Showing papers by "Sequans published in 2011"

MU-MIMO in LTE Systems

Abstract: A relatively recent idea of extending the benefits of MIMO systems to multiuser scenarios seems promising in the context of achieving high data rates envisioned for future cellular standards after 3G (3rd Generation). Although substantial research has been done on the theoretical front, recent focus is on making Multiuser Multiple-Input Multiple-Output (MUMIMO) practically realizable. This paper presents an overview of the different MU-MIMO schemes included/being studied in 3GPP standardization from LTE (long-term evolution) to LTE Advanced. MU-MIMO system concepts and implementation aspects have been studied here. Various low-complexity receiver architectures are investigated, and their performance assessed through link-level simulations. Appealing performance offered by low-complexity interference aware (IA) receivers is notably emphasized. Furthermore, system level simulations for LTE Release 8 are provided. Interestingly, it is shown that MU-MIMO only offers marginal performance gains with respect to single-user MIMO. This arises from the limited MU-MIMO features included in Release 8 and calls for improved schemes for the upcoming releases.

Blind source separation of convolutive mixtures of non circular linearly modulated signals with unknown baud rates

Abstract: This paper addresses the problem of blind separation of convolutive mixtures of BPSK and circular linearly modulated signals with unknown (and possibly different) baud rates and carrier frequencies. In previous works, we established that the Constant Modulus Algorithm (CMA) is able to extract a source from a convolutive mixture of circular linearly modulated signals. We extend the analysis of the extraction capabilities of the CMA when the mixing also contains BPSK signals. We prove that if the various source signals do not share any non zero cyclic frequency nor any non conjugate cyclic frequencies, the local minima of the constant modulus cost function are separating filters. Unfortunately, the minimization of the Godard cost function generally fails when considering BPSK signals that have the same rates and the same carrier frequencies. This failure is due to the existence of non-separating local minima of the Godard cost function. In order to achieve the separation, we propose a simple modification of the Godard cost function which only requires knowledge of the BPSK sources frequency offsets at the receiver side. We provide various simulations of realistic digital communications scenarios that support our theoretical statements.

Method of exploitation of Co-channel Interference, apparatus and computer program

Abstract: A method of canceling co-channel interference of a signal received by a mobile station having at least two receiving antennas, in a cellular OFDMA-TDD communication network having a serving base station and at least one interfering base station. The signal is a result of a serving signal transmitted by the serving base station which is distorted by at least one interfering signal transmitted by the interfering base station. The serving signal is transmitted with a first modulation scheme and the interfering signal is transmitted with a second modulation scheme. The method includes: a channel estimation phase delivering channel estimations of the serving signal and the interfering signal; and a MIMO detecting phase, detecting symbols by using the channel estimations of the serving signal and the interfering signal. The MIMO detection phase is realized using a non-linear method which uses the first modulation scheme and the second modulation scheme.

Cell search method for a downlink channel of an OFDMA transmission system

Abstract: A cell search method in a multiple carrier wireless network includes a step of trying to select, among a list of candidate couples [carrier frequency; bandwidth], at least one candidate associated with at least one cell of the multiple carrier wireless network. The cell search method includes: a step of obtaining and ordering an initial list of possible candidate couples [carrier frequency; bandwidth] according to the multiple carrier wireless network; a step of reducing the initial list, delivering a reduced list, the reduced list including only candidates compatible with a predetermined communication mode of the multiple carrier wireless network, through a differential cross correlation of the candidate couples [carrier frequency; bandwidth] of the initial list; a cell search step in which a cross-correlation is performed by testing all time/frequency hypothesis for at least one candidate of the reduced list of candidates, delivering synchronisation information.

Spectrum Aggregation: Initial outcomes from SAMURAI project

Abstract: Multi-user MIMO and Spectrum Aggregation (also referred to Carrier Aggregation) are two key enablers of next generation wireless systems. Although those techniques were already quite well investigated at theoretical level, their practical implementation is not immediate and raises numerous challenges. The SAMURAI project aims at investigating such challenges as well as at providing realistic performance results. This paper presents first outcomes of the project, with a specific focus of Carrier Aggregation (CA). Two aspects are discussed: CA at PHY/RF level with implementation challenges. Then, the system aspect of CA, providing new flexibility in terms of radio resource management is investigated. System simulations show capacity improvement when CA is enabled. SAMURAI outcomes in the MU-MIMO aspects are reported in a companion paper [9].

Simplified EM channel estimation in LTE systems

Abstract: In this paper, an iterative channel estimation algorithm is considered in LTE systems. A simplified expectation maximization (EM) based algorithm is proposed for channel estimation in the frequency domain. In order to deal with “null” sub-carriers in the guard band and make the algorithm simpler, the truncated singular value decomposition method is integrated to EM (TSVD-EM). Together with a simple linear interpolation in time domain, channel estimates are obtained in an iterative way with low complexity. The mean square error of the proposed algorithm is also analyzed, and, based on this analysis, an additional constant variance is proposed to be used in the detection process. Simulation results show that the proposed channel estimation and detection methods have very close performance to the perfect case.

Interference Cancellation Method with Multiple Data Layer MIMO Transmission

Abstract: A method and apparatus are provided for cancelling interferences of a received MIMO RF signal. The method includes: receiving a signal y; an initial step of obtaining, from the received signal y, at least one data representative of the received signal, called decoded data; computing the decoded data as a function of at least one predetermined criterion, delivering feedback data and a feedback decision; and, as a function of the feedback decision, at least one iteration of the following steps: constructing a set of feedback symbols from the feedback data; obtaining, from the set of feedback symbols, at least one data representative of a feedback decoded signal, called feedback decoded data; computing the at least one feedback decoded data as a function of the at least one predetermined criterion, delivering the feedback data.

Channel estimation method

Abstract: A channel estimating method is provided for a signal transmitted in an orthogonal frequency division multiplexing (OFDM) system, wherein an OFDM symbol of the signal has at least one null subcarrier set in its defined bandwidth. The method includes: obtaining at least one initial channel estimate over at least one OFDM symbol of a received signal; processing the at least one initial channel estimate for delivering at least one soft information in a form of a complete matrix; and updating the at least one initial channel estimate by performing a truncated singular value decomposition of a partial FFT matrix. The partial FFT matrix includes only the first L DP columns and the rows N DP of an FFT matrix of the complete matrix, the N DP rows corresponding to modulated subcarriers of received signal, including data and pilot symbols and excluding the at least one null subcarrier, delivering an improved channel estimate.

Device for simulating MIMO environment

Abstract: The invention relates to a device which comprises a plurality of input ports for accepting corresponding input RF signals corresponding to a plurality of transmitting antenna elements and a plurality of output ports for generating coherent output signals to a corresponding plurality of receiving antenna elements, whereby the output ports and the input ports are interconnected by a module comprising a network of interconnected hybrid elements. According to the invention, said module further comprises at least one attenuator being connected to at least one input and or output of at least one of hybrid element of said network of hybrid elements.

Simple iterative channel estimation in LTE systems

Abstract: In this paper, a simple iterative channel estimation algorithm is studied in LTE systems. For estimation in the frequency domain, a simple iterative interpolation method is proposed by using the soft information from the channel decoder. Together with a simple linear interpolation in time domain, channel estimates are obtained in an iterative way with low complexity. Simulation results show that even simple iterative approaches provide considerable improvement.

Method for testing a device bound to be inserted in a mobile terminal, as well as corresponding system, device and computer program product

Abstract: The invention relates to a method for testing a device (DUT) bound to be inserted in a wireless mobile terminal. According to the invention, said method comprises the following steps : - building of a test signal from an RF signal, said building step comprising an insertion step, in said RF signal, of instructions for a self-test procedure of said device, - transmitting said test signal to a device to test (DUT), - receiving said test signal by said device to test, - intercepting in said test signal by the device to test, of said instructions, and - implementing of a self-test procedure delivering settings (parameters) values to an automatic test equipment (ATE), as a function of said instructions intercepted in said test signal.

Method and apparatus for optimizing transmission diversity

Abstract: A method and apparatus are provided for transmission in a communication system. The system includes a transmitter having at least one first and second antenna and a receiver, which includes at least one receiving antenna. The method includes determining a transmit diversity scheme for at least one group of symbols from among a plurality of modulated symbols. This step includes at least one step of obtaining a phase shift of at least one symbol of the at least one group of symbols. The phase shift is based on: at least one phase shift parameter N; and responses (h k1 , h k2 ), respectively, between the at least one first antenna and the at least one receiving antenna and between the at least one second antenna and the at least one receiving antenna. The method further includes a step of phase shifting, allowing rotation of said symbol at a multiple of a π/N value.

Method and apparatus for enhancing ul throughput in ofdma systems

Abstract: A method is provided for enhancing uplink frame throughput in an OFDMA system. The method includes an uplink map fill phase for at least one mobile station including allocation of grants in said uplink frame for the at least one mobile station. After the fill phase, the method includes at least one iteration of: a subchannel expansion in which at least one slot is allocated for transmission, for a current grant, to a mobile station in the layer of the uplink frame, the expansion being performed for at least one of the at least two mobile stations, and the expansion delivering an expanded uplink map; and a slot compression step, from the expanded uplink map, in which an improvement of a modulation and coding scheme is realized by changing the modulation and coding scheme of a grant dependent on an current pair's optimizing mutual capacity, delivering an improved uplink map.