Uplink Pilots for Multiuser MIMO with Mixed Grant Free and Grant Based Transmissions

Abstract : Pilot based acquisition of channel state information (CSI) is a challenging problem in multiuser multiple-input multiple-output (MU-MIMO) systems that allow unscheduled intermittent transmissions from user terminals (UTs). These challenges stem mainly from the need to account for i) possible intra-cell pilot collisions or interference which are characteristic of unscheduled uplink transmissions and ii) the fact that these same pilots are typically also needed for user activity detection at the base station (BS). The problem gets even more challenging in situations with both scheduled (grant based) and unscheduled (grant free) concurrent uplink transmissions because of the fact that grant free and grant based transmissions are typically different in terms of key performance indicators (KPIs). In this article, we present an assignment and multiplexing scheme for uplink pilot sequences from grant free and grant based transmissions that take place on the same time-frequency resources and we numerically assess its performance in terms of both CSI quality and user activity detection probability. The scheme is based on a novel method that can extend any set of state-of-the-art orthogonal pilots to generate a hybrid (orthogonal/non-orthogonal) pilot set with a built-in trade-off between the maximum achievable sum rate for grant based transmissions and the maximum tolerable level of pilot interference for grant free transmissions.
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Nassar Ksairi, Merouane Debbah. Uplink Pilots for Multiuser MIMO with Mixed Grant Free and Grant Based Transmissions. 2018 IEEE 87th Vehicular Technology Conference (VTC Spring), Jun 2018, Porto, Portugal. ⟨hal-01962091⟩

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