Showing papers by "Timothy A. Thomas published in 2007"

Method and apparatus for performing spatial-division multiple access

Abstract: A method and apparatus for performing spatial-division multiple access within a communication system is provided herein. During operation, nodes will return null subspace information along with their channel subspace information. The null subspace is similar to the channel subspace except that the null subspace information directs a base station to the matrix in a predefined codebook that results in minimum power being received at the node. The null subspace for each node is taken into consideration when communicating with a particular node.

Methods for Switching Between Long Term and Short Term Transmit Beamforming in OFDM

Abstract: Obtaining channel state information (CSI) at the base station in an OFDM system can significantly improve both the capacity and reliability of the downlink. However, this CSI will suffer from estimation error plus possible quantization error for certain feedback methods and will also become outdated because of time variations in the channel. If the channel variations are reasonably slow and the estimation (or quantization) errors are small, then per-subcarrier maximal ratio transmission (MRT) is the optimal single-stream beamforming technique (MRT is designed to match the instantaneous channel and hence is a short-term transmit beamformer). When the estimation (or quantization) error is high, then eigen beamforming (EBF) is typically better than MRT because it uses long-term statistics that can be estimated more accurately by averaging over many samples. In addition, if the channel variations are high, then EBF is typically better than MRT because again EBF is matched to the long-term channel statistics (e.g., channel covariance matrix) that do not change rapidly. Since MRT is better than EBF under some conditions and EBF is better than MRT in others, it may be necessary for the base to choose between the two. This paper introduces three methods to switch between MRT and EBF and shows through simulation results that the methods are fairly effective at choosing the best method under various channel conditions.

Method and apparatus for finite impulse response cyclic-shift diversity

Abstract: A generalized form of cyclic shift diversity is described for use in an OFDM system with multiple transmit antennas. Multiple cyclic shifts are performed for each transmit antenna and the shifted signals are scaled and summed to form a time-domain data stream for each transmit antenna. A cyclic extension is added to each data stream prior to transmission.

Pilot signal transmission method and apparatus

Abstract: A pilot (or reference) transmission scheme is utilized where different transmitters are assigned pilot sequences with possibly different cyclic time shifts. A pilot signal is transmitted concurrently by the transmitters in a plurality of pilot blocks, and a receiver processes the plurality of received pilot blocks to recover a channel estimate for at least one of the transmitters while suppressing the interference due to the pilot signals from the other transmitters.

Method and apparatus for performing stream weighting in an sdma communication system

Abstract: A method and apparatus for performing stream weighting in a spatial-division multiple access communication system is provided herein. During operation the base station receiver receives channel information from mobiles (also known as users or nodes) and calculates transmit antenna weights that maximize the power delivered to each mobile while allowing for a small amount of crosstalk to exist between users. Information or data is transmitted to the nodes utilizing the appropriate stream weights. By allowing tolerable amounts of cross talk, the coherent gain to the desired SDMA user can be increased, and thus the overall performance is improved.

User Selection for SDMA Beamforming with Imperfect CSI in MIMO-OFDM Systems

Abstract: Leveraging multi-user diversity by selecting users from a user pool has been a critical strategy for realizing a fraction of the MIMO broadcast channel capacity. This paper considers exploiting multi-user diversity in a unique and practical setting that includes - significant transmit correlation, a broadband channel using OFDM, and imperfect CSI at the transmitter derived from channel reciprocity techniques. These imply a higher correlation amongst user channels and an increase in the volume of CSI per user. Under these assumptions, algorithms are presented for selecting G out of Nu users for applying SDMA beamforming and also for switching off SDMA when the G selected user channels may not support simultaneous transmission. In a EUTRA-type simulation setting, the algorithms lead to gains on the order of 5-7 dB in coded FER with reasonable costs in terms of CSI, sum-rate and computational complexity.

An Algorithm for 2-User Downlink SDMA Beamforming with Limited Feedback for MIMO-OFDM Systems

Abstract: Recent information theoretic results on MIMO broadcast channels have promoted research attention on devising practical transmit-receive strategies that achieve a reasonable fraction of the broadcast capacity. This paper proposes an algorithm for SDMA weight construction for two users. The weights are chosen to represent a subspace that lies between the dominant signal subspace of one user and the nullspace of the other. This proposal is unique due to its practical considerations - a broadband MIMO broadcast channel employing OFDM, imperfect CSI at the transmitter in the form of codebook based limited feedback, coded BER as a performance metric, and the ability to choose the users for SDMA arbitrarily. Simulation results in an IEEE 802.16e-style setup show that a coded BER within 1-2 dB of the case of perfect CSI can be achieved with a feedback of less than 1 bit per subcarrier. Additionally, maximum ratio combining receivers suffice and they may be oblivious to the multi-user nature of the SDMA transmission.

Transmit Beamforming Weights for Channel Estimation Error and Feedback Latency in OFDM

Abstract: This paper presents a new method for computing transmit beamforming weights in OFDM when there is channel state information (CSI) uncertainty at the base. The channel uncertainty may be caused by channel estimation error, quantization effects, and/or channel variations from when the CSI is measured and when the beamforming weights are applied. The weight calculation automatically trades off using long-term statistics (e.g., the average spatial covariance matrix) and short-term beamforming (i.e., maximal ratio transmission). Simulation results show that the new weights have very promising performance with the benefit of not requiring a means to decide whether the short-term or long-term beamforming should be used.

An algorithm for 2-userdownlink sdma beamforming with limitedfeedback for mimo-ofdm systems

Abstract: Recent information theoretic results onMIMO broadcast channelshavepromoted research attention ondevising practical transmit-receive strategies that achieve areasonable fraction of thebroadcast capacity. Thispaperproposes analgorithm for SDMA weight construction fortwousers. Theweights arechosentorepresent asubspace that lies between thedominant signal subspace ofoneuserandthenullspace oftheother. Thisproposal isunique duetoitspractical considerations -abroadband MIMO broadcast channel employing OFDM,imperfect CSIat thetransmitter intheformofcodebook based limited feedback, codedBER asaperformance metric, andtheability tochoose theusers forSDMA arbitrarily. Simulation results inanIEEE 802.16e-style setup showthat acoded BERwithin 1-2dBofthe caseofperfect CSIcanbeachieved withafeedback ofless than 1bitpersubcarrier. Additionally, maximumratio combining receivers suffice andtheymaybeoblivious tothemulti-user nature oftheSDMA transmission. IndexTerms-space division multiplexing, MIMO systems.