Showing papers on "Multi-user MIMO published in 2008"

The secrecy capacity of the MIMO wiretap channel

Abstract: We consider the MIMO wiretap channel, that is a MIMO broadcast channel where the transmitter sends some confidential information to one user which is a legitimate receiver, while the other user is an eavesdropper. Perfect secrecy is achieved when the transmitter and the legitimate receiver can communicate at some positive rate, while insuring that the eavesdropper gets zero bits of information. In this paper, we compute the perfect secrecy capacity of the multiple antenna MIMO broadcast channel, where the number of antennas is arbitrary for both the transmitter and the two receivers. Our technique involves a careful study of a Sato-like upper bound via the solution of a certain algebraic Riccati equation.

Generalized Design of Multi-User MIMO Precoding Matrices

Abstract: In this paper we introduce a novel linear precoding technique. The approach used for the design of the precoding matrix is general and the resulting algorithm can address several optimization criteria with an arbitrary number of antennas at the user terminals. We have achieved this by designing the precoding matrices in two steps. In the first step we minimize the overlap of the row spaces spanned by the effective channel matrices of different users using a new cost function. In the next step, we optimize the system performance with respect to specific optimization criteria assuming a set of parallel single- user MIMO channels. By combining the closed form solution with Tomlinson-Harashima precoding we reach the maximum sum-rate capacity when the total number of antennas at the user terminals is less or equal to the number of antennas at the base station. By iterating the closed form solution with appropriate power loading we are able to extract the full diversity in the system and reach the maximum sum-rate capacity in case of high multi-user interference. Joint processing over a group of multi-user MIMO channels in different frequency and time slots yields maximum diversity regardless of the level of multi-user interference.

Block diagonalization for multi-user MIMO with other-cell interference

Abstract: Block diagonalization is one approach for linear preceding in the multiple-input multiple-output broadcast channel that sends multiple interference free data streams to different users in the same cell. Unfortunately, block diagonalization neglects other-cell interference (OCI), which limits the performance of users at the edge of the cell. This paper presents an OCI-aware enhancement to block diagonalization that uses a whitening filter for interference suppression at the receiver and a novel precoder using the interference-plus-noise covariance matrix for each user at the transmitter. For complex Gaussian matrix channels, the asymptotic sum rate of the proposed system is analyzed under a large antenna assumption for isotropic inputs and compared to conventional block diagonalization. The capacity loss due to OCI is quantified in terms of results from single-user MIMO capacity. Several numerical examples compare achievable sum rates, the proposed asymptotic rates, and the capacity loss, in low and high interference regimes.

A Novel Approach to MIMO Transmission Using a Single RF Front End

Abstract: In this paper we introduce a new perspective to the implementation of wireless MIMO transmission systems with increased bandwidth efficiency. Unlike traditional spatial multiplexing techniques in MIMO systems, where additional information can be sent through the wireless channel by feeding uncorrelated antenna elements with diverse bitstreams, we use the idea of mapping diverse bitstreams onto orthogonal bases defined in the beamspace domain of the transmitting array far-field region. Using this approach we show that we can increase the capacity of wireless communication systems using compact parasitic antenna architectures and a single RF front end at the transmitter, thus paving the way for integrating MIMO systems in cost and size sensitive wireless devices such as mobile terminals and mobile personal digital assistants.

Multi-user mimo feedback and transmission in a wireless communication system

Abstract: The present invention provides a method for feedback and transmission of multi-user (MU) multiple input multiple output (MIMO) in a wireless communication system. The method includes steps of selecting subset codebook or full code book based on traffic load of a base station, and broadcasting the selected codebook to user equipments. In high traffic load, subset codebook is selected, and in low traffic load, full codebook is selected. User stations calculated a channel quality indicator of a spatial codeword vector that is included in the selected codebook. Information of the maximum channel quality indicator is sent to the base station together with a precoder of the user equipment. The base station selects user equipments based on the information of the maximum channel quality indicator and precoder, and transmits precoder signal and data signal to the user equipments. The present invention also provides a system for the base station that causes the base station to perform the above mentioned operations.

Vector Precoding for Wireless MIMO Systems and its Replica Analysis

Abstract: This paper studies a nonlinear vector precoding scheme which inverts the wireless multiple-input multiple-output (MIMO) channel at the transmitter so that simple symbol-by-symbol detection can be used in lieu of sophisticated multiuser detection at the receiver. In particular, the transmit energy is minimized by relaxing the transmitted symbols to a larger alphabet for precoding, which preserves the minimum signaling distance. The so-called replica method is used to analyze the average energy savings with random MIMO channels in the large-system limit. It is found that significant gains can be achieved with complex-valued alphabets. The analysis applies to a very general class of MIMO channels, where the statistics of the channel matrix enter the result via the R-transform of the asymptotic empirical distribution of its eigenvalues. Moreover, we introduce polynomial-complexity precoding schemes for binary and quadrature phase-shift keying in complex channels by using convex rather than discrete relaxed alphabets. In case the number of transmit antennas is more than twice the number of receive antennas, we show that a convex precoding scheme, despite its polynomial complexity, outperforms NP-hard precoding using the popular Tomlinson-Harashima signaling.

Method for reducing inter-cell interference

Abstract: A method for reducing inter-cell interference and a method for transmitting a signal by a collaborative MIMO scheme, in a communication system having a multi-cell environment are disclosed. An example of a method for transmitting, by a mobile station, precoding information in a collaborative MIMO communication system includes determining a precoding matrix set including precoding matrices of one more base stations including a serving base station, based on signal strength of the serving base station, and transmitting information about the precoding matrix set to the serving base station. A mobile station in an edge of a cell performs a collaborative MIMO mode or inter-cell interference mitigation mode using the information about the precoding matrix set collaboratively with neighboring base stations.

Modeling and Capacity of Polarized MIMO Channels

Abstract: In this work the modeling and capacity of a dual polarized (DP) MIMO channel is addressed. The modeling includes channel parameters such as receive and transmit correlation, channel cross-polarization discrimination (XPD), and antenna parameters such as polarization state, polarization parallelity and coupling. The capacity of the DP MIMO channel is evaluated and compared to the capacity of a single polarized (SP) MIMO system. The SP MIMO system with spatially separated antenna sensors has the advantage that it also offers array gain and will therefore during idealized conditions outperform the DP MIMO system. However, in this work it is found that this advantage often is reduced and sometimes even lost when channel and antenna imperfections such as, e.g., correlation, channel XPD, and polarization mismatch are introduced. The main conclusion is that DP antennas might not always yield the best MIMO performance, but instead offer compact antenna solutions for mobile devices and robust performance that is more insensitive to the aforementioned imperfections.

Correlation and capacity of measured multi-user MIMO channels

Abstract: In multi-user multiple-input multiple-output (MU-MIMO) systems, spatial multiplexing can be employed to increase the throughput without the need for multiple antennas and expensive signal processing at the user equipments. In theory, MU-MIMO is also more immune to most of propagation limitations plaguing single-user MIMO (SU-MIMO) systems, such as channel rank loss or antenna correlation. However, in this paper we show that this is not always true. We compare the capacity and the correlation of measured MU-MIMO channels for both outdoor and indoor scenarios. The measurement data has been acquired using Eurecompsilas MIMO openair sounder (EMOS). The EMOS can perform real-time MIMO channel measurements synchronously over multiple users. The results show that in most scenarios MU-MIMO provides a higher throughput than SU-MIMO also in the measured channels. However, in outdoor scenarios with a line of sight, the capacity drops significantly when the users are close together, due to high correlation at the transmitter side of the channel. In such a case, the performance of SU-MIMO and MU-MIMO is comparable.

Apparatus and method for medium access control in wireless communication networks

Abstract: A method includes identifying a white space at a first wireless node and selecting a channel in the identified white space. The white space includes at least one frequency or frequency band not in use (like a licensed frequency or frequency band). The method also includes identifying at the first wireless node a channel access factor for each of multiple wireless nodes including the first wireless node. The method further includes determining if the first wireless node has a specified channel access factor. In addition, the method includes transmitting data from the first wireless node on the channel when the first wireless node has the specified channel access factor. The channel access factors can be identified and the determination whether the first wireless node has the specified channel access factor can be performed without using control signals transmitted between the wireless nodes. The channel access factor could represent a hash function value.

Minimum redundancy MIMO radars

Abstract: The multiple-input multiple-output (MIMO) radar concept has drawn considerable attention recently. In the traditional single-input multiple-output (SIMO) radar system, the transmitter emits scaled versions of a single waveform. However, in the MIMO radar system, the transmitter transmits independent waveforms. It has been shown that the MIMO radar can be used to improve system performance. Most of the MIMO radar research so far has focused on the uniform array. However, it is in general a loss of optimality to assume the array to be uniform. In this paper, the nonuniform array design problem in the MIMO radar is studied. In the SIMO radar, it has been shown that there is a class of linear arrays which minimizes the number of redundant spacings in the array. These are called minimum redundancy linear arrays. It has been shown that this class of arrays has excellent performance in rejection of mainlobe interferences. In this paper, the idea of minimum redundancy linear array is extended to the MIMO radar case. The numerical examples show that the proposed minimum redundancy MIMO radar results in improved rejection of mainlobe interferences, with negligible degradation in sidelobe interference rejection capabilities.

Channel access protocol for wireless communication

Abstract: Method, apparatus, and computer program product embodiments are disclosed to enable an access point in a wireless network to use Orthogonal Frequency-Division Multiple Access (OFDMA) reserve subcarriers of OFDM symbols for transmitting high priority voice data and video data to particular wireless devices, while the access point allocates the remaining available subcarriers for contention-based access by other wireless devices.

Low complexity MIMO scheduling with channel decomposition using capacity upperbound

Abstract: In multiuser MIMO systems, the base station schedules transmissions to a group of users simultaneously. Since the data transmitted to each user are different, in order to avoid the inter-user interference, a transmit preprocessing technique which decomposes the multiuser MIMO downlink channel into multiple parallel independent single-user MIMO channels can be used. When the number of users is larger than the maximum that the system can support simultaneously, the base station selects a subset of users who have the best instantaneous channel quality to maximize the system throughput. Since the exhaustive search for the optimal user set is computationally prohibitive, a low complexity scheduling algorithm which aims to maximize the capacity upper bound is proposed. Simulation results show that the proposed scheduling algorithm achieves comparable total throughput as the optimal algorithm with much lower complexity.

Mode Switching for MIMO Broadcast Channel Based on Delay and Channel Quantization

Abstract: Imperfect channel state information degrades the performance of multiple-input multiple-output (MIMO) communications; its effect on single-user (SU) and multi-user (MU) MIMO transmissions are quite different. In particular, MU-MIMO suffers from residual inter-user interference due to imperfect channel state information while SU-MIMO only suffers from a power loss. This paper compares the throughput loss of both SU and MU MIMO on the downlink due to delay and channel quantization. Accurate closed-form approximations are derived for the achievable rates for both SU and MU MIMO. It is shown that SU-MIMO is relatively robust to delayed and quantized channel information, while MU MIMO with zero-forcing precoding loses spatial multiplexing gain with a fixed delay or fixed codebook size. Based on derived achievable rates, a mode switching algorithm is proposed that switches between SU and MU MIMO modes to improve the spectral efficiency, based on the average signal-to-noise ratio (SNR), the normalized Doppler frequency, and the channel quantization codebook size. The operating regions for SU and MU modes with different delays and codebook sizes are determined, which can be used to select the preferred mode. It is shown that the MU mode is active only when the normalized Doppler frequency is very small and the codebook size is large.

One- and Two-Way Decode-and-Forward Relaying for Wireless Multiuser MIMO Networks

Abstract: In this paper, we propose multiuser multiple-input multiple-ouput (MIMO) one-and two-way relaying protocols. Several wireless MIMO node pairs are establishing either unidirectional or bidirectional transmission links via a single decode-and-forward MIMO relay without causing interference to each other. For both half-duplex two-phase relaying protocols, the relay separates messages of different node pairs spatially by using zero-forcing based methods, whereas, in order to cancel the interference within the bidirectional links, XOR preceding followed by self-interference cancellation is employed. Both sum rate and maxmin fairness optimizations, as well as quality-of-service assurance, are considered for the design of the precoders at the relay. The corresponding optimization algorithms for both one-and two-way relaying have been developed using semideflnite programming, which consider the intersection of multiple access and broadcast phases' rate regions.

Method for transmitting and receiving signals using collaborative mimo scheme

Abstract: A method for reducing inter-cell interference and a method for transmitting a signal by a collaborative MIMO scheme, in a communication system having a multi-cell environment are disclosed. An example of a method for transmitting, by a mobile station, precoding information in a collaborative MIMO communication system includes determining a precoding matrix set including precoding matrices of one more base stations including a serving base station, based on signal strength of the serving base station, and transmitting information about the precoding matrix set to the serving base station. A mobile station in an edge of a cell performs a collaborative MIMO mode or inter-cell interference mitigation mode using the information about the precoding matrix set collaboratively with neighboring base stations.

Apparatus and method for supporting a plurality of MIMO modes in a wireless communication system

Abstract: An apparatus and method for supporting a plurality of MIMO modes in a wireless communication system are provided, in which a receiver selects one of the plurality of MIMO modes according to at least one of a received signal strength, a speed of the receiver, a correlation between a transmitter and the receiver, a presence or absence of channel quality information, a precoding index, and an antenna information, a rank, and a number of users, generates feedback information in a transmission format corresponding to the selected MIMO mode, and transmits the feedback information to the transmitter using a feedback resources allocated according to the selected MIMO mode.

Scheduling method for multi-user mimo

Abstract: A priority computation process computes a priority of each user for each RB (Resource Brock) using a reception SINR. A maximum priority user selection/RB allocation process selects a user with the maximum priority for an unallocated RB and allocates the RB to the user. A frequency axis/space axis unallocated RB presence determination process proceeds to scheduling for a next user if there is an unallocated RB on the frequency or space axis. A projected channel vector update process updates a projected channel vector of an unselected user by GS orthogonalization. An orthogonal coefficient computation process computes an orthogonal coefficient. A corrected SINR computation process computes a corrected SINR. A next MIMO layer priority computation process computes priorities of unselected users for a corresponding RB in the next multiple MIMO layer. The priorities of the unselected users are used in the next user scheduling processing.

Network Coding meets MIMO

Abstract: In this paper, MIMO techniques are applied to network coding in order to exploit the spatial diversity inherent in packet combining. This is possible because network coding and MIMO solve similar problems, namely to decode a vector of transmitted symbols given a vector of received samples. This observation leads to a new approach for NC coding/decoding. Our basic idea is to move network coding functionalities towards the physical layer in order to jointly perform NC decoding and MIMO detection and gain the advantages of the two approaches. We refer to this new strategy as MIMO_NC. Theoretical analysis and simulations prove that our system is more robust than traditional network coding to fading and packet losses.

Eigen-beamforming for wireless communication systems

Abstract: Techniques for sending multiple-input multiple-output (MIMO) transmissions in wireless communication systems are described. In one design, a transmitter sends a first reference signal via a first link, e.g., a cell-specific reference signal via the downlink. The transmitter receives channel quality indicator (CQI) information determined by a receiver based on the first reference signal. The transmitter also receives a second reference signal from the receiver via a second link, e.g., a sounding reference signal via the uplink. The transmitter obtains at least one MIMO channel matrix for the first link based on the second reference signal. The transmitter determines at least one precoding matrix based on the at least one MIMO channel matrix, e.g., in accordance with ideal eigen-beamforming or pseudo eigen-beamforming. The transmitter then sends a data transmission to the receiver based on the at least one precoding matrix and the CQI information.

Spatial correlation and capacity measurements for wideband MIMO channels in indoor office environment

Abstract: This paper describes a broadband multiple input, multiple output (MIMO) channel characterization platform and capacity measurement results in indoor office environment. The MIMO testbed has been designed for broadband MIMO channel sounding, capacity measurements and for characterizing the directional-multipaths of the radio propagation channel. The MIMO channel data have been collected in 5 GHz band inside a modern office environment. Capacity results from these experiments are discussed for different propagation conditions, including non line-of-sight (NLOS) and LOS propagations with various spacings between array elements. Spatial correlations are analyzed from the measured data and a frequency selective MIMO channel model based on the correlation statistics is validated. Post-processing of the measured data with a sequential ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm is utilized to extract the directions of departures and arrivals of multipath components at the transmitting and receiving arrays, respectively. A method of obtaining broadband MIMO capacity, indirectly, from a single directional measurement of MIMO propagation channel is also proposed and verified from the measurement results.

MIMO wireless communication system

Abstract: A wireless communication system is described which includes a transmitter operable to transmit a transmitted signal, the transmitter having one or more transmitting antennae, a receiver operable to receive a received signal, the receiver having one or more receiving antennae, wherein lattice reduction is used in obtaining, at the receiver, an estimate of the transmitted signal based on the received signal, characterised in that the lattice reduction utilises a lattice reduction matrix, a decomposed representation of which is transmitted between the transmitter and the receiver.

A Distributed Multi-User MIMO MAC Protocol for Wireless Local Area Networks

Abstract: Multi-user multiple-input multiple-output (MIMO) systems have been emerging and attracting considerable attention recently for its potential to substantially improve system capacity via space division multiple access. In this paper, we propose a distributed multi-user (MU) medium access control (MAC) protocol for wireless local area networks (WLANs) with MIMO capability, using a leakage-based preceding scheme. By exploiting the multi-user degree of freedom in a MIMO system to allow the access point (AP) to communicate with multiple users in the same frequency band simultaneously, the proposed MU MAC can effectively minimize the AP-bottleneck effect in legacy WLANs. We then develop an analytical model to study the performance of the proposed MU MAC, in terms of the maximum number of users that can be supported and the network throughput. The analysis and simulation results show that the proposed MU MAC significantly outperforms the single-user MAC.

Performance Evaluation of MIMO in IEEE802.16e/WiMAX

Abstract: The methodology used to evaluate the performance of the various MIMO schemes in orthogonal frequency division multiplexing (OFDM) WiMAXWiMAX stands for Worldwide Interoperability for Microwave Access systems is described The per-tone signal-to-noise plus interference ratio (SINR) of a MIMO OFDM system is derived in a multi user, multicell and multisector communication system The sector average spectral efficiency for a coded MIMO OFDM system is evaluated under a single frequency reuse deployment scenario It is shown that the second antenna at the subscriber station receiver provides significant gains over single transmit single receive antenna (SISO) systems The rate 2 spatial multiplexing MIMO scheme on the downlink improves the sector spectral efficiency by 10% over single transmit and two receive antenna (SIMO) system in a single frequency reuse deployment It is also shown that in a time-division duplex (TDD) WiMAX system, the gain in downlink spectral efficiency from beamforming can be very large if the delay between the uplink channel estimate and the downlink beamforming is small compared to the coherence time of the channel On the uplink, it is shown that the collaborative spatial multiplexing increases the spectral efficiency by 9% over a SIMO system with the same number of receive antennas at the base station (BS) Significant gains in uplink spectral efficiency are seen with more antennas at the BS and by using uplink beamforming as compared to SISO

Multi-user MIMO relay protocol with self-interference cancellation

Abstract: Systems and methodologies for employing a multi-user Multiple Input Multiple Output (MIMO) relay protocol with self-interference cancellation in a wireless communication environment are provided herein Data streams sent between user device(s) and base station(s) can traverse through a relay Further, a decode and forward protocol can be employed by the relay rather than merely amplifying and forwarding the data streams Moreover, each user device can listen to and decode data stream(s) sent by other user device(s) to the relay, and these decoded data stream(s) can be utilized for interference cancellation Further, the user device(s) and the base station(s) can employ self-interference cancellation upon signals received from the relay According to another example, an adaptive relay power allocation algorithm, which enhances power efficiency, can be leveraged by the relay for transmitting signals to the user device(s) and the base station(s)

Method and apparatus for managing coexistence interference

Abstract: The present disclosure provides for a wireless device that can include a transceiver that communicates with a network through an access point using a radio frequency (RF) signal on a channel. The RF signal propagates through an RF environment shared by multiple interference-susceptible wireless devices. The wireless device can also include an interference determination module that 1) detects a coexistence interference profile involving the wireless device and the multiple interference-susceptible wireless devices and 2) determines a channel change for the transceiver to communicate with the network. The wireless device can also include a channel adjuster that changes the channel accordingly to increase the integrity of signals of interference-susceptible wireless devices.

CQI Table for Wireless MIMO Networks

Abstract: A Channel Quality Indicator table for wireless multiple input multiple output (MIMO) networks is disclosed. In one embodiment, a method of generating a table for channel quality indicator (CQI) for an open loop MIMO transmission includes calculating performance of a link between a transmitter and a user end unit for each MIMO transmission mode over a range of average signal-to-noise ratio, and selecting the MIMO transmission mode that maximizes performance for each subset of the range of average signal-to-noise ratio. The method further includes storing the selected MIMO transmission mode and the corresponding subset of the range of average signal-to-noise ratio in a CQI table, the CQI table being stored in an user end unit and a base transceiver station of the open loop MIMO network.

Performance of linear multi-user MIMO precoding in LTE system

Abstract: For scenarios with a large number of users to be served in one cell, high capacity gains can be achieved by transmitting independent data streams to different users sharing the same time-frequency resources. This technique is known as Multi-User MIMO (MU-MIMO). In this paper we investigate the performance of MU-MIMO operation in 3GPP LTE for different frequency granularities of the precoder at the OFDM transmitter. We also investigate the impact of channel correlation on the performance of the receiver when it is unaware of the interfererpsilas precoding vector. The performance is evaluated by means of semistatic system simulations.

System and method for incorporating dynamic orthogonal frequency-division multiplexing into wireless network protocols

Abstract: A method for incorporating dynamic orthogonal frequency-division multiplexing (“OFDM”) in wireless networks is provided. Information about a wireless channel between a transmitter and one or more receivers communicating according to a wireless protocol is acquired. Dynamic OFDM allocations are generated based on the acquired information about the wireless channel. The dynamic OFDM allocations are transmitted to the one or more receivers according to the wireless protocol.

Capacity of linear multi-user MIMO precoding schemes with measured channel data

Abstract: In multi-user multiple-input multiple-output (MU-MIMO) systems, spatial multiplexing can be employed to increase the throughput without the need for multiple antennas and expensive signal processing at the user equipments. In theory, MU-MIMO is also more immune to most of propagation limitations plaguing single-user MIMO (SU-MIMO) systems, such as channel rank loss or antenna correlation. In this paper we compare the performance of different linear MU-MIMO precoding schemes using real channel measurement data. The measurement data has been acquired using Eurecompsilas MIMO Openair Sounder (EMOS). The EMOS can perform real-time MIMO channel measurements synchronously over multiple users. The results show that MU-MIMO provides a higher throughput than SU-MIMO also in the measured channels. However, the throughput in the measured channels is by far worse than the one in channels without spatial correlation. Of all the evaluated linear precoding schemes, the MMSE precoder performs best in the measured channels.