Carmen E. Rodríguez

Real-time H.263+ video transmission on 802.11 wireless LANs

Wireless video services in packet-switched environments constitute a significant challenge. While radio channels present limited capacity high error rates and time-varying and asymmetric propagation properties, video applications are bandwidth-killing, error-sensitive and delay-intolerant. This situation forces wireless video systems to dynamically adapt to changes in the network resources. In the recent past there have been some papers investigating video transmission over wireless networks (H. Liu and M. El Zarki, 1997; N. Chan and P.T. Mathiopoulos, 2000), performance of the transfer protocols proposed in the IEEE 802.11 standard (H.S. Chhaya and S. Gupta, 1996; J. Weinmiller et al., 1997), and real time video communications in packet-switched infrastructure networks (P. Bahl, 1998). However, no previous experimental work on video conferencing systems for IEEE 802.11 Ad Hoc WLANs supporting IP multicast extensions has been reported so far. The paper describes Kinesis, an H.263+video transmission system for 802.11 network. Kinesis supports IP multicast extension and implements real time transport protocols to manage synchronization and QoS issues. We focus on the system architecture as a general communication framework for distributed and interactive audiovisual applications. Kinesis has been tested under different working conditions, performing H.263+ video sequences with excellent results.

On the relationship between the block error and channel-state Markov models in transmissions over slow-fading channels

Block-error processes in transmissions over slow-fading channels can be accurately modeled by a two-state Markov chain [the block Markov model (BMM)]. Another line of research has focused on the use of a channel-state Markov model (CSMM) to analyze block transmissions. Although both techniques provide results that agree well with observations, the relationship between both Markov models has not been recognized in the previous literature. In this letter, we show that the BMM for slow-fading channels can be directly derived from the CSMM. In addition, we introduce a greatly simplified channel-modeling methodology. In the new methodology, the BMM is the primary channel characterization tool, and the CSMM becomes essentially an estimation technique that provides parameters for the BMM. Results of packet transmissions in slow-fading channels show that our approach provides significant improvements in both accuracy and simplicity over previously proposed techniques.

Enhanced-performance video transmission in multicode CDMA wireless systems using a feedback error control scheme

We propose a new bounded delay retransmission scheme that enhances the reliability of compressed video transmission in multicode DS-CDMA (MC-CDMA) systems. We consider H.263/H.263+ video at low data rates (i.e., <64 kbps) in an IS-95B system over slow fading Rayleigh channels. Our approach is designed to reduce not only packet loss, time delays, and complexity, but also interference effects. The latter is critical in CDMA systems. Based both on theoretical analysis and entire system simulation, we investigate the improvements in packet transmission and system capacity achieved by the new scheme, and provide results for H.263 video transmission over IS-95B. Our results show that the scheme proposed significantly improves the error resilience of video transmission over MC-CDMA systems with small additional complexity. We also introduce new techniques for the analysis of video transmission in MC-CDMA with error control based on ARQ protocols, and present the application of these techniques to the proposed retransmission scheme.

A New Parallel Carrier Recovery Architecture for Intradyne Coherent Optical Receivers in the Presence of Laser Frequency Fluctuations

This paper introduces a new parallel carrier recovery architecture suitable for ultra- high speed intradyne coherent optical receivers (e.g., >=40Gb/s). The proposed scheme combines a novel low-latency parallel digital phase locked loop (DPLL) with a feedforward carrier phase recovery (CPR) algorithm. The new low-latency parallel DPLL is designed to compensate not only frequency offset, but also frequency fluctuations such as those induced by mechanical vibrations. It is well-known that nonzero frequency offset leads to higher phase error variance in a feedforward CPR [1]. Furthermore, it has been recently shown that laser frequency instability caused by mechanical vibrations significantly degrades the performance of CPR [2]. Other effects such as power supply noise may also introduce laser frequency fluctuations. To avoid receiver performance degradations, both frequency offset and frequency fluctuations should be compensated before the feedforward CPR block. We show that this task can be achieved by using a typical decision-directed serial DPLL. Then, a new approximation to the DPLL computation is introduced to enable a parallel-processing implementation in multigigabit per second receivers. The proposed technique reduces the latency within the feedback loop of the DPLL introduced by parallel processing, while providing a bandwidth and capture range close to those achieved by a serial DPLL. Simulation results demonstrate that the effects caused by frequency deviations can be eliminated with the proposed parallel carrier recovery architecture.

MMSEC-RAKE receivers with resolution reduction of the diversity branches: analysis, simulation and applications

The DS-CDMA IS-95 standard with 1.23 MHz bandwidth was originally designed for an outdoor cellular system where the delay spread is usually in the range of 10 /spl mu/s. The delay spread for an indoor environment is typically around 100 ns, which cannot be resolved by a CDMA receiver with an 813 ns chip interval. In this situation, the conventional maximal ratio combiner (MRC) RAKE receiver sets to unity the number of branches and diversity gain is not available. Resolution reduction (RR) of the diversity receiver branches is a technique that improves the performance of CDMA systems in channels where the delay spread is smaller than the chip duration. A new resolution reduction technique based on the use of a minimum mean square error diversity combiner (MMSEC) is proposed in this work. A detailed study of the performance of a dual-branch MMSEC-RAKE receiver with RR in a typical indoor office environment is presented. We show that, under very general assumptions, this new method of RR is optimal and provides a 1.2 dB improvement over the previously RR proposed technique based on MRC, and 3.6 dB improvement over conventional MRC-RAKE receiver without RR, at a frame error rate (FER) of 0.01 for the IS-95 system downlink.

MMSEC-RAKE receivers with resolution reduction of the diversity branches: analysis, simulation, and applications

In channels where the delay spread is smaller than the chip interval (e.g., an IS-95 system operating in indoor environments), spread-spectrum signals do not give rise to path diversity. In this situation, maximal-ratio combiner (MRC) RAKE receivers with resolution reduction (RR) of the diversity receiver branches may be used by the mobile stations to provide diversity gain, significantly improving system performance. A new resolution reduction technique based on the use of a minimum mean-square-error diversity combiner (MMSEC) is proposed in this work. We show that, under very general assumptions, this new method of RR is optimal. A detailed study of the performance of a dual-branch MMSEC-RAKE receiver with RR in a typical indoor office environment is presented. In order to allow a simple practical implementation, a suboptimal structure of the MMSEC is also proposed. Numerical results show that this new receiver scheme provides a 1.2-dB improvement over the previously proposed RR technique based on MRC, and a 4.9-dB improvement over conventional MRC-RAKE receiver without RR, at a frame-error rate of 0.01 for the downlink of the IS-95 system in a typical indoor office environment.

Performance evaluation in multimedia CDMA wireless transmission

Investigates the behavior of block errors in multimedia direct-sequence (DS) CDMA transmission systems operating over slow fading channels. We show that block errors can be accurately modeled by a first-order Markov process. Although Markov models have been used before to analyze packet transmission over radio channels, they have not been applied to CDMA systems that incorporate convolutional codes (CC) with soft-decision decoding. A major contribution of this work is to include in the new Markov model the effects of the CC, soft-decision decoding, and the RAKE diversity fingers with maximal ratio combining. The validity and accuracy of the model are established based on information-theoretic arguments and confirmed by computer simulation. A method to estimate the Markov process parameters is also presented. Another major contribution of this paper is the analysis of the throughput of the go-back-N and selective-repeat automatic repeat request (ARQ) protocols over DS-CDMA systems using the proposed Markov model. Several cases involving different numbers of RAKE fingers and various values of round-trip delays are analyzed. Our approximation is found to be excellent in all cases. The study of multimedia transmission, especially video, over DS-CDMA channels is greatly simplified using the model introduced in this work.

Compensation of Laser Frequency Fluctuations and Phase Noise in 16-QAM Coherent Receivers

Frequency fluctuations caused by mechanical vibrations, power supply noise, and other mechanisms are detrimental to the phase estimator performance in high speed intradyne coherent optical receivers. In this letter, we propose the use of a low-latency parallel digital phase lock loop in combination with common feed-forward carrier phase recovery algorithms in order to compensate both the phase noise and laser frequency fluctuation effects on 16-quadrature amplitude modulation receivers. Numerical results demonstrate the excellent behavior of the proposed two-stage carrier recovery scheme.

Reduced complexity multiuser receivers for asynchronous trellis coded DS-CDMA systems

Previous works have shown that multiuser receivers with trellis-coded modulation significantly improve performance in CDMA transmissions over bandwidth limited AWGN channels. In Corral-Briones et al., a reduced-state multiuser receiver (RSMR) that jointly decodes the trellis code and equalizes the multiuser interference of the channel has been introduced. In this paper the performance of this reduced-state family is studied for a heavily loaded bandwidth limited communication system. Unlike the previous work we address the performance of RSMR in situations of practical interest such as in high bandwidth efficiency situations and in near-far situations. Simulation results show that performance near the trellis-coded single user bound is achieved with this scheme at a drastically reduced complexity compared with that of the optimal receiver. Moreover we analyze the near-far resistance of RSMR and other suboptimal schemes. Results show that RSMR significantly outperforms previously known suboptimal structures.

A new information theoretic test of the Markov property of block errors in fading channels

This paper introduces a new and effective information theoretic test of the accuracy of the Markov property of block errors in fading channels. We apply the test to verify the validity of a first-order Markov model for block error processes on Rayleigh and Ricean fading channels. Unlike previous work, we address the effects of maximal ratio diversity-combining at the receiver. As a second application, we investigate the behavior of the block success/failure process in transmission systems that incorporate closed-loop power control. This topic is of great interest in the design of next-generation wireless networks. Numerical results show that our approach provides significant improvements in accuracy over previously proposed methods.