Abstract
This letter tackles the problem of optimizing the parameters of a symbol-level precoder for downlink multi-antenna multi-user systems in the finite block length regime. Symbol-level precoding (SLP) is a non-linear technique for multi-user wireless networks, which exploits constructive interference among co-channel links. Current SLP designs, however, implicitly assume asymptotically infinite blocks, since they do not take into account that the design rules for finite and especially short blocks might significantly differ. This letter fills this gap by introducing a novel SLP design based on discrete constellation rotations. The rotations are the added degree of freedom that can be optimized for every block to be transmitted, for instance, to save transmit power. Numerical evaluations of the proposed method indicate substantial power savings, which might be over 99% compared to the traditional SLP, at the expense of a single additional pilot symbol per block for constellation de-rotation.
Highlights
The capacity gains stemming from full frequency-reuse downlink transmissions is significantly impacted by the underlying multi-user interference (MUI)
If symbol-level precoding (SLP) is applied, the transmit vector x[k] is optimized in such a way to guarantee that the scaled version of the received signal at the nth user pertains to the extended constellation region Sn[k] ⊂ C associated with symbol sn[k]
Due to the complexity of the proposed precoder, which scales exponentially with the number of constellation points, we focus on quadrat√ure PSK (QPSK) transmissions only
Summary
The capacity gains stemming from full frequency-reuse downlink transmissions is significantly impacted by the underlying multi-user interference (MUI). In [7], [8], the authors have shown different optimization strategies, including sum power minimization and max-min fairness problem for phase shift keying (PSK) modulations Those results have been extended to multi-level modulations in [9]. The authors in [13] have discovered a new degree of freedom for the design of SLP-based multi-user systems, namely: the rotation of the symbol constellation of each user as part of the overall precoding procedure They have assumed a continuous-phase rotation for each user and formulated a non-convex optimization problem to determine the optimal transmit vectors in each symbol interval along with the optimal constellation rotation for all symbol intervals within the coherence time.
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