Receiver Feedback Research Articles

Multichannel Detection for Wideband Underwater Acoustic CDMA Communications

Direct-sequence (DS) code-division multiple access (CDMA) is considered for future wideband mobile underwater acoustic networks, where a typical configuration may include several autonomous underwater vehicles (AUVs) operating within a few kilometers of a central receiver. Two receivers that utilize multichannel (array) processing of asynchronous multiuser signals are proposed: the symbol decision feedback (SDF) receiver and the chip hypothesis feedback (CHF) receiver. Both receivers use a chip-resolution adaptive front end consisting of a many-to-few combiner and a bank of fractionally-spaced feedforward equalizers. In the SDF receiver, feedback equalization is implemented at symbol resolution, and receiver filters, including a decision-directed phase-locked loop, are adapted at the symbol rate. This limits its applicability to the channels whose time variation is slow compared to the symbol rate. In a wideband acoustic system, which transmits at maximal chip rate, the symbol rate is down-scaled by the spreading factor, and an inverse effect may occur by which increasing the spreading factor results in performance degradation. To eliminate this effect, feedback equalization, which is necessary for the majority of acoustic channels, is performed in the CHF receiver at chip resolution and receiver parameters are adjusted at the chip rate. At the price of increased computational complexity (there are as many adaptive filters as there are symbol values), this receiver provides improved performance for systems where time variation cannot be neglected with respect to the symbol rate [e.g., low probability of detection (LPD) acoustic systems]. Performance of the two receivers was demonstrated in a four-user scenario, using experimental data obtained over a 2-km shallow-water channel. At the chip rate of 19.2 kilochips per second (kc/s) with quaternary phase-shift keying (QPSK) modulation, excellent results were achieved at an aggregate data rate of up to 10 kb/s

Unquantized and uncoded channel state information feedback in multiple-antenna multiuser systems

We propose a channel state information (CSI) feedback scheme based on unquantized and uncoded (UQ-UC) transmission. We consider a system where a mobile terminal obtains the downlink CSI and feeds it back to the base station using an uplink feedback channel. If the downlink channel is an independent Rayleigh fading channel, then the CSI may be viewed as an output of a complex independent identically distributed Gaussian source. Further, if the uplink feedback channel is an additive white Gaussian noise channel, and the downlink CSI is perfectly known at the mobile terminal, it can be shown that UQ-UC CSI transmission (that incurs zero delay) is optimal in that it achieves the same minimum mean-squared error distortion as a scheme that optimally (in the Shannon sense) quantizes and encodes the CSI, while theoretically incurring infinite delay. Since the UQ-UC transmission is suboptimal on correlated wireless channels, we propose a simple linear CSI feedback receiver that can be used to improve the performance of UQ-UC transmission while still retaining the attractive zero-delay feature. We provide bounds on the performance of such UQ-UC CSI feedback and study its impact on the achievable information rates. Furthermore, we explore its application and performance in multiple-antenna multiuser wireless systems, and also propose a corresponding pilot-assisted channel-state estimation scheme

Scalability improvement of the real time control protocol

Scalability problems arise when the Real Time Control Protocol (RTCP), which is the control protocol of the Real-time Transport Protocol (RTP), is used in large multicast groups. The problems include: increased feedback delay, increased storage state at every member, and ineffective RTCP bandwidth usage. We have designed a Scalable RTCP (S-RTCP) which is based on a hierarchical structure in which members are grouped into local regions. For every region, there is an Aggregator (AG) which receives the feedback Receiver Reports (RRs) sent by its local members. The AG summarizes important information in the RRs, derives some statistics, and sends them to a Manager. The Manager performs additional statistical analysis to monitor the transmission quality and to identify regions suffering massively from congestion. A simulation of S-RTCP using the Network Simulator (NS) showed that S-RTCP alleviates some of the RTCP scalability problems and makes effective use of RRs. Consequently, the feedback provides timely and useful QoS information required for network monitors and for adaptive applications. Details of the simulations and performance analysis are presented and described which show the advantages of using S-RTCP over the original RTCP.

Blind adaptive decision feedback CDMA receivers for dispersive channels

Blind adaptive decision feedback (DF) receivers for direct sequence code division multiple access (DS-CDMA) systems in dispersive channels are proposed. Blind adaptive stochastic gradient algorithms are developed for use with the constrained minimum variance and constrained constant modulus receivers along with successive and parallel DF structure.

Asymptotic analysis of the conventional decision feedback receiver in fading channels

Spectral efficiency presents the ultimate limit on data rate per unit bandwidth of a certain communication system. For direct-sequence code-division multiple access systems, the spectral efficiency has been derived in case of synchronous reception for optimal and linear multiuser receivers, flat-fading and nonfading environments, as well as single and multicell cellular networks. The most pervasive model employed in all these analyses is the large system random signature model. For the decision feedback receivers, previous research handled only the nonfading case while the case of fading channels remained unknown. This paper analyzes the spectral efficiency of the popular conventional decision feedback receiver (CDFR) in flat- and frequency-selective fading channels with and without power ordering. Results show that in the case of power ordering before cancellation and very large system loads, the spectral efficiency of this receiver in fading channels can be even larger than in the case of channels with no fading. We also discuss and identify optimal power control laws for the CDFR with and without power ordering. The power control law which equalizes single-user capacities in the case of power ordering is also discussed.

Decision-feedback interference suppression in CDMA systems: a ML-based semiblind approach

This paper addresses the problem of interference suppression in direct sequence code division multiple access systems. We propose a novel semiblind decision feedback (DF) receiver based on the maximum likelihood principle that simultaneously exploits the transmission of training sequences and the statistical information of the unknown transmitted symbols. Both iterative and adaptive implementations of the proposed receiver, derived within the framework of the expectation maximization algorithm, are presented. Computer simulations show that the resulting multiuser detectors attain practically the same performance as the theoretical DF minimum mean square error receiver.

Spectral efficiency of the feedback receiver for two sets of orthogonal sequences

We present the spectral efficiency analysis of the recently proposed feedback receiver for two sets of orthogonal sequences (TSOS) scheme which is capable of supporting higher number of users than the spreading gain of the DS/CDMA system. The added complexity is minimal compared to the conventional matched filter. We show that in the case of coded system, spectral efficiency of this scheme is close to that of a Gaussian multiple access channel if the incomplete group of orthogonal sequences is detected first. We also discuss the choice of orthogonal sequences that maximizes the spectral efficiency. This analysis is extended for the case of flat fading channels.

Letter: An Improved Decision-Feedback Receiver for CDMA IS-95 Reverse Link

In this paper, an improved decision feedback receiver is presented for the CDMA IS-95 reverse link. Two techniques are proposed to improve the receiver performance: (1) noncoherent block detection that uses multiple Walsh codes for joint detection, and (2) iterative decision feedback detection. Comparing with conventional "decision feedback receiver," a significant performance improvement is observed via the computer simulations for both AWGN and Rayleigh flat fading channels.

Load-Adaptive MUI/ISI-Resilient Generalized Multi-Carrier CDMA with Linear and DF Receivers

A plethora of single-carrier and multi-carrier (MC) CDMA systems have been proposed recently to mitigate intersymbol interference (ISI) and eliminate multiuser interference (MUI). We present a unifying all-digital Generalized Multicanier CDMA framework which enables us to describe existing CDMA schemes and to highlight thorny problems associated with them. To improve the bit error rate (BER) performance of existing schemes, we design block FIR transmitters and decision feedback (DF) receivers based on an inner-code/outer-code principle, which guarantees MUI/ISI-elimination regardless of the frequency-selective physical channel. The flexibility of our framework allows further BER enhancements by taking into account the load in the system (number of active users), while blind channel estimation results in bandwidth savings. Simulations illustrate the superiority of our framework over competing MC CDMA alternatives especially in the presence of uplink multipath channels.

Communication Theory: Letter: Multiuser Detection Analysis in DS-CDMA Channel with Nakagami Fading

In this paper the multiuser detection performance analysis- in synchronous direct sequence code division multiple access (DS-CDMA) system using decorrelating and decision feedback (DFB) receivers, including single path Nakagami fading channel is presented. The lower bounds of the receivers bit error probability (BER) are derived, and the comparison of these receivers' performances to the conventional CDMA receiver is made. The analytical and numerical results show that the DFB receiver performs better in the presence of the high Nakagami fading channels.

A simulation comparison of multiuser receivers for cellular CDMA

Multiuser detection has gained significant notoriety as a potential advanced enabling technology for the next generation of CDMA systems. Due to the limitations of the conventional correlation receiver, the capacity of a single cell using CDMA is limited by self-interference and is subject to the near-far problem. To overcome these drawbacks, several advanced receiver structures have been proposed. Unlike the conventional receiver which treats multiple access interference (MAI) as if it were AWGN, multiuser receivers treat MAI as additional information to aid in detection. Although each of the multiuser types have been the subject of much literature, there is little published work comparing all structures on the basis of common assumptions. We present a comparison of five of the most discussed receiver structures: the decorrelator, the minimum mean square error (MMSE) receiver, the multistage parallel interference cancellation receiver, the successive interference cancellation receiver, and the decorrelating decision feedback receiver. Comparisons are based on both theoretical analysis and simulation results, examining bit error rate (BER) performance in AWGN, Rayleigh fading, and near/far channels. Additionally, receiver structures are compared on the basis of computational complexity as well as robustness to code phase misalignment. Finally, we present simulation results for noncoherent architectures of the aforementioned receivers.

A decision feedback decorrelator for a dual rate synchronous DS/CDMA system

A dual rate synchronous DS/CDMA system provides service to low bit rate and high bit rate users. In a fixed duration interval, a low rate user transmits one bit while a high rate user transmits M bits. The differences in the bit transmission rates result in different processing gains for each class of user. In this paper, we propose a decision feedback decorrelator for the dual rate synchronous DS/CDMA system which uses modified correlators and initiates the bit decision process at the end of the high rate bit interval. Each step of the decision process is executed by utilizing the decisions of all previously decoded users. This dual rate decision feedback receiver (evaluated by simulation) is found to outperform two types of decorrelators for dual rate CDMA systems. It is also observed that as the interferers grow stronger relative to the desired user, the performance of the decision feedback receiver for decoding the desired user approaches the single user bound.

Decision feedback detection of minimum shift keying

A new decision feedback (DF) receiver for minimum shift keying (MSK) is derived, which uses the previously detected symbols to estimate the initial phase in the current symbol for optimum coherent detection. Some of theoretical and simulated bit error rate (BER) performance results are presented. The results show that the DF detection provides good bit error performance in the AWGN channel, and is also an efficient detection method for MSK in a fast Rayleigh fading mobile channel.

Efficiently structured CDMA receiver with near–far immunity

A Gaussian code division multiple access (CDMA) channel is shared by K active users who transmit asynchronously with BPSK modulation by independent binary data streams. A conventional direct sequence CDMA receiver has only limited capability for suppressing cochannel multiple user interference (MUI) from the K – 1 other CDMA signals while attempting to receive and demodulate each CDMA signal. A new and efficient feedback receiver structure is described for coherent demodulation of the K asynchronous CDMA signals. By incorporating adaptive cancellation of the cochannel MU1 in a feedback structure, the new design offers the following advantages: protection of the receiver's synchronization loops as well as its data demodulators against MUI; lower bit error rate (BER) than the conventional receiver for equal-strength as well as near-far MUI; and implementation simplicity close to that of the conventional CDMA receiver. It is shown that all the CDMA signals are canceled to a common level determined by N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , the one-sided power spectral density of the input AWGN; the bandwidth spreading ratio N; and K. The BER for any signal may be computed as if the N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> at its matched filter input were multiplied by the factor (6N/π)(1.5N + 1 – K) for K ≤ 1.5N. Computer simulations corroborate this result. A straightforward enhancement to the receiver structure is shown to allow it to overcome the deleterious affects of multipath propagation.

Adaptive receiver structures for asynchronous CDMA systems

Adaptive linear and decision feedback receiver structures for coherent demodulation in asynchronous code division multiple access (CDMA) systems are considered. It is assumed that the adaptive receiver has no knowledge of the signature waveforms and timing of other users. The receiver is trained by a known training sequence prior to data transmission and continuously adjusted by an adaptive algorithm during data transmission. The proposed linear receiver is as simple as a standard single-user detector receiver consisting of a matched filter with constant coefficients, but achieves essential advantages with respect to timing recovery, multiple access interference elimination, near/far effect, narrowband and frequency-selective fading interference suppression, and user privacy. An adaptive centralized decision feedback receiver has the same advantages of the linear receiver but, in addition, achieves a further improvement in multiple access interference cancellation at the expense of higher complexity. The proposed receiver structures are tested by simulation over a channel with multipath propagation, multiple access interference, narrowband interference, and additive white Gaussian noise. >

The effects of informational feedback and self-beliefs on the motivation to perform a self-regulated task

College students were given the opportunity to perform a self-regulated task, specifically to voluntarily write test items in return for grade points based on number and type of items written. High-third point-getters received a double grade bonus, middle-third point-getters a single bonus, and low-third point-getters no bonus. One group was given no informational feedback regarding their weekly relative position in points while a second group was given explicit informational feedback. Subjects in the no feedback condition who were high in their belief in their own performance capability outperformed their counterparts in the feedback condition. The reverse was true for subjects of middle and low self-perceived capability where feedback receivers outperformed those receiving no feedback. Student self-competence beliefs moderated the relationship between amount of informational feedback and performance suggesting that different types of students need to be motivated in different ways.

Erratum: Optical feedback receiver with sensitivity of −55.2 dBm at 8 Mbit/s

Erratum: Optical feedback receiver with sensitivity of −55.2 dBm at 8 Mbit/s

Optical feedback receiver with sensitivity of −55.2 dBm at 8 Mbit/s

An optical receiver operating at 1.55 mu m with optical feedback at 0.85 mu m has been proposed to extend bit rate operation beyond 2 Mbit/s. Results of sensitivity (10/sup -9/ BER) of -61.8 dBm at 2.048 Mbit/s, -55.2 dBm at 8.192 Mbit/s and -48.4 dBm at 34 Mbit/s are demonstrated.

−70dBm APD optical feedback receiver at 2.048 Mbit/s

We demonstrate an optical feedback (OFB) transimpedance receiver employing a GaInAs avalanche photodiode (APD) input device and an Si pin diode feedback element. The highest reported sensitivity of −70dBm at 2.048 Mbit/s was achieved.

Error propagation in decision feedback receivers

The problem of assessing error propagation in decision-directed feedback receivers is considered. It is shown that useful bounds on error statistics can be obtained by modelling the error mechanism using stochastic state-transition diagrams and then employing state reduction techniques to enable the complexity of the problem to be contained. This provides a more widely applicable approach to obtaining bounds on performance measures for such receivers than was previously available. The techniques are illustrated for representative error burst statistics of interest in practical applications.