Branimir Stantchev

Time-variant distortions in OFDM

We analyze the degradation of orthogonal frequency division multiplexing (OFDM) signals from time-variant distortions like carrier and sampling frequency offset, Doppler spread and oscillator phase noise. We propose a novel analytical framework in the discrete-time domain based on the FFT demodulation of a single OFDM symbol and unify previous methods with some new results. Exact definitions of the signal-to-subcarrier-interference ratio (SIR) and the effective signal-to-noise ratio (SNR) are given and evaluated for a HIPERLAN/2 system.

Burst synchronization for OFDM-based cellular systems with separate signaling channel

A novel burst synchronization scheme suitable for a high rate OFDM/TDMA/TDD (pico)cellular system with a centralized dynamic slot allocation MAC protocol is presented. The physical layer may have an optional low-rate separate signaling channel (SSC) used in the standby mode and implemented on a subset of OFDM subcarriers. A one-shot feedforward synchronization concept for the OFDM-based active transmission mode is designed using the functionality of the SSC and a special periodical OFDM preamble. Joint symbol and frequency synchronization as well as channel estimation can be achieved simultaneously. The synchronizer features low complexity and low processing delay. Simulations verify its robust operation for indoor channel power delay profiles at 5 GHz.

An integrated FSK-signaling scheme for OFDM-based advanced cellular radio

Future broadband picocellular networks demand powerful modulation and coding methods under the tight constraints of affordable modem complexity and low power consumption for the mobile terminal (MT). In order to increase battery life and allow low-cost mobile equipment to be effectively integrated in such a broadband system we propose the use of a separate low-rate signaling channel (SSC) on the physical layer. This additional channel has the primary function of power saving. For this purpose it takes on connection and mobility control functions as well as further tasks on broadcasting and synchronization. A new concept of a broadband TDMA system is presented, incorporating OFDM for the physical high rate data channel (D-channel) and CPFSK modulation with non-coherent detection for the low-rate SSC, whereby both channels are separated in frequency. Modem and system design issues are discussed and evaluated by simulation.

Optimum frame synchronization for orthogonal FSK in flat fading channels and one burst application

We derive a maximum likelihood (ML) rule for one-shot frame synchronization of a random data burst with non-coherent orthogonal M-FSK signaling in a flat fading or AWGN channel. It uses a known frame synchronization pattern and, optionally, channel state information at the receiver. High SNR approximations are developed that are well suited for a low-cost implementation. A burst synchronizer structure is proposed with an ML frame synchronizer as the last processing stage. Simulations reveal the performance gain of the ML rules over conventional correlation rules for a 2-CPFSK modulated separate signaling channel in an OFDM FTDMA broadband cellular system.

Synchronizability of general periodic pattern signals

An optimum and sub-optimum maximum likelihood (ML) rule for joint frame and carrier frequency offset estimation are derived, which rely on the transmission of a periodic pattern signal with an arbitrary number of periods and period length, embedded in the random data signal of a single user. Ad-hoc modification of the ML rule yields a robust metric for pattern search in a multi-user signal in a time division multiple access and time division duplex (TDMA/TDD) system, and a receiver with automatic gain control keeping constant average signal power at the synchronizer input. The mean, variance and Cramer-Rao Bound of the frequency offset estimate as well as the mean and variance of the frame estimate are theoretically evaluated and verified by simulation. The presented material proposes a unified framework for synchronization by periodic pattern signals and provides some new results, which are compared to known references.