ViPer NL-COMM: Making Vector Perturbation Precoding Practical

TL;DR

This work tackles the practical gap for downlink vector perturbation precoding by introducing ViPer NL-COMM, a scalable, hardware-friendly framework that extends NL-COMM to the downlink. It combines a preprocessing stage that identifies promising perturbation vectors with an improved SRQD-based downlink formulation to enable VP in overloaded MIMO, all implemented on an FPGA to meet real-time constraints. The FPGA-enabled ViPer NL-COMM supports up to 16×16 MIMO, 256-QAM, and 100 MHz bandwidth, delivering higher throughput than MMSE and existing NL precoders while achieving substantial RF power savings due to reduced antenna counts. The results demonstrate viable, energy-efficient non-linear precoding suitable for future Open RAN and dense connectivity scenarios.

Abstract

Large MIMO systems rely on efficient downlink precoding to enhance data rates and improve connectivity through spatial multiplexing. However, currently employed linear precoding techniques, such as MMSE, significantly limit the achievable spectral efficiency. To meet practical error-rate targets, existing linear methods require an excessively high number of access point antennas relative to the number of supported users, leading to disproportionate increases in power consumption.Efficient non-linear processing frameworks for uplink MIMO transmissions, such as NL-COMM, have been proposed. However, downlink non-linear precoding methods, such as Vector Perturbation (VP), remain impractical for real-world deployment due to their exponentially increasing computational complexity with the number of supported streams. This work presents ViPer NL-COMM, the first practical algorithmic and implementation framework for VP-based precoding. ViPer NL-COMM extends the core principles of NL-COMM to the precoding problem, enabling scalable parallelization and real-time computational performance while maintaining the substantial spectral-efficiency benefits of VP precoding. ViPer NL-COMM consists of a novel mathematical framework and an FPGA prototype capable of supporting large MIMO configurations (up to 16x16), high-order modulation (256-QAM), and wide bandwidths (100 MHz) within practical power and resource budgets. System-level evaluations demonstrate that ViPer NL-COMM achieves target error rates using only half the number of transmit antennas required by linear precoding, yielding net power savings on the order of hundreds of Watts at the RF front end. Moreover, ViPer NL-COMM enables supporting more information streams than available AP antennas when the streams are of low-rate, paving the way for enhanced massive-connectivity scenarios in next-generation wireless networks.

Figures (16)

• Figure 1: Coded bit error rate performance comparison of ViPer husmann'18 and ViPer-NLCOMM, assuming 8 $\times$ 8 Rayleigh fading channels with 64-QAM modulation and LDPC code rate 5/6.
• Figure 2: Spectral efficiency comparison of MMSE, THP, FSE, and ViPer for (a) 8 AP antennas and (b) 10 AP antennas, assuming Rayleigh fading channels with 64-QAM modulation and LDPC code rate 5/6 at an intermediate SNR of 17 dB. Dashed lines represent the 10% PER boundaries for different number of concurrently transmitted streams.
• Figure 3: Top-level architecture of the ViPer NL-COMM based non-linear precoder.
• Figure 4: Spectral efficiency comparison of linear (MMSE) and non-linear (ViPer) precoding for an 8-antenna AP serving single-antenna low-rate users employing 4-QAM and 1/2 rate LDPC codes with 1944 block length at an SNR of 17 dB.
• Figure 5: Coded BER performance for an 8x8 MU-MIMO MMSE vs ViPer NL-COMM under 3GPP CDL-B (5 km/h) using QPSK and 0.5 code rate.