Browsing by Author "Mitra, Urbashi"

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    Channel Estimation for Multirate DS-CDMA Systems
    (2000-11-20) Sabharwal, Ashutosh; Mitra, Urbashi; Bhashyam, Srikrishna; Center for Multimedia Communications (http://cmc.rice.edu/)
    The problem of channel estimation for multiple data rate Direct-Sequence Code-Division Multiple-Acess (DS-CDMA) systems is addressed. The derived algorithms are suitable for both variable chipping rate and/or variable spreading multirate systems. Furthermore, methods for uplink and downlink channel estimation are investigated. It is observed that direct exploitation of the multirate nature of the signal yields improved performance. The algorithms are evaluated via simulation.
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    Rate-constrained Relaying: A Model for Cooperation with Limited Relay Resources
    (2005-06-01) Sabharwal, Ashutosh; Mitra, Urbashi; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, the impact of limited resources on achievable rates in relay channels is investigated. Resource limitation is modeled as a rate constraint, Rbar, which constrains the rate at which a relay can reliably decode or source-code data. With this constraint in hand, the achievable rates for several protocols are derived and compared: decode-and-forward, estimate-and-forward, a Markovian amplify-and-forward and a hybrid scheme that combines decode-and-forward and amplify-and-forward. The key result is that the relative performance of these protocols in the unconstrained case is not maintained in rate constrained scenarios. The case of severe resource limitation (Rbar --> 0) is also considered. Previously established rates for unconstrained decode-and-forward and estimate-and-forward are re-examined in the context of optimal parameters and asymptotic performance. For most relay locations (topologies), either decode-and-forward and estimate-and-forward offers superior performance. However, for severe rate constraints, amplify-and-forward generally offers the best performance.