Showing papers by "Timothy A. Thomas published in 2010"

LTE-advanced: next-generation wireless broadband technology [Invited Paper]

Abstract: LTE Release 8 is one of the primary broadband technologies based on OFDM, which is currently being commercialized. LTE Release 8, which is mainly deployed in a macro/microcell layout, provides improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operation and seamless integration with existing systems. LTE-Advanced (also known as LTE Release 10) significantly enhances the existing LTE Release 8 and supports much higher peak rates, higher throughput and coverage, and lower latencies, resulting in a better user experience. Additionally, LTE Release 10 will support heterogeneous deployments where low-power nodes comprising picocells, femtocells, relays, remote radio heads, and so on are placed in a macrocell layout. The LTE-Advanced features enable one to meet or exceed IMT-Advanced requirements. It may also be noted that LTE Release 9 provides some minor enhancement to LTE Release 8 with respect to the air interface, and includes features like dual-layer beamforming and time-difference- of-arrival-based location techniques. In this article an overview of the techniques being considered for LTE Release 10 (aka LTEAdvanced) is discussed. This includes bandwidth extension via carrier aggregation to support deployment bandwidths up to 100 MHz, downlink spatial multiplexing including single-cell multi-user multiple-input multiple-output transmission and coordinated multi point transmission, uplink spatial multiplexing including extension to four-layer MIMO, and heterogeneous networks with emphasis on Type 1 and Type 2 relays. Finally, the performance of LTEAdvanced using IMT-A scenarios is presented and compared against IMT-A targets for full buffer and bursty traffic model.

Closed-loop transmission feedback in wireless communication systems

Abstract: A method and apparatus for providing channel feedback is provided herein. During operation a covariance matrix at time t (R) is calculated by the mobile as a function of a received downlink signal. In order to reduce overhead, R is normalized and quantized by the mobile using multiple codebook entries plus at least one constant for quantization. The mobile then transmits the normalized and quantized covariance matrix back to the base station as bit values indicating the selected entries from the codebook plus bit values corresponding to the at least one constant. The base unit then uses the covariance matrix estimate to determine appropriate channel beamforming weights, and instructs transmit beamforming circuitry to use the appropriate weights.

Method and apparatus for pilot signal processing in a wireless communication system

Abstract: A wireless communication system is provided that spreads pilot signals, or channel state information reference signals (CSI-RSs), using a spreading code chosen from a set of mutually unbiased bases (MUBs) The advantages of such spreading with MUBs are that multiple base stations can send their pilot signals on a same time-frequency resources, making the pilot signal design very efficient and also improving channel estimation at a user equipment through orthogonal and quasi-orthogonal spreading which gives a gain above noise and interference A short spreading code chosen from MUBs may be used for spreading pilot signals transmitted from each antenna of a base station within a time-frequency resource comprising multiple closely-spaced subcarriers in frequency and/or multiple closely-spaced symbols in time

Closed-loop feedback in wireless communications system

Abstract: A method and apparatus for providing channel feedback is provided herein. During operation a covariance matrix at time t (R) is calculated as a function of a received downlink signal. In order to reduce overhead, R is normalized and quantized. The base unit then uses the covariance matrix estimate to determine appropriate channel beamforming weights, and instructs transmit beamforming circuitry to use the appropriate weights. In an embodiment, circuitry performs a method of calculating a first precoding matrix index I from a codebook using a received signal, calculating a second codebook index J* using the first precoding matrix index I to approximate a covariance matrix and calculating a quantized coefficient α* to approximate the covariance matrix wherein the quantized coefficient α* is determined using the first precoding matrix index I.

MU-MIMO system performance analysis in LTE evolution

Abstract: It has been recognized that a primary method of further enhancing the spectral efficiency of LTE Release-8 is to improve support for multi-user (MU) MIMO transmission in the downlink in an efficient manner. This paper investigates the performance improvements due to the enhanced support of single-cell MUMIMO within the framework of LTE evolution (specifically LTE Release-9 and Release-10 also known as LTE-Advanced) considering a 4Tx-2Rx system. A realistic estimate of achievable MU-MIMO full-buffer throughput performance is provided with respect to variations in the propagation environment, feedback accuracy, antenna polarizations, antenna calibration as well as receiver type. It is observed that a closely spaced uniform linear array (ULA) at the base station and interference rejection capability at the mobile is essential to maximize MU-MIMO gains. Further, channel reciprocity based spatial channel feedback in a TDD scenario can provide significantly higher MU-MIMO throughput compared to codebook based feedback.

CSI Reference Signal Designs for Enabling Closed-Loop MIMO Feedback

Abstract: Obtaining reliable channel state information (CSI) in future wireless standards is critical for the operation of downlink closed-loop techniques such as SU-MIMO, MU-MIMO, and coordinated multipoint (CoMP). However, this CSI is difficult to obtain especially when considering the potentially strong interference from neighboring sectors/base stations on the downlink reference symbols (RSs, aka pilot symbols) used to obtain the CSI. This paper describes three possible CSI RS designs for enabling such closed- loop feedback in light of the strong interference environment and discusses some tradeoffs between the designs. First, two code-division multiplexing (CDM) designs are proposed where one has nine orthogonal sequences and the other has three orthogonal sequences. Next a new CSI-RS design is described which uses mutually unbiased bases (MUB) to provide a 6 dB quasi-orthogonal gain over neighboring cells. The MUB design is shown in system-level simulation to have performance close to a design with 9 orthogonal sequences while providing a finer frequency-domain sampling than the 9- orthogonal approach.

Differential closed-loop transmission feedback in wireless communication systems

Abstract: A method and apparatus for providing channel feedback is provided herein. During operation a covariance matrix at time t (R) is calculated as a function of a received downlink signal. Matrix C t is also calculated and is based on a previous quantized covariance matrix (R q t-1 ), the covariance matrix (R) at time t, and a forgetting factor (γ) that is applied to R q t-1 . The C t is then used to create a DERC feedback message (signal or waveform) and may be transmitted with pilots on a proper feedback channel to a base unit. The base unit receives the feedback (C t ) as a DERC waveform on a proper feedback channel. The base unit uses non-coherent or coherent detection to detect the DERC values send by the remote unit and uses the DERC values with a previous quantized covariance matrix estimate, a forgetting factor, and a weighting value to compute a covariance matrix estimate to use for beamforming. The base unit then uses the covariance matrix estimate to determine appropriate channel beamforming weights, and instructs transmit beamforming circuitry to use the appropriate weights.

Error-resistant differential feedback of long-term covariance matrix in closed-loop MIMO

Abstract: One method of obtaining channel information for closed-loop SU-MIMO or MU-MIMO is for the mobile to feed back a covariance matrix based on the measured downlink channel However for MU-MIMO to be able to operate properly (ie, have low crosstalk), the covariance matrix must be fed back with fairly high precision which would result in a very high level of overhead This paper describes two new methods for feeding back accurate covariance matrix estimates from a mobile to a base station with low overhead, while being very robust to transmission errors System-level results show that one of the proposed feedback methods has performance very close to an unquantized covariance matrix when the feedback bit error rate is less than 10% When the feedback bit error rate is 20% one of the proposed feedback methods has better performance than an existing single-shot covariance matrix quantization method operating with no feedback errors while additionally requiring less feedback overhead than the existing method (16 bits versus 28 bits)