1,720,974 research outputs found
Blind Multi-User Equalisation for a Dispersive DS-CDMA Downlink under Carrier Frequency Offset Conditions
No full textWe address blind multiuser detection in a DS-CDMA downlink channel in the presence of carrier frequency offset. The synchronous users are separated by re-establishing orthogonality of their spreading sequences in a shared equaliser at the chip level. The adaptation algorithm is based on a constant modulus criterion of the various users, for which a stochastic gradient descent algorithm can be derived. We show that the resulting filtered-error filtered-regressor algorithm requires modifications in order to cope with carrier offset, and propose an combined blind multiuser equaliser with blind carrier frequency offset estimation. Simulations demonstrating the algorithm's convergence and BER performance are presented
Adaptive Blind Chip-Level Multiuser Detection in Multi-Rate Synchronous DS-CDMA System with Partial Loading
No full textThis paper addresses an adaptive blind multiuser detection strategy for a multi-rate direct sequence CDMA downlink channel. Specifically, two chip-level equalisation schemes, using either variable spreading length (VSL) or multi-code (MCD) multi-rate access modes, are proposed and tested. Both equalisers can be updated by minimising a hybrid CM/MSE cost function based on the constant modulus (CM) criterion for active users and a mean square error (MSE) criterion for inactive users in a partially loaded system. The BER performance abd the convergence of the proposed algorithms are analysed and compared through various simulations in both partially and fully loaded systems. Essentially, our results show that the VSL algorithm exhibits faster convergence than the MCD scheme while similar BER performance over different multi-path channels is achieved
A Fast Converging Concurrent Affine Projection Algorithm for Blind Multiuser Equalisation
No full textIn this paper, we derive a fast converging concurrent affine projection algorithm for blind multiuser equalisation, suitable for downlink DS-CDMA systems. Adaptation is performed by concurrently minimising two cost functions based on either a CM criterion or a DD scheme for all active users. Concurrent schemes are known to provide faster convergence. Similarly, affine projection algorithms have been incorporated into blind algorithms elsewhere with good improvement of convergence speed. We show by computer simulation that substantial performance improvements can be gained over either technique on its own
A Semi-Blind Channel Equalisation Strategy for the UMTS-TDD Downlink
No full textIn this paper, we propose a semi-blind channel equalisation scheme for the downlink time-division duplex (TDD) component of the universal mobile telecommunication system (UMTS). The main two concepts introduced consist of: first exploiting the constant modulus carried by the decoded symbols of all active users; and second making use of the inactive users to load a suitable number of pilot signals. In response to this scenario, a semi-blind equalisation is performed in the data fields, whereby the equaliser weights are updated by minimising a hybrid CM/MSE cost function based on the constant modulus (CM) criterion for all active users and a mean square error (MSE) criterion for both absent users and pilot signals. Computer simulations are used to assess and analyse the channel equalisation strategy in terms of MSE and BER performance over a quasi-static dispersive channel
A Blind Synchronous Multi-User DS-CDMA Equaliser With Partial Loading
No full textIn this paper, we propose a blind multiuser detection scheme in a direct sequence CDMA downlink scenario by means of a chip-level equaliser, which can be updated even if not all possible users are active, i.e. the system is partially loaded. The active synchronous users are separated by re-establishing orthogonality of their spreading sequences in a common chip-level equaliser. The adaptation algorithm is mainly based on a constant modulus (CM) criterion applied to the active users. The inactive codes in the system must be considered, for which we propose and compare three different methods: (i) a mean square error criterion for absent users, and a CM approach with (ii) zero modulus or (iii) the transmission of arbitrary signals with small code amplitude. For all three cases, stochastic gradient descent algorithms are derived. The proposed algorithms are analysed and compared through various simulations, which demonstrate the algorithms' convergence and BER performance
A Coding and Equalisation Approach for Dispersive Channels with Structured Additive Noise
No full textOversampled filter banks (OSFB) have recently been considered for channel coding applications where the noise is correlated. In this paper, we address the additional task of blindly mitigating the effects of channel dispersion, which would otherwise destroy the orthogonality of the channel coder. An example for a power line communication setting is provided, comparing the proposed method to existing solutions
Blind Chip-Rate Multiuser Equalisation
No full textThis paper addresses blind multiuser detection in a DS-CDMA downlink channel. The synchronous users are separated by re-establishing orthogonality of their spreading sequences in a common chip-rate equaliser. The adaptation algorithm can be based either on a constant modulus (CMA) criterion of the various users, or on a decision directed (DD) scheme. In either case, a stochastic gradient descent algorithm will result, as derived, in a multiple error filtered-X LMS type approach, whereby the equaliser input to both the multichannel CMA or DD algorithm are replaced by spreading code filtered versions. Adaptation examples are given underlining some of the characteristics of the proposed algorithm
Switching between two Different Fractionally Spaced Subband Adaptive Equalisation Structures
No full textThis paper discusses methods for the fast adaptation of fractionally spaced equalisers. Subband methods offer advantages in adapting large scale equaliser, whereby two structures have been previously proposed. A first structure is faster, but can only be operated in training mode. A second structure permits the use in both training and decision directed mode at the cost of slower convergence. We propose and demonstrate a switching method to exploit the various benefits as best as possible
Blind Chip-Rate Equalisation for DS-CDMA Downlink Receiver
No full textA dispersive channel in the DS-CDMA downlink destroys the orthogonality of the synchronous users' spreading sequences. In this paper, we aim to re-establish this orthogonality blindly by means of a common chip-level equaliser. The adaptation algorithm is based on a constant modulus criterion forcing the various user symbols onto a constant modulus, for which a stochastic gradient descent algorithm is derived. This algorithm is structurally similar to a multiple error filtered-X LMS type approach, whereby the equaliser input CM update is replaced by a spreading code filtered version. Various simulations demonstrating the algorithm's convergence and noise performance are presented
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