TCM coding of PPM based modulations for Infrared WLAN's impaired by ISI

In this communication we consider the use of PPM based modulation methods, such as the hybrid modulation method called Amplitude and Pulse-Position Modulation (APPM) and Overlapping Pulse-Position Modulation (OPPM) to improve the performance of infrared WLAN’s, by the use of Trellis-Coded Modulation (TCM) codes. We describe the best code search and results, which demonstrate that, even with trellis codes of moderate complexity, non-negligible coding gains can be obtained without bandwidth expansion. Monte Carlo simulations have been done to compare uncoded PPM against APPM and OPPM TCM coded systems performances, on ceiling-bounce channel models for various values of delay spread. Although our codes were derived for the AWGN channel without Inter-Symbol Interference (ISI) awareness, their behavior in multipath dispersion channels is quite effective

Performance of extended space-time coding techniques for MIMO MC-CDMA systems

In this paper we consider a transmission system based on MC-CDMA, where signal spreading is performed entirely in the frequency domain. An extended spacetime coding technique, the Double Alamouti, is evaluated considering a MIMO channel. This assessment is made against standard Alamouti coding, for two different Tx/Rx antenna schemes (2x1 and 2x2). Numerical results, attained through system model simulations, are presented for performance evaluation under realistic scenarios considering some typical system impairments. These results show that in practical systems significant improvements can be achieved by using the Double Alamouti coding scheme

Simulation-based evaluation of spectrum opportunities in UMTS cellular networks

A scenario based on an UMTS TDD opportunistic cellular system that operates over UMTS FDD licensed cellular networks is considered. Therefore we develop a simulation tool that addresses the goal of analysis and assessment of UMTS TDD opportunistic radio system in a coexistence environment with UMTS FDD primary cellular networks. The communication presents the scenario considered, the main features of the tool, discusses and proposes metrics to evaluate the communication opportunities in UMTS FDD primary cellular networks, and presents numerical simulation results. These show that a moderate number of UMTS TDD available frequencies and a reasonable UMTS TDD opportunistic radio transmission power will allow the deployment of UMTS TDD opportunistic radio networks in a coexistence environment with existing licensed systems

MIMO-OFDM channel estimation: a pilot sequence design for time-domain processing

A pilot sequence design for MIMO-OFDM systems is investigated where all transmit antennas share the same sub-carriers to convey pilot symbols. The pilot sequences are endowed with phase-shifting properties, granting the receiver the possibility of extracting the impulse responses for all channels directly from the antennas’ time-domain received signals, without co-channel interference. A linear processing of the timedomain samples is all that is needed to get the channel impulse responses, resulting in an algorithm with very low computational load. The feasibility of the investigated method is substantiated by system simulation using indoor and outdoor broadband wireless channel models

Frequency domain equalizer for multirate UMTS-TDD systems

Direct-sequence spread spectrum (DS-SS) signals exhibit cyclostationary properties which imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper we present a multiple access interference canceller that explores this property and applies to UMTS-TDD. This linear frequency domain canceller operates in the spreaded signal in such a way that the interference and noise at its output is minimized (minimum mean squared error criterium). The performance is evaluated in two configurations: one including the frequency shift canceller (FSC) and the other concatenated with parallel interference canceller (PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector

Lossy source coding using belief propagation and soft-decimation over LDGM codes

This paper focus on the lossy compression of a binary symmetric source. We propose a new algorithm for binary quantization over low density generator matrix (LDGM) codes. The proposed algorithm is a modified version of the belief propagation (BP) algorithm used in the channel coding framework and has linear complexity in the code block length. We also provide a common framework under which the proposed algorithm and some previously proposed algorithms fit. Simulation results show that our scheme achieves close to state-of-the-art performance with reduced complexity

Mitigating CFO in OFDM systems by exploring the symbol structure

In this paper we revisit the carrier frequency offset (CFO) estimation and removal for broadband orthogonal frequency division multiplexing (OFDM) systems. We propose an algorithm that explores the time-domain (TD) properties of transmitted symbols carrying pilots and data. A careful examination of the components that constitute the TD received symbols leads to the definition of a simple, yet effective, cost function. In a real scenario, the cost function presents a bowl shape with a single global minimum easily identifiable (no local minima). The proposed method is independent of the channel estimation and decision, presenting a very low computational load. The feasibility of the investigated method is substantiated by system simulation using BRAN-A broadband wireless channel model. Simulation results show that the impact of the residual CFO in the system¿s performance is minimal

Frequency shift based multiple access interference canceller for multirate UMTS-TDD systems

Direct Sequence Spread Spectrum (DS-SS) signals exhibit cyclostationary properties which imply a redundancy between frequency components separated by multiples of the symbol rate. In this paper we present a Multiple Access Interference Canceller that explores this property and applies to UMTS-TDD. This frequency domain canceller operates in the spreaded signal in such way that the interference and noise at its output is minimized (Minimum Mean Squared Error Criterium). The performance is evaluated in two con gurations: one including the Frequency Shift Canceller (FSC) and the other concatenated with Parallel Interference Canceller(PIC). The results are benchmarked against the performance of the conventional RAKE detector and the conventional PIC detector

On the impact of the pilot density in the channel estimation of MC-CDMA systems

In this paper we investigate the impact of the pilot density (and associated pilot distances for both frequency and time domains) in the performance of the channel estimation for MC-CDMA systems, when using dedicated sub-carriers to transmit pilots with 2-D rectangular patterns. The analytical formulation of the channel estimation MSE for the single-user downlink scenario with a generic channel estimation scheme for a given power efficiency is presented. The attained results go beyond the commonly accepted rule of thumb that suggests that an oversampling factor of two will result in a good trade-off between performance and pilot overhead. The analytical formulation was validated by system simulations. A scenario using the 2-D LS-DFT channel estimation scheme confirmed the analytical result. The lowest channel estimation MSE for a given value of power efficiency is achieved when the pilot density is the one that closely fulfils the 2-D sampling theorem

Multi-cell cooperation for future wireless systems

Portuguese CADWIN - PTDC/EEA TEL/099241/2008Portuguese Foundation for Science and Technology (FCT