Man-On Pun

Maximum-likelihood synchronization and channel estimation for OFDMA uplink transmissions

Maximum-likelihood estimation of the carrier frequency offset (CFO), timing error, and channel response of each active user in the uplink of an orthogonal frequency-division multiple-access system is investigated in this study, assuming that a training sequence is available. The exact solution to this problem turns out to be too complex for practical purposes as it involves a search over a multidimensional domain. However, making use of the alternating projection method, we replace the above search with a sequence of mono-dimensional searches. This results in an estimation algorithm of a reasonable complexity which is suitable for practical applications. As compared with other existing semi-blind methods, the proposed algorithm requires increased overhead but has more flexibility as it can be used with any subcarrier assignment scheme. Simulations indicate that the accuracy of the CFO estimates asymptotically achieves the Cramer-Rao bound.

Iterative detection and frequency synchronization for OFDMA uplink transmissions

The problem of frequency synchronization, channel estimation, and data detection for all active users in the uplink of an OFDMA system is investigated in this work. Since the exact maximum likelihood (ML) solution to this problem turns out to be too complex for practical purposes, we derive an alternative scheme that operates in an iterative fashion. At each step, the superimposed signals arriving at the base station (BS) are separated by means of the space-alternating generalized expectation-maximization (SAGE) algorithm. Each separated signal is then passed to an expectation-conditional maximization (ECM)-based processor that updates frequency estimates and performs channel estimation and data detection for each user. The resulting architecture is reminiscent of the parallel interference cancellation (PIC) receiver, where interference is generated and removed from the received signal to improve the system performance. Simulations indicate that the proposed scheme outperforms other benchmark solutions at the price of increased computational complexity

An EM-based joint maximum likelihood estimation of carrier frequency offset and channel for uplink OFDMA systems

A maximum likelihood estimator (MLE) based on expectation maximization (EM) method is presented to jointly estimate the carrier frequency offset (CFO) and the channel of each user in uplink OFDMA systems. The proposed MLE distinguishes itself from existing methods by its applicability to any arbitrary carrier assignment schemes. The proposed MLE achieves high computational efficiency by transforming a multidimensional maximization problem into a number of substantially smaller separate maximization problems. Computer experiments have been conducted to confirm the effectiveness of the proposed estimators in estimation accuracy and robustness against the near-far effect.

Secure and efficient cryptosystem for smart grid using homomorphic encryption

A secure and efficient cryptography-based data exchange scheme for the smart grid based on homomorphic encryption is investigated in this work. Public key cryptography has been proposed as the main tool to ensure the security of a smart grid [1]. We first examine the shortcomings of conventional public key cryptography to motivate our current study. For the uplink communication (i.e. from customers to the utility) in the proposed scheme, smart meters homomorphically encrypt privacy-sensitive customer data before sending them over the grid. To prevent message forgery, a simple authentication scheme is designed specifically for uplink communication. For the downlink communication (i.e. from the utility to customers), the utility transmits the signed control messages to customers using digital signatures to ensure that messages from utilities are not tampered. For the data exchange scheme, we propose a practical cryptosystem based on a partially homomorphic encryption algorithm. The proposed solution has several appealing features such as strengthened security, high efficiency and user privacy preservation.

Joint synchronization and channel estimation in uplink OFDMA systems

We consider the uplink of an OFDMA system and address the problem of estimating the carrier frequency offset, timing error and channel response of each active user. In doing so we follow a maximum likelihood (ML) approach and assume that a training sequence is available. Unfortunately, joint ML estimation of all the above parameters involves a multi-dimensional grid-search that is difficult to implement in practical systems. Therefore we resort to the alternating-projection algorithm and replace the multidimensional search with a sequence of one-dimensional searches. Compared to other existing methods, the proposed estimator has more flexibility since it can be used with any subcarrier assignment scheme. Simulations indicate that the accuracy of the frequency estimates asymptotically achieve the relevant Cramer-Rao bound (CRB).

Optimized opportunistic multicast scheduling (OMS) over heterogeneous cellular networks

Optimized opportunistic multicast scheduling (OMS) has been studied previously by the authors for homogeneous cellular networks, where the problem of efficiently transmitting a common set of data from a single base station to multiple users that have identical channel statistics was examined. It has been demonstrated that OMS can achieve significant performance improvement by exploiting the optimal tradeoff between multiuser diversity and multicast gain. In this work, we extend our studies to heterogeneous networks with users subject to different channel statistics. Specifically, we consider a single cell wireless network with users uniformly distributed in a circular region around the base station. Since users with low SNR are the ones that hinder system throughput, we argue that system performance may be predicted by the behavior of users in the outmost ring of the cell, which are approximately homogeneous. Using extreme value theory and results obtained from the homogeneous case, we determine the optimal user selection ratio for a homogeneous ring of users near the edge of the cell and then use it to derive the optimal selection ratio over the entire heterogeneous network. Simulations confirm theoretical results and illustrate the effectiveness of the proposed scheme.

Codebook-based quantized MIMO feedback for closed-loop transmit precoding

Advanced quantization schemes are proposed for closed-loop transmit precoding over correlated multiple-input multiple-output (MIMO) channels in this work. Unlike the conventional schemes that directly quantize the MIMO channel covariance matrix and feed back each quantized matrix element, the proposed schemes quantize the channel covariance matrix by exploiting the common rank-one codebook shared by mobile stations and base stations. Compared to the conventional quantization schemes, the proposed quantization schemes can achieve comparable throughput performance with more than 50% overhead reduction at the cost of affordable increase in computational complexity.

WLC23-6: Performance Analysis of Ultra-Wide Band Impulse Radio (UWB-IR) with Super-Orthogonal Turbo Codes (SOTC)

Low-complexity low-rate super-orthogonal turbo codes (SOTC) are proposed in this work to replace the implicit repetition code (RC) in an ultra-wide band impulse radio (UWB-IR) system to improve BER, transmission range and system throughput. Various receivers including matched-filter and RAKE receivers are adopted for data detection in the additive white Gaussian noise (AWGN) channel and the indoor IEEE 802.15.3a channel. The performance of SOTC-encoded UWB is analyzed and validated by computer simulation. It is shown that the SOTC-based UWB-IR system can achieve significant performance improvement over the conventional DS- UWB system encoded by RC.

Ultra-Wide Band Impulse Radio (UWB-IR) with Super-Orthogonal Turbo Codes (SOTC)

The performance of a direct-sequence ultra-wide- band (DS-UWB) system coded by super-orthogonal turbo codes (SOTC) over simulated UWB channels is studied in this work. The SOTC scheme is attractive due to its low rate and low- decoding-complexity features. Several receivers are compared for data detection over an indoor IEEE 802.15.3a channel, including the SOTC encoded matched-filter (MF) receiver and the RAKE receiver. The performance of the SOTC-encoded UWB system with different receivers is evaluated by computer simulation. It is shown that the proposed system can achieve significant performance improvement over the DS-UWB system encoded by conventional repetition codes (RC).

Joint maximum likelihood estimation of carrier frequency offset and channel in uplink OFDMA systems

A maximum likelihood estimator (MLE) that jointly estimates the carrier frequency offset (CFO) and the channel response of each user in uplink OFDMA systems is investigated in this research. The proposed MLE distinguishes itself from existing methods by its applicability to more flexible carrier assignment schemes. It achieves high computational efficiency by transforming a multidimensional optimization problem into a one-dimensional optimization problem. A suboptimal method is developed to further reduce the computational complexity. It is demonstrated by simulation results that the proposed MLE can provide accurate CFO and channel estimation in both SISO and SIMO environments.