Kambiz Zangi

LTE-Advanced - Evolving LTE towards IMT-Advanced

This paper provides a high-level overview of some technology components currently considered for the evolution of LTE including complete fulfillment of the IMT-advanced requirements. These technology components include extended spectrum flexibility, multi-antenna solutions, coordinated multipoint transmission/reception, and the use of advanced repeaters/relaying. A simple performance assessment is also included, indicating potential for significantly increased performance.

A framework for future radio access

A framework for future radio-access systems, enabling user data rates up to 100 Mbps with wide-area coverage and up to 1 Gbps for local-area coverage, is presented. The proposed framework supports flexibility in system bandwidth and duplex arrangements. Furthermore, support for relaying and advanced antenna solutions is integrated in the design of the framework. We believe that the proposed framework provides a suitable platform for development and evaluation of novel techniques required for future radio-access solutions.

Enhanced DC estimation via sequence-specific frequency offset

A method for improving DC estimation over any particular training sequence is presented for use in direct conversion receivers. It is known that direct-conversion receivers suffer from a large DC-offset, which often cannot be adequately removed by a simple average of the received signal. To alleviate this problem, the DC offset can be jointly estimated with the channel over the training sequence in baseband. However, training sequences designed to facilitate channel estimation are not necessarily good for the estimation of DC offset. In the proposed method, an intentional frequency offset tailored to each training sequence is imposed at the radio front-end in order to improve the ability of estimating DC over that training sequence in digital baseband. We analyze the resulting DC estimation error and show analytically how the optimal frequency offset can be derived for each training sequence. Using the EDGE air interface, we demonstrate that significant gains can be achieved with this new method.

Impact of Transmit Array Geometry on Downlink System-Level Performance of MIMO Systems

For a cellular system with a fixed number of transmit antennas at each base station, we investigate how the system-level performance varies as a function of the geometry of the transmit array used at each base station. Our results show that the system-level performance of a cellular system can be significantly improved by using transmit antenna geometries other than the uniform linear arrays that have been investigated extensively in the literature so far. For example with 4 transmit antennas at each base station, we show that the spectral efficiency for achieving a 5 percentile user data rate of 2 Mbps (in a 5 MHz bandwidth) is improved by 58% when a non-uniform, linear array is used instead of a traditional uniform linear array.

Feedback of channel state information using the digital reverse link

In this paper, we focus on a cellular system with M transmit antennas at the base station (BTS) and one receive antenna at the mobile (i.e. an M-input/single-output (MISO) channel), where the BTS commands each mobile to transmit its channel state information back to the BTS. Our main result is a specific method for each mobile to use the uplink channel to inform the BTS of the state of its M downlink channels. This feedback method is particularly well-suited for wireless packet data cellular systems on the downlink where the entire downlink channel is allocated to one downlink user at a time (e.g. a cellular packet data system employing proportional fairness type scheduling on the downlink). We show that with a small portion of uplink channel resources devoted to feedback of channel state information per site, our feedback method results in a performance very close (within 1dB) to the performance that can be obtained with perfect channel knowledge at the BTS.

Maximum likelihood channel estimation with noise of unknown power spectrum

In this paper, we present an algorithm for obtaining maximum-likelihood (ML) channel estimates when the baseband noise is colored with an unknown power spectrum. The algorithm jointly estimates the channel and the spectrum of the noise in an iterative manner. We use the EDGE (enhanced data rates for global evolution) air interface to evaluate the impact of the proposed channel estimation technique in both interference dominated and thermal noise dominated situations. Simulation results show that the algorithm is very effective in suppressing the adjacent channel interference. For example, about 5 dB reduction in C/I required to achieve 10% block error rate at coding rate of 0.76 can be achieved.

An adaptive maximum-likelihood receiver for colored noise and interference

A receiver that adapts to the second-order statistics of the disturbance (interference or noise) is presented. By adapting to the statistics of the disturbance, this receiver produces the maximum-likelihood (ML) estimate of the transmitted sequence for any type of disturbance, in contrast to a conventional receiver that only produces the ML estimate when the discrete-time disturbance is white. This disturbance signal could result from (1) cochannel interference, (2) adjacent-channel interference or (3) thermal noise. The proposed receiver consists of a Nyquist analog receive filter and an adaptive maximum-likelihood sequence estimator for colored noise. We show that the samples of the output of this analog receive filter, at one sample per symbol, are sufficient statistics for finding the most likely transmitted sequence regardless of the type of disturbance present at the input to the receiver. An important feature of this adaptive receiver is that the analog part of the this receiver is fixed, i.e. adaption to the statistics of the disturbance is accomplished by processing in the digital domain only (at one sample per symbol). Using the EGPRS air interface as an example, we show that this adaptive receiver performs substantially better than a conventional non-adaptive receiver: (1) cochannel dominated case: 4.0 dB of gain, (2) adjacent-channel dominated case: 3.5 dB of gain, (3) thermal-noise dominated case: 1.2 dB of gain.

Direct Geolocation of Spread-Spectrum Emitters

Position estimation (geolocation) of a direct-sequence, spread-spectrum radio emitter is particularly challenging because of low signal-to-noise ratio at the radio sensors. In this letter, we develop a maximum-likelihood approach to geolocation that utilizes knowledge of the spreading sequence. Simulation results are used to show that spreading sequence knowledge can provide significant gains in position accuracy.

Error Compensated MMSE-Based Multi-User Precoding for Coordinated Multi-Point Transmission

Coordinated multi-point (CoMP) transmission is a general method for mitigating the inter-cell interference in a cellular network through the use of multi-user precoding across neighboring base stations. In this paper, we investigate the system performance of MMSE-based multi-user precoding in a cellular network with CoMP transmission. An iterative algorithm that exploits the duality between the downlink broadcast channel and the uplink multi-access channel is used to compute the multi-user precoding matrices. The algorithm uses the knowledge of the channel error covariance during the precoder computation to alleviate the impact of channel uncertainty at the network. We simulate the system performance of various MMSE-based precoders under the CoMP setting using a realistic MIMO channel model and compare them with those of the zero-forcing precoders. We show that the error-compensated MMSE precoder is more robust towards channel uncertainty and can significantly outperform zero-forcing precoding at high loads.

Using side information to improved channel estimation for channels with fractionally-spaced medium rays

We present a technique to incorporate the knowledge of the transmit filter and the receive filter into the channel estimation process, when the spacing between the medium rays is shorter than the symbol duration. We use the EDGE (Enhanced Data rates for Global Evolution) air interface to evaluate the impact of this new channel estimation technique. For the EDGE air interface, we derive analytical expressions showing that the error in estimating the channel coefficients is reduced by incorporating side information in the estimation operation. Simulation results with the EDGE air interface (8PSK modulation) are presented, and these simulations show a 1 dB reduction in E/sub b//N/sub 0/ required to achieve 1% block error rate at coding rate of 0.76 (i.e. MCS-7 mode of EDGE).