This work presents a reduced complexity architecture for the backpropagation-based compensation of frequency interleaved analog to digital converters (FI-ADC). Earlier literature has described a promising adaptive background compensation technique based on the backpropagation algorithm from machine learning, which is suitable to mitigate errors of the analog signal path of FI-ADCs. This technique is applicable to high speed digital receivers such as those used in coherent optical communications. The key ingredients of the aforementioned technique are MIMO equalization and the backpropagation algorithm used to adapt the coefficients of the equalizer. The work presented here modifies the earlier architecture to reduce the number of required analog mixers by a factor two, and to simplify, also by a factor two, the compensation equalizer responsible for correcting the errors of the analog signal path. This results in a much simpler and significantly improved FI-ADC system. Simulations show that the analog impairments are accurately compensated and their impact on the performance of the receiver is essentially eliminated.

This paper presents an experimental evaluation of the backpropagation-based technique for the background compensation of time-interleaved analog to digital converters (TI-ADC). Measurements obtained on an 8 bit, 4 GS/s TI-ADC test chip fabricated in 130 nm CMOS technology demonstrate the effectiveness of this technique. Backpropagation-based background compensation of TI-ADC is typically used in high-speed digital communication receivers. Historically, the error backpropagation algorithm has been used to train neural networks in machine learning applications. In a TI-ADC compensation application, the error at the symbol detection stage of a digital receiver is processed by the backpropagation algorithm to compute an equivalent error at the TI-ADC output, which otherwise would not be directly observable. Then, the backpropagated error can be applied to train a low-complexity adaptive equalizer which compensates the TI-ADC mismatches. In comparison with prior art, the main advantages of this technique are its robustness, its high convergence speed, and its intrinsic background operation. Measurements with the aforementioned test chip show that the impact of the TI-ADC mismatch on the receiver performance is eliminated. In addition, an improvement in both the SNDR and SFDR of almost 24 dB and 25.6 dB, respectively, is observed at the Nyquist frequency when the compensation is applied.

This paper introduces an adaptive background compensation technique to mitigate errors of the analog signal path of Frequency-Interleaved ADCs (FI-ADC). The key ingredients of the proposed technique are MIMO equalization and the backpropagation algorithm often used to train neural networks. The technique introduced here is applicable to digital receivers such as those used in coherent optical communications. Although compensation of FI-ADCs has been discussed in the technical literature, adaptive background techniques have not been reported so far. The latter are important to automatically compensate errors caused by process, voltage, and temperature variations in CMOS implementations of the data converters and therefore ensure high manufacturing yield. Simulations show that the impairments of the analog signal path are accurately compensated and their effect essentially eliminated, resulting in a high performance ADC system.

Backpropagation is a machine learning algorithm used to train neural networks. This paper introduces a back-propagation based technique for the calibration of the mismatch errors of time-interleaved analog to digital converters (TI-ADCs). It is applicable to digital receivers such as those used in coherent optical communications. The error at the slicer of the receiver is processed by the backpropagation algorithm, and applied to compensate the TI-ADC mismatches using an adaptive equalizer. Alternatively, the back propagated error is used to estimate the mismatches and correct them with analog techniques. Implementation complexity is low. The main advantages of the technique proposed here compared to prior art are its robustness, its speed of convergence, and the fact that it always works in background mode, independently of the oversampling factor and the properties of the input signal, as long as the receiver converges. Simulations are presented to demonstrate the effectiveness of the technique.

We propose a low-complexity implementation architecture for turbo product code (TPC) suitable for next-generation fiber optic networks (e.g. ≳ 100 Gb/s). The proposed code makes use of expurgated Bose-Chaudhuri-Hocquenghem (BCH) codes to improve the performance and reduce implementation complexity. In comparison with existing solutions, our results show that the propos d TPC architecture is able to reduce approximately to a half the gate count and the memory size. This feature makes the new TPC an excellent candidate for practical VLSI implementation in commercial transceivers.

This paper presents an improved decision feedforward equalizer (DFFE) for high speed receivers operating on highly dispersive channels. The DFFE has been recently proposed for multigigabit communication receivers, where the use of parallel processing is required. Well-known parallel architectures for the traditional decision feedback equalizer (DFE) have a complexity which grows exponentially with the channel memory. Instead, the new DFFE avoids that exponential increase in complexity by using tentative decisions to iteratively cancel intersymbol interference (ISI). Additional complexity reduction can be achieved by improving the reliability of the initial tentative decisions. To provide more reliable initial tentative decisions to the DFFE, a simple reduced-state Viterbi algorithm (VA) is proposed in this work. Computer simulations demonstrate that the combination of DFFE and VA not only allows a similar performance to the typical DFE to be achieved, but it also results in a significant complexity reduction.

This work introduces a novel serial code concatenation (SCC) scheme to combat the error floor problem experienced in iterated sparse graph-based error correcting codes such as turbo product (TP) codes and low density parity check (LDPC) codes. SCC has been widely used in the past to reduce the error floor in iterative decoders. However, the main stumbling block for its practical application in high speed communication systems has been the need for long and complex outer codes. The use of short outer block codes with interleaving has been shown to provide a good tradeoff between complexity and performance. Nevertheless, its application to next-generation ultra high-speed communication systems is still a major challenge as a result of the careful design of long complex interleavers needed to meet the requirements of these applications (e.g., a net coding gain >10 dB at a bit error rate of 10-15with an overhead of ~ 20% for 100 Gb/s optical transport networks [1]). In this paper we present a new SCC scheme built from short outer block codes. Unlike previous proposals, the long interleaver is replaced by a simple block code combined with a novel encoding/decoding strategy. Based on this finding, we show that complexity and latency can be drastically reduced with negligible penalty. The SCC technique introduced here provides a new general framework for solving the error floor problem induced by low-weight error patterns of any coding scheme.

A joint sampling-time error and channel skew background calibration technique for time interleaved analog to digital converters (TI-ADC) is presented. The technique is aimed at applications in dual-polarization QPSK/QAM receivers for coherent optical communications at high data rates (e.g., 40Gb/s and beyond). Unlike previous proposals, the calibration algorithm introduced here is used to jointly compensate for sampling-time and channel skew errors. Estimates of the gradient of the mean squared error (MSE) or the bit error rate (BER) with respect to the sampling phases of the different signal lanes and interleaves are computed and used to iteratively minimize a cost function (i.e., MSE or BER). Computer simulations demonstrate the excellent behavior of the proposed compensation technique. The calibration algorithm can be implemented with minimal hardware requirements and with a slow clock. This allows power dissipation in a CMOS VLSI implementation to be minimized.

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. Furthermore, it has been recently shown that laser frequency instability caused by mechanical vibrations significantly degrades the performance of CPR. 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.

This paper presents a non-concatenated forward error correction (FEC) code suitable for applications in 100Gb/s optical transport networks (OTN). A typical requirement in this application is a net coding gain (NCG) >;10 dB at a bit error rate (BER) of 10-15with an overhead (OH) of ~20%. As discussed in [1], non-concatenated codes are the ultimate frontier in terms of performance for OTN applications, because of their superior performance, lower latency, and lower overhead than concatenated codes. However, a major stumbling block for the use of these codes has been the existence of BER floors at levels significantly higher than the required 10-15(typically 10-10). In this paper we present a new coding scheme based on a low density parity check (LDPC) code with an expected net coding gain of 11.30dB at 10-15, 20% OH, and a block size of 24576 bits. This represents a significant improvement over the previous state of the art [2], based on a concatenated code with a block size of 74844 bits and 20.5% OH. The code is designed to minimize the BER floor while simultaneously reducing the memory requirements and the interconnection complexity of the iterative decoder [3]. Experimental results obtained with an FPGA-based hardware emulator demonstrate an NCG of 10.70 dB at a BER of 10-13and no error floors. These experimental results are extrapolated to 10-15using importance sampling techniques, resulting in the expected performance stated above. Moreover, we find that fixed-point implementation is the main cause of error floors below 10-13Based on this finding, we introduce a new low complexity postprocessing technique to push BER floors down to 10-15.

Baseline wander (BLW) is caused by transmission zeros at DC in the frequency response of the communication channel [1]. One of its common sources is a high-pass filter used to AC-couple the received signal. Numerous BLW compensation (BLWC) schemes for baseband and homodyne communication systems have been proposed in past literature. However, a preferred architecture in modern coherent optical communications is the intradyne receiver, because it avoids complex optical phase locked loops. As a result of the carrier frequency offset experienced at the input of intradyne coherent receivers, existing BLWC techniques are not sufficient. In this work we propose an adaptive parallel-processing BLW compensation architecture for high-speed coherent intradyne fiber optic receivers. Parallel processing for the BLWC implementation is imposed by the high data rate required in current optical applications. The new compensator estimates the BLW interference based on both the detected symbols and the carrier frequency offset provided by the carrier recovery loop. Numerical results demonstrate the effectiveness of the proposed technique to mitigate BLW effects.

This paper introduces the decision feedforward equalizer (DFFE), a new low complexity iterative equalization architecture for high speed receivers used on channels with large intersymbol interference (ISI). The decision feedback equalizer (DFE) is one of the preferred receivers for high ISI channels. The high data and symbol rate of current communications systems often require parallel processing receiver implementations [1]. Unfortunately, the complexity of parallel architectures of the DFE grows exponentially with the channel memory [2] and limits their application to low ISI channels. The DFFE avoids the exponential growth of the DFE by using tentative decisions to iteratively cancel the ISI. We show that the DFFE can achieve performance similar to the DFE. These benefits make it an excellent choice for high-speed receivers required to operate over highly dispersive channels.

In this paper we present a theory of the bit error rate (BER) of Euclidean metric-based maximum likelihood sequence detectors (EM-MLSD) in the presence of channel mismatch caused by nongaussian noise. Although the theory is general, here we focus on the effects of quantization noise (QN) added by the front-end analog-to-digital converter (ADC) typically used in DSP based implementations of the receiver. Numerical results show a close agreement between the predictions of the theoretical analysis and computer simulations. As a practical application of the proposed theory, we investigate the performance of EM-MLSD in 10Gb/s Ethernet receivers for multimode optical fibers. Since the BER required in this application is below 10-12, which precludes the use of computer simulations to estimate BER, a theoretical study of the MLSD performance including the combined effects of the channel dispersion and QN, becomes necessary. We present numerical results for the three stressors specified by the 10GBASE-LRM standard. Our study shows that the impact of the QN added by the ADC on the performance depends strongly on the channel dispersion (i.e., the stressor).

In this paper we present the architecture and experimental evaluation of a single-chip CMOS maximum-likelihood sequence detection (MLSD) transceiver for electronic dispersion compensation (EDC) of multimode optical fibers (MMF) at 10 Gb/s data rate. The primary application of this transceiver is the 10 GBASE-LRM standard for 10 Gb/s Ethernet over MMF [1]. Application to single-mode fiber (SMF) channels and/or other standards is also possible. Both optical module and line card applications are discussed. The architecture of the receiver is discussed in considerable depth, including implementation details. Extensive experimental results on several channels are presented. For the precursor, postcursor, and symmetric stressors specified by the 10 GBASE-LRM standard we demonstrate receiver sensitivities of -14.84 dBm, -14.37 dBm, and -13.68 dBm, respectively, in a line card application with 6 inches of electrical interconnect These sensitivities exceed the requirements of the standard by 8.34 dBo, 7.87 dBo, and 7.68 dBo', respectively. Measured results on industry defined worst-case fibers and on nonstationary channels such as the dynamic stressor defined by the 10 GBASE-LRM standard are also presented.

In this paper we propose a multiple-input, multiple-output (MIMO) maximum-likelihood sequence estimation (MLSE) receiver for electronic dispersion compensation (EDC) of multimode optical fibers at 10 Gb/s data rate. As the primary example of the effectiveness of the techniques introduced here we present a receiver for the emergent 10 GBASE-LRM standard for 10 Gb/s Ethernet over multimode optical fibers [1]. We show that an MLSE receiver provides an advantage of at least 2 dB over other receivers, such as the decision-feedback equalizer (DFE), on 99% of the local area network (LAN) multimode fiber population, as modeled by a database proposed by the IEEE 802.3aq Task Force [2]. The paper deals with both theoretical and practical aspects of the architecture. To enable the complete integration of this receiver as a single chip in current 90 nm CMOS technology, we propose the parallel-processing digital implementation of a feedforward equalizer (FFE) and a Viterbi decoder together with an interleaved front-end analog to digital converter (ADC). The MIMO structure jointly compensates for fundamental channel impairments such as multimode dispersion, and implementation-related effects such as the limitations of the analog front end (AFE), particularly the one known as fixed-pattern noise (FPN) [3]. As a result of the high speed operation, FPN has a major impact on performance. We show that, without compensation, FPN could result in a degradation of 7.86 dB. With the compensation proposed here, the latter is reduced to 0.23 dB.

In this paper, we investigate the effect of mismatches in the frequency responses of the track-and-hold (T&H) amplifiers of an interleaved array of analog-to-digital converters (ADC) used as the front-end of a high-speed communications receiver. Furthermore, we introduce digital signal processing (DSP) techniques to compensate the performance loss caused by these mismatches. These techniques take advantage of the specific application of the ADC as a front-end of a digital communication receiver, assumed to be based on a parallel processing implementation of a decision-feedback equalizer (DFE) (Kasturia and Winters, 1991; Parhi, 1999). With a relatively simple modification, the DFE can be transformed into a multiple-input, multiple-output (MIMO) equalizer. The latter effectively compensates the effect of the T&H frequency response mismatch. The performance measure used in this paper is the signal to noise ratio (SNR) at the slicer. An SNR loss of 2.5 dB or more could result from the T/H mismatch. Most of this loss is compensated by the MIMO equalizer. Our conclusions can be easily extended to receiver architectures other than the DFE, for example maximum-likelihood sequence estimation (MLSE) (Agazzi, 2005). As an example of the effectiveness of the techniques introduced here, we present an optical receiver with electronic dispersion compensation (EDC) for the emergent IEEE 802.3aq standard for 10 Gb/s Ethernet over multimode fibers

Reduced-complexity multiuser interference equalization and group decoding (ME-GD) is an attractive way to improve performance of multiuser transmissions for narrowband additive white Gaussian noise channels. ME-GD receivers have been investigated in the past for convolutionally coded synchronous multiple-access channels (MACs). In this paper, we propose a new ME-GD for trellis-coded asynchronous MACs. The coding information of a subset of users and the multiuser interference are jointly taken into account in the branch metric computation using per-survivor processing. The novel approach leads to a family of Viterbi decoders which offers a good tradeoff between performance and complexity. As a second contribution of this paper, we derive theoretical bounds for the performance of ME-GD considering the effect of error propagation, which is critical for asynchronous MACs. Numerical results show the excellent accuracy of the proposed bounds. Finally, based on the novel theoretical analysis, we derive a new criterion to achieve good tradeoffs between performance and complexity.

As the spacing between channels in dense wavelength division multiplexed (DWDM) links decreases and their number increases, nonlinear coupling effects such as four wave mixing (FWM) and cross-phase modulation (CPM) become more important. These impairments, combined with chromatic dispersion (CD), polarization mode dispersion (PMD), and amplified spontaneous emission (ASE) noise ultimately limit the maximum reach of the links. A receiver robust to these impairments would increase the maximum possible length of links without regeneration, reduce the number of required compensation elements such as dispersion compensation fibers (DCF) and optical amplifiers, and simplify link provisioning by making its fine tuning less critical and labor intensive. Maximum likelihood sequence estimation (MLSE) receivers have been previously proposed to combat CD and PMD in the presence of ASE noise. In this paper we propose to extend the use of MLSE to DWDM links to combat the combined effect of nonlinear crosstalk, dispersion and noise. MLSE receivers can incorporate detailed knowledge of the statistical properties of noise and crosstalk into the decision process, therefore improving performance in the presence of these impairments. We derive a new analytical expression (necessary to implement the MLSE receiver) for the statistics of the received signal in the presence of FWM. Although in long haul DWDM links DCF cannot be avoided even when electronic dispersion compensation is used, dispersion compensation with DCF is not complete because even a compensated link exerts different dispersion effects on different wavelengths. Our results show that the performance of DWDM systems can be significantly improved by using an MLSE receiver, particularly in the presence of residual dispersion. For example, a 2000 km 20-channel link can operate within the correction capabilities of a forward error correction (FEC) code (G.975 RS) in the presence of 1700 ps/nm of residual dispersion even with an extremely high value of FWM crosstalk using the MLSE receiver proposed in this work. As a second important contribution of this paper, we present a theoretical performance analysis of MLSE in DWDM optical channels. This analysis is built upon the theory we developed previously. An excellent agreement is found between the prediction of the theory and simulation results.

Joint reduced-state detection and group decoding (RSD-GD) is a promising approach to increase the spectral efficiency of multiple-access systems. Performance of RSD-GD receivers improves as coding information of more users is taken into account in the detection algorithm. For asynchronous channels, however, there is another approach to improve performance: use of more states of the multiple-access channel (MAC). Performance of RSD-GD receivers for asynchronous channels has been investigated using computer simulations; however, analytical studies of the performance of RSD-GD have not been reported so far. In this paper we derive upper bounds for the performance of RSD-GD over asynchronous MAC. The new results are based on an improved upper bound on the error event probability, which is significantly tighter than the traditional bound based on pairwise error events. Comparisons with values obtained from computer simulations show the excellent accuracy of the proposed bound.

Chromatic dispersion (CD) and polarization mode dispersion (PMD) severely limit the performance of optical transmission systems operating at and above 10 Gb/s. Electrical equalization techniques have been proposed to compensate dispersion in both coherent and intensity modulation/direct-detection (IM/DD) systems. We investigate the combined adaptive digital equalization of all-order PMD, CD, and laser phase noise in high-speed coherent optical transmission systems. Simultaneous equalization of these impairments has not been reported previously and is particularly important in modulation systems that exploit polarization to increase modulation efficiency. We propose a novel 4-dimensional equalizer structure for joint polarization modulation and M-ary differential phase shift keying (JPMDPSK) systems. The specific example considered is 40 Gb/s transmission with a 10 GBaud symbol rate, using DQPSK modulation on each axis of polarization. Our results show that the new four-dimensional equalizer can compensate channel dispersion of up to 1000 km of standard single-mode fiber, with less than 3 dB penalty in signal-to-noise ratio (SNR). This is a dramatic improvement over 40 Gb/s IM/DD systems. The feasibility of the very large scale integration (VLSI) of coherent receivers in current technology is also discussed.

Maximum likelihood sequence estimation (MLSE) has been proposed in earlier literature to combat the effects of nonlinear dispersion in intensity modulation/direct detection (IM/DD) optical channels. We develop a theory of the bit error rate (BER) of MLSE-based IM/DD receivers operating in the presence of nonlinear intersymbol interference (ISI) and amplified spontaneous emission (ASE) noise. Numerical results confirm the predictions of our theory. Based on the new analysis, we also investigate the log-likelihood ratio (LLR) of the received signal yielded by a soft-input/soft-output (SISO) front-end decoder. Our study shows that traditional channel codes designed for transmissions over AWGN channels, in combination with SISO front-end detectors, achieve an asymptotically optimal performance in transmissions over IM/DD optical 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.

Group decoding receivers (GDRs) are an attractive way to improve the performance of multiuser systems with highly correlated users. These reduced complexity receivers use jointly the coding information of a subset of users and multiuser information. Performance of GDR for synchronous multiple-access channels is upper bounded by E.A. Fain and M. Varanasi (see IEEE Trans. on Commun., vol.49, no.6, p.1021-32, 2001). However, upper bounds of GDR for asynchronous multiple-access channels (where the effect of error propagation is critical), have not been reported so far. We derive an upper bound for the performance of GDR for asynchronous multiple-access channels considering error propagation effects. We show that performance analysis based exclusively on minimum distance error events is poor, even at high signal-to-noise ratios (SNRs). To improve the accuracy of analytical estimates, new bounds accounting for the contribution of non-minimum distance error events and error propagation are derived. Comparisons with values obtained from computer simulations show the excellent accuracy of the proposed bounds. Our results also reveal that performance of GDR in channels with highly correlated users is severely affected by error propagation. To mitigate this problem, we propose a new scheme based on the combination of the generalized Viterbi algorithm and the GDR. We show that the proposed receiver is able nearly to eliminate error propagation with low additional complexity.

Reliable interactive video transmission in CDMA cellular networks is difficult since residual channel errors cannot be avoided and the available bandwidth is scarce. In order to improve quality of the decoded video, this paper introduces a novel architecture for reliable transmission in multicode DS-CDMA (MC-CDMA) systems. Unlike previous protection schemes, the new architecture not only improves video quality, but it also achieves an efficient use of the available resources in the network. As an application of this concept, we propose a robust scheme for reliable video communication from wired users to mobile users in slow fading environments. Our approach consists of two main ingredients: a novel closed-loop error control technique to reduce packet loss and interference, and a new scheme to stop error propagation in the decoded sequence originating from residual channel errors. Based both on theoretical analysis and numerical results, we show that our approach achieves important improvements in the quality of the decoded video with low additional complexity.

We show that block errors in multicarrier DS-CDMA (MC-CDMA) systems with transmit diversity operating over slow fading channels are accurately modeled by a Markov process. Although earlier contributions have shown that packet transmission over radio channels can be modeled by Markov processes, error models for DS-CDMA systems using multicarrier transmit diversity, convolutional coding with soft-decision Viterbi decoding, and RAKE diversity fingers with maximal ratio combining, have not been reported so far. A major contribution of this work is a novel method to estimate the parameters of the Markov model. Comparisons with results of detailed simulations of the entire system confirm that this is an excellent packet-level error model for MC-CDMA with transmit diversity.

This paper introduces a novel and efficient method based on information theoretic metrics to evaluate the accuracy of the first-order Markov approximation for block errors in transmissions over fading channels. Numerical results show that our approach provides significant improvements in both accuracy and simplicity over previously proposed methods.

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.

We propose a new reduced-complexity multiuser receiver for asynchronous trellis-coded CDMA systems in AWGN channels. Based on the coding information of a subset of users and the set partitioning principles inherent in TCM, reduced-state trellises are constructed. The Viterbi algorithm is thus used to jointly decode the trellis codes of this subset of users and equalize the multiuser interference of the channel. As a special case of the proposed receiver, we study a structure that decodes only one user and equalizes the multiuser interference. Simulation results show that the performance of the new reduced complexity receiver approaches the trellis-coded single-user bound for both low and high-interference channels. Moreover, we compare the performance of the proposed receiver with that of previously proposed suboptimal structures. Our scheme is found to significantly outperform previously known suboptimal structures.

Block error processes in transmissions over radio channels can be accurately modeled by a two-state Markov chain (Zorzi et al. 1997) (the block-error Markov model, or BMM). An independent line of research has focused on the use of hidden Markov models to describe channel states (channel-state Markov models, or CSMM) (Turin 1999). Although both techniques provide results that agree well with observations, their relationship has not been recognized in the previous literature. In this paper we show that the BMM can be directly derived from the CSMM. Based on this analysis, 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.

We present a simple and accurate semi-analytic methodology for performance evaluation of wireless video transmission in multicode DS-CDMA (MC-CDMA). Based on the novel approach (as well as on entire system simulations), we analyze performance of H.263 in an IS-95B system over slow fading channels at low bit rates. Peak signal-to-noise ratio (PSNR) values for various video test sequences demonstrate that important improvements of the video quality can be achieved by using feedback error control schemes. In addition, we investigate the effectiveness of the combined use of error correction and error resilience techniques for reliable video transmission in MC-CDMA systems.

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.

In channels where the delay spread is smaller than the chip interval, spread spectrum signals do not give rise to path diversity. In this situation, distributed antennas systems may be used to provide path diversity, significantly improving the system performance. The interference component caused by the same cell-site users, or interpath interference (IPI), is a dominant factor in the performance of these systems. In their analysis it is typically assumed that the probability density function (PDF) of the IPI can be approximated by the Gaussian distribution. This is called the Gaussian approximation (GA). We show that the GA for the IPI does not provide an accurate characterization of the performance of the downlink of DS-CDMA indoor systems with distributed antennas. The bit error rate (BER) at the output of a practical RAKE receiver is computed using an accurate approach for the IPI. We show that when the IPI density is thus modeled, the signal to noise ratio (SNR) at the output of the combiner is well approximated by a random variable with a Weibull distribution. Comparison of the BER obtained by the new approach for the IPI with those obtained by the GA shows that the latter method incurs in considerable error when used to evaluate performance of DS-CDMA indoor systems with distributed antennas.

We study the performance of the downlink of a DS-CDMA indoor system that incorporates distributed antennas and the convolutional code (CC) as error correction method. Based on the improved approach for the interpath interference (IA-IPI) introduced by Hueda (see Proc. 11/sup th/ PIRMC'00, London, 2000), we propose analytical expressions for the bit and frame error rate (BER/FER) at the output of the Viterbi decoder with soft decision decoding. The BER and FER of the forward traffic channel of an IS-95 system are then derived by computer simulation of the entire system and by numerical evaluation of analytic expressions. Results for several cases involving different numbers of antenna elements and different levels of interference from other cells are presented. The analytical models are shown to be most efficient in terms of computational cost while preserving excellent accuracy.

In channels where the delay spread is smaller than the chip interval, spread spectrum signals do not give rise to path diversity, thus a classical maximal ratio combiner (MRC)-RAKE receiver sets to unity the number of branches. In this situation, MRC-RAKE receivers with resolution reduction (RR) of the diversity branches may be used to significantly improve performance in the forward link of DS-CDMA systems (Yang 1995). As a result of the use of RR techniques, the noise components at the finger outputs are correlated. It has been shown that the MRC is suboptimal in the presence of correlated noise components. In this paper, a minimum mean squared error combiner (MMSEC) RAKE receiver with RR is proposed. The performance of the RAKE receiver with two branches is studied by means of theoretical analysis and computer simulations. To allow a simple practical implementation, a suboptimal structure of the MMSEC is also proposed. Frame error rate (FER) is obtained by using a simulation system based on the forward link of the digital cellular standard IS-95 in a typical indoor radio channel. Our results show that this new receiver scheme can achieve a gain of 3.6 dB over a conventional MRC-RAKE receiver at an FER of 0.01, with small increase in complexity.