Joint Scheduling and Precoding Scheme in MU-MIMO System with Limited Feedback

We present a generalized singular value decomposition (GSVD) based multi-user (MU) multi-stream multiple-input multiple-output (MIMO) precoding scheme with limited feedback. Our precoding scheme maximizes the signal to leakage and noise ratio (SLNR), so that it does not require any iterations to obtain precoding matrices. However, in MU-MIMO system, the precoding matrix should be computed at transmit side, because the precoding matrix of a user affects those of other users. Accordingly, full channel state information (CSI) for all users is required at the transmitter, which prevents MU-MIMO precoding implementable. To overcome this problem, an efficient limited feedback algorithm optimized to the GSVD based MU-MIMO precoding is proposed. Here, by extracting the parameters to be utilized in GSVD based precoding from the full CSI, the amount of feedback information is reduced. Furthermore, by analyzing their statistical properties, the parameters are efficiently quantized according to their probability densities. Finally, we develop a modified GSVD based precoding considering the channel quantization errors due to the limited feedback.

Joint linear beamforming and scheduling are performed in a system where limited feedback is present at the transmitter side. The feedback conveyed by each user to the base station consists of channel direction information (CDI) based on a predetermined codebook and a scalar metric with channel quality information (CQI) used to perform user scheduling. In this paper, we present a design framework for scalar feedback in MIMO broadcast channels with limited feedback. An approximation on the sum rate is provided for the proposed family of metrics, which is validated through simulations. For a given number of active users and average SNR conditions, the base station is able to update certain transmission parameters in order to maximize the sum-rate function. On the other hand, the proposed sum-rate function provides a means of simple comparison between transmission schemes and scalar feedback techniques. Particularly, the sum rate of SDMA and time division multiple access (TDMA) is compared in the following extreme regimes: large number of users, high SNR, and low SNR. Simulations are provided to illustrate the performance of various scalar feedback techniques based on the proposed design framework.

We propose a low-complexity vector precoding (VP) scheme for the downlink of multi-user multiple input single output (MU-MISO) systems with limited feedback. Conventional VP requires the use of modulo operation and knowledge of the scaling factor used at the transmitter in order to remove the perturbation quantity at the receiver. The latter may be problematic in certain limited feedback scenarios where only quantized versions of the scaling factor can be made available at the receiver. To circumvent this shortcoming, we propose a modified VP technique where the search of perturbing vectors is limited to the area in the symbol constellation which is constrictive to the information symbols, i.e., the area where the distances from the decision thresholds are increased with respect to a distance threshold. By doing this, the perturbation quantities can only enhance detection and need not be removed at the receiver. Successful detection can therefore be done without the use of the modulo operation and the scaling factor, which makes the proposed VP scheme applicable to limited feedback scenarios. Analytical and simulation results show that the performance of the proposed scheme is free of the error floor typically encountered with conventional VP in the presence of limited feedback.

Near-orthogonal transmission over MIMO broadcast channels with limited feedback is addressed, identifying the intrinsic tradeoffs that govern the system performance in terms of sum rate. A low-complexity scheme is proposed for joint scheduling and beamforming, which employs feedback in the form of quantized channel directions and lower bounds on each user's received SINR, given a maximum orthogonality factor epsi between transmit beamforming vectors (epsi = 0 if orthogonal beamforming). The authors assume a simple power allocation, which consists of distributing the available power equally over the active beams. In a system with K active users and a given average SNR, the authors show that the sum rate is a function of the number of antennas, the number of active beams (considered less than or equal to the number of antennas), the orthogonality factor epsi and the quantization codebook size. A practical rate function is derived for the described system which approximates the average sum rate accurately as validated through computer simulations. Based on the proposed rate function, optimality of SDMA vs. TDMA systems with asymptotically large number of users is shown. In addition, the authors provide a performance comparison between the proposed algorithm with limited feedback and other approaches with different level of channel state information at the transmitter.

Coordinated Multi-Point transmission/reception (CoMP) can increase the performance of the cell edge users, the spectrum efficiency and mitigate the inter-cell interference among cells. In the CoMP scenario, several base stations which usually need satisfy the cooperating condition serve a single or several cell-edge users cooperatively. As we know, the CoMP techniques can be characterized into many classes. For the joint transmission system which is a classical CoMP system, a large number of data sharing and channel information exchange will be needed, where the system inevitably become comparatively complex. However, in this paper, the coordinated beam forming system will be considered, where several base stations cope with the inter-cell interference among cells, and all the base stations will only exchange the channel information. Further, the downlink multi-cell coordinated beam forming system is considered in this paper. In this downlink multi-cell coordinated beam forming system, it is crucial to design appropriate pre-coding schemes that are able to mitigate the inter-cell interference. In obtainable pre-coding schemes, the Signal-to-leakage-and-noise ratio (SLNR) pre-coding scheme is a promising pre-coding scheme in downlink multi-cell coordinated beam forming system, which makes closed-form solution accessible. Moreover, the problem in designing the pre-coding vectors is not a coupled problem since the cooperative base stations need not optimize the pre-coding vectors jointly, which are responsible for their own design of pre-coding vectors, that is, they will just carry out independent optimization. In this paper, we proposed a novel pre-coding scheme based on the SLNR scheme, which will determine the pre-coding vectors via iteration. The simulation results show that, the proposed method outperforms the conventional SLNR pre-coding scheme in bit error rate performance.

ABSTRACTIn this paper, several multi‐user scheduling algorithms with limited channel feedback are proposed for multi‐antenna downland channels. With little channel quality information (CQI) and channel direction information (CDI) at multi‐antenna base station, multi‐user scheduling algorithms are carefully designed to maximise the potential system throughput while reducing the feedback load. We first propose a scheduling algorithm that requires a quantised CDI and a threshold CQI. Then we propose a scheduling that needs limited feedback of a threshold CDI and a perfect CQI. Both scheduling algorithms are studied in terms of the achievable throughput and the feedback load. Analytical results show that the proposed scheduling with limited channel feedback has a small performance degradation compared with the scheduling with unlimited channel feedback when the thresholds for user scheduling are judiciously designed. Moreover, under a per user total bit rate constraint of the feedback channel, we propose a scheduling with the quantisation of both CQI and CDI. It is demonstrated that allocating 1 bit to the CQI with an optimal threshold and the remaining bits to the CDI achieves the near optimal sum‐rate performance. Copyright © 2011 John Wiley & Sons, Ltd.

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Moving relay (MR) is a relay node deployed on public transportation such as cars and trains, which can effectively combat the vehicle penetration loss (VPL) problem in vehicle communication. outage probability (OP) is an expression of link capacity. When the link capacity cannot meet the required user rate, an outage will occur. For the downlink multi-user network, we compared the performance of direct transmission and MR-assisted transmission with the probability of interruption as a performance indicator. Subsequently, we design a joint optimization scheme for transmission mode selection and user scheduling, select the transmission mode suitable for the user, schedule the user with the largest information rate, and derive the closed-form expression of the scheduling scheme. And compared with random selection scheduling. The experiment compares random scheduling with the scheduling scheme we proposed. Experimental results show that compared with random selection and scheduling scheme, the proposed scheduling scheme can reduce OP, thereby improving system performance.

Many opportunistic scheduling techniques are impractical because they require accurate channel state information (CSI) at the transmitter. In this paper, we investigate the scheduling of unicast and multicast services in a downlink network with a very limited amount of feedback information. Specifically, unicast users send imperfect (or no) CSI and infrequent acknowledgements (ACKs) to a base station, and multicast users only report infrequent ACKs to avoid feedback implosion. We consider the use of physical-layer rateless codes, which not only combats channel uncertainty, but also reduces the overhead of ACK feedback. A joint scheduling and power allocation scheme is developed to realize multiuser diversity gain for unicast service and multicast gain for multicast service. We prove that our scheme achieves a near-optimal throughput region. Our simulation results show that our scheme significantly improves the network throughput over schemes employing fixed-rate codes or using only unicast communications.

The multiuser MIMO (MU-MIMO) systems tend to have inter-user interference in practical systems with limited feedback. Practical MU-MIMO systems with limited feedback can be divided into two classes in general. One is zero-forcing beamforming (ZFBF), and the other is per user unitary rate control (PU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> RC) which is an extended version of random beamforming (RBF). To improve performance, ZFBF needs more feedback bits, and PU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> RC needs a larger number of users. We propose a new hybrid MU-MISO system which has the advantages of the two MU-MIMO schemes simultaneously, and analyze the sum-rate performance and the quantization error of the hybrid scheme. The analysis shows how the number of feedback bits, the number of users, and the signal power affect the sum-rate. The throughput scaling laws are also derived in the high and the medium SNR regimes. Simulation results show that the proposed hybrid MU-MISO system performs better than either of the two MU-MIMO systems with the same number of feedback bits. Another advantage is that it has lower complexity for codeword search since it has a hierarchical codebook structure.

To maintain a specific degree of multiplexing gain with limited feedback in multiple-input/multiple-output broadcast channels, the number of feedback bits has been increased linearly with the signal-to-noise ratio (SNR) measured in decibel, and conditions for the slope of this increase have been derived in previous studies. However, previous studies mostly focused on the chordal distance (ChD) for channel quantization, although it is not an optimally designed distance measure. Thus, the possibility exists that the same degree of multiplexing gain can be obtained by using fewer feedback bits than the existing method requires. In this paper, we propose an optimal distance measure that can maximize the multiplexing gain achieved by limited-feedback-based block diagonalization over broadcast channels. Then, we provide a sufficient condition for the number of feedback bits to achieve a specific degree of multiplexing gain with the proposed method. The sufficient number of feedback bits is given by a linear function of the SNR (decibel) as in previous studies, but the slope of the linear increase can be much less than that obtained in previous studies. As a consequence, the proposed method achieves higher multiplexing gain and corresponding throughput than the existing methods with the same number of feedback bits.

User scheduling in multiuser multiple-input-multiple-output (MU-MIMO) systems is fundamentally different with single-user systems 1 , in the sense that without spatial multiplexing, users in single-user systems are sharing the time-frequency degree-of-freedoms (DoFs), whereas in MU-MIMO systems, due to the fact that the number of spatial DoFs scales with the number of users (assuming sufficient base station (BS) antennas), users are not sharing the DoFs, but rather creating additional DoFs for their own use. However, instead of limited by the available DoFs, the number of simultaneous users are limited by the channel state information (CSI) acquisition overhead in pilot-assisted MU-MIMO systems. In this paper, we investigate the user scheduling scheme in pilot-assisted time-division-duplex (TDD) MU-MIMO systems. Leveraging the Lyapunov optimization techniques, we derive the throughput-optimal scheduling policy which serves as a performance bound due to its non-causality and high complexity. We then propose a heuristic scheme, which is causal and substantially decreases the complexity. Moreover, it performs fairly close to the optimum.

User scheduling and multiuser multi-antenna (MU-MIMO) transmission are at the\ncore of high rate data-oriented downlink schemes of the next-generation of\ncellular systems (e.g., LTE-Advanced). Scheduling selects groups of users\naccording to their channels vector directions and SINR levels. However, when\nscheduling is applied independently in each cell, the inter-cell interference\n(ICI) power at each user receiver is not known in advance since it changes at\neach new scheduling slot depending on the scheduling decisions of all\ninterfering base stations. In order to cope with this uncertainty, we consider\nthe joint operation of scheduling, MU-MIMO beamforming and Automatic Repeat\nreQuest (ARQ). We develop a game-theoretic framework for this problem and build\non stochastic optimization techniques in order to find optimal scheduling and\nARQ schemes. Particularizing our framework to the case of "outage service\nrates", we obtain a scheme based on adaptive variable-rate coding at the\nphysical layer, combined with ARQ at the Logical Link Control (ARQ-LLC). Then,\nwe present a novel scheme based on incremental redundancy Hybrid ARQ (HARQ)\nthat is able to achieve a throughput performance arbitrarily close to the\n"genie-aided service rates", with no need for a genie that provides\nnon-causally the ICI power levels. The novel HARQ scheme is both easier to\nimplement and superior in performance with respect to the conventional\ncombination of adaptive variable-rate coding and ARQ-LLC.\n

The precoding technique is an effective and flexible way for reducing the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. However, different precoding schemes will increase error probabilities of the system. With the knowledge of the channel information and the receiver filter, we derive a necessary condition on the chosen precoding matrices for minimizing error probability of the OFDM system in the additive white Gaussian noise (AWGN) channel. A systematic procedure in designing such an optimal precoding matrix is provided. With a proper selection, the optimal precoding matrix can meet the requirements of PAPR reduction and achieve the minimum error probability in white Gaussian noise. Our simulation results show that the chosen precoding matrix notably outperforms other general precoding matrices in both AWGN and multipath fading channels. We also proved that the precoding matrix with all the singular values equal to 1 is one of the optimal solutions.

In this paper, we consider practical multiuser multiple-input multiple-output (MU-MIMO) systems where each user has multiple receive antennas and limited channel feedback is available at the base station (BS). The joint combiner and precoding scheme for downlink transmission is proposed, in which we consider the joint use of the receive antenna combiner at each user and precoding at the BS. In the proposed scheme, each user independently chooses the codeword as its preferred-beam vector from the predefined codebook, which maximizes the effective channel gain after the receive antenna combiner. Utilizing the received feedback from users, the BS can easily construct the precoding matrix by zero-forcing beamforming (ZFBF) criterion. In addition, a user selection algorithm is provided to select served users, when the number of users exceeds the number of transmit antennas. Numerical results show that the proposed scheme improves the sum-rate performance of MU-MIMO downlink systems.