve the sum rate substantially at low to moderate signal-to-noise ratios.

Transmitter preceding is a crucial technique for harnessing the potential of multiple-input multiple-output (MIMO) fading channels. In many practical wireless systems, a limited amount of feedback from the receiver is available at the transmitter, which can be used to direct the choice of the precoder from a codebook to match the channel state. Assuming noiseless, limited-rate feedback, this work studies the design of simple, efficient quantization and feedback schemes which achieve near-optimal ergodic channel capacity. In the case the precoder takes the form of a beamforming vector for modulating a single symbol stream, it is found that simple scalar quantization of the elements of the vector is nearly optimal over a wide range of feedback rates; it typically costs a fraction of a dB higher SNR to achieve the same capacity as that of far more sophisticated vector quantization schemes. In the case a precoding matrix consisting of multiple beams is used to modulate multiple symbol streams, separate encoding of the beams using scalar quantization also performs well. Roughly speaking, the rate loss due to separate encoding of the beams increases linearly with the number of beams but appears to be constant over a wide range of SNRs. The loss can be reduced substantially by more sophisticated encoding of each beam, e.g., two-state trellis coded quantization. The complexity of such quantization schemes is linear in the number of antennas and the number of feedback bits.

The achievable rate of a wideband multi-input single-output channel with multi-carrier transmission is studied with limited feedback of channel state information (CSI). The set of sub-channel vectors are assumed to be jointly quantized and relayed back to the transmitter. Given a fixed feedback rate, the performance of an optimal joint quantization scheme can be characterized by the rate-distortion bound. The distortion metric is the average loss in capacity (forward rate) relative to the capacity with perfect channel state information at the transmitter and receiver. The corresponding rate distortion function gives the forward capacity as a function of feedback rate, and is determined explicitly by casting the minimization of mutual information in the rate-distortion problem as an optimal control problem. Numerical results show that when the feedback rate is relatively small, the rate-distortion bound significantly outperforms separate quantization of the state information of each sub-channel.

In this paper, we propose two non-orthogonal approaches to design linear transmit preceding matrices for the downlink of multi-user multi-input multi-output (MU-MIMO) communication over fading wireless channels. Both are better suited to solve the power control problem of minimizing the power output subject to a fixed set of rate requirements and do not have the strict constraint on the transmit and receive antenna numbers as the block-diagonalization (BD) algorithm does. One is named modified MRC scheme in which an iterative approach is introduced and for each user we compute the optimal preceding matrix to compensate the noise and the interference caused by other users without considering the interference it causes to others. The other one is named modified BD scheme in which for each user we only consider the dominant sub-channels that have relatively large channel gains when using the conventional BD algorithm. Theoretic analysis and simulation results reveal that the modified MRC scheme can achieve a better performance than BD algorithm under lower SNR regime and the modified BD scheme outperforms BD algorithm almost in all cases

In this paper, we aim to design a set of linear transmitter preceding vectors for the downlink of multiuser MIMO system, using the maximal signal to jamming and noise ratio (SJNR) criterion for each user, subject to a transmit power constraint. Unlike some conventional methods, the proposed scheme does not impose any restrictions on system configuration, i.e., the number of antennas at the transmitter and receivers. Furthermore, this new algorithm can mitigate the co-channel interference (CCI) and noise at the same time. Both analysis and simulation results have shown the superiority of the proposed method over conventional transmit precoders.

In this paper, we address the problem of designing jointly the linear transmitter and receiver for the downlink of multiuser MIMO system, using minimum the total mean square error criterion (T-MMSE), subject to a total transmit power constraint. We show that the optimum transmitter and receiver under such design criterion could be realized through a joint iterative algorithm. The convergence of the proposed algorithm is proved. The analysis and simulation results have both shown the feasibility of this new method.