Abstract
Cell-free massive MIMO (CF-mMIMO) provides wireless connectivity for a large number of user equipments (UEs) using access points (APs) distributed across a wide area with high spectral efficiency (SE). The energy efficiency (EE) of the uplink is determined by (i) the transmit power control (TPC) algorithms, (ii) the numbers, configurations, and locations of the APs and the UEs, and (iii) the propagation channels between the APs and the UEs. This paper investigates all three aspects, based on extensive (~30,000 possible AP locations and 128 possible UE locations) channel measurement data at 3.5 GHz. We compare three different TPC algorithms, namely maximization of transmit power (max-power), maximization of minimum SE (max-min SE), and maximization of minimum EE (max-min EE) while guaranteeing a target SE. We also compare various antenna arrangements including fully-distributed and semi-distributed systems, where APs can be located on a regular grid or randomly, and the UEs can be placed in clusters or far apart. Overall, we show that the max-min EE TPC is highly effective in improving the uplink EE, especially when no UE within a set of served UEs is in a bad channel condition and when the BS antennas are fully-distributed.
Highlights
We recently proposed that a large amount of channel data for CF-mMIMO systems can be measured using a compact channel sounder with a drone acting as a virtual array and released open-source channel data [2]
We show that the max-min EE is very effective, especially when no user equipments (UEs) within a set of served UEs is in a bad channel condition, minimum mean square error (MMSE) combining is applied, when more base station (BS) antennas are used in comparison to the number of UEs, when the BS antennas are fully-distributed evenly across the coverage area, and when the UEs are far apart in the case of distributed BS antennas
For CF-mMIMO, evaluating the trade-offs between the spectral efficiency (SE) and EE for different types of transmit power control (TPC) algorithms is very important for a large number of battery-powered UEs which the system serves
Summary
C ELL-FREE massive MIMO (CF-mMIMO), which combines various wireless communication system concepts such as mMIMO, ultra-dense networks, and cooperative multi-point (CoMP), exploits a large number of access points (APs) distributed across a wide area to reliably serve a large number of user equipments (UEs) while suppressing the inter-cell interference conventional cellular systems suffer from [1]. In a recent conference paper [24], we suggested the maxmin EE method, which optimizes the power allocation to maximize the minimum uplink EE over all UEs, at a given target SE This algorithm improved EE for UEs with the lowest EE in comparison to the max-power and max-min SE algorithms. C. CONTRIBUTIONS To provide a more realistic assessment of TPC algorithms, and bridge the gap between the theory and practical implementation of CF-mMIMO, we apply three different TPC algorithms (max-power, max-min SE, and max-min EE) to a large number of measured propagation channel data at 3.5 GHz to analyze the trade-offs between the EE and SE for CF-mMIMO systems with varying numbers, configurations, and locations of the APs and UEs. The amount of data used for these analyses is very large, featuring ∼30,000 possible AP locations and 128 possible UE locations across a 200m×200m area, providing statistical confidence of the evaluated performances in a realistic deployment setting. We show that the max-min EE is very effective, especially when no UE within a set of served UEs is in a bad channel condition, minimum mean square error (MMSE) combining is applied, when more BS antennas are used in comparison to the number of UEs, when the BS antennas are fully-distributed evenly across the coverage area, and when the UEs are far apart in the case of distributed BS antennas
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