The mobile outdoor radio environment is challenging for vehicular communications. Although multipath propagation offers diversity and benefits in non-line-of-sight (NLOS) conditions, simultaneous multipath and mobility results in a doubly-selective fading channel. In practice, this means that the channel parameters vary significantly in both time and frequency within the bandwidth and typical packet durations used in 802.11p/WAVE standards for short-range vehicular communications. This paper presents the results of extensive field trial campaigns conducted in several countries, totaling over 1100 km. These field trials are scenario based, focusing on challenging low-latency, high-reliability vehicle-to-vehicle (V2V) safety applications including intersection collision warning, turn across path, emergency electronic brake light, do not pass warning, and precrash sensing. Vehicle-to-infrastructure (V2I) applications are also considered. The field trials compared the performance of off-the-shelf WiFi-based radio equipment with a more advanced 802.11p compliant radio employing more sophisticated channel estimation and tracking. Field trial results demonstrate significantly improved performance using the advanced radio, translating into greatly increased driver warning times and stopping distances. In fact the results show that off-the-shelf WiFi equipment fails to provide sufficient stopping distance to avert accidents in some cases. During the field trials, channel sounding data were also captured. Analysis of these channel measurements reveals the critical importance of accurate channel estimation, tracking the channel in both time and frequency within each packet. Delay spread and Doppler spread statistics computed from the channel measurements validate previously reported results in the literature. The results in this paper, however, provide the first instance of channel measurements performed simultaneously to application performance evaluation. The objective is to firmly establish the link between radio channel characteristics and the performance of critical V2V safety applications.

A K-symmetric channel is a K-user linear multiple-access channel in which the cross correlations between each pair of users are identical. The main contribution of this correspondence is an algorithm which finds the P sequences with highest a posteriori probability (APP) in the case of binary transmission over a K-symmetric channel with additive white Gaussian noise. This list detector is applied to the problem of iterative multiple-user decoding, approximating the APP computation by marginalization over these P sequences, rather than all possible 2/sup K/ sequences. Simulation results indicate that using only small values of P, very good performance may be obtained. It is also demonstrated how to incorporate prior probabilities (a requirement for iterative decoding). The overall per-bit computational complexity of the approach is O(K/sup 2/+PlogP). It is also shown that for any multiuser system possessing a polynomial complexity optimal detection algorithm it is possible to obtain the P most probable sequences with polynomial complexity.

One powerful approach for multiuser decoding is to iterate between a linear multiuser filter (which ignores coding constraints) and individual decoders (which ignore multiple-access interference). Subject to clearly formulated statistical assumptions and the history of input signals provided by the outer decoders over all previous iterations, an extrinsic Kalman filter is suggested. This approach is motivated by the recent observation that decoder outputs are loosely correlated during initial iterations. Numerical results show that iterative decoding using this filter provides better performance in terms of the supportable load and convergence speed as compared to previously suggested linear-filter-based iterative decoders.

We propose a low complexity iterative multiuser decoder. We perform the multiuser a-posteriori probability calculation based on the marginalization of probabilities over a subset of the P highest probable sequences. Given a K user symmetric channel this list may be approximated closely with per-bit computational complexity O(K + P + 2K log K). We further show that for any multiuser system possessing a polynomial complexity optimal detection algorithm it is possible to obtain the P highest probable sequences with polynomial complexity. We further show that for any multiuser channel it is possible to obtain P highly probable sequences in polynomial time and hence utilize the list detection procedure for iterative decoding.

In this paper we investigate the performance of a CDMA iterative multi-user receiver with non-binary modulation. The receiver incorporates iterative channel estimation and is applied to multipath fading channels. The particular algorithm investigated is an extension of the work presented in Alexander et al., (2000), and Grant et al., (2000). Results are presented based on analysis, simulation and fixed point DSP implementation.

We investigate a sub-optimal iterative receiver for joint equalization and decoding called a turbo-equalizer. We view the evolution of the error variance of the transmitted symbols through iterative processing, obtaining the convergence analysis. This allows us to predict the asymptotic performance (when the turbo-equalizer has converged) but also the trigger point observed in its performance.

The downlink capacity of WCDMA systems is highly important due to the expected asymmetrical nature of the traffic. It is thus of paramount importance that highly efficient downlink receivers are implemented. The conventional RAKE receivers, as are likely to be initially implemented in UMTS networks, suffer significant performance degradation under high traffic loads. A candidate receiver for the downlink of such networks is the LMMSE chip level equaliser. The principal of such receivers is to restore the orthogonality of users from one cell by equalising the received signal from that cell at chip level before despreading. This technique is not only able to suppress the intra-cell multiple access interference (MAI) but may also be used to reduce the inter-cell MAI. This paper addresses the use of the minimum mean-squared error (MMSE) criteria for the construction of a linear MMSE chip equaliser for a WCDMA downlink receiver. Channel coding, as specified in the 3GPP technical specifications, is employed in both single and multi-cell scenarios. Simulation demonstrate such receivers are able to achieve considerable performance gains when compared to the conventional RAKE receiver.

We investigate a sub-optimal reduced-complexity iterative technique for joint detection and estimation for sets of constrained sequences and derive analytical results concerning convergence regions and fixed points. We apply this theory to the problem of multiple-user decoding.

This paper investigates a multiple-access communication receiver system that receives coded data modulated using either direct-sequence code division multiple access or narrowband binary phase shift keying, with an antenna array in a multipath propagation environment. We describe an iterative receiver that improves the initial estimates from the antenna array, and therefore reduces the multiple access interference. Simulation results show that the bit error rate performance approaches that obtained when only one user's signal is incident on the array. This occurs even with a large number of users in comparison to the product of the spreading gain and array size.

We view the asynchronous random code division multiple-access (CDMA) channel as a time-varying convolutional code. We study the case where the users encode their data, and, therefore, the single user transmitters and the CDMA channel appear as the concatenation of two coding systems. At the receiver we employ serial turbo decoding strategies. Unlike conventional turbo codes where both the inner and outer code may be selected, in our case, the inner code is due to the CDMA channel which we assume to be random. Nevertheless, the decoding system resembles the decoder of a serial turbo code and single-user performance is obtained even for numbers of users approaching the spreading code length.

In this letter we present a simplified proof for the convergence of the windowed Cholesky factorization of the time-varying asynchronous code-division multiple-access (CDMA) channel.

This paper introduces an iterative multiuser receiver for direct sequence code-division multiple access (DS-CDMA) with forward error control (FEC) coding. The receiver is derived from the maximum a posteriori (MAP) criterion for the joint received signal, but uses only single-user decoders. Iterations of the system are used to improve performance, with dramatic effects. Single-user turbo code decoders are utilized as the FEC system and a complexity study is presented. Simulation results show that the performance approaches single-user performance even for moderate signal-to-noise ratios.

The effect of using randomly selected sequence multisets for the uplink of a synchronous code-division multiple-access channel is considered. A tight lower bound on the expected value of the sum capacity over the ensemble of randomly selected sequence multisets is given. For large systems, the sum rate penalty for using randomly selected multisets is shown to be at most 1 nat and to vanish as the number of users becomes large, compared to the sequence length.

In this paper, receiver design and performance analysis for coded asynchronous code-division multiple access (CDMA) systems is considered. The receiver front-end consists of the near-far resistant multiuser detector known as the projection receiver (PR). The PR performs multiple-access interference resolution and is followed by error-control decoding. The output of the projection receiver yields the appropriate metric (i.e., soft information) for decoding of the coded sequences. An expression for the metric is derived that allows the use of a standard sequence decoder (e.g., Viterbi algorithm, M-algorithm) for the error-control code. It is then shown that the metric computer has an elegant adaptive implementation based on an extension of the familiar recursive least squares (RLS) algorithm. The adaptive PR operates on a single sample per chip and achieves a performance virtually identical to the algebraic PR, but with significantly less complexity. The receiver performance is studied for CDMA systems with fixed and random spreading sequences, and theoretical performance degradations with regard to the single-user bound are derived. The near-far resistance of the PR is also proven, and demonstrated by simulation.

This paper discusses a code-division multiple access (CDMA) iterative multi-user receiver with forward error control (FEC) decoding. The maximum a-posteriori probability (MAP) criteria is used to derive the receiver. The decoding is done using a turbo-code decoder with modifications which are discussed. Iterations of the system are used to attain large performance improvements over conventional systems.

We consider a CDMA system with error-control coding. Optimal joint decoding is prohibitively complex. Instead, we propose a sequential approach for handling multiple-access interference and error-control decoding. Error-control decoding is implemented via single-user soft-input decoders utilizing metrics generated by linear algebraic multiuser metric generators. The decorrelator, and a new scheme termed the projection receiver, are utilized as metric generators. For a synchronous system, the coded performance of the projection receiver metric is shown to be superior to the decorrelator even though they are equally complex. Also, the theoretical degradation relative to the single user bound is derived.

Multicarrier CDMA and direct sequence CDMA have been suggested for both narrowband and broadband multiple access systems. A series of concepts have further been proposed for variable rate broadband transmission within a CDMA system. These principally different techniques exhibit certain common characteristics that can be exploited. We propose a generalised unifying discrete-time model for describing both MC-CDMA, DS-CDMA and hybrid DSMC-CDMA narrowband and variable rate broadband schemes. The model leads to a system description which is similar to the matrix algebraic formulation of a traditional DS-CDMA system.

In this paper, we align a class of known multiuser detectors for code-division multiple-access (CDMA) along with some new multiuser detectors in a unified group. The new multiuser detectors are devised by completing the structure of the group. The unified group structure is a binary tree obtained by splitting sequence and single symbol detection, maximum aposteriori (MAP) and maximum likelihood (ML) detection, and unconstrained as well as constrained detection. The link between sequence and single symbol estimators is explained revealing the increased complexity of single symbol estimators as compared to sequence estimators. Finally, the aforementioned group structure is supported by a geometrical technique depicting the detection process performed by the multiuser detectors.

A new multiuser receiver for synchronous code-division multiple-access (CDMA) systems with error control coding is proposed. The receiver achieves interference cancellation by projecting the undesired users onto the space spanned by the desired users' signal vectors. The detector calculates the least squares (LS) estimate of the interfering users data, that is used to yield an adjusted metric for maximum likelihood sequence estimation (MLSE) for the desired users' sequences. Simulation results indicate that close to optimal performance can be achieved when all but one of the users are projected using only a single user decoder for the desired user. Further, an adaptive receiver structure based on the recursive LS update is presented that is well-suited for DSP implementation due to it's computational efficiency.

In this paper, we align a class of known multiuser detectors for code-division multiple-access (CDMA) along with some new multiuser detectors in a unified group. The new multiuser detectors are devised by completing the structure of the group. The unified group structure is a binary tree obtained by splitting sequence and single symbol detection, maximum upostel-iori (MAP) and maximum likelihood (ML) detection, and unconstrained as well as constrained detection. The link between sequence and single symbol estimators is ex- plained revealing the increased complexity of single symbol es- timators as compared to sequence estimators. Finally, the afore- mentioned group structure is supported by a geometrical tech- nique depicting the detection process performed by the multiuser detectors.

We examine the behaviour of the information theoretic capacity of jointly detected (multiuser) symbol synchronous DS-SSMA systems, when randomly selected spreading sequences are used. We find upper and lower bounds on capacity for a certain cross-correlation measure, and show that if the number of users is larger than the sequence length, that the lower bound tends to the maximum capacity with increasing sequence length. This implies that for large systems, random spreading sequences are optimal in terms of capacity. We also examine more realistic cases for the number of users and sequence length, and find that in certain cases that the use of random spreading sequences results only in a very small decrease in capacity. I. Introduction In a recent paper by Rupf and Massey [1], it was shown that any sequence multiset that achieves Welch's lower bound on total squared correlation [2] (denoted a WBE set) maximises capacity for the Gaussian direct sequence spread spectrum multiple access ...

It has been shown that the performance of sub-optimal multiuser receivers for synchronous DS/SSMA is improved by using a noise whitening matched filter (WMF) as a receiver filter. In the asynchronous case the performance is also improved through the use of a WMF. We present an efficient technique for deriving the WMF for an asynchronous DS/SSMA system. The WMF is determined as the Cholesky decomposition of X where X is the solution to X=R/sub 0/-R/sub 1//sup T/X/sup -1/R/sub 1/. A recursive algorithm for determining X is proposed. Constraints on the spreading codes that ensure convergence of the algorithm are derived and the efficiency of the technique is demonstrated by simulation.