Evaluation of Decision Feedback Equalization and Viterbi Algorithm Detection for Voiceband Data Transmission--Part II

This paper describes experimental results of simulated 12000 and 14400 bits/s quadrature amplitude-modulated (QAM) systems (which were described in detail in Part I [1] ) operating on real and simulated voiceband channels. These experimental results show that both Viterbi algorithm detection and decision feedback equalization allow digital data transmission at symbol rates exceeding the nominal bandwidth capabilities of typical telephone channels. It is demonstrated in these experiments that the Viterbi algorithm receiver structure tested is less affected by linear distortion than the decision feedback receiver but exhibits greater sensitivity to phase jitter. On conditioned voiceband channels where there may be substantial phase jitter, a reasonable compromise between sensitivity to distortion and noise and sensitivity to phase jitter is made by a modem employing decision feedback equalization and decision-directed carrier phase tracking.     

Single Carrier Modulation With Nonlinear Frequency Domain Equalization: An Idea Whose Time Has Come—Again

In recent years single carrier modulation (SCM) has again become an interesting and complementary alternative to multicarrier modulations such as orthogonal frequency division multiplexing (OFDM). This has been largely due to the use of nonlinear equalizer structures implemented in part in the frequency domain by means of fast Fourier transforms, bringing the complexity close to that of OFDM. Here a nonlinear equalizer is formed with a linear filter to remove part of intersymbol interference, followed by a canceler of remaining interference by using previous detected data. Moreover, the capacity of SCM is similar to that of OFDM in highly dispersive channels only if a nonlinear equalizer is adopted at the receiver. Indeed, the study of efficient nonlinear frequency domain equalization techniques has further pushed the adoption of SCM in various standards. This tutorial paper aims at providing an overview of nonlinear equalization methods as a key ingredient in receivers of SCM for wideband transmission. We review both hybrid (with filters implemented both in time and frequency domain) and all-frequency-domain iterative structures. Application of nonlinear frequency domain equalizers to a multiple input multiple output scenario is also investigated, with a comparison of two architectures for interference reduction. We also present methods for channel estimation and alternatives for pilot insertion. The impact on SCM transmission of impairments such as phase noise, frequency offset and saturation due to high power amplifiers is also assessed. The comparison among the considered frequency domain equalization techniques is based both on complexity and performance, in terms of bit error rate or throughput.     

Block channel equalization in the presence of a cochannelinterferent signal

Novel algorithms for block equalization of M-ary phase shift keying (PSK) signals transmitted over multipath fading channels in the presence of an interferent cochannel signal are introduced and analyzed. The algorithms exploit the intrinsic statistical properties of cochannel interference (CCI) in order to mitigate its effects. Both linear and decision feedback equalizers (DFEs) are derived under the assumption that the overall channel impulse responses of both the useful and the inteferent signal are known. Simulation results show that: (a) whereas zero-forcing block equalizers yield a large noise enhancement effect, a minimum mean-square block DFE (MMSE-BDFE) can efficiently compensate for the distortion in the useful channel and reduce the effect of CCI at the same time, and (b) the MMSE-BDFEs outperform conventional DFEs, at least in the idealized conditions of our analysis     

Optimum power allocation for a V-BLAST system with two antennas at the transmitter

In this letter we propose a power optimization scheme for 2/spl times/N (where N is the number of receive antennas) Vertical Bell Laboratories Layered Space-Time (V-BLAST) systems. The proposed scheme minimizes the uncoded probability of vector error for each channel realization, considering error propagation. Simulation results show that this approach can yield SNR gain of up to about 3 dB for a 2/spl times/2 system at probability of vector error 10/sup -3/ and can partially compensate for the performance loss due to error propagation.     

Use of cell-site diversity in millimeter-wave fixed cellularsystems to combat the effects of rain attenuation

An examination of potential advantages of cell-site diversity with selection combining in the context of MM-wave fixed cellular systems is reported. The study involves simulation of converging radio links over weather radar images of the radar reflectivity factor, from which the specific attenuation of rain at 30 GHz is derived. The average correlation of attenuation on two converging links as a function of their angular separation is shown to indicate the potential benefits in the use of cell-site diversity, especially in heavy rain. Results show that diversity gain exhibits a dependence on angular separation &thetas; in the general form of sin^k (&thetas;/2). For links of identical lengths the model reduces to a root-sinusoidal shape (k=0.5), whereas links of unequal lengths lead to the ITU-R recommended model (k=1). Based on the sin^k (&thetas;/2) model and observation of the length ratio of the links, a set of criteria for determining the benefit of cell-site diversity for a given subscriber location is proposed     

Performance evaluation of candidate MAC protocols for LMCS/LMDSnetworks

Broadband wireless access (BWA) offers attractive features such as ease and speed of deployment, fast realization of revenues, and low infrastructure cost. This paper focuses on medium access control (MAC) alternatives that can find application in an LMDS/LMCS network capable of supporting multimedia traffic. Multifrequency demand assignment TDMA-based schemes appear (at the moment) to be a suitable choice in this context. The selected protocol should be dynamically capable of providing multirate capabilities and quality-of-service guarantees. An obvious candidate for the aforementioned purpose is the LMDS-specific MAC proposal in the DAVIC 1.2 recommendation. For purposes of comparison, we also examine the evolving IEEE 802.14 MAC convergence protocol and MCNS (multimedia cable network system) DOCSIS (ITU J-112) MAC standard that are intended to support similar applications and services in an HFC (hybrid fiber coax) environment. The three protocols are examined under noiseless and noisy channel conditions. Previous results on LMDS channel characteristics are summarized and used for modeling noisy channel conditions. The candidate protocols are compared in terms of mean access delay, throughput, and collision multiplicity statistics, when contention of users is involved. The effect of dynamic slot allocation on the performance of the candidate protocols is also examined     

Decorrelating detectors for quasi-synchronous CDMA

To date, two forms of spread spectrum code division multiple access (CDMA) have been considered in the literature. Either synchronous CDMA, where all users are both chip and bit synchronized relative to each other, or asynchronous CDMA, where all users are neither chip nor bit synchronized relative to each other. Between these two extremes lies quasi-synchronous CDMA (QS-CDMA) where users are not chip synchronized but are approximately bit synchronized. Quasi-synchronous CDMA arises in microcell systems where the combined propagation time and delay spread produces a variation in the round trip delay time limited to a few chips.Detection of QS-CDMA at the base station is considered. It is shown that memoryless detection can be achieved by appending blank chips to the user's spreading codes.Two decorrelating detectors are proposed and analyzed. The first uses a matched filter bank to produce a discrete time signal, and the second uses a single oversampling integrate-and-dump filter to produce a discrete time signal. For the first detector, each matched filter is synchronized to its respective user, and for the second detector, the integrate-and-dump filter is synchronized to the base station transmissions. The performance of the two detectors is shown to be approximately equal provided the integrate-and-dump filter is sampled at least two times the chip rate.     

A delay independent decorrelating detector for quasi-synchronousCDMA

The decorrelating detector is a near-far resistant linear joint detector for a code-division multiple-access (CDMA) system. It consists of a bank of matched filters followed by a decorrelating matrix. For proper operation, both the matched filter bank and the decorrelating matrix require knowledge of the delays of all users. The delays are due to the different propagation times from each user to the base station. Delay estimation is a weak link in this system since it is complex and prone to error. The proposed decorrelating detector does not require exact knowledge of the user delays, but instead requires that the delays be bounded to a fraction of a symbol interval. The delays are naturally bounded in this way in many microcell and picocell systems where the round trip propagation time is small compared to the symbol interval. The new delay independent decorrelating detector is shown to be near-far resistant and, through appropriate spreading code selection, to experience a modest 3 dB signal-to-noise ratio (SNR) loss relative to orthogonal access schemes. It is also shown to limit capacity to a maximum of 50% of the spreading gain when the delays are bounded by a single chip interval. The complexity is similar to the conventional correlating receiver which is far less complex than the joint detection schemes proposed to date     

Fibrinogen synthesis is elevated in fasting cancer patients with an acute phase response

Paired immunoglobulin-like receptor B (PIR-B) (p91) molecule has been proposed to function as an inhibitory receptor in B cells and myeloid lineage cells. We demonstrate here that the cytoplasmic region of PIR-B is capable of inhibiting B cell activation. Mutational analysis of five cytoplasmic tyrosines indicate that tyrosine 771 in the motif VxYxxL plays the most crucial role in mediating the inhibitory signal. PIR-B-mediated inhibition was markedly reduced in the SH2-containing protein tyrosine phosphatases SHP-1 and SHP-2 double-deficient DT40 B cells, whereas this inhibition was unaffected in the inositol polyphosphate 5'-phosphatase SHIP-deficient cells. These data demonstrate that PIR-B can negatively regulate B cell receptor activation and that this PIR-B-mediated inhibition requires redundant functions of SHP-1 and SHP-2.