David Mottier

A spreading sequence allocation procedure for MC-CDMA transmission systems

A novel spreading sequence allocation procedure for multi-carrier code division multiple access (MC-CDMA) systems is proposed and investigated. This new technique, which relies on an analytical evaluation of the multiple access interference (MAI) mitigates the interference between different users by optimizing the spreading sequence selection within a given spreading sequence family. For low-loaded transmissions over different realistic frequency correlated channels, it is shown that this new selection procedure outperforms systems where no specific allocation rule is employed. Furthermore, this technique affects the emitted signals and its performance improvement is observed whatever the detection technique used at the receiver side. Thus, this technique improves the capacity of MC-CDMA systems.

SINR-based channel pre-equalization for uplink multi-carrier CDMA systems

The paper investigates channel pre-equalization for the uplink of multi-carrier code division multiple access (MC-CDMA) mobile cellular systems. This technique solves the crucial problem of uplink channel estimation by keeping the high spectral efficiency of MC-CDMA with no need of pilot symbols. We study an optimisation criterion based on the maximization of the signal-to-interference-plus-noise ratio (SINR) at the base station while constraining the power transmitted by the mobile stations. The optimum algorithm is derived and a sub-optimum version is proposed to reduce the computational complexity in the mobile station. Simulation results on Rayleigh fading channels show that SINR-based pre-equalization systems achieve performance that are much better than systems using either conventional pre-equalization techniques at the transmitter or multiuser detection techniques at the receiver.

New PAPR-preserving mapping methods for single-carrier FDMA with space-frequency block codes

Innovative mapping schemes for Space-Frequency Block Codes (SFBC) which are compatible with the structure of Single-Carrier Frequency Division Multiple Access (SC-FDMA) systems are introduced. We first show that existing space-time and space-frequency block codes lack flexibility in terms of framing or cause a degradation of the signal envelope properties when combined with SC-FDMA. Then, we present an Alamoutibased orthogonal code designed for 2 transmit antennas that makes use of an innovative mapping in the frequency domain to preserve the low envelope properties of SC-FDMA. Next, an extension of this concept to a quasi-orthogonal code for 4 transmit antennas is presented and analyzed. We prove the good performance of the proposed schemes over multiple-input multiple-output (MIMO) channels both in static and in highmobility scenarios.

A Doppler estimation for UMTS-FDD based on channel power statistics

A Doppler estimation based on a statistical analysis of the channel power variations is proposed for the frequency division duplex (FDD) mode of Universal Mobile Telecommunication Systems (UMTS). Its implementation is considered in both cases of constant power and closed-loop power controlled transmissions. Performance is analyzed in terms of velocity estimation error under various conditions of speed, estimation delay and signal-to-interference-plus-noise ratio (SINR). The influence of a SINR-based power channel estimation is also taken into account. Results show the reliability of this Doppler estimation technique that yields estimates with less than 15 km/h error in a velocity range from 0 km/h to 100 km/h. Results also present the impact of the closed-loop power control on the Doppler estimation reliability.

An Analysis of Three Multiple Access Techniques for the Uplink of Future Cellular Mobile Systems

Orthogonal frequency division multiple access (OFDMA) is today a part of many standard specifications. Despite its attractive features, it has two main drawbacks when employed in the uplink: the high peak-to-average power ratio (PAPR) inherited from orthogonal frequency division multiplexing (OFDM) and the inherent frequency diversity loss. The loss of frequency diversity can be alleviated by precoding. Variants of precoded OFDMA include spread spectrum multi-carrier multiple access (SS-MC-MA) and the frequency-domain implementation of single-carrier (SC) FDMA. The latter transforms OFDMA into a SC transmission system, avoiding the PAPR problem. This paper analyses OFDMA, several variants of precoded OFDMA and SC-FDMA in its frequency-domain implementations and compares them for uplink transmission. Copyright

Single-Carrier Space-Frequency Block Coding: Performance Evaluation

In this paper we investigate the performance of single-carrier space-frequency block coding (SC-SFBC), a new diversity technique compatible with single-carrier frequency division multiple access (SC-FDMA). SC-FDMA has been adopted as a possible air interface for future wireless networks as it combines the advantages of orthogonal frequency division multiple access (OFDMA) and the low envelope variation properties of single carrier (SC) transmission. Existing transmit antenna diversity techniques as space-time block coding (STBC) and space-frequency block coding (SFBC) are incompatible either with the system constraints or with the low envelope variations of SC-FDMA. We describe the new proposed SC-SFBC technique and we prove its good performance both in terms of peak to average power ratio (PAPR) and bit error rate (BER) performance on frequency selective multiple input multiple output channel. Our new scheme is compared to existing open-loop transmit antenna selection, cyclic delay diversity (CDD), STBC and classical SFBC schemes.

A Novel Space-Frequency Coding Scheme for Single Carrier Modulations

Single-carrier frequency division multiple access (SC-FDMA) has been adopted as a possible air interface for future wireless networks. It combines most of the advantages of orthogonal frequency division multiple access and the low peak to average power ratio (PAPR) of single carrier (SC) transmission. Existing transmit antenna diversity techniques such as space-time block coding and space-frequency block coding are incompatible either with the system constraints or with the SC nature of SC-FDMA. We propose a novel space-frequency flexible coding scheme compatible with SC-FDMA and we prove its good performance both in terms of PAPR and bit error rate (BER) on frequency selective multiple input multiple output channels.

Comparison of antenna array techniques for the downlink of multi-carrier CDMA systems

The paper deals with transmit pre-filtering using antenna arrays in the downlink of wireless cellular systems based on multi-carrier code division multiple access (MC-CDMA). In addition to the signal separation in frequency obtained by spreading codes, the base station exploits the spatial separation of users to improve interference mitigation, which enables the use of low-complexity mobile receivers. We propose transmit filtering techniques optimizing the signal separation in space only, or jointly in space and frequency, according to single- and multi-user criteria. These techniques assume channel knowledge at transmission, and are particularly destined for time division duplex systems, where we may benefit from channel reciprocity between up- and downlink. We compare the performance of the proposed techniques in terms of interference reduction and robustness to time variant channels. Simulation results emphasize the benefit of transmit filtering optimized jointly in space and frequency, which allows a striking increase on the user capacity thanks to space division multiple access.

Influence of system load on channel estimation in MC-CDMA mobile radio communication systems

This paper investigates the influence of a variable system load on the performance of the channel estimation in a multicarrier code division multiple access (MC-CDMA) transmission scheme. We consider channel estimation based on pilot symbols, which are periodically inserted into the MC-CDMA frame. Starting from the analytical expression of the decision variable, we show the impact of erroneous channel estimation as a function of system load. We derive a simple adaptation rule for the pilot symbol power in order to maintain the signal-to-noise ratio (SNR) loss due to channel estimation errors constant for all system loads. We show that this simple rule enables us to minimize the total transmitted power required at a given bit error rate (BER), particularly for systems with high spreading gain as used in mobile radio channels with high frequency diversity.

Optimum and sub-optimum linear MMSE multi-user detection for multi-carrier CDMA transmission systems

This paper investigates optimum and sub-optimum linear minimum mean square error (MMSE) multi-user detection (MUD) techniques for multi-carrier code division multiple access transmission systems. In a first step, we present two formulations of MMSE MUD techniques that are optimum for any number of users and any power distribution and achieve exactly the same performance with a different computational complexity. In a second step, we present a simplification of optimum MMSE MUD techniques that achieves sub-optimum performance but at a reduced complexity that is similar to single-user detection techniques. Simulation results on indoor channels show that optimum and sub-optimum MMSE MUD techniques outperform the MMSE single-user detection technique, especially for low and medium system loads as well as in the case of the near-far effect.