International audience

The paper introduces the latest Multiple Input Multiple Output (MIMO) codebook specified within the 3rd Generation Partnership Project (3GPP). It relies on channel prediction and precoder compression in the Doppler domain at the UE for better up-to-date CSI provisioning at the Base Station. A channel prediction strategy in the angle-delay domain is detailed together with a novel physical layer abstraction needed for precoder selection. Monte Carlo simulations demonstrate the effectiveness of the Rel. 18 codebook with respect to the UE speed in terms of tradeoff between performance enhancement and feedback overhead increase.

In this paper, we consider an orthogonal Multiple Access Multiple Relay Network (MAMRN), where several nodes cooperate to send their messages to a single destination. Frequency Division Multiplexing (FDM) is adopted for the sources and the relays, and the destination acts as the central node responsible for scheduling the cooperative retransmissions. Since FDM is used for orthogonality, different nodes are allocated at each sub-band of the transmission and retransmission phases. In this work, we present the system model of the considered orthogonal MAMRN when the FDM mechanism is adopted, while including the analytical derivations of the utility metrics (spectral efficiency and outage events). Then, two centralized node selection strategies are proposed. Moreover, we present the control information exchange process between the destination and the different nodes. The proposed strategies allocate for each sub-band the node that will transmit (or retransmit) with the goal of maximizing the spectral efficiency. Our numerical analysis considers both symmetric and asymmetric channel and source rate scenarios, and shows that the proposed algorithms outperform the algorithms used in the prior-art, and achieve a spectral efficiency that is close to the upper bound calculated by an exhaustive search approach while reducing the complexity and the overhead.

In this paper, we propose a novel selection strategy for the orthogonal Multiple Access Multiple Relay Networks (MAMRN). Rather than selecting a single relaying node to help one source node at a given retransmission time slot, we propose allocating one source to be helped by multiple relaying nodes. The idea is to exploit the multipath diversity of the different relaying nodes in order to optimize the spectral efficiency. We present the control exchange process in the novel selection strategy and we compare it to that of the prior art. In addition, we investigate the effect of equal gain combining on the performance, as well as the effect of the rates and the channel configuration. The numerical results show that the proposed strategy outperforms the prior art by exploiting the power budget available at each relaying node included in the system.

In this paper, we investigate centralized scheduling strategies for cooperative incremental redundancy retransmissions in the slow-fading half-duplex multiple access multiple relay channel. Time Division Multiple Access is assumed for the sources and the relays. Sources transmit successively in time slots for the first phase. The second phase consists of a limited number of time slots for retransmissions. In each time slot, the destination schedules a node (being a relay or a source) to retransmit, conditional on the knowledge of the correctly decoded source sets of each node (which is itself for a source). A scheduled relay retransmission uses Joint Network and Channel Coding on its correctly decoded source messages (cooperative retransmission). Several node selection strategies are proposed based on the maximization of the long-term aggregate throughput under a given constraint of fairness. Monte-Carlo simulations show that these strategies outperform the state of the art one based on the minimization of the probability of the common outage event after each time-slot. Moreover, the long-term aggregate throughput reached with these strategies is close to the upper-bound, calculated by the exhaustive search approach. The same conclusion remains valid for both symmetric and asymmetric source rate scenarios.

In this paper, we compare the performance of three different cooperative Hybrid Automatic Repeat reQuest (HARQ) protocols for slow-fading half-duplex orthogonal multiple access multiple relay channel. Channel State Information (CSI) is available at the receiving side of each link only. Time Division Multiplexing is assumed, where each orthogonal transmission occurs during a time-slot. Sources transmit in turns in consecutive time slots during the first transmission phase. During the second phase, the destination schedules in each time-slot one node (source or relay) to transmit redundancies based on its correctly decoded source messages (its decoding set) with the goal to maximize the average spectral efficiency. Bidirectional limited control channels are available from sources and relays towards the destination to implement the necessary control signaling of the HARQ protocols. Among the three proposed HARQ, two follow the Incremental Redundancy (IR) approach. One consists in sending incremental redundancies on all the messages from the scheduled node decoding set (Multi-User encoding) while the other one helps a single source (Single User encoding) chosen randomly. The third one is of the Chase Combining (CC) type, where the selected node repeats the transmission (including modulation and coding scheme) of one source chosen randomly from its decoding set. Monte-Carlo simulations confirm that the IR-type of HARQ with Multi-User encoding offers the best performance, followed by IR-type of HARQ with Single User encoding and CC-type of HARQ. We conclude that IR-type of HARQ with Single User encoding offers the best trade-off between performance and complexity for a small number of sources in our setting.

A novel technique is proposed, which enables each transmitter to acquire global channel state information (CSI) from the sole knowledge of individual received signal power measurements, which makes dedicated feedback or inter-transmitter signaling channels unnecessary. To make this possible, we resort to a completely new technique whose key idea is to exploit the transmit power levels as symbols to embed information and the observed interference as a communication channel the transmitters can use to exchange coordination information. Although the used technique allows any kind of low-rate information to be exchanged among the transmitters, the focus here is to exchange local CSI. The proposed procedure also comprises a phase which allows local CSI to be estimated. Once an estimate of global CSI is acquired by the transmitters, it can be used to optimize any utility function which depends on it. While algorithms, which use the same type of measurements, such as the iterative water-filling algorithm, implement the sequential best-response dynamics (BRD) applied to individual utilities, here, thanks to the availability of global CSI, the BRD can be applied to the sum-utility. Extensive numerical results show that significant gains can be obtained and, this, by requiring no additional online signaling.

In this paper, we study the benefit of Joint Network Channel Coding (JNCC) and Decoding (JNCD) for half-duplex slow fading Multiple-Access Multiple-Relay Channels (MAMRC), defined as follows: (1) Multiple statistically independent sources communicate with a single destination with the help of multiple relays; (2) Each relay is half-duplex; (3) The links between the different nodes are subject to slow fading and additive white Gaussian noise; (4) Some links interfere. Channel State Information (CSI) is available only at the receivers (relays or destination) and assumed perfect. While the Multiple Access (MA) function is presented generically depending on how the resources are allocated to the sources and relays, the emphasis is on the most spectrally-efficient Non-Orthogonal Multiple-Access (NOMA) where sources and cooperating relays interfere. The relays perform JNCC in combination with Selective Relaying (SR), meaning that they forward a function of the correctly decoded sources' packets to the destination. Practical linear JNCC schemes and the corresponding JNCD architectures are described in detail for NOMA, and shown to perform close to the theoretical limits.

Modern wireless communication systems rely on limited feedback for Fast Link Adaptation (FLA). FLA in presence of advanced receivers needs new physical layer abstractions (PHY abstractions) that are able to capture their performance in order to derive the limited feedback metrics (e.g., Channel Quality Indicators). Recently, a PHY abstraction was proposed for the class of iterative linear minimum mean square error based soft interference cancellation and decoding also referred to as turbo Linear Codeword Interference Cancellation (turbo L-CWIC) receivers in 3GPP. This PHY abstraction needs to be calibrated and a first one-dimensional (1D) calibration method was proposed. In this paper, we revisit the calibration method by proposing a more rigorous framework to minimize the predicted throughput error that dramatically influences the FLA mechanisms. We also suggest a novel multi- dimensional calibration approach. Monte Carlo simulations show that it outperforms the 1D at the expense of the knowledge of the number of independent fading blocks experienced by a codeword.

Assessing the performance of new technologies before deploying it is key for wireless network operators. It allows making the right decision when considering the balance between the gain that can be expected from the new technologies, and the associated deployment cost. In this paper, we propose a method that consists in combining field measurements on LTE live networks, and predictions from link-level simulations. We then validate it with end-to-end measurements carried out on different areas of a live network with LTE category 3 devices. The prediction method turns out to give reliable results. Simulated and measured throughputs are in average close to one another: the prediction error distribution has a low mean (close to 0 dB) and a small standard deviation (less than 4 dBs). Predictions with this method can now be done to obtain highly reliable performance estimates for yet-to-be released or future devices: category 5 ones with MIMO 4×2 or 4×4 capabilities, and LTE-advanced devices.

In this paper, we study the individual and common symmetric ϵ-outage achievable rates of various cooperative strategies adapted to the slow fading Multiple Access Multiple Relay Channel (MAMRC). Channel links are independent and subject to slow Rayleigh fading and additive white Gaussian noise. No Channel State Information at the Transmitter (CSIT) is available. Common to all investigated cooperation schemes are the following assumptions: (1) The sources are independent and want to communicate their messages to a single destination with the help of multiple relays using Joint Network Channel Coding (JNCC) and Decoding (JNCD); (2) Each relay is half-duplex and applies a Selective Decode and Forward (SDF) cooperative strategy, i.e., it forwards a deterministic function of the source messages that are decoded without errors. The cooperation schemes differ in the assignment of the available channel uses to the sources and the relays, ranging from the less efficient orthogonal assignment where sources and relays are given non-overlapping channel uses to the most efficient one where sources and relays can transmit simultaneously in all available channel uses.

In this paper, we tackle the problem of link adaptation in closed-loop turbo coded multiple-input multiple-output antenna systems with non-linear turbo receivers performing iterative linear minimum mean-square error (soft) interference cancellation (LMMSE-IC) and turbo decoding. The proposed link adaptation scheme relies on a low rate feedback and operates joint spatial precoder selection (e.g., antenna selection) and modulation-coding scheme selection so as to maximize the average rate subject to a target block error rate constraint. The paper first revisits in detail the architecture of the whole turbo receiver, then presents a semi-analytical performance prediction method to analyze its evolution through the stochastic modeling of each of the components. Based on this PHY-layer abstraction, average predicted and simulated throughputs are shown for different communication scenarios.

In MIMO LTE Networks, closed loop link adaptation together with precoding matrix selection try to reduce the interference at the receiver side while maximizing the data rate for a given QoS. However, due the constraints of limited feedback and finite size precoding codebooks, the residual spatial layer interference remains a major impediment to reach high spectral efficiency. The aim of this paper is to evaluate for LTE downlink both at link and system levels the potential gain of non linear receivers with respect to their Linear Minimum Mean Square Error (LMMSE) counterpart. Within the considered 4×4 MIMO LTE network, adopting a Cyclic Redundancy Check (CRC) based Successive Interference Cancellation receiver potentially improves the average rate by 5% and 10% in peak and off-peak traffic hours, respectively. However, the interference-free upper bound promises a gain of 45% and 60% for the same load conditions. These results militate for the adoption of more sophisticated receivers, e.g., turbo-SIC, for large MIMO systems.

In this paper, we investigate the benefit of Joint Network Channel Coding (JNCC) for a new class of Multiple Access Relay Channel (MARC), defined as follows: (1) Independent sources communicate with a single destination in the presence of a relay; (2) The relay is half-duplex and applies a Selective Decode and Forward (SDF) strategy, i.e, it forwards only a deterministic function of the error-free decoded messages; (3) The sources are allowed to transmit simultaneously during both the listening phase and the transmission phase of the relay. We refer this model to as Non-Orthogonal MARC (NOMARC), which better leverages the broadcast nature of the radio channel than its more conservative orthogonal counterpart, i.e., Orthogonal MARC (OMARC). This paper aims at measuring the advantage brought by NOMARC/JNCC over OMARC/JNCC from both an information-theoretic and a practical code design perspectives.

The aim of this paper is to provide a reliable performance evaluation of 3GPP Long Term Evolution (LTE) networks. A new kind of simulation method, based on drive test measurements on a live network, is used. The measurements are combined with link level simulation results for different LTE device categories. Simulation results show that all MIMO 2x2 LTE devices have the same performance in a macro-cell environment: the average user throughput is around 35Mbps, but there are still differences depending on the user's location. Network upgrade to MIMO 4x4 leads to a 60% improvement of average throughput.

In this contribution, we assess a performance prediction method for turbo successive interference cancellation (turbo-SIC) receivers with the help of lab measurements from a quasi-compliant LTE platform. The presented results show that the calibrated prediction method is able to predict the system performance satisfactorily for the considered modulation and coding scheme.

In this paper, a novel semi-analytical performance prediction method is proposed for iterative MMSE-IC detection and semi-blind channel estimation. It is examplified in the context of single-user MIMO-OFDM transmission which we model by a discrete-input MIMO P-block fading flat Rayleigh channel in the frequency domain. The proposed method extends existing symbolwise Mutual Information Effective SNR Metric (MIESM) link-to system approach to the context of imperfect channel state information and semi-blind channel estimation at the receiver side. It allows to compute the average Block Error Rate (BLER) conditional on an initial pilot-assisted channel estimation and long term channel distribution information. Thus, the proposed method takes advantage of all the available a priori information before actual data transmission and could be equally employed for slow and fast link adaptation.

In this paper, we investigate the benefit of joint network-channel distributed coding for the multiple-access half-duplex relay channel. In contrast with previous work, the medium access is not orthogonal and the source-to-relay links are not error-free, which render the reality of wireless environments and notably complicate the problem. Indeed, due to the broadcast nature of wireless media, transmitted signals interfere at the relay and at the destination and form multiple-access channels. This entails the need of more sophisticated joint multiuser detection and decoding procedures. Moreover, improper (erroneous) distributed codes may result from unsuccessful decoding at the relay. This second issue is handled with selection relaying. We describe in detail the structure of the encoders, when and how JNC coding is performed, and the structure of the joint multiuser detectors and decoders. For a given spectral efficiency, we show through simulations that our approach provides substantial performance gains compared to conventional joint network-channel distributed coding for the orthogonal multiple-access half-duplex relay channel. Equivalently, for a targeted performance, our approach allows to transmit at higher rates.

In this paper, a joint channel-network coding is proposed for the multiple access half duplex relay channel. To improve performance, the classical constraints of orthogonality between links are partially relaxed, i.e., in the first transmission phase the two sources are allowed to transmit simultaneously, while in the second phase, the relay transmits alone and the sources are silent. In order to resolve the subsequent collisions occurring at the relay and destination, related iterative multiuser detection structures are suggested. Simulation results demonstrate the superiority of the proposed semi-orthogonal transmission protocol compared to the orthogonal one.

In this paper, two semi-analytical performance prediction methods are proposed and compared for multiuser MIMO transmission over block-fading multipath channels and iterative MMSE successive interference cancellation based joint decoding. In the first method, we develop a completely analytical parameterisation of the whole turbo receiver up to the users' channel decoders. The method which follows the classical framework of extrinsic information transfer charts tracks the evolution of the average mutual information defined at coded bit level and circulating between the multiuser detector and the bank of channel decoders. The modelling of the demapping output in the multiuser detector is based on an explicit calculation of the (conditional) mutual information for each symbol digit in a symbol label under arbitrary (possibly non-uniform) a priori probability distributions on others. Then, shifting in viewpoint and considering user demapping and decoding as a joint process, we present an alternative method which tracks the evolution of the average mutual information defined at symbol level and circulating between the MMSE successive interference based interface and the bank of joint demappers and channel decoders. This allows to avoid the critical issue of parameterising the demapping. Numerical results demonstrate the superiority of the second method for high-order non-linear mapping.

High Speed Uplink Packet Access (HSUPA) is designed to increase the uplink data rate. To reach this goal, a new uplink transport channel, called enhanced dedicated channel (E-DCH), is introduced to the UMTS physical layer. This channel is used in a multicodes transmission principle with very low spreading factors. Furthermore, it supports Hybrid Automatic Repeat reQuest (HARQ), and a shorter 2 ms transmission time interval (TTI) optionally with 10 ms TTI. For determining the expected performance of HSUPA in a multipath channel, the sophisticated signal processing techniques supported by E-DCH channel must be simulated. Especially, the turbo decoding process makes the simulation very prohibitive and CPU-time consuming. In this paper, we propose a simplified simulation methodology which gives an abstraction of the system performance. Computer simulations show that the required CPU-time is significantly reduced.

We consider an idealized synchronous single-carrier single-cell uplink communication where K users equipped with multiple antennas transmit their data to a base station also equipped with multiple antennas. Users have no (or very limited) channel state information. They employ space-time trellis codes to improve the reliability of their transmission link. Transmission occurs on fading multipath channels. Prior to transmission, users' spacetime codewords are interleaved in order to break channel memory. The multiuser joint decoding at the base station (assuming perfect channel state information) is reformulated as the computation of marginal probabilities on dense layered factor graphs with loops. This computation is done by means of the sum-product algorithm. We derive several instances of the sum-product algorithm by modifying the topology of the loopy graph and the definition of the corresponding exchanged messages and show through simulations that those variations can substantially impact the final delivered marginals.

Hybrid automatic repeat request (HARQ) is one of the main technologies used in HSUPA system to increase the uplink data throughput. Chase combining (CC) and incremental redundancy (IR) are the most known HARQ algorithms. The first provides a time diversity, while the second offers a coding gain. In this paper, we propose a method to increase the chase combining efficiency. In this method, all HARQ transmissions are considered as additional receive antennas, and combined at the chip-level using a linear minimum mean square error (LMMSE). It is shown that with our approach, the Chase combining diversity gain is considerably increased especially in the case of high order modulation.

Abstract-This paper presents a semi-analytical method to predict the performance of iterative interference cancellers used in advanced multiuser MIMO CDMA systems. At the receiver side, the orthogonality between codes is restored by a chip rate equalization allowing the transmitted symbols to be subsequently estimated by a simple correlations. The proposed prediction method is based on Signal to Interference-plus-Noise Ratio (SINR) tracking and Average Mutual Information (AMI) compression. Therefore, the SINR at symbol level after despreading is analytically derived and injected into the algorithm. Simulations confirm that the method provides accurate performance prediction and is fast enough to be integrated in radio resource management and link adaptation mechanisms of advanced MIMO CDMA/MC-CDMA systems using powerful interference cancellation techniques.

This paper generalizes the information-theoretic optimality of minimum mean square error successive interference cancellation in layered space-time coding with rate and power control. Based on this derivation, a new concept relying on partial feedback is introduced, whose core idea is to exploit an additional degree of freedom relative to the partitioning of transmit antennas. Taking into account this additional degree of freedom, together with power control and decoding order, allows the reduction of the quantization noise induced by the use of finite discrete-rate sets at the transmitter. However, the simultaneous optimization of all those degrees of freedom proves to be computationally intensive and would result in a tremendous feedback load. Practical algorithms are thus proposed to achieve this optimization with a reasonable complexity and a limited amount of feedback. Monte-Carlo simulations show that those algorithms perform close to the theoretical limits.

This paper presents a novel fast performance prediction method for iterative minimum-mean square error interference cancellation based joint decoding in a very general multiuser MIMO setting considering a transmission over block-fading frequency-selective channels. A Gaussian approximation is made for all propagated messages between the outer codes and the space-time multiuser detector. The method relies on a signal-to-interference-plus-noise ratio compression, by means of the mutual information effective signal-to-noise metric (MIESM), which allows a one-dimensional AWGN look-up table to capture the soft decoding result of each distinct user. Simulations show that this method while being extremely simple and fast keeps very accurate and is thus adequate for radio resource management and link adaptation purposes.

This paper investigates the effect of channel estimation errors in presence of spatial correlation for fading MIMO multiple access channels (MACs). Specifically, the capacity is derived under the notion of reliable communication based on the average of the error probability over all channel estimation errors. Although not necessarily optimality in presence of channel estimation errors, we restrict the channel inputs to be Gaussians, allowing a closed-form expression for the mutual information, which leads to an inner bound of the capacity region. For different types of transmitter information (e.g. instantaneous and covariance information), we study optimal transmission schemes that permit to maximize the sum-rate. Numerical results show that the performance of covariance feedback is quite sensitive to channel accuracy while the gap between the open-loop and the closed-loop scenario increases with the channel uncertainty. Simulations also assess the performance degradation of transmit/receive correlation and reveal that their impact behaves independently of the estimation errors.

Waterfilling enables a dramatic increase in capacity of multiple antenna systems, but requires perfect knowledge at transmitter.When a finite discrete rate is available, this algorithm is no more optimal since the interest lies in maximizing the sum discrete-rate. In this paper, motivated by a feedback reduction, we introduce a transceiver that partially decouples the channel into parallel subchannels. From the analogy between multi-user and single-user channels, we propose a discrete rate iterative waterfilling procedure. It first relies on the classical iterative waterfilling, and then rounds continuous rates to discrete rates with an efficient use of residual power. Simulations show that a substantial enhancement of the discrete bit loading process is provided. We further investigate the trade-off feedback-performance of different waterfilling-based systems. Simulations point out that the proposed scheme outperforms the waterfilling with nearly 60% of feedback reduction in the moderate to high SNR region, while achieving nearly 90% of the sum discrete-rate waterfilling at low SNR.

Today link-to-system (L2S) interfaces are more and more used in order to speed up complete system-level simulations. In this paper a simple extension to generic incremental redundancy (IR) hybrid automatic repeat request (HARQ) strategies is presented for two well-known L2S interfaces: the exponential effective SNR metric (EESM) and the mutual information effective SNR metric (MIESM). Then we focus on the problem of the L2S interface tuning, which is necessary to achieve the highest accuracy of the prediction models. The standard calibration procedure is compared with a new method, based on the average (over channel and noise) physical (PHY) layer performance. The latter, called average calibration procedure, is less time consuming than the standard procedure. Moreover, we show by simulation that the optimal calibration factors, calculated with the two methods, converge to close values thus obtaining equivalent prediction accuracy for the same L2S interface.

This paper investigates a new concept that combines space-time coding with the layered architecture of successive interference cancellation with per antenna rate control (PARC- SIC) for wireless MIMO channels, relying on partial feedback. The key element is to exploit an additional degree of freedom (DoF), namely antenna partitioning, in addition to decoding order and power control considering that the set of transmit antennas can be grouped into subsets. It is proved that this design theoretically achieves the open-loop capacity. In presence of a finite discrete modulation and coding scheme (MCS) rate set at transmitter, this new DoF allows to either fight back the resulting quantization noise or to reduce the feedback without much performance loss.

High speed downlink packet access (HSDPA) is the major evolution of UMTS downlink. It provides peak data rates up to 10.8 Mbps. This is reached by radio link adaptivity, which includes fast scheduling, adaptive modulation and coding (AMC), and hybrid automatic repeat request (HARQ). The performance of AMC suffers from multipath interference introduced by the channel especially when using 16-QAM modulation. This is because of the variation in bit reliabilities caused by the bit-mapping onto the signal constellation. In this paper, we present a soft iterative multipath interference canceller (MPIC) for suppressing the multipath interference. We also propose to use HARQ Chase combining with the constellation rearrangement technique in joint application with MPIC. This can equalize the variation of bit reliabilities by using different mapping rules over HARQ transmissions. It is proven that the performance of adaptive modulation and coding is significantly improved after only two HARQ transmissions.

Antenna selection, as a special case of power adaptation, can enhance the capacity of layered space time codes under rate control without any overhead feedback. To efficiently fight back rates quantization, this contribution considers a joint optimization of the best active transmit antennas associated to rate control together with antenna partitioning (i.e., layered multidimensional coding) and decoding order degrees of freedom. We propose an iterative antenna selection, which initializes the decoding order, so as to keep a low computational complexity while achieving near-optimal performance. Gains get all the more important as the signal-to-noise ratio increases.

This paper investigates the combination of spatial multiplexing and overloading as an evolution of the UMTS-HSDPA (high speed downlink packet access) toward very high spectral efficiencies in the case of no (or very limited) channel state information at transmitter. Overloaded spreading is carried out in the time domain only (independent per antenna), transmit space diversity is partially recovered employing Space-Time BICM for each user. A low complexity soft interference cancellation based iterative receiver for channel estimation, space-time chip-equalization and multiuser detection is proposed. Two scenarios are considered for the intracell interference: either the spreading codes and the modulation of the interfering users are known or no-knowledge of the interfering signals is available at the receiver whatsoever.

This paper investigates the combination of spatial multiplexing and overloading as an evolution of the UMTSHSDPA (High Speed Downlink Packet Access) toward very high spectral efficiencies. The base station employs single-carrier space-time bit-interleaved coded modulations with linear precoding for each user. From an information-theoretic point of view, this scheme is potentially capacity achieving but puts high stakes on the receiver complexity. Channel estimation is made particularly difficult by the superimposition of pilot symbols and data. Indeed, the classical channel estimation method based on matched filtering and accumulation clearly fails to give satisfactory performance in a spatial multiplexing and overloading context. Thus, we propose a sophisticated iterative receiver which includes channel estimation, MMSE-based chip-equalization and MMSE-based multiuser detection together with decoding of the turbo-code. Simultaneous downlink transmission to several users is investigated in this paper which brings out the overloading flexibility in terms of Space-Time Modulation and Coding Scheme (ST-MCS) design and radio resource allocation.

In this paper, frequency-domain implementation of a block turbo-equalizer where ISI suppression and CAI resolution are performed separately is presented. This functional split allows deriving efficient low complexity structures based on a modified doubly iterative architecture which ensure better tradeoff between performance and complexity. We focus on a modified doubly-iterative and hybrid structure where further complexity reduction is obtained by relaxing the criterion retained for ISI suppression (max-SNR instead of MMSE) from a given iteration. Simulation results show the efficiency of such modified architectures in the case of a realistic spatially correlated MIMO channel

This paper investigates the combination of spatial multiplexing and overloading as an evolution of the UMTS-HSDPA (high speed downlink packet access) toward very high spectral efficiencies. The base station employs single-carrier space-time bit-interleaved coded modulations with linear preceding for each user. From an information-theoretic point of view, this scheme is potentially capacity achieving but puts high stakes on the receiver complexity. Channel estimation is made particularly difficult by the superimposition of pilot symbols and data. Indeed, the classical channel estimation method based on matched filtering and accumulation clearly fails to give satisfactory performance in a spatial multiplexing and overloading context. Thus, we propose a sophisticated iterative receiver which includes channel estimation, MMSE-based chip-equalization and MMSE-based multiuser detection together with decoding of the turbo-code. Simultaneous downlink transmission to several users is investigated in this paper which brings out the overloading flexibility in terms of space-time modulation and coding scheme (ST-MCS) design and radio resource allocation

This paper presents a new class of block turbo-equalizers for single-carrier transmission over multiple-input multiple-output (MIMO) broadband wireless channels. The key underlying idea consists in equalizing (nonoverlapping) groups of symbols and detecting their individual space-time components in a disjoint and iterative fashion. This functional split naturally induces new design options that have been accurately listed and described, i.e., choice of distinct criteria for intergroup interference (IGI) equalization and intragroup components detection, yielding hybrid structures, multiple iterative loops, and related scheduling variants. Selected algorithms in the proposed class are compared in terms of performance under various transmission scenarios. For all of them, minimum mean square error IGI equalization certainly occupies a central role (at least for the first iteration) and may be identified as the computational bottleneck. Fortunately, block spread transmission together with cyclic prefix operations make the channel matrix block circulant, thus allowing low-complexity inversion in the Fourier domain

This paper addresses the problem of transmitting space-time interleaved coded signals over Multiple-Input Multiple-Output (MIMO) block fading channels with InterSymbol Interference (ISI). At the the receiver, we investigate a cooperative approach in which data detection, decoding, and channel estimation are performed separately and iteratively. Computationally efficient methods based on the Expectation-Maximization (EM) algorithm are developed for multiple path gain estimation.

This paper addresses the issue of advanced equalization methods for space-time communications over multiple-input multiple-output block fading channel with intersymbol interference. Instead of resorting to conventional multiuser detection techniques (based on the straightforward analogy between antennas and users), we adopt a different point of view, and separate time equalization from space equalization, thus introducing a higher degree of freedom in the overall space-time equalizer design. Time-domain equalization relies on minimum mean-square error criterion and operates on multidimensional modulation symbols, whose individual components can be detected in accordance with another criterion. In particular, when the optimum maximum a posteriori criterion is chosen, substantial performance gains over conventional space-time turbo equalization have been observed for different transmission scenarios, at the price of an increased, albeit manageable, computational complexity.

In this paper, we propose a class of iterative equalizers for space-time bit interleaved coded modulation over MIMO block fading multipath AWGN channel. Simulations show that the proposed receivers are able to reach the matched-filter bound and that the tested space-time coding schemes have the potential to get close to the Shannon theoretical limit.

This paper addresses the problem of decoding the space-time bit-interleaved coded signals transmitted over multiple-input multiple-output block fading multipath channel when channel state information is not perfect at the receiver. We develop a cooperative approach in which the channel estimation, channel equalization, data detection, and outer decoding are performed separately and iteratively. A computationally efficient method is proposed where most of the inner modules rely on the suboptimal MMSE criterion. The approach is validated through Monte-Carlo simulations.

The paper aims at comparing the respective benefits of STBICM (space-time bit-interleaved coded modulation) and space-time trellis codes (STTC) for a MIMO block fading multipath channel. Giving a general framework for the calculation of the outage probability, we try to point out the best transmit scheme that approaches the closest the outage probability, assuming the most efficient receiver (i.e., disregarding any complexity aspect). We conclude that iteratively decoded STBICM should be used in preference to STTC.

We propose the building of high rate space-time codes for multiple-input multiple-output (MIMO) block fading multipath additive white Gaussian noise channels by layering small compound space-time trellis codes. To recover part of the spatial diversity between the different layers, we also study their concatenation with an outer code. Naturally, our space-time coding architecture puts the burden on the receiver complexity. As a main contribution of this paper, a near-optimal soft-input soft-output reduced-complexity trellis search algorithm is suggested for joint MIMO detection and inner decoding.

This letter presents a new receiver for Q-ary transmission, where all receiver blocks are embedded in an iterative structure. Packet data transmission in Global Systems for Mobile communications (GSM) and Enhanced Data rates for Global Evolution (EDGE) are considered as examples. A low-complexity soft-in-soft-out detector for EDGE is introduced and its modification suitable for iterative detection is derived. Application of iterative detection and channel estimation techniques in GSM/EDGE shows a significant performance enhancement. Additional improvement may be obtained if the iterative processing is applied to packet retransmission schemes.

In this paper, we consider a generic model of space-time bit-interleaved coded modulation (ST-BICM) on a multiple-input multiple-output (MIMO) Rayleigh fading multipath channel. A practical low-complexity receiver structure performing iteratively MIMO data detection, channel decoding and channel estimation, is presented. The MIMO data detection, employing a reduced-state list-type soft output Viterbi algorithm enables to cope with severe channel intersymbol interference (ISI) without MIMO prefiltering. Among other results, simulations show that our approach can dramatically improve the downlink performance of time-division multiple access (TDMA) systems with high order modulation, keeping a reasonable complexity at the receiver side.

We propose two pilot symbol aided iterative algorithms for channel re-estimation to be used in iterative detection and decoding (turbo-detection) of signals transmitted over frequency selective channels. The first algorithm is based on the expectation-maximization (EM) algorithm that makes use of a signal decomposition to compute separate dosed form recursion expressions for the channel coefficient re-estimates. The a posterior conditional channel state probabilities required by the EM-based turbo-estimation can be easily provided by the forward-backward trellis search performed by the soft-in soft-out (SISO) ISI decoding task. The second algorithm is based on the well-known bootstrap algorithm where soft information from the outer decoder is used to make hard decisions on transmitted coded data, and the detected coded data are used in the channel turbo-estimation process via the conventional linear pseudo-inverse method. Both of the proposed channel turbo-estimation algorithms are naturally embedded in the framework of the turbo-detector, bringing almost no overhead to the receiver complexity.

A generic model of bit-interleaved coded modulation (BICM) on a multipath Rayleigh fading multiple-input multiple-output (MIMO) channel is derived. A practical low-complexity trellis-based receiver performing iteratively channel estimation, multilayer coded data detection and channel decoding is introduced. The inner multilayer data detector, employing reduced-state techniques together with generalized per-survivor processing, presents two-fold advantages. It enables one to cope with severe channel intersymbol interference and allows the use of more transmit antennas than receive antennas. Simulations show that our approach can dramatically improve the downlink performance of time division multiple access (TDMA) systems with high order modulation, such as EDGE, keeping a reasonable complexity at the receiver side.

In this letter, we propose to assess the extra power needed in wide band code division multiple access (WB-CDMA) systems due to perfect power control. First, we derive analytically the expression of the average of this extra power. Then we study its evolution with respect to the number of paths, their average power and their delay spacing.

The UMTS radio interface based on CDMA, has been designed to offer a wide range of data rates using variable spreading factor. It is ongoing standardization effort to include a high data rate packet mode in the downlink direction. This new mode is called high speed downlink packet access (HS-DPA) and is characterized by the giving up of closed loop power control, link adaptation using variable constellations (QPSK, MAQ16,MAQ64), and low spreading factor. The proposed receiver is particularly adapted to HSDPA mode has it gives close to optimal performance even in the case of high modulation order and low spreading factor while keeping a reasonable complexity.

We investigate a communication model made of an outer code interleaved with a space-time trellis coded modulation (ST-TCM). Such a scheme is very relevant for highly frequency selective and time-varying channels using turbo-detection at the receiver side, but inherently suffers from a low coding rate. One possible solution to alleviate this impairment (i.e., to increase spectral efficiency) is to use parallel (or multilayered) transmission as done in recent approaches (BLAST). However, parallel transmission and highly frequency selective channels involve tremendous complexity at the receiver side. We propose a low complexity turbo-detector based on reduced-state trellis search and generalized per survivor processing that proves to be very efficient in this resulting multiple-input multiple-output (MIMO) ISI channel context.

We analyze and compare several strategies for iteratively decoding trellis-encoded signals over channels with memory. Soft-in/soft-out extensions of reduced-complexity trellis search algorithms such as delayed decision-feedback sequence estimating (DDFSE) or parallel decision-feedback decoding (PDFD) algorithms are used instead of conventional BCJR and min-log-BCJR algorithms. It has been shown that for long channel impulse responses and/or high modulation orders where the BCJR algorithm becomes prohibitively complex, the proposed algorithms offer very good performance with low complexity. The problem of channel estimation in practical implementation of turbo detection schemes is studied in the second part. Two methods of channel reestimation are proposed: one based on the expectation-maximization (EM) algorithm and the second on a simple Bootstrap technique. Both algorithms are shown to dramatically improve the performance of the classical pseudo inverse channel estimation performed initially on a training sequence.

We investigate two iterative methods for decoding serially concatenated convolutional codes (SCCC) on multipath Rayleigh fading channels. One method employs optimal maximum a posterior (MAP) decoders or sub-optimum versions. The second slightly differs and is based on a soft-in soft-out (SISO) parallel decision-feedback decoder (PDFD), used for iteratively performing joint equalization and inner decoding together with outer decoding. Simulation results show that the first iterative process is only 0.7 dB better than the second one, which is however far less complex.

A turbo-detector concept assuring a favorable tradeoff between performance and complexity is investigated for the novel radio access scheme at enhanced data rates for GSM evolution (EDGE). The high level modulation employed in EDGE (8-PSK) precludes MAP or MLSE turbo-detection, and consequently, computationally efficient sub-optimum solutions have to be considered. A turbo-detector based on a generic SISO delayed decision-feedback sequence estimator (DDFSE) is shown to be a very promising candidate, so long as we couple it with a discrete-time allpass pre-filter turning channel impulse response into minimum phase.