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ABSTRACT: This letter considers the design problem of coordinated downlink minimum power beamforming for a multiuser multi-cell network, where each multi-antenna base station (BS) serves multiple single antenna users. We propose a decentralized primal decomposition based algorithm where limited amount of information is exchanged between coupled BSs at each iteration. Algorithm converges to the globally optimal solution for a static scenario. Unlike most of the previous decentralized methods, a feasible set of beamformers is guaranteed at each iteration even when the exchanged backhaul information is outdated. Consequently, the proposed approach naturally lends itself to realistic time-correlated fading scenarios.
IEEE Signal Processing Letters 12/2011; · 1.39 Impact Factor
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ABSTRACT: A three-dimensional (3D) multi-cell multi-user system model along with spatial array processing are presented and studied in this paper. Most of the radio propagation in the literature has been studied and modeled in a two dimensional plane. The 3D propagation channel modeling has drawn attentions recently. This is due to the fact that, in some scenarios, the assumption of small elevation domain angular spread does not hold. With multiple antenna elements displaced in a 3D space, e.g., uniform linear array (ULA) or uniform planar array (UPA), the base station (BS) is capable of forming beams in vertical domain. This is particularly interesting for the interference limited scenario, since the 3D array processing provides another degree of freedom to transmit signal and eliminate the interference. Using the 3D multicell channel model, the impact of elevation domain angular spread and antenna analog beam pattern on the base station antenna design, digital beamforming and power control are studied.
Communications (ICC), 2011 IEEE International Conference on; 07/2011
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ABSTRACT: Transmission power allocation in single-carrier multiple-input multiple-output (MIMO) systems with iterative frequency-domain (FD) soft cancellation (SC) minimum mean-squared error (MMSE) equalization is considered. A novel framework for transmission power minimization subject to equalizer convergence constraints, referred as convergence constrained power allocation (CCPA) method, is proposed based on extrinsic information transfer (EXIT) chart analysis. The proposed method decouples the spatial interference between the streams using singular value decomposition (SVD), and minimizes the transmission power while achieving the target mutual information for each stream after iterations at the receiver. We show that the transmission power optimization can be formulated as a convex optimization problem. Three CCPA methods, one approximately optimal, and other two heuristic methods inspired by the Lagrange duality are derived. The numerical results demonstrate that the proposed scheme outperforms the existing linear precoding schemes. Moreover, the proposed heuristic schemes can achieve performance close with that of the approximately optimal method in terms of the equalizer convergence properties as well as transmission power.
IEEE Transactions on Signal Processing 05/2011; · 2.63 Impact Factor
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ABSTRACT: Decentralized base station (BS) assignment methods with limited backhaul information exchange are proposed to be combined with decentralized optimal multi-cell minimum power beamforming and Zero-Forcing (ZF) beamforming techniques. The aim of the combined BS assignment and downlink beamforming is to minimize sum power while guaranteeing a minimum SINR per user. A major finding is that a simple ZF beamforming with the optimal decentralized BS assignment, and with limited backhaul information exchange obtains near optimal performance near the cell edge when the user-specific SINR targets are high. Thus, only marginal gains are available from more complex BS assignment and beamforming approaches. However, in the case of low user-specific SINR targets, there is a significant gain available at the cell edge from the optimal beamforming and BS assignment. For example, a simple maximum channel norm selection based BS assignment method with the optimal minimum power beamforming can obtain much of this gain.
Signal Processing Advances in Wireless Communications (SPAWC), 2010 IEEE Eleventh International Workshop on; 07/2010
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ABSTRACT: Coordinated linear transmit-receive processing with beam selection is a method to utilize all degrees of freedom available in multiuser multiple-input multiple-output (MIMO) cellular systems in order to increase system capacity. By applying channel state information in the transmitter (CSIT), the base station (BS) is able to spatially multiplex downlink data streams for different users. In the time division duplex (TDD) mode, CSIT for the BS is provided by means of uplink CSI sounding pilots. However, antenna-specific uplink pilot streams cause an extensive overhead that restricts the size of the practical user group and the terminal antenna setup that can be handled within the same time-frequency slot. We propose to reduce the required overhead by letting the terminals form pilot beams by transmit precoding, based on the knowledge of the user-specific MIMO channels obtained via a downlink common pilot signal. The achievable rate of the system is evaluated in conjunction with channel estimation and linear receivers. According to the results, the performance loss induced by the incomplete sounding is minor, as the beamforming gain provided by multiple terminal antennas, and the multiuser diversity seen by the BS are retained. When taking into account the CSI estimation error in the BS, the overhead reduction turns out to improve robustness and even increase the average system capacity.
GLOBECOM Workshops, 2009 IEEE; 01/2010
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ABSTRACT: A new power allocation method for irregularly modulated signaling in single carrier point-to-point multiple input multiple output (MIMO) systems with an iterative frequency-domain (FD) soft cancellation (SC) minimum mean squared error (MMSE) equalization is proposed. The proposed method explicitly takes into account the convergence properties of the iterative equalizer while transmission power is minimized. The proposed scheme is based on the combination of irregular modulation, different signal constellations within one code word, and convergence constraint precoding (CCP), technique that decouples the spatial interference between streams using singular value decomposition (SVD), and minimizes the transmission power while achieving the target mutual information for each stream after iterations at the receiver side. Numerical results show that the proposed scheme improves the transmission rate adaptivity of the original CCP while ensuring the convergence with reduced transmission power.
Signals, Systems and Computers, 2009 Conference Record of the Forty-Third Asilomar Conference on; 12/2009
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ABSTRACT: This paper proposes a novel linear precoder design technique for single carrier single-user multiple input multiple output (MIMO) systems with frequency-domain (FD) soft cancellation (SC) minimum mean squared error (MMSE) iterative equalization where the convergence properties of the equalizer are taken into account. The proposed precoder design technique, convergence constrained precoding (CCP), minimizes the transmission power while it achieves the target mutual information for each stream after the iterations at the receiver side. We show that the optimality criterion for the proposed design can be formulated as a convex optimization problem. The results demonstrate that our proposed technique outperforms the existing linear precoding techniques by ensuring the convergence with a reduced transmission power. Furthermore, we show that with CCP we can adjust transmission according to convergence properties of the iterative equalizer in a more flexible way than, e.g., minimum sum mean squared error (MinSumMSE) and maximum information rate (MaxRate) precoding.
Information Theory, 2009. ISIT 2009. IEEE International Symposium on; 08/2009
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ABSTRACT: Joint design of linear multiuser multiple-input-multiple-output (MIMO) transceiver subject to different quality of service (QoS) constraints per user and with per-antenna or antenna group power constraints is considered. Solutions for two linear transceiver optimization problems are proposed, i.e., balancing the weighted signal-to-interference-plus-noise ratio (SINR) per data stream and balancing the weighted rate for each scheduled user with minimum rate requirements per user. The proposed joint transceiver optimization algorithms are compared to corresponding optimal nonlinear transmission methods as well as to generalized zero-forcing transmission solutions. Unlike the optimal nonlinear schemes, the optimization problems employed in the linear multiuser MIMO transceiver design are not convex in general. However, the proposed algorithms are shown to provide very efficient solutions despite of the fact that the global optimum cannot be guaranteed due to nonconvexity of the problems.
IEEE Transactions on Signal Processing 08/2008; · 2.63 Impact Factor
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ABSTRACT: The uplink-downlink SINR duality theorem is a key tool which simplify substantially the problem of joint design of the linear transmit and receive beamformers in multiple-input multiple-output (MIMO) downlink channels. The theorem has been proved previously under the assumption that the cross- coupling matrix between the users is primitive or, alternatively, by postulating the nonnegativity of the resolvent. By using the Lagrange duality theory, we first give an alternative proof which holds for arbitrary cross-coupling matrices. The proof does not only extend the result to a larger set of practical applications, but it also reveal more insight on the sum power minimization problem under a set of minimum SINR requirements for data streams. As a practical application, we apply the uplink-downlink SINR duality to derive a general method for MIMO downlink linear transceiver optimization according to different system performance criteria, including weighted sum rate maximization, weighted sum mean square error minimization, and minimum SINR maximization. The proposed method can handle multiple antennas at the BS and at the mobile user with single and/or multiple data streams per scheduled user. The numerical simulations show that the sum rate achieved by the sum rate maximization algorithm is within 0.5-1.5 bits/second/Hz close to the sum capacity. When compared to the traditional zero forcing based solutions, the proposed method provides more than 4 dB SNR gain and up to 3.5 bits/sec/Hz better spectral efficiency.
Wireless Communications and Networking Conference, 2008. WCNC 2008. IEEE; 05/2008
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ABSTRACT: Cooperative processing of transmitted signal from several multiple-input multiple-output (MIMO) base stations (BS) is considered for users located within a soft handover (SHO) region. The downlink resource allocation problem with different BS power constraints is studied for the orthogonal frequency division multiplexing system with adaptive MIMO transmission. Joint design of the linear transmit and receive beamformers in a MIMO multiuser transmission subject to per BS power constraints is considered. A solution for the weighted sum rate maximization problem is proposed. The proposed algorithm is shown to provide a very efficient solution despite of the fact that the global optimality cannot be guaranteed due to the non-convexity of the optimization problem. Moreover, efficient resource allocation method based on zero forcing transmission is provided. The impact of the size of a SHO region, the overhead from the increased resource utilization, and different inter-cell interference distributions due to the SHO are evaluated by system level simulations. Although the overhead from the SHO processing can be significant, it can be mitigated by using space division multiple access for users having an identical SHO active set composition. The users located at the SHO region may enjoy from greatly increased transmission rates. This translates to significant overall system level gains from the cooperative SHO processing.
IEEE Transactions on Wireless Communications 05/2008; · 2.59 Impact Factor
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ABSTRACT: Joint design of linear multiuser multiple-input multiple-output (MIMO) transceiver subject to different quality of service constraints per user and with per antenna or antenna group power constraints is considered. A solution for finding a maximum weighted common rate achievable for each scheduled user with minimum rate requirements per user is proposed. The proposed joint transceiver optimization algorithms are compared to corresponding optimal nonlinear transmission methods as well as to zero forcing transmission solutions. The proposed algorithms provide efficient solutions for difficult non-convex transceiver optimization problems.
Global Telecommunications Conference, 2007. GLOBECOM '07. IEEE; 12/2007
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ABSTRACT: We consider a single-cell multiple-input multiple-output (MIMO) downlink channel where linear transmission and reception strategy is employed. The base station (BS) transmitter is equipped with a scheduler using a simple opportunistic beamforming strategy, which associates an intended user for each of the transmitted data streams. For the case when the channel of the scheduled users is available at the BS, we propose a general method for joint design of the transmit and the receive beamformers according to different optimization criteria, including weighted sum rate maximization, weighted sum mean square error minimization, minimum signal-to-interference-plus-noise ratio (SINR) maximization and sum power minimization under a minimum SINR constraint. The proposed method can handle multiple antennas at the BS and at the mobile user with single and/or multiple data streams per scheduled user. The optimization problems encountered in the beamformer design (e.g., covariance rank constraint) are not convex in general. Therefore, the problem of finding the global optimum is intrinsically nontractable. However, by exploiting the uplink-downlink SINR duality, we decompose the original optimization problem as a series of simpler optimization problems which can be efficiently solved by using standard convex optimization tools. Even though each subproblem is optimally solved, there is no guarantee that the global optimum has been found due to the nonconvexity of the problem. However, the simulations show that the algorithms converge fast to a solution, which can be a local optimum, but is still efficient.
IEEE Transactions on Signal Processing 10/2007; · 2.63 Impact Factor
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ABSTRACT: We consider a single-cell multiple-input multiple-output (MIMO) downlink channel where linear transmission and reception strategy is employed. The base station (BS) transmitter is equipped with a scheduler using a simple opportunistic beamforming strategy, which associates an intended user for each of the transmitted data streams. For the case when the channel of the scheduled users is available at the BS, we propose a joint power control and linear transmit and receive beamformers design to optimally balance the SINR values of data streams under a total power budget. The proposed method can handle multiple antennas at the BS and at the mobile users with single and/or multiple data streams per scheduled user. We consider a general problem formulations which includes minimum SINR constraints on a subset of data streams and discuss possible particularizations which can be solved more efficiently. By exploiting the uplink-downlink SINR duality, we decompose the original optimization problem as a series of remarkably simpler optimization problems which can be efficiently solved by using standard convex optimization solvers. The simulations show that the algorithm converges fast to a solution, which can be a local optimum, but is still efficient. Only one iteration of the proposed method is enough to substantially outperform the zero forcing based solution.
Communications and Networking in China, 2007. CHINACOM '07. Second International Conference on; 09/2007
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ABSTRACT: Joint design of linear multiuser multiple-input multiple-output (MIMO) transceiver subject to different optimization criteria and with per base station (BS) and/or per antenna power constraints is considered. We assume a scenario, where cooperative processing of transmitted signal from several BS antenna heads is applied to users located within a soft handover region. Solutions for three optimization criteria are derived: maximization of the weighted sum rate, finding a maximum weighted common rate achievable for each scheduled user and maximization of minimum weighted SINR per stream. The linear joint transceiver optimization algorithms are compared to each other, as well as, to corresponding optimal nonlinear transmission methods. The proposed algorithms provide very efficient solutions despite of the non-convexity of the problems.
Communications and Networking in China, 2007. CHINACOM '07. Second International Conference on; 09/2007
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ABSTRACT: Joint cooperative processing of transmitted signal from several multiple-input multiple-output (MIMO) base station (BS) antenna heads is considered for users located within a soft handover (SHO) region. Downlink resource allocation problem with different BS power constraints is studied. The mathematical framework for the SHO based MIMO system is derived and the joint design of linear transmit and receive beamformers in a MIMO multiuser transmission according to weighted sum rate maximization criterion and subject to per BS power constraints is considered. The proposed algorithm is shown to provide very efficient solutions despite of the fact that there is no guarantee of achieving the global optimum due to the non-convexity of the problem. Moreover, practical and efficient resource allocation method based on generalized zero forcing transmission is provided.
Communications, 2007. ICC '07. IEEE International Conference on; 07/2007
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ABSTRACT: We consider a single-cell multiple-input multiple-output (MIMO) downlink channel where linear transmission and reception strategy is employed. The base station (BS) transmitter is equipped with a scheduler using a simple opportunistic beamforming strategy, which associates an intended user for each of the transmitted data streams. For the case when the channel of the scheduled users is available at the BS, we propose a general method for joint design of linear transmit and receive beamformers, according to weighted sum rate maximization criteria. The proposed method can handle multiple antennas at the BS and at the mobile users with an arbitrary number of data streams per scheduled user. It can also handle a fairly general set of practical power constraints for the transmit beamformers, i.e., we can impose sum power constraints for different subsets of the transmit antennas.
Vehicular Technology Conference, 2007. VTC2007-Spring. IEEE 65th; 05/2007
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ABSTRACT: System level simulations in a realistic multi-cell model are needed to evaluate the performance of the adaptive radio link and radio resource management (RRM) algorithms for future wireless communication systems. In this paper, we consider spatial mode adaptation, modulation adaptation, hybrid automatic repeat request (HARQ) and scheduling in a cellular network. In order to simulate a considered multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) physical layer, a novel link frame error rate (FER) prediction method is presented. Link-to-system level method is based on the mutual information metric which takes multiple antenna receiver and multiple antenna channel coding into account. The proposed metric is shown to provide an accurate FER approximation for diverse range of MIMO-OFDM channels also with inter-cell interference and HARQ. System level results illustrate that the MIMO-OFDM system with adaptive space-frequency turbo coded modulation (SFTuCM) provides flexibility and high spectral efficiency for interference limited system. The MIMO-OFDM system with interference averaging frequency hopping channel allocation achieves performance close to the channel condition aware dynamic channel allocation (DCA) with time division multiple access (TDMA).
Vehicular Technology Conference, 2007. VTC2007-Spring. IEEE 65th; 05/2007
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ABSTRACT: The joint cooperative processing of transmitted signal from several multiple-input multiple-output (MIMO) base station (BS) antenna heads is considered for users located within a soft handover (SHO) region. The mathematical framework for the SHO based MIMO system is derived and the joint design of linear transmit and receive beamformers in a MIMO multiuser transmission subject to per BS power constraints is considered. Solution for the maximization of the minimum weighted SINR per data stream criterion is proposed. The proposed algorithm is shown to provide very efficient solutions despite of the fact that the global optimum cannot be guaranteed due to the non-convexity of the problem. Moreover, a less complex but still efficient allocation method based on zero forcing transmission is provided for the same optimization criterion.
Wireless Communications and Networking Conference, 2007.WCNC 2007. IEEE; 04/2007
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ABSTRACT: The joint cooperative processing of transmitted signal from several multiple-input multiple-output (MIMO) base station (BS) antenna heads is considered for users located within a soft handover (SHO) region. The system level gains and trade-offs from cooperative SHO processing are investigated. The impact of the size of the SHO region, overhead from the increased hardware and physical (time, frequency) resource utilization, and different non-reciprocal inter-cell interference distributions due to SHO are evaluated. Practical user, bit and power allocation method with different BS power constraints is provided for the proposed cooperative multiuser MIMO transmission. The overhead from SHO processing can be significant, and the call blocking probability can be dramatically increased. However, the overhead can be mitigated by using space division multiple access for users that have identical SHO active set composition. Also, the dropping probability is decreased, and thus, the total outage probability with SHO is less than without SHO. The users located at the SHO region may enjoy from greatly increased transmission rates. This translates to significant overall system level gains from the cooperative SHO processing. The proposed soft handover scheme can be used to provide more evenly distributed service over the entire cellular network.
Global Telecommunications Conference, 2006. GLOBECOM '06. IEEE; 01/2007
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ABSTRACT: The joint cooperative processing of transmitted signal from several multiple-input multiple-output (MIMO) base station (BS) antenna heads is considered for users located within a soft handover (SHO) region. Downlink space-frequency bit and power allocation problem with different BS power constraints is studied for the considered adaptive MIMO-OFDM system. The performance of the proposed heuristic loading method is shown to be close to the optimal convex optimization method with per BS power constraints. It is shown that the highest SHO gains are achieved with a small power imbalance between the received BS powers at low signal-to-noise ratio (SNR), where the achievable rates can be even doubled. On the other hand, the gain from joint processing in SHO quickly diminishes as the imbalance increases, especially at low SNR. Moreover, the results indicate that the joint processing can be even detrimental for the system performance if a coarse phase synchronization between BS antenna head is not guaranteed, as some additional fading on the target SNR values is introduced
Personal, Indoor and Mobile Radio Communications, 2006 IEEE 17th International Symposium on; 10/2006
