Abstract-In this work we tackle the problem of mitigating the multi-user interference by optimizing the time-hopping codes, in an asynchronous impulse radio multiple access scheme. We derive the expression of the multi-user interference variance at the output of a rake receiver assuming that the codes are deterministic, for both pulse position modulation and pulse amplitude modulation formats, when propagating through multipath channels. The result shows that the code contribution is independent of the other parameters. We derive from this expression a practical criterion that enables us to find a set of optimal codes that ensures minimal multi-user interference variance at the receiver output. We check through simulations, that the set of optimal codes found using the criterion, leads to bit error rate improvement.

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In the context of PAM Time-Hopping/Direct Sequence Impulse Radio Ultra Wide Band (IR-UWB), a residual interference occurs at the output of the Rake Receiver when realistic multipath propagation channel is considered even if a large guard-time interval is used. As a consequence, the performance is limited and exhibits BER floor. In this paper, we explicitally explains what causes such a residual interference. Secondly, we propose a simple way to modify the Rake receiver in order to totally remove this residual interference. The results are illustrated by simulations that validate the proposed method.

Abstract-We consider an ultra wide band impulse radio multiple access scheme based on time-hopping code under a realistic multipath channel model. As a novelty, we provide a closed-form expression for the variance of the multi-user interference at the output of the rake receiver of the user of interest. That enables us to characterize the optimal time-hopping codes minimizing the multi-user interference variance. Finally we observe through simulations that minimizing the multi-user interference variance translates into better performance by significantly decreasing the error probability.